For an effective conversion of solar energy to electric power or a chemical fuel a lot of different materials as well as device structures have been suggested but only very few provide technological competitive conversion efficiencies so far. Especially thin film devices originally considered to provide the most cost competitive solution have not fulfilled their promises. Limitations and loss processes can be deduced from a detailed consideration of the involved photovoltaic and electrochemical elementary steps. Optimized performance can only be reached when the photovoltaic and electrolytic boundary conditions of integrated systems are comparable to those of separated devices with no extra loss due to the coupling process.

Feasible solutions seem to be possible for thin film PV and water splitting generation, as we will show with a number of investigations performed recently combining thin film synthesis, electrical or electrochemical investigations with surface science studies. Especially photoemission results on the demands on the bulk and surface electronic structure provide clear boundary conditions on the material’s and surface properties: i) the semiconductors must provide a wide splitting of quasi Fermi levels which will be provided by multi-junctions of classical semiconductors but are not to be expected for semiconductors with localized electron states as e. g. oxides as e. g. hematite due to polaron formation. ii) The growth of the materials must provide compact, grain boundary poor absorber layers to avoid charge recombination and/or transport barriers to adjust the diffusion lenghts to the absorption lengths. Iii) The interfaces between the semiconductor absorber material and the contact layers also including the electrocatalyst must be prepared avoiding Fermi level pinning and non-adjusted electron transfer states by using proper aligned passivation layers. As a consequence a buried junction is needed. We will compare PV and/or PEC cells based on Si, GaInP, and SnS to illustrate our conceptual considerations.

In summary, we do believe that effective thin film PV and PEC devices need the input and coupled scientific approach from different disciplines and expertise ranging from solid state physics, surface science to electrochemistry. Promising advanced technological solutions seem possible based on improved materials science approaches in correlating material’s engineering, manufacturing and resulting properties to the involved physical boundary conditions of the elementary steps of energy conversion.

Photoelectrochemical (PEC) water splitting with Earth abundant and/or low-cost materials can potentially lead to low-cost green hydrogen fuel. One material that will be discussed is copper oxide (Cu₂O)–fabricated by thermal oxidation of copper foils–for use as a photocathode for solar hydrogen production. Capacitance-based methods are used to identify the origin of the improvement of an etching treatment on the cuprous oxide and a performance record is achieved. The orientation of the millimeter-sized single crystal grains, which can be seen with the naked eye due to selective etching and the resulting optical effects on the different facets, enable the impact of crystal orientation on the photocathodes to be assessed. It is found that the high index crystal orientations yield statistically higher performance than low index facets, and the origins of this improvement will be discussed. Another material under investigation in our lab is Sb₂Se₃, and I will discuss recent developments. I will close with an outlook on how thin film studies can inform the development of photocatalytic particles, which have disruptive potential in the green hydrogen space.

Quasi one-dimensional (Q1-D) structures based on van der Waals materials such as (Sb,Bi)2(S,Se)3 have demonstrated impressive progresses over the past 5 years. These types of materials typically form nano/micro-ribbons with strong covalent bonding in one direction, and at the same time, weak van der Waals bonding in the others two. This fingerprint feature confers unusual optoelectronic properties to these materials when properly oriented, and has allowed fast progresses in terms of conversion efficiency of solar cell devices, with a current record exceeding 10%. Beyond Sb and Bi-chalcogenide compounds, there is a limited body of knowledge on other Q1-D systems such as mixed chalco-halide van der Waals compounds, that combine chalcogens (S,Se) with halogens (Br,I) in the same structure. In fact, recent theoretical studies suggests that mixed chalco-halides can simultaneously achieve the robustness and stability of chalcogenide materials, along with the excellent optical and electrical properties of halides, and in particular showing high defect tolerance similarly to halide-perovskites.

This presentation will introduce a novel family of materials based on mixed Sb and Bi chalco-halides [(Sb,Bi)(S,Se)(Br,I)]. The first part will be devoted to reviewing the most relevant results reported so far for the few examples available in the literature, that have reached encouraging conversion efficiencies around 5% within a very limited timeframe, with a meso-porous solar cell configuration. In addition, the fundamental properties of these compounds obtained by DFT modelling will be discussed. The calculation of the bandgaps, band structures, optical and transport properties will be presented, discussing the trends of these properties depending on the chalcogen or the halogen introduced into the structure. The results will show that all these compounds can be relevant for different thin film photovoltaic applications, with bandgaps ranging from 1.6 eV for SbSeBr up to 2.3 eV for BiSBr.

In the second part of the presentation, the complexity of the synthesis of mixed chalco-halides will be discussed, and a new methodology for their synthesis, developed by the authors of this work, and based on the combination of co-evaporation of chalcogenides and high-pressure reactive annealing under halogen atmosphere, will be presented. This methodology allowed for the first time the demonstration of working solar cells with absorbers produced by vapor deposition techniques with these compounds. The tunability of the Q1-D structures by changing the synthesis temperature and pressure will be demonstrated. It will be shown that the synthesis of bromine-based compounds requires higher temperatures and pressures than iodine ones, due to the thermodynamics associated with the incorporation of Br and I in the chalcogenide phase. A detailed analysis of the mechanisms behind the formation of the different compounds will be presented by implementing interrupted-synthesis processes, and supported by the combination of thermo-gravimetric analysis, differential scanning calorimetry, and advanced structural/compositional characterizations.

The last part of the presentation will be devoted to the challenges and possible technological solutions for the fabrication of planar-heterojunction solar cell devices with these innovative photovoltaic absorbers. The use of different electrons and holes transport layers will be discussed, demonstrating for the first-time conversion efficiencies with these architectures between 1-5% and with very encouraging Voc values above 600 mV in some cases. Finally, the perspective of these materials and the possible advantages with respect to current chalcogenide and halide technologies, will be presented and discussed.

Although lead-halide perovskites have demonstrated astonishing increases in efficiency in solar cells, they contain lead in a soluble form and are processed with toxic solvents. Bismuth-based compounds have gained increasing attention as a non-toxic alternative because bismuth is a heavy-metal cation that can replicate many of the features of lead thought to enable defect-tolerance. Defect-tolerance is the ability of materials to achieve long charge-carrier transport lengths despite the presence of defects, potentially enabling efficient performance to be achieved in materials made by low-cost methods. This talk explores our recent work on two such materials, bismuth oxyiodide (BiOI) and sodium bismuth sulfide (NaBiS₂). We demonstrate BiOI to be tolerant towards its most common point defects, and develop an all-inorganic device structure to achieve external quantum efficiencies up to 80% in photovoltaics [1]. Next, we develop this into a photocathode, making use of a graphite epoxy to further protect the devices from degradation. In doing so, we show that we can improve the stability of the devices from a couple of minutes (BiOI in direct contact with the electrolyte) to a couple of months of operation for water splitting [2]. We further show that integrating these BiOI photocathodes in tandem with BiVO₄ photoanodes leads to fully oxide-based photoelectrochemical tandems capable of self-driven water splitting and syngas production [2]. Finally, we cover our recent work on NaBiS₂, demonstrating this material to be stable in air for at least 11 months, and a stronger light harvester than established thin-film absorbers, such that a 30 nm thick film has a spectroscopic limited maximum efficiency of 26% [3]. Through detailed computations, we rationalise the cause behind this strong light absorption as resulting from cation disorder. We further show that cation disorder has significant consequences on charge-carrier transport by enabling the formation of small hole polarons.

The generation of chemical fuels (e.g., hydrogen, hydrocarbons) using sunlight offers a carbon-neutral route to meet the ever-increasing world’s energy needs. In recent years, significant progress has been reported, especially in solar water splitting. Compared to indirectly coupling photovoltaic cells with water electrolyzers, direct photoelectrochemical (PEC) water splitting offers potential advantages in terms of system and thermal integration. Nevertheless, the demonstrated solar-to-hydrogen (STH) efficiencies are still insufficient to obtain technological competitiveness. Efforts in increasing efficiency are, unfortunately, faced with a classic material science dilemma: high STH efficiencies (~20%) are achieved in devices employing high quality semiconductors that are cost-prohibitive,[1-2] while devices based on low-cost metal oxide semiconductors have only demonstrated modest STH efficiencies (<10%).[3-4] These metal oxide-based PEC water splitting devices typically utilize BiVO₄ as the absorber, and further increase of the efficiency is hindered by the optical absorption limit of BiVO₄ that has a ~2.4 eV bandgap. Novel metal oxide semiconductors with a bandgap of 1.7-1.9 eV, which are stable and efficient, are therefore desired. In this talk, our recent efforts in developing two complex metal oxides will be presented. The first metal oxide is alpha-SnWO₄ photoanode with a bandgap of ~1.9 eV. We deposited alpha-SnWO₄ thin films using pulsed laser deposition and investigated the pH stability window using a combination of inductively coupled plasma optical emission spectroscopy, x-ray photoelectron spectroscopy and in situ spectro(photo)electrochemistry measurements.[5] We found that photocorrosion occurs in alkaline pH electrolytes, while at acidic to neutral pH, a self-passivating oxide layer is formed on the surface of alpha-SnWO₄ that blocks the transfer of photogenerated holes to the electrolyte. The latter could be overcome by depositing NiO_x as a protection layer, but it is also accompanied by the reduction of the photovoltage. A thorough interface investigation using synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) and surface photovoltage spectroscopy (SPV) reveals the presence of an interfacial SnO₂ layer,[6] which must be avoided to overcome the photovoltage limitation. The second metal oxide is modified BaSnO₃. While pristine BaSnO₃ has a large bandgap (> 3 eV), the introduction of defects, such as oxygen vacancy and sulfur substitution, extends the optical absorption onset to ~1.7 eV and increases the charge carrier mobility and photocurrent by a factor of ~20.[7] By combining analyses from photoluminescence spectroscopy, modulated SPV, and wavelength-dependent time-resolved microwave conductivity, these defects can be attributed to the generation of delocalized mid-bandgap states through which charge transport via a long-lived carrier-hopping mechanism is enabled. Further outlook on these two promising complex metal oxides, alpha-SnWO₄ and modified BaSnO₃, will be discussed.

Electron transport layers (ETLs) have been used in photovoltaics (PV) cells particularly for silicon PV for efficient charge collection resulting in significantly improving its power conversion efficiency. However, when employing ETLs for Si photoelectrochemical cells particularly for CO₂ reduction (CO₂R) there are additional constraints to consider. First the ETLs must be stable to the reduction reaction (for example CO₂R) under operation. Second, the ETLs when interfaced with a catalyst layer needs to be selective to the reduction reaction of interest (CO₂R) and inert to other reactions (like hydrogen evolution reaction (HER)). Design and exploration of new ETLs for CO₂R photocathodes must take into account these considerations and in this work, we show that TaO_x satisfies all of the above criteria. TaOx films were synthesized by both pulsed laser deposition (PLD) and RF sputtering. In both cases, careful control of the oxygen partial pressure during growth was required to produce ETLs with acceptable electron conductivity. p-Si/TaO_x photocathodes were interfaced with ca. 10 nm of a CO₂R catalyst: Cu or Ag. Under front illumination with simulated AM 1.5G in CO₂-saturated bicarbonate buffer, we observed, for both metals, faradaic efficiencies for CO₂R products of ~50% and ~ 30% for PLD TaO_x and RF sputtered TaO_x, respectively, at photocurrent densities up to 8 mA cm^-2. p-Si/TiO₂/Cu photocathodes were also evaluated but produced mostly H₂ (> 97%) due to reduction of the TiO₂ to Ti metal under CO₂R conditions. In contrast, a dual ETL photocathode (p-Si/TiO₂/TaO_x/Cu) was selective for CO₂R, which suggests a strategy for separately optimizing selective charge collection and the stability of the ETL/water interface. The RF sputtered TaO_x ETL based Si photocathode was also found to be stable for CO₂R for about 240 mins, with metal crossover from the counter electrode being a limiting factor.  Our techno-economic analysis shows that the reported system, if scaled, could allow for an economically viable production of feedstocks for chemical synthesis under the adoption of specific CO₂ credit schemes, thus become a significant component to carbon-neutral manufacturing. Further evidence of efficient metal oxide catalyst support for CO₂R is unravelled by ambient pressure x-ray photoelectron spectroscopy.

Cu-Ga-Se chalcopyrite structures with a band gap of 1.68 eV (CuGaSe2) to 1.85 eV (CuGa3Se5) are considered to be promising materials to be used as the photocathode in a tandem photoelectrochemical (PEC) water splitting configuration. Therefore, we prepared polycrystalline Cu-Ga-Se films with different compositions ranging from Cu-poor CuGaSe2 (Cu/Ga = 0.85) to extremely Cu-poor CuGa3Se5 (Cu/Ga = 0.33) and investigated the effect of the Cu/Ga ratio on the crystal structure, morphology and PEC performance of the films. Without any surface treatment or formation of a p-n junction, we report remarkable saturated photocurrent densities of -19.0 and -12.1 mA/cm2 (measured at -0.40 V vs. RHE) for our films with Cu/Ga = 0.85 and Cu/Ga = 0.33, respectively, using an LED-based solar simulator. These outstanding results cover 86% and 68% of the maximum theoretical photocurrents for materials with a band gap of 1.68 eV and 1.85 eV, respectively. Furthermore, we were able to obtain and validate a realistic equivalent circuit via potentiodynamic electrochemical impedance spectroscopy (P-EIS), which among others confirmed that the obtained difference in onset potential (270 mV) between these two films was in agreement with the obtained difference in flat-band potential (290 mV).

A unique approach to exploring non-equilibrium synthesis parameter spaces of oxide thin film photoelectrodes will be presented, broadening the pathway toward discovering new chemical spaces inaccessible through conventional solid-state reactions.

There is an increasingly urgent need for disruptive and innovative materials that satisfy the chemical and physical requirements to reduce global warming through sustainable development. Fortunately, only a fraction of the possible combinations were studied, making it likely that the best materials are still awaiting discovery. Unfortunately, designing controlled synthesis routes of single-phase oxides with low defects concentration will become more difficult as the number of elements increases;^1,2 and there are currently no robust and proven strategies for identifying promising multi-elemental systems.

These challenges demand an initial focus on synthesis parameters of novel non-equilibrium synthesis approaches rather than chemical composition parameters by high-throughput combinatorial investigations of synthesis-parameter spaces, opening new avenues for stabilizing metastable materials, discovering new chemical spaces, and obtaining light-absorbers with enhanced properties to study their physical working mechanisms in photoelectrochemical energy conversion.

Using two non-equilibrium synthesis tools: pulsed laser deposition and flash photonic sintering, can form gradients in synthesis parameters without modifying composition parameters, which enables reproducible, high-throughput combinatorial synthesis over large-area substrates and high-resolution observation and analysis. Even minor changes in synthesis significantly impact material properties, physical working mechanisms, and performances, demonstrated by the relationship between synthesis conditions, crystal structures of α-SnWO4,³ and properties over a range of thicknesses of CuBi2O4,¹ both emerging light-absorbers for photoelectrochemical water-splitting used as model multinary oxides. Our approach addresses an immediate need by focusing on novel non-equilibrium synthesis approaches of disruptive and innovative materials that meet the chemical and physical requirements for reducing global warming through sustainable development.

Solar driven photoelectrochemical (PEC) reduction to produce chemical fuel is a promising strategy for solar energy harvesting, which could address the intermittent and dispersed supply of solar energy and provide a high-density solar energy storage method in the form of chemical bonds. PEC CO₂ reduction, converting CO₂ into value-added chemical, has attracted significant attention because of its potential to capture and reduce the massive emission of CO₂. Another desirable PEC application is NH₃ synthesis from NO_x reduction reaction (NO_xRR), which provides not only a promising alternative to the conventional energy-intensive NH₃ production process, but a sustainable solution for balancing the global nitrogen cycle by restoring ammonia from wastewater.

Kesterite Cu₂ZnSnS₄ (CZTS)-based photocathodes have attracted increasing research interest due to its excellent light harvesting capability, earth-abundance, and environmental-friendliness. After constructing p–n junction with CdS buffer layer, CZTS photocathode exhibits robust charge separation efficiency, which can enable an energetic photoinduced reaction driving force to power the PEC reactions. This work will present the strategies we applied to achieve high PEC reduction performance and controllable PEC reduction selectivity in both CO₂ and NO_x reduction.

In CO₂ reduction, by introducing heat treatment (HT) strategy on CZTS/CdS photocathodes, interface charge transfer optimization and surficial S vacancy engineering have been simultaneously realized (Figure 1). HT improves the CZTS/CdS heterojunction interface by introducing elemental inter-diffusion between Cd in CdS and Cu/Zn in CZTS, leading to a more favorable p-n junction with enlarged built-in potential, prolonged carrier lifetime and suppressed charge recombination.

In NO_x reduction for NH₃ production, the uniquely designed CZTS photocathode by loading defect engineered TiOx cocatalyst enables selective NH₃ production from NOxRR, yielding up to 89.1% Faradaic efficiency (FE) with a remarkable positive onset potential (Figure 2). By tailoring the amount of surface defective Ti³⁺ species, the adsorption of reactant NO³⁻ and NO₂ intermediate is significantly promoted while the full coverage of TiOx also suppresses NO²⁻ liberation as a by-product, contributing to high NH₃ selectivity.

Kesterite materials are promising alternatives to conventional Cu(In,Ga)Se₂ and CdTe compounds for thin-film photovoltaics (PV) based on abundant and non-toxic elements. However, the efficiency of kesterite solar cells is still lagging due to large open-circuit voltage (V_oc) losses that are mainly ascribed to electrostatic fluctuations and high density of point defects, related to their typical off-stoichiometric composition. Counteracting such losses can be achieved to a certain extent by substituting Ge to Sn in Cu₂Zn(Sn_1-x,Ge_x)Se₄ (CZTGSe) compounds, while also allowing Sn-Ge bandgap tuning through the x=Ge/(Ge+Sn) ratio, which is the focus of this work.

We implement a sequential process involving a physically evaporated Ge/Zn/Sn/Cu/Sn/Zn/Ge metallic precursor stack. This stack is pre-annealed in N₂ to favour crystallization, and afterwards annealed with an optimized recipe in a Se-containing environment to activate and enhance grain growth. Polycrystalline CZTGSe absorbers with the expected composition, low surface roughness and close to micron-size crystalline grains are obtained, according to SEM-EDS and Raman data. TOF-SIMS measurements calibrated by EDX results demonstrate that a Sn-Ge back gradient is reached, with an average Ge/(Ge+Sn) ratio around 40%, corresponding to about 60% at the back and 20% at the front. This bandgap gradient likely has its minimum bandgap at the top surface, estimated to be 1.04eV from the PL spectrum. So as to attempt solving the TRPL-measured limited carrier lifetime, front surface sulfurization is investigated but still requires optimization.

Using these CZTGSe absorbers, complete solar cells are processed and their IV characteristics are measured under AM1.5G illumination, leading to values of 487 mV and 28.1 mA/cm² for V_oc and the short-circuit current density (J_sc), respectively. These encouraging values beyond 60% of the Shockley-Queisser limits are possibly the result of enhanced carrier collection by the Sn-Ge bandgap gradient, supporting the potential of Ge inclusion to boost the efficiency of kesterite solar cells. However, these devices are affected by a large fill factor deficit, which cannot be counteracted without a deeper understanding of the mainly responsible loss mechanisms.

This work was recently submitted for peer-review as a Feature Paper in the “Materials for Energy Applications 2022-2023” Special Issue of the Crystals journal.

Identifying new photovoltaic absorbers purely experimentally is a very time-consuming and expensive process involving complex synthesis and characterization. Recently, the progress in first-principles simulation codes and supercomputing capabilities have given birth to the so-called high-throughput ab initio approach, thus allowing for the identification of many new compounds.
In this talk, I will present the materials screening that was performed including for the first time an estimate of carrier lifetime based on defect computations. Among 7000 known copper-based compounds, a few unsuspected solar absorber
candidates have been identified which combine a potential for high efficiency and earth-abundance. Further analysis of the data highlights two challenges in discovering Cu-based solar absorbers: deep anti-site defects lowering the carrier lifetime and low formation-energy copper vacancies leading to metallic behavior. The alkali copper phosphides and pnictides offer unique chemistries that tackle these two issues.
The high-throughput ab initio approach offers a new avenue to search rapidly for new solar absorbers and
highlights new promising chemistries especially alkali-based phosphides to be targeted by future solar cell research.

Lone pair containing solar absorbers are of interest due to their beneficial charge and defect screening properties which are similar to those of the exceptionally efficient lead halide hybrid perovskites. Both bismuth and antimony absorbers are composed of earth-abundant materials and experience the same beneficial relativistic effects that act to increase the width of the conduction band. Unlike lead, however, bismuth and antimony are non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced. In this talk, we outline general design rules for emerging earth-abundant chalcogenide photovoltaics. We demonstrate how these rules apply to the most promising PV absorbers. Using a data driven approach, we identify a new class of layered lone-pair materials that have not yet been tested as photovoltaics but which possess promising optoelectronic and charge transport properties. We highlight the potential of this materials class as PV absorbers using relativistic hybrid density functional theory and simulations of device performance.

Photocatalytic technologies are growing at higher technology readiness levels to mediate the reduction of CO2 to generate alternatives for fossil fuels with the assistance of outdoor solar light. Good designed photocatalytic structures have a great potential in splitting of water for the production of hydrogen and reduction of carbon dioxide to generate hydrocarbon fuels. Among all the photocatalytic structures that can mediate this process the Z-scheme equivalent to photosynthesis looks the most promising. Such a structure can optimize the photocatalytic activity due to the effectiveness of separating the electron hole pair compared to a single photocatalyst. The efficiency of the generation of carriers by this dual n-type photocatalysts as in the Z-scheme that combine into a heterojunction, supporting the oxidation and reduction reaction are studied. In this work a theory is presented for the electronic characteristics of a direct Z-scheme semiconductor heterojunction under solar irradiation. Based on the idealization of this theory we predicted the maximal efficiency for the conversion of solar energy to chemical energy of such a direct Z-scheme to be 11.4 %.

A tandem photoelectrochemical (PEC) water-splitting device for solar hydrogen production consists of two light absorbers with different bandgaps. It is important to enhance the performance of both cells to achieve high solar-to-hydrogen (STH) conversion efficiency. In this regard, silicon photoelectrodes have been widely investigated because of their bandgap (1.12 eV), which is suitable for the low bandgap bottom cell of a tandem device. Herein, we apply a tunnel oxide passivated contact (TOPCon) on the front and back sides of a Si wafer to prepare a TOPCon Si PEC device. Photocathodes and photoanodes based on TOPCon Si are demonstrated over a broad pH range (0–14), and they produce photovoltages of 640–650 mV under 1 sun illumination, which are the highest values obtained from crystalline Si photoelectrodes. TOPCon Si demonstrates excellent thermal stability, enduring a high processing temperature of up to 600 °C for 1 h in air. These advantages of TOPCon Si provide high efficiency and great design flexibility for monolithic tandem cells. TOPCon Si was coupled with BiVO₄, a large bandgap top cell consisting of earth-abundant and non-toxic elements, in monolithic construction, i.e., a wireless PEC tandem cell. A photovoltage exceeding 1.7 V was produced by the BiVO₄/Si with a successful demonstration of unbiased PEC water splitting. I will discuss details of the challenges and our solutions in the monolithic integration of BiVO₄ and Si.

Epitaxial semiconductors involving III-V compounds, germanium, and silicon provide the highest performance levels in optoelectronic applications such as in solar cells and photoelectrochemical cells [1-3]. However, highest performance in solar energy conversion can only be achieved, when using optimum absorber layers and sophisticated contact formation for electronic and chemical passivation, i.e. for the protection of the solid-liquid interface against corrosion as well as preventing interfacial non-radiative recombination. In order to address the surface and interface properties of III-V semiconductor layer structures in relation to their performance, we present the synthesis, theoretical modelling and properties of critical and well-defined interfaces such as GaInP/AlInP. Here, lattice matched n-type AlInP(100) charge selective contacts are commonly grown on n-p GaInP(100) top absorbers in highest-efficiency III–V multijunction solar or photoelectrochemical cells, where the cell performance can be greatly limited by missing electron selectivity and detrimental valance band offsets. Hence, understanding of the atomic and electronic properties of the GaInP/AlInP heterointerface, for instance, is crucial for the reduction of photocurrent losses in III–V multijunction devices [4]. We discuss the essential considerations on the properties of critical interfaces in relation to photoelectrochemical cells from a conceptual and from a theoretical modeling point of view assuming mostly idealized surface conditions. We also address latest progress on the important III-V/Si interface, modifications by fine-tuning of the preparation and describe experimental model experiments on the surface reactivity of III-phosphide surfaces to H₂O exposure. These different surface science approaches are then related to photoelectrochemical cells for H₂ evolution and CO₂ reduction using different III-V based tandem cells and providing highest conversion yields.

References

[1] M.M. May, H.-J. Lewerenz, D. Lackner, F. Dimroth, T. Hannappel, Nature Communications 6 (2015) 8256;
[2] W.-H. Cheng, M.H. Richter, M. M. May, J. Ohlmann, D. Lackner, F. Dimroth, T. Hannappel, H.A. Atwater, H.-J. Lewerenz; Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency, ACS Energy Letters 3 (2018) 1795
[3] W.H. Cheng, M.H. Richter, R. Müller, M. Kelzenberg, S. Yalamanchili, P.R. Jahelka, A.N. Perry, P.C. Wu, R. Saive, F. Dimroth, B.S. Brunschwig, T. Hannappel, H.A. Atwater,  Advanced Energy Materials 12 (36) (2022) 2201062

[4] O. Romanyuk, A. Paszuk, I. Gordeev, R.G. Wilks, S. Ueda, C. Hartmann, R. Felix, M. Bär, C. Schlueter, A. Gloskovskii, I. Bartoš, M. Nandy, J. Houdková, P. Jiříček, W. Jaegermann, J.P. Hofmann, T. Hannappel;

Applied Surface Science 605 (2022) 154630

Green hydrogen will play an important role in the energy transition as a renewable energy vector for long-duration energy storage and as feedstock chemical for the industry. To reduce the price below 1.5 €/ kg H₂, competitive to production from fossil fuels, PV-powered efficient anion exchange membrane (AEM) water electrolysis is a promising combination. Practical implementation of such a PV-EC technology requires standard area-sized solar cells and electrolyzers operated at large current densities (>400mA/cm²). Nonetheless, state-of-the-art research often employs <10cm² PV devices and electrolyzers operated at current densities <10mA/cm².

In this work a commercially relevant PV-EC system that couples shingled standard silicon technology with AEM electrolysis based on high surface area nickel nanomesh electrodes is presented. AEM water electrolysis combines the advantage of using low-cost materials such as nickel electrodes with the high operation current densities shown in proton-exchange membrane (PEM) water electrolysis. The nanomesh electrodes developed in our group consist of a regular 3D-network of interconnected nanowires with a large surface area of 26m²/cm³ and a porosity of 70%. [1] These exceptional properties result in >100x current enhancement for the hydrogen evolution reaction compared to planar nickel electrodes due to the accessibility of a high number of active surface sites and outperforms commercial nickel foams. The photovoltaic module and the electrolyzer were wire-connected over a custom-build monitoring unit that in-situ measures the amount of produced H₂, operating current & voltage of the system over the entire operation time. We demonstrate stable solar-to-hydrogen efficiency (η_STH) of 10% at highest electrolyzer current densities seen in literature (58mA/cm²) over >20h.

Based on the measured PV-EC system data best practices to accurately determine the η_STH for PV-powered and (photo)electrocatalytic water splitting devices and the validation of this benchmark against important component parameters for practical technology implementation are discussed.

Barium zirconium sulfide (BaZrS3) is an earth-abundant and environmentally friendly chalcogenide perovskite with promising properties for various energy conversion applications. Recently, sulfurization of oxide precursors has been suggested as a viable solution for effective synthesis, especially from the perspective of circumventing the difficulty of handling alkali earth metals. In this work, we explore in detail the synthesis of BaZrS₃ from Ba-Zr-O oxide precursor films sulfurized at temperatures ranging from 700 ^∘C to 1000 ^∘C. We propose a formation mechanism of BaZrS₃ based on a two-step reaction involving an intermediate amorphization step of the BaZrO₃ crystalline phase. We show how the diffusion of sulfur (S) species in the film is the rate-limiting step of this reaction. The processing temperature plays a key role in determining the total fraction of conversion from oxide to sulfide phase at a constant flow rate of the sulfur-containing H2S gas used as a reactant. Finally, we observe the formation of stoichiometric BaZrS₃ (1:1:3), even under Zr-rich precursor conditions, with the formation of ZrO₂ as a secondary phase. This marks BaZrS₃ quite unique among the other types of chalcogenides, such as chalcopyrites and kesterites, which can instead accommodate quite a large range of non-stoichiometric compositions. This work opens up a pathway for further optimization of the BaZrS₃ synthesis process, straightening the route towards future applications of this material.

Atomic layer deposition (ALD) and variants of the technique are ideally suited to the generation of ‘extremely thin absorber’ (ETA) solar cells, in which three distinct semiconductors are combined as electron transport layer (SnO₂, TiO₂, ZnO), light absorption layer (Sb₂S₃, Sb₂Se₃), and hole transport layer (V₂O₅ or spin-coated organics). We have demonstrated six main advantages.

(1) ALD can deliver a material quality significantly improved with respect to solution processing techniques, with crystals of 10 µm lateral size obtained in planar films of 50 nm thickness.

(2) It enables the experimentalist to vary the thickness of layers within the semiconductor stack systematically and identify the geometric parameters that optimize the overall energy conversion efficiency.

(3) It offers the opportunity to engineer interfaces with the use of interfacial layers providing chemical or physical functions (adhesion layers, tunnel barriers) with extreme thickness sensitivity, on the length scale of 0.5 to 1.5 nm.

(4) It provides the capability, unique among the deposition techniques from the gas phase, to deliver conformal coatings of non-planar substrates. Parallel arrays of cylindrical p-i-n heterojunctions in a coaxial geometry allow for decoupling the path lengths for light absorption and charge separation. Ordered monolayers of nanospheres can be exploited to scatter light of near-bandgap energy, enhancing the conversion of light incident in the red spectral range and/or under oblique angle.

(5) Our extension of ALD to the use of precursors dissolved in the liquid phase, instead of classically delivered from the gas phase (‘solution ALD’ or sALD) expands the range of materials accessible by ALD. It also provides additional experimental tools to adjust the deposit’s morphology, and it provides an inexpensive access to ALD.

(6) The recent invention of ‘atomic-layer additive manufacturing’ (ALAM) circumvents the limitations associated with traditional blanket layering methods. It opens the door to rapid prototyping approaches in the photovoltaics field that mirror the use of 3D printing in manufacturing.

In summary, each major material family exploited in photovoltaics has been associated with a certain set of processing techniques, and our research indicates that atomic-layer processing provides all ingredients required for the success of the group 5 chalcogenides and similar light absorbers.

The photovoltaic (PV) market has long been dominated by crystalline silicon-based technologies. Their success is rooted also in the laboratory, with record efficiencies ultimately approaching the theoretical limit for monocrystalline silicon [1]. Market competitors are Cu(In,Ga)(S,Se)₂ [2,3,4] (CIGS aka chalcopyrite) and CdTe [5,6,7,8] thin films. Currently, the commercial attractiveness of CIGS is curtailed by the module efficiency gap and by the supply of critical raw materials, with respect to silicon.

Overcoming the Shockley-Queisser efficiency gap requires the pursuit of strategic approaches, leaving plenty of room for research at both industrial and laboratory scale. Likewise, the availability of critical raw materials may constrain the future deployment of CIGS at GW scales needed for the ecological transition [9]. The progress on both fronts poses compelling scientific challenges.

This contribution starts with outlining the large opportunities offered by a concerted effort on wide-gap chalcopyrite. It then reveals a novel strategy making use of ferrofluids to explore the large scale fabrication of microconcentrator solar cells, promising to reduce critical raw materials needs by one to two orders of magnitude. Here, insights are drawn from recent and older literature on narrow-gap chalcopyrite with the ambition to fully unlock the potential of the material [10–12] and afford efficient dual junction chalcopyrite PV devices through a paradigm shift in microfabrication [13].