WO2011085196A2 - Quantum well waveguide solar cells and methods of constructing the same - Google Patents

Quantum well waveguide solar cells and methods of constructing the same Download PDF

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Publication number
WO2011085196A2
WO2011085196A2 PCT/US2011/020513 US2011020513W WO2011085196A2 WO 2011085196 A2 WO2011085196 A2 WO 2011085196A2 US 2011020513 W US2011020513 W US 2011020513W WO 2011085196 A2 WO2011085196 A2 WO 2011085196A2
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thin film
solar cell
layer
film solar
emitter
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PCT/US2011/020513
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French (fr)
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WO2011085196A3 (en
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Roger E. Welser
Ashok K. Sood
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Magnolia Solar, Inc
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Publication of WO2011085196A3 publication Critical patent/WO2011085196A3/en

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    • H01L31/02—Details
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    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H01L31/02168—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells the coatings being antireflective or having enhancing optical properties for the solar cells
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
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    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
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    • H01L31/0735—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers the potential barriers being only of the PN heterojunction type comprising only AIIIBV compound semiconductors, e.g. GaAs/AlGaAs or InP/GaInAs solar cells
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    • H01L31/1892—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof methods involving the use of temporary, removable substrates
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    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • This invention relates to semiconductor-based photovoltaic energy converters, also known as “solar cells,” and to the design and fabrication of the same.
  • photovoltaic solid state semiconductor devices convert sunlight into electrical power by generating both a current and a voltage upon illumination.
  • the current source in a solar cell is the charge carriers that are created by the absorption of photons. These photogenerated carriers are typically separated and collected by the use of PN or PIN junctions in semiconductor materials.
  • the operational voltage of the device is limited by the dark current characteristics of the underlying PN or PIN junction.
  • This invention overcomes the disadvantages of the prior art by providing a solar cell design and a process for constructing a solar cell that includes two sets of design elements that separately, or in combination, can increase the power output of semiconductor solar cells.
  • an illustrative embodiment combines the two sets of design elements together to increase both the voltage and current output of thin film solar cells and enable them to approach the performance limits calculated by detailed balance methodologies that assume a quasi-Fermi level separation.
  • One set of design elements relate to the material structure of the active region of the solar cell device where photo-generated carriers are created and separated.
  • the basic active region structure consists of a PN or PIN junction-containing materials of different energy gap within the junction depletion region.
  • a novel feature of the design is the positioning of the different energy gap material within the active region.
  • a wider energy gap barrier layer can also be positioned within the depletion region adjoining either the P-type layer or the N-type layer or both in order to suppress carrier injection across the junction.
  • wider energy gap material can be located within the depletion region in the zone of maximum space charge recombination, where the injected electron and hole concentrations are comparable.
  • the dimensions of the narrower energy gap material can be widely varied.
  • the structure can employ quantum structured active regions as an option.
  • the structure positions at least one of the transitions from the narrowest energy gap material in active region to a wider energy gap material so that that it remains within the depletion region over a wide range of bias levels, even at forward bias levels appropriate for photovoltaic power generation.
  • a second set of design elements relate to light-guiding layers placed above and/or below the active region of the device in order to enhance optical absorption within the active region.
  • the application of anti-reflection layers above the active region of the device can be employed to maximize the number of incident photons that are directed into the device active region. Scattering of the incident light into the plane of the active region absorber layers can be accomplished by the adding nanostructured layers above the photon absorbing layer and/or the application of metallic or dielectric nanoparticles to the top semiconductor surface.
  • Photons that pass through the device absorber layers can be redirected back into the active regions of the device for a second chance at absorption by including a reflector structure on the back side of the photovoltaic device.
  • efficient photovoltaic devices both maximize the creation and collection of photo-generated carriers and enhance the voltage at which photo-generated carriers are extracted.
  • the following detailed discussion describes both a novel, broadly-defined material structure for a thin film solar cell that can enhance the voltage performance, along with design strategies for maximizing the current generating capability of thin film photovoltaic devices.
  • a thin film photovoltaic solar cell device structure and method of manufacturing comprises at least one narrow energy gap, higher refractive index semiconductor well placed within a wider energy gap, lower refractive index semiconductor junction depletion region; located between an overlying, forward-scattering, antireflective top structure comprising an optical coating of at least one layer, such that the refractive index of each layer in the optical coating is intermediate between the refractive index of the top side of the semiconductor thin film solar cell and air; and an underlying, back-scattering bottom structure comprising a metal film and an optical coating of at least one layer located between the metal film and the back side of the semiconductor thin film solar cell structure.
  • a III-V quantum well thin film solar cell device comprises: a III-V semiconductor extended emitter PN heterojunction, in which the extended emitter heterojunction defines an emitter layer of one dopant type overlying a smaller band gap base layer of the opposite dopant type, which furthermore comprises a region of the PN junction depletion region adjacent to the emitter layer that contains material with higher energy gap than the underlying base layer; at least one narrow energy gap well inserted into the depletion region of the extended emitter heterojunction structure adjacent to the base layer; a top window layer comprised of a III-V semiconductor of higher energy gap than the emitter layer and appropriate top metal contacts; a bottom back surface filed layer comprised of a III-V semiconductor of higher energy gap than the base layer and appropriate bottom metal contacts; and a top antireflective structure comprising an optical coating of at least one layer, such that the refractive index of each layer in the optical coating is intermediate between the refractive index of the top side of the III-V semiconductor thin film
  • Fig. 1 is a schematic cross-sectional side view of a thin film solar cell device according to an illustrative embodiment incorporating a lower band gap, waveguiding absorption layer and optical coatings to increase the optical path length of incident light through the active region of the device;
  • Fig. 2 is a graphical diagram of the energy band versus position for a material structure of a thin film semiconductor solar cell, illustrating the desirable design elements necessary to achieve a high operating voltage in a structure having both narrow and wide energy gap material, in accordance with illustrative embodiments;
  • Fig. 3 is a flow chart of a procedure for manufacturing a thin film waveguide solar cell in accordance with illustrative embodiments
  • Fig. 4 is a flow chart of a procedure for manufacturing a thin film quantum waveguide solar cell in accordance with illustrative embodiments
  • Fig. 5 is graphical diagram of the energy band versus position for a material structure of a quantum well III-V semiconductor solar cell, illustrating the desirable design elements necessary to achieve a high operating voltage in a structure having both narrow and wide energy gap material, in accordance with illustrative embodiments;
  • Fig. 5A is graphical diagram of the energy band versus position for a material structure of a thin film semiconductor solar cell, illustrating various dark diode current mechanisms, in accordance with illustrative embodiments.
  • Fig. 5B is graphical diagram of the energy band versus position for part of the material structure of a thin film semiconductor solar cell, illustrating various carrier escape mechanisms, in accordance with illustrative embodiments.
  • a typical thin film solar cell structure contains a limited volume of low band gap material, and thus requires advanced light trapping structures to reach its potential performance levels.
  • Light management is achieved by assuring that incident photons are not lost due to reflections but are instead directed into the semiconductor absorbing layers.
  • the scattering of incident light to ensure each photon has a non-normal trajectory is a strategy for increasing the optical path length of photons within the absorption layer.
  • the application of a back reflector to bounce any unabsorbed photons back up into the active layers of the device is a beneficial aspect of any effective photovoltaic light trapping scheme.
  • the most effective light trapping schemes will also direct light horizontally into the plane of the absorbing layer. Waveguide structures in which thin layers of high refractive index material are surrounded by low refractive index material provide a physical mechanism by which to achieve this type of in-plane light trapping.
  • FIG. 1 A schematic diagram of an exemplary waveguide solar cell structure is depicted in Fig. 1.
  • This thin film waveguide solar cell incorporates lower band gap, higher index of refraction materials in the active region of the device, along with tailored, nanostructured optical coatings.
  • the optical path length of light incident upon this novel device can be dramatically enhanced via coupling into laterally propagating waveguide modes.
  • This combination of active device structure and passive coatings can redirect normally incident light into laterally propagating waveguides modes, and represents a dramatic change in thin film solar cell design.
  • a thin film waveguide photovoltaic device is depicted according to an illustrative embodiment.
  • incident light 100 first encounters a top covering surface 110, which can be a top cover glass, transparent epoxy or other light transmitting covering surface.
  • the top covering surface 110 is located above a PIN diode semiconductor material device structure 130.
  • the PIN diode structure is coated with a transparent optical coating 120 that minimizes reflection losses and scattering incident light into the underlying PIN diode 130.
  • the refractive index of the top optical coating 120 is illustratively adapted to generate a graded index of refraction antireflection coating, consisting of one or more layers with refractive index intermediate between the covering surface material 110 and the PIN diode semiconductor material 130.
  • one or more of the layers in the top optical coating 120 also incorporate nanoparticles or nanorods which differ in refractive index from that of their surrounding material. Nanostructured optical coatings 120 provide a mechanism by which incident light can be scattered horizontally into the plane of the underlying PIN diode 130.
  • the PIN diode device structure consists of top window/contact layers 132, back surface field/contact layers 136, and incorporates lower energy gap material 134 within the depletion region of the PIN diode structure 130.
  • Lower energy gap material also tends to have a higher index of refraction, thereby resulting in the formation of a waveguide structure.
  • the PIN diode device structure can consist of any common semiconductor materials, including but not limited to group IV materials (Si, Ge, SiGe, SiC, etc.), group III-V materials (GaAs, AlGaAs, InGaP, InGaAs, InP, AlInAs, GaAsSb, InAsSb, AlAsSb, GaN, InGaN, AlGaN, etc.), group II-VI materials (CdS, CdTe, etc.), and group I-III-VI 2 materials (CIGS, etc.).
  • the PIN diode device structure 130 comprises two or more PIN junctions.
  • the refractive index and thickness of the semiconductor materials used in the top window/contact layers 132 is tailored to function as part of a step graded refractive index antireflection structure. Electrical contact is made to the top window/contact layers 132 via metal contacts 125.
  • the back of the semiconductor PIN diode 130 is coated with a conductive, transparent optical coating 140.
  • the refractive index of the bottom optical coating 140 has a value of approximately 1.5 or lower, thereby creating an Omni-directional reflector when combined with the underlying metallic layer 150.
  • the bottom optical coating 140 consists of multiple layers differing in refractive index to form a distributed Bragg reflector.
  • one or more of the layers in the bottom optical coating 140 also incorporate nanoparticles or nanorods.
  • the back-scattering structure consisting of the back optical coating 140 and back metal contact 150, employ plasmonic structures.
  • Plasmonic structures closely coupled to absorbing semiconductors can be used to increase the photocurrent in a variety of thin film solar cells.
  • the peak wavelength of the plasmon resonance is adjustable to match the absorption band of the nearby semiconductor layers, particularly the lower band gap, higher index material 134.
  • optical scattering by the nanoparticles or nanorods above the semiconductor device structure can lead to coupling of photons incident normal to the device surface into lateral optical propagation paths, i.e., paths parallel to the device surface.
  • These parallel optical modes 170 result from the introduction of a lateral wave vector component into the forward scattered wave 160, and can dramatically enhance the optical path length of photons through thin film solar cell device structures. Unabsorbed, lower energy photons that are not coupled into the waveguide modes 170 pass through the PIN diode 130 device before striking a back scattering optical coating 140.
  • Back-scattered light 180 is directed into the active, absorbing layers of the device by the presence of the back-scattering structure, which consists of the back optical coating 140 and back metal contact 150.
  • the front optical coating 120 is configured and arranged with transparent antireflection coating structures to reduce the reflection of incident photons at the material interface between the light transmitting covering surface 110 and semiconductor device structure 130.
  • the back optical coating 140 is configured and arranged to maximize the reflection of unabsorbed photons back into the semiconductor device structure.
  • the front coating 120 and the back coating 140 are implemented in accordance with industry standard processes and materials known to those skilled in the art. These materials include, but are not limited to, titanium dioxide, silicon dioxide, indium tin oxide, zinc oxide, and other transparent conductive oxides (TCOs).
  • TCOs transparent conductive oxides
  • the antireflection coating can be synthesized using a variety of techniques, including sputtering, evaporation, and oblique-angle deposition.
  • Transparent antireflection coating structures can comprise a single layer or multiple layers of materials having an index of refraction intermediate between the semiconductor structure 140 and the media in which the incident photons are delivered, which by way of example is illustrated as a cover glass or encapsulant 110 in Fig. 1.
  • Back reflector structures can comprise either a single metallic layer, or a plurality of layers consisting of a metallic layer in combination with one or more layers of transparent optical material having an index of refraction lower than the semiconductor material.
  • Omni-directional reflectors which combine a metal layer with a low-refractive index layer, provide ultra-high reflectivity over a wide range of wavelengths and incident angles.
  • the forward- and back-scattered light is well known to depend upon the optical properties of the thin film and surrounding environments which dictate the reflection, refraction, and absorption characteristics of the light.
  • Employing an array of nanoparticles or nanorods can provide unique and desirable physical phenomena, particularly when the particle size is very small compared to the incident wavelength.
  • the scattering and absorption characteristics of the forward wave front depend upon the size, shape, density, and permittivity of the nanoparticles. See for example, by way of useful background information, P. Matheu, S. H. Lim, D. Derkacs, C. McPheeters, and E. T.
  • Nanoparticle coatings can provide additional light- trapping benefits when the adjoining semiconductor device structure contains distinct index of refraction steps.
  • optical scattering by nanoparticles atop a semiconductor device structure can lead to coupling of photons incident normal to the device surface into lateral optical propagation paths, i.e., paths parallel to the device surface.
  • These parallel optical modes result from the introduction of a lateral wave vector component into the scattered wave, and can dramatically enhance the optical path length of photons through thin film solar cell device structures.
  • Oblique-angle deposition is a method of growing arrays of nanorods in a wide variety of materials, enabled by surface diffusion and self-shadowing effects during the deposition process. Because the resulting thin films are porous, oblique-angle deposition is utilized as an effective technique for tailoring the refractive index of a variety of thin film materials (see for example, by way of useful background, J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F.
  • the bottom optical coating 140 comprises of a layer nanostructured, porous indium tin oxide layer with a refractive index of 1.5 or lower deposited by oblique angle deposition.
  • the top optical coating 120 comprises multiple layers, with at least one layer of dense indium tin oxide and at least one layer of porous indium tin oxide or porous titanium dioxide deposited by oblique-angle deposition.
  • the top optical coating 120 and the bottom optical coating 140 comprise dielectric and/or metallic nanoparticles embedded within a dense optical film material.
  • examples include Si0 2 nanoparticles embedded within a dense layer of indium tin oxide, Si0 2 nanoparticles embedded within a dense layer of Ti0 2 , Ti0 2 nanoparticles embedded within a transparent encapsulant, Ti0 2 nanoparticles embedded within a dense layer of Si0 2 , and metallic nanoparticles embedded within a dense layer of ITO. Note that deposition of the nanoparticles can occur according to conventional techniques in illustrative embodiments.
  • the operating voltage of a semiconductor PIN diode solar cell 130 is generally dictated by the underlying dark diode current of the device.
  • the dark diode current of semiconductor devices is composed of several different components, all of which are dependent upon the energy gap of the material used in the active junction of the device.
  • each cell in a solar cell consists of one type of material, and the energy gap of that material influences both the current and voltage output of the device.
  • Lower energy gap material can enhance the current generating capability, but typically results in a lower operating voltage. Therefore, it is desirable to provide a device structure 130 that can harness the current generating capabilities of narrow energy gap material while also maintaining a high operating voltage.
  • the layered structures and associated energy bands are depicted in the graphical diagram of Fig. 2, which is simplified to facilitate understanding, showing the elements of semiconductor thin film solar cell device structure 130 according to the illustrative embodiment.
  • the structure 130 simultaneously suppresses the dark diode current and enhances the photocurrent of a thin film cell or subcells.
  • the structure 130 comprises of a window 200, a PIN junction 205, and a back surface field 280.
  • the PIN junction 205 consists of an emitter 220, a base 270, and a depletion region 225 illustratively shown with a wide energy gap (E g ) barrier layer 235, an extended wide energy gap (E g ) emitter layer 240, a narrow energy gap (E g ) well or dot region layer 250, and a depleted base layer region 260.
  • the depletion region is distinguished by the presence of a built-in electric field, a non-zero slope in the conduction band 285 (E c ) and valence band 290 (E v ), induced by the juxtaposition of p-type and n-type semiconductor material.
  • the depletion region is on the order of approximately 0.1 to 2 microns for thin film solar cells. It should be apparent to those skilled in the art that the thickness of the depletion region can be adjusted by varying the thickness of unintentionally doped material between the p-type and n-type material.
  • Window 200 and back surface field 280 layers are typical solar cell structures deposited above and below the PIN junction 205.
  • lightly-doped or undoped higher energy gap material is inserted into depletion region 225 adjacent to the emitter 220 to form an extended wide energy gap (E g ) emitter layer 240 and a barrier layer 235.
  • E g extended wide energy gap
  • the physical boundary between the different energy gap materials is offset from the physical boundary between the emitter and intrinsic/base materials.
  • the wide energy gap material layers adjacent to the emitter within the depletion region reduce both space charge recombination and carrier injection from the emitter.
  • the depletion region consists of multiple layers with different energy gap materials.
  • a region of narrow energy gap material wells or dots 250 is also inserted into in the depletion layer 225.
  • the transitions in energy gap between wide and narrow energy gap material can either be abrupt, step graded, or continuously graded, as illustrated in the conduction band 285 and valence band 290 profiles shown in Fig. 2.
  • the use of wider energy gap material in the emitter layer 220 provides various known benefits for photovoltaic devices.
  • Theories may vary with respect to the physical principals that govern, but in general, a wide energy gap emitter 220 can reduce diffusion-driven injection of majority carriers from the base 270 into the emitter 220. However, other significant dark diode current components, such as space charge recombination within the depletion region 225 and majority carrier injection from the emitter 220 into the base 270, are generally not improved with a typical heterojunction design.
  • both wide and narrow energy gap material within the junction depletion region 225 illustrated in Fig. 2 enables the photovoltaic device to absorb a wider portion of the solar spectrum while simultaneously lowering the dark current to increase the operating voltage of the photovoltaic device.
  • Placing a wide energy gap barrier layer 235 within the depletion region adjoining the emitter provides a means to suppress majority carrier injection out of the emitter 220.
  • Shiotsuka have also suggested that the radiative recombination in quantum wells can be further reduced by employing a step graded profile, as taught in Y. Okada and N. Shiotsuka, "Fabrication of Potentially Modulated Multi-Quantum Well Solar Cells," Proceedings of the 31st IEEE Photovoltaic Specialists Conference, December 2005.
  • a III-V quantum well thin film solar cell comprises an n-type InGaP the emitter 220, a p-type GaAs base layer 260/270, an undoped AlGaAs wide band gap region in the depletion region adjacent to the emitter 235/240, and InGaAs quantum well layers or InAs quantum dot layers 250.
  • a III-V quantum well thin film solar cell device comprises a p-type AlGaAs emitter 220, an n-type GaAs base layer 260/270, wide band gap undoped InGaP and AlGaAs in the depletion region adjacent to the emitter 235/240, and InGaAs quantum well or InAs quantum dot layers 250.
  • the photovoltaic device design illustrated in Figs. 1 - 2 addresses three requirements for quantum well and quantum dot solar cells to achieve their promised levels of performance.
  • the carriers generated by photon absorption in the lower energy-gap material can escape before recombining by positioning the narrow energy gap well or dot region 250 within the depletion region 225.
  • the dark diode current is minimized by suppressing both radiative and non-radiative recombination by the use of a unique structure incorporating both a wide band gap barrier layer within a heterojunction depletion region and step graded wells.
  • absorption in the narrow energy gap well or dot region is maximized by the use of light trapping coatings that can scatter light into the plane of the quantum wells and dots.
  • a method of manufacturing a thin film waveguide solar cell comprises locating, or sandwiching, a thin film solar cell structure between an overlying, forward-scattering, antireflective top structure and an underlying, back-scattering bottom structure. According to the procedure, a plurality of electrical contacts are formed on the predetermined top region of the semiconductor thin film device structure and a predetermined bottom region of the semiconductor thin film device structure.
  • industry standard photolithography process can be employed to selectively etch and metalize the semiconductor thin film device structure.
  • Fig. 3 showing a procedure 300 for manufacturing, or otherwise constructing, a thin film waveguide solar cell as described herein.
  • the procedure provides a thin film solar cell structure at step 310 and provides an overlying, forward-scattering anti- reflective top structure at step 320.
  • An underlying, back-scattering bottom structure is provided at step 330.
  • the thin film solar cell structure is located between the top structure and the bottom structure so as to create a semiconductor thin film device.
  • a plurality of electrical contacts are formed on a predetermined top region of the semiconductor thin film device and a predetermined bottom region of the semiconductor thin film device.
  • a procedure 400 for manufacturing a III-V thin film quantum well or quantum dot waveguide solar cell begins at step 410 by growing a III-V epitaxial structure inverted on a GaAs or InP substrate. Then, at step 420, depositing the back optical coating and metal film, at step 430 attaching a new supporting substrate.
  • a new supporting substrate include metal foil, stainless steel, titanium, polyurethane and other high durability flexible polymers.
  • the procedure removes the III-V epitaxial layers from the substrate, at step 450 the procedure continues by depositing the front optical coating, at step 460 forming metallic contacts and gridlines, and at step 470 encapsulating the device.
  • a method of manufacturing a thin film waveguide solar cell comprises providing a flexible substrate having a front surface and a back surface; depositing an optical film followed by a semiconductor thin film solar cell device structure at least partially on a top surface of the flexible substrate thereby leaving a predetermined top region of the semiconductor thin film device structured exposed; coating the semiconductor thin film solar cell with optical coatings and encapsulant so as to create an optical coating layer and an encapsulant layer on the semiconductor thin film solar cell; and forming a plurality of electrical contacts on the predetermined top region of the semiconductor thin film device structure and a predetermined bottom region of the semiconductor thin film device structure.
  • a method of manufacturing a thin film waveguide solar cell comprises: providing a glass sheet having a front surface and a back surface; coating a transparent film on at least one of the front surface and the back surface of the glass sheet to form a coated glass superstate; depositing a semiconductor thin film solar cell device structure partially on the back surface of the coated glass superstate leaving a predetermined top region of the semiconductor thin film device structured exposed; depositing a reflective optical coating structure; and forming a plurality of electrical contacts on the predetermined top region of the semiconductor thin film device structure and a predetermined bottom region of the semiconductor thin film device structure.
  • the metal contacts can be formed using conventional semiconductor processing technology known to those of ordinary skill.
  • the optical coatings can be applied using oblique angle deposition, sputtering, or evaporation.
  • the semiconductor thin films can be deposited via a variety of conventional material synthesis techniques, including sputtering, evaporation, and chemical vapor deposition. It is further contemplated that other processes can be adapted for thin film deposition including such semiconductor deposition tool as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Commercially available epitaxial lift-off (ELO) processes can be employed to remove the III-V epitaxial layers from their underlying substrate. It should be apparent to those skilled in the art that various other techniques when made available can be employed to fabricate the structures of the described embodiment, herein.
  • the structure 130 comprises a high Al composition (Al% > 50) window 500, an AlGaAs PIN junction 505, and an InGaP back surface field 580.
  • the depletion region is distinguished by the presence of a built-in electric field, a non-zero slope in the conduction band 585 (E c ) and the valence band 590 (E v ), induced by the juxtaposition of p-type and n- type semiconductor material.
  • the depletion region is on the order of approximately 0.1 to 2 microns for III-V thin film solar cells.
  • the thickness of the depletion region can be adjusted by varying the thickness of unintentionally doped material between the p-type and n-type material, and that a similar structure comprises an n-type InGaP emitter and p-type AlGaAs base, according to conventional techniques readily apparent to those of ordinary skill.
  • the illustrative device structure shown in Fig. 5 provides several mechanisms by which to suppress the diode dark current, as illustrated in Fig. 5A, but without being bound to a particular theory.
  • the larger energy gap of emitter 520 relative to the base 570 reduces the diffusion of electrons 592 out of the n-type base layer.
  • hole diffusion 594 is suppressed in this structure by placing a thin InGaP layer 535 adjacent to the p-type AlGaAs emitter 520.
  • the large valence band offset ( ⁇ ⁇ ) at the type II InGaP barrier 535 / AlGaAs emitter 520 interface also alters the distribution of holes 594 within the depletion region.
  • the region of enhanced recombination within the depletion region can be pulled closer to the p-side of the structure, away from the InGaAs well region 550 and into the wide band gap material 540 in the depletion region adjacent to the emitter, thus reducing space charge recombination 596.
  • radiative recombination within the InGaAs wells can still fundamentally limit the dark current, and hence the operating voltage, of the device.
  • One illustrative mechanism by which radiative recombination rates inside the wells can be reduced employs spatial separation of electron and hole wavefunctions, which can result from placing the quantum wells within the built-in field of the depletion region of a P- N or P-I-N junction device.

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Abstract

A material structure and device design are provided that produce efficient photovoltaic power conversion. Materials of different energy gap are combined in the depletion region of a semiconductor junction. A wider energy gap barrier layer is positioned to reduce the diode dark current by suppressing both carrier injection across the junction and recombination rates within the junction. Light guiding layers are placed above and below the active region of the device in order to enhance optical absorption in the lower energy gap material.

Description

QUANTUM WELL WAVEGUIDE SOLAR CELLS AND
METHODS OF CONSTRUCTING THE SAME
FIELD OF THE INVENTION
This invention relates to semiconductor-based photovoltaic energy converters, also known as "solar cells," and to the design and fabrication of the same.
BACKGROUND OF THE INVENTION
With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly known as solar cells, convert sunlight into electrical power by generating both a current and a voltage upon illumination. The current source in a solar cell is the charge carriers that are created by the absorption of photons. These photogenerated carriers are typically separated and collected by the use of PN or PIN junctions in semiconductor materials. The operational voltage of the device is limited by the dark current characteristics of the underlying PN or PIN junction. Thus improving the power output performance of any solid state solar cell generally entails simultaneously maximizing absorption and carrier collection while minimizing dark diode current.
Detailed balance calculations are typically used to compute the ideal, limiting performance of semiconductor solar cell devices (see for example, C. H. Henry, Limiting Efficiencies of Ideal Single and Multiple Energy-gap Terrestrial Solar Cells, J. Appl. Phys., vol. 51, pp. 4494-4500, August 1980). Two fundamental assumptions are traditionally made in these theoretical calculations. First, it is assumed that the diode dark current is limited by radiative recombination, and that the radiative recombination rate is set by the energy gap of the semiconductor material used to fabricate the device. Second, all of the photons in the incident spectrum with energy above the energy gap of the device material are assumed to create a charge carrier pair that is successfully separated and collected. In recent years, several groups have argued that even higher photovoltaic conversion efficiencies can be achieved in structures that mix narrow energy gap wells or dots within a wider energy gap matrix (see for example, G. Wei, K.-T. Shiu, N. C. Giebink, and S. R. Forrest,
Thermodynamic Limits of Quantum Photovoltaic Cell Efficiency, Applied Physics Letters, vol. 91, no. 223507, November 2007, and references therein). Higher theoretical efficiencies are predicted for quantum structures due to a suppression of the radiative recombination rate in the narrow band gap material due to the presence of the wide energy gap material and the generation of a quasi-Fermi level separation.
While Wei et al. teach combinations of material energy gaps that can in theory be combined to achieve theoretical one-sun conversion efficiencies in excess of 40%, they do not provide any specific or practical device designs that would be capable of achieving efficient solar-electric conversion. In practice, the dark currents of almost all semiconductor diodes are limited by non-radiative recombination processes. Therefore, it is desirable to provide for device designs and resulting devices that can effectively suppress dark currents in semiconductor diodes that can be used for solar cell applications. Moreover, the complete absorption and collection assumptions made in detailed balance calculations are not readily achieved in practical devices, particularly ones employing thin absorber layers. Therefore, it is also desirable to provide design strategies and processes that can maximize the
photocurrent generating capability of thin film solar cell devices.
SUMMARY OF THE INVENTION
This invention overcomes the disadvantages of the prior art by providing a solar cell design and a process for constructing a solar cell that includes two sets of design elements that separately, or in combination, can increase the power output of semiconductor solar cells. When fully functionalized, an illustrative embodiment combines the two sets of design elements together to increase both the voltage and current output of thin film solar cells and enable them to approach the performance limits calculated by detailed balance methodologies that assume a quasi-Fermi level separation.
One set of design elements relate to the material structure of the active region of the solar cell device where photo-generated carriers are created and separated. Notably, the basic active region structure consists of a PN or PIN junction-containing materials of different energy gap within the junction depletion region. Moreover, a novel feature of the design is the positioning of the different energy gap material within the active region. In an illustrative embodiment, a wider energy gap barrier layer can also be positioned within the depletion region adjoining either the P-type layer or the N-type layer or both in order to suppress carrier injection across the junction. In addition, wider energy gap material can be located within the depletion region in the zone of maximum space charge recombination, where the injected electron and hole concentrations are comparable. The dimensions of the narrower energy gap material can be widely varied. Illustratively, the structure can employ quantum structured active regions as an option. In an illustrative embodiment, the structure positions at least one of the transitions from the narrowest energy gap material in active region to a wider energy gap material so that that it remains within the depletion region over a wide range of bias levels, even at forward bias levels appropriate for photovoltaic power generation.
A second set of design elements relate to light-guiding layers placed above and/or below the active region of the device in order to enhance optical absorption within the active region. The application of anti-reflection layers above the active region of the device can be employed to maximize the number of incident photons that are directed into the device active region. Scattering of the incident light into the plane of the active region absorber layers can be accomplished by the adding nanostructured layers above the photon absorbing layer and/or the application of metallic or dielectric nanoparticles to the top semiconductor surface.
Photons that pass through the device absorber layers can be redirected back into the active regions of the device for a second chance at absorption by including a reflector structure on the back side of the photovoltaic device.
Illustratively, efficient photovoltaic devices both maximize the creation and collection of photo-generated carriers and enhance the voltage at which photo-generated carriers are extracted. The following detailed discussion describes both a novel, broadly-defined material structure for a thin film solar cell that can enhance the voltage performance, along with design strategies for maximizing the current generating capability of thin film photovoltaic devices.
In an illustrative embodiment, a thin film photovoltaic solar cell device structure and method of manufacturing comprises at least one narrow energy gap, higher refractive index semiconductor well placed within a wider energy gap, lower refractive index semiconductor junction depletion region; located between an overlying, forward-scattering, antireflective top structure comprising an optical coating of at least one layer, such that the refractive index of each layer in the optical coating is intermediate between the refractive index of the top side of the semiconductor thin film solar cell and air; and an underlying, back-scattering bottom structure comprising a metal film and an optical coating of at least one layer located between the metal film and the back side of the semiconductor thin film solar cell structure. In another illustrative embodiment, a III-V quantum well thin film solar cell device comprises: a III-V semiconductor extended emitter PN heterojunction, in which the extended emitter heterojunction defines an emitter layer of one dopant type overlying a smaller band gap base layer of the opposite dopant type, which furthermore comprises a region of the PN junction depletion region adjacent to the emitter layer that contains material with higher energy gap than the underlying base layer; at least one narrow energy gap well inserted into the depletion region of the extended emitter heterojunction structure adjacent to the base layer; a top window layer comprised of a III-V semiconductor of higher energy gap than the emitter layer and appropriate top metal contacts; a bottom back surface filed layer comprised of a III-V semiconductor of higher energy gap than the base layer and appropriate bottom metal contacts; and a top antireflective structure comprising an optical coating of at least one layer, such that the refractive index of each layer in the optical coating is intermediate between the refractive index of the top side of the III-V semiconductor thin film solar cell and air
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:
Fig. 1 is a schematic cross-sectional side view of a thin film solar cell device according to an illustrative embodiment incorporating a lower band gap, waveguiding absorption layer and optical coatings to increase the optical path length of incident light through the active region of the device;
Fig. 2 is a graphical diagram of the energy band versus position for a material structure of a thin film semiconductor solar cell, illustrating the desirable design elements necessary to achieve a high operating voltage in a structure having both narrow and wide energy gap material, in accordance with illustrative embodiments;
Fig. 3 is a flow chart of a procedure for manufacturing a thin film waveguide solar cell in accordance with illustrative embodiments;
Fig. 4 is a flow chart of a procedure for manufacturing a thin film quantum waveguide solar cell in accordance with illustrative embodiments; Fig. 5 is graphical diagram of the energy band versus position for a material structure of a quantum well III-V semiconductor solar cell, illustrating the desirable design elements necessary to achieve a high operating voltage in a structure having both narrow and wide energy gap material, in accordance with illustrative embodiments;
Fig. 5A is graphical diagram of the energy band versus position for a material structure of a thin film semiconductor solar cell, illustrating various dark diode current mechanisms, in accordance with illustrative embodiments; and
Fig. 5B is graphical diagram of the energy band versus position for part of the material structure of a thin film semiconductor solar cell, illustrating various carrier escape mechanisms, in accordance with illustrative embodiments.
The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.
DETAILED DESCRIPTION
Embodiments of the invention are discussed below with reference to Figs. 1-5. Those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes, however, because the invention extends beyond these recited embodiments.
A typical thin film solar cell structure contains a limited volume of low band gap material, and thus requires advanced light trapping structures to reach its potential performance levels. Light management is achieved by assuring that incident photons are not lost due to reflections but are instead directed into the semiconductor absorbing layers. The scattering of incident light to ensure each photon has a non-normal trajectory is a strategy for increasing the optical path length of photons within the absorption layer. In addition, the application of a back reflector to bounce any unabsorbed photons back up into the active layers of the device is a beneficial aspect of any effective photovoltaic light trapping scheme. However, the most effective light trapping schemes will also direct light horizontally into the plane of the absorbing layer. Waveguide structures in which thin layers of high refractive index material are surrounded by low refractive index material provide a physical mechanism by which to achieve this type of in-plane light trapping.
A schematic diagram of an exemplary waveguide solar cell structure is depicted in Fig. 1. This thin film waveguide solar cell incorporates lower band gap, higher index of refraction materials in the active region of the device, along with tailored, nanostructured optical coatings. The optical path length of light incident upon this novel device can be dramatically enhanced via coupling into laterally propagating waveguide modes. This combination of active device structure and passive coatings can redirect normally incident light into laterally propagating waveguides modes, and represents a dramatic change in thin film solar cell design.
With reference to Fig. 1, a thin film waveguide photovoltaic device is depicted according to an illustrative embodiment. In operation, incident light 100 first encounters a top covering surface 110, which can be a top cover glass, transparent epoxy or other light transmitting covering surface. The top covering surface 110 is located above a PIN diode semiconductor material device structure 130. The PIN diode structure is coated with a transparent optical coating 120 that minimizes reflection losses and scattering incident light into the underlying PIN diode 130. The refractive index of the top optical coating 120 is illustratively adapted to generate a graded index of refraction antireflection coating, consisting of one or more layers with refractive index intermediate between the covering surface material 110 and the PIN diode semiconductor material 130. In an illustrative embodiment one or more of the layers in the top optical coating 120 also incorporate nanoparticles or nanorods which differ in refractive index from that of their surrounding material. Nanostructured optical coatings 120 provide a mechanism by which incident light can be scattered horizontally into the plane of the underlying PIN diode 130.
In the illustrative embodiment shown in Fig. 1, the PIN diode device structure consists of top window/contact layers 132, back surface field/contact layers 136, and incorporates lower energy gap material 134 within the depletion region of the PIN diode structure 130. Lower energy gap material also tends to have a higher index of refraction, thereby resulting in the formation of a waveguide structure. The PIN diode device structure can consist of any common semiconductor materials, including but not limited to group IV materials (Si, Ge, SiGe, SiC, etc.), group III-V materials (GaAs, AlGaAs, InGaP, InGaAs, InP, AlInAs, GaAsSb, InAsSb, AlAsSb, GaN, InGaN, AlGaN, etc.), group II-VI materials (CdS, CdTe, etc.), and group I-III-VI2 materials (CIGS, etc.). In another illustrative embodiment, the PIN diode device structure 130 comprises two or more PIN junctions. In yet another illustrative embodiment, the refractive index and thickness of the semiconductor materials used in the top window/contact layers 132 is tailored to function as part of a step graded refractive index antireflection structure. Electrical contact is made to the top window/contact layers 132 via metal contacts 125.
In the illustrative embodiment shown in Fig. 1, the back of the semiconductor PIN diode 130 is coated with a conductive, transparent optical coating 140. In an illustrative embodiment, the refractive index of the bottom optical coating 140 has a value of approximately 1.5 or lower, thereby creating an Omni-directional reflector when combined with the underlying metallic layer 150. In another illustrative embodiment, the bottom optical coating 140 consists of multiple layers differing in refractive index to form a distributed Bragg reflector. In yet another illustrative embodiment one or more of the layers in the bottom optical coating 140 also incorporate nanoparticles or nanorods. In yet another illustrative embodiment, the back-scattering structure, consisting of the back optical coating 140 and back metal contact 150, employ plasmonic structures. Plasmonic structures closely coupled to absorbing semiconductors can be used to increase the photocurrent in a variety of thin film solar cells. In particular, the peak wavelength of the plasmon resonance is adjustable to match the absorption band of the nearby semiconductor layers, particularly the lower band gap, higher index material 134.
In the illustrative embodiment shown in Fig. 1, optical scattering by the nanoparticles or nanorods above the semiconductor device structure can lead to coupling of photons incident normal to the device surface into lateral optical propagation paths, i.e., paths parallel to the device surface. These parallel optical modes 170 result from the introduction of a lateral wave vector component into the forward scattered wave 160, and can dramatically enhance the optical path length of photons through thin film solar cell device structures. Unabsorbed, lower energy photons that are not coupled into the waveguide modes 170 pass through the PIN diode 130 device before striking a back scattering optical coating 140.
Back-scattered light 180 is directed into the active, absorbing layers of the device by the presence of the back-scattering structure, which consists of the back optical coating 140 and back metal contact 150.
According to various embodiments, the front optical coating 120 is configured and arranged with transparent antireflection coating structures to reduce the reflection of incident photons at the material interface between the light transmitting covering surface 110 and semiconductor device structure 130. The back optical coating 140 is configured and arranged to maximize the reflection of unabsorbed photons back into the semiconductor device structure. In the various embodiments, the front coating 120 and the back coating 140 are implemented in accordance with industry standard processes and materials known to those skilled in the art. These materials include, but are not limited to, titanium dioxide, silicon dioxide, indium tin oxide, zinc oxide, and other transparent conductive oxides (TCOs). The antireflection coating can be synthesized using a variety of techniques, including sputtering, evaporation, and oblique-angle deposition. Transparent antireflection coating structures can comprise a single layer or multiple layers of materials having an index of refraction intermediate between the semiconductor structure 140 and the media in which the incident photons are delivered, which by way of example is illustrated as a cover glass or encapsulant 110 in Fig. 1. Back reflector structures can comprise either a single metallic layer, or a plurality of layers consisting of a metallic layer in combination with one or more layers of transparent optical material having an index of refraction lower than the semiconductor material. In particular, Omni-directional reflectors (ODRs), which combine a metal layer with a low-refractive index layer, provide ultra-high reflectivity over a wide range of wavelengths and incident angles.
When light is incident upon a semiconductor device coated with a continuous thin film material, the forward- and back-scattered light is well known to depend upon the optical properties of the thin film and surrounding environments which dictate the reflection, refraction, and absorption characteristics of the light. Employing an array of nanoparticles or nanorods can provide unique and desirable physical phenomena, particularly when the particle size is very small compared to the incident wavelength. In this case, the scattering and absorption characteristics of the forward wave front depend upon the size, shape, density, and permittivity of the nanoparticles. See for example, by way of useful background information, P. Matheu, S. H. Lim, D. Derkacs, C. McPheeters, and E. T. Yu, Metal and dielectric nanoparticle scattering for improved optical absorption in photovoltaic devices, Appl. Phys. Lett. 93, 113108-1-3 (2008). Nanoparticle coatings can provide additional light- trapping benefits when the adjoining semiconductor device structure contains distinct index of refraction steps. In this case, optical scattering by nanoparticles atop a semiconductor device structure can lead to coupling of photons incident normal to the device surface into lateral optical propagation paths, i.e., paths parallel to the device surface. These parallel optical modes result from the introduction of a lateral wave vector component into the scattered wave, and can dramatically enhance the optical path length of photons through thin film solar cell device structures.
Oblique-angle deposition is a method of growing arrays of nanorods in a wide variety of materials, enabled by surface diffusion and self-shadowing effects during the deposition process. Because the resulting thin films are porous, oblique-angle deposition is utilized as an effective technique for tailoring the refractive index of a variety of thin film materials (see for example, by way of useful background, J.-Q. Xi, M. F. Schubert, J. K. Kim, E. F.
Schubert, M. Chen, S.-Y. Lin, W. Liu, and J. A. Smart, Optical Thin-Film Materials with Low Refractive Index for Broad-Band Elimination of Fresnel Reflection, Nat. Photon., vol. 1 , pp. 176-179, 2007). In one illustrative embodiment, the bottom optical coating 140 comprises of a layer nanostructured, porous indium tin oxide layer with a refractive index of 1.5 or lower deposited by oblique angle deposition. In another illustrative embodiment, the top optical coating 120 comprises multiple layers, with at least one layer of dense indium tin oxide and at least one layer of porous indium tin oxide or porous titanium dioxide deposited by oblique-angle deposition.
In another illustrative embodiment, the top optical coating 120 and the bottom optical coating 140 comprise dielectric and/or metallic nanoparticles embedded within a dense optical film material. Examples include Si02 nanoparticles embedded within a dense layer of indium tin oxide, Si02 nanoparticles embedded within a dense layer of Ti02, Ti02 nanoparticles embedded within a transparent encapsulant, Ti02 nanoparticles embedded within a dense layer of Si02, and metallic nanoparticles embedded within a dense layer of ITO. Note that deposition of the nanoparticles can occur according to conventional techniques in illustrative embodiments.
The operating voltage of a semiconductor PIN diode solar cell 130 is generally dictated by the underlying dark diode current of the device. The dark diode current of semiconductor devices is composed of several different components, all of which are dependent upon the energy gap of the material used in the active junction of the device. Typically, each cell in a solar cell consists of one type of material, and the energy gap of that material influences both the current and voltage output of the device. Lower energy gap material can enhance the current generating capability, but typically results in a lower operating voltage. Therefore, it is desirable to provide a device structure 130 that can harness the current generating capabilities of narrow energy gap material while also maintaining a high operating voltage.
The layered structures and associated energy bands are depicted in the graphical diagram of Fig. 2, which is simplified to facilitate understanding, showing the elements of semiconductor thin film solar cell device structure 130 according to the illustrative embodiment. Notably, the structure 130 simultaneously suppresses the dark diode current and enhances the photocurrent of a thin film cell or subcells. The structure 130 comprises of a window 200, a PIN junction 205, and a back surface field 280.
In the illustrative embodiment, the PIN junction 205 consists of an emitter 220, a base 270, and a depletion region 225 illustratively shown with a wide energy gap (Eg) barrier layer 235, an extended wide energy gap (Eg) emitter layer 240, a narrow energy gap (Eg) well or dot region layer 250, and a depleted base layer region 260. The depletion region is distinguished by the presence of a built-in electric field, a non-zero slope in the conduction band 285 (Ec) and valence band 290 (Ev), induced by the juxtaposition of p-type and n-type semiconductor material. In the illustrative embodiment, the depletion region is on the order of approximately 0.1 to 2 microns for thin film solar cells. It should be apparent to those skilled in the art that the thickness of the depletion region can be adjusted by varying the thickness of unintentionally doped material between the p-type and n-type material. Window 200 and back surface field 280 layers are typical solar cell structures deposited above and below the PIN junction 205.
In the illustrative embodiment, lightly-doped or undoped higher energy gap material is inserted into depletion region 225 adjacent to the emitter 220 to form an extended wide energy gap (Eg) emitter layer 240 and a barrier layer 235. In contrast to prior art, the physical boundary between the different energy gap materials is offset from the physical boundary between the emitter and intrinsic/base materials. The wide energy gap material layers adjacent to the emitter within the depletion region reduce both space charge recombination and carrier injection from the emitter. In further embodiments, the depletion region consists of multiple layers with different energy gap materials. By way of example, a region of narrow energy gap material wells or dots 250 is also inserted into in the depletion layer 225. The transitions in energy gap between wide and narrow energy gap material can either be abrupt, step graded, or continuously graded, as illustrated in the conduction band 285 and valence band 290 profiles shown in Fig. 2. The use of wider energy gap material in the emitter layer 220 provides various known benefits for photovoltaic devices. Theories may vary with respect to the physical principals that govern, but in general, a wide energy gap emitter 220 can reduce diffusion-driven injection of majority carriers from the base 270 into the emitter 220. However, other significant dark diode current components, such as space charge recombination within the depletion region 225 and majority carrier injection from the emitter 220 into the base 270, are generally not improved with a typical heterojunction design. The careful and unique placement of both wide and narrow energy gap material within the junction depletion region 225 illustrated in Fig. 2 enables the photovoltaic device to absorb a wider portion of the solar spectrum while simultaneously lowering the dark current to increase the operating voltage of the photovoltaic device. Placing a wide energy gap barrier layer 235 within the depletion region adjoining the emitter provides a means to suppress majority carrier injection out of the emitter 220.
By extending the region of wide energy gap material, as described herein, into the depletion region, space charge recombination can be also reduced. In particular, by reducing majority carrier injection out of the emitter 220, the region of enhanced recombination within the depletion region 225 can be pulled into the extended wide energy gap emitter layer 240 and away from the region of narrow energy gap material 250. Radiative recombination within the narrow energy gap material can be minimized by placing the narrow energy gap material region 250 within the built-in field of the depletion region 225. Okada and
Shiotsuka have also suggested that the radiative recombination in quantum wells can be further reduced by employing a step graded profile, as taught in Y. Okada and N. Shiotsuka, "Fabrication of Potentially Modulated Multi-Quantum Well Solar Cells," Proceedings of the 31st IEEE Photovoltaic Specialists Conference, December 2005.
By way of further background, for photovoltaic applications, energy-gap differences at heterointerfaces within the device structure can act as unwanted barriers to the extraction of photo-generated carriers. However, field-assisted thermionic emission and tunneling are well-established mechanisms by which carriers can escape from a potential well (see for example, by way of useful background information, A. Alemu, J. A. H. Coaquira, and A. Freundlich, Dependence of Device Performance on Carrier Escape Sequence in Multi- Quantum-Well p-i-n Solar Cells, J. Appl. Phys., vol. 99, no. 084506, May 2006). While various theories of operation may be applicable, in the interfaces between the narrow energy gap material 250 and wider energy gap materials 240 / 260 occur in a region containing a non-zero built-in electric field 225.
In one illustrative embodiment, a III-V quantum well thin film solar cell comprises an n-type InGaP the emitter 220, a p-type GaAs base layer 260/270, an undoped AlGaAs wide band gap region in the depletion region adjacent to the emitter 235/240, and InGaAs quantum well layers or InAs quantum dot layers 250. In another illustrative embodiment, a III-V quantum well thin film solar cell device comprises a p-type AlGaAs emitter 220, an n-type GaAs base layer 260/270, wide band gap undoped InGaP and AlGaAs in the depletion region adjacent to the emitter 235/240, and InGaAs quantum well or InAs quantum dot layers 250.
Note that the photovoltaic device design illustrated in Figs. 1 - 2 addresses three requirements for quantum well and quantum dot solar cells to achieve their promised levels of performance. First, the carriers generated by photon absorption in the lower energy-gap material can escape before recombining by positioning the narrow energy gap well or dot region 250 within the depletion region 225. Second the dark diode current is minimized by suppressing both radiative and non-radiative recombination by the use of a unique structure incorporating both a wide band gap barrier layer within a heterojunction depletion region and step graded wells. Third, absorption in the narrow energy gap well or dot region is maximized by the use of light trapping coatings that can scatter light into the plane of the quantum wells and dots.
The structures described herein can be constructed employing conventional techniques known to those of ordinary skill. In one illustrative embodiment, a method of manufacturing a thin film waveguide solar cell comprises locating, or sandwiching, a thin film solar cell structure between an overlying, forward-scattering, antireflective top structure and an underlying, back-scattering bottom structure. According to the procedure, a plurality of electrical contacts are formed on the predetermined top region of the semiconductor thin film device structure and a predetermined bottom region of the semiconductor thin film device structure. In particular, industry standard photolithography process can be employed to selectively etch and metalize the semiconductor thin film device structure.
Reference is made to Fig. 3 showing a procedure 300 for manufacturing, or otherwise constructing, a thin film waveguide solar cell as described herein. The procedure provides a thin film solar cell structure at step 310 and provides an overlying, forward-scattering anti- reflective top structure at step 320. An underlying, back-scattering bottom structure is provided at step 330. At procedure step 340 the thin film solar cell structure is located between the top structure and the bottom structure so as to create a semiconductor thin film device. At step 350, a plurality of electrical contacts are formed on a predetermined top region of the semiconductor thin film device and a predetermined bottom region of the semiconductor thin film device.
As shown in Fig. 4, according to an illustrative embodiment, a procedure 400 for manufacturing a III-V thin film quantum well or quantum dot waveguide solar cell is shown. The procedure begins at step 410 by growing a III-V epitaxial structure inverted on a GaAs or InP substrate. Then, at step 420, depositing the back optical coating and metal film, at step 430 attaching a new supporting substrate. Some examples of a new supporting substrate include metal foil, stainless steel, titanium, polyurethane and other high durability flexible polymers. At step 440 the procedure removes the III-V epitaxial layers from the substrate, at step 450 the procedure continues by depositing the front optical coating, at step 460 forming metallic contacts and gridlines, and at step 470 encapsulating the device.
In one embodiment, a method of manufacturing a thin film waveguide solar cell comprises providing a flexible substrate having a front surface and a back surface; depositing an optical film followed by a semiconductor thin film solar cell device structure at least partially on a top surface of the flexible substrate thereby leaving a predetermined top region of the semiconductor thin film device structured exposed; coating the semiconductor thin film solar cell with optical coatings and encapsulant so as to create an optical coating layer and an encapsulant layer on the semiconductor thin film solar cell; and forming a plurality of electrical contacts on the predetermined top region of the semiconductor thin film device structure and a predetermined bottom region of the semiconductor thin film device structure.
In another embodiment, a method of manufacturing a thin film waveguide solar cell comprises: providing a glass sheet having a front surface and a back surface; coating a transparent film on at least one of the front surface and the back surface of the glass sheet to form a coated glass superstate; depositing a semiconductor thin film solar cell device structure partially on the back surface of the coated glass superstate leaving a predetermined top region of the semiconductor thin film device structured exposed; depositing a reflective optical coating structure; and forming a plurality of electrical contacts on the predetermined top region of the semiconductor thin film device structure and a predetermined bottom region of the semiconductor thin film device structure. According to the various embodiments, the metal contacts can be formed using conventional semiconductor processing technology known to those of ordinary skill. The optical coatings can be applied using oblique angle deposition, sputtering, or evaporation. The semiconductor thin films can be deposited via a variety of conventional material synthesis techniques, including sputtering, evaporation, and chemical vapor deposition. It is further contemplated that other processes can be adapted for thin film deposition including such semiconductor deposition tool as metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Commercially available epitaxial lift-off (ELO) processes can be employed to remove the III-V epitaxial layers from their underlying substrate. It should be apparent to those skilled in the art that various other techniques when made available can be employed to fabricate the structures of the described embodiment, herein.
The layer structures and associated energy bands graph depicted in Fig. 5, which is simplified to facilitate understanding, show the elements of a III-V semiconductor quantum well solar cell device structure 130 according to an illustrative embodiment wherein the structure 130 comprises a high Al composition (Al% > 50) window 500, an AlGaAs PIN junction 505, and an InGaP back surface field 580. In this illustrative embodiment, the PIN junction 505 consists of a p-type AlGaAs emitter 520 with Al% > 10; an n-type AlGaAs base 570 of lower Al composition than the emitter (such as Al% = 0); and a depletion region 525 illustratively shown with a wide energy gap (Eg) InGaP barrier layer 535, an extended wide energy gap (Eg) AlGaAs emitter layer 240 with Al% > 10, a narrow energy gap (Eg) InGaAs well region layer 550, and a depleted AlGaAs base layer region 560. The depletion region is distinguished by the presence of a built-in electric field, a non-zero slope in the conduction band 585 (Ec) and the valence band 590 (Ev), induced by the juxtaposition of p-type and n- type semiconductor material. In the illustrative embodiment, the depletion region is on the order of approximately 0.1 to 2 microns for III-V thin film solar cells. It should be apparent to those skilled in the art that the thickness of the depletion region can be adjusted by varying the thickness of unintentionally doped material between the p-type and n-type material, and that a similar structure comprises an n-type InGaP emitter and p-type AlGaAs base, according to conventional techniques readily apparent to those of ordinary skill.
The illustrative device structure shown in Fig. 5 provides several mechanisms by which to suppress the diode dark current, as illustrated in Fig. 5A, but without being bound to a particular theory. As with conventional heterojunction structures, the larger energy gap of emitter 520 relative to the base 570 reduces the diffusion of electrons 592 out of the n-type base layer. In addition, hole diffusion 594 is suppressed in this structure by placing a thin InGaP layer 535 adjacent to the p-type AlGaAs emitter 520. The large valence band offset (ΔΕν) at the type II InGaP barrier 535 / AlGaAs emitter 520 interface also alters the distribution of holes 594 within the depletion region. By reducing the hole concentration injected from the p-type AlGaAs emitter 520, the region of enhanced recombination within the depletion region can be pulled closer to the p-side of the structure, away from the InGaAs well region 550 and into the wide band gap material 540 in the depletion region adjacent to the emitter, thus reducing space charge recombination 596. The extended AlGaAs emitter 540 / InGaP barrier 535 structure illustrated in Fig. 5 thus provides a means of reducing both the non-radiative n=l diffusion and n=2 space charge recombination components of the diode dark current.
While non-radiative recombination can be minimized by the use of the extended emitter heterojunction structure depicted in Fig. 5, radiative recombination within the InGaAs wells can still fundamentally limit the dark current, and hence the operating voltage, of the device. One illustrative mechanism by which radiative recombination rates inside the wells can be reduced employs spatial separation of electron and hole wavefunctions, which can result from placing the quantum wells within the built-in field of the depletion region of a P- N or P-I-N junction device. Higher escape rates, and further reductions in the wavefunction overlap, can illustratively be engineered by employing a step graded compositional profile to allow photogenerated carriers to readily hop out of the InGaAs well, as illustrated in Fig. 5B. In Fig. 5B, an incident photon 598 creates an electron 592 - hole 594 pair that escapes from the InGaAs well region 550 through a series of field-assisted thermionic emission steps. Similar escape mechanisms allow carriers being swept towards either the emitter or the base to avoid being capture in the InGaAs well region and potentially recombining radiatively and lowering the dark current.
The many features and advantages of the illustrative embodiments described herein are apparent from the above written description and thus it is intended by the appended claims to cover all such features and advantages of the invention. Further, because numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. For example, the illustrative embodiments can include additional layers to perform further functions or enhance existing, described functions. Likewise, while not shown, the electrical connectivity of the cell structure with other cells in an array and/or an external conduit is expressly contemplated and highly variable within ordinary skill. More generally, while some ranges of layer thickness and illustrative materials are described herein. It is expressly contemplated that additional layers, layers having differing thicknesses and/or material choices can be provided to achieve the functional advantages described herein. In addition, directional and locational terms such as "top", "bottom", "center", "front", "back", "above", and "below" should be taken as relative conventions only, and not as absolute. Furthermore, it is expressly contemplated that various semiconductor and thin films fabrication techniques can be employed to form the structures described herein. Accordingly, this description is to be taken only by way of example and not to otherwise limit the scope of the invention.
What is claimed is:

Claims

1. A photovoltaic device comprising:
a thin film solar cell structure including a semiconductor junction depletion region with at least one narrow energy gap material layer having a higher refractive index than the surrounding wider energy gap / lower refractive index material;
an overlying, forward-scattering, antireflective top structure comprising a top optical coating including at least one layer having a refractive index intermediate between a refractive index of a top side of the thin film solar cell structure and a refractive index of air; and
an underlying, back-scattering bottom structure comprising a metal film and a bottom optical coating including at least one layer located between the metal film and a back side of the thin film solar cell structure.
2. The photovoltaic device of claim 1 wherein the top optical coating includes a plurality of layers and at least one of the plurality of layers in the top optical coating comprises porous material with a nanorod structure.
3. The photovoltaic device of claim 2 wherein the porous material comprises one of indium tin oxide, titanium dioxide, silicon dioxide, and zinc oxide.
4. The photovoltaic device of claim 1 wherein at least one of the layers in the top optical coating contains nanoparticles embedded within a dense optical film material.
5. The photovoltaic device of claim 4 wherein at least one of the layers in the optical coating contains at least one of: Si02 nanoparticles embedded within a dense layer of indium tin oxide, Si02 nanoparticles embedded within a dense layer of Ti02, Ti02 nanoparticles embedded within a transparent encapsulant, and Ti02 nanoparticles embedded within a dense layer of Si02.
6. The photovoltaic device of claim 1 wherein at least one layer in the bottom optical coating comprises a transparent conductive oxide film, including one of indium tin oxide and aluminum doped zinc oxide.
7. The photovoltaic device of claim 6 wherein at least one layer in the bottom optical coating comprises a porous material having a nanorod structure.
8. The photovoltaic device of claim 7 wherein the bottom optical coating comprises alternating layers of material having different porosity.
9. The photovoltaic device as set forth in claim 1 wherein the metal film contains microstructures that enhance the formation of surface plasmons.
10. The photovoltaic device as set forth in claim 1 wherein at least one layer in the bottom optical coating contains embedded metal nanoparticles.
11. The photovoltaic device as set forth in claim 1 wherein the thin film solar cell structure comprises an extended emitter heterojunction, in which the extended emitter heterojunction defines an emitter layer and a region of the depletion region adjacent to the emitter layer that both contain a higher energy gap material than the underlying base layer material.
12. The photovoltaic device of claim 11 wherein at least one quantum well is inserted into the depletion region of an extended emitter heterojunction structure adjacent to the base layer.
13. The photovoltaic device of claim 12 further comprising multiple quantum well layers inserted into the depletion region of the extended emitter heterojunction structure adjacent to the base layer.
14. The photovoltaic device as set forth in claim 13 wherein at least one of the multiple quantum well layers further comprises embedded quantum dots.
15. The photovoltaic device as set forth in claim 11 wherein a plurality of quantum dots are inserted into the depletion region.
16. A III-V quantum well thin film solar cell device comprising: a III-V semiconductor extended emitter PN heterojunction, in which the extended emitter heterojunction defines an emitter layer of one dopant type overlying a smaller band gap base layer of another dopant type, which further comprises a PN junction depletion region adjacent to the emitter layer that contains material with higher energy gap than the base layer; and
at least one narrow energy gap well inserted into a depletion region of the extended emitter heterojunction structure adjacent to the base layer; and
a top window layer comprised of a III-V semiconductor having a higher energy gap than the emitter layer and top metal contacts; and
a bottom back surface field layer comprising a III-V semiconductor having a higher energy gap than the base layer and a bottom metal contact; and
a top antireflective structure comprising an optical coating of at least one layer, such that the refractive index of the optical coating is intermediate between a refractive index of a top side of the III-V semiconductor thin film solar cell and a refractive index of air.
17. The III-V quantum well thin film solar cell device of claim 16 wherein the top antireflective structure comprises an optical coating having at least two layers, one composed of nanorods of material selected from one of indium tin oxide, titanium dioxide, silicon dioxide, and zinc oxide.
18. The III-V quantum well thin film solar cell device of claim 16 wherein a transition from a wide band gap material to a narrow band gap material in the depletion region is compositionally graded.
19. The III-V quantum well thin film solar cell device of claim 18 further comprising a plurality of narrow energy gap material layers, and wherein the at least one narrow energy gap well comprises a compositionally step graded structure.
20. The III-V quantum well thin film solar cell device of claim 19 wherein the emitter comprises n-type InGaP, the base layer comprises p-type GaAs, the wide band gap region of the depletion region adjacent to the emitter comprises undoped AlGaAs, and the at least one narrow energy gap well comprises InGaAs.
21. The III-V quantum well thin film solar cell device of claim 19 wherein the emitter comprises p-type AlGaAs, the base layer comprises n-type GaAs, the wide band gap region of the depletion region adjacent to the emitter comprises undoped InGaP and AlGaAs, and the at least one narrow energy gap well comprises InGaAs.
22. The III-V quantum well thin film solar cell device of claim 16 further comprising a bottom contact structure that comprises a metal film and a bottom optical coating comprising at least one layer located between the metal film and a back side of the semiconductor thin film solar cell structure.
23. The III-V quantum well thin film solar cell device of claim 22 wherein at least one layer in the bottom optical coating comprises a porous transparent conductive oxide film having a nanorod structure, including one of indium tin oxide and aluminum doped zinc oxide.
24. The III-V quantum well thin film solar cell device of claim 16 wherein electrical contacts to the top emitter side of the extended heterojunction are formed by etching holes in a backside of the semiconductor thin film solar cell structure.
25. The photovoltaic device as set forth in claim 16 further comprising multiple quantum dot layers inserted into the depletion region of the extended emitter heterojunction structure.
26. A method of manufacturing a photovoltaic device comprising:
locating a thin film solar cell structure between an overlying, forward-scattering, antireflective top structure and an underlying, back-scattering bottom structure; and
forming a plurality of electrical contacts on a predetermined top region of the thin film solar cell structure and a predetermined bottom region of the thin film solar cell structure.
27. The method as set forth in claim 26 wherein the locating is performed by (a) growing an epitaxial structure inverted on a GaAs substrate via metal organic chemical vapor deposition, (b) depositing the back optical coating and metal film, (c) attaching a new supporting substrate, (d) removing the III-V epitaxial layers from the GaAs substrate, and (e) depositing the front optical coating, and the forming is performed by forming metallic contacts and gridlines, and further comprising encapsulating the device.
28. The method of 27 wherein the porous optical coatings are deposited via oblique- angle deposition. 29 The method of claim 26 wherein the thin film solar cell structure is connected to a coated glass substrate. 30. The method of claim 26 wherein the thin film solar cell structure is connected to a flexible substrate.
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