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We have investigated the influence of simulated outdoor illumination conditions on the functionality of photovoltaic-biased electrosynthetic (PV–EC) systems used for the production of hydrogen as a...

Parasitic absorption and limited fill factor (FF) brought in by the use of amorphous silicon layers are efficiency-limiting challenges for the silicon heterojunction (SHJ) solar cells. In this work, postdeposition phosphorus (P) catalytic doping (Cat-doping) on intrinsic amorphous silicon (a-Si:H(i)) at a low substrate temperature was carried out and a P concentration of up to 6 × 1021 cm-3 was reached. The influences of filament temperature, substrate temperature, and processing pressure on the P profiles were systemically studied by secondary-ion mass spectrometry. By replacing the a-Si:H(n+er with P Cat-doping of an a-Si:H(i) layer, the passivation quality was improved, reaching an iVOC of 741 mV, while the parasitic absorption was reduced, leading to an increase in JSC by ∼1 mA/cm2. On the other hand, the open-circuit voltage and the FF of a conventional SHJ solar cell (with the a-Si:H(n) layer) can be improved by adding a Cat-doping process on the a-Si:H(i) layer, resulting in an increase in FF by 4.7%abs and in efficiency by 1.5%abs.

Reversible and irreversible changes due to long term air and short term de-ionized water (DIW) or pure oxygen exposure were investigated in about 1 mu m thick hydrogenated microcrystalline silicon (mu c-Si:H) films deposited on rough glass substrates, thereby comparing highly crystalline with compact material. Time and temperature dependent dark conductivity, steady-state photoconductivity, the steady-state photocarrier grating and dual-beam photoconductivity methods have been used to study the effects. Standard measurement procedures defined previously have been carefully applied to record the changes after different treatments using the steady-state methods under light. After long term air exposure of highly crystalline mu c-Si:H films, a thermal annealing step leads to an increase in dark conductivity (sigma(D)) and steady-state photoconductivity (sigma(ph)) as well as to a significant increase in the sub-bandgap absorption. These effects are likely due to a reversible recovery from surface adsorbents in a porous microstructure after air exposure resulting in surface charge and Fermi level shifts in agreement with earlier results. Compact mu c-Si:H films showed only marginal effects upon an annealing after long term air exposure suggesting much reduced susceptibility to surface adsorbent induced by Fermi level shifts. Five hours exposure to de-ionized water at 80 degrees C caused more than an order of magnitude increase in sigma(D) and sigma(ph) and a substantial decrease in the sub-bandgap absorption spectrum in highly crystalline as well as in compact mu c-Si:H films. In addition, minority carrier diffusion lengths measured by the steady-state photocarrier grating method improved significantly. The changes after exposure to water were not reversible upon our standard annealing procedure. Exposure to high purity oxygen gas at 150 degrees C resulted in similar effects like the exposure to DIW. Also here the changes in material properties were not reversible upon annealing. Results are discussed in terms of adsorption and chemical reactions on surfaces in the porous highly crystalline material versus the materials with more compact structures. Results are compared to earlier observations and consequences for device application will be indicated.

N-type microcrystalline silicon carbide (μc-SiC:H(n)) is a wide bandgap material that is very promising for the use on the front side of crystalline silicon (c-Si) solar cells. It offers a high optical transparency and a suitable refractive index that reduces parasitic absorption and reflection losses, respectively. In this work, we investigate the potential of hot wire chemical vapor deposition (HWCVD)–grown μc-SiC:H(n) for c-Si solar cells with interdigitated back contacts (IBC). We demonstrate outstanding passivation quality of μc-SiC:H(n) on tunnel oxide (SiO2)–passivated c-Si with an implied open-circuit voltage of 742 mV and a saturation current density of 3.6 fA/cm2. This excellent passivation quality is achieved directly after the HWCVD deposition of μc-SiC:H(n) at 250°C heater temperature without any further treatments like recrystallization or hydrogenation. Additionally, we developed magnesium fluoride (MgF2)/silicon nitride (SiNx:H)/silicon carbide antireflection coatings that reduce optical losses on the front side to only 0.47 mA/cm2 with MgF2/SiNx:H/μc-SiC:H(n) and 0.62 mA/cm2 with MgF2/μc-SiC:H(n). Finally, calculations with Sentaurus TCAD simulation using MgF2/μc-SiC:H(n)/SiO2/c-Si as front side layer stack in an IBC solar cell reveal a short-circuit current density of 42.2 mA/cm2, an open-circuit voltage of 738 mV, a fill factor of 85.2% and a maximum power conversion efficiency of 26.6%.

Herein, we have developed an integrated photoanode for solar water splitting based on an ‘‘Earthabundant’’ Ni–Fe based electrocatalyst combined with a versatile multijunction Si-based photovoltaic device, designed in such a way to allow a direct coupling with the electrocatalyst with minimal losses. The water oxidation catalyst was prepared by electrochemical deposition of iron on a nickel foil, followed by thermal annealing, leading to the formation of NiO, a-Fe2O3, and NiFe2O4 phases. Detailed structural and surface characterization revealed the effect of the addition of different Fe contents and the subsequent implications on the electrocatalytic performance. The optimized integrated photoanode delivered a maximum photocurrent density of 6.2 mA cm2 at 0 V applied bias, which corresponds to a 7.7% of Solar-To-Hydrogen conversion efficiency, which remained stable for more than 20 hours. These results pave the way towards large-scale, efficient and low-cost solar energy conversion solutions based on non-critical raw materials.

A detailed investigation of the effects of prolonged postdeposition annealing on the performance of amorphous silicon (a-Si:H) solar cells and the properties of individual a-Si:H layers that are fabricated at low temperature of 120 degrees C is presented. A substantial improvement in all parameters of the current-voltage curves of these solar cells is observed upon annealing, consistent with an improvement in the collection voltage of the solar cells. Modifications of p-type layers during deposition of the solar cells are found to make no significant contribution to the annealing behavior of solar cells, while variations in the properties of n-type and intrinsic layers contribute substantially. The results indicate that the largest contribution to the annealing effect originates from changes in the electron mu tau-product in the intrinsic absorber layer upon annealing, while changes in hole mu tau-products have a minor contribution to the annealing effect in the solar cell. Besides a lack of significant changes in the number of recombination centers upon annealing, an improvement in the external quantum efficiency curves upon annealing may be accurately reproduced in computer simulations by assuming an increase in the band mobilities of both electrons and holes.

Using transmission and scanning electron microscopy, we study mechanisms which determine the stability of Silicon photocathodes for solar driven water splitting. Such tandem or triple devices can show a promising stability as photocathodes if the semiconductor surface is protected by an ultrathin TiO2 protection layer. Using atomic layer deposition (ALD) with Cl-precursors, 4–7 nm thick TiO2 layers can be grown with high structural perfection. The layer can be electrochemically covered by Pt nanoparticels serving as electro-catalysts. However, Cl-remnants which are typically present in such layers due to incomplete oxidation, are the origin of an electrochemical degradation process. After 1 h AM1.5G illumination in alkaline media, circular shaped corrosion craters appear in the topmost Si layer, although the TiO2 layer is intact in most parts of the crater. The crater development is stopped at local inhomogenities with a higher Pt coverage. The observations suggests that reduced Titanium species due to Cl−/O2− substitution are nucleation sites of the initial corrosion steps due to enhanced solubility of reduced Ti in the electrolyte. This process is followed by electrochemical dissolution of Si, after direct contact between the electrolyte and the top Si layer surface. To increase the stability of TiO2 protected photocathodes, formation of reduced Ti species must be avoided.

Silicon (Si) has been investigated as a promising photoelectrode material for use in photoelectrochemical water splitting. However, development of Si photocathodes that can operate at a high photoc...

A silicon heterojunction solar cell using silicon carbide as front contact is presented, which features the main advantage of high transparency. To enhance this advantage, an optical loss analysis is performed. It is found that reflection losses play an important role for the solar cell, which can easily be reduced by applying an additional MgF2 coating. The deposition of the coating degrades the passivation quality of the contact but can be cured, eventually leading to a certified short circuit current density of 40.9 mA/cm² and efficiency of 23.99%. Afterwards, a roadmap to a theoretical efficiency of 25% is presented.

Summary In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both sides of contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design for SHJ solar cells achieving a low series resistivity of 0.32 Ωcm2 and a good fill factor of 80.7% with a conventional finger pitch of 1.8 mm. Achieving high efficiencies in TCO-free SHJ solar cells requires suppressing deterioration of the passivation quality induced by the direct metal-to-a-Si:H contacts. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion into the a-Si:H layer and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of >22%.

Nature energy (2021). doi:10.1038/s41560-021-00806-9, Published by Nature Publishing Group, London

Hot-wire chemical vapor deposition was utilized to develop rapidly grown and high-quality phosphorus-doped amorphous silicon (a-Si:H) thin films for poly-crystalline silicon on tunnel oxide carrier-selective passivating contacts. Deposition rates higher than 150 nm/min were obtained for the in situ phosphorus-doped a-Si:H layers. To optimize the passivating contact performance, material properties such as microstructures as well as hydrogen content were characterized and analyzed for these phosphorus-doped a-Si:H films. The results show that a certain microstructure of the films is crucial for the passivation quality and the conductance of passivating contacts. Porous silicon layers were severely oxidized during high-temperature crystallization, giving rise to very low conductance. The insufficient effective doping concentration in these layers also yields inferior passivation quality due to lack of field-effect passivation. On the other hand, dense silicon layers are insensitive to oxidation but very sensitive to blistering of the films during the subsequent high-temperature process steps. By optimizing the deposition parameters, a firing-stable-implied open-circuit voltage of 737 mV and a contact resistivity of 10 mΩ·cm2 were achieved at a high deposition rate of 100 nm/min while 733 mV and 90 mΩ·cm2 were achieved at an even higher deposition rate of 150 nm/min.

The thermal stability of thick amp; 8776;4 amp; 8201; amp; 956;m plasma grown hydrogenated amorphous silicon a Si H layers on glass upon application of a rather rapid annealing step is investigated. Such films are of interest as precursor layers for laser liquid phase crystallized silicon solar cells. However, at least half day annealing at T amp; 8776;550 amp; 8201; C is considered to be necessary so far to reduce the hydrogen H content and thus avoid blistering and peeling during the crystallization process due to H. By varying the deposition conditions of a Si H, layers of rather different thermal stability are fabricated. Changes in the surface morphology of these a Si H layers are investigated using scanning electron microscopy and profilometry measurements. Hydrogen effusion, secondary ion mass spectrometry SIMS depth profiling, and Raman spectroscopy measurements are also carried out. In summary, amorphous silicon precursor layers are fabricated that can be heated within 30 amp; 8201;min to a temperature of 550 amp; 8201; C without peeling and major surface morphological changes. Successful laser liquid phase crystallization of such material is demonstrated. The physical nature of a Si H material stability instability upon application of rapid heating is studied

In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both-sides contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for efficient carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design. A series resistance of 0.32 Ωcm2 and a fill factor of 80.7% were measured for a TCO-free back-junction SHJ solar cell with a conventional finger pitch of 1.8 mm, thereby proving that relying on lateral conduction in the c-Si bulk is compatible with low series resistances. Achieving high efficiencies in SHJ solar cells with TCO-free front contacts requires suppressing deterioration of the passivation quality induced by direct metal-a-Si:H contacts and in-diffusion of metal into the a-Si:H layer. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of > 22%.

Conductive hydrogenated silicon carbide (SiCx:H) is discovered as a promising hydrogenation material for tunnel oxide passivating contacts (TOPCon) solar cells. The proposed SiCx:H layer enables a good passivation quality and features a good electrical conductivity, which eliminates the need of etching back of SiNx:H and indium tin oxide (ITO)/Ag deposition for metallization and reduces the number of process steps. The SiCx:H is deposited by hot wire chemical vapor deposition (HWCVD) and the filament temperature (Tf) during deposition is systematically investigated. Via tuning the SiCx:H layer, implied open-circuit voltages (iVoc) up to 742 ± 0.5 mV and a contact resistivity (ρc) of 21.1 ± 5.4 mΩ·cm2 is achieved using SiCx:H on top of poly-Si(n)/SiOx/c-Si(n) stack at Tf of 2000 °C. Electrochemical capacitance–voltage (ECV) and secondary ion mass spectrometry (SIMS) measurements were conducted to investigate the passivation mechanism. Results show that the hydrogenation at the SiOx/c-Si(n) interface is responsible for the high passivation quality. To assess its validity, the TOPCon stack was incorporated as rear electron selective-contact in a proof-of-concept n-type solar cells featuring ITO/a-Si:H(p)/a-Si:H(i) as front hole selective-contact, which demonstrates a conversion efficiency up to 21.4%, a noticeable open-circuit voltage (Voc) of 724 mV and a fill factor (FF) of 80%.

Conductive hydrogenated silicon carbide (SiC

Photovoltaic driven electrochemical (PV-EC) water splitting technology is considered as one of the solutions for an environmental-friendly hydrogen supply. In a PV-EC system, efficient catalysts are required to increase the rate of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, we present the development of a CoFeVO$_x$ bifunctional catalyst produced by a simple electrodeposition method. We have found that after the water splitting reaction vanadium is almost completely depleted in the mixture of elements for OER while its concentration at the HER catalyst is similar or even higher after the reaction. For the OER catalyst the depletion of vanadium might lead to the formation of pores, which could be correlated with the activity enhancement. The developed catalyst is integrated into PV-EC devices, coupled with different types of silicon PV. An average solar to hydrogen efficiency of 13.3 % (9.6 cm$^2$ PV aperture area) is achieved with a shingled module consisting of three laterally series connected silicon heterojunction solar cells.

As is obvious from previous work on semiconductor photoelectrochemistry, single junction semiconductors do not provide either the required maximum photovoltage or a high photocurrent for solar water splitting, which is required for efficient stand-alone devices. From these experiences we conclude, that multi-junction devices must be developed for bias-free water splitting. In this article we present our design considerations needed for the development of efficient photo-electro-synthetic cells, which have guided us during the DFG priority program 1613. At first, we discuss the fundamental requirements, which must be fulfilled to lead to effective solar water splitting devices. Buried junction and photoelectrochemical arrangements are compared. It will become clear, that the photovoltaic (PV) and electrochemical (EC) components can be optimized separately, but that maximized conversion efficiencies need photovoltages produced in the photovoltaic part of the device, which are adapted to the electrochemical performance of the electrolyzer components without energetic losses in their coupling across the involved interfaces. Therefore, in part 2 we will present the needs to develop appropriate interface engineering layers for proper chemical and electronic surface passivation. In addition, highly efficient electrocatalysts, either for the hydrogen or oxygen evolution reaction (HER, OER), must be adjusted in their energetic coupling to the semiconductor band edges and to the redox potentials in the electrolyte with minimized losses in the chemical potentials. The third part of our paper describes at first the demands and achievements on developing multijunction thin-film silicon solar cells. With different arrangements of silicon stacks a wide range of photovoltages and photocurrents can be provided. These solar cells are applied as photocathodes in integrated directly coupled PV-EC devices. For this purpose thin Pt and Ni catalyst layers are used on top of the solar cells for the HER and a wire connected RuO2 counter electrode is used for the OER. Electrochemical stability has been successfully tested for up to 10,000 s in 0.1 M KOH. Furthermore, we will illustrate our experimental results on interface engineering strategies using TiO2 as buffer layer and Pt nanostructures as HER catalyst. Based on the obtained results the observed improvements, but also the still given limitations, can be related to clearly identified non-idealities in surface engineering either related to recombination losses at the semiconductor surface reducing photocurrents or due to not properly-aligned energy states leading to potential losses across the interfaces.

A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/µc-SiC:H( n )] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the µc-SiC:H( n ) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of µc-SiC:H( n ) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/µc-SiC:H( n ) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization.

In this study, we present different silicon based integrated devices for photoelectrochemical water splitting, which provide enough photovoltage to drive the reaction without an external bias. Thin films of titanium dioxide, prepared by atomic layer deposition (ALD), are applied as a surface passivation and corrosion protection. The interfaces between the multi-junction cells and the protective coating were optimized individually by etching techniques and finding optimal parameters for the ALD process. The energy band alignment of the systems was studied by X-ray photoelectron spectroscopy (XPS). Electrochemically deposited platinum particles were used to reduce the HER overpotential. The prepared systems were tested in a three-electrode arrangement under AM 1.5 illumination in 0.1 M KOH. In final tests the efficiency and stability of the prepared devices were tested in a two-electrode arrangement in dependence of the pH value with a ruthenium-iridium oxide counter electrode. For the tandem-junction device solar to hydrogen efficiencies (STH) up to 1.8% were reached, and the triple-junction device showed a maximum efficiency of 4.4%.

Paper-based flexible thin film solar cells are promising power sources for the emerging portable and wearable electronics, for which high conversion efficiency and flexibility are demanded. Herein, flexible thin film silicon solar cells using textured aluminum-doped zinc oxide (AZO) for light scattering, and ultrathin silver electrodes combined with cellophane paper substrates, were demonstrated. A universal route to form textured AZO thin film by chemical etching with hydrochloric acid vapor was proposed. The AZO layers textured in this way exhibited similar light scattering properties as compared with state-of-the-art AZO thin films. Applying textured AZO to solar cells on cellophane paper substrates resulted in current density and conversion efficiency increase from 8.7 to 9.5 mA/cm2 and from 4.1% to 5.7%, respectively. Thin film solar cells on cellophane exhibited excellent mechanical flexibility, which maintained 90% of their initial efficiency after bending at a radius of 1 mm for 50 cycles.

We report on the preparation and performance of catalysts from earth abundant materials and their implementation in a stand-alone photovoltaic-electrochemical (PV-EC) module with 64 cm2 active area. NiFeOX as the oxygen evolution reaction catalyst and NiMo as the hydrogen evolution reaction catalyst were electrodeposited on nickel sheets. We compare the NiFeOX/NiMo catalysts to a noble metal catalyst system consisting of IrOX and Pt regarding their potential for upscaling to large areas and their application and performance in the PV-EC module with a triple junction thin film silicon based solar cell. Additionally, we present long-term stability measurements of the catalyst systems (i) NiMo/NiFeOX and (ii) Ni/Ni under simulated day-night cycles. Overall, we show the feasibility of using earth abundant catalysts in an upscaled stand-alone PV-EC module. The NiMo/NiFeOX catalyst pair outperforms the precious metal catalysts with a solar-to-hydrogen efficiency of ηSTH(NiMo/NiFeOX) = 5.1% (ηSTH(Pt/IrOX) = 4.8%) and shows an excellent long-term stability in the simulated day-night cycles.

We present a stand-alone integrated solar water-splitting module with an active area of 64 cm2 and a long-term stable operation. As a photocathode we employ multijunction thin film silicon solar cells that were optimized to deliver a suitable output voltage for spontaneous water-splitting. Two approaches for the design of a suitable front contact are presented to reduce series resistance losses related to the upscale of the photoelectrodes. The photoelectrode is protected from the electrolyte by a sheet metal which connects the rear contact of the solar cell with the hydrogen evolving catalyst. Thereby, the sheet metal ensures long-term stability while the electrical and thermal coupling of the solar cell and the electrolysis cell is maintained. Due to the modular setup, which allows us to vary and optimize the device components (i.e. the solar cell, catalysts, membrane, and electrolyte) individually, the presented water-splitting device provides a convenient toolbox for the optimization of such systems.

Hot-wire chemical vapor deposition was used to deposit in situ-doped amorphous silicon layers for poly-Si/SiOx passivating contacts at a high deposition rate of 42 nm/min. We investigated the influence of a varied phosphine gas (PH3) concentration during deposition on (i) the silicon film properties and (ii) the passivating contact performances. The microstructural film properties were characterized before and after a high-temperature crystallization step to transform amorphous silicon films into polycrystalline silicon films. Before crystallization, the silicon layers become less dense as the PH3 concentrations increase. After crystallization, an increasing domain size is derived for higher PH3 concentrations. Sheet resistance is found to decrease as domain size increased, and the correlation between mobility and domain size was discussed. The performances of the passivating contact were measured, and a firing stable open circuit voltage of 732 mV, a contact resistivity of 8.1 mΩ·cm2, and a sheet resistance of 142 Ω/□ could be achieved with the optimized PH3 concentration. In addition, phosphorous doping tails into the crystalline silicon were extracted to evaluate the Auger recombination of the passivating contact.

The concept of hybrid tandem device structures that combine metal oxides with thin‐film semiconducting photoabsorbers holds great promise for large‐scale, robust, and cost‐effective bias‐free photoelectrochemical water splitting (PEC‐WS). This work highlights important steps toward the efficient coupling of high‐performance hematite photoanodes with multijunction thin‐film silicon photocathodes providing high bias‐free photocurrent density. The hybrid PEC‐WS device is optimized by testing three types of multijunction silicon photocathodes with the hematite photoanode: amorphous silicon (a‐Si:H) tandem: a‐Si:H/a‐Si:H and triple junction with microcrystalline silicon (μc‐Si:H): a‐Si:H/a‐Si:H/μc‐Si:H and a‐Si:H/μc‐Si:H/μc‐Si:H. The results provide evidence that the multijunction structures offer high flexibility for hybrid tandem devices with regard to tunable photovoltages and spectral matching. Furthermore, both photoanode and photocathode are tested under various electrolyte and light concentration conditions, respectively, with respect to their photoelectrochemical performance and stability. A 27 % enhancement in the solar‐to‐hydrogen conversion efficiency is observed upon concentrating light from 100 to 300 mW cm−2. Ultimately, bias‐free water splitting is demonstrated, with a photocurrent density of 4.6 mA cm−2 (under concentrated illumination) paired with excellent operation stability for more than 24 h of the all‐earth‐abundant and low‐cost hematite/silicon tandem PEC‐WS device., The authors acknowledge funding from Generalitat de Catalunya through the CERCA program, 2017 SGR 1246, 2017 SGR 327 and the Spanish MINECO projects MAT2014‐59961, ENE2016‐80788‐C5‐5‐R and ENE2017‐85087, together with the support from REPSOL, S. A. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV‐2017‐0706). IREC also acknowledges additional support from the European Regional Development Funds (ERDF, FEDER), (S)TEM part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program and the rest in the Nanoscience program of the University of Barcelona. The authors thank S. Moll (IEK‐5), M. Biset‐Peiró (IREC), and H. Xie (IREC) for their contribution to this work. F.U. acknowledges financial support from MINECO through Juan de la Cierva fellowship (FJCI‐2016–29147).V.S., K.W., and F.F. (authors from IEK‐5) thank the Deutsche Forschungsgemeinschaft (DFG) (Priority Program SPP 1613).

We investigate cobalt ferrite nanoparticles (NPs) supported on large-scale electrodes as oxygen evolution reaction (OER) catalysts. Colloidal CoFe2O4 NPs were loaded on low-cost and high surface area nickel foam (NF) scaffolds. The coating process was optimized for large electrode areas, ensuring a proper distribution of the NPs on the NF that allowed overcoming the electrical conductivity limitations of oxide NPs. We were able to produce CoFe2O4-coated NFs having 10 cm2 geometric surface areas with overpotentials below 300 mV for the OER at a current density of 50 mA/cm2. Such impressively low overpotentials suggested using CoFe2O4 NP-based electrodes within a water electrolysis device. In this prototype device, stable operating currents up to 500 mA at remarkably low cell-voltages of 1.62 and 1.53 V, at ambient and 50 °C electrolyte temperatures, respectively, were reached during operation periods of up to 50 h. The high electrochemical energy efficiencies reached at 50 mA/cm2, 75% and 81% respectively, rendered these devices particularly appealing to be combined with low-cost photovoltaic systems for bias-free hydrogen production. Therefore, CoFe2O4 NP-based electrolysers were coupled to low-cost thin-film silicon solar cells with 13% efficiency to complete a system that afforded solar-to-fuel efficiencies above 10%., Authors acknowledge funding from Generalitat de Catalunya through the CERCA program, 2017 SGR 1246, 2017 SGR 327 and the Spanish MINECO projects MAT2014-59961, ENE2016-80788-C5-5-R, ENE2016-77798-C4-3-R and ENE2017-85087, together with the support from Repsol S. A. Likewise, the authors thank Enagás S.A. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). IREC also acknowledges additional support from the European Regional Development Funds (ERDF, FEDER), (S)TEM part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program and the rest in the Nanoscience program of the University of Barcelona. The authors thank S. Moll (IEK-5), M. Biset-Peiró (IREC), and H. Xie (IREC) for their contribution to this work. F.U. acknowledges financial support from MINECO through Juan de la Cierva fellowship (FJCI-2016-29147).V.S. and F.F. (authors from IEK-5) thank the Deutsche Forschungsgemeinschaft (DFG) (Priority Program SPP 1613). J. Llorca is a Serra Húnter Fellow and is grateful to ICREA Academia program and funding from Generalitat de Catalunya 2017 SGR 128.

We present a study of the effect of illumination intensity on the performance of a photovoltaic-biased electrochemical (PV-EC) device for solar hydrogen production based on a triple junction thin film silicon solar cell. The influence of the illumination intensity was studied for the solar cell as well as for an integrated PV-EC device. We show that while the open circuit voltage decreases with a reduction in intensity, the triple junction solar cell still provides a sufficient voltage to drive spontaneous water splitting. Moreover, a slight improvement in the fill factor at lower intensities can relax the requirements to the utilized co-catalysts. As a consequence, the difference in the performance of PV-EC devices featuring very active (Pt/RuO 2 ) and less active (Ni/Co 3 O 4 ) catalyst materials decreases when the illumination intensity is reduced.

We present the study of high efficiency single and multi-junction solar cells, focusing on the stability against degradation under illumination. In both single and multijunction solar cells, the thickness of the a-Si:H absorber layer was varied over a wide range up to 790 nm. While single junction a-Si:H solar cells show reduced stability against prolonged light illumination with an increase in layer thickness, themultijunction solar cells are significantly more stable. In these cells the total thickness of the a-Si:H absorber layers (first and second sub-cells) can be significantly increased up to 790 nm while keeping the degradation level low (below 13%) after 1000 hrs of light soaking. The possible origins of an improved stability against degradation are addressed in the paper.

TiO2 is a common protection layer on semiconductor electrodes for photoelectrochemical water splitting. We investigate the interface formation of TiO2 on amorphous silicon tandem solar cells by X-ray photoelectron spectroscopy. In order to optimize the contact properties, we prepare TiOx interface layers with various oxygen content by reactive magnetron sputter deposition. We observe, that a TiOx interface layer can reduce the silicon oxide growth during the film deposition on the amorphous silicon, but it forms a non-ohmic contact. The electrochemical investigation shows, that the benefit due to the reduction of the silicon oxide is counteracted by the unfavorable contact formation of TiOx interface layers prepared with low oxygen content.

The unique microstructure of nanocrystalline silicon oxide (nc-SiO X :H) thin films results in excellent optoelectronic properties that can be tuned in a wide range to fulfill the requirements of the specific application. For photovoltaic applications, this material is used as doped layers in silicon heterojunction solar cells and intermediate reflectors in multijunction thin-film solar cell. In this paper, we present a microstructure model based on a large number of n- and p-doped nc-SiO X :H films that were deposited under various deposition pressures, plasma powers, plasma frequencies and gas mixtures. This model is meant to provide guidelines for the systematic classification of the complex material system nc-SiO X :H by establishing a link between the structure of the deposited films and the optoelectronic performance of nc-SiO X :H. Based on this model, the deposition of nc-SiO X :H films can be divided into four characteristic regions: (i) fully amorphous region, (ii) onset of nc-Si formation, (iii) oxygen and nc-Si enrichment region, and (iv) deterioration of nc-Si. According to our microstructure model, an optimal phase composition with respect to the optoelectronic performance can be achieved with a high amount of highly conductive nc-Si percolation paths embedded in an oxygen rich a-SiO X :H matrix.

Doped microcrystalline silicon oxide (μc-SiOx:H) alloys attract significant attention as a functional material in photovoltaic devices. By using various advanced characterization methods, we have studied the relationship between optoelectronic properties, chemical composition, and structure of p-type µc-SiOx:H deposited by plasma enhanced chemical vapor deposition (PECVD). For a wide range of compositions with varying oxygen content, we show that the dominant components are Si and a-SiO2, while the fraction of suboxides is minor. The μc-SiOx:H material with sufficient oxygen content (x = 0.35) exhibits an enlarged optical gap E04 > 2.2 eV and sufficiently high dark conductivity >10−6 S cm−1; the crystalline silicon fraction has a filament-like shape (with a typical width of around 10 nm) forming a branch-like structure elongated in the growth direction over several hundreds of nanometers.

In order to increase the efficiency of high performance silicon heterojunction solar cells even further, it is paramount to increase the photoelectric current by enhancing the amount of light being captured within the absorber. Therefore, to reduce the parasitic absorption in the other layers, optoelectronically favorable hydrogenated nanocrystalline silicon oxide films can substitute the commonly used hydrogenated amorphous silicon layers. In this work, we systematically investigate the combination of hydrogenated nanocrystalline silicon oxide and front side nano-imprint textures as anti-reflection layers in silicon heterojunction solar cells. Ultimately, we were able to tune the parasitic absorption via variation of the front surface field layer and enhance the short-circuit current of the planar solar cells by about 2 mA cm−2 due to a random silicon pyramid textured imprint layer. A maximum active area efficiency of 20.4% was achieved with a short-circuit current of 37.7 mA cm−2.

Maintaining efficient light management is an essential part for obtaining high efficiency flexible thin-film solar cells. Transparent plastic films like polyethylene terephthalate (PET) are a cost efficient substrate alternative but also impose additional constraints on the light management approaches available. In this study, we investigate and compare two approaches to prepare substrates with light scattering characteristics. We have developed low temperature ZnO:Al layers, which are fully compatible with PET substrates, and are textured by wet-chemical methods. These low temperature textured ZnO:Al layers implemented in amorphous silicon solar cells on glass substrates result in similar efficiencies as highly optimized high temperature-etched ZnO:Al layers. Nanoimprint lithography was used as an alternative light management approach and we show that an improved solar cell performance can be achieved on flexible PET substrates with both methods. Besides the effect on solar cell performance, pros and cons of both approaches with respect to flexibility in choice of materials and textures, fabrication process, stress evolution, and reproducibility are discussed.

Microcrystalline silicon carbide alloys (μc-SiC:H) have attracted attention as a potential window layer for thin film silicon solar cells and silicon heterojunction solar cells. Thanks to the combination of high transparency and conductivity. At present, identification of electronically active impurities and defects is highly relevant for application of μc-SiC:H. Electron paramagnetic resonance has been successfully applied to investigate defects and impurities in crystalline SiC. Continuous wave X-band EPR measurements performed earlier on a number of μc-SiC:H samples show intensive and complex spectra with multiple overlapped components. In order to resolve components of the μc-SiC:H spectra, we performed a series of pulsed EPR measurements in S-, X-, and Q-band completed with a Q-band ENDOR experiment. The experiment was successful to resolve hidden complexity of the EPR spectra of μc-SiC:H and determine the nucleus responsible for the hyperfine pattern observed in previous studies.

An integrated solar water splitting device based on thin film silicon multijunction photocathodes is presented. A graphical representation of the photovoltaic current–voltage data is introduced which allows for an estimation of the maximum achievable solar-to-hydrogen efficiency of the integrated device. Furthermore, a simple yet very useful series circuit model is used to predict the photoelectrochemical performance of the integrated device in a more elaborate way when the j–V characteristics of the individual components are known. Within the model, the j–V characteristics of each component can be either modeled with parameters from the literature or measured. The photocathode, the electrolyte concentration, and the hydrogen and oxygen evolving catalysts were varied exemplarily and the impact of each component on the integrated device performance was evaluated. A maximum solar-to-hydrogen efficiency of 9.5% was found using a triple junction solar cell functionalized with a Pt catalyst for the hydrogen evolution and a RuO2 catalyst for the oxygen evolution reaction in a 1 M KOH electrolyte. This result was confirmed experimentally and is compared to efficiencies reported in the literature.

The fabrication process of high performance quadruple junction thin film silicon solar cells is described and the application of the solar cells in an integrated photoelectrochemical water splitting device is demonstrated. It is shown that the performance of solar cells can be adjusted by varying the process parameters and the thickness of the absorber layers of the individual sub cells and by integrating microcrystalline silicon oxide as intermediate reflecting layers. Thereby current matching of the sub cells was improved and a high open-circuit voltage of 2.8 V was achieved. Furthermore, the solar cell stability against light-induced degradation was investigated. Efficiencies of 13.2% (initial) and 12.6% (after 1000 h of light-soaking) were achieved. Bias-free water splitting with a solar-to-hydrogen efficiency of 7.8% was demonstrated in an integrated photovoltaic–electrochemical device using the developed quadruple junction photocathode. Finally, it is shown that in the case of quadruple junction solar cells the light-induced degradation has a lower effect on the photovoltaic–electrochemical efficiency as on the photovoltaic efficiency.

This paper highlights the effect of the operation temperature on the performance of a photovoltaic-biased electrosynthetic cell (PV-EC) device for solar hydrogen production based on a triple junction thin film silicon solar cell. The influence of the temperature in the range from 25 °C to 60 °C was studied individually for all components of the device: the solar cell, the hydrogen evolving cathode, the oxygen evolving anode, and the electrolyte. Based on the experimental data, the overall temperature-dependent current–voltage characteristics of the complete PV-EC device was modeled by merging the current–voltage characteristics of the individual components in an empirical series circuit model. We found that a decrease in the photovoltage of the solar cells with increasing temperature can be compensated by an improved electrochemical kinetics with temperature. This lead to a slight improvement in the performance of the integrated PV-EC device. Under an assumption of 100% faradaic efficiency, a maximum solar-to-hydrogen efficiency of 9.5% was found in 1 M KOH at an operation temperature of 50 °C.

Although photovoltaic–electrochemical (PV–EC) water splitting is likely to be an important and powerful tool to provide environmentally friendly hydrogen, most developments in this field have been conducted on a laboratory scale so far. In order for the technology to make a sizeable impact on the energy transition, scaled up devices must be developed. Here a scalable (64 cm2 aperture area) artificial PV–EC device composed of triple-junction thin-film silicon solar cells in conjunction with an electrodeposited bifunctional nickel iron molybdenum water-splitting catalyst is shown. The device shows a solar to hydrogen efficiency of up to 4.67% (5.33% active area, H2 production rate of 1.26 μmol H2/s) without bias assistance and wire connection and works for 30 min. The gas separation is enabled by incorporating a membrane in a 3D printed device frame. In addition, a wired small area device is also fabricated in order to show the potential of the concept. The device is operated for 127 h and initially 7.7% solar to hydrogen efficiency with a PV active area of 0.5 cm2 is achieved.

Al-doped Zinc Oxide (AZO) is an attractive substitution of Tin-doped Indium Oxide in silicon heterojunction (SHJ) solar cells due to its low cost and benign nature. In our work room temperature (RT) sputtered AZO has been introduced into SHJ solar cells in view of industrialization and cost reduction. In addition, n type nc-Si:H is employed as window layer to reduce parasitic absorption. The influence of AZO deposition and post-deposition annealing on effective carrier lifetime is studied. A 20.2% cell efficiency with 20 × 20 mm2 aperture area has been achieved with RT sputtered AZO and n type nc-Si:H layer.

We present a new transparent passivated contact concept utilizing microcrystalline silicon carbide and an ultra-thin silicon tunnel oxide $( \mu \mathrm{c} -$SiC:H(n)/SiO 2 ) for the front side of silicon heterojunction solar cells. We investigated different oxidation agents in combination with selected deposition conditions of the $\mu \mathrm{c} -$SiC:H(n) to find the ideal parameters for high passivation quality and high conductivity. Implied open-circuit voltages up to 728 mV were achieved without any post-deposition treatment e.g. high temperature or forming gas annealing. The transparent passivated contact solar cells show increased quantum efficiency in the short wavelength range as compared to the conventional silicon heterojunction solar cells. These insights show the great potential for the transparent passivated contact front side.

Transparent passivated contacts (TPCs) using a wide band gap microcrystalline silicon carbide (μc-SiC:H(n)), silicon tunnel oxide (SiO2) stack are an alternative to amorphous silicon-based contacts for the front side of silicon heterojunction solar cells. In a systematic study of the μc-SiC:H(n)/SiO2/c-Si contact, we investigated selected wet-chemical oxidation methods for the formation of ultrathin SiO2, in order to passivate the silicon surface while ensuring a low contact resistivity. By tuning the SiO2 properties, implied open-circuit voltages of 714 mV and contact resistivities of 32 mΩ cm2 were achieved using μc-SiC:H(n)/SiO2/c-Si as transparent passivated contacts.

Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring the silicon melting point and with calculated data using a physical model as well as published work. Since D-H interdiffusion measures hydrogen diffusion length and temperature within the silicon films by a memory effect, the method is capable of determining both quantities precisely also in multilayer structures, as is demonstrated for films underneath metal contacts. Several applications are discussed. Employing literature data of laser-induced temperature rise, laser scanning is used to measure the H diffusion coefficient at T > 500 °C in a-Si:H. The model-based high temperature hydrogen diffusion parameters are confirmed with important implications for the understanding of hydrogen diffusion in the amorphous silicon material.Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring ...

This work demonstrates that the combination of a wet-chemically grown SiO2 tunnel oxide with a highly-doped microcrystalline silicon carbide layer grown by hot-wire chemical vapor deposition yields an excellent surface passivation for phosphorous-doped crystalline silicon (c-Si) wafers. We find effective minority carrier lifetimes of well above 6 ms by introducing this stack. We investigated its c-Si surface passivation mechanism in a systematic study combined with the comparison to a phosphorous-doped polycrystalline-Si (pc-Si)/SiO2 stack. In both cases, field effect passivation by the n-doping of either the µc-SiC:H or the pc-Si is effective. Hydrogen passivation during µc-SiC:H growth plays an important role for the µc-SiC:H/SiO2 combination, whereas phosphorous in-diffusion into the SiO2 and the c-Si is operative for the surface passivation via the Pc-Si/SiO2 stack. The high transparency and conductivity of the µc-SiC:H layer, a low thermal budget and number of processes needed to form the stack, and the excellent c-Si surface passivation quality are advantageous features of µc-SiC:H/SiO2 that can be beneficial for c-Si solar cells. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)