The ideal high-bandgap partner for a tandem solar absorber using c-Si as the low-bandgap bottom cell must have a bandgap of 1.6 eV to 1.9 eV and excellent optoelectronic properties.

Selenium, perhaps the very first material to be studied for its photovoltaic properties, is emerging as an interesting candidate for this application. Indeed Se has a bandgap of around 1.95 eV [1] and a steep increase in absorption above this photon energy. Todorov et al showed [2] in 2017 that single-junction Se solar cells could reach 6.5% efficiency using a new cell architecture with a very thin absorber layer. Recently we have increased the OCV to a new record of 991 mV [2] and mapped the main shortfalls of state-of-the-art Se-cells preventing them from approaching their theoretical potential OCV. The talk will introduce Se as a solar absorber and discuss we know about its properties and progress towards use in solar devices such as tandem photovoltaics and PEC stacks [3].

The high efficiency thin film technologies available or emerging (e.g., CIGS, CdTe or halide perovksites) have all issues in terms of cost, element abundance or long-term stability. Finding new solar absorber is a cumbersome process involving complex synthesis and characterization. First principles computations on the other hand can asses important solar absorber properties such as band gap, mobilities or defects and offer an attractive way to speed up this process. Here, we will report on a large scale high-throughput computational search for new solar absorbers among known inorganic materials. Importantly, the need for high carrier lifetime is taken into account by including in the screening intrinsic defects and their role as potential Shockley-Read-Hall recombination centers. Screening 30,000 known inorganic compounds, we identify a handful very promising solar absorbers. I will discuss the chemistries that we identified and highlight a few interesting new materials. I will especially focus on BaCd₂P₂, a new phosphide where our experimental follow-up work confirms the promising properties including adequate band gap but also long carrier lifetime and very high stability. Beyond BaCd₂P₂, our work highlights the discovery of an entire family of Zintl phosphides with exciting recent results on CaZn₂P₂ thin films. I will finish my talk highlighting the opportunities and challenges ahead in computationally-driven discovery of new solar absorbers.

The synthesis of multinary semiconductors for solar energy conversion applications such as kesterite (Cu₂ZnSn(S,Se)₄, CZTSSe) is extremely challenging due to the complexity of this type of compounds. Having multiple elements in their structure the formation of secondary phases, punctual or extended detrimental defects, and/or singular interfaces is commonly very problematic. In particular, quaternary kesterite-type compounds are not the exception, and all these detrimental issues explain why during almost 10 years the world record efficiency was unchanged. But, the very recent development of molecular inks route with special precursors, allows the accurate control of single kesterite phase with high crystalline quality. In addition, the use of selective diluted alloying has shown a high potential for minimizing detrimental punctual defects formation, contributing to increase the conversion efficiency record of kesterite based solar cells up to 15% in a short time.

This presentation will be focused first in demonstrating how the molecular inks synthesis route was of key relevance for the control of high quality single phase kesterite, through the modification of the synthesis mechanisms. The relevance of the composition of the ink, the precursor salts, and the interaction between the solvent and the cations in the solution is key for a reliable and reproducible high efficiency kestetite production baseline. Then, diluted alloying/doping strategies will be presented including Cu, Zn and Sn partial substitution with elements such as Ag, Li, Cd or Ge [1]. The positive impact of these cation substitutions will be discussed in regards of their impact on the kesterite quality, as well as on the annihilation of detrimental punctual defects, allowing for new efficiency records at 15% level.

Finally, very recent and innovative interface passivation strategies will be discussed, showing the pathway to increase the record efficiency beyond 20%.

The Internet of Things (IoT) is a rapidly growing ecosystem of billions of smart devices connected together via the cloud, embedding intelligence into infrastructure. But a significant challenge is the reliance of these devices on batteries as the power supply [1]. This talk explores the use of indoor photovoltaics (IPVs) as an alternative, either directly powering the IoT devices, or working in synergy with energy storage devices. Lead-halide perovskites currently demonstrate the highest power conversion efficiency (PCE), now approaching 45% [2]. However, the high lead content could act as a barrier to use in consumer electronics. This talk explores the development of IPVs from lead-free perovskite-inspired materials. We start with our early efforts on bismuth- and antimony-halide materials [3], discussing the challenges and opportunities with silver bismuth halide compounds [4], before covering our recent work with Sb₂S₃ IPV [5]. In the latter case, we achieve 17.55% PCE under indoor lighting, the highest yet reported for this material, and develop 5 cm² minimodules as a prototype to power a multisensory device. We finish with the broader challenges and opportunities of the emerging area of perovskite-inspired materials for IPVs.

Solar energy has experienced remarkable growth in the past decades, but innovative PV technologies and architectures must be developed to reduce the cost/watt utilization. Thin-film solar cells with high absorption coefficients and direct band gaps have emerged as promising candidates for achieving high efficiency at low cost. Thin-film solar cells, usually necessitate light-trapping strategies to reduce optical losses at the nanoscale, but similar nanoscale strategies can be used to reduce also the electrical losses. Recombination losses, Shunt, and Series resistances can be controlled by engineering the surface contact areas with appropriate nanopatterning designs. In this work, we explore the effects of reducing the junction area in thin film solar cells by using coupled 3D simulations based on FDTD, Schrodinger-Poisson solver, and Drift diffusion models to predict the performance of different stacks and 3D patterns.  As a case study, we focus on Zinc-Phosphide (Zn₃P₂) as an emerging earth-abundant, p-type absorber material with a direct band gap. Different n-type heterojunction partners such as Indium Phosphide (InP), Silicon  (Si), and Titanium Oxide (TiO₂) have been explored, and an intermediate Silicon oxide (SiO₂) layer with Zn₃P₂-filled holes in a periodic configuration have been introduced to modulate the n-contact area. The junction area fraction has been varied from 100% (no openings) down to 1.4% for each configuration, showing an overall improvement of the cell performance in efficiency due to a strong increase in the open circuit voltage. The dark JV curves, simulated and modeled with a 2/3Diode model, showed the linear relationship between saturation current and junction area fraction. The reverse saturation current, mainly represented by recombination mechanisms, scales down with the contact area. The beneficial effect of this method has been proved independently by the heterojunction contact leading to an increase of the V_oc from 0.07V up to 0.12 V. With this method Zn₃P₂ based solar cell with a record efficiency of 5.96%[1] can be potentially improved up 12-13% trying different kind of junctions and demonstrating the versatility of this approach.

The mitigation of climate change requires major transformations in the ways we generate energy and

operate technologies that release CO2. Photonic concepts and novel light-driven technologies provide many opportunities to mitigate CO2 emissions, transforming our current modes of energy use into more effective and sustainable ones. In a recent review paper, we describe several of these concepts that are in the early stage of scientific discovery, with at the same time great technological potential.1

In this presentation, we focus on how to create photovoltaics with improved properties that have potential for large-scale implementation. I will present an integrated near field/far-field multiple scattering formalism to control the absorption of light in multijunction solar cells. As a model system we use III-V/Si multi-junction solar cell and enhance the light trapping inside the silicon bottom cell by multiple scattering, creating a record photovoltaic energy conversion efficiency for silicon-based multijunction solar cells of 36.1%.2 A similar light trapping concept can be applied in other multijunction solar cell geometries, such as perovskite/silicon tandem solar cells.

We then present a study on the nanoscale incoupling of light in textured perovskite/silicon solar cells, and show how optical Mie resonances create strong light inhogeneities in the tandem solar cell that can affect its performance. In addition we create micron-scale light scattering structures in solar cells to enhance emission in the (far-)infrared to create passive radiative cooling, enhancing the efficiency of and long-term stability of the solar cell. Light-driven processes can also help fabricate novel photovoltaics materials, and we show our most recent work on laser-induced crystallization of methyl-ammonia lead iodide perovskite directly from solution, with the crystal formation monitored in-situ through photoluminescence and Raman spectroscopy.

I will also present the 900 M€ Dutch national research, innovation and industrial development program SolarNL, in which universities, research institutes, and companies work together to develop photovoltaics technology and industry to help create a fully sustainable energy generation system in our society by 2040.3

References

1)   Photonic solutions to fight climate change, G. Tagliabue, H.A. Atwater, A. Polman, and E. Cortes, Nature Photon. (2024), in press. See here for a preprint of this article.

2)   Wafer-bonded two-terminal III-V//Si triple-junction solar cell with power conversion efficiency of 36.1 % at AM1.5g, P. Schygulla, R. Müller, O. Höhn, M. Schachtner, D. Chojniak, A. Cordaro, S. Tabernig, B. Bläsi, A. Polman, G. Siefer, D. Lackner, and F. Dimroth, Progr. Photovolt. 32, 1-9 (2023); Nano-patterned back-reflector with engineered near-field/far-field light scattering for enhanced light trapping in silicon-based multi-junction solar cells, A. Cordaro, R. Müller, S. Tabernig, N. Tucher, P. Schygulla, O. Höhn, B. Bläsi, and A. Polman, ACS Photon. 10, 4061 (2023)

3)  SolarNL: www.solarnl.eu

Transparent Photovoltaic (TPV) technologies represent a promising branch within photovoltaics, seeking to expand their applications by overcoming challenges related to on-site integration, especially within architectural elements related to BIPV, and more recently, also in the areas of IoT and Agrivoltaics. Unlike conventional approaches solely focused on efficiency, TPV introduces two additional dimensions: transparency and aesthetics, which pose added challenges to the device architecture. Moreover, for TPV technologies to be translated into competitive products it is critical to work on sustainable and stable materials that at least meet the stringent requirements for any PV technologies in conjunction with the transparency and aesthetic value that allow for seamless integration.

This challenge is being actively investigated using different materials, such as organic materials and perovskites. However, oxide-based structures constitute a very attractive prospect as they can be integrated as different functional layers in the solar cell architecture (i.e. as absorber, Charge Transport Layer and transparent electrical contacts). Additionally, many binary or ternary oxide compounds present high bandgap, tuneable conductivity, low deposition temperatures and can be deposited by a plethora of techniques that are possible to upscale for industrial purposes. Another key aspect is that many oxide materials are stable, cheap and CRM-free. Given these aspects the challenge is on how to combine them in advanced device architectures (with other oxides or materials) to develop a final device that is efficient, transparent and aesthetically pleasing that can be integrated in architectural components (windows, canopies, façades) or even on devices that present low power draws such as smartphones or screens and IoT devices and sensors.

Herein, we will discuss the basic principles of TPV as well as the state of the art of oxide-based strategies. This includes two main approaches that are based on either the development of Zn(O.S) UV-selective absorbers and on the optimisation of oxide-based architectures integrating nanometric a-Si:H layers. Main challenges and late results achieved in both strategies will be reviewed, including the achievement of record devices with Light Utilisation Efficiency up to 1.3%, transparency in the range between 30% and 70% and photoconversion efficiencies up to 5%.

A method for improving the quality of heterointerfaces which has been increasingly investigated is selective area epitaxy (SAE) as it can help reduce interface defect formation and defect propagation between layers. SAE relies on a mask layer, such as silicon dioxide, patterned with (nano)holes where the growth is limited to under appropriate growth conditions. First, by reducing the area of the holes down to the nanoscale it can have a significant impact in the defect formation mechanisms, such as misfit dislocations, during epitaxial growth. Moreover, any threading dislocations will also be stopped by the mask layer, significantly reducing the amount propagating into the epilayer. SAE therefore holds potential in facilitating the epitaxial growth of materials lacking a lattice matched substrate as well as allowing for new material combinations for enhanced photovoltaic device performance. However, most of the techniques used for SAE using nanoscale holes rely on low-throughput techniques, such as molecular beam epitaxy and electron beam lithography. In our recent work we have shown how to overcome these limitations using e.g. metalorganic chemical vapour deposition (MOCVD) and Talbot Displacement lithography for the case of the earth-abundant photovoltaic absorber zinc phosphide (Zn₃P₂).

Zn₃P₂ is an emerging photovoltaic absorber for single-junction devices with a direct bandgap (1.5 eV) and other promising optoelectronic properties. However, its large lattice parameter and coefficient of thermal expansion has complicated its incorporation in heterojunctions, while the lack of controlled n-type doping has hindered the creation of homojunctions. Previous work has shown that SAE allows for high quality epitaxial growth of Zn₃P₂ nanopyramids and textured thin films. Unfortunately, the approach used relied on the aforementioned low-throughput techniques in addition to the use of scarce elements (In) in the substrate. In our recent work we have demonstrated how to overcome the first two limitations through the compatibility of SAE grown Zn₃P₂ with MOCVD, as well as scaling the nanopatterned areas using Talbot Displacement lithography. Through a combinatorial study we have explored the effect of temperature, precursor partial pressures and pitch on factors such as growth selectivity, defect formation and functional properties that were evaluated using a range of microscopy and spectroscopy techniques.

Zinc phosphide (Zn₃P₂) constitutes as a promising solar absorber material due to its high absorption and carrier moblity as well as due to the abundance of zinc and phosphorous in the earth crust. Since the first published studies few decades ago, the efficiencies of Zn₃P₂-based solar cells have remained below its potential. We believe this is mostly due to the limited understanding of how to tune its optoelectronic properties.

In this talk we report on the progress towards the understanding of the growth and functional properties of the material such as light absorption, carrier concentration and mobility [1-5]. We report selective area growth as the method that results into the highest quality material as well as with the highest conversion efficiency, beyond the previous published record ~6%. We finalize by providing the main design rules for next-generation Zn₃P₂-based heterojunction solar cells, which should allow us to go beyond the current conversion values.

Certain phosphorus-containing III-V semiconductors (GaP, InP and related alloys) are among the best-performing PV absorbers. Yet, there is hardly any other phosphide material that has received extensive attention for applications in PV or optoelectronics in general. Exciting progress has been reported within the family of Zn-based phosphide compounds (Zn₃P₂, ZnGeP₂, ZnSnP₂ etc.), but high PV efficiencies are yet to be demonstrated. In this talk, I will discuss two radically different classes of semiconductors that harness the unique ability of phosphorus to exist in a broad range of oxidation states.

The first class is “P-rich phosphides”. In stark contrast to almost any other compound semiconductor previously investigated for PV applications, P-rich phosphides contain bonds between nonmetallic atoms (in their specific case, phosphorus-phosphorus bonds). I will present the first successful thin-film synthesis [1] of any polycrystalline P-rich phosphide. The synthesized material is CuP₂, a 1.5 eV band gap semiconductor with strong optical absorption and native p-type doping in an attractive range for thin-film heterojunction solar cells.

A second intriguing family of materials can be obtained by combining phosphorus with a more electropositive and a more electronegative species. Of particular interest are “phosphosulfides”, where sulfur is the more electronegative species. Many phosphosulfides are predicted to be stable semiconductors with direct band gaps in the visible and disperse band edges. Yet, there are less than five reports of phosphosulfides in thin-film form and hardly any optoelectronic characterization [2].

Backed by a unique suite of combinatorial thin-film deposition setups with access to S and P sources, we have explored three ternary phosphosulfide phase diagrams by high-throughput experiments. In this process, we have synthesized several semiconducting compounds that were previously unknown or that had only been synthesized in bulk form. We will show that the photoluminescence decay time of some of these phosphosulfides is already above 100 ns, demonstrating that phosphosulfides also deserve close attention by the PV research community.

To understand how good (or bad) these early-stage PV materials are at their current development stage, I will discuss a recently proposed figure of merit to assess the quality of a generic PV absorber [3], [4].

From solar photovoltaics to batteries to sustainable fuels, many advances in renewable energy technology are enabled by the availability of high-quality functional materials. In the past decade, innovations such as computational materials design and combinatorial synthesis techniques have rapidly expanded our ability to predict new materials, screen for targeted properties, and identify promising candidates to accelerate the energy transition.

However, although the goal of many of these studies is to contribute to global sustainability, rarely do scientists explicitly include sustainability metrics in early-stages of the materials design process. Often, life cycle assessments (LCAs) are not performed until after a material or device has been scaled up to high technology readiness levels (TRLs), at which it may be too late to make significant changes (this is a phenomenon known as “technology lock-in”). The exclusion of life cycle design at early stages is likely not intentional, but rather because we have not yet established the tools and infrastructure to do so, and because of the disconnects between the fields of materials science and life cycle assessment.

In this talk, I will discuss the promises and challenges of integrating life cycle assessment into early-TRL materials design and discovery. I will give a brief overview of what LCA can and cannot do, share tools I have developed to connect the brightway LCA infrastructure to materials design infrastructure, and discuss case studies from my research on inorganic solar materials (focusing specifically on the role of process uncertainty). Lastly, we’ll look towards the future and explore how the materials scientist community can center life cycle thinking in our own research.

   Selenium is an elemental semiconductor with a wide bandgap appropriate for a range of optoelectronic and solar energy conversion technologies [1]. Developing high-performance selenium-based devices requires an in-depth understanding of both majority and minority carriers [2]. However, characterizing these carrier properties necessitates a wide range of experimental techniques with different sample configurations and illumination levels, complicating the analysis. This often results in discrepancies in the literature and values that fail to accurately reproduce experimental performance in device simulations. Thus, more reliable methods for extracting charge carrier information are highly sought after in the study of emerging optoelectronic materials.

   We study the properties of both carriers in selenium simultaneously using a high-sensitivity, variable temperature photo-Hall system with a rotating parallel dipole line (PDL) magnet [3]. These results are compared with those from other advanced characterization tools, including transient THz spectroscopy, capacitance-based techniques, and voltage-dependent quantum efficiency measurements. To address discrepancies, we construct semiconductor physics models to account for non-idealities at interfaces and surfaces, and assess the validity of commonly used assumptions in standard analysis models, such as complete ionization of acceptors and donors at room temperature. This study is complemented by device simulations, resulting in a unique combination of material properties for high-performance selenium photoabsorbers that accurately reproduce experimental JV-curves and EQE-spectra.

Developing new photovoltaic materials historically has been a time-consuming process, and only few materials have cleared the path into commercialization. It has also been shown that increasing device efficiency is strongly correlated with the number of publications [1] and thus also with the overall research effort in a particular material class. High-throughput synthesis and characterization can be a path to accelerate material development. Recently an empirical figure of merit (FOM) based on 8 material parameters has been proposed by Crovetto [2], to predict potential device efficiency based on measured or estimated materials. This can be useful tool to screen potential materials or direct research efforts. In all of this the application advanced characterization and interpretation of data plays an important role. Determining reliable numbers for carrier lifetime, doping density [3,4], mobility, or band offsets (among others) can be challenging, and misinterpretation of data or unsuitable measurement conditions may result in misguided conclusions about research priorities. We propose that development and publication of best practices [5] as well as numerical simulation-aided analysis methodologies in advanced characterization techniques can help to reduce unwanted variance in results and focus research efforts.

[1] Dale and Scarpulla, Solar Energy Materials and Solar Cells, 251 (2023) 112097

[2] Crovetto, J. Phys. Energy 6 (2024) 025009

[3] Hages, et al, Adv. En. Mat. 7 (2017), 1700167S

[4] Ravishankar, Unold, Kirchartz, Science 371 (2021), eabd8014

[5] Hempel et al, Adv. En. Mat. 12 (2022) 2102776

We fabricate Cu(In,Ga)(S,Se)2 (CIGS) solar cells using the two step-selenization method. First a 500 nm thick 10x multilayer of CuGa/In is sputtered on a soda-lime glass substrate with a Mo back contact layer, followed by evaporation of a 2 µm thick Se layer. This multi-stack is then annealed in a graphite box in a rapid-thermal annealing oven. During the anneal H₂S can be added and removed at different times during the anneal. In a first step we add H₂S in the first phase of the anneal to reduce Ga-diffusion towards the backside of the absorber layer and to include S in the bulk of the absorber layer.  We also add H₂S at the end of the anneal to create a higher band gap S-rich CIGS layer to reduce the front interface recombination. The final absorber layers show a minority carrier recombination time as measured by time resolved photoluminescence measurements of up to 100 ns. The absorber layers are then stored for a few days. Then they are cleaned in an ammonium sulfide solution 1 hour before the CdS top layer is deposited by chemical bath deposition. The solar cells are finished with sputtered ZnO and ITO transparent conductive oxides. The finished solar cells show a best total area efficiency of 16% under AM1.5G illumination. To find out where the major recombination in the devices is located, we performed bias- and temperature-dependent admittance spectroscopy measurements on the cells and compared the results to simulated admittance profiles of CIGS cells [1]. In addition to these devices, the admittance response of devices from Avancis was measured as well.

The results show two distinct recombination patterns, one present at room temperature in forward bias and the second becoming visible in the measurement range only at lower temperatures. The room temperature response showed a bias- and frequency-dependency which in simulations could only be replicated with interface defects. The bias and frequency-dependency of the second recombination response could be replicated with either bulk recombination or a barrier at the CIGS-CdS interface. A clear distinction between the two recombination channels could not be made as these two responses are very similar in the bias voltage versus measurement frequency space. Considering that the series resistance of the devices shows an exponential increase as the temperature is reduced, the barrier at the CIGS-CdS interface seems to be more likely as a cause. The activation energy of the interface defect is of the order of 200 meV, whereas the activation energy of the second recombination response was measured to be about 100 meV deep.

It appears therefore that further improvements to the power conversion efficiency of the devices should involve an improvement of the CIGS-CdS interface properties. This is also in agreement with observations that show that a short anneal at about 200°C after the CdS deposition can improve the fill factor of the devices strongly, likely due to some limited interdiffusion at the interface reducing the recombination behavior at that location.

Engineering of Single atom photocatalysts is a novel yet very challenging pathway that serves the frontline of catalysis field for over the last 10 years. As a result, intriguing performances have been seen due to their unique electronic structures and maximized atomic utilization. However, the major challenge lies in forming a desirable support surface that can allow stable single atom trapping with isolated dispersion of those active sites. So far many strategies for either single atom anchoring or support system have been reported but often with special requirements. Here in, we report, a simple electrochemical deposition approach for anchoring the single atoms in a controlled and desirable way. Titanium dioxide (TiO₂) nanotubes [TiNT] in general on annealing are known to have surface defects (like Ti³⁺-O_v [oxygen vacancy]) are capable of acting as a suitable traps for single atoms. Therefore, different amount of single atom dispersion have been achieved by electrochemically depositing copper single atoms (CuSA) on the TiNTs using CuCl₂ solution of concentration as low as 0.1 mM. Such SA decorated TiNTs have exhibited stronger driving force for photogenerated charge carrier separation and transfer, as a function of the amount of SA dispersion and its deposition condition. Consequently, CuSA/TiNTs have led to maximum photocurrent of 20 mA/cm², thereby attaining significant photoelectrochemical efficiency of 6% which is 1.2 times higher than the so far reported non noble metal based single atom photocatalysts. This study not only reveals the excellent ability of CuSAs to boost the overall charge carrier kinetics but also paves the way for designing advanced non noble metal based single atom photocatalysts that can attain remarkable efficiencies.