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A major challenge for achieving energy transition and transforming the current energy model into distributed production is the development of efficient artificial leaf-type devices capable of directly converting carbon dioxide (CO2), water and sunlight into sustainable fuels and chemicals under ambient conditions. These devices should avoid using critical raw materials to be sustainable and cost-competitive. We report top-level results for the first time in converting CO2 and H2O to fuels (formate and H-2) using sunlight and electrodes based solely on earth-abundant materials. The cell provides a solar-to-fuel efficiency of >10% combined with world-record current densities to comparable devices operating at room temperature, without adding sacrificial donors or electrical bias. In addition, we present the novel concept of producing at the same time H-2 and an H-2-storage element (formate), the latter used to produce H-2 when light is absent. This solution allows continuous (24 h) hydrogen production using an artificial-leaf device. For the first time, we show the feasibility of this solution. The experimental results were obtained in an optimised, compact electrochemical flow cell, with electrodes based on Cu-S and Ni-Fe-Zn oxide (for CO2 reduction and oxygen evolution reactions, respectively) supported on gas-diffusion substrates, integrated with a low-cost Si-based photovoltaic module. The cell design allows for easy scale-up and low manufacturing and operating costs. The cell operates at a current density of about 17 mA cm(-2) and a full-cell voltage of 2.5 V (stable for at least ten hours and in on-off operations), providing formate productivity of 193 mu mol h(-1) cm(-2), paving the way towards the implementation of affordable artificial-leaf type systems in the future energy scenario., Energy & Environmental Science, 16 (4), ISSN:1754-5692, ISSN:1754-5706

Solar energy 249, 233-241 (2023). doi:10.1016/j.solener.2022.11.040, Published by Elsevier Science, Amsterdam [u.a.]

ACS omega 7(31), 27532-27541 (2022). doi:10.1021/acsomega.2c02940, Published by ACS Publications, Washington, DC

Direct solar-powered hydrogen generation (so-called “green” hydrogen) is promising as a renewable fuel that can be generated anywhere there is sunshine and water. Many attempts are made to integrate a water electrolyzer (EC) and solar cell at different levels (a so-called artificial leaf) to take advantage of the reduced losses from the lack of wiring and optionally increased portability afforded by an integrated unit. However, in many cases, EC catalysts degrade as electrodes depolarize when shut down at night. Much less attention is paid to the need for a minimum current across the EC under insufficient illumination to prevent excessive cyclic degradation. Directly coupling a battery to keep an artificial leaf running at night can address this need and, in theory, also increase solar-to-hydrogen (STH) efficiency. A seven-cell silicon heterojunction module, two bifunctional NiFeMo ECs in series, and a commercial Li-ion NMC battery are selected to provide the same amount of solar output power despite different working voltages and tested in a series of simulated diurnal cycles. The increased average STH efficiency per cycle (11.4% vs. 10.5% without the battery) is analyzed and discussed with implications for future artificial leaf design and implementation.

The maximum solar-to-hydrogen efficiency (STH) in directly coupled photovoltaic-assisted water-splitting systems is achieved when the photovoltaic (PV) and electrochemical (EC) devices are power matched precisely. This matching requires that the polarization curve of the EC device crosses the current–voltage (IV) characteristics of the PV device at its maximum power point (MPP). Conversely, each point on the EC polarization curve can be considered the MPP of a PV device optimally coupled to the EC device. Therefore, at each point on the polarization curve, the minimum PV efficiency and maximum EC efficiency can be calculated for a specific irradiance. The product of both efficiencies generates the STH limit that can be attained at that specific point on the polarization curve. This “reverse analysis,” carried out with elementary math, does not involve any modeling or analysis of PV IV characteristics. Herein, this reverse analysis is described and how it can be used to quantify losses in PV–EC systems and the effect of mutual scaling of PV and EC devices is shown. This method is presented using a NiMo/NiFeOX catalyst pair as an example and was applied to a variety of PV–EC combinations described in the literature.

Solar RRL 2100594 (2021). doi:10.1002/solr.202100594, Published by Wiley-VCH, Weinheim

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.

Efficient solar charging of a battery has been demonstrated in the past by sizing batteries many times that of a solar cell to reduce the effective current density experienced by the battery. Altho...

Cell reports / Physical science 3(11), 101123 (2022). doi:10.1016/j.xcrp.2022.101123, Published by Elsevier, [New York, NY]

Multijunction solar cells, proven technological route for achievements of highest PV conversion efficiencies, require accurate tuning of the sub-cell absorption to ensure that every cell in the stack delivers the same current density. Even though currents of the sub-cells can be precisely matched for a fixed illumination spectrum, current mismatch cannot be avoided in real terrestrial applications due to variations of the irradiance spectrum. The issue becomes critical when a solar cell has to cover wider range of applications such as a mixture of direct sun / shadow / artificial light – the case for various distributed PV-powered electronics. Furthermore the current matching constrains choice of materials and designs of sub-cells for a tandem device. A straightforward basic configuration with decoupling the sub-cell's currents is a 3-terminal configuration with one additional contact sheared by the top and bottom cell. This concept requires voltage-matching between the top and bottom cell when these cells are integrated in modules. A realistic concept for the voltage-matched 3-terminal cell reported recently includes a wide gap top cell combined with a tandem bottom cell made of 2 sub-cells with lower bandgaps. The concept is a hybrid between 2 and 3-terminal configurations with voltage matching and relaxed current matching constrains. Established thin film silicon solar cell technology provides interesting option to realize the hybrid 3-terminal cell with amorphous Si top cell (VOC ≈ 0.9 V) and two microcrystalline Si cells (VOC ≈ 0.5 V). In this work we present proof of concept of the voltage matched 3-terminal tandem cell prepared with highly transparent and conductive IOH intermediate contact. The efficiency of 10.4% has been achieved made up of independently operating 7.9% efficient top cell and 2.5% efficient bottom tandem cell. The paper summarizes the development and discusses optical losses identified in the 3-T devices.

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.

This work focuses on the potentials of monolithic integrated thin-film silicon solar cell and lithium ion cell in a simple cell-to-cell integration without any control electronics as a compact power solution for portable electronic devices. To demonstrate this we used triple-junction thin-film silicon solar cell connected directly to a lithium ion battery cell to charge the battery and in turn discharge the battery through the solar cell. Our results show that with appropriate voltage matching the solar cell provides efficient charging for lab-scale lithium ion storage cell. Despite the absence of any control electronics the discharge rate of the Li-ion cell through the non-illuminated solar cell can be much lower than the charging rate when the current voltage (IV) characteristics of the solar cell is matched properly to the charge-discharge characteristics of the battery. This indicates good sustainability of the ultimately simple integrated device. At the maximum power point, solar energy-to-battery charging efficiency of 8.5% which is nearly the conversion efficiency of the solar cell was obtained indicating potential for loss-free operation of the photovoltaic (PV)-battery integration. For the rest of the charging points, an average of 8.0% charging efficiency was obtained.

Solar cell in combination with a secondary battery is a straightforward power supply solution for portable electronics. When used indoors or outdoors under sufficient illumination conditions the combination is attractive to realize self-sustained devices or extend off-grid operation time. However, only little fraction of widely spread portable devices worldwide are equipped with PV energy harvesters. Besides low-average power, combination of discrete PV module and battery is bulky for extreme demands in modern portable electronics. Integration of thin-film solar cell with a thin-film storage cell in a monolithic, customizable module is a viable option. The low level cell-to-cell integration is possible with thin-film silicon multi-junction solar cells providing sufficient voltage to charge Li-ion storage cell. In this work, we focus on the development of triple-junction thin-film silicon solar cells for monolithic integration with lithium ion storage cells. We show that with appropriate voltage matching a triple junction thin-film silicon solar cell provides efficient charging for lab-scale Li-ion storage cell under a range of illumination intensities. Maximum solar energy-to-battery charging efficiency of 8.8% was obtained.

High efficiency thin-film silicon solar cells require texturing of the front contacts for advanced light management. However, optically favorable textures may be challenging in terms of the electrical performance. Solar cells deposited on very rough front contacts show a reduced open-circuit voltage, Voc, as compared to flat TCOs. This contribution focuses on the impact of rough surfaces on the Voc in a-Si:H solar cells deposited on six different textured front TCOs. With various i-layer thicknesses from 100 to 400 nm the origin of the Voc loss is investigated. Based on our results we propose that the reduction of Voc in a-Si:H solar cells are mainly due to defective regions in the bulk material induced by very rough textures. It is also seen that enlarging the i-layer thickness can improve the Voc of solar cells when grown on very rough textures. By using variable illumination intensity measurements the existence of shunts could be evidenced.

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.

Over the last couple of years, photonic materials with tailored -i.e. with deliberately introduced- structural disorder have attracted considerable interest in photovoltaics due to their extended spectral and angular range of effectiveness [1]. Notably, quasi-random nanostructures realized by e-beam lithography (EBL) have been integrated in solar cells as broadband light trapping elements, and have proved to approach the theoretical (Lambertian) limit [2]. Despite recent research efforts aiming at increasing the EBL writing speed [3], alternative routes based on self-assemblies still possess major advantages for an industrial implementation of disordered structures as they allow to rapidly process them over large areas (>>cm2). In this communication, we show that the up-scalable polymer blend lithography technique can be used as a versa-tile platform for fabricating 2D planar, disordered nanostructures that can be exploited in both top-down and bottom-up strategies. Tailored disorder is achieved here by adjusting the process parameters (polymer blend composition and deposition conditions), enabling to tune the morphology and the spatial distribution of the nanostructures produced, and in turn their light harvesting properties. We first use our approach to pattern a resist etching mask, which is employed for transferring disordered nanoholes into a thin hydrogenated amorphous silicon layer by dry etching (top-down route). We report an enhancement of its integrated absorption of +90% under normal incidence, and of up to +200% at large incident angles with respect to an unprocessed absorber [4]. In a second example, we demonstrate that similar structures can serve as a template in a bottom-up configuration, whereby copper indium diselenide nanocrystals are infiltrated into the disordered nano-holes formed in a resist layer. This route, paving the way to wet-processable "photonized" absorbers, is compared to a previous work relying on a serial writing process [5], and the optical properties of the resulting patterned absorbing layers are analysed. We finally elaborate on the significance of these findings for the reverse problem, namely for light extraction in broadband light-emitting diodes. References [1] Burresi, M., Pratesi, F., Riboli, F., & Wiersma, D. S. (2015). Complex photonic structures for light harvesting. Advanced Optical Materials, 3(6), 722-743. [2] Martins, E. R., Li, J., Liu, Y., Depauw, V., Chen, Z., Zhou, J., & Krauss, T. F. (2013). Deterministic quasi-random nanostructures for photon control. Nature communications, 4, 2665. [3] Li, K., Li, J., Reardon, C., Schuster, C. S., Wang, Y., Triggs, G. J., ... & Krauss, T. F. (2016). High speed e-beam writing for large area photonic nanostructures—a choice of parameters. Scientific reports, 6. [4] Siddique, R. H., Donie, Y. J., Gomard, G., Yalamanchili, S., Merdzhanova, T., Lemmer, U., & Holscher, H. (2017). Bioinspired phase-separated disordered nanostructures for thin photovoltaic absorbers. Science Advances, 3(10), e1700232. [5] Dottermusch, S., Quintilla, A., Gomard, G., Roslizar, A., Voggu, V. R., Simonsen, B. A., ... & Richards, B. S. (2017). Infiltrated photonic crystals for light-trapping in CuInSe2 nanocrystal-based solar cells. Optics Express, 25(12), A502-A514.

We investigate the light-induced degradation of a-Si:H/µc-Si:H tandem solar cells by applying the Power-Matching-Method (PMM). The PMM allows for a characterization of a cell under various matching conditions by systematically varying the spectral distribution of the illumination. The top cell's thickness was varied from 100 nm to 500 nm for cells with and without SiO x intermediate reflector layer (IRL). The method provides a powerful means of comparing the degradation of differently matched tandem cells. We conclude that the IRL does not significantly affect the stability of our sub cells. The method reveals that only the top-cell degrades significantly under illumination. In addition, the study shows that the PMM is useful for outdoor performance estimations of tandem solar cells or modules.

Laser and oven annealing effects on hydrogen concentration, hydrogen diffusion and material microstructure in hydrogenated amorphous silicon films deposited on crystalline silicon substrates are compared. For laser annealing, a 6 W green (532 nm) continuous wave laser with 100 µm focus diameter was applied and samples of about 1 cm2were scanned in ambient with a line distance of 50 µm and at a speed of 1 – 100 mm/s. Hydrogen content and microstructure were measured by infrared spectroscopy, and hydrogen diffusion was investigated by secondary ion mass spectroscopy (SIMS) measurements of depth profiles of deuterium and hydrogen in layered structures of deuterated and hydrogenated material. The results show that in both annealing experiments hydrogen diffuses predominantly in form of atoms although some formation of H2molecules cannot be excluded. By comparison of laser and oven treatment, an effective temperature describing the laser treated state can be defined. Furthermore, the temperature of the thin silicon film during laser treatment is estimated.

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO 2 :F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

We present here the performance analysis of PV-battery 3-terminal integration concept. In this arrangement, a PV device is integrated monolithically with a battery stack separated only by a common contact layer which makes the use of a control electronic e.g. a DC-DC converter possible. The concept is a versatile power solution for self-sustained operation of small-scale electronics. In this work we investigate this concept by using thinfilm multi-junction solar cell to directly charge a Li battery with the aid of a control electronic. Operation of the PV-battery combination with and without charge controller is analyzed under standard AM1.5 illumination.

Intermediate reflector layers are commonly used for light man-agement purposes in multi-junction silicon based devices containing a-Si:H top- and µc-Si:H bottom-sub-cells. A low resistance of such layers can have a severe impact on the solar module performance due to shunting of the bottom sub-cell by the P2 scribe. A common solution for this problem is the use of an additional scribe line. However, not only the additional processing step is disadvantageous but also the dead area losses are increased as well by the additional scribe. This work introduces a novel solar cell stripe interconnection scheme that requires only three scribing processes with similar dead area losses as they would be apparent in the standard interconnection scheme. An implementation to mini modules shows no negative impact on the electrical properties and simultaneously reducing the required number of scribing steps. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

The optical performance of tandem a-Si:H/mu c-Si:H (micromorph) thin film solar cell was investigated experimentally and by means of rigorous 3-D optical simulation. The interplay of intermediate reflectors, with different refractive indices and thicknesses, and front electrode surface texture was studied. Experiments and simulations show that LPCVD ZnO based front electrodes have the highest optical potential together with a low refractive index of the intermediate reflector. The intermediate reflector layer serves for redistribution of the mid-range solar spectrum between the top and bottom cell, while the sum of the top and bottom cell currents decreases with increasing IRL thickness. Additionally, promising concepts to increase the short-circuit current of the tandem solar cell are shown. The most important steps are related to lowering parasitic absorption in supportive layers by the introduction of silicon oxide layers and improving the light incoupling by introduction of anti-reflective layers. (C) 2014 Elsevier B.V. All rights reserved.

In this study, nanoimprint processing was used to realize various multiscale textures on glass substrates for application in thin-film photovoltaic devices. The multiscale textures are formed by a combination of large and small features, which proofed to be beneficial for light trapping in silicon thin-film solar cells. Two approaches for the fabrication of multiscale textures are presented in this study. In the first approach, the multiscale texture is realized at the lacquer/transparent conductive oxide (TCO) interface, and in the second approach, the multiscale texture is realized at the TCO/Si interface. Various types of multiscale textures were fabricated and tested in microcrystalline thin-film silicon solar cells in p-i-n configuration to identify the optimal texture for the light management. It was found that the best light-scattering multiscale texture was realized using an imprint-textured glass substrate, which contains large craters, in combination with HF-etched TCO (ZnO:Al), which contains small features, on top of the imprint. With this structure (of the second approach), the short-circuit current density of the solar cell devices was improved by 0.6 mA/cm-2 using multiscale textures realized by nanoimprint processing.

In this work, we report on the development of thin-film silicon solar cells on textured glass substrates. The textured glass substrates are fabricated by ion beam etching using a wet-chemically textured three-dimensional etching mask. The development of transparent and conductive front contact ZnO:Al films on textured glass is presented. The optimum of the front contact layer thickness was found to be 60 nm. For this thickness deteriorating reflection maxima are avoided, which occur due to the interferences in the front contact layer. The glass texture is adjusted to achieve better light trapping in the near infrared range. In addition, ITO instead of ZnO:Al film was investigated to surpass the decreased fill factor of solar cells deposited on ZnO:Al thin front contacts.

Microcrystalline silicon oxide (µc-SiO x :H) alloys prepared by plasma enhanced chemical vapor deposition (PECVD) represent a versatile material class for opto-electronic applications especially for thin-film and wafer based silicon solar cells. The material is a phase mixture of microcrystalline silicon (µc-Si:H) and amorphous silicon oxide (a-SiO x :H). The possibility to enhance the optical band gap energy and to adjust the refractive index over a considerable range, together with the possibility to dope the material p-type as well as n-type, makes μc-SiO x :H an ideal material for the application as window layer, as intermediate reflector (IR), and as back reflector in thin-film silicon solar cells. Analogously, μc-SiO x :H is a suitable material for p- and n-type contact layers in silicon hetero junction (SHJ) solar cells. The present paper gives an overview on the range of physical parameters (refractive index, optical band gap, conductivity) which can be covered by this material by variation of the deposition conditions. The paper focuses on the interdependence between these material properties and optical improvements for amorphous silicon/microcrystalline silicon (a-Si:H/µc-Si:H) tandem solar cells prepared on different substrates, such as Asahi (VU) and sputtered ZnO:Al. It gives a guideline on possible optical gains when using doped µc-SiO x :H in silicon based solar cells. As intermediate reflector in a-Si:H/µc-Si:H tandem cells µc-SiO x :H leads to an effective transfer of short circuit current generation from the bottom cell to the top cell resulting in a possible thickness reduction of the top cell by 40%. Within another series of solar cells shown in this paper a short circuit current density of 14.1 mA/cm² for an a-Si:H/µc-Si:H tandem solar cell with a µc-SiO x :H intermediate reflector is demonstrated. A SHJ solar cell on a flat (non-textured) wafer using p- and n-type doped µc-SiO x :H contact layers with an effective area efficiency of 19.0% is also presented.

Single-chamber processes for the deposition of high efficiency thin-film silicon tandem cells of an a-Si:H p-i-n (top cell)/μc-Si:H p-i-n (bottom cell) structure involving short fabrication time are reported. An industry relevant reactor and an excitation frequency of 13.56 MHz were used. The conversion efficiency is found to be highly sensitive to dopant cross contamination into the μc-Si:H i-layer of the bottom cell and within the n/p-interface of the tunnel recombination junction (TRJ). Different reactor treatments at the p/i-interfaces of the top and bottom cells and at the n/p-interface of the TRJ were applied, aiming to prevent dopant cross contamination. The phosphorus and the boron concentrations were evaluated by secondary ion mass spectrometry measurements. Phosphorus cross contamination after TRJ n-layer deposition is found to result in significant n-type doping of the μc-Si:H i-layer of the bottom cell if no reactor treatment is applied. In situ reactor treatment via an Ar flush and pumping step of 15 min applied at the n/p-interface of TRJ results in reduction of the μc-Si:H i-layer phosphorus concentration to values below 1017 cm− 3. A conversion efficiency of 11.8% for such tandem cells is demonstrated. Shorter interface treatment time with phosphorus concentrations in the μc-Si:H i-layer of about 5 × 1017 cm− 3 results in lower conversion efficiencies of 10.6%, mainly due to the decrease of open-circuit voltage and fill factor.

To effectively mitigate oxygen induced degradation of electronic properties in hydrogenated nanocrystalline silicon (nc-Si:H) the microscopic origins of the associated defects must be determined. This work elucidates the origin of a 0.7 eV photoluminescence (PL) band observed in thermally annealed, oxygen contaminated nc-Si:H. Raman and PL spectroscopy data demonstrate the deep defects responsible for the 0.7 eV PL reside in the crystalline phase. The electronic levels enabling the emission exist 0.4 eV below the conduction band edge, or 0.4 eV above the valence band edge, or both, with transitions into or out of extended states as opposed to band tail recombination. The defect formation energy is 0.6 eV, which suggests that the formation involves a local reconfiguration of oxygen and hydrogen as opposed to bulk diffusion. Hydrogen plays multiple roles in the microstructural evolution and electronic activity of these centers. TEM micrographs show dislocations in the nanocrystalline regions, which suggests that oxygen gettering occurs. It seems in the absence of hydrogen dislocations in Si nanocrystals scavenge oxygen impurities from grain boundaries to become optically active, enabling emission at 0.7 eV.

In this work, we report on the fabrication of microcrystalline thin-film silicon solar cells on textured glass substrates. The development of transparent and conductive front contacts for these solar cells is presented. State-of-the-art random textures for light-trapping were replicated into a glass-like resist on glass substrates with an imprint process. We applied an industrial relevant soft polymer mold that gives excellent replication accuracy. The necessity of applying thin front contacts for enhanced incoupling of the incident light is shown. An increased series resistance of these thin front contacts caused a decrease of the fill factor of the solar cells. One way to surpass this decrease in fill factor by reducing the solar cell width is demonstrated. In addition, the light-trapping and the light-incoupling for solar cells deposited on three different types of random textures were compared.

In this work, we present a technology for a high precision nanostructure replication process based on ultraviolet nanoimprint lithography for the application in the field of thin-film photovoltaics. The potential of the technology is demonstrated by the fabrication of microcrystalline silicon thin-film prototype solar cells. The high accuracy replication of random microstructures made from sputtered and etched ZnO:Al, used to scatter the incident light in thin solar cells, is shown by local topography investigations of the same 7.5 × 7.5 µm2 area on the master and the replica. Different types of imprint resists and imprint moulds were investigated to find the optimal, high precision replication technology. Two types of thin-film silicon solar cells, in p-i-n and n-i-p configuration, were fabricated to study the potential of the imprint technology for different applications. It is shown that solar cells deposited on an imprinted glass hold similar performances compared with reference solar cells fabricated with a standard process on textured ZnO:Al. Thus, it is demonstrated that the replication of light scattering structures by using an imprint process is an attractive method to decouple the scattering properties from the layer forming the electrical front contact. Because a simple and cheap high throughput process is used, this study additionally proves the relevance for the industrial mass production in the field of photovoltaics. Copyright © 2013 John Wiley & Sons, Ltd.

In this study angular resolved scattering measurements are used to monitor the surface texture properties of sputter-etched ZnO:Al and to identify the influences of process parameter changes on the surface texture. Variations in pressure and temperature are possible drifts in the industrial fabrication that can influence the ZnO:Al properties. Therefore – with respect to industrial relevance – this study focuses on the influence of these parameters. Deposition parameter dependent trends in the angular resolved scattering are studied. Correlations between changes in the results of the angular resolved scattering measurements and the performance of a-Si:H/μc-Si:H tandem solar cells are shown. Additionally, it is demonstrated that the measurements can be used to optimize the surface texture of ZnO:Al for single-junction μc-Si:H solar cells by adjusting the deposition conditions according to the previously identified trends.

Inspired by black butterfly scales, self-assembled phase-separated nanostructures form efficient photovoltaic absorbers., The wings of the black butterfly, Pachliopta aristolochiae, are covered by micro- and nanostructured scales that harvest sunlight over a wide spectral and angular range. Considering that these properties are particularly attractive for photovoltaic applications, we analyze the contribution of these micro- and nanostructures, focusing on the structural disorder observed in the wing scales. In addition to microspectroscopy experiments, we conduct three-dimensional optical simulations of the exact scale structure. On the basis of these results, we design nanostructured thin photovoltaic absorbers of disordered nanoholes, which combine efficient light in-coupling and light-trapping properties together with a high angular robustness. Finally, inspired by the phase separation mechanism of self-assembled biophotonic nanostructures, we fabricate these bioinspired absorbers using a scalable, self-assembly patterning technique based on the phase separation of binary polymer mixture. The nanopatterned absorbers achieve a relative integrated absorption increase of 90% at a normal incident angle of light to as high as 200% at large incident angles, demonstrating the potential of black butterfly structures for light-harvesting purposes in thin-film solar cells.

The influence of oxygen and nitrogen impurities on the material properties of a-Si:H and μc-Si:H films and on the corresponding solar cell performances was studied. For intentional contamination of the i-layer the impurities were inserted by two contamination sources: (i) directly into the plasma through a leak at the chamber wall or (ii) into the gas supply line. The critical oxygen and nitrogen concentrations for silicon solar cells were determined as the lowest concentration of these impurities in the i-layer causing a deterioration of the cell performance. Similar critical concentrations for a-Si:H and μc-Si:H cells in the range of 4–6 × 1018 cm− 3 for nitrogen and 1–5 × 1019 cm− 3 for oxygen by applying a chamber leak are observed. Similar increase of conductivity with increasing impurity concentration in the a-Si:H and μc-Si:H films is found. A more detailed study shows that the critical oxygen concentration depends on the contamination source and the deposition parameters. For a-Si:H cells, the application of the gas pipe leak leads to an increased critical oxygen concentration to 2 × 1020 cm− 3. Such an effect was not observed for nitrogen. For μc-Si:H, a new deposition regime with reduced discharge power was found where the application of the gas pipe leak can also result in an increase of the oxygen concentration to 1 × 1020 cm− 3.

Boron (B) and phosphorus (P) cross-contamination for single-chamber deposited a-Si:H, μc-Si:H, and a-Si:H/μc-Si:H tandem solar cells has been investigated by studying their impact on the different layers of solar cells. To reduce the B and P cross-contamination into the i-layer and p-layer, respectively, to a tolerable level, for a-Si:H and μc-Si:H cells a 15' evacuation cycle prior to the i-layer deposition is applied. The effect of P cross-contamination into the i-layer is strongly reduced by the p-layer deposition and a 15’ evacuation cycle prior to the i-layer deposition. The p-layer is assumed to cover up or to fix (in form of P-B complexes) most of the P at the chamber walls. This leads to high quality μc-Si:H cells and a-Si:H cells with only slightly reduced performance. Here, a soft-start of the a-Si:H i-layer led to high quality cells, presumably due to reduced P recycling. Further, there is no need to clean the process chamber with, e.g. NF3, after each p-layer, as applied in many industrial processes. Instead, many cells are deposited without cleaning the process chamber. We established a single-chamber tandem cell process with 15' evacuation cycles prior to the μc-Si:H p-layer and to each i-layer with a cell efficiency of ~ 11.1%.

Hydrogenated amorphous silicon (a-Si:H) is conventionally deposited using static plasma-enhanced chemical vapor deposition (PECVD) processes. In this work, a very high frequency (VHF) dynamic deposition technique is presented, on the basis of linear plasma sources. This configuration deploys a simple reactor design and enables continuous deposition processes, leading to a high throughput. Hence, this technique may facilitate the use of flexible substrates. As a result, the production costs of thin-film silicon solar cells could be reduced significantly. We found a suitable regime for the homogeneous deposition of a-Si:H layers for growth rates from 0.35–1.1 nm/s. The single layer properties as well as the performance of corresponding a-Si:H solar cells are investigated and compared with a state-of-the-art radio frequency (RF) PECVD regime. By analyzing the Fourier transform infrared spectroscopy spectra of single layers, we found an increasing hydrogen concentration with deposition rate for both techniques, which is in agreement with earlier findings. At a given growth rate, the hydrogen concentration was at the same level for intrinsic layers deposited by RF-PECVD and VHF-PECVD. The initial efficiency of the corresponding p–i–n solar cells ranged from 9.6% at a deposition rate of 0.2 nm/s (RF regime) to 8.9% at 1.1 nm/s (VHF regime). After degradation, the solar cell efficiency stabilized between 7.8% and 5.9%, respectively. The solar cells incorporating intrinsic layers grown dynamically using the linear plasma sources and very high frequencies showed a higher stabilized efficiency and lower degradation loss than solar cells with intrinsic layers grown statically by RF-PECVD at the same deposition rate. Copyright © 2012 John Wiley & Sons, Ltd.

In situ optical transmission measurements performed during thin-film silicon plasma-enhanced chemical vapor deposition (PECVD) are presented. Hereto, the plasma emission was used as light source. With this setup information about thickness, crystallinity and absorption characteristic of the growing intrinsic silicon thin film can be obtained. By integrating the intrinsic layers in solar cells with p-i-n configuration, the layer information gained in situ during the PECVD process can be directly correlated to the generated short-circuit current of the solar cell. The intention of this paper is to show that, by using these transmission measurements for the estimation of solar cell currents, an in situ current matching of stacked a-Si/μc-Si tandem devices is possible, which is a useful extension of the process control techniques.

For deposition of a-Si:H p–i–n solar cells, a single-chamber plasma enhanced chemical vapor deposition process at the frequency of 13.56 MHz is developed. A 40×40 cm² deposition chamber, which represents typical industry reactors equipped with showerhead electrodes is employed. Various methods are applied to reduce the boron-cross contamination from the boron-doped p-layer into the intrinsic layer, which is considered to reduce solar cell efficiency by losses especially in the short wavelength range. Three different device configurations and four different chamber treatment methods are studied, aimed to reach stable device efficiencies comparable to multi-chamber systems at minimum chamber treatment effort and treatment time. An ex-situ CO2–plasma treatment applied after deposition of the p-doped layer is found to be effective to reduce boron-cross contamination. However, this CO2-treatment is a time-consuming process step for production. We found a less time consuming treatment: by a chamber evacuation to 9×10−7 mbar subsequent to p-layer deposition. Initial and stable efficiencies of 10.2% and 7.7%, respectively, were obtained. This latter treatment results in a sharp drop of the boron concentration from ∼5×1020 cm−3 in the p-doped layer to ∼1017 cm−3 in the intrinsic layer. For comparison of different reactor geometries and their influence on the cross-contamination we used a small-area (10×10 cm2) lab-type reactor.

Thin‑film silicon solar cells based on hydrogenated amorphous silicon (a‑Si:H) and hydrogenated microcrystalline silicon (μc‑Si:H) absorber layers are typically deposited using static plasma-enhanced chemical vapor deposition (PECVD) processes. It has been found that the use of very‑high frequencies (VHF) is beneficial for the material quality at high deposition rates when compared to radio-frequency (RF) processes. In the present work a dynamic VHF‑PECVD technique using linear plasma sources is developed. The linear plasma sources facilitate the use of very-high excitation frequencies on large electrode areas without compromising on the homogeneity of the deposition process. It is shown that state-of-the-art a‑Si:H and μc‑Si:H single-junction solar cells can be deposited incorporating intrinsic layers grown dynamically by VHF-PECVD at 0.35 nm/s and 0.95 nm/s, respectively.

For different process conditions, series of hydrogenated amorphous silicon p-i-n solar cells with various oxygen concentrations in the intrinsic absorber layer were fabricated by plasma-enhanced chemical vapor deposition at 13.56 MHz using process gas mixtures of SiH 4 and H 2 . Oxygen was introduced into the gas phase during the deposition process by a controllable leak in the chamber wall and the amount of oxygen supply is characterized by the oxygen base pressure p b . It is found that for a certain deposition regime defined by silane and H 2 flows, deposition pressure and substrate temperature the oxygen incorporation follows an expected dependence on the ratio p b / r d with r d the deposition rate. This relation is not valid for the comparison of different deposition regimes. A high hydrogen flow is found to reduce the oxygen incorporation strongly. The photovoltaic parameters of the solar cells were measured in the initial state as well as after 1000 h of light-soaking. The critical oxygen concentration (i.e. the upper limit of incorporated oxygen not leading to a decay of the solar cell performance) was determined for each regime in the initial and light-soaked state. For all deposition regimes, the results show no difference in these critical oxygen concentrations for the initial and light-soaked state. The critical oxygen concentration, is found to differ for the different process regimes and turns out to be the highest (approximately 1×10 20 cm −3 ) for the deposition regime with the highest hydrogen flow rate, which interestingly is the regime with the lowest oxygen incorporation at a given p b / r d ratio. This combination makes the regime of high hydrogen gas flow suitable for depositing high-efficiency solar cells at high base pressure.

We developed a model describing the photoluminescence spectra from hydrogenated microcrystalline silicon (μc-Si:H). From the model we derived analytical relations between the separation of the quasi-Fermi levels and PL-peak energy and intensity. These relations may be useful when photoluminescence or electroluminescence based methods are applied for characterization of μc-Si:H solar cells and modules. We compared the model with experimental PL spectra from a μc-Si:H solar cell. Our model can consistently explain the relation between the measured PL-peak intensity, energy and the measured open circuit voltage of the μc-Si:H solar cell.