NL2025744B1 - Methodology for efficient hole transport layer using transition metal oxides - Google Patents
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—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
- H01L31/04—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
- H01L31/06—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
- H01L31/072—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
- H01L31/0745—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 a AIVBIV heterojunction, e.g. Si/Ge, SiGe/Si or Si/SiC solar cells
- H01L31/0747—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 a AIVBIV heterojunction, e.g. Si/Ge, SiGe/Si or Si/SiC solar cells comprising a heterojunction of crystalline and amorphous materials, e.g. heterojunction with intrinsic thin layer
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- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
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- H01L31/02—Details
- H01L31/0216—Coatings
- 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
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- H01L31/00—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
- H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
- H01L31/186—Particular post-treatment for the devices, e.g. annealing, impurity gettering, short-circuit elimination, recrystallisation
- H01L31/1868—Passivation
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract
The present invention is in the field of a process for making solar cells, or photovoltaic (PV) cell, with trans— parent contacts and an improved hole transport layer. Said solar cells comprise at least one hetero junction and typi— cally two hetero junctions. The invention provides solar cells with good operating characteristics, such as in terms of conversion efficiency, fill factor, and current gain.
Description
P100424NL00 1 Methodology for efficient hole transport layer using transi- tion metal oxides
FIELD OF THE INVENTION The present invention is in the field of a process for making solar cells, or photovoltaic (PV) cell, with trans- parent contacts and an improved hole transport layer. Said solar cells comprise at least one hetero junction and typi- cally two hetero junctions. The invention provides solar cells with good operating characteristics, such as in terms of conversion efficiency, fill factor, and current gain.
BACKGROUND OF THE INVENTION A solar cell, or photovoltaic (PV) cell, is an electri- cal device that converts energy of light, typically sun light (hence “solar”), directly into electricity by the so- called photovoltaic effect. The solar cell may be considered a photoelectric cell, having electrical characteristics, such as current, voltage, resistance, and fill factor, which vary when exposed to light and which vary from type of cell to type.
Solar cells are described as being photovoltaic irre- spective of whether the source is sunlight or an artificial light. They may also be used as photo detector.
When a solar cell absorbs light it may generate either electron-hole pairs or excitons. In order to obtain an elec- trical current charge carriers of opposite types are sepa- rated. The separated charge carriers are “extracted” to an external circuit, typically providing a DC-current. For practical use a DC-current may be transformed into an AC- current, e.g. by using a transformer.
Typically solar cells are grouped into an array of ele- ments. Various elements may form a panel, and various panels may form a system.
Wafer based c-Si solar cells contribute to more than 90% of the total PV market. According to recent predictions, this trend will remain for the upcoming years towards 2020 and many years beyond. Due to their simplified process,
P100424NL00 2 conventional c-Si solar cells dominate a large part of the market. As alternative to the industry to improve the power to cost ratio, the silicon heterojunction approach has be- come increasingly attractive for PV industry, even though the relatively complicated process to deploy the proper front layers, such as a transparant conductive oxide (TCO) and an inherent low thermal budget of the cells limiting us- age of existing production lines and thus result in a negli- gible market share so far. A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. A homo- junction relates to a semiconductor interface formed by typ- ically two layers of similar semiconductor material, wherein these semiconductor materials have equal band gaps and typi- cally have a different doping (either in concentration, in type, or both). A common example is a homojunction at the interface between an n-type laver and a p-type layer, which is referred to as a p-n junction. In heterojunctions ad- vanced techniques are used to precisely control a deposition thickness of layers involved and to create a lattice-matched abrupt interface. Three types of heterojunctions can be dis- tinguished, a straddling gap, a staggered gap, and a broken gap.
A disadvantage of solar cells is that the conversion per se is not very efficient, typically, for Si-solar cells, limited to some 20%. Theoretically a single pn junction crystalline silicon device has a maximum power efficiency of about 30%. An infinite number of layers may reach a maximum power efficiency of 86%. The highest ratio achieved for a solar cell per se at present is about 44%. For commercial silicon solar cells the record is about 25.6%. In view of efficiency the front contacts may be moved to a rear or back side, eliminating shaded areas. In addition thin silicon films were applied to the wafer. Solar cells also suffer from various imperfections, such as recombination losses, reflectance losses, heating during use, thermodynamic losses, shadow, internal resistance, such as shunt and
P100424NL00 3 series resistance, leakage, etc.
A qualification of perfor- mance of a solar cell is the fill factor (FF). The fill fac- tor may be defined as a ratio of an actual maximum obtaina- ble power to the product of the open circuit voltage and short circuit current.
It is considered to be a key parame- ter in evaluating performance.
A typical advanced commercial solar cell has a fill factor > 0.75, whereas less advanced cells have a fill factor between 0.4 and 0.7. Cells with a high fill factor typically have a low equivalent series re- sistance and a high equivalent shunt resistance; in other words less internal losses occur.
Efficiency is nevertheless improving gradually, so every relatively small improvement is welcomed and of significant importance.
In aspect of multi-layer structures relates to the so- called work-function.
In physics the work function relates to a minimum thermodynamic work (i.e., energy) needed to re- move an electron from a solid to a point in the vacuum out- side the solid surface.
Here "outside" means that the final electron position is far from the surface on the atomic scale, but still too close to the solid to be influenced by ambient electric fields in the vacuum.
The work function is considered not to be a characteristic of a bulk material, but rather a property of the surface of the material and hence depending on crystal face and possible contamination, surface charge, etc.
The work-function may be expressed in eV.
Typically at an interface between two different material there is a mismatch, such as in terms of the work-function.
A “loss” in work-function may occur at the interface.
At present a solar cell having a full area front passiv- ating contact is not attractive, such as due to highly ab- sorptive materials used to build such a structure.
That is the case of heavily doped poly-silicon and a-Si layers.
In a poly-silicon case, the process requires a very thin polysil- icon film for minimizing parasitic absorption loss, and in case of a-Si, the process requires e.g. an extra transparent conductive oxide (TCO) layer for supporting the carrier lat- eral transport.
Transition metal oxides (TMOs)}) may be considered for c-
P100424NL00 4 Si based heterojunction (SHJ) solar cells in view of their ability to induce efficient carrier selectivity and mitigate parasitic absorption losses resulting in clear current gain. Among TMOs, molybdenum oxide (MoO.) is promising for appli- cations as hole transport layer (HTL). MoO, layer, in combi- nation with a thin intrinsic passivation a-Si:H layer and a transparent conductive oxide (TCO) has in fact demonstrated conversion efficiency of 23.5%. However, (i)a-Si:H/MoO. ex- hibits a weak thermal stability in air/moisture hindering the carrier selectivity in contrast to conventional SHJ cells. Consequently, devices with TMOs usually suffer from lower fill factor (FF) and possibly S-shaped J-V character- istics as compared to solar cells with doped silicon carrier selective HTLs. The present invention relates to an increased effi- ciency Si-based solar cell and various aspects thereof and a simplified process for manufacturing the solar cell which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.
SUMMARY OF THE INVENTION The present invention relates in a first aspect to Si-based solar cell according to claim 1, and in a second aspect to a method for making such a solar cell according to claim 14. The present invention is also subject of a scientific publication of L. Mazzarella et al, entitled “Strategy to mitigate the dipole interfacial states in (i)a- Si:H/MoO, passivating contacts solar cells”, which publica- tion and its contents are incorporated by reference. It is found that with the present invention it is possible to de- posit hole transport layers, such as MoO, layers in a wider operational range (such as by PECVD, thermal evaporation, atomic layer deposition, PVD, and sputtering) by introducing e.g. a prior PECVD plasma pre-treatment. This pre-treatment (i) mitigates the interaction of the hole transport layer with (i)a-Si:H and (ii) strongly supports the charge transport ascribing this to the reduction of the dipole strength. An optical gain is obtained with lower parasitic
P100424NL00 absorption.
The optimized plasma treatment gave a S-shape- free J-V curve with FF comparable to the SHJ reference de- vice.
Further thickness optimization demonstrated that the MoO: layer can be further reduced down to 3.5 nm with no 5 electrical losses by the presence of the plasma treatment (PT). This result is in agreement with the simulated optimal hole transport layer thickness for FF maximization.
Finally, both optical simulations and experimental EQE showed that the proposed approach consisting of plasma treatment (PT) + the hole transport layer (treatment time of 130 s) results in very limited losses as compared to the cell with only the hole transport layer , such as MoO... The present solar cells have as advantages e.g. a good work-function and/or a limited loss of work-function over the interface of the hole transport layer/pre-treated layer, a mitigated dipole on the interface, a good conversion effi- ciency, a good transparency, a low parasitic absorption, good carrier collection, a not very complex structure, a high Voc, a high Jsc, and a high fill factor.
The present invention makes use of various techniques in order to solve one or more of the prior art problems and provides further advantages; these advantages relate to measurable character- istics (see above effects) of the obtained devices and hence constitute noticeable physical differences over e.g. the prior art.
No annealing step is required.
A high efficiency solar cell (> 22% efficiency) is provided.
With some simple optimization steps an efficiency of 25-26% is feasible.
In the present solar cells front and rear (also indicated as back) contacts are present.
The present solar cell typically comprises at least one hetero junction and typically two hetero junctions.
The present solar cell (100) comprises a hole transport layer (12), which sometimes is also referred to as contact layer, or collector layer, characterized in that the hole transport layer (12) comprises at least one transition metal oxide, wherein the hole transport layer (12) is provided on a plasma pre-treated surface layer (12a), such as a surface passivation layer, such as a-Si:H and/or silicon pre-treated layer (12a). Therewith in
P100424NL00 6 particular a loss in work-function is minimized, and the di- pole at the plasma treated surface is limited as well. The above and other improvements lead to a short cir- cuit current of > 39 mA/cm:, and/or an FF of > 70%, prefera- bly FF>753, preferably >77%, such as > 80%, and/or a Voc of 700-730 mV, and/or a conversion efficiency of > 21%, and/or an absorption coefficient <20 x10% cm! in the range 3-4eV. Such relates to an improvement of 1-3% over comparable prior art devices, which is a relative improvement of 5- 15%. For return of investment such a difference is con- sidered huge.
The present method is considered to be relatively simple and reduces process time and use of equipment, as pre-treatment can take place in the same tool as wherein the surface pre-treatment occurs, without a breach of vacuum. The present method comprises the steps of providing a Si-substrate (10), such as a crystalline Si-sub- strate, depositing (forming) an a-Si:H layer (11) on said Si-substrate, without a vacuum break plasma pre-treating the a-Si:H layer (11) with a plasma mixture with a positive do- pant comprising gas, such as a B-, Al-or Ga-comprising do- pant gas, such as B:ls, preferably comprising SiHi, Hz, and the gaseous p-dopant, hence there is no need for chemical etching, preferably at a frequency of 12-15 MHz, and/or preferably during 10-1000 sec, and/or preferably at power density of 50-350 mW/cm?, and/or preferably at a temperature < 523 K (< 250 °C), and/or preferably at a pressure from 50- 400 Pa (0.5-4 mbar), and/or preferably with a gas mixture comprising 0.2-2sccm SiH4, 50-400 sccm Hz, and 1-20sccm B:Hs (200ppm in H:), depositing a transition metal oxide layer {12) on the treated a-Si:H layer, depositing a transparent conductive oxide layer (13) on the transition metal oxide layer, and providing at least one contact (14) on the trans- parent conductive oxide layer. Mostly fabrication tools thereof are already part of standard production lines. Therefore the present invention may be considered commer- cially available from the start since it does not require development of additional process tools.
P100424NL00 7 In summary the present invention provides a simpli- fied fabrication process wherein solar cells can be fin- ished within a couple of steps, and which is a low cost and high throughput process, using compatible industrial standard metallization steps, solar cells featuring a high Voc due to the full passivated contacts, solar cells featuring a high Jsc & Voc due to the high transparency of the passivating contacts, solar cells featuring a rel- atively high fill factor (FF) due to the lowly doped c-Si regions near the interfaces, and wherein the design is applicable to both a front/rear contacted conventional solar cell architecture, a bifacial solar cell architec- ture and for both n-type and p-type bulk material. Thereby the present invention provides a solution to one or more of the above mentioned problems.
Advantages of the present description are detailed throughout the description. References to the figures are not limiting, and are only intended to guide the person skilled in the art through details of the present invention.
DETAILED DESCRIPTION OF THE INVENTION The present invention relates in a first aspect to a single or hetero junction Si-based solar cell according to claim 1, and in a second aspect to a process for making such a solar cell according to claim 14.
In an exemplary embodiment of the present solar cell the work-function loss of the combined hole transport layer (12) /pre-treated layer (12a) is < 1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, such as < 0.35 eV.
In an exemplary embodiment of the present solar cell the dipole of the plasma pre-treated surface is < 4 C/m, preferably < 2 C/m, more preferably < 1 C/m, such as < 0.7 C/m.
In an exemplary embodiment of the present solar cell the pre-treated layer (12a) is obtained by PECVD treatment with a plasma mixture comprising a positive dopant compris- ing gas, such as a B-, Al-or Ga-comprising dopant gas, such as B:hs, preferably comprising SiH4, Hz, and the gaseous p-do- pant.
P100424NL00 8 In an exemplary embodiment of the present solar cell the pre-treated layer (12a) comprises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive elec- trical charge, or a combination thereof.
In an exemplary embodiment of the present solar cell pre-treatment is performed during 10-1000 sec, preferably 20-300 sec, such as 30-100 sec.
In an exemplary embodiment of the present solar cell a power density during pre-treatment is 50-350 mW/cm?, pref- erably 70-200 mW/cm?, more preferably 80-100 mW/cm?, such as 90 mW/cm?.
In an exemplary embodiment of the present solar cell pre-treatment is performed at a temperature < 523 K (< 250 °C), preferably < 473 K (< 200 °C), more preferably < 443 K (< 170 °C).
In an exemplary embodiment of the present solar cell a plasma pressure is from 50-400 Pa (0.5-4 mbar), preferably 100-300 Pa {1-3 mbar), more preferably 150-250 Pa (1.5-2.5 mbar), such as 220 Pa (2.2 mbar).
In an exemplary embodiment of the present solar cell the pre-treated layer is substantially free of Si0:, such as having less than 1% Si0:/pre-treated layer (atom/atom), more preferably <1000 ppm, even more preferably <100 ppm, such as < 10 ppm.
In an exemplary embodiment of the present solar cell is with the proviso that no annealing of the a-Si:H layer is provided, prior to deposition of the hole transport layer.
In an exemplary embodiment of the present solar cell is with the proviso that no chemical etching of the a-Si:H layer is provided, preferably no chemical etching at all.
In an exemplary embodiment of the present solar cell the hole transport layer (12) has a thickness of <10 nm, preferably 1.5-9 nm, more preferably 2-7 nm, even more pref- erably 2.5-5 nm, such as 3-4 nm.
In an exemplary embodiment of the present solar cell the hole transport layer (12) has a an absorption coeffi- cient <20 x10% cmt in the range 3-4eV, preferably <10 x104
P100424NL00 9 cmt.
In an exemplary embodiment of the present solar cell the hole transport layer (12) has a current gain of 1-2 mA/ cm.
In an exemplary embodiment of the present solar cell the hole transport layer (12) is structured, such as com- prising a zig/zag structure, comprising random pyramids, texturing, preferably with a height of 1-7 pm, such as 2-5 um, and combinations thereof.
In an exemplary embodiment of the present solar cell the hole transport layer (12) is provided under a transpar- ent conducting material (13).
In an exemplary embodiment of the present solar cell the transition metal is selected from period 4 or period 5 transition metals, such as Ti, V, Cr, Co, Ni, Cu, Zn, Cs, Nb, Mo, W, and alloys thereof.
In an exemplary embodiment of the present solar cell the hole transport layer (12) is dopant free.
In an exemplary embodiment of the present solar cell the hole transport layer (12) is deposited on a pre-treated a-Si:H layer, such as by PECVD, thermal evaporation, atomic layer deposition, PVD, and sputtering.
In an exemplary embodiment the present solar cell further comprising at least one of a metal contact (14), and a stack of layers comprising a transparent conducting layer (13) of 40-200 nm, preferably 50-100 nm, such as 60-75 nm, in electrical contact with the metal contact, preferably with a carrier concentration 1-10*102%cm?, the transparent conducting layer above the < 10 nm hole transport layer (12), the hole transport layer (12) above the 1-10 nm treated layer (lla), a 100-500 um doped crystalline silicon substrate (10), and on a back side of the doped crystalline silicon substrate a second 1-10 nm a-Si:H layer (21), above the second a-Si:H layer an 1-10 nm electron transport layer, preferably with an activation energy <350 meV, such as an n- doped a-Si:H layer (22) and/or n-doped nc-Si:H and/or al- loyed with 0, N or C, a second 20-300 nm transparent
P100424NL00 10 conducting layer (23), such as an ITO layer, and a metal contact layer (24) above the transparent layer.
In an exemplary embodiment the present solar cell has a short circuit current of > 39 mA/cm?, and/or an FF of > 70%, preferably FF>75%, preferably >77%, such as > 80%, and/or a Voc of 700-730 mV, and/or a conversion efficiency of > 21%.
In an exemplary embodiment of the present solar cell the solar cell is selected from single junction solar cells, hetero junction solar cells, multi-junction solar cells, thin film solar cells, wherein the silicon is crystalline silicon, n-doped or p-doped crystalline silicon.
In a second aspect the present invention relates to a method of producing a solar cell according to the inven- tion.
The invention is further detailed by the accompanying figures and examples, which are exemplary and explanatory of nature and are not limiting the scope of the inven- tion. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceiv- able falling within the scope of protection, defined by the present claims.
SUMMARY OF FIGURES Figure 1 shows a schematic representation of the present so- lar cell, and figs. 2Za-c a comparison with prior art solar cells. Figs. 3-6 show experimental results of the present solar cell.
DETAILED DESCRIPTION OF FIGURES In the figures: 100 solar cell 10 Si substrate 11 first a-Si:H layer; 12 hole transport layer (12) 12a treated layer (12a) 13 transparent conductive layer 14 electrical contact 16 nc-Si:H, possibly p- or n-doped
P100424NL00 11 21 second a-Si:H layer 22 n-doped a-Si:H layer 23 transparent conductive layer 24 electrical contact The figures are further detailed in the description of the experiments below.
Figure 1 shows a schematic representation of the present solar cell, as explained throughout the description, and figs. 2a-c a comparison of an enlarged section of fig. 1 with prior art solar cells, wherein fig. 2a relates to a prior art silicon hetero junction solar cell, fig. 2b to a solar cell with an MO, layer, without pre-treatment, and fig. Zc the present solar cell with pre-treated layer.
Figs. 3-6 show simulations and experimental results of the present solar cell.
Fig.3 shows simulations of (a) Voc and (b} FF as func- tion of MoO, thickness and for different work-functions (WF)MoO,, including the dipole at (i)a-Si:H/MoQ, interface. The inset shows the dipole layer. A strong difference of work function (WF) between MoO. and (i)a-Si:H is found to cause accumula- tion/depletion of holes at this interface with formation of a thin dipole (see inset in fig.3 b). It is observed that for higher WFMoO. there is a clear optimal MoO, thickness below 5 nm which is considered a trade-off between dipole and c-Si band bending. On the contrary, typically measured for non-stoichiometric MoO, the simulated trends progres- sively change leading to higher FF and VOC for thicker MoO, layers.
Fig. 4 shows HTL as depicted in the inset: SHJ reference with 20-nm thick (p)nc-Si:H, 5.7 nm thick MoO, and PT+5.7 nm thick MoO:. Fig. 4 shows illuminated J-V curves and Fig. 5 the corresponding electrical parameters for various HTLs, respectively, showing the effect of plasma treatment on Voc and FF. The cell with only MoOx (red) exhibits lower Voc and FF (708 mV, 74.2%) originated from the S-shape J-V curve as compared to SHJ reference cell (green). Treating the (i)a- Si:H layer with PT, before the MoO, layer deposition, is
P100424NL00 12 found to progressively recover the electrical properties with an optimum at 130 s of PT time with measured Vee of 715 mV and FF above 77%. In Fig. 6 the MoO, thickness optimiza- tion is shown using the optimized PT. The results demon- strate that MoO; layer thickness can be reduced down to 3 nm in the presence of PT without Voc loss (715 mV) and with a progressive gain in FF up to 77.7%. The optimum MoO, thick- ness is in agreement with the trend observed in our simula- tions discussed above.
Fig. 6 shows solar cell parameters with different MoO. thickness and constant PT compared to a SHJ reference. (a) VOC and i-VOC, (b) JSC-EQE, (c} FF and p-FF, and (d) Dace. Note that all the cells (except the SHJ ref.) feature an un- intentionally thicker ITO (90 nm) that reduces JSC by ~0.55 mA/cm2. EXAMPLES/EXPERIMENTS In an example the following solar cell was made: ITO, 65nm MoOx, 3.5 nm (i)a-Si:H 5nm (n)c-Si wafer, 250um (iYa-Si:H 5nm (n)a-Si:H 6nm ITO, 150nm. Electrical properties of layers MoOyx no doping ITO: carrier concentration 5x10%%cm3 (nja-Si:H: activation energy <350 meV Conditions of the method PECVD: treatment frequency: 13.56 MHz Pressure: Z.2mbar Power density: 90 mW/cm? Time: 130s Gas mixture SiH: 0.8sccm, Hz: 170sccm, B:Hs (200ppm in Hs) :10sccm The invention although described in detailed explana- tory context may be best understood in conjunction with
P100424NL00 13 the accompanying figures.
It should be appreciated that for commercial application it may be preferable to use one or more variations of the present system, which would similar be to the ones disclosed in the present application and are within the spirit of the invention.
For the purpose of searching the following section is added, of which the subsequent section is a translation into Dutch.
1. Single or hetero junction Si-based solar cell (100) com- prising a hole transport layer (12), characterized in that the hole transport layer (12) comprises at least one transition metal oxide, wherein the hole transport layer (12) is provided on a plasma pre-treated surface layer (12a), such as a surface passivation layer, such as a-Si:H and/or silicon pre-treated layer (12a).
2. Solar cell according to embodiment 1, wherein the work- function loss of the combined hole transport layer (12) /pre- treated layer (12a) is < 1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, such as < 0.35 eV, and/or wherein the dipole of the plasma pre-treated surface is < 4 C/m, preferably < 2 C/m, more preferably < 1 C/m, such as <
0.7 C/m.
3. Solar cell according to embodiment 1 or 2, wherein the pre-treated layer (12a) is obtained by PECVD treatment with a plasma mixture comprising a positive dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as B2Hs, preferably comprising SiH, Hs, and the gaseous p-do- pant.
4. Solar cell according to any of embodiments 1-3, wherein the pre-treated layer (12a) comprises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive elec- trical charge, or a combination thereof.
5. Solar cell according to any of embodiments 1-4, wherein pre-treatment is performed during 10-1000 sec, preferably 20-300 sec, such as 30-100 sec, and/or
P100424NL00 14 wherein a power density during pre-treatment is 50-350 mW/cm?, preferably 70-200 mW/cm?, more preferably 80-100 mW/cm?, such as 90 mW/cm?, and/or wherein pre-treatment is performed at a temperature < 523 K (< 250 °C), preferably < 473 K (< 200 °C), more preferably < 443 K (< 170 °C), and/or wherein a plasma pressure is from 50-400 Pa {0.5-4 mbar), preferably 100-300 Pa {1-3 mbar), more preferably 150-250 Pa (1.5-2.5 mbar), such as 220 Pa (2.2 mbar), and/or wherein the pre-treated layer is substantially free of Si0©;, such as having less than 1% Si0:/pre-treated layer (atom/atom), more preferably <1000 ppm, even more preferably <100 ppm, such as < 10 ppm, and/or with the proviso that no annealing of the a-Si:H layer is provided, and/or with the proviso that no chemical etching of the a-Si:H layer is provided, preferably no chemical etching at all.
6. Solar cell according to any of embodiments 1-5, wherein the hole transport layer (12) has a thickness of <10 nm, preferably 1.5-9 nm, more preferably 2-7 nm, even more pref- erably 2.5-5 nm, such as 3-4 nm, and/or wherein the hole transport layer (12) has a an absorption coefficient <20 x10% cmt in the range 3-4eV, preferably <10 x10% cmt, and/or wherein the hole transport layer (12) has a current gain of 1-2 mA/cm?.
7. Solar cell according to any of embodiments 1-6, wherein the hole transport layer (12) is structured, such as com- prising a zig/zag structure, comprising random pyramids, texturing, preferably with a height of 1-7 um, such as 2-5 um, and combinations thereof.
8. Solar cell according to any of embodiments 1-7, wherein the hole transport layer (12) is provided under a transpar- ent conducting material (13).
9. Solar cell according to any of embodiments 1-8, wherein the transition metal is selected from period 4 or period 5 transition metals, such as Ti, V, Cr, Co, Ni, Cu, Zn, Cs, Nb, Mo, W, and alloys thereof, and/or
P100424NL00 15 wherein the hole transport layer (12) is dopant free.
10. Solar cell according to any of embodiments 1-9, wherein the hole transport layer (12) is deposited on a pre-treated a-Si:H layer.
11. Solar cell according to any of embodiments 1-10, further comprising at least one of a metal contact (14), and a stack of layers comprising a transparent conducting layer (13) of 40-200 nm in electrical contact with the metal contact, preferably with a carrier concentration 1-10*10%%cm?®, the transparent conducting layer above the < 10 nm hole transport layer (12), the hole transport layer (12) above the 1-10 nm treated layer (lla), a 100-500 pm doped crystal- line silicon substrate (10), and on a back side of the doped crystalline silicon substrate a second 1-10 nm a-Si:H layer (21), above the second a-Si:H layer an 1-10 nm electron transport layer, preferably with an activation energy <350 meV, such as an n-doped a-Si:H layer (22) and/or n-doped nc- Si:H and/or alloyed with O, N or C, a second 20-300 nm transparent conducting layer (23), such as an ITO layer, and a metal contact layer (24) above the transparent layer.
12. Solar cell according to any of embodiments 1-11, having a short circuit current of > 39 mA/cm?, and/or an FF of > 70%, preferably FF>75%, preferably >77%, such as > 80%, and/or a Vee of 700-730 mV, and/or a conversion efficiency of > 21%.
13. Solar cell according to any of embodiments 1-12, wherein the solar cell is selected from single junction solar cells, hetero junction solar cells, multi-junction solar cells, thin film solar cells, wherein the silicon is crystalline silicon, n-doped or p-doped crystalline silicon.
14. Method of producing a solar cell according to any of em- bodiments 1-12, comprising the steps of providing a Si-substrate (10), such as a crystalline Si-sub- strate, depositing an a-Si:H layer (11) on said Si-substrate, without a vacuum break plasma pre-treating the a-Si:H layer (11) with a plasma mixture with a positive dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as
P100424NL00 16
B:Hs, preferably comprising SiH4, Hz, and the gaseous p-do- pant, preferably at a frequency of 12-15 MHz, and/or prefer- ably during 10-1000 sec, and/or preferably at power density of 50-350 mW/cm?, and/or preferably at a temperature < 523 K
(< 250 °C), and/or preferably at a pressure from 50-400 Pa (0.5-4 mbar), and/or preferably with a gas mixture compris- ing 0.2-2sccm SiH4, 50-400 sccm He, and 1-20sccm B:Hs (200ppm in Hs), depositing a transition metal oxide layer (12) on the treated a-Si:H layer, depositing a transparent conductive oxide layer (13) on the transition metal oxide layer, such as by PECVD, thermal evaporation, atomic layer deposition, PVD, and sputtering, and providing at least one contact (14) on the transparent con- ductive oxide layer.
Claims (14)
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| NL2025744A NL2025744B1 (en) | 2020-06-03 | 2020-06-03 | Methodology for efficient hole transport layer using transition metal oxides |
| EP21730689.3A EP4162534A1 (en) | 2020-06-03 | 2021-05-30 | Methodology for efficient hole transport layer using transition metal oxides |
| CN202180032494.XA CN115516648A (en) | 2020-06-03 | 2021-05-30 | Method of high efficiency hole transport layer using transition metal oxides |
| PCT/NL2021/050345 WO2021246865A1 (en) | 2020-06-03 | 2021-05-30 | Methodology for efficient hole transport layer using transition metal oxides |
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Non-Patent Citations (5)
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| KAWA ELIORA: "Molybdenum Oxide in Hole-selective Contacts for Silicon-based Solar Cells", MASTER THESIS, 6 September 2019 (2019-09-06), pages 1 - 84, XP055773960, Retrieved from the Internet <URL:https://repository.tudelft.nl/islandora/object/uuid:a9467d6c-33ec-4ce1-9cbf-409ddd8953cd/datastream/> [retrieved on 20210209] * |
| L. MAZZARELLA ET AL., STRATEGY TO MITIGATE THE DIPOLE INTERFACIAL STATES IN (I)A-SI:H/MOO PASSIVATING CONTACTS SOLAR CELLS |
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| SHI JIANHUA ET AL: "MoO x modified ITO/a-Si:H(p) contact for silicon heterojunction solar cell application", MATERIALS RESEARCH BULLETIN, vol. 97, 4 September 2017 (2017-09-04), GB, pages 176 - 181, XP055773946, ISSN: 0025-5408, DOI: 10.1016/j.materresbull.2017.09.005 * |
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