EP4162534A1 - Methodology for efficient hole transport layer using transition metal oxides - Google Patents

Methodology for efficient hole transport layer using transition metal oxides

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Publication number
EP4162534A1
EP4162534A1 EP21730689.3A EP21730689A EP4162534A1 EP 4162534 A1 EP4162534 A1 EP 4162534A1 EP 21730689 A EP21730689 A EP 21730689A EP 4162534 A1 EP4162534 A1 EP 4162534A1
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EP
European Patent Office
Prior art keywords
layer
solar cell
hole transport
transport layer
treated
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP21730689.3A
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German (de)
French (fr)
Inventor
Paul Alejandro PROCEL MOYA
Olindo ISABELLA
Miroslav Zeman
Luana MAZZARELLA
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Technische Universiteit Delft
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Technische Universiteit Delft
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Application filed by Technische Universiteit Delft filed Critical Technische Universiteit Delft
Publication of EP4162534A1 publication Critical patent/EP4162534A1/en
Pending legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F10/00—Individual photovoltaic cells, e.g. solar cells
    • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
    • H10F10/16—Photovoltaic cells having only PN heterojunction potential barriers
    • H10F10/164—Photovoltaic cells having only PN heterojunction potential barriers comprising heterojunctions with Group IV materials, e.g. ITO/Si or GaAs/SiGe photovoltaic cells
    • H10F10/165—Photovoltaic cells having only PN heterojunction potential barriers comprising heterojunctions with Group IV materials, e.g. ITO/Si or GaAs/SiGe photovoltaic cells the heterojunctions being Group IV-IV heterojunctions, e.g. Si/Ge, SiGe/Si or Si/SiC photovoltaic cells
    • H10F10/166—Photovoltaic cells having only PN heterojunction potential barriers comprising heterojunctions with Group IV materials, e.g. ITO/Si or GaAs/SiGe photovoltaic cells the heterojunctions being Group IV-IV heterojunctions, e.g. Si/Ge, SiGe/Si or Si/SiC photovoltaic cells the Group IV-IV heterojunctions being heterojunctions of crystalline and amorphous materials, e.g. silicon heterojunction [SHJ] photovoltaic cells
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F71/00—Manufacture or treatment of devices covered by this subclass
    • H10F71/129—Passivating
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/30—Coatings
    • H10F77/306—Coatings for devices having potential barriers
    • H10F77/311—Coatings for devices having potential barriers for photovoltaic cells
    • 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
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • 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
    • 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
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention is in the field of a process for making solar cells, or photovoltaic (PV) cell, with transparent contacts and an improved hole transport layer. Said solar cells com prise at least one hetero junction and typically two hetero junctions.
  • the invention provides solar cells with good operating characteristics, such as in terms of conversion efficiency, fill factor, and current gain.
  • a solar cell, or photovoltaic (PV) cell is an electrical 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 irrespective of whether the source is sun light or an artificial light. They may also be used as photo detector.
  • a solar cell When a solar cell absorbs light it may generate either electron-hole pairs or excitons. In or der to obtain an electrical current charge carriers of opposite types are separated. The separated charge carriers are “extracted” to an external circuit, typically providing a DC-current. For prac tical use a DC-current may be transformed into an AC-current, e.g. by using a transformer.
  • solar cells are grouped into an array of elements.
  • 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. Accord ing to recent predictions, this trend will remain for the upcoming years towards 2020 and many years beyond. Due to their simplified process, conventional c-Si solar cells dominate a large part of the market. As alterna tive to the industry to improve the power to cost ratio, the silicon heterojunction approach has become increasingly attractive for PV industry, even though the relatively complicated process to deploy the proper front layers, such as a transparent conductive oxide (TCO) and an inherent low thermal budget of the cells limiting usage of existing production lines and thus result in a negligible market share so far.
  • TCO transparent conductive oxide
  • a heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors.
  • a homojunction relates to a semiconductor interface formed by typically two layers of similar semiconductor material, wherein these semiconductor materials have equal band gaps and typically have a different doping (either in concentration, in type, or both).
  • a common example is a homojunction at the interface between an n-type layer and a p-type layer, which is referred to as a p-n junction.
  • advanced 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 distinguished, 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 p-n junction crystalline silicon de vice 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 series resistance, leakage, etc.
  • the fill factor may be defined as a ratio of an actual maximum obtainable power to the product of the open circuit voltage and short circuit current. It is considered to be a key parameter 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 resistance 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 im portance.
  • the work function In aspect of multi-layer structures relates to the so-called work-function.
  • the work function In physics the work function relates to a minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum outside the solid surface.
  • 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.
  • a solar cell having a full area front passivating contact is not attractive, such as due to highly absorptive materials used to build such a structure. That is the case of heavily doped poly-silicon and a-Si layers.
  • 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 lateral transport.
  • TCO transparent conductive oxide
  • Transition metal oxides may be considered for c-Si based heterojunction (SHJ) solar cells in view of their ability to induce efficient carrier selectiv ity and mitigate parasitic absorption losses resulting in clear current gain.
  • MoO x molyb denum oxide
  • HTL hole transport layer
  • MoO x layer in combination with a thin intrinsic passivation a-Si:H layer and a transparent conductive oxide (TCO) has in fact demonstrated conversion efficiency of 23.5%.
  • a-Si:H/MoO x exhibits 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 characteristics as compared to solar cells with doped silicon car rier selective HTLs.
  • FF fill factor
  • the transient photo-conductance decay measurements showed that both ef fective carrier lifetime and implied open-circuit voltage increased abruptly after inserting an ul trathin MoOx buffer layer, from 1.43 ms to 2.28 ms and 735 mV to 744 mV, respectively, which is mainly attributed to modification of MoOx buffer layer on the Schottky barrier at the interface of ITO/a-Si:H(p) contact.
  • the MoOx based SHJ solar cell illustrated a champion efficiency up to 21.8%, companying with an absolute V oc gain over 8 mV, and fill factor gain over 1.5%, respectively.
  • the present invention relates to an increased efficiency 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.
  • 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.
  • 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 x passivating contacts solar cells”
  • hole transport layers such as MoO x layers in a wider operational range (such as by PECVD, thermal evaporation, atomic layer depo sition, PVD, and sputtering) by introducing e.g. a prior PECVD plasma pre-treatment.
  • Inventors confirmed using XPS that even after the plasma treatments they do not find B (doping) of the plasma treated layer, so they do not have the deposition of a thin doped layer as results of the plasma treatment. That is considered important, because they do not have a doped emitter under neath the MoOx transport layer (see fig. 7).
  • This pre-treatment 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 absorption.
  • the optimized plasma treatment gave a S-shape-free J-V curve with FF comparable to the SHJ reference device. Further thickness optimization demonstrated that the MoO x layer can be further reduced down to 3.5 nm with no 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 maxi mization.
  • 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 di pole on the interface, a good conversion efficiency, a good transparency, a low parasitic absorp tion, 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 characteristics (see above effects) of the obtained devices and hence constitute noticeable physi cal differences over e.g. the prior art. No annealing step is required. A high efficiency solar cell (> 22% efficiency) is provided.
  • the present solar cell typically comprises at least one hetero junction and typically two hetero junc tions.
  • 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) has a thick ness of 1.5-9 nm, wherein the hole transport layer (12) is provided on a plasma pre-treated sur face layer (12a), wherein the plasma pre-treated surface is a surface passivation layer, wherein the surface passivation layer is an a-Si:H pre-treated layer (12a).
  • a loss in work-function is minimized, and the dipole at the plasma treated surface is limited as well.
  • 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-substrate, 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 dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as B2H5 , preferably comprising S1H4, 3 ⁇ 4, and the gaseous p-dopant, hence there is no need for chemical etching, preferably at a frequency of 12-15 MHz, and/or preferably dur ing 10-1000 sec, and/or preferably at power density of 50-350 mW/cm 2 , and/or
  • the present invention provides a simplified fabrication process wherein solar cells can be finished 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 rela tively 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 ar chitecture, a bifacial solar cell architecture and for both n-type and p-type bulk material.
  • FF tively high fill factor
  • the present invention provides a solution to one or more of the above-mentioned problems.
  • the present invention relates in a first aspect to a single or hetero junction Si-based so lar cell according to claim 1, and in a second aspect to a process for making such a solar cell.
  • the surface passivation layer further comprises a silicon pre-treated layer provided on the a-Si:H pre-treated layer.
  • 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.
  • 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.
  • the pre-treated layer (12a) is ob tained 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 B2H6 , preferably comprising S1H4, 3 ⁇ 4, and the gaseous p-dopant.
  • a plasma mixture comprising a positive dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as B2H6 , preferably comprising S1H4, 3 ⁇ 4, and the gaseous p-dopant.
  • the pre-treated layer (12a) com prises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive electrical charge, or a combination thereof.
  • the present solar cell pre-treatment is performed dur ing 10-1000 sec, preferably 20-300 sec, such as 30-100 sec.
  • a power density during pre-treat ment is 50-350 mW/cm 2 , preferably 70-200 mW/cm 2 , more preferably 80-100 mW/cm 2 , such as 90 mW/cm 2 .
  • 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).
  • 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).
  • the pre-treated layer is substan tially free of S1O2, such as having less than 1% SiC /pre-treated layer (atom/atom), more prefera bly ⁇ 1000 ppm, even more preferably ⁇ 100 ppm, such as ⁇ 10 ppm.
  • 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.
  • the present solar cell is with the proviso that no chemi cal etching of the a-Si:H layer is provided, preferably no chemical etching at all.
  • the hole transport layer (12) has a thickness of 2-7 nm, preferably 2.5-5 nm, such as 3-4 nm.
  • the hole transport layer (12) has an absorption coefficient ⁇ 20 xlO 4 cm 1 in the range 3-4eV, preferably ⁇ 10 xlO 4 cm 1 .
  • the hole transport layer (12) has a current gain of 1-2 mA/cm 2 .
  • the hole transport layer (12) is structured, such as comprising a zig/zag structure, comprising random pyramids, texturing, pref erably with a height of 1-7 pm, such as 2-5 pm, and combinations thereof.
  • the hole transport layer (12) is provided under a transparent conducting material (13).
  • 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.
  • the hole transport layer (12) is dopant free.
  • 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.
  • 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 l-10*10 2 °cm 3 , the transparent conducting layer above the ⁇ 10 nm hole transport layer (12), the hole transport layer (12) above the 1-10 nm treated layer (12a), a 100-500 pm 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 alloyed with O, N or C
  • the present solar cell has a short circuit current of > 39 mA/cm 2 , 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%.
  • the solar cell is selected from sin glejunction 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.
  • the present invention relates to a method of producing a solar cell according to the invention.
  • Figure 1 shows a schematic representation of the present solar cell, and figs. 2a-c a comparison with prior art solar cells.
  • Figs. 3-7 show experimental results of the present solar cell.
  • Figure 1 shows a schematic representation of the present solar cell, as explained through out the description, and figs. 2a-c a comparison of an enlarged section of fig. 1 with prior art so lar cells, wherein fig. 2a relates to a prior art silicon hetero junction solar cell, fig. 2b to a solar cell with an MO x layer, without pre-treatment, and fig. 2c 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 function of MoO x thickness and for dif ferent work-functions (WF)MoO x , including the dipole at (i)a-Si:H/MoO x interface.
  • the inset shows the dipole layer.
  • a strong difference of work function (WF) between MoO x and (i)a-Si:H is found to cause accumulation/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 x there is a clear optimal MoO x thick ness 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 x , the simulated trends progressively change leading to higher FF and VOC for thicker MoO x 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 x and PT+5.7 nm thick MoO x .
  • 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 MoO x (central layout) exhibits lower Voc and FF (708 mV, 74.2%) originated from the S-shape J-V curve as compared to SHJ reference cell (left columns).
  • Treating the (i)a-Si:H layer with PT, before the MoO x layer deposition, is found to progressively recover the electrical properties with an optimum at 130 s of PT time with meas ured Voc of 715 mV and FF above 77%.
  • the MoO x thickness optimization is shown us ing the optimized PT.
  • the results demonstrate that MoO x 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 x thickness is in agreement with the trend observed in our simulations discussed above.
  • Fig. 6 shows solar cell parameters with different MoO x thickness and constant PT com pared to a SHJ reference (a) VOC and i-VOC, (b) JSC-EQE, (c) FF and p-FF, and (d) h ; ,a. Note that all the cells (except the SHJ ref.) feature an unintentionally thicker ITO (90 nm) that reduces JSC by -0.55 mA/cm 2 .
  • Figure 7 XPS full survey spectra for (7a) c-Si/(i)a-Si:H/MoOx and (7b) c-Si/(i)a-Si:H/PT/MoOx stacks.
  • XPS X-ray photoelectron spectroscopy
  • XPS analysis is carried out using a PHI-TFA XPS spectrometer (Physical Electronic Inc.), equipped with an X-ray A1 -monochromatic source.
  • the vacuum level during XPS analysis is 10 9 mbar.
  • the analyzed area is 0.4 mm in diameter and the analysis depth is 3 - 5 nm.
  • Narrow mul tiplex scans of the peaks are recorded using a pass energy of 23.5 eV with a step size 0.1 eV, at a take-off angle of 45° with respect to the sample surface.
  • Low energy electron gun is used for sur face charge neutralization XPS.
  • Spectra are processed using Multipak v8.0 (Physical Electronics Inc.).
  • XPS Survey spectra are used to obtain surface composition.
  • the XPS survey spectra show no presence of boron for the sample with plasma treatment
  • ITO 65nm MoOx, 3.5 nm (i)a-Si:H 5nm (n)c-Si wafer, 250pm (i)a-Si:H 5nm (n)a-Si:H 6nm ITO, 150nm.
  • ITO carrier concentration 5xl0 20 cm 3 (n)a-Si:H: activation energy ⁇ 350 meV
  • PECVD treatment frequency: 13.56 MHz Pressure: 2.2mbar Power density: 90 mW/cm 2 Time: 130s

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Abstract

The present invention is in the field of a process for making solar cells, or photovoltaic (PV) cell, with transparent contacts and an improved hole transport layer. Said solar cells comprise at least one hetero junction and typically 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

Methodology for efficient hole transport layer using transition 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 transparent contacts and an improved hole transport layer. Said solar cells com prise at least one hetero junction and typically 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 electrical 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 irrespective of whether the source is sun light 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 or der to obtain an electrical current charge carriers of opposite types are separated. The separated charge carriers are “extracted” to an external circuit, typically providing a DC-current. For prac tical 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 elements. 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. Accord ing to recent predictions, this trend will remain for the upcoming years towards 2020 and many years beyond. Due to their simplified process, conventional c-Si solar cells dominate a large part of the market. As alterna tive to the industry to improve the power to cost ratio, the silicon heterojunction approach has become increasingly attractive for PV industry, even though the relatively complicated process to deploy the proper front layers, such as a transparent conductive oxide (TCO) and an inherent low thermal budget of the cells limiting usage of existing production lines and thus result in a negligible 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 homojunction relates to a semiconductor interface formed by typically two layers of similar semiconductor material, wherein these semiconductor materials have equal band gaps and typically have a different doping (either in concentration, in type, or both). A common example is a homojunction at the interface between an n-type layer and a p-type layer, which is referred to as a p-n junction. In heterojunctions advanced 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 distinguished, 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 p-n junction crystalline silicon de vice 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 series resistance, leakage, etc. A qualification of perfor mance of a solar cell is the fill factor (FF). The fill factor may be defined as a ratio of an actual maximum obtainable power to the product of the open circuit voltage and short circuit current. It is considered to be a key parameter 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 resistance 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 im portance.
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 remove an electron from a solid to a point in the vacuum outside 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 passivating contact is not attractive, such as due to highly absorptive 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 lateral transport.
Transition metal oxides (TMOs) may be considered for c-Si based heterojunction (SHJ) solar cells in view of their ability to induce efficient carrier selectiv ity and mitigate parasitic absorption losses resulting in clear current gain. Among TMOs, molyb denum oxide (MoOx) is promising for applications as hole transport layer (HTL). MoOx layer, in combination 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/MoOx exhibits 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 characteristics as compared to solar cells with doped silicon car rier selective HTLs.
Ziegler et al. in “Plasma-enhanced atomic-layer-deposited MoOx emitters for silicon het erojunction solar cells”, Appl. Phys. A (2015), 120:811-816, recite a method for the deposition of molybdenum oxide (MoOx) with high growth rates at temperatures below 200 °C based on plasma-enhanced atomic layer deposition (PE-ALD) is presented. The stoichiometry of the of the over-stoichiometric MoOx films can be adjusted by the plasma-parameters. First results of these layers acting as hole-selective contacts in silicon heterojunction (SHJ) solar cells are presented and discussed. Kawa in “Molybdenum Oxide in Hole-selective Contacts for Silicon-based Solar Cells”, Master Thesis. TUDelft, September 2019, recites an investigation of the introduction of MoOx in the front contact of solar cells. To this purpose, the evolution of the passivation is eval uated along the fabrication process together with temperature sensitivity. The application of the layer negatively influences the passivation quality. Moreover, the passivation quality decreases more after annealing and transparent conductive oxide (TCO) deposition. We mitigate such is sues by treating the surface of the passivation layer before MoOx application. Shi et al. in “MoOx modified ITO/a-Si:H(p) contact for silicon heterojunction solar cell application”, Materi als Research Bulletin 97, January 2018, P. 176-181 developed a facile and effective method to modify the electrical properties of ITO/a-Si:H(p) contact by thermal evaporation an ultrathin MoOx buffer layer. The transient photo-conductance decay measurements showed that both ef fective carrier lifetime and implied open-circuit voltage increased abruptly after inserting an ul trathin MoOx buffer layer, from 1.43 ms to 2.28 ms and 735 mV to 744 mV, respectively, which is mainly attributed to modification of MoOx buffer layer on the Schottky barrier at the interface of ITO/a-Si:H(p) contact. By modulating the thickness of MoOx buffer layer, the MoOx based SHJ solar cell illustrated a champion efficiency up to 21.8%, companying with an absolute Voc gain over 8 mV, and fill factor gain over 1.5%, respectively.
The present invention relates to an increased efficiency 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. 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/MoOx passivating contacts solar cells”
(DOT10.1002/pip.3381), which publication and its contents are incorporated by reference. It is found that with the present invention it is possible to deposit hole transport layers, such as MoOx layers in a wider operational range (such as by PECVD, thermal evaporation, atomic layer depo sition, PVD, and sputtering) by introducing e.g. a prior PECVD plasma pre-treatment. Inventors confirmed using XPS that even after the plasma treatments they do not find B (doping) of the plasma treated layer, so they do not have the deposition of a thin doped layer as results of the plasma treatment. That is considered important, because they do not have a doped emitter under neath the MoOx transport layer (see fig. 7). 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 absorption. The optimized plasma treatment gave a S-shape-free J-V curve with FF comparable to the SHJ reference device. Further thickness optimization demonstrated that the MoOx layer can be further reduced down to 3.5 nm with no 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 maxi mization. Finally, both optical simulations and experimental EQE showed that the proposed ap proach 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 MoOx.
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 di pole on the interface, a good conversion efficiency, a good transparency, a low parasitic absorp tion, 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 characteristics (see above effects) of the obtained devices and hence constitute noticeable physi cal 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 junc tions. 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) has a thick ness of 1.5-9 nm, wherein the hole transport layer (12) is provided on a plasma pre-treated sur face layer (12a), wherein the plasma pre-treated surface is a surface passivation layer, wherein the surface passivation layer is an a-Si:H pre-treated layer (12a). Therewith in particular a loss in work-function is minimized, and the dipole at the plasma treated surface is limited as well.
The above and other improvements lead to a short circuit current of > 39 mA/cm2, 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%, and/or an absorption coefficient <20 xlO4 cm 1 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 differ ence is considered 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-substrate, 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 dopant comprising gas, such as a B-, Al-or Ga-comprising dopant gas, such as B2H5, preferably comprising S1H4, ¾, and the gaseous p-dopant, hence there is no need for chemical etching, preferably at a frequency of 12-15 MHz, and/or preferably dur ing 10-1000 sec, and/or preferably at power density of 50-350 mW/cm2, 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 S1H4, 50-400 seem ¾, and l-20sccm B2H5 (200ppm in ¾), 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 transparent conductive oxide layer. Mostly fabrication tools thereof are already part of standard production lines. Therefore, the present inven tion may be considered commercially available from the start since it does not require de velopment of additional process tools.
In summary the present invention provides a simplified fabrication process wherein solar cells can be finished 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 rela tively 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 ar chitecture, a bifacial solar cell architecture 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 so lar cell according to claim 1, and in a second aspect to a process for making such a solar cell.
In an exemplary embodiment of the present solar cell the surface passivation layer further comprises a silicon pre-treated layer provided on the a-Si:H pre-treated layer.
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 ob tained 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 B2H6, preferably comprising S1H4, ¾, and the gaseous p-dopant.
In an exemplary embodiment of the present solar cell the pre-treated layer (12a) com prises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive electrical charge, or a combination thereof.
In an exemplary embodiment of the present solar cell pre-treatment is performed dur ing 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-treat ment is 50-350 mW/cm2, preferably 70-200 mW/cm2, more preferably 80-100 mW/cm2, such as 90 mW/cm2.
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 substan tially free of S1O2, such as having less than 1% SiC /pre-treated layer (atom/atom), more prefera bly <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 chemi cal 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 2-7 nm, preferably 2.5-5 nm, such as 3-4 nm.
In an exemplary embodiment of the present solar cell the hole transport layer (12) has an absorption coefficient <20 xlO4 cm 1 in the range 3-4eV, preferably <10 xlO4 cm 1.
In an exemplary embodiment of the present solar cell the hole transport layer (12) has a current gain of 1-2 mA/cm2.
In an exemplary embodiment of the present solar cell the hole transport layer (12) is structured, such as comprising a zig/zag structure, comprising random pyramids, texturing, pref erably with a height of 1-7 pm, such as 2-5 pm, and combinations thereof.
In an exemplary embodiment of the present solar cell the hole transport layer (12) is provided under a transparent 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 l-10*102°cm3, the transparent conducting layer above the < 10 nm hole transport layer (12), the hole transport layer (12) above the 1-10 nm treated layer (12a), a 100-500 pm 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 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.
In an exemplary embodiment the present solar cell has a short circuit current of > 39 mA/cm2, 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 sin glejunction 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 invention.
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 invention.
To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims. SUMMARY OF FIGURES
Figure 1 shows a schematic representation of the present solar cell, and figs. 2a-c a comparison with prior art solar cells.
Figs. 3-7 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
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 through out the description, and figs. 2a-c a comparison of an enlarged section of fig. 1 with prior art so lar cells, wherein fig. 2a relates to a prior art silicon hetero junction solar cell, fig. 2b to a solar cell with an MOx layer, without pre-treatment, and fig. 2c 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 function of MoOx thickness and for dif ferent work-functions (WF)MoOx, including the dipole at (i)a-Si:H/MoOx interface. The inset shows the dipole layer. A strong difference of work function (WF) between MoOx and (i)a-Si:H is found to cause accumulation/depletion of holes at this interface with formation of a thin dipole (see inset in fig.3 b). It is observed that for higher WFMoOx there is a clear optimal MoOx thick ness below 5 nm which is considered a trade-off between dipole and c-Si band bending. On the contrary, typically measured for non-stoichiometric MoOx, the simulated trends progressively change leading to higher FF and VOC for thicker MoOx layers.
Fig. 4 shows HTL as depicted in the inset: SHJ reference with 20-nm thick (p)nc-Si:H, 5.7 nm thick MoOx and PT+5.7 nm thick MoOx. 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 (central layout) exhibits lower Voc and FF (708 mV, 74.2%) originated from the S-shape J-V curve as compared to SHJ reference cell (left columns). Treating the (i)a-Si:H layer with PT, before the MoOx layer deposition, is found to progressively recover the electrical properties with an optimum at 130 s of PT time with meas ured Voc of 715 mV and FF above 77%. In Fig. 6 the MoOx thickness optimization is shown us ing the optimized PT. The results demonstrate that MoOx 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 MoOx thickness is in agreement with the trend observed in our simulations discussed above.
Fig. 6 shows solar cell parameters with different MoOx thickness and constant PT com pared to a SHJ reference (a) VOC and i-VOC, (b) JSC-EQE, (c) FF and p-FF, and (d) h;,a. Note that all the cells (except the SHJ ref.) feature an unintentionally thicker ITO (90 nm) that reduces JSC by -0.55 mA/cm2. Figure 7 XPS full survey spectra for (7a) c-Si/(i)a-Si:H/MoOx and (7b) c-Si/(i)a-Si:H/PT/MoOx stacks.
X-ray photoelectron spectroscopy (XPS)
XPS analysis is carried out using a PHI-TFA XPS spectrometer (Physical Electronic Inc.), equipped with an X-ray A1 -monochromatic source. The vacuum level during XPS analysis is 10 9 mbar. The analyzed area is 0.4 mm in diameter and the analysis depth is 3 - 5 nm. Narrow mul tiplex scans of the peaks are recorded using a pass energy of 23.5 eV with a step size 0.1 eV, at a take-off angle of 45° with respect to the sample surface. Low energy electron gun is used for sur face charge neutralization XPS. Spectra are processed using Multipak v8.0 (Physical Electronics Inc.). XPS Survey spectra are used to obtain surface composition.
The XPS survey spectra show no presence of boron for the sample with plasma treatment
(PT).
EXAMPLES/EXPERIMENTS In an example the following solar cell was made:
Thickness of layers, from front to back:
ITO, 65nm MoOx, 3.5 nm (i)a-Si:H 5nm (n)c-Si wafer, 250pm (i)a-Si:H 5nm (n)a-Si:H 6nm ITO, 150nm.
Electrical properties of layers MoOx no doping
ITO: carrier concentration 5xl020cm3 (n)a-Si:H: activation energy <350 meV Conditions of the method PECVD: treatment frequency: 13.56 MHz Pressure: 2.2mbar Power density: 90 mW/cm2 Time: 130s
Gas mixture SiH*: 0.8sccm, ¾: 170sccm, B2H5 (200ppm in H2):10sccm
The invention although described in detailed explanatory context may be best under stood in conjunction with 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 pre sent application and are within the spirit of the invention.

Claims

1. Single or hetero junction Si-based solar cell (100) comprising 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) has a thickness of 1.5-9 nm, wherein the hole transport layer (12) is provided on a plasma pre-treated surface layer (12a), wherein the plasma pre-treated surface is a surface passivation layer, such as wherein the surface passivation layer is an a-Si:H pre-treated layer (12a).
2. Solar cell according to claim 1, wherein the surface passivation layer further comprises a silicon pre-treated layer provided on the a-Si:H pre-treated layer.
3. Solar cell according to claim 1 or 2, 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 prefer ably < 1 C/m, such as < 0.7 C/m.
4. Solar cell according to any of claims 1-3, 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 B2H6, preferably comprising SiLL, ¾, and the gase ous p-dopant.
5. Solar cell according to any of claims 1-4, wherein the pre-treated layer (12a) comprises nanocrystalline Si, a relaxed interface, p-dopants, amorphous Si, a positive electrical charge, or a combination thereof.
6. Solar cell according to any of claims 1-5, wherein pre-treatment is performed during 10- 1000 sec, preferably 20-300 sec, such as 30-100 sec, and/or wherein a power density during pre-treatment is 50-350 mW/cm2, preferably 70-200 mW/cm2, more preferably 80-100 mW/cm2, such as 90 mW/cm2, 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 S1O2, such as having less than 1% SiCk/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.
7. Solar cell according to any of claims 1-6, wherein the hole transport layer (12) has a thick ness of 2-7 nm, preferably 2.5-5 nm, such as 3-4 nm, and/or wherein the hole transport layer (12) has a an absorption coefficient <20 xlO4 cm 1 in the range 3-4eV, preferably <10 xlO4 cm 1, and/or wherein the hole transport layer (12) has a current gain of 1-2 mA/cm2.
8. Solar cell according to any of claims 1-7, wherein the hole transport layer (12) is struc tured, such as comprising a zig/zag structure, comprising random pyramids, texturing, preferably with a height of 1-7 pm, such as 2-5 pm, and combinations thereof.
9. Solar cell according to any of claims 1-8, wherein the hole transport layer (12) is provided under a transparent conducting material (13).
10. Solar cell according to any of claims 1-9, wherein the transition metal is selected from pe riod 4 or period 5 transition metals, such as Ti, V, Cr, Co, Ni, Cu, Zn, Cs, Nb, Mo, W, and alloys thereof, and/or wherein the hole transport layer (12) is dopant free.
11. Solar cell according to any of claims 1-10, wherein the hole transport layer (12) is depos ited on a pre-treated a-Si:H layer.
12. Solar cell according to any of claims 1-11, further comprising at least one of a metal con tact (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 l-10*102°cm3, the transparent conducting layer above the < 10 nm hole transport layer (12), the hole transport layer (12) above the 1-10 nm treated layer (12a), a 100-500 pm doped crystalline silicon sub strate (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.
13. Solar cell according to any of claims 1-12, having a short circuit current of > 39 mA/cm2, 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%.
14. Solar cell according to any of claims 1-13, 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.
15. Method of producing a solar cell according to any of claims 1-14, comprising the steps of providing a Si-substrate (10), such as a crystalline Si-substrate, 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 B2H5, preferably comprising S1H4, FF, and the gaseous p-dopant, 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/cm2, 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 S1H4, 50-400 seem ¾, and l-20sccm B2H5 (200ppm in ¾), depositing a transition metal oxide layer (12) on the pre-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 conductive oxide layer.
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