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Abstract Surface defects in semiconducting materials, though they have been widely studied, remain a prominent source of loss in optoelectronic devices; here we sought a new angle of approach, looking into the dynamic roles played by surface defects under atmospheric stressors and their chemical passivants in the lifetime of optoelectronic materials. We find that surface defects possess properties distinct from those of bulk defects. ab initio molecular dynamics simulations reveal a previously overlooked reversible degradation mechanism mediated by hydrogen vacancies. We find that dynamic surface adsorption affinity (DAA) relative to surface treatment ligands is a surrogate for passivation efficacy, a more strongly-correlated feature than is the static binding strength emphasized in prior reports. This guides us to design targeted passivator ligands with high molecular polarity: for example, 4-aminobutylphosphonic acid exhibits strong DAA and provides defect passivation applicable to a range of perovskite compositions, including suppressed hydrogen vacancy formation, enhanced photovoltaic performances and operational stability in perovskite solar cells.

Abstract Tin (Sn)‐containing perovskite solar cells (PSCs) have gained significant attention in the field of perovskite optoelectronics due to lower toxicity than their lead‐based counterparts and their potential for tandem applications. However, the lack of stability is a major concern that hampers their development. To achieve the long‐term stability of Sn‐containing PSCs, it is crucial to have a clear and comprehensive understanding of the degradation mechanisms of Sn‐containing perovskites and develop mitigation strategies. This review provides a compendious overview of degradation pathways observed in Sn‐containing perovskites, attributing to intrinsic factors related to the materials themselves and environmental factors such as light, heat, moisture, oxygen, and their combined effects. The impact of interface and electrode materials on the stability of Sn‐containing PSCs is also discussed. Additionally, various strategies to mitigate the instability issue of Sn‐containing PSCs are summarized. Lastly, the challenges and prospects for achieving durable Sn‐containing PSCs are presented.

Bandgap gradient is a promising approach to improve the open-circuit voltage in thin film solar cells. Here, authors incorporate a Cd(O,S,Se,Te) region to realize the bandgap gradient at front interface and demonstrate Cd(Se,Te) solar cells with reduced recombination and a champion efficiency of 20.03%.

Spatiotemporal dynamic of charge carriers is commonly studied with optical or photoconductivity measurements, yet these techniques come with their own limitations. To circumvent these limits, the authors probe the free-carrier diffusion dynamics of microsecond lifetimes via laser-illuminated microwave impedance microscopy.

Symbolic regression holds big promise for guiding materials design, yet its application in materials science is still limited. Here the authors use symbolic regression to introduce an activity descriptor predicting new oxide perovskites with improved oxygen evolution activity as corroborated by experimental validation.

Perovskite solar cells (PSCs) represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of the relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path toward commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and the progress made toward addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field, which covers a broad range of topics related to PSC commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and move forward toward addressing the outstanding challenges in this fast-developing field.

Despite the rapid progress on perovskite light emitting diodes (PeLEDs), the efficiency of blue PeLEDs is lagging behind. Here Wang et al. employ yttrium (III) chloride additive to yield enhanced photoluminescence in the perovskite materials and thus record high device efficiencies for sky-blue and blue PeLEDs.

Strong terahertz (THz) emission from the methylammonium lead iodide (MAPbI3) perovskite semiconductors has been observed following above-bandgap photoexcitation, yet local THz responses of crystalline microstructures are absent. We implement laser THz emission microscope (LTEM), yet-to-be applied to the perovskite semiconductors, as a novel and complementary tool to evaluate the electronic and grain heterogeneity of MAPbI3 thin films. Two MAPbI3 samples with different grain sizes are studied. Using this approach, we show that the one with a larger grain size gives more uniform THz radiation. More significant spatial THz intensity fluctuation is observed for the sample with a smaller grain size.

All-inorganic cesium lead bromide perovskite based light-emitting diodes show improved operational stability but the film quality limits their performance. Here Wang et al. use trifluoroacetate anions to passivate defects and achieve excellent device performance and stability.

We use cryogenic spatial–temporal photoluminescence (PL) imaging measurements down to 10 K and with short-wavelength, 405 nm laser excitation to study surface charge generation, trapping, and recombination at single bright and dark grains as well as their boundaries in model methylammonium lead iodide (MAPbI3) polycrystalline thin films. These salient conditions are shown to be critical for identifying both the detrimental and cooperative roles of grain microstructures where the dark grains serve as the PL quenching center, while the grain boundaries are largely benign and may promote electron–hole separation.

Optoelectronic devices based on organic–inorganic halide perovskites show promising performance, but their poor stability impedes the commercialization. Here Lin et al. show that excess free charges are detrimental and efficient charge-carrier extraction is necessary for improved device stability.

Summary: Electrodeposition (ED) technology is a low-cost industrial candidate for solar cell fabrication. However, the practical aspects of controlling deposit morphology and composition have not been significantly addressed because of the complex co-plating variables that still need to be understood for multinary alloy ED. This work addresses these practical aspects on how to control composition and deposit morphology using co-electrodeposited kesterite alloy precursors as a case study. The alloy precursors co-plated under the optimized conditions from a mixed thiosulfate-sulfite electrolyte bath show uniform, smooth, and compact film morphology as well as uniform distribution of composition, well suited for efficient kesterite absorbers, finally delivering a Cu2ZnSnS4 (CZTS) thin-film solar cell with 7.4% efficiency based on a configuration Mo/CZTS/CdS/ZnO/aluminum-doped ZnO. This work underscores that alloy ED, with the advantage of controllable composition and morphology, holds promise for low-cost industrial manufacture of thin-film solar cells. : Energy Materials; Energy Sustainability; Materials Chemistry Subject Areas: Energy Materials, Energy Sustainability, Materials Chemistry

In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.

Abstract Up to now, multijunction cell design is the only successful way demonstrated to overcome the Shockley–Quiesser limit for single solar cells. Perovskite materials have been attracting ever‐increasing attention owing to their large absorption coefficient, tunable bandgap, low cost, and easy fabrication process. With their rapidly increased power conversion efficiency, organic–inorganic metal halide perovskite‐based solar cells have demonstrated themselves as the most promising candidates for next‐generation photovoltaic applications. In fact, it is a dream come true for researchers to finally find a perfect top‐cell candidate in tandem device design in commercially developed solar cells like single‐crystalline silicon and CIGS cells used as the bottom component cells. Here, the recent progress of multijunction solar cells is reviewed, including perovskite/silicon, perovskite/CIGS, perovskite/perovskite, and perovskite/polymer multijunction cells. In addition, some perspectives on using these solar cells in emerging markets such as in portable devices, Internet of Things, etc., as well as an outlook for perovskite‐based multijunction solar cells are discussed.

Using a CdTe x Se1−x alloy in CdTe solar cells leads to better performances. Here, Poplawsky et al. show that the photoactive properties of the alloy depend on its chemical composition and structural properties, and derive a phase diagram to further optimize alloy growth for photovoltaic devices.

Atom probe tomography (APT) data acquired from a CAMECA LEAP 4000 XHR for the CdS/CdTe interface for a non-CdCl2 treated CdTe solar cell as well as the mass spectrum of an APT data set including a GB in a CdCl2-treated CdTe solar cell are presented. Scanning electron microscopy (SEM) data showing the evolution of sample preparation for APT and scanning transmission electron microscopy (STEM) electron beam induced current (EBIC) are also presented. These data show mass spectrometry peak decomposition of Cu and Te within an APT dataset, the CdS/CdTe interface of an untreated CdTe solar cell, preparation of APT needles from the CdS/CdTe interface in superstrate grown CdTe solar cells, and the preparation of a cross-sectional STEM EBIC sample. Keywords: Scanning electron microscopy, Atom probe tomography, Mass spectroscopy, Solar cells

The replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm2, and a fill factor of 74.0%.

Mixed tin (Sn) and lead (Pb) based perovskite thin films have been prepared by solution processing combining methylammonium lead iodide (MAPbI3) and formamidinium tin iodide (FASnI3) precursors. Optical response in the form of complex dielectric function (ε = ε1 + iε2) spectra and absorption coefficient (α) spectra of (FASnI3)1-x(MAPbI3)x based perovskite films have been extracted over a spectral range 0.74 to 5.89 eV using spectroscopic ellipsometry. Absorption band edge energy changes as a function of composition for films including FASnI3, MAPbI3, and mixed x = 0.20, 0.35, 0.40, and 0.6 (FASnI3)1-x(MAPbI3)x perovskites. (FASnI3)0.60(MAPbI3)0.4 is found to have the minimum absorption band edge energy near ∼1.2 eV.

Nature Communications 7: Article number: 12537 (2016); Published: 27 July 2016; Updated: 2 September 2016 In Fig. 6 of this article, the division between the zincblende and wurtzite phases is misplaced in the low temperature range. The correct version of Fig. 6 appears below as Fig. 1.

We report the band structures and excitonic properties of delafossites CuMO2 (M=Al, Ga, In, Sc, Y, Cr) calculated using the state-of-the-art GW-BSE approach. We evaluate different levels of selfconsistency of the GW approximations, namely G0W0, GW0, GW, and QSGW, on the band structures and find that GW0, in general, predicts the band gaps in better agreement with experiments considering the electron-hole effect. For CuCrO2, the HSE wave function is used as the starting point for the perturbative GW0 calculations, since it corrects the band orders wrongly predicted by PBE. The discrepancy about the valence band characters of CuCrO2 is classified based on both HSE and QSGW calculations. The PBE wave functions, already good, are used for other delafossites. All the delafossites are shown to be indirect band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV, in consistent with experimental findings. The excitation mechanisms are explained by examining the exciton amplitude projections on the band structures. Discrepancies compared with experiments are also addressed. The lowest and strongest exciton, mainly contributed from either Cu 3d→Cu 3p (Al, Ga, In) or Cu 3d→M 3d (M=Sc, Y, Cr) transitions, is always located at the L point of the rhombohedral Brillouin zone. [ABSTRACT FROM AUTHOR]

Zinc nanobelts and nanodisks are studied. It is demonstrated that ZnO nanobelts and metallic Zn nanobelts and nanodisks can be formed in-situ in a transmission electron microscope (TEM) by electron-beam irradiation of single-crystalline Zn nanoparticles.

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) and CdTe photovoltaic semiconductors have reached remarkable laboratory efficiencies. It is surprising that these thin-film polycrystalline solar cells can reach such high efficiencies despite containing a high density of grain boundaries (GBs), which would seem likely to be nonradiative recombination centers for photo-generated carriers. In this paper, we review our atomistic theoretical understanding of the physics of grain boundaries in CIGS and CdTe absorbers. We show that intrinsic GBs with dislocation cores exhibit deep gap states in both CIGS and CdTe. However, in each solar cell device, the GBs can be chemically modified to improve their photovoltaic properties. In CIGS cells, GBs are found to be Cu-rich and contain O impurities. Density-functional theory calculations reveal that such chemical changes within GBs can remove most of the unwanted gap states. In CdTe cells, GBs are found to contain a high concentration of Cl atoms. Cl atoms donate electrons, creating n-type GBs between p-type CdTe grains, forming local p-n-p junctions along GBs. This leads to enhanced current collections. Therefore, chemical modification of GBs allows for high efficiency polycrystalline CIGS and CdTe thin-film solar cells. [ABSTRACT FROM AUTHOR]

High efficiency CdTe solar cells typically use oxygenated CdS (CdS:O) window layers. We synthesize CdS:O window layers at room temperature (RT) and 270 °C using reactive sputtering. The band gaps of CdS:O layers deposited at RT increase when O2/(O2+Ar) ratios in the deposition chamber increase. On the other hand, the band gaps of CdS:O layers deposited at 270°C decrease as the O2/(O2+Ar) ratios increase. Interestingly, however, our high temperature closed-space sublimation (CSS) processed CdTe solar cells using CdS:O window layers deposited at RT and 270 °C exhibit very similar cell performance, including similar short-circuit current densities. To understand the underlying reasons, CdS:O thin films deposited at RT and 270 °C are annealed at temperatures that simulate the CSS process of CdTe deposition. X-ray diffraction, atomic force microscopy, and UV-visible light absorption spectroscopy characterization of the annealed films reveals that the CdS:O films deposited at RT undergo grain regrowth and/or crystallization and exhibit reduced band gaps after the annealing. Our results suggest that CdS:O thin films deposited at RT and 270 °C should exhibit similar optical properties after the deposition of CdTe layers, explaining the similar cell performance. [ABSTRACT FROM AUTHOR]

Aluminum doped zinc oxide (AZO) has attained prominence as being a very good transparent conducting oxide for optoelectronics and photovoltaic applications. In this work, we report on the synthesis and characterization of AZO films with c-axis preferred orientation using magnetron radio frequency (RF) sputtering. It was found that the degree of the c-axis preferred orientation can be controlled by varying the growth conditions, such as working pressure, RF power, and substrate temperatures. The preferred orientation increased as the working pressure decreased, while it increased as the RF power and substrate temperature increased. Electrical and optical measurements have revealed that the growth conditions and c-axis preferred orientation have strong influence on the physical properties of the synthesized AZO thin films. The thin films with increased c-axis preferred orientation exhibited enhanced carrier mobility. [ABSTRACT FROM AUTHOR]

CdCl2 activation near 400 °C is known to be critically important for obtaining high efficiency CdS/CdTe solar cells. However, this treatment step behaves differently on high-temperature-grown CdTe than on lower-temperature-grown CdTe layers such as those grown by sputtering. On sputtered films, the post-deposition activation produces grain-boundary passivation, sulfur diffusion into CdTe, and substantial grain growth. Nevertheless, we find the CdCl2 process for sputtered films to be characterized by a single activation energy that we interpret as applying to S diffusion into CdTe. We find this activation energy to hold for CdCl2 treatments from 370 to 440 °C. The completed CdS/CdTe solar-cell structures showed somewhat poorer initial performance with activation above 420 °C, but, in this case, the cell efficiency increased after accelerated life testing at 85 °C, open-circuit biasing and one-sun illumination. With an optimized CdCl2 activation process, the use of oxygenated sputtered CdS, and low-iron soda-lime glass, cell efficiencies of 14.5% were achieved. [ABSTRACT FROM AUTHOR]

The stability, transparency, and conductivity of ZnO are suggested to be tailored by alloying with MgO or CdO to meet wide applications. Our Monte Carlo simulation based on first-principle cluster expansion methods partially explain the solubility and stability data scattered in extensive experiments and further reveal that CdxZn1-xO has much higher solubility than prevalent MgxZn1-xO in a large range of experimentally achievable temperature (400K-1200K). Furthermore, first-principles calculations based on hybrid functional methods show that CdxZn1-xO has better n-type doping properties than MgxZn1-xO. The optical gap of CdxZn1-xO could be ~1.5 eV higher than its fundamental gap due to large Moss-Burstein shift. We thus predict that CdxZn1-xO has great potential to be a better transparent conducting oxide than MgxZn1-xO. [ABSTRACT FROM AUTHOR]

Recently, Cu delafossites, CuMO2, have received much attention as photo-catalysts for hydrogen production by water splitting due to their unique properties such as stability in most aqueous solutions and p-type conductivity with excellent hole mobility. However, due to their large optical bandgaps, they can absorb sun light only in the ultraviolet region. Hence, it is necessary to tailor their optical properties to enhance their optical absorption in the visible light regions. In this paper, we report on the effects of Bi alloying on the electronic and optical properties of Cu delafossites by density functional theory. We find that Bi alloying can lead to improved optical absorption as compared to the pristine Cu delafossites. We further find that the lone pair Bi-6s electrons create occupied delocalized anti-bonding states on the top of the valence band, leading to further improved hole mobility. [ABSTRACT FROM AUTHOR]

We report on the synthesis of amorphous copper tungsten oxide thin films with tunable band gaps. The thin films are synthesized by the magnetron cosputtering method. We find that due to the amorphous nature, the Cu-to-W ratio in the films can be varied without the limit of the solubility (or phase separation) under appropriate conditions. As a result, the band gap and conductivity type of the films can be tuned by controlling the film composition. Unfortunately, the amorphous copper tungsten oxides are not stable in aqueous solution and are not suitable for the application of photoelectrochemical splitting of water. Nonetheless, it provides an alternative approach to search for transition metal oxides with tunable band gaps. [ABSTRACT FROM AUTHOR]

We present a density-functional theory study on the electronic structure of pure and 3d transition metal (TM) (Sc, Ti, Cr, Mn, and Ni) incorporated α-Fe2O3. We find that the incorporation of 3d TMs in α-Fe2O3 has two main effects such as: (1) the valence and conduction band edges are modified. In particular, the incorporation of Ti provides electron carriers and reduces the electron effective mass, which will improve the electrical conductivity of α-Fe2O3. (2) The unit cell volume changes systematically such as: the incorporation of Sc increases the volume, whereas the incorporation of Ti, Cr, Mn, and Ni reduces the volume monotonically, which can affect the hopping probability of localized charge carriers (polarons). We discuss the importance of these results in terms of the utilization of hematite as a visible-light photocatalyst. [ABSTRACT FROM AUTHOR]

We present density-functional total-energy calculations to investigate the passivation effects of impurity elements such as Br, Cl, S, O, H, and Na on double-positioning twin boundaries in CdTe. We find that Br, Cl, S, and O atoms present passivation effects on the boundaries with different degrees, whereas H and Na do not passivate the boundaries. Of all these impurities, Cl atoms present the best passivation effects on the double-positioning twin boundaries. The superior passivation effects are realized by either terminating the Cd atoms with dangling bonds, or substituting the Te atoms with dangling bonds in the double-positioning twin boundaries in CdTe by Cl atoms. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Using the combination of high-resolution transmission electron microscopy, first-principles density-functional total-energy calculations, and image simulations, we studied the atomic structure and effects of double-positioning twin boundaries in CdTe. The structure for the double-positioning twin boundaries is found to contain more Te dangling bonds than Cd dangling bonds. The two sides of the boundaries have the same polarities. The structure produces energy states in the band gap that are detrimental to the electronic properties of CdTe. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Scanning Kelvin probe microscopy (SKPM) is used for examining cross sections of working CdTe/CdS solar cells. The distribution of the potential and electrical field inside a CdTe/Sn[O.sub.2] device, without the CdS layer, is examined and PoissonEs equation is used for estimating the doping inside the CdTe film in both devices.

Several first-principles studies are conducted to explain the mechanisms involved in the iron segregation into silicon [summation]5 grain boundary. The stress is shown to be the main factor deciding the choice of the segregation site.

The study employs the first-principle density-functional theory to explain the structure, as well as the various effects of the vacancies observed in the [delta]3 (112) grain boundaries in Si. These vacancies are shown to be extremely significant for the analysis of grain boundary physics and passivation behavior.

The synthesis of p-type ZnO films with similar bandgaps but varying carrier concentrations through codoping of Cu and Ga is described. The tuning of carrier concentration is realized by varying the Ga concentration, but the carrier concentration tuning has not changed the bandgap and crystallinity.

The preparation of p-type Zno thin films with significantly reduced band gaps by using Cu incorporation at room temperature followed by postdeposition annealing at 500 degree Celsius in air for 2 h is described.

Metallic zinc wires are synthesized by evaporating ZnO powder mixed with graphite in an NH3 flowing carrying gas environment. Most of the as-synthesized Zn nanowires are found to be single crystalline and free from structural defects.

The basic structural, optical, and electron-transport properties of rf magnetron sputter-deposited Zn2SnO4 (ZTO) films onto glass substrates are presented. The result shows that the relatively low electron effective mass in ZTO should contribute to potentially high mobilities, if higher-quality grain growth could be achieved.

Using ab initio density function theory total energy calculations, we have investigated the mechanism of Fe segregation into Si Σ5(310) grain boundary (GB). We find that the segregation is site selective at the GB-Fe will only segregate to specific sites. We further find that the choice of the segregation site is determined by the segregation-induced stress and effective crystal-field-induce splitting of Fe d orbital at that site. Our results suggest that the revealed mechanism of Fe segregation into the GB should be general for other 3d transition metals with partially filled 3d orbits and for other grain boundaries. [ABSTRACT FROM AUTHOR]

Mixed tin (Sn) and lead (Pb) based perovskite thin films have been prepared by solution processing combining methylammonium lead iodide (MAPbI3) and formamidinium tin iodide (FASnI3) precursors. Optical response in the form of complex dielectric function (ε = ε1 + iε2) spectra and absorption coefficient (α) spectra of (FASnI3)1-x(MAPbI3)x based perovskite films have been extracted over a spectral range 0.74 to 5.89 eV using spectroscopic ellipsometry. Absorption band edge energy changes as a function of composition for films including FASnI3, MAPbI3, and mixed x = 0.20, 0.35, 0.40, and 0.6 (FASnI3)1-x(MAPbI3)x perovskites. (FASnI3)0.60(MAPbI3)0.4 is found to have the minimum absorption band edge energy near ~1.2 eV. [ABSTRACT FROM AUTHOR]

The dynamics of partial dislocations in CdTe have been observed at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM), allowing the mobility of different dislocations to be directly compared: Cd-core Shockley partial dislocations are more mobile than Te-core partials, and dislocation cores with unpaired columns have higher mobility than those without unpaired columns. The dynamic imaging also provides insight into the process by which the dislocations glide. Dislocations with dangling bonds on unpaired columns are found to be more mobile because the dangling bonds mediate the bond exchanges required for the dislocations to move. Furthermore, a screw dislocation has been resolved to dissociate into a Shockley partialdislocation pair along two different directions, revealing a way for the screw dislocation to glide in the material. The results show that dynamic STEM imaging has the potential to uncover the details of dislocation motion not easily accessible by other means. [ABSTRACT FROM AUTHOR]