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Angewandte Chemie / International edition e202218850 (2023). doi:10.1002/anie.202218850 special issue: "Hot Topic: Water Splitting", Published by Wiley-VCH, Weinheim

The outdoor operation of an up scaled thermally photovoltaic electrolyzer PV EC , constructed using a heat exchanger HE made of low cost materials, compared to its nonintegrated counterpart to quantify heat transfer and its effects, is studied. Thermal coupling of the PV and EC can reduce the difference between their temperatures, benefitting device performance. Such devices can produce hydrogen at rooftop installations of small to medium sized nonindustrial buildings. The devices are tested outdoors using automated real time monitoring. Under amp; 8776;880 amp; 8201;W amp; 8201;m amp; 8722;2 peak irradiance, they produced hydrogen at amp; 8776;120 and amp; 8776;110 amp; 8201;mL amp; 8201;min amp; 8722;1 rate with and without HE, respectively, corresponding to about 8.5 and 7.8 solar to hydrogen efficiencies. During about 700 amp; 8201;h of testing, the HE is beneficial at over amp; 8776;500 amp; 8201;W amp; 8201;m amp; 8722;2 due to cyclic device operation. Under lower irradiance levels, pumping previously heated electrolyte through the HE increases the PV and reduces the electrolyte temperature, reducing the device performance. The HE increases the cumulative hydrogen production amp; 8776;800 amp; 8201;L from both devices , so even relatively modest heat transfer rates can improve the PV EC operation. Improving the HE should further increase the benefits, but additional measures may be needed to maximize the hydrogen production

The transition towards a renewable energy-based society is challenged by spatial and temporal imbalances of energy demand and supply. Storage properties and versatility may favor hydrogen to serve as the linking element between renewable energy generation and a variety of sector coupling options. This paper examines four alternative solar-based hydrogen production concepts based on concentrated solar (CSP) or photovoltaic (PV) power generation and solid oxide (SOE) or polymer electrolyte membrane (PEM) electrolysis, namely, CSP-SOE and CSP-PEM, as well as PV-PEM concepts with (PV-PEM I) or without (PV-PEM II) power converters coupling both devices. In this paper, we analyze these concepts in terms of their techno-economic performance in order to determine the levelized cost of hydrogen (LCOH) for the target year 2025, based on different locations with different climate conditions. The analysis was carried out using a broadly applicable computer model based on an hourly resolved time-series of temperature and irradiance. The lowest LCOH was identified in the case of the CSP-SOE and CSP-PEM concepts with 14–17 €-ct per kW per h at high-irradiance locations, which clearly exceed the US Department of Energy (DOE) target of 6 $-ct per kW per h for the year 2020. Moreover, CSP-SOE also shows the highest hydrogen production volumes and, therefore, solar-to-hydrogen efficiencies. Considering the PV-PEM concepts, we found that the application of power converters for the electrical coupling of PV modules and electrolyzers does not contribute to cost reduction due to the higher related investment costs. A further system optimization is suggested regarding the implementation of short-term energy storage, which might be particularly relevant at locations with higher fluctuations in power supply.

Direct solar hydrogen generation via a combination of photovoltaics PV and water electrolysis can potentially ensure a sustainable energy supply while minimizing greenhouse emissions. The PECSYS project aims at demonstrating a solar driven electrochemical hydrogen generation system with an area gt;10 amp; 8201;m2 with high efficiency and at reasonable cost. Thermally integrated PV electrolyzers ECs using thin film silicon, undoped, and silver doped Cu In,Ga Se2 and silicon heterojunction PV combined with alkaline electrolysis to form one unit are developed on a prototype level with solar collection areas in the range from 64 to 2600 amp; 8201;cm2 with the solar to hydrogen StH efficiency ranging from amp; 8776;4 to 13 . Electrical direct coupling of PV modules to a proton exchange membrane EC to test the effects of bifaciality 730 amp; 8201;cm2 solar collection area and to study the long term operation under outdoor conditions 10 amp; 8201;m2 collection area is also investigated. In both cases, StH efficiencies exceeding 10 can be maintained over the test periods used. All the StH efficiencies reported are based on measured gas outflow using mass flow meters

The conventional energy system is undergoing a transformation towards renewable energy technologies, as society strives for sustainable and green energy supply. This has created challenges, such as spatial and temporal imbalances of energy demand and feed-in arising from volatile renewable energy resources. A possible solution to this challenge is presented by hydrogen as a versatile chemical storage medium. Promising technologies for producing hydrogen from renewable energy include the production pathways photoelectrolysis (PEC) and photovoltaic–electrolysis (PV–EL). This paper examines three production pathways which differ in the connection and integration of the constituent photovoltaic (PV) and electrolysis (EL) subsystems by modelling the integrated system's behaviour under the various device designs and operational conditions. The model is based on the electrochemical processes and addresses losses and how the overall performance can be enhanced, in contrast to literature-based models. The efficiency of the subsystems, as well as the coupling efficiency, are predicted under various conditions, enabling the determination of optimum design and operational parameters. This analysis is enhanced by an application of the PV–EL pathways to the hourly weather conditions of Julich, Germany. The solar to hydrogen efficiency was found to drop as the level of integration increased. The study showed that varying weather conditions strongly affect the efficiency of integrated systems and should be further taken into account for future improvement and cost estimations of integrated device performance.

Direct coupling of photovoltaic (PV) modules to electrolyser(s) (ECs) can benefit from reduced component costs by omitting power electronics. Thermal integration of the PV to the EC could potentially enhance performance by cooling the PV and heating up the EC. However, conventional PV and EC constructions need an additional heat exchanger to make this possible. Without concentrating optics, the temperature of the PV remains comparatively low, which could be an added challenge for effective use of the waste heat. Considering this, the question is, how much thermal integration can benefit the PV-EC operation, and how complex a sufficiently efficient heat exchanger would be. To study the effect of heat exchange on the device operation, we compared the simultaneous operation of two identical sets of PV modules directly electrically coupled to EC stacks. One of the devices was also thermally coupled using a heat exchanger at the back of the PV module to heat up the electrolyte (1.0 M KOH) before it enters the EC (Figure 1.a, the thermally coupled device is on the left in Figure 1.b). The heat exchanger prevented contact between the corrosive KOH and the PV module but enabled heat transfer from the PV module to the electrolyte. The PV modules consisted of nine series-connected 6-inch wafer silicon heterojunction solar cells, and the total collection area, of each, was ca. 2600 cm2 (51 cm × 51 cm), of which ca. 2480 cm2 was active. The devices were operated outdoors in Berlin, Germany (52° 25ʹ 53.3ʺ N, 13° 31ʹ 25.9ʺ E) for a total of about 700 hours, of which the last about 500 hours were continuous, except for few short maintenance breaks. During testing, the solar to hydrogen efficiency of both devices was typically in the 8 – 12 % range, reducing with increasing irradiance. Typical peak hydrogen production rate on a sunny day (800 – 850 W/m2) was about 120 ml/min with the heat exchanger and about 110 ml/min without, the highest measured values being about 10 ml/min higher. The heat exchanger improved the performance under irradiance over about 500 W/m2, and at over 800 W/m2 irradiance, the enhancement corresponded to about 10 % increase in the hydrogen production rate. On the other hand, interestingly, the heat exchanger also reduced the hydrogen production rate at low irradiance conditions. This, together with the fact that most of our testing days were comparatively cloudy, probably explains why the total hydrogen yields over the 700-hour period were very similar for both devices, with a slight 10 litre advantage for thermal integration (ca. 770 litres vs ca. 760 litres). Nevertheless, based on our results, even a moderately efficient heat exchanger enhances the PV-EC operation in sunny conditions. Since much of the annual hydrogen yield would be produced in such conditions, the concept of transferring heat from PV to EC shows definite promise, but further development and optimization is needed to extract the full benefits of our approach. The authors acknowledge support from the German Federal Ministry of Education and Research in the framework of the project CatLab (03EW0015A). The present study benefits from work started under the PECSYS project (ended December 2020) funded by the FUEL CELLS AND HYDROGEN 2 JOINT UNDERTAKING under grant agreement No. 735218. This Joint Undertaking receives support from the EUROPEAN UNION’S HORIZON 2020 RESEARCH AND INNOVATION programme and Hydrogen Europe and N.ERGHY. Figure 1

The Horizon 2020 project PECSYS aims to build a large area demonstrator for hydrogen production from solar energy via integrated photovoltaic (PV) and electrolysis systems of different types. In this study, Si- and CIGS-based photovoltaics are developed together with three different electrolyzer systems for use in the corresponding integrated devices. The systems are experimentally evaluated and a general model is developed to investigate the hydrogen yield under real climatic conditions for various thin film and silicon PV technologies and electrolyser combinations. PV characteristics using a Si heterojunction (SHJ), thin film CuInxGa1-xSe2, crystalline Si with passivated emitter rear totally diffused and thin film Si are used together with temperature dependent catalyst load curves from both acidic and alkaline approaches. Electrolysis data were collected from (i) a Pt-IrO2-based acidic electrolysis system, and (ii) NiMoW-NiO-based and (iii) Pt-Ni foam-based alkaline electrolysis systems. The calculations were performed for mid-European climate data from Julich, Germany, which will be the installation site. The best systems show an electricity-to-hydrogen conversion efficiency of 74% and over 12% solar-to-hydrogen (STH) efficiencies using both acidic and alkaline approaches and are validated with a smaller lab scale prototype. The results show that the lower power delivered by all the PV technologies under low irradiation is balanced by the lower demand for overpotentials for all the electrolysis approaches at these currents, with more or less retained STH efficiency over the full year if the catalyst area is the same as the PV area for the alkaline approach. The total yield of hydrogen, however, follows the irradiance, where a yearly hydrogen production of over 35 kg can be achieved for a 10 m(2) integrated PV-electrolysis system for several of the PV and electrolyser combinations that also allow a significant (100-fold) reduction in necessary electrolyser area for the acidic approach. Measuring the catalyst systems under intermittent and ramping conditions with different temperatures, a 5% lowering of the yearly hydrogen yield is extracted for some of the catalyst systems while the Pt-Ni foam-based alkaline system showed unaffected or even slightly increased yearly yield under the same conditions.

An integrated photovoltaic electrolyser with a solar collection area of 294 cm2 was constructed and its performance, represented by the solar to hydrogen STH conversion efficiency and hydrogen production rate mH2 in various outdoor and indoor conditions, was investigated by measurements of the product gas streams. The device was composed of silicon heterojunction photovoltaic cells integrated with an electrolysis cell using nickel foam electrodes coated with nickel iron oxide and nickel molybdenum on the anode and cathode side, respectively, with 1.0 M KOH electrolyte. Depending on the operating conditions, the STH efficiency was typically 3.4 10 and typical mH2 of 30 60 mg h amp; 8722;1, approximately corresponding to 1 2 W output power. When considering non concentrating devices, the STH efficiency is one of the best for solar collection areas exceeding 100 cm2 and the hydrogen production rate the highest reported for devices smaller than 1 m2. The efficiency seems to depend mainly on the temperature, and less on the irradiance, reducing at a rate of about 0.35 absolute per 1 C increase in ambient temperature, significantly steeper than the efficiency of the used photovoltaic cells, making it a possible concern for practical hydrogen generation

Photoelectrochemical systems have the potential to contribute to de-carbonation of the energy system because solar energy can be directly converted to hydrogen which can be burnt without the release of greenhouse gases. However, meaningful deployment of photoelectrochemical (PEC) technology in the global energy system, even when highly efficient scaled up devices become available, shall only be a reality when their safe and reliable operation can be guaranteed over serveral years of service life. The first part of the review discusses the importance of hermetic sealing of up scaled PEC device provided by the casing and sealing joints from a reliability and risk perspective. The second part of the review presents a survey of fully functional devices and early demonstrators and uses this to establish the extent to which the state of the art in PEC device design address the issue of hermetic sealing. The survey revealed that current material choices and sealing techniques are still unsuitable for scale –up and commercialization. Accordingly, we examined possible synergies with related photovoltaic and electrochemical devices that have been commericalised and derived therefrom, recommendations for future research routes that could accelerate the development of hermetic sealings of PEC devices.

The passivation quality at the interface between dielectric interlayer (IL) stacks and liquid-phase crystallized silicon (LPC-Si) was investigated by means of high-frequency capacitance–voltage (C–V) measurements. The developed device structure was based on a molybdenum layer sandwiched between the glass substrate and the IL/LPC-Si stack. C–V curves were discussed in terms of hysteresis formation and capacitance relaxation. We varied the nitrogen and hydrogen content in the a-SiOxNy:H layer adjacent to the LPC-Si and studied the effects on the defect state density at the IL/LPC-Si interface (Dit) as well as on the effective charge density in the IL (QIL,eff). Both parameters are crucial for the analysis of chemical and field-effect passivation. Furthermore, the effect of an additional hydrogen plasma treatment (HPT) on Dit and QIL,eff was investigated. A Gaussian-like defect distribution at around 0.1 eV above the mid gap energy is significantly reduced by the additional HPT. With additional HPT, the lowest Dit and highest QIL,eff at mid gap, i.e., Dit = (3.5 ± 0.7) × 1011 eV−1 cm−2 and QIL,eff = (1.6 ± 0.3) × 1012 cm−2, correspond to the passivation by an a-SiOxNy:H layer with a low nitrogen and high hydrogen content.

We present a detailed analysis of the photo-current potentials and losses in thin film crystalline silicon solar cells on glass. The effects of texturing the silicon backside, applying a diffuse back reflector and a textured anti-reflection foil were analysed. Light beam induced current measurements were used to determine the losses due to local effects like the absorber contact, cracks in the absorber and grain boundaries. Detailed loss analysis in combination with ray-tracing simulations showed that the maximum light trapping potential imposed by geometrical optics has nearly been achieved. The photocurrent losses due to incomplete carrier collection and parasitic absorption were accounted for using a theoretical model. For the investigated, textured, n-doped cell with reflector and anti-reflection foil, the short circuit current density ( J SC ) was 28.9 mA/cm 2 and the main loss factors were direct reflection (3.4 mA/cm 2 ), electrical shading effects due to the absorber contact (3.1 mA/cm 2 ) and incomplete carrier collection due to surface/bulk recombination (1.6 mA/cm 2 ). Using the presented light trapping scheme we obtained the following efficiencies: 11.8% for a p-doped and 12.1% for an n-doped crystalline silicon absorber. Finally, the potentials for efficiencies beyond 14% are discussed.

Thin crystalline silicon solar cells prepared directly on glass substrates by means of liquid-phase crystallization of the absorber utilize only a small fraction of the silicon material used by standard wafer-based silicon solar cells. The material consists of large crystal grains of up to square centimeter area and results in solar cells with open-circuit voltages of 650 mV, which is comparable with results achieved with multi-crystalline silicon wafers. We give a brief status update and present new results on the electronic interface and bulk properties. The interrelation between surface passivation and additional hydrogen plasma passivation is investigated for p-type and n-type absorbers with different doping concentrations. Internal quantum efficiency measurements from both sides on bifacial solar cells are used to extract the bulk-diffusion length and surface-recombination velocity. Finally, we compare various types of solar cell devices based on 10 µm thin crystalline silicon, where conversion efficiencies of 11–12% were achieved with p–type and n-type liquid-phase crystallized absorbers on glass. Copyright © 2015 John Wiley & Sons, Ltd.

The effect of varying the boron and aluminium content of the starting electrolyte for extrinsically doped ZnO films grown on SnO 2 :F substrates by electrochemical deposition was investigated. The ZnO:B film surface was characterized by grains with mainly hexagonal faces exposed while the exposed faces of the ZnO:Al grains were rectangular. Whereas a B 3 + /Zn 2 + ratio of up to 10 at.% in the electrolyte had no significant effect on the crystalline structure of the ZnO films, an Al 3 + /Zn 2 + ratio above 0.25 at.% increased the disorder in the crystalline structure. All the boron doped films exhibit a strong E 2 -high Raman mode related to wurtzite ZnO structure but this peak was much weaker for ZnO:Al and diminished with increasing Al incorporation in the films. Exposing the films to ultra-violet light reduced their effective sheet resistance from values beyond measurement range to values between 40 and 5000 kΩ/sq for film thicknesses of 200–550 nm. Inspection of the optical spectra near the bandgap edge and the plasma edge in the mid infrared range, showed that the Al-doping resulted in a higher carrier concentration ~ 10 20 cm − 3 than B-doping. X-ray electron spectroscopy showed that the dopant efficiency was limited by the absence of dopant atoms near the surface of all the ZnO:B films and of the lightly doped ZnO:Al and, by the formation of aluminium oxide at the surface of the more highly doped ZnO:Al films.

On a-Si:H/µc-Si:H thin film solar cells, the back-contact and reflector typically consist of a thin ZnO:Al (AZO) buffer layer and a silver layer. The high cost of silver as well as plasmonic absorption make it desirable to use a silver-free backside reflector. We report that a backside contact composed of an AZO transparent conducting oxide (TCO) layer deposited by DC-magnetron-sputtering at 180 °C combined with a white paint backside reflector can achieve almost the same solar cell performance as a TCO/silver back contact. The properties of the reflector materials, i.e. silver and commercially available white paint were studied by total and diffuse reflection measurements. The effect of the reflector materials on the performance of the solar cells in mini-module design were evaluated by current–voltage (I–V) and external quantum efficiency-measurements. Moreover, the AZO layer used as back contact was optimized to obtain high near-infrared (NIR)-transparency while preserving a low resistivity. Altogether, the approach of constructing an a-Si:H/µc-Si:H solar cell with a silver-free backside reflector has been successfully implemented and can still be improved by using a TCO with higher carrier mobility and lower carrier density, thus improving NIR-light transmission.

Metalloid and metal based oxides are an almost unavoidable component in the majority of solar cell technologies used at the time of writing this review. Numerous studies have shown increases of ≥1% absolute in solar cell efficiency by simply substituting a given layer in the material stack with an oxide. Depending on the stoichiometry and whether other elements are present, oxides can be used for the purpose of light management, passivation of electrical defects, photo-carrier generation, charge separation, and charge transport in a solar cell. In this review, the most commonly used oxides whose benefits for solar cells have been proven both in a laboratory and industrial environment are discussed. Additionally, developing trends in the use of oxides, as well as newer oxide materials, and deposition technologies for solar cells are reported.

The influence of the transparent conducting oxide (TCO) topography was studied on the performance of a silicon oxide intermediate reflector layer (IRL) in a-Si/μc-Si tandem cells, both experimentally and by 3-D optical simulations. Therefore, cells with varying IRL thickness were deposited on three different types of TCOs. Clear differences were observed regarding the performance of the IRL as well as its ideal thickness, both experimentally and in the simulations. Optical modeling suggests that a small autocorrelation length is essential for a good performance. Design rules for both the TCO topography and the IRL thickness can be derived from this interplay.

Hydrogen generated by electrochemical water splitting driven by solar energy presents a near zero carbon emission means of energy storage. For the wide scale deployment of this technology, it is necessary to reduce costs of the components by reducing the amount of scarce materials used on one hand on the other hand, simplifying the device design. In this work, a fully integrated photovoltaic driven electrolysis system is designed in which the electrical connection of the device is wholly internal and the two energy conversion components are inseparable. Also transfer of heat from the back of the PV minimodule to the electrolyte is facilitated by thermal contact. Computational fluid dynamics simulations were used to optimise the geometry as well as the feedstock and thermal management in the device. The PV minimodule with a total area of 294 cm², consisted of silicon heterojunction solar cells while the electrolysis cell used 1.0M KOH electrolyte and 100 cm² nickel foam electrodes for water splitting. The hydrogen production efficiency of the entire device was tested under different irradiation and ambient temperature conditions without additional heating. Outdoor tests under irradiation levels of about 500 W/m² recorded hydrogen production rates of reaching 85 µg cm² s-1, corresponding to solar-to-hydrogen conversion efficiencies around 5 %. The use of better catalysts and higher irradiance conditions are expected to contribute to improved device performance. Figure 1: Photograph of the photovoltaic integrated electrolyser during outdoor tests Figure 1

This paper addresses the achievements and challenges in a-Si:H/µc-Si:H tandem solar cell technology including production aspects. As an example we show figures of the module production at Masdar PV, with module conversion efficiencies reaching 10%. The process integration was supported by PVcomB, namely, by developing improved PECVD processes and transferring them to Masdar PV. The important role of the TCO substrate for producing high-efficiency modules at lowest possible costs is discussed. We show the results of tandem cells on thermally annealed ZnO:Al substrates, resulting in an improvement of 0.4–0.5% (abs) as a result of the TCO annealing, reaching a stable efficiency of 12.1% for a 1 cm² solar cell and 11.6% for a mini module. The challenges in developing thin film silicon solar modules with higher efficiency than presently reached are discussed.

Techniques, such as photoluminescence (PL) and electron-beam-induced current (EBIC), have already proven their effectiveness and applicability for solar materials. Although, the methods are standard techniques for multicrystalline Si PV, their application to thin films is challenging and requires special adjustment and a careful selection of the measuring parameters. Here we report on the investigation of thin-film tandem solar cells consisting of hydrogenated amorphous (a-Si:H) and microcrystalline silicon (µc-Si:H) on glass substrates. We observe a homogeneous spatial distribution of the PL signals caused by the dominance of the surface recombination. A typical PL spectrum exhibits sub-band gap features of a-Si:H. We relate the sub-band-gap spectral features mainly to transitions of carriers trapped in deep states. Observations on partially processed stacks support this supposition. PL is only detectable on the a-Si:H layer, while EBIC signal is generated mainly in the µc-Si:H layer. It is found out that the luminescence features of the thin a-Si:H layer resemble those on bulk a-Si:H.

Ultra-thin silicon oxide (a-SiO x :H) films have been grown by means of plasma enhanced chemical vapor deposition (PECVD) to replace the standard hydrogenated amorphous silicon (a-Si:H) passivation layer for silicon heterojunction solar cells to reduce parasitic absorption. Additionally, silicon oxide surfaces are well known as superior substrates for the nucleation enhancement for nanocrystalline silicon doped films. Symmetrical passivation samples were fabricated with variable a-SiO x :H layers with a thickness of 10–1.5 nm and characterized after several annealing steps (25–650 °C). The best value reached so far on oriented Si wafers is: implied open circuit voltage of 686 mV and minority carrier lifetime of 1.6 ms after annealing at 300 °C. Such values were found to be reproducible even for ultra-thin a-SiO x :H layers (1.5 nm).

In this paper, we report on the development of an n-type silicon oxide intermediate reflector (SOIR) for a-Si:H/µc-Si:H tandem solar cells produced in an industrial-type AKT1600 PECVD reactor. A comparison to a tunnel recombination junction with µc-SiOx in the p-layer is made. Lower fill factors, resulting from the implementation of the SOIR, could be avoided by the deposition of a thin n-doped microcrystalline silicon (µc-Si) recombination layer after the SOIR. A cell efficiency of 9.5% after 168 h of light soaking at 50 °C and 1 sun was reached on commercially available SnO2:F front TCO, which is a 2% relative increase over a similar cell without the SOIR. Possible explanations for the role of this recombination layer are discussed (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A special class of transparent conducting oxides (TCO) with high mobility of > 65 cm 2 V − 1 s − 1 allows film resistivity in the low 10 − 4 Ω cm range and a high transparency of > 80% over a wide spectrum, from 300 nm to beyond 1500 nm. This exceptional coincidence of desirable optical and electrical properties provides opportunities to improve the performance of opto-electronic devices and opens possibilities for new applications. Strategies to attain high mobility (HM) TCO materials as well as the current status of such materials based on indium and cadmium containing oxides are presented. Various concepts used to understand the underlying mechanisms for high mobility in HMTCO films are discussed. Examples of HMTCO layers used as transparent electrodes in thin film solar cells are used to illustrate possible improvements in solar cell performance. Finally, challenges and prospects for further development of HMTCO materials are discussed.

The optical transmission of dye-sensitised solar cells (DSCs) can be tuned by altering the dye and/or particle size of the mesoporous TiO2 layers, to allow their application as the top device in tandem solar cells. To benefit from this semi-transparency, parasitic optical losses by the transparent electrodes must be minimised. This work investigates the influence of using two different transparent conductors, namely, the high mobility material titanium doped indium oxide (ITiO) and fluorine doped tin oxide (FTO) as electrodes for semi-transparent DSCs. The overall NIR transparency through the DSCs increased significantly as each FTO electrode was replaced by an ITiO electrode. This increase was from 20–45% in the 1300–700 nm wavelength range for fully FTO-based cells, to about 60% for fully ITiO-based cells, across the same spectrum. DSCs prepared on these electrodes exhibited short circuit currents ranging from 14·0–14·9 mA/cm2. The conversion efficiency of the cell with ITiO as both the front and rear electrodes was 5·8%, which though significant, was lower than the 8·2% attained by the cell using FTO electrodes, as a result of a lower fill factor. Improvements in the ITiO thermal stability and in the processing of the TiO2 interfacial layer are expected to improve the cell efficiency of such single DSC devices. The high current density and optical transparency of ITiO-based DSCs make them an interesting option for tandem solar cells. Copyright © 2008 John Wiley & Sons, Ltd.

CuIn1-xGaxSe2 (CIGS) solar cells show a good spectral response in a wide range of the solar spectrum and the bandgap of CIGS can be adjusted from 1.0 eV to 1.7 eV by increasing the gallium-to-indium ratio of the absorber. While the bandgaps of Ga-rich CIGS or CGS devices make them suitable for top or intermediate cells, the In rich CIGS or CIS devices are well suited to be used as bottom cells in tandem solar cells. The photocurrent can be adapted to the desired value for current matching in tandem cells by changing the composition of CIGS which influences the absorption characteristics. Therefore, CIGS layers with different [Ga]/[In+Ga] ratios were grown on Mo and ZnO:Al coated glass substrates. The grain size, composition of the layers, and morphology strongly depend on the Ga content. Layers with Ga rich composition exhibit smaller grain size and poor photovoltaic performance. The current densities of CIGS solar cells on ZnO:Al/glass varied from 29 mA cm(-2) to 13 mA cm(-2) depending on the Ga content, and 13.5% efficient cells were achieved using a low temperature process (450 degrees C). However, Ga-rich solar cells exhibit lower transmission than dye sensitized solar cells (DSC). Prospects of tandem solar cells combining a DSC with CIGS are presented. (C) 2008 Elsevier B.V. All rights reserved.

Aluminium-doped zinc oxide films were deposited on glass substrates at high rates by reactive mid-frequency sputtering. The in-line sputter system allows oxygen influx along the middle and sides of a dual-cathode system. The effect of varying the oxygen flow from the sides on the electrical and optical properties together with the surface morphology after wet chemical etching was investigated. Increasing the amount of oxygen flow from the sides improved the resistivity profile of static prints and gave highly conductive and transparent films in dynamic deposition mode. The etched films developed rough surface textures with effective light scattering that could be controlled by the oxygen balance between the middle and sides. Optimally textured films were used as front contacts in 1 cm2 single junction microcrystalline solar cells yielding an initial efficiency of 8.4%. The improvement in light trapping led to short circuit densities higher than that of the reference solar cells.

Silicon nitride (SiN x ) thin films have been deposited by a new remote plasma deposition system HiTUS (High Target Utilisation Sputtering). The remote plasma geometry allows, pseudo separation of plasma/target-bias parameters, lower ion bombardment, and effectively eliminates poisoning, making it an attractive option for stable reactive sputtering of important electronic and photovoltaic films including silicon nitride. Transmission and absorbance measurements on glass were performed in order to evaluate the absorbance of the silicon nitride thin layer. The SiN x is produced by reactive sputtering from a silicon target in an Ar/N2 atmosphere, negating the use of silane gas in difference to the more commonly used PECVD method. A deposition rates up to 0.7 nm/s have been obtained. Control of refractive index from 1.9 to 2.3 was achieved by varying the RF target bias, meeting the requirements for silicon passivation in PV applications, with a growth rate independent of refractive index across a wide range. The carrier life time over a range of specified minority carrier densities was measured using a contactless inductively coupled photoconductance tester for 50 ohm cm 〈100〉 oriented silicon wafers coated with different types of SiN x . We found that the passivation action of the silicon nitride increased the carrier lifetime over one order of magnitude.

Indium tin oxide (ITO) films were deposited by reactive High Target Utilisation Sputtering (HiTUS) onto glass and polyimide substrates. The ion plasma was generated by an RF power source while the target bias voltage was varied from 300 V to 500 V using a separate DC power supply. The deposition rate, at constant target power, increased with DC target voltage due to increased ion energy reaching 34 nm/min at 500 V. All the films were polycrystalline and showed strong (400) and (222) reflections with the relative strength of latter increasing with target bias voltage. The resistivity was lowest at 500 V with values of 1.8 × 10 − 4 Ω cm and 2.4 × 10 − 4 Ω cm on glass and polyimide, respectively but was still less than 5 × 10 − 4 Ω cm at 400 V. All films were highly transparent to visible light, (> 80%) but the NIR transmittance decreased with increasing target voltage due to higher free carrier absorption. Therefore, ITO films can be deposited onto semiconductor layers such as in solar cells, with minimal ion damage while maintaining low resistivity.

Indium tin oxide (ITO) films were deposited on soda lime glass and polyimide substrates using an innovative process known as High Target Utilisation Sputtering (HiTUS). The influence of the oxygen flow rate, substrate temperature and sputtering pressure, on the electrical, optical and thermal stability properties of the films was investigated. High substrate temperature, medium oxygen flow rate and moderate pressure gave the best compromise of low resistivity and high transmittance. The lowest resistivity was 1.6 × 10− 4 Ω cm on glass while that on the polyimide was 1.9 × 10− 4 Ω cm. Substrate temperatures above 100 °C were required to obtain visible light transmittance exceeding 85% for ITO films on glass. The thermal stability of the films was mainly influenced by the oxygen flow rate and thus the initial degree of oxidation. The film resistivity was either unaffected or reduced after heating in vacuum but generally increased for oxygen deficient films when heated in air. The greatest increase in transmittance of oxygen deficient films occurred for heat treatment in air while that of the highly oxidised films was largely unaffected by heating in both media. This study has demonstrated the potential of HiTUS as a favourable deposition method for high quality ITO suitable for use in thin film solar cells.

In this study, various silicon dielectric films, namely, a-SiOx:H, a-SiNx:H, and a-SiOxNy:H, grown by plasma enhanced chemical vapor deposition (PECVD) were evaluated for use as interlayers (ILs) between crystalline silicon and glass. Chemical bonding analysis using Fourier transform infrared spectroscopy showed that high values of oxidant gases (CO2 and/or N2), added to SiH4 during PECVD, reduced the Si-H and N-H bond density in the silicon dielectrics. Various three layer stacks combining the silicon dielectric materials were designed to minimize optical losses between silicon and glass in rear side contacted heterojunction pn test cells. The PECVD grown silicon dielectrics retained their functionality despite being subjected to harsh subsequent processing such as crystallization of the silicon at 1414 °C or above. High values of short circuit current density (Jsc; without additional hydrogen passivation) required a high density of Si-H bonds and for the nitrogen containing films, additionally, a high N-H bond density. Concurrently high values of both Jsc and open circuit voltage Voc were only observed when [Si-H] was equal to or exceeded [N-H]. Generally, Voc correlated with a high density of [Si-H] bonds in the silicon dielectric; otherwise, additional hydrogen passivation using an active plasma process was required. The highest Voc ∼ 560 mV, for a silicon acceptor concentration of about 10(16) cm(-3), was observed for stacks where an a-SiOxNy:H film was adjacent to the silicon. Regardless of the cell absorber thickness, field effect passivation of the buried silicon surface by the silicon dielectric was mandatory for efficient collection of carriers generated from short wavelength light (in the vicinity of the glass-Si interface). However, additional hydrogen passivation was obligatory for an increased diffusion length of the photogenerated carriers and thus Jsc in solar cells with thicker absorbers.

Microcrystalline silicon (μc-Si:H) thin-film solar cells are processed on glass superstrates having both micro- and nanoscale surface textures. The microscale texture is realised at the glass surface, using the aluminium-induced texturing (AIT) method, which is an industrially feasible process enabling a wide range of surface feature sizes (i.e., 700 nm–3 μm) of the textured glass. The nanoscale texture is made by conventional acid etching of the sputter-deposited transparent conductive oxide (TCO). The influence of the resulting “double texture” on the optical scattering is investigated by means of atomic force microscopy (AFM) (studying the surface topology), haze measurements (studying scattering into air), and short-circuit current enhancement measurements (studying scattering into silicon). A predicted enhanced optical scattering efficiency is experimentally proven by a short-circuit current enhancementΔIscof up to 1.6 mA/cm2(7.7% relative increase) compared to solar cells fabricated on a standard superstrate, that is, planar glass covered with nanotextured TCO. Enhancing the autocorrelation length (or feature size) of the AIT superstrates might have the large potential to improve theμc-Si:H thin-film solar cell efficiency, by reducing the shunting probability of the device while maintaining a high optical scattering performance.

The use of relatively low purity zinc nitrate for electrochemical deposition of compact ZnO films is attractive for large scale production because of the cost saving potential. ZnO films were grown on SnO2:F and magnetron sputtered ZnO:Al templates using a three electrode potentiostatic system in galvanic mode. The electrolyte consisted of a 0.1 M zinc nitrate solution (either 99.998% or 99% purity) and 1 mM aluminium nitrate for extrinsic doping, when required. Moderate deposition rates of up to 0.9 nm s−1 were achieved on ZnO:Al templates with lower rates of up to 0.5 nm s−1 on SnO2:F templates. Observation of SEM images of the films revealed a wall-like morphology whose lateral thickness (parallel to the substrate) reduced as aluminium was added to the system either in the electrolyte or from the substrate. However, pre- deposition activation of the template by applying a negative voltage (approximately −2 V) allowed the growth of compact films even for the low purity electrolyte. The optical band gap energy of intrinsically doped films was lower than that of the Al doped films. The composite electrical conductivity of all the films studied, as inferred from sheet resistance and Hall effect measurements of the ZnO/template stacks was much less than that of the uncoated templates. A strong E2 (high) mode at around 437 cm−1 was visible in the Raman spectra for most films confirming the formation of ZnO. However, both the Raman modes and XRD reflections associated with wurtzite ZnO diminished for the Al doped films indicating a high level of mainly oxygen related defects. Based on these data, further studies are underway to improve the doping efficiency of aluminium, the crystalline structure and thus the conductivity of such films.

Light trapping techniques are one of the key research areas in thin film silicon photovoltaics. Since the 1980s randomly rough textured front transparent oxides (TCOs) have been the methods of choice as light trapping strategies for thin-film devices. Light-trapping efficiency can be optimized by means of optical simulations of nano-structured solar cells. We present a FEM based simulator for 3D rigorous optical modeling of amorphous silicon / microcrystalline silicon tandem thin-film solar cells with randomly textured layer interfaces. We focus strongly on an error analysis study for the presented simulator to demonstrate the numerical convergence of the method and investigate grid and finite element degree refinement strategies in order to obtain reliable simulation results.

We present a FEM based simulator for 3D rigorous optical modeling of a-Si/μc-Si tandem thin-film solar cells with randomly textured layer interfaces. Our focus lies on a detailed analysis of the numerical error.

In this contribution we compare the optical, electronic and surface morphological properties of various textured transparent conducting oxides (TCO) based on zinc oxide and fluorine doped tin oxide. Since the TCO material properties tend to be interrelated, comparison of such films is a rather complex exercise. The TCO films were characterised by atomic force microscopy, scanning electron microscopy, spectrophotometry, angle resolved scattering, four point probe and Hall Effect measurements. Thereafter, an attempt was made to correlate the TCO material properties with the corresponding current-voltage characteristics and quantum efficiency in actual solar cells. It was found that the solar cells were more sensitive to changes in the optical properties of the TCO substrates than to those in the sheet resistance. Based on these results, we recommend that when optimising TCO films for thin film silicon production, the first priority should be to obtain the highest transmittance possible and then to tune the electrical properties accordingly.

The best light scattering properties for ZnO:B films grown by Low Pressure Chemical Vapor Deposition (LPCVD) require low doping in the precursors and films. Such films must be thicker than 2 µm to achieve sheet resistances ≤ 20 Ω/sq, suitable for thin film silicon solar cells. In this study, ZnO:B films, grown by LPCVD with different doping levels, were etched using several solutions so as to modify their surface morphology in a quick and simple way. The haze ratio, at 800 nm, of the films increased from values below 10 % to about 20 % after etching. Observation of the SEM micrographs confirmed that the film surface had been modified by the etching process. Highly doped films could be etched to achieve a high haze while keeping the sheet resistance below 15 Ω/sq. Tandem junctions consisting of amorphous silicon p-i-n and micro-crystalline silicon p-i-n solar cells grown on etched highly doped ZnO:B films exhibited higher open circuit voltage than those on a natively textured lower doped films. Further optimization of both the etching process and the starting ZnO:B material is expected to lead to higher solar cell efficiency. These results show that the optimum thickness of ZnO:B front contacts in thin film silicon can be reduced, potentially cutting production time and material costs.

Light scattering superstrates are important for thin-film a-Si:H solar cells. In this work, aluminum-induced texture (AIT) glass, covered with nonetched Al-doped ZnO (AZO), is investigated as an alternative to the commonly used planar glass with texture-etched AZO superstrate. Four different AIT glasses with different surface roughnesses and different lateral feature sizes are investigated for their effects on light trapping in a-Si:H solar cells. For comparison, two reference superstrates are investigated as well: planar glass covered with nonetched AZO and planar glass covered with texture-etched AZO. Single-junction a-Si:H solar cells are deposited onto each superstrate, and the scattering properties (haze and angular resolved scattering) as well as the solar cell characteristics (current-voltage and external quantum efficiency) are measured and compared. The results indicate that AIT glass superstrates with nonetched AZO provide similar, or even superior, light trapping than the standard reference superstrate, which is demonstrated by a higher short-circuit current Jsc and a higher external quantum efficiency. Using the trapped light fraction δ, a quantity based on the integrated light scattering at the AZO/a-Si:H interface, we show that Jsc linearly increases with δ in the scattering regime of the samples, regardless of the type of superstrate used.

Understanding the oxygen evolution reaction (OER) activity and stability of the NiFe-based materials is important for achieving low-cost and highly efficient electrocatalysts for practical water splitting. Here, we report the roles of Ni and Fe on the OER activity and stability of metallic NiFe and pure Ni thin films in alkaline electrolyte. Our results support that the Ni(OH)2/NiOOH does not contribute to the OER directly, but it serves as an ideal host for Fe incorporation, which is essential for obtaining high OER activity. Furthermore, the availability of Fe in the electrolyte is found to be important and necessary for both NiFe and pure Ni thin films to maintain enhanced OER performance, while the presence of Ni is detrimental to the OER kinetics. The impacts of Fe and Ni species present in KOH on the OER activity are consistent with the dissolution/re-deposition mechanism we proposed. Stability studies show that the OER activity will degrade under prolonged continuous operation. Satisfactory stability can, however, be achieved with intermittent OER operation in which the electrocatalyst is cycled between a degraded and recovered state. Accordingly, two important ranges, i.e., the recovery range and the degradation range, are proposed. Compared to intermittent OER operation, prolonged continuous OER operation (i.e. in the degradation range) generates a higher NiOOH content in the electrocatalyst, which is likely related to the OER deactivation. If the electrode works in the recovery range for a certain period, i.e., at a sufficiently low reduction potential, where the Ni3+ is reduced to Ni2+, the OER activity can be maintained and even improved, if also Fe is present in the electrolyte.