In semi-transparent perovskite solar cells (PSCs) comprising a sputtered top electrode, the minimisation of detrimental sputter damage and/or its curing by post-processing treatment is essential to reach high efficiencies. In this work, we investigate the influence of sputter damage and post-deposition annealing steps for different electron transport layers (ETLs) in PSCs without the need of a protective buffer layer such as tin oxide deposited by atomic layer deposition. We compare solution-processed 6,6-Phenyl C61 butyric acid methyl ester (PCBM) to thermally evaporated C60, each in combination with a bathocuproine (BCP) layer deposited by spin coating or thermal evaporation including a thickness variation. In general, we find that C60 is more resilient against sputter damage and thus C60-based cells show higher as-grown power conversion efficiencies (PCEs). Post-deposition annealing of the complete cell stack increases the PCE further to values > 16 % on 0.5 cm2 active area. However, we observe that the remaining solvent in the spin-coated PCBM layer is highly beneficial for the curing of the sputter damage during post-deposition annealing and we achieve even higher PCEs for cells incorporating solution-processed PCBM with up to > 18 % on 0.5 cm2. We show an alternative way to reach high efficiency semi-transparent perovskite solar cells without using thermally evaporated C60 and/or a buffer layer like tin oxide deposited by atomic layer deposition (ALD).

(Ag,Cu)(In,Ga)Se2-based solar cells can be optimized for different applications by adjusting the composition. A wide range of compositions has been explored in a production-relevant deposition line with minor adjustments to the process. The best efficiencies are achieved for the (Ag,Cu)(In,Ga)Se2 material optimized for single-junction applications, where most optimization efforts have been directed. For tandem devices the achievable bandgap range is suitable for both top and bottom cells. In both cases, material quality must be improved and further modifications to the growth process are necessary. For the bottom cell, standard and reduced-Ga compositions could already be successfully implemented in high-efficiency two-terminal tandems with solution-processed perovskite top cells. In this contribution we review our R&D line with mostly in-line deposition steps, discuss the performance of chalcopyrite solar cells at different compositions, and present modifications which are beneficial for the development of tandem devices. Best results for completely in-house processing are over 18 % for two-terminal perovskite/(Ag,Cu)(In,Ga)Se2 tandems and over 26.6 % for the four-terminal configuration.

Two types of Cu(In,Ga)Se2 (CIGS) thin-film solar modules, differing only in the patterning procedures, were exposed to a high voltage (1 kV) across the thickness of the soda-lime glass substrate. Both module types utilized a cell stack, and in particular, a molybdenum back contact, which was optimized for CIGS solar cells with enhanced stability against potential-induced degradation (PID). A standard module with regular patterning lines for monolithic interconnection consists of P1, P2, and P3 lines, with P1 separating the back contact, P2 establishing the contact between the front contact TCO of one cell to the Mo back contact of the next cell, and P3 isolating the front contact from one cell to the next cell. However, modules employing this cell stack with regular patterning lines P1, P2, and P3 suffered considerably from PID while modules with a P1 groove filled with an insulator showed greatly enhanced stability similar to the pure single cell without P1 patterning. PID manifests once a specific quantity of charge has been transmitted through the soda-lime glass substrate, while the molybdenum back contact of the cell functions as the cathode, maintaining a negative bias relative to the substrate's backside. As a consequence of PID, sodium is increased in the adsorber for susceptible cells. Therefore, inhibiting sodium transport through the P1 groove by filling it with an insulating material enhances the PID stability of the modules considerably. As a result, the modules with a filled P1 groove showed similar stability to the single solar cells with an improved PID stable cell stack, while modules without a filled P1 patterning were much more susceptible to PID, although a cell stack with greatly enhanced PID stability was used. In summary, the presented strategy to fill the P1 groove offers a viable and novel path to improved PID stability of CIGS modules.

bottom solar cells featuring an interdigitated back contact (IBC) based on laser processes can be integrated in highly efficient 3-Terminal perovskite/silicon tandem solar cells. While surface texturing of the Si bottom cell on the front side is essential for light trapping and thus enhanced absorption of long-wavelength light, it can hamper the conformal wet-chemical deposition of the perovskite top cell. Modification of our texture-etch allows a reduction of the pyramids size without affecting the light absorption in the Si cell. Our laser-processed double-side textured Si IBC cells reach efficiencies up to 22.6% under 1-sun illumination when using a non-optimized laser doping process. Perovskite layers deposited on these modified bottom cells cover their pyramids, which is necessary for shunt-free tandem devices. Since tandem operation requires a low-resistive electrical connection of the subcells, passivation properties of wet-processed SnO2 and TiO2 layers on Si are evaluated. To achieve higher tandem cell efficiencies, IBC solar cells with suitable passivating front contact layers need to be developed.

In order to improve the power conversion efficiency of thin-film solar cells, it is essential to identify and quantify their dominant loss mechanisms and, thus, guide experimental device optimization. We provide this functionality via loss analyses determined from computer-aided modeling and numerical device simulations. Since electrical and optical effects influence each other within solar cells, the consideration of an isolated parameter is often not sufficient for maximizing cell performance. Therefore, a holistic perspective, including both the effects, is developed. Its modeling is achieved by an interplay of an optical transfer-matrix method and a quasi-3-D electrical finite-element method. Optical refractive information and electrical resistivity data have been measured experimentally and put into the model in order to forecast device $I\text{--}V$ curves. Finally, these $I\text{--}V$ curves are used to assign and quantitatively calculate the loss mechanisms of grid shading, reflection, parasitic and incomplete absorption, local maximum power point mismatches, undergrid reverse currents, and ohmic losses. We verified our approach using thin-film CuIn $_{1-x}$ Ga $_{x}$ Se $_{2}$ cells with a variable thick front contact made of aluminum-doped zinc oxide. This discussion guides laboratory work to the relevant loss mechanisms and enables to rapidly test new innovation ideas by means of numerical simulations.

Over the past decade, the impressive progress in power conversion efficiency (PCE) of organometallic halide perovskite solar cells (PSCs), coupled with their ready integration into tandem solar cells, has led them to approach PCEs of 30% for tandem solar cells with a silicon bottom subcell. However, the complementary technology of perovskite/copper indium gallium selenide (CIGS) tandem solar cells has been thus far unable to reach similar efficiency values. Herein, a further advance in the efficiency of 4T perovskite/CIGS tandems is demonstrated, increasing the PCE up to 27.3% via systematic optimization of the top semitransparent PSC. Improvements in light management through the optimization of anti‐reflection coatings, coupled with the development of transparent conductive oxides that incur very low parasitic absorption are reported. It is revealed that both are crucial for maximizing efficiency and, by utilizing additional optical simulations, a detailed loss analysis that enables us to outline a path toward approaching 30% PCE for 4T perovskite/CIGS tandem devices is developed.

Narrow-band gap (NBG) Sn-Pb perovskites with band gaps of ∼1.2 eV, which correspond to a broad photon absorption range up to ∼1033 nm, are highly promising candidates for bottom solar cells in all-perovskite tandem photovoltaics. To exploit their potential, avoiding optical losses in the top layer stacks of the tandem configuration is essential. This study addresses this challenge in two ways (1) removing the hole-transport layer (HTL) and (2) implementing highly transparent hydrogen-doped indium oxide In2O3:H (IO:H) electrodes instead of the commonly used indium tin oxide (ITO). Removing HTL reduces parasitic absorption loss in shorter wavelengths without compromising the photovoltaic performance. IO:H, with an ultra-low near-infrared optical loss and a high charge carrier mobility, results in a remarkable increase in the photocurrent of the semitransparent top and (HTL-free) NBG bottom perovskite solar cells when substituted for ITO. As a result, an IO:H-based four-terminal all-perovskite tandem solar cell (4T all-PTSCs) with a power conversion efficiency (PCE) as high as 24.8% is demonstrated, outperforming ITO-based 4T all-PTSCs with PCE up to 23.3%.

We have investigated the influence of simulated outdoor illumination conditions on the functionality of photovoltaic-biased electrosynthetic (PV-EC) systems used for the production of hydrogen as a renewable and storable fuel via solar water splitting. Thin-film multijunction solar cells were adopted for the PV part of the device together with an electrosynthetic cell with a Pt/IrOx catalyst pair in a 1 molar potassium hydroxide electrolyte solution. We studied the influence of the incident illumination angle on the solar-to-hydrogen efficiency and have given a first evaluation of the long-term (1 year) performance of PV-EC systems in terms of the hydrogen volume produced for a given geographical location. In this approach, variations from the standard AM1.5G type illumination expressed as changes in the average photon energy of the spectra were used to simulate different geographical locations as well as seasonal and daily changes in the illumination spectra. Finally, we compared the impact of various types of multijunction photovoltaic devices (tandem, triple, and quadruple junctions) on the annual solar hydrogen production.

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 cm 2 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, H 2 production rate of 1.26 μmol H 2 /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.

Although photoelectrochemical 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 made of inexpensive and earth abundant materials must be developed. In this work, we demonstrate a scalable (64 cm2 aperture area) artificial photoelectrochemical device composed of triple-junction thin-film silicon solar cells in conjunction with an electrodeposited bifunctional nickel iron molybdenum water splitting catalyst. Our device shows a solar to hydrogen efficiency of up to 4.67% (5.33% active area) without bias assistance and wire connection. Furthermore, gas separation was enabled by incorporating a membrane in a 3D printed device frame.

Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/ or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.

Stable performance of a solar water splitting device with an area of 64 cm2 and catalysts from earth abundant materials.

We present a stand-alone integrated solar water-splitting device with an active area of 64 cm2 and a long-term stable operation. The modular setup of the device provides a versatile tool to integrate and evaluate various combinations of photoelectrodes and catalysts.

Renewable and storable fuel hydrogen can be produced through light-driven water splitting using photovoltaic-biased electrosynthetic (PV-EC) devices. The required voltage to drive the reaction is approximately 1.5 V, which can be provided using multi-junction silicon solar cells. To generate these voltages at the operation point, the solar cells are electrically optimized with respect to the standard test conditions. However, if such devices were to be used outdoors, a wide range of different illumination conditions has to be considered. Herein, we discuss the dependence of the solar-to-hydrogen efficiency on the spectral quality, the incident illumination intensity and the operation temperature. It is found that in the case of high irradiation intensities (e.g. 1 sun), high operation temperatures reduce the PV performance, but the overall PV-EC device performance remains almost unchanged due to improved kinetics in the EC part. In contrast, when the illumination intensity is reduced, the loss in PV performance cannot be compensated by improved EC performances due to higher temperatures.

TiO2 is a common protection layer on semiconductor electrodes for photoelectrochemical water splitting. We investigate the interface formation of TiO2 on amorphous silicon tandem solar cells by X-ray photoelectron spectroscopy. In order to optimize the contact properties, we prepare TiOx interface layers with various oxygen content by reactive magnetron sputter deposition. We observe, that a TiOx interface layer can reduce the silicon oxide growth during the film deposition on the amorphous silicon, but it forms a non-ohmic contact. The electrochemical investigation shows, that the benefit due to the reduction of the silicon oxide is counteracted by the unfavorable contact formation of TiOx interface layers prepared with low oxygen content. (C) 2016 Elsevier B.V. All rights reserved.

The impact of light-induced degradation (LID) of silicon photoelectrodes on the solar-to-hydrogen efficiency of photoelectrochemical (PEC) devices is investigated. To evaluate the effect, stabilized state-of-the-art thin-film silicon solar cells (after 1000 h of light soaking) were used as photocathodes in photovoltaic-electrochemical (PV-EC) device assemblies and their performances were compared to the performances of the initial solar-cell-based PV-EC devices. A wide range of photoelectrode configurations (tandem, triple, quadruple) was addressed. With regard to the widespread use of multijunction-based photoelectrodes in the literature, the results presented herein will have a high impact and may serve as guidelines for the design of photovoltaic devices particularly tailored for PEC applications, with high stabilities and efficiencies. It is shown that LID affects the performances of PV and PV-EC devices in different ways and strongly depends on the photovoltage of the applied solar cell.