Method of using a transition metal oxide as an efficient hole transport layer

Abstract

The present invention belongs to the field of methods for manufacturing solar cells or photovoltaic (PV) cells having transparent contacts and improved hole transport layers. The solar cells include at least one heterojunction and typically include two heterojunctions. The present invention provides solar cells having good operating characteristics, such as good operating characteristics in terms of conversion efficiency, fill factor, and current gain.

Description

FIELD OF THE INVENTION

[0001] The present invention belongs to the field of methods for manufacturing solar cells or photovoltaic (PV) cells having transparent contacts and improved hole transport layers. The solar cells comprise at least one heterojunction and typically comprise two heterojunctions. The present invention provides solar cells having good operating characteristics, such as good operating characteristics in terms of conversion efficiency, fill factor, and current gain. BACKGROUND OF THE INVENTION

[0002] A solar cell or photovoltaic (PV) cell is an electrical device that directly converts light energy, typically sunlight energy (hence the term "solar"), into electrical energy by means of the so-called photovoltaic effect. A solar cell can be regarded as a photoelectric cell, which has electrical characteristics, such as current, voltage, resistance, and fill factor, that change when exposed to light and vary depending on the type of cell.

[0003] Solar cells are described as photovoltaic cells regardless of whether the source is sunlight or artificial light. They can also be used as photodetectors.

[0004] When a solar cell absorbs light, it can generate electron-hole pairs or excitons. To obtain an electric current, charge carriers of opposite types are separated. The separated charge carriers are "extracted" to an external circuit, typically providing a DC current. In practical use, the DC current can be converted into an AC current, for example, by using a transformer.

[0005] Typically, solar cells are assembled into an array of elements. Multiple elements can form a panel, and multiple panels can form a system.

[0006] Wafer-type c-Si solar cells account for over 90% of the total PV market. According to recent forecasts, this trend will continue in 2020 and for many years to come. Due to the simplified process, conventional c-Si solar cells dominate the market. As an alternative for the industry to improve the power-to-cost ratio, the silicon heterojunction approach has become increasingly attractive for the PV industry. However, the method of deploying appropriate front layers, such as transparent conductive oxides (TCOs), is relatively complex and the inherently low thermal budget of the cells limits the use of existing production lines and thus has led to a negligible market share to date. A heterojunction is an interface that appears between two dissimilar crystalline semiconductor layers or regions. In contrast to a homojunction, these semiconductive materials have unequal bandgaps. A homojunction involves a semiconductor interface typically formed by two layers of similar semiconductor materials, where these semiconductor materials have equal bandgaps and typically have different doping (different in concentration, type, or both). A common example is the homojunction at the interface between an n-type layer and a p-type layer, called a p-n junction. In a heterojunction, advanced techniques are used to precisely control the deposition thickness of the layers involved and to create a lattice-matched abrupt interface. Heterojunctions can be classified into three types, namely straddling gap, staggered gap, and broken gap.

[0007] One disadvantage of solar cells is that the conversion itself is not very efficient. Typically, for Si-solar cells, it is limited to about 20%. Theoretically, a single p-n junction crystalline silicon device has a maximum power efficiency of about 30%. An infinite number of layers can achieve a maximum power efficiency of 86%. Currently, the highest ratio achieved by solar cells themselves is about 44%. For commercially available silicon solar cells, the record is about 25.6%. Considering efficiency, the front contact can be moved to the rear or back side, thereby eliminating the shadow area. In addition, a thin silicon film is also applied to the wafer. There are also various defects in solar cells, such as recombination losses, reflection losses, heating during use, thermodynamic losses, shadow, internal resistances such as shunt resistance and series resistance, leakage, etc. A technical indicator of solar cell performance is the fill factor (FF). The fill factor can be defined as the ratio of the actual maximum achievable power to the product of the open-circuit voltage and the short-circuit current. This ratio is considered a key parameter for performance evaluation. Typical advanced commercially available solar cells have a fill factor >0.75, while 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, there are fewer internal losses. The efficiency is still continuously improving, so each relatively small improvement is welcome and significant.

[0008] In terms of the multilayer structure, the so-called work function is involved. In physics, the work function involves the minimum thermodynamic work (i.e., energy) required to move 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 at the atomic scale, but still too close to the solid to be affected by the ambient electric field in the vacuum. The work function is considered not to be a property of the bulk material, but actually a property of the material surface, and thus depends on the crystal plane and possible contamination, surface charge, etc. The work function can be expressed in eV. Typically, there is a mismatch at the interface between two different materials, for example, in terms of the work function. A work function "loss" occurs at the interface.

[0009] Currently, solar cells with full-area front passivating contacts are not attractive, for example, because of the high absorptivity of the materials used to construct such structures. This is the case for heavily doped polysilicon and a-Si layers. In the case of polysilicon, the method requires an extremely thin polysilicon film to minimize parasitic absorption losses, while in the case of a-Si, the method requires, for example, an additional transparent conductive oxide (TCO) layer to support the lateral transport of carriers.

[0010] Given that transition metal oxides (TMOs) can induce efficient carrier selectivity and reduce parasitic absorption losses, resulting in a significant current gain, TMOs can be considered for use in c-Si type heterojunction (SHJ) solar cells. Among the numerous TMOs, molybdenum oxide (MoO x ) is promising as a hole transport layer (HTL). In fact, the combination of MoO x layer with a thin intrinsic passivating a-Si:H layer and a transparent conductive oxide (TCO) exhibits a conversion efficiency of 23.5%. However, compared with conventional SHJ cells, (i) a-Si:H / MoO x exhibits weaker thermal stability in air / moisture, hindering carrier selectivity. Therefore, compared with solar cells with doped silicon carrier-selective HTLs, devices with TMOs typically have a lower fill factor (FF) and a possible S-shaped J-V characteristic.

[0011] Ziegler et al. described in "Plasma - enhanced atomic-layer-deposited MoOx emitters for silicon heterojunction solar cells", Appl. Phys. A (2015), 120: 811 - 816, a method for depositing MoO x to deposit molybdenum oxide (MoO x ) at a temperature below 200 °C based on plasma-enhanced atomic layer deposition (PE-ALD). The stoichiometry of the over-stoichiometric MoO x film can be adjusted according to the plasma parameters. The results of these layers acting as hole-selective contacts in silicon heterojunction (SHJ) solar cells were first proposed and discussed. Kawa described in "Molybdenum Oxide in Hole-selective Contacts for Silicon-based Solar Cells", Master Thesis, TU Delft, September 2019, a study on introducing MoO x into the front contact of a solar cell. For this purpose, the evolution of passivation was evaluated along with temperature sensitivity during the manufacturing process. The application of the layer has a negative impact on the passivation quality. In addition, after annealing and transparent conductive oxide (TCO) deposition, the passivation quality is further reduced. We investigated the influence of the deposition temperature of MoO x It should be noted that there seems to be an incomplete sentence at the end of the original text. I have translated as much as possible based on the existing content. If you can provide the complete text, I will be able to offer a more accurate translation.The surface of the passivation layer is processed beforehand to reduce such problems. Shi et al. developed a convenient and effective method to change the electrical properties of the ITO / a-Si:H(p) contact by thermally evaporating an ultrathin MoOx buffer layer in "MoOx modified ITO / a-Si:H(p) contact for silicon heterojunction solar cell application" (Materials Research Bulletin 97, January 2018, pp. 176-181). The results of transient photoconductivity decay measurements showed that the effective carrier lifetime and the implied open-circuit voltage increased suddenly from 1.43 ms to 2.28 ms and from 735 mV to 744 mV, respectively, after inserting the ultrathin MoOx buffer layer, which was mainly attributed to the change of the Schottky barrier at the interface of the MoOx buffer layer and the ITO / a-Si:H(p) contact. By adjusting the thickness of the MoOx buffer layer, the MoOx-type SHJ solar cell showed a champion efficiency of up to 21.8%, accompanied by an absolute Voc gain of more than 8 mV and a fill factor gain of more than 1.5%, respectively.

[0012] The present invention relates to an Si-based solar cell with increased efficiency and various aspects thereof, as well as a simplified method for manufacturing such a solar cell, which overcomes one or more of the above disadvantages without compromising functionality and advantages. Summary of the Invention

[0013] In a first aspect, the present invention relates to an Si-based solar cell, and in a second aspect, to a method for manufacturing such a solar cell. The present invention is also the subject of a scientific publication by L. Mazzarella et al. entitled "Strategy to mitigate the dipole interfacial states in (i)a-Si:H / MoO x passivating contacts solar cells" (DOI: 10.1002 / pip.3381), the publication and its content being incorporated by reference. It has been found that by using the present invention, it is possible to deposit a hole transport layer, such as MoO x in a wide operating range by introducing, for example, a previous PECVD plasma pretreatment. xLayers (e.g., by PECVD, thermal evaporation, atomic layer deposition, PVD, and sputtering). The inventors used XPS to confirm that even after plasma treatment, they did not find B (doping) in the plasma-treated layer, and thus, they did not deposit a thin doped layer due to plasma treatment. This is considered important because they do not have Doped Emitter (see FIG. 7). This pre-treatment (i) reduces the interaction between the hole transport layer and (i) a-Si:H, and (ii) strongly supports charge transport, which is attributed to the reduction of dipole strength. Optical gain is obtained at lower parasitic absorption. The optimized plasma treatment results in a non-S-shaped J-V curve with an FF comparable to that of the SHJ reference device. Further optimization of the thickness shows that the MoO x layer can be further reduced to 3.5 nm due to the presence of plasma treatment (PT) without electrical loss. This result is consistent with the simulated optimal hole transport layer thickness at maximum FF. Finally, both optical simulation and experimental EQE show that the loss caused by the proposed method consisting of plasma treatment (PT) + hole transport layer (130-second treatment time) is extremely limited compared to a cell with only a hole transport layer such as MoO x The present invention's solar cell has, for example, the following advantages: good work function and / or limited work function loss at the interface of the hole transport layer / pre-treatment layer, reduced dipole moment at the interface, good conversion efficiency, good transparency, low parasitic absorption, good carrier collection, less complex structure, high Voc, high Jsc, and high fill factor. The present invention utilizes various techniques to solve one or more prior art problems and provides additional advantages; these advantages relate to the measurable characteristics of the resulting device (see the above effects) and thus constitute an obvious physical difference compared to, for example, the prior art. No annealing step is required. High-efficiency solar cells (>22% efficiency) are provided. With some simple optimization steps, an efficiency of 25 - 26% is feasible. In the solar cell of the present invention, there are front and back contacts (also referred to as back contacts). The solar cell of the present invention typically includes at least one heterojunction and typically includes two heterojunctions. The solar cell (100) of the present invention includes a hole transport layer (12), sometimes also referred to as a contact layer or a collector layer, characterized in that the hole transport layer (12) includes 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 disposed on a plasma-pretreated surface layer (12a), wherein the plasma-pretreated surface is a surface passivation layer, and wherein the surface passivation layer is an a-Si:H pre-treated layer (12a). Thus, specifically, the loss of work function is minimized, and the dipole moment at the plasma-treated surface is also limited.

[0014] ​

[0015] The above and other improvements result in a short-circuit current > 39 mA / cm 2 , and / or FF > 70%, preferably FF > 75%, preferably FF > 77%, for example FF > 80%, and / or V OC is 700 - 730 mV, and / or a conversion efficiency > 21%, and / or an absorption coefficient < 20×10 4 cm -1 . This represents an improvement of 1 - 3% relative to comparable prior art devices, and also a relative improvement of 5 - 15%. In terms of return on investment, this difference is considered significant.

[0016] The method of the present invention is considered relatively simple and reduces processing time and equipment usage, since the pre-treatment can be carried out in the same tool where the surface pre-treatment occurs without breaking the vacuum. The method of the present invention comprises the following steps:

[0017] Providing a Si substrate (10), such as a crystalline Si substrate; depositing (forming) an a-Si:H layer (11) on said Si substrate; without breaking the vacuum, subjecting said a-Si:H layer (11) to plasma pre-treatment with a plasma mixture having a gas containing a positive dopant, such as a dopant gas containing B, Al or Ga, such as B2H6, preferably comprising SiH4, H2 and a gaseous p-dopant, so that chemical etching is not required, said plasma pre-treatment preferably at a frequency of 12 - 15 MHz, and / or preferably within 10 - 1000 seconds, and / or preferably at a power density of 50 - 350 mW / cm 2 , and / or preferably at a temperature < 523 K (< 250 °C), and / or preferably at a pressure of 50 - 400 Pa (0.5 - 4 mbar), and / or preferably with a gas mixture containing 0.2 - 2 sccm SiH4, 50 - 400 sccm H2 and 1 - 20 sccm B2H6 (200 ppm in H2); depositing a transition metal oxide layer (12) on the treated a-Si:H layer; depositing a transparent conductive oxide layer (13) on said transition metal oxide layer; and providing at least one contact (14) on said transparent conductive oxide layer. Most of its manufacturing tools have become part of a standard production line. Thus, the present invention can be considered commercially available from the start, since it does not require the development of additional processing tools.

[0018] In general, the present invention provides a simplified manufacturing method, in which a solar cell can be completed in several steps, and the method is a low-cost and high-yield method, using compatible industrial standard metallization steps. The solar cell has a high Voc due to fully passivated contacts, a high Jsc and Voc due to the high transparency of the passivated contacts, a relatively high fill factor (FF) due to a lowly doped c-Si region near the interface, and wherein the design is applicable to conventional front-contact / rear-contact solar cell architectures, bifacial solar cell architectures, and n-type and p-type bulk materials.

[0019] Thus, the present invention provides a solution to one or more of the above problems.

[0020] The advantages of the present specification will be described in detail throughout the description. The drawings mentioned are not intended as a limitation, but only as a guide to the person skilled in the art through the details of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram showing the solar cell of the present invention, and Figure 2a -c is a comparison with a prior art solar cell.

[0022] Figure 3 -7 shows the experimental results of the solar cell of the present invention.

[0023] DETAILED DESCRIPTION OF THE DRAWINGS

[0024] In the drawings:

[0025] 100, solar cell;

[0026] 10, Si substrate;

[0027] 11, first a-Si:H layer;

[0028] 12, hole transport layer (12);

[0029] 12a, pretreatment layer (12a);

[0030] 13, first transparent conductive layer;

[0031] 14, first electrical contact;

[0032] 16, nc-Si:H, which can be p-doped or n-doped;

[0033] 21, second a-Si:H layer;

[0034] 22, n-doped a-Si:H layer;

[0035] 23, second transparent conductive layer;

[0036] 24. Second electrical contact. Detailed implementation

[0037] In a first aspect, the present invention relates to a single-junction or heterojunction Si-based solar cell according to claim 1, and in a second aspect, to a method for manufacturing such a solar cell.

[0038] In an exemplary embodiment of the solar cell of the present invention, the surface passivation layer further includes a silicon-pretreated layer disposed on the a-Si:H pretreated layer.

[0039] In an exemplary embodiment of the solar cell of the present invention, the work function loss of the combined hole transport layer 12 / pretreated layer 12a < 1.0 eV, preferably < 0.6 eV, more preferably < 0.5 eV, for example < 0.35 eV.

[0040] In an exemplary embodiment of the solar cell of the present invention, the dipole moment of the plasma-pretreated surface < 4 C / m, preferably < 2 C / m, more preferably < 1 C / m, for example < 0.7 C / m.

[0041] In an exemplary embodiment of the solar cell of the present invention, the pretreated layer 12a is obtained by PECVD treatment with a plasma mixture including a gas containing a positive dopant, such as a dopant gas containing B, Al, or Ga, such as B2H6, preferably including SiH4, H2, and a gaseous p-dopant.

[0042] In an exemplary embodiment of the solar cell of the present invention, the pretreated layer 12a includes nanocrystalline Si, a relaxed interface, a p-dopant, amorphous Si, positive charges, or a combination thereof.

[0043] In an exemplary embodiment of the solar cell of the present invention, the pretreatment is carried out within 10 - 1000 seconds, preferably 20 - 300 seconds, for example 30 - 100 seconds.

[0044] In an exemplary embodiment of the solar cell of the present invention, the power density during the pretreatment is 50 - 350 mW / cm 2 , preferably 70 - 200 mW / cm 2 , more preferably 80 - 100 mW / cm 2 , for example 90 mW / cm 2 .

[0045] In an exemplary embodiment of the solar cell of the present invention, the pretreatment is carried out at a temperature of <523 K (<250 °C), preferably <473 K (<200 °C), more preferably <443 K (<170 °C).

[0046] In an exemplary embodiment of the solar cell of the present invention, the plasma pressure is 50 - 400 Pa (0.5 - 4 mbar), preferably 100 - 300 Pa (1 - 3 mbar), more preferably 150 - 250 Pa (1.5 - 2.5 mbar), for example 220 Pa (2.2 mbar).

[0047] In an exemplary embodiment of the solar cell of the present invention, the pretreated layer is substantially free of SiO2, for example having less than 1% SiO2 / pretreatment layer (atom / atom), more preferably <1000 ppm, even more preferably <100 ppm, for example <10 ppm.

[0048] In an exemplary embodiment of the solar cell of the present invention, the condition is that annealing of the a-Si:H layer is not provided before deposition of the hole transport layer.

[0049] In an exemplary embodiment of the solar cell of the present invention, the condition is that chemical etching of the a-Si:H layer is not provided, preferably chemical etching is not carried out at all.

[0050] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 12 has a thickness of 2 - 7 nm, preferably 2.5 - 5 nm, for example 3 - 4 nm.

[0051] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 12 has an absorption coefficient of <20×10 4 cm -1 、preferably <10×10 4 cm -1 in the range of 3 - 4 eV.

[0052] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 1 has a current gain of 1 - 2 mA / cm 2 .

[0053] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 12 is structured, for example includes a zigzag structure, includes random cones, is textured, preferably having a height of 1 - 7 µm, for example 2 - 5 µm, and combinations thereof.

[0054] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 12 is disposed under the transparent conductive material 13.

[0055] In an exemplary embodiment of the solar cell of the present invention, the transition metal is selected from transition metals of the 4th or 5th period, such as Ti, V, Cr, Co, Ni, Cu, Zn, Cs, Nb, Mo, W, and their alloys.

[0056] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 12 does not contain a dopant.

[0057] In an exemplary embodiment of the solar cell of the present invention, the hole transport layer 12 is deposited on the pre-treated a-Si:H layer by, for example, PECVD, thermal evaporation, atomic layer deposition, PVD, and sputtering.

[0058] In an exemplary embodiment, the solar cell of the present invention further comprises at least one of the following: a first electrical contact 14, and a layer stack comprising a first transparent conductive layer 13 of 40 - 200 nm, preferably 50 - 100 nm, such as 60 - 75 nm, in electrical contact with the metal contact, preferably having a carrier concentration of 1 - 10*10 20 cm 3 above the hole transport layer 12 of <10 nm, the hole transport layer 12 being above a treatment layer 12a of 1 - 10 nm; a 100 - 500 µm doped crystalline silicon substrate 10; and a second a-Si:H layer 21 of 1 - 10 nm on the back side of the doped crystalline silicon substrate; an electron transport layer of 1 - 10 nm above the second a-Si:H layer, preferably having an activation energy of <350 meV, such as an n-doped a-Si:H layer 22 and / or an n-doped nc-Si:H and / or alloyed with O, N, or C; a second transparent conductive layer (23) of 20 - 300 nm, such as an ITO layer; and a second electrical contact 24 above the transparent layer.

[0059] In an exemplary embodiment, the solar cell of the present invention has a short-circuit current of >39 mA / cm 2 ; and / or an FF of >70%, preferably >75%, preferably >77%, such as >80%; and / or a V of 700 - 730 mV OC ; and / or a conversion efficiency of >21%.

[0060] In an exemplary embodiment of the solar cell of the present invention, the solar cell is selected from single-junction solar cells, heterojunction solar cells, multi-junction solar cells, thin-film solar cells, where the silicon is crystalline silicon, n-doped crystalline silicon, or p-doped crystalline silicon.

[0061] In a second aspect, the present invention relates to a method for producing a solar cell according to the present invention.

[0062] The present invention will be described in further detail with reference to the accompanying drawings and examples, which are illustrative and explanatory and do not limit the scope of the present invention. Those skilled in the art can clearly understand that many obvious or non-obvious changes can be envisaged within the scope of protection defined by the claims of the present invention.

[0063] The accompanying drawings will be further described in detail in the following experimental description.

[0064] Figure 1 A schematic diagram showing the solar cell of the present invention as described throughout this specification, and Figure 2a -c is Figure 1 A comparison of the enlarged portion with a prior art solar cell, where Figure 2a relates to a prior art silicon heterojunction solar cell, Figure 2b relates to a prior art solar cell having an unpretreated MO x layer, and Figure 2c relates to the solar cell of the present invention having a pretreated layer.

[0065] Figures 3 - 6 Shows the simulation and experimental results of the solar cell of the present invention.

[0066] Figure 3 Shows (a) V OC and (b) FF as a function of the MoO x thickness and different work functions (WF) of MoO x (including the dipole at the (i) a-Si:H / MoO x interface). The inset shows the dipole layer. It is found that the strong difference in work function (WF) between MoO x and (i) a-Si:H causes the accumulation / depletion of holes at this interface and the formation of a thin dipole (see Figure 3 the inset in b). It is observed that for a higher WF MoO x , the clearly optimal MoO x thickness is less than 5 nm, which is considered a trade-off between the dipole and the c-Si band bending. In contrast, typically measured for non-stoichiometric MoO x , the simulated trend gradually changes such that a thicker MoO x layer has a higher FF and VOC.

[0067] Figure 4 Shows the HTL depicted in the inset: a SHJ reference with 20 nm thick (p) nc-Si:H, 5.7 nm thick MoO x and PT + 5.7 nm thick MoO x . Figure 4shows the illuminated J-V curves and Figure 5 are the corresponding electrical parameters of various HTLs, showing the effect of plasma treatment on V OC and FF. According to the S-shaped J-V curves, compared to the SHJ reference cell (left bar), the cell with only MoO x exhibits lower V OC and FF (708 mV, 74.2%). It is found that before the deposition of the MoO x layer, treating the (i)a-Si:H layer with PT gradually restores the electrical properties, reaching the optimum at 130 s of PT time, and the measured V OC is 715 mV and the FF is greater than 77%. In Figure 6 , the MoO x thickness optimization using optimized PT is shown. The results show that in the presence of PT, the MoO x layer thickness can be reduced to 3 nm without V OC loss (715 mV), and the progressive gain in FF is up to 77.7%. The optimum MoO x thickness is consistent with the trend observed in the simulations discussed above.

[0068] Figure 6 Comparison of solar cell parameters of the SHJ reference with those at different MoO x thicknesses and constant PT. (a) VOC and i-VOC, (b) JSC-EQE, (c) FF and p-FF, and (d) η act . It should be noted that all cells (except the SHJ reference) have inadvertently thicker ITO (90 nm), reducing JSC by approximately 0.55 mA / cm 2 . Figure 7 is the XPS full survey spectra of the (7a) c-Si / (i)a-Si:H / MoOx and (7b) c-Si / (i)a-Si:H / PT / MoOx stacks.

[0069] X-ray photoelectron spectroscopy (XPS)

[0070] XPS analysis was performed using a PHI-TFA XPS spectrometer (Physical Electronic Inc.) equipped with an X-ray Al monochromatic source. The vacuum during XPS analysis was 10^-9 mbar. The diameter of the analysis area was 0.4 mm and the analysis depth was 3 - 5 nm. Narrow multiple scans of each peak were recorded at an emission angle of 45° relative to the sample surface using a pass energy of 23.5 eV and a step size of 0.1 eV. Surface charge neutralization XPS was performed using a low-energy electron gun. The spectra were processed using Multipak v8.0 (Physical Electronics Inc.). The surface composition was obtained using XPS measurement spectra.

[0071] The XPS measurement spectra showed the absence of boron in the samples treated with plasma (PT).

[0072] Examples / Experiments

[0073] In one example, the following solar cells were fabricated:

[0074] Layer thickness, from front to back:

[0075] ITO, 65 nm

[0076] MoOx, 3.5 nm

[0077] (i)a-Si:H, 5 nm

[0078] (n)c-Si wafer, 250 µm

[0079] (i)a-Si:H, 5 nm

[0080] (n)a-Si:H, 6 nm

[0081] ITO, 150 nm.

[0082] Electrical properties of each layer

[0083] MoO x , undoped

[0084] ITO: Carrier concentration 5×10 20 cm 3

[0085] (n)a-Si:H: Activation energy <350 meV

[0086] Conditions of the PECVD method:

[0087] Processing

[0088] Frequency: 13.56 MHz

[0089] Pressure: 2.2 mbar

[0090] Power density: 90 mW / cm 2

[0091] Time: 130 seconds

[0092] Gas mixture SiH4: 0.8 sccm, H2: 170 sccm, B2H6 (200 ppm in H2): 10 sccm

[0093] Although described in an illustrative context of a detailed description, the present invention may be best understood in conjunction with the accompanying drawings.

[0094] It should be understood that for commercial applications, one or more variations of the present invention system similar to and within the spirit of the present invention disclosed in this application may be preferably used.

Claims

1. A single-junction or multi-junction Si-based solar cell (100), the solar cell 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 disposed on a plasma pretreatment layer (12a), wherein the plasma pretreatment layer is a surface passivation layer, and wherein the surface passivation layer is an a-Si:H pretreated layer; wherein the dipole moment of the plasma-pretreated surface layer < 4 C / m; wherein the pretreatment layer (12a) is obtained by PECVD treatment with a plasma mixture, the plasma mixture comprising a gas containing a positive dopant.

2. The solar cell according to claim 1, wherein the work function loss of the combined hole transport layer (12) / pretreatment layer (12a) < 1.0 eV.

3. The solar cell according to claim 1 or 2, wherein the dipole moment of the plasma-pretreated surface layer < 2 C / m.

4. The solar cell according to claim 1, wherein the pretreatment layer (12a) comprises nanocrystalline Si or amorphous Si.

5. The solar cell according to claim 1, wherein the pretreatment time is 10 - 1000 seconds, and / or wherein the power density during pretreatment is 50 - 350 mW / cm 2 , and / or wherein the pretreatment is carried out at a temperature < 523 K, and / or wherein the plasma pressure is 50 - 400 Pa, and / or wherein the pretreatment layer has less than 1% SiO2, and / or provided that annealing of the a-Si:H layer is not provided, and / or provided that chemical etching of the a-Si:H layer is not provided.

6. The solar cell according to claim 1, wherein the hole transport layer (12) has a thickness of 2 - 7 nm, and / or wherein the hole transport layer (12) has an absorption coefficient of <20×10 4 cm -1 in the range of 3 - 4 eV, and / or wherein the hole transport layer (12) has a current gain of 1-2 mA / cm 2 2.

7. The solar cell according to claim 1, wherein the hole transport layer (12) is structured.

8. The solar cell according to claim 1, wherein the hole transport layer (12) is disposed under a first transparent conductive layer (13).

9. The solar cell according to claim 1, wherein the transition metal is selected from transition metals of the 4th or 5th period, and / or wherein the hole transport layer (12) does not contain a dopant.

10. The solar cell according to claim 1, wherein the hole transport layer (12) is deposited on a pretreated a-Si:H layer.

11. The solar cell according to claim 1, wherein the solar cell further comprises at least one of the following: a first electrical contact (14), and a layer stack including a first transparent conductive layer (13) of 40 - 200 nm in electrical contact with the first electrical contact (14), the first transparent conductive layer being above a hole transport layer (12) of < 10 nm, the hole transport layer (12) being above a pretreatment layer (12a) of 1 - 10 nm; a doped crystalline silicon substrate (10) of 100 - 500 μm; and a second a-Si:H layer (21) of 1 - 10 nm on the back side of the doped crystalline silicon substrate; an electron transport layer of 1 - 10 nm above the second a-Si:H layer; a second transparent conductive layer (23) of 20 - 300 nm; and a second electrical contact (24) above the second transparent conductive layer.

12. The solar cell according to claim 1, the solar cell having a short-circuit current > 39 mA / cm 2 ; and / or an FF > 70%; and / or a VOC of 700 - 730 mV; and / or a conversion efficiency > 21%.

13. The solar cell according to claim 1, wherein the solar cell is selected from a single-junction solar cell or a multi-junction solar cell, and wherein the silicon base is crystalline silicon.

14. A method of manufacturing a solar cell according to any one of claims 1 to 13, the method comprising the steps of: providing a Si substrate (10), depositing a first a-Si:H layer (11) on the Si substrate, Without breaking the vacuum, the first a-Si:H layer (11) is subjected to plasma pretreatment with a plasma mixture having a gas containing a positive dopant, the pretreatment being at a frequency of 12 - 15 MHz, and / or within 10 - 1000 seconds, and / or at a power density of 50 - 350 mW / cm 2 2, and / or at a temperature of <523 K, and / or at a pressure of 50 - 400 Pa, and / or carried out with a gas mixture comprising 0.2 - 2 sccm SiH4, 50 - 400 sccm H2 and 1 - 20 sccm B2H6. depositing a hole transport layer (12) on the pretreated first a-Si:H layer, depositing a first transparent conductive layer (13) on the hole transport layer (12), and providing a first electrical contact (14) on the first transparent conductive layer (13).

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