Wide-bandgap, chalcopyrite-type materials are attractive candidates for top cells of tandem-type solar cells because of their high absorption coefficients and excellent chemical stability. Nevertheless, achieving high opencircuit voltages (VOC) and high conversion efficiencies using wide-bandgap Cu(In,Ga)Se2 (CIGS) absorbers remains challenging. Conventionally, wide-bandgap CIGS solar cells are fabricated by depositing polycrystalline CIGS (poly-CIGS) layers with high [Ga] on glass substrates, and these poly-CIGS layers feature high densities of grain boundaries at which enhanced nonradiative recombination occurs. Epitaxial CIGS (epi-CIGS) solar cells do not contain any grain boundaries and can potentially overcome the VOC limitation. In the present study, we fabricated epi-CIGS layers with high [Ga] by molecular beam epitaxy, and compared the microstructural, compositional, and optoelectronic characteristics of poly-CIGS and epi-CIGS solar cells by electron backscatter diffraction, energy dispersive X-ray spectroscopy, cathodoluminescence, and secondary ion mass spectrometry. For poly-CIGS, Ga segregation occurred during three-stage processing, and layers with high [In] decreased the bandgap energies and the VOC. In contrast, uniformly distributed [Ga] were confirmed in the epi-CIGS layers. For epi-CIGS solar cells, a conversion efficiency of 14.5 % and VOC of 0.93 V were achieved using absorbers with a bandgap energy of 1.6 eV. The external quantum efficiency spectra of the epi-CIGS solar cells showed a sharp absorption edge, and their radiative and nonradiative VOC losses were lower than those of the poly-CIGS solar cells. These results suggest that CIGS solar cells with uniformly distributed [Ga] are optimal for top cells of tandem-type solar cells.

The present work reports on microscopic analyses of recombination at grain boundaries (GBs) in polycrystalline Li‐doped (Ag,Cu) 2 ZnSn(S,Se) 4 (Li‐ACZTSSe) and Cu 2 ZnSnS 4 (CZTS) absorber layers in high‐efficiency solar cells (conversion efficiencies of 14.4% and 10.8%). Recombination velocities s GB were determined at a large number of GBs by evaluating profiles extracted from cathodoluminescence intensity distributions across GBs in these polycrystalline layers. In both Li‐ACZTSSe and CZTS absorber layers, the s GB values exhibited wide ranges over several orders of magnitude with a median values of 680 and 1100 cm s −1 for the Li‐ACZTSSe and CZTS absorbers. A model that provides a comprehensive explanation for this finding is presented and discussed in detail. Correspondingly, wide ranges for s GB can be explained by different positive or negative excess charge densities present at different GBs, leading to different downward or upward band bending on the order of several ±10 meV, provided that the net‐doping density of the absorber layers is sufficiently large. As a result of the evaluation of the s GB , input parameters for multidimensional device simulations are obtained. It is revealed that the grain boundary lifetime closely matches the overall effective lifetime, indicating that grain boundary recombination is a key factor limiting the effective carrier lifetime of both Li‐ACZTSSe and CZTS absorbers. The estimated V OC losses due to GBs reach up to 126 mV for Li‐ACZTSSe and 88 mV for CZTS. This work highlights that reducing grain boundary recombination via improved passivation and increasing grain size is an effective strategy for achieving further efficiency improvements.

The present work revisits the recombination velocities (sGB) of minority-charge carriers determined at grain boundaries in polycrystalline absorber materials for solar cells. The equations describing sGB as well as the barriers for electrons and holes were derived. It is shown that for given net-doping density and absolute temperature, the experimentally determined recombination velocity of a specific grain boundary can be described by sGB=sGB,0nexp⁡[−ΦGB(NGB,charge)/(kBT)], where ΦGB is the band bending induced by the excess-charge density NGB,charge at the grain boundary, and kB as well as T are the Boltzmann constant and the absolute temperature; i.e., sGB depends only on the excess-charge density at this planar defect as well as on the prefactor sGB,0n describing the nonradiative recombination. Value ranges for these two quantities can be determined for any measured sGB value. When analyzing sGB datasets acquired on various (Ag,Cu)(In,Ga)Se2 and microcrystalline Si absorbers, it is apparent that both the excess-charge density and the prefactor sGB,0n remain within about the same orders of magnitude for all grain boundaries analyzed in a specific absorber. The broad range of the recombination velocities over several orders of magnitude indicates upward as well as downward band bending, and the band-bending values are on the order of several ±10 meV for all materials analyzed.

The present study reports on the structural changes and related photovoltaic properties resulting from the addition of Ag in Cu(In,Ga)Se _2 (CIGS)-based thin-film solar cells analysed using two-dimensional (2D) and three-dimensional (3D) analytical and modelling techniques. The microstructures of CIGS and Ag-containing CIGS (ACIGS) layers were characterized by means of 2D and 3D electron backscatter diffraction (EBSD). Ag addition significantly increased grain sizes, which is differently observed in the 2D and 3D EBSD analyses due to the different methodology. Moreover, opto-electronic 3D device simulations, which incorporate real 3D structural data obtained from 3D EBSD, confirmed that 2D models tend to underestimate the impact of grain boundaries on the device performance. Device simulations showed that the increased grain size and thus, the decreased grain-boundary density, resulting from Ag addition had a positive effect on the open-circuit voltage ( V _OC ) of the solar cell. The V _OC improvement was more pronounced in ACIGS layers of high minority-carrier lifetimes. These findings highlight the necessity of combining 3D EBSD analysis with 3D device simulations to optimize the relationship between microstructural characteristics and photovoltaic performance in solar cells to understand opto-electronic properties more quantitatively.

This study introduces the effect of growth atmosphere and pressure on the as-grown crystals and the physical characterization of the double perovskite compound Ba2NiWO6 (BNWO). The Ni2+ ions form a face-centered cubic (FCC) lattice with an ideal octahedral arrangement. Our growth trials by the floating zone method revealed that BNWO exhibits two distinct colors depending on the growth atmosphere, suggesting that it influences its stoichiometry and phase purity. The antiferromagnetic properties of BNWO align with conventional antiferromagnetic behavior with a J = 1 ground state, as determined by entropy calculation. Additionally, surface photovoltage (SPV) spectroscopy indicates that the impurity levels within the crystal influence the electronic properties of BNWO.

Based on the high power conversion efficiencies and compatibility toward large-area deposition techniques, Cu(In,Ga)(S,Se) _2 (CIGSSe) phototvoltaic absorbers are currently at the forefront of chalcopyrite thin-film solar cell technology. The performance of these solar cells is critically dependent on the properties of the interface between the p-type chalcopyrite absorber and the n-type buffer and window layers. Due to the complex defect physics of the chalcopyrites in general, the defect-electronic properties of the absorber surface is of particular concern. In this regard, the CIGSSe surfaces are considerably less understood compared with their S-free counterparts (e.g. Cu(In,Ga)Se _2 ). In the present work, by applying high-resolution scanning probe techniques such as atomic force microscopy and scanning tunneling spectroscopy (STS), combined with electron backscatter diffraction, the morphology, the crystallographic orientation, and the defect electronic properties of CIGSSe thin-film surfaces were investigated. Our work highlights distinct differences as well as similarities between S-containing and S-free chalcopyrite thin films. Three types of features were found on the CIGSSe surface, which were found to be exclusively made of polar facets. This is different from S-free absorbers that are known to facet in both, polar and non-polar planes with distinct electronic properties. Defect density mapping using STS revealed a highly defective surface with significant lateral inhomogeneities. Furthermore, grain boundary band bending detected in S-free absorber surfaces was absent. However, similar to S-free absorbers, annealing under ultra-high vacuum conditions was found to electronically passivate the CIGSSe surface. Our results shed light on the fundamental properties of these S-containing chalcopyrite-type surfaces and demonstrate a valuable platform for further optimization of this promising solar cell technology.

Polycrystalline materials are ubiquitous in technology, and grain boundaries have long been known to affect materials properties and performance. First principles materials modeling and electron microscopy methods are powerful and highly complementary for investigating the atomic scale structure and properties of grain boundaries. In this review, we provide an introduction to key concepts and approaches for investigating grain boundaries using these methods. We also provide a number of case studies providing examples of their application to understand the impact of grain boundaries for a range of energy materials. Most of the materials presented are of interest for photovoltaic and photoelectrochemical applications and so we include a more in depth discussion of how modeling and electron microscopy can be employed to understand the impact of grain boundaries on the behavior of photoexcited electrons and holes (including carrier transport and recombination). However, we also include discussion of materials relevant to rechargeable batteries as another important class of materials for energy applications. We conclude the review with a discussion of outstanding challenges in the field and the exciting prospects for progress in the coming years.

An essential step toward enabling the production of renewable and cost-efficient fuels is an improved understanding of the performance of energy conversion materials. In recent years, there has been growing interest in ternary metal oxides. Particularly, α-SnWO4 exhibited promising properties for application to photoelectrochemical (PEC) water splitting. However, the number of corresponding studies remains limited, and a deeper understanding of the physical and chemical processes in α-SnWO4 is necessary. To date, charge-carrier generation, separation, and transfer have not been exhaustively studied for SnWO4-based photoelectrodes. All of these processes depend on the phase composition, not only α-SnWO4 but also on the related phases SnW3O9 and WO3, as well as on their spatial distributions resulting from the coating synthesis. In the present work, these processes in different phases of tin tungstate films were investigated by transient surface photovoltage (TSPV) spectroscopy to complement the analysis of the applicability of α-SnWO4 thin films for practical PEC oxygen evolution. Pure α-SnWO4 films exhibit higher photoactivities than those of films containing secondary SnW3O9 and WO3 phases due to the higher recombination of charge carriers when these phases are present.

Polycrystalline widegap Cu(In,Ga)Se-2 (CIGSe) absorbers for top cells in photovoltaic tandem devices can be synthesized via [Ga]/([Ga] + [In]) (GGI) ratios of > 0.5. However, the power conversion efficiencies of such high-GGI devices are smaller than those of the record cells with GGI < 0.5. In the present work, the effects of the GGI ratio on various CIGSe material properties were studied and correlated with the radiative and nonradiative open-circuit voltage (V-OC) deficits of the thin-film solar cells. Average grain sizes, grain boundary (GB) recombination velocities, fluctuations in luminescence energy distribution, barrier heights at GBs, effective electron lifetimes, and Urbach energies were investigated in five solar cells with GGI ratios from 0.13 to 0.83. It was found that the GGI variation affects GB recombination velocities, fluctuations in spatial luminescence distributions, the average grain size, the electron lifetime, and the Urbach energy. In contrast, the detected ranges of barrier heights at GBs are independent of the GGI ratio. Mainly Ga/In gradients give rise to substantial radiative V-OC losses in all solar cells. Nonradiative V-OC deficits are dominant especially for solar cells with GGI > 0.5, which can be attributed to low bulk lifetimes and enhanced recombination at GBs in CIGSe absorbers in this compositional range.

The present work provides an overview of radiative performance losses in thin-film solar cells, focusing on those related to the open-circuit voltage, using (Ag,Cu)(In,Ga)Se(2 )devices as examples. The microscopic origins of these losses are outlined, highlighting the presence of compositional variations, strain, and inhomogeneously distributed point defects on various length scales as contributors to band-gap and electrostatic potential fluctuations, which both contribute to the broadening of the absorption edge in the absorptance or quantum efficiency spectra of the semiconductor absorber layer or the completed solar-cell device. The relationship between this broadening and Urbach tails is discussed. It is shown that the photovoltaic band-gap energy as well as the broadening can be reliably determined from the arithmetic mean and standard deviation extracted from Gaussian fits to the first derivative of the absorptance or quantum efficiency spectra around the absorption edge. The more enhanced the broadening, the more the local maximum in the luminescence spectrum shifts to smaller energies with respect to the band-gap energy of the absorber layer, as verified for about 30 (Ag,Cu)(In,Ga)Se-2 solar cells.

Ag alloying of Cu(In,Ga)Se-2 (CIGSe) absorbers in thin-film solar cells leads to improved crystallization of these absorber layers at lower substrate temperatures than for Ag-free CIGSe thin films as well as to enhanced cation interdiffusion, resulting in reduced Ga/In gradients. However, the role of Ag in the microscopic structure-property relationships in the (Ag,Cu)(In,Ga)Se-2 thin-film solar cells as well as a correlation between the various microscopic properties of the polycrystalline ACIGSe absorber and open-circuit voltage of the corresponding solar cell device has not been reported earlier. In the present work, we study the effect of Ag addition by analyzing the differences in the various bulk, grain-boundary, optoelectronic, emission, and absorption-edge properties of ACIGSe absorbers with that of a reference CIGSe absorber. By comparing thin-film solar cells with similar band-gap energies ranging from about 1.1 to about 1.2 eV, we were able to correlate the differences in their absorber material properties with the differences in the device performance of the corresponding solar cells. Various microscopic origins of open-circuit voltage losses were identified, such as strong Ga/In gradients and local compositional variations within individual grains of ACIGSe layers, which are linked to absorption-edge broadening, lateral fluctuations in luminescence-energy distribution, and band tailing, thus contributing to radiative V-OC losses. A correlation established between the effective electron lifetime, average grain size, and lifetime at the grain boundaries indicates that enhanced nonradiative recombination at grain boundaries is a major contributor to the overall V-OC deficit in ACIGSe solar cells. Although the alloying with Ag has been effective in increasing the grain size and the effective electron lifetime, still, the Ga/In gradients and the grain-boundary recombination in the ACIGSe absorbers must be reduced further to improve the solar-cell performance.

A new method for efficiently converting electron backscatter diffraction data obtained using serial sectioning by focused ion beam of a polycrystalline thin film into a computational, three-dimensional (3D) structure is presented. The reported data processing method results in a more accurate representation of the grain surfaces, reduced computer memory usage, and improved processing speed compared to traditional voxel methods. The grain structure of a polycrystalline absorption layer from a high-efficiency Cu(In,Ga)Se2 solar cell (19.5%) is reconstructed in 3D and the grain size and surface distribution is investigated. The grain size distribution is found to be best fitted by a log-normal distribution. We further find that the grain size is determined by the [Ga]/([Ga] + [In]) ratio in vertical direction, which was measured by glow discharge optical emission spectroscopy. Finally, the 3D model derived from the structural information is applied in optoelectronic simulations, revealing insights into the effects of grain boundary recombination on the open-circuit voltage of the solar cell. An accurate 3D structure like the one obtained with our method is a prerequisite for a detailed understanding of mechanical properties and for advanced optical and electronic simulations of polycrystalline thin films.

A dimethylformamide (DMF) and thiourea (TU)‐based ink deposition route is used to fabricate narrow bandgap (≈1.0 eV) CuIn(S,Se) 2 (CISSe) films with Cu‐poor ([Cu]/[In] = 0.85), stoichiometric ([Cu]/[In] = 1.0), and Cu‐rich ([Cu]/[In] = 1.15) compositions for photovoltaic applications. Characterization of KCN‐ or (NH 4 ) 2 S‐treated Cu‐rich absorber films using X‐ray diffraction and scanning electron microscopy confirms the removal of copper‐selenide phases from the film surface, while electron backscatter diffraction measurements and depth‐dependent energy‐dispersive X‐ray spectroscopy indicate remnant copper‐selenides in the absorber layer bulk. Contrary to best practice for vacuum‐processed cells, optimum [Cu]/[In] ratios appear to be stoichiometric, rather than Cu‐poor, in DMF–TU‐based CISSe devices. Accordingly, stoichiometric film compositions yield large‐grained (≈2 μm) absorber layers with smooth absorber surfaces (root mean square roughness <20 nm) and active area device efficiencies of 13.2% (without antireflective coating). Notably, these devices reach 70.0% of the Shockley–Queisser limit open‐circuit voltage (i.e., 526 mV at E g of 1.01 eV), which is among the highest for ink‐based CISSe devices.

We investigated triple-halide perovskite (THP) absorber layers with 5 mol % MAPbCl3 added to the double-halide perovskite (Cs0.22FA0.78)Pb(I0.85Br0.15)3. As a deposition method, a highly scalable printing technique, slot-die coating, with a subsequent annealing step was used. We found a strong power conversion efficiency (PCE) dependence of the corresponding solar cells on the annealing temperature. The device performance deteriorated when increasing the annealing temperature from 125 to 170 °C, mainly via losses in the open-circuit voltage (Voc) and in the fill factor (FF). To understand the mechanisms behind this performance loss, extensive characterizations were performed on both, the THP thin films and the completed solar-cell stacks, as a function of annealing temperature. Correlative scanning electron microscopy analyses, i.e., electron backscatter diffraction, energy-dispersive X-ray spectroscopy, and cathodoluminescence, in addition to X-ray diffraction and photoluminescence, confirmed the presence of PbI2 platelets on the surface of the THP thin films. Moreover, the area fraction of the PbI2 platelets on the film surface increased with increasing annealing temperature. The deteriorated device performance when the annealing temperature is increased from 125 to 170 °C is explained by the increased series resistance and increased interface recombination caused by the PbI2 platelets, leading to decreased Voc and FF values of the solar-cell devices. Thus, the correlative analyses provided insight into microscopic origins of the efficiency losses.

MoS2 is a two-dimensional layered transition metal dichalcogenide with unique electronic and optical properties. The fabrication of ultrathin MoS2 is vitally important, since interlayer interactions in its ultrathin varieties will become thickness-dependent, providing thickness-governed tunability and diverse applications of those properties. Unlike with a number of studies that have reported detailed information on direct bandgap emission from MoS2 monolayers, reliable experimental evidence for thickness-induced evolution or transformation of the indirect bandgap remains scarce. Here, the sulfurization of MoO3 thin films with nominal thicknesses of 30 nm, 5 nm and 3 nm was performed. All sulfurized samples were examined at room temperature with spectroscopic ellipsometry and photoluminescence spectroscopy to obtain information about their dielectric function and edge emission spectra. This investigation unveiled an indirect-to-indirect crossover between the transitions, associated with two different Λ and K valleys of the MoS2 conduction band, by thinning its thickness down to a few layers.

Metal oxides are considered as stable and low‐cost photoelectrode candidates for hydrogen production by photoelectrochemical solar water splitting. However, their power conversion efficiencies usually suffer from poor transport of photogenerated charge carriers, which has been attributed previously to a variety of effects occurring on different time and length scales. In search for common understanding and for a better photo‐conducting metal oxide photoabsorber, CuFeO 2 , α‐SnWO 4 , BaSnO 3 , FeVO 4 , CuBi 2 O 4 , α‐Fe 2 O 3 , and BiVO 4 are compared. Their kinetics of thermalization, trapping, localization, and recombination are monitored continuously 100 fs–100 µs and mobilities are determined for different probing lengths by combined time‐resolved terahertz and microwave spectroscopy. As common issue, we find small mobilities < 3 cm 2 V ‐1 s ‐1 . Partial carrier localization further slows carrier diffusion beyond localization lengths of 1–6 nm and explains the extraordinarily long conductivity tails, which should not be taken as a sign of long diffusion lengths. For CuFeO 2 , the localization is attributed to electrostatic barriers that enclose the crystallographic domains. The most promising novel material is BaSnO 3 , which exhibits the highest mobility after reducing carrier localization by annealing in H 2 . Such overcoming of carrier localization should be an objective of future efforts to enhance charge transport in metal oxides.

Epitaxially grown Cu(In,Ga)Se 2 (CIGS) absorber layers were analyzed by various techniques in scanning electron microscopy in order to reveal microstructure-property relationships in these thin films. Owing to their epitaxial nature, these CIGS absorber layers do not contain any grain boundaries, but only anti-phase domains (APDs) and dislocations. By combining electron channeling-contrast imaging, electron backscatter diffraction, and cathodoluminescence (CL), in some cases on identical specimen positions of polished cross-sections of CIGS/Mo/glass stacks, it was possible to correlate the presence and orientations of APDs and dislocations with the lateral distributions of the CL intensity and emission-peak energy. We studied CIGS layers with three different [Ga]/([Ga]+[In]) ratios as well as with and without NaF/KF treatments. Considerable differences between the CIGS layer properties in the microstructure-property relationships were found, depending on the growth parameters. Dislocations in the epitaxial CIGS layers do not tend to exhibit strong CL intensity decreases, which contrasts with the situation in numerous other semiconductor materials.

Ag alloying of CIGSe absorbers in thin film solar cells provides the means to fabricate absorber layers with slightly wider band-gap energies at lower substrate temperatures than for CIGSe layers without Ag. In the present study, three solar cells with ACIGSe photoabsorber of same band-gap energies of about 1.2 eV exhibit different open-circuit voltages Voc. The influence of the microscopic properties of the ACIGSe absorber on the Voc of the solar cell was investigated. Several characterization techniques in scanning electron microscopy were applied in a correlative manner on the identical specimen positions in addition to time-resolved photoluminescence and external quantum efficiency measurements. Differences in microstructural and optoelectronic properties such as average grain size, effective electron lifetime, absorption edge broadening by compositional gradients, and fluctuations in the spatial luminescence distribution were identified as origins of radiative and nonradiative loss mechanisms.