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Summary Recently, most of the highly efficient perovskite optoelectronic devices have been reported to employ effective surface passivation strategies, further confirming the significance of surface states in regulating their device performance. Therefore, an in-depth understanding and a systematic approach toward comprehensive investigations on perovskite surface states are urgently required. Here, we present methodical studies toward understanding the surface states in perovskite thin films utilizing a molecular “positively charged defect indicator” strategy. In formamidinium (FA)-methylammonium (MA) mixed-cation perovskite thin films, a nonuniform distribution of cations is uncovered with FA cations being close to the top and MA close to the bottom of the film, which leads to unique surface defect energetics. Antisite FAI was found to become a dominant deep trap on the surface of this system. As a result, a surface recombination velocity as low as 20 cm/s was achieved in such FA-based perovskite photovoltaic devices.

Intrinsic ion migration in the metal halide perovskite (MHP) absorber layer and its interfaces seriously limits the device stability of perovskite solar cells (PSCs). Despite considerable efforts to mitigate the ion migration issue, it remains a formidable challenge in the commercialization of PSCs. Here, we provide a short review of the device failure mechanisms induced by intrinsic ion migration and discuss the detrimental effects of ion migration on the different component layers of PSCs. We outline the corresponding strategies to mitigate ion migration in PSCs and provide an insight on materials engineering to attain long-term stabilized perovskite photovoltaics (PVs).

CdTe solar cell technology is one of the lowest-cost methods of generating electricity in the solar industry, benefiting from fast CdTe absorber deposition, CdCl2 treatment and Cu doping. However, Cu doping has low photovoltage and issues with instability. Doping group V elements into CdTe is therefore a promising route to address these challenges. Although high-temperature in situ group V doped CdSeTe devices have demonstrated efficiencies exceeding 20%, they face obstacles including post-deposition doping activation processes, short carrier lifetimes and low activation ratios. Here, we demonstrate low-temperature and effective ex situ group V doping for CdSeTe solar cells using group V chlorides. For AsCl3 doped CdSeTe solar cells, the dopant activation ratio can be 5.88%, hole densities reach >2 × 1015 cm−3 and carrier lifetime is longer than 20 ns. Thus, ex situ As doped CdSeTe solar cells show open-circuit voltages ~863 mV, compared to the highest open-circuit voltage of 852 mV for Cu doped CdSeTe solar cells. Doping CdTe solar cells with group V elements could overcome the limitations in voltage output and device stability of copper doping, yet implementation remains challenging. Now, Li et al. have devised an ex situ doping approach that is based on group V chloride solutions and low-temperature annealing.

Sb2Se3 has become a promising absorber layer material for photovoltaic applications because of its attractive optical properties and rapid improvement in Sb2Se3 solar cell efficiency in recent years. The indirect bandgap and Urbach energy of Sb2Se3 have been determined to be 1.12 eV and 21.1 meV, respectively, using photothermal deflection spectroscopy. Above bandgap critical points at 1.48, 2.00, 2.35, 2.87, and 3.86 eV are identified from the complex dielectric function (e = e1 + ie2) spectra of thin film Sb2Se3 obtained from spectroscopic ellipsometry. Spectra in e serve as input for external quantum efficiency (EQE) simulations of substrate type Sb2Se3 solar cells. Comparison of experimental and simulated EQE quantifies carrier collection losses and identifies a carrier collection length in the absorber layer of 439 ± 4 nm and a 96.4 ± 0.2 % carrier collection probability near the heterojunction.

Antimony selenide (Sb2Se3) solar cells are an environmentally friendly and cost-effective photovoltaic technology. In this work, we fabricate Sb2Se3 solar cells using closed-space sublimation follo...

In this study, we demonstrate a new hybrid three-dimensional (3D) nanostructure system as an efficient hole transport layer (HTL) by a facile design of a low-temperature solution process. It is realized by integrating high-conductive chromium-doped CuGaO2 nanoplates synthesized with choline chloride (denoted as Cr/CuGaO2-CC) into ultrasmall NiOx nanoparticles. First, we propose to incorporate a Cr-doped strategy under hydrothermal synthesis conditions together with controllable intermediates and surfactants' assistance to synthesize fine-sized Cr/CuGaO2-CC nanoplates. Subsequently, these two-dimensional (2D) nanoplates serve as the expressway for improving hole transportation/extraction properties. Meanwhile, the ultrasmall-sized NiOx nanoparticles are employed to modify the surface for achieving unique surface properties. The HTL formed from the designed hybrid 3D-nanostructured system exhibits the advantages of smooth and full-covered surface, remarkable charge collection efficiency, energy level alignment between the electrode and perovskite layer, and the promotion of perovskite crystal growth. Consequently, nearly 20% of power conversion efficiency with negligible hysteresis is achieved in inverted perovskite solar cells (PSCs). This work not only demonstrates the potential applications of a 3D-nanostructured Cr/CuGaO2-CC/NiOx hybrid HTL in PSCs but also provides a fundamental insight into the design of hybrid material systems by manipulating electric behavior and morphology structure for achieving high-performance photovoltaic devices.

Electrical doping (that is, intentional engineering of carrier density) underlies most energy-related and optoelectronic semiconductor technologies. However, for the intensely studied halide perovskite family of semiconductors, reliable doping remains challenging, owing to, for example, compensation from and facile migration of intrinsic defects. In this Review, we first discuss the underlying fundamentals of semiconductor doping and then investigate different doping strategies in halide perovskites, including intrinsic defect, extrinsic defect and charge transfer doping, from an experimental as well as a theoretical perspective. We outline the advantages and pitfalls of different characterization techniques to assess doping and examine the impact of doping on optoelectronic properties. Finally, we highlight challenges that need to be overcome to gain control over the electronic properties of this important material class. Halide perovskites exhibit outstanding semiconductor properties and are a key component of a variety of devices, including solar cells and light-emitting diodes. This Review discusses electrical doping strategies for halide perovskites and takes a critical look at the challenges that need to be overcome to control the electronic properties of these semiconducting materials.

Perovskite solar cell device performance is affected by optical and electronic losses. To minimize these losses in solar cells, it is important to identify their sources. Here, we report the optical and electronic losses arising from physically mixed interfacial layers between the adjacent component materials in highly efficient two terminal (2T) all-perovskite tandem, single-junction wide-bandgap, and single-junction narrow-bandgap perovskite-based solar cells. Physically mixed interfacial layers as the sources of optical and electronic losses are identified from spectroscopic ellipsometry measurements and data analysis followed by comparisons of simulated and measured external quantum efficiency spectra. Parasitic absorbance in the physically mixed regions between silver metal electrical contacts and electron transport layers (ETLs) near the back contact and a physical mixture of commercial indium tin oxide and hole transport layers (HTL) near the front electrical contact lead to substantial optical loss. A lower-density void + perovskite nucleation layer formed during perovskite deposition at the interface between the perovskite absorber layer and the HTL causes electronic losses because of incomplete collection of photogenerated carriers likely originating from poor coverage and passivation of the initially nucleating grains.

An accurate assessment of bifacial solar cells under concurrent bifacial illumination is critical to evaluate their real photovoltaic performance. In this work, we demonstrate bifacial perovskite solar cells with bifacial equivalent power conversion efficiencies of 21% and 26% under concurrent bifacial illumination with an albedo of 0.2 and 0.5, respectively. A detailed balance efficiency limit analysis further reveals the full potential of bifacial perovskite solar cells.

Although back-surface passivation plays an important role in high-efficiency photovoltaics, it has not yet been definitively demonstrated for CdTe. Here, we present a solution-based process, which achieves passivation and improved electrical performance when very small amounts of oxidized Al3+ species are deposited at the back surface of CdTe devices. The open circuit voltage (Voc) is increased and the fill factor (FF) and photoconversion efficiency (PCE) are optimized when the total amount added corresponds to ∼1 monolayer, suggesting that the passivation is surface specific. Addition of further Al3+ species, present in a sparse alumina-like layer, causes the FF and PCE to drop as the interface layer becomes blocking to current flow. The optimized deposit increases the average baseline PCE for both Cu-free devices and devices where Cu is present as a dopant. The greatest improvement is found when the Al3+ species are deposited prior to the CdCl2 activation step and Cu is employed. In this case, the best-cell efficiency was improved from 12.6 to 14.4%. Time-resolved photoluminescence measurements at the back surface and quantum efficiency measurements performed at the maximum power point indicate that the performance enhancement is due to a reduction in the interface recombination current at the back surface.

The effects of post-deposition CdCl2 annealing temperature on the electronic properties of CdTe solar cells were investigated. CdTe devices incorporate Mg doped ZnO as a buffer layer and selenization using a CdSe interlayer for reducing the buffer/absorber interface recombination and increasing solar energy absorption respectively. The post-deposition CdCl2 annealing treatments were done under separate, inert atmospheres of nitrogen and helium across the temperature range 380–430 °C. Electrical characterization of devices is carried out including temperature dependent current-voltage characteristics, admittance spectroscopy, and Shockley-Read-Hall recombination analysis. The best improvements in device efficiency are obtained upon annealing at temperature 410 °C. This anneal correlated with reduced back contact barrier in CdTe and reduced grain-boundary barrier height which is beneficial for enhanced charge transport.

High-performance perovskite/perovskite tandem solar cells require high-efficiency and stable low-bandgap perovskite subcells. State-of-the-art low-bandgap mixed tin–lead iodide perovskite solar cells exhibit either a high power-conversion efficiency or improved stability, but not both. Here we report a two-step bilayer interdiffusion growth process to simultaneously meet both requirements for formamidinium-based low-bandgap mixed tin–lead iodide perovskite solar cells. The bilayer interdiffusion growth process allows for the formation of high-quality and large-grained perovskite films with only 10 mol% volatile methylammonium. Additionally, one-dimensional pyrrolidinium perovskite was applied to passivate the perovskite film and improve the junction quality, which resulted in a carrier lifetime of 1.1 μs and an open circuit voltage of 0.865 V for our perovskite film and device with a bandgap of 1.28 eV. Our strategies enabled a power-conversion efficiency of 20.4% for low-bandgap perovskite solar cells under AM 1.5G illumination. More importantly, an encapsulated device can retain 92% of its initial efficiency after 450 h of continuous 1 sun illumination. Low-bandgap tin–lead perovskites are key to all-perovskite tandem solar cells but simultaneous improvement in efficiency and stability has proven challenging. Now, Li et al. fabricate tin–lead perovskite cells with reduced methylammonium content that are 20.4% efficient and stable under illumination for 450 h.

Summary With rapid progress in perovskite light-emitting diodes (PeLEDs), the electroluminescence performance of large-area is of increasing interest. We investigated why large-area performance lags behind that achieved in laboratory-scale devices and found that defects in perovskite films—emerging from thermal convection during solvent evaporation, as well as electronic traps formed during perovskite crystallization—are chief causes. Here, we report a molecular modification strategy that simultaneously eliminates pinholes in perovskite layers by controlling the dynamics of film formation and that passivates defects in perovskites by incorporating Br species, thereby preventing shorts and non-radiative recombination. The molecular modifier 1,3,5-tris (bromomethyl) benzene (TBB) also modulates the electronic structure of injection or transport materials to achieve improved charge injection and balanced charge transport. As a result, we demonstrate 20 mm × 20 mm green perovskite nanocrystal LEDs that achieve an external quantum efficiency (EQE) of over 16%, a record for large-area PeLEDs.

The advent of the two-dimensional (2D) family of halide perovskites and their demonstration in 2D/three-dimensional (3D) hierarchical film structures broke new ground toward high device performance and good stability. The 2D Dion-Jacobson (DJ) phase halide perovskites are especially attractive in solar cells because of their superior charge transport properties. Here, we report on 2D DJ phase perovskites using a 3-(aminomethyl)piperidinium (3AMP) organic spacer for the fabrication of mixed Pb/Sn-based perovskites, exhibiting a narrow bandgap of 1.27 eV and a long carrier lifetime of 657.7 ns. Consequently, solar cells employing mixed 2D DJ 3AMP-based and 3D MA

Surface treatment using large alkyl/aryl ammonium cations has demonstrated reduced open-circuit voltage (VOC) deficits in perovskite solar cells (PSCs), but the origin of the improvements has been vaguely attributed to defect passivation. Here, we combine microscopic probing of the local electrical properties, thermal admittance spectroscopic analysis, and first-principles calculations to elucidate the critical role of arylammonium interface layers in suppressing ion migration in wide-bandgap (WBG) PSCs. Our results reveal that arylammonium surface treatment using phenethylammonium iodide increases the activation energy barrier for ion migration on the surface, which suppresses the accumulation of charge defects at surface and grain boundaries, leading to a reduced dark saturation current density in WBG PSCs. With device optimization, our champion 1.73 eV PSC delivers a power conversion efficiency of 19.07% with a VOC of 1.25 V, achieving a VOC deficit of 0.48 V.

In a quest for Pb-free perovskites suitable for solar energy applications, Cs2TiBr6 has recently been reported as a promising compound, with appropriate optical and electrical properties as well as high stability under environmental stresses. In this study, we pursue investigation on this compound, demonstrating phase pure Cs2TiBr6 powder formation using solution synthesis and providing complementary experimental characterization and theoretical calculations. An experimental absorption onset of around 2.0 eV is extracted and a weak broad photoluminescence is measured. Density functional theory calculations predict an indirect bandgap, parity-forbidden for both the direct and indirect transitions, which explains the weak and Stokes shifted luminescence. Additionally, we highlight the strong instability of Cs2TiBr6 powder in ambient atmosphere. Therefore, our experimental results supported by theoretical calculations differ from previous results and raise doubts on the suitability of Cs2TiBr6 in its pristine form for solar energy applications.

Nickel oxide (NiOx) as a hole-transporting layer (HTL) in perovskite solar cells (PSCs) has been studied extensively in recent years. However, unlike the solution-processed NiOx films, magnetron sputtered NiOx exhibits relatively low conductivity and imperfect band alignment with perovskites, severely limiting the device performance of PSCs. In this study, a synergistically combined strategy consisting of triple interface treatments – including post-annealing, O2-plasma, and potassium chloride treatments – is employed to modulate the optoelectronic properties of the sputtered NiOx films. Through this approach, we successfully obtained NiOx films with increased carrier density and conductivity, better energy level alignment with the perovskite absorber layer, reduced interface trap density, and improved interfacial charge extraction. PSCs using this modified sputtered NiOx as the HTL deliver a highest stabilized efficiency of 18.7%. Our result offers an alternative method to manipulate sputtered NiOx thin film properties and thereby sheds light on a manufacturing pathway to perovskite solar cells featuring sputtered NiOx HTL.

Metal halide perovskite solar cells have progressed rapidly over the past decade, providing an exceptional opportunity for space photovoltaic (PV) power applications. However, the solar cells to be...

The efficiencies of green and red perovskite light-emitting diodes (PeLEDs) have been increased close to their theoretical upper limit, while the efficiency of blue PeLEDs is lagging far behind. Here we report enhancing the efficiency of sky-blue PeLEDs by overcoming a major hurdle of low photoluminescence quantum efficiency in wide-bandgap perovskites. Blending phenylethylammonium chloride into cesium lead halide perovskites yields a mixture of two-dimensional and three-dimensional perovskites, which enhances photoluminescence quantum efficiency from 1.1% to 19.8%. Adding yttrium (III) chloride into the mixture further enhances photoluminescence quantum efficiency to 49.7%. Yttrium is found to incorporate into the three-dimensional perovskite grain, while it is still rich at grain boundaries and surfaces. The yttrium on grain surface increases the bandgap of grain shell, which confines the charge carriers inside grains for efficient radiative recombination. Record efficiencies of 11.0% and 4.8% were obtained in sky-blue and blue PeLEDs, respectively., 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.

Thin film solar cells have, in general, three recombination locations that can limit the device performance – the bulk, front interface, and back interface. Unfortunately, it is difficult to determine which of these mechanisms limit any given device. Time resolved photoluminescence measurements provide an estimate for the carrier lifetime and, therefore, bulk recombination. Determining the role of the front and back interface in device performance, though, is more complicated. With this in mind, we investigated the use of current densityvoltage measurements with front and back illumination at varying intensities. Using numerical modeling, we show that recombination information about each interface can be gleaned, and the limiting interface can be determined. The response curve signatures learned from modeling are then applied to measurements of real devices.

The intermittent nature of solar energy has made it necessary for photovoltaic (PV) systems to rely on external energy storage when deployed off-the-grid. In recent years, solar flow batteries (SFBs) have emerged as a potential alternative, which integrates energy production and storage in an integrated device. Here we performed an environmental assessment by highlighting potential hotspots that might hinder their acceptance, offering less pollutive alternatives. Specifically, we analyzed the environmental impacts of perovskite/silicon tandem based SFB. Our results show that the PV cell is responsible for three fourths of the impact of the device at lab scale, while the stack frame is responsible for the remaining impact. At this scale, the impact of the electrolytes is negligible. We expect the impact of the frame to reduce as the scale increases, while we anticipate an increase for the importance of the electrolytes as more energy is stored. For the future, the impact of the PV cell could be reduced by using a back electrode different than gold, and by substituting the perovskite/silicon tandem configuration with a perovskite/perovskite one.

CdSe thickness and Cu doping play significant roles in achieving a highly efficiency in CdTe solar cells. Using an evaporated CdSe/CdTe device stack to avoid vacuum breaks between layers, we investigate the role of the CdSe thickness on the device performance and show that when the CdSe thickness is above a critical value the device performance, specifically the short circuit current (J SC ), suffers. From these measurements, we determine the critical thickness of CdSe to be 120 nm. However, in some cases the CdSe thicknesses can be increased above the critical value and increasing the Cu doping process temperature can lead to an increase in J SC and an overall improvement in device efficiency. Specifically, for a device with ~270 nm of CdSe, the J SC increases from 7.9 mAcm-2 with CuCl 2 processing temperature of 200 °C to ~29 mAcm-2 when the processing temperature is increased to 250 °C.

Monolithic perovskite/perovskite tandem solar cells have attracted substantial attention owing to their promise for high power conversion efficiency (PCE) and low fabrication cost. Conventional all-perovskite tandem solar cells employ a mixed tin (Sn)-lead (Pb) low-bandgap subcell, which suffers from instability issues in the ambient air. Here, we fabricate two-terminal perovskite/perovskite tandem solar cells consisting of two solution-processed perovskite subcells based on the more airstable pure Pb-based perovskites. The champion perovskite tandem cell exhibits a PCE of 19.2% with a high open-circuit voltage of 2.15 V. We further demonstrate photovoltaicelectrolysis (PV-EC) water-splitting applications using these high-photovoltage all-perovskite tandem solar cells.

Although Sn-Pb perovskites sensing near-ultraviolet-visible-near-infrared light could be an attractive alternative to silicon in photodiodes and imaging, there have been no clear studies on such devices constructed on metal/silicon substrates, hindering their direct integration with complementary metal-oxide semiconductor (CMOS) and silicon electronics. Typically, high surface roughness and severe pinholes of Sn-rich binary perovskites make it difficult for them to fulfill the requirements of efficient photodiodes and imaging. These issues cause inherently high dark current and poor (dark and photo-) current uniformity. Herein, we propose and demonstrate the room-temperature crystallization in the Sn-rich binary perovskite system to effectively control film crystallization kinetics. With experimental and theoretical studies of the crystallization mechanism, we successfully tune the density and location of nanocrystals in precursor films to achieve compact nanocrystals, which coalesce into high-quality (smooth, dense, and pinhole-free) perovskites with intensified preferred orientation and decreased trap density. The high-quality perovskites reduce dark current and improve (dark and photo-) current uniformity of perovskite photodiodes on CMOS-compatible metal/silicon substrates. Meanwhile, self-powered devices achieve a high responsivity of 0.2 A/W at 940 nm, a large dynamic range of 100 dB, and a fast fall time of 2.27 μs, exceeding those of most silicon-based imaging sensors. Finally, a 6 × 6 pixel integrated photodiode array is successfully demonstrated to realize the imaging application. The work contributes to understanding the fundamentals of the crystallization of Sn-rich binary perovskites and advancing perovskite integration with Si-based electronics.

Electronic properties of a CdTe solar cell are reported using temperature-dependent capacitance spectroscopy and current-voltage characteristics, the latter in dark and illuminated conditions. The baseline solar cell material stack investigated is comprised of soda-lime-glass/SnO2:F/SnO2/CdS:O-buffer/CdTe-absorber/Cu/Au. Device properties are compared with CdTe solar cells in which the back surface was hydroiodic acid etched, before the back-contact formation, and a CdTe device in which Mg-doped ZnO (MZO) replaces buffer layers. Reduced back-contact barrier height and grain boundary barrier height are observed in the hydroiodic acid treated CdTe cell. Improved device performance in the MZO-based CdTe device is attributed to reduced emitter/absorber interface recombination when using the MZO window layer.

Band offsets at the back contact and front window layer in CdTe-based solar cells affect photovoltaic performance and challenge standard characterization methods. By analyzing the temperature and excitation dependence of both open circuit voltage and absolute photoluminescence intensity, we show that the effects band offsets can be separated from the effects of recombination and shunting. Solar cells were grown with MgZnO window layers and compared to cells with CdS window layers containing varying amounts of oxygen. It was demonstrated that band alignment rather than reduced recombination velocity is the reason for the success of MgZnO as a front interface contact. An assortment of thin back contact interlayers were also deposited, and a PbTe interlayer showed some promise as an Ohmic contact to the CdTe, though it appears to induce a photoconductive shunt. Finally, we show that the shunting resistance given by a standard current-voltage curve technique generally does not represent a physically meaningful quantity unless it is well below one kiloOhm square cm.

Organic p-type semiconductors with tunable structures offer great opportunities for hybrid perovskite solar cells (PVSCs). We report herein two dithieno[3,2-b:2',3'-d]pyrrole (DTP) cored molecular semiconductors prepared through π-conjugation extension and an N-alkylation strategy. The as-prepared conjugated molecules exhibit a highest occupied molecular orbital (HOMO) level of -4.82 eV and a hole mobility up to 2.16×10-4 cm2 V-1 s-1 . Together with excellent film-forming and over 99 % photoluminescence quenching efficiency on perovskite, the DTP based semiconductors work efficiently as hole-transporting materials (HTMs) for n-i-p structured PVSCs. Their dopant-free MA0.7 FA0.3 PbI2.85 Br0.15 devices exhibit a power conversion efficiency over 20 %, representing one of the highest values for un-doped molecular HTMs based PVSCs. This work demonstrates the great potential of using a DTP core in designing efficient semiconductors for dopant-free PVSCs.

Wide-bandgap (∼1.7–1.8 eV) perovskite solar cells have attracted substantial research interest in recent years due to their great potential to fabricate efficient tandem solar cells via combining with a lower bandgap (1.1–1.3 eV) absorber (e.g., Si, copper indium gallium diselenide, or low-bandgap perovskite). However, wide-bandgap perovskite solar cells usually suffer from large open circuit voltage (Voc) deficits caused by small grain sizes and photoinduced phase segregation. Here, we demonstrate that in addition to large grain sizes and passivated grain boundaries, controlling interface properties is critical for achieving high Voc's in the inverted wide-bandgap perovskite solar cells. We adopt guanidinium bromide solution to tune the effective doping and electronic properties of the surface layer of perovskite thin films, leading to the formation of a graded perovskite homojunction. The enhanced electric field at the perovskite homojunction is revealed by Kelvin probe force microscopy measurements. This advance enables an increase in the Voc of the inverted perovskite solar cells from an initial 1.12 V to 1.24 V. With the optimization of the device fabrication process, the champion inverted wide-bandgap cell delivers a power conversion efficiency of 18.19% and sustains more than 72% of its initial efficiency after continuous illumination for 70 h without encapsulation. Additionally, a semitransparent device with an indium tin oxide back contact retains more than 88% of its initial efficiency after 100 h maximum power point tracking.

Summary Tandem solar cells coupling narrow- and wide-band-gap thin-film polycrystalline absorbers are attractive for achieving ultrahigh efficiency with low manufacturing cost. For established narrow-band-gap CIGS thin-film bottom cells, a challenge is to develop highly efficient polycrystalline wide-band-gap top cells. Here, we demonstrate a 1.68-eV (FA0.65MA0.20Cs0.15)Pb(I0.8Br0.2)3 wide-band-gap perovskite solar cell with an efficiency of ∼20% enabled by using PEAI and Pb(SCN)2 complementary additives in the perovskite precursor. The coupling of PEA+ and SCN− provides a synergistic effect that overcomes growth challenges with either additive individually and improves perovskite film quality with enhanced crystallinity, reduced formation of excess PbI2 (in comparison to using Pb(SCN)2 additive alone), lower defect density and energetic disorder, and an improved carrier mobility (∼47 cm2 V−1s−1) and lifetime (∼2.9 μs). When coupling a semi-transparent 1.68-eV perovskite top cell fabricated by this approach with a 1.12-eV CIGS bottom cell, we achieve 25.9%-efficient polycrystalline perovskite/CIGS 4-terminal thin-film tandem solar cells.

Efficient all-perovskite tandem cells Organic-inorganic perovskite films can boost the output of conventional silicon solar cells in tandem geometries by utilizing more of the light at the blue end of the solar spectrum. Tandem cells that use only perovskite films have been less successful because of the lack of a suitable material with a low bandgap that can replace silicon. Tong et al. report that a mixed tin-lead organic-inorganic material containing a small fraction of guanidinium thiocyanate has a low bandgap, long charge-carrier lifetime, and efficiencies around 25%. Science , this issue p. 475

Comparing to the traditional CdS buffer layer, zinc magnesium oxide (ZMO) offers the following advantages for CdTe-based thin-film solar cells: it introduces a spike to conduction band offset, whic...

All-inorganic metal halide perovskites have attracted great interest in recent years due to their good device performance with higher thermal stability than that of their organic–inorganic perovskite counterparts. However, the all-inorganic perovskite polycrystalline films prepared by the conventional spin-coating method possess many pinholes, nonuniform surface with many small crystals, and irregular agglomerates, limiting their device performance. Herein, we introduced a monolayer nano-polystyrene (PS) sphere confined growth method for obtaining CsPbBr3 nanonet films (NFs) with ordered nanostructures grown in the preferred (110) orientation, which is beneficial for the charge carrier transport and the light-harvesting efficiency. The (110) peak intensity of CsPbBr3 NFs increased with the increase of the diameter of the monolayer sphere, while the (001) peak intensity was suppressed greatly, indicating the more preferred (110) oriented growth. The PDs based on (110)-orientation-preferred CsPbBr3 NFs prepared by using 850 nm PS spheres showed the best performance. The best performing device displayed the biggest linear dynamic range of up to 120 dB. In addition, a responsivity of 2.84 A W−1 and a detectivity of 5.47 × 1012 Jones were also achieved.

Searching for novel functional materials represents an important direction in the research and development of renewable energy. Due to their unique structural and compositional flexibility and high material stability, oxide perovskites and their derivatives have recently been extensively explored as a class of versatile materials for applications in electrocatalysis (EC), photocatalysis (PC) and photovoltaics (PV), showing great promise in terms of catalytic activity and device stability. In this review, we firstly discuss the extreme flexibilities of oxide perovskites in terms of their structures and compositions, which lead to a treasure trove of materials for diverse applications. Secondly, the current status of their applications and challenges in EC, PC and PV are reviewed. We attempt to build the connections between the structural and compositional flexibility and the tunable material properties desirable for various applications.

The annual energy yields for metal halide perovskite/copper indium gallium diselenide (CIGS) tandem photovoltaics have been calculated for 16 different bandgap combinations, using both 2 terminal and 4 terminal device designs, with fixed-tilt mounting as well as 1- and 2-axis tracking. Measured complex index of refraction data were used for the materials comprising the devices, and hourly irradiance data were extracted from Version 3 of the National Solar Radiation Database. Simulations were performed for Toledo OH, Golden CO, Phoenix AZ, and New Orleans LA, and the effect of local temperature variation was also considered. The combination of irradiance and temperature variations throughout the year cause different devices to be optimal at different times of the year. Interestingly, devices constructed to maximize AM1.5 photoconversion efficiency do not necessarily maximize the annual energy yield. A detailed analysis of the monthly energy yields at the different locations reveals the interplay between the changing light and temperature conditions. Over the course of the year these effects average to some degree so that annual energy yields that are close to the maximum possible value can be achieved by several different tandem device designs. The conclusions are valid for devices made with relatively thin perovskite layers, such as those used in champion efficiency devices. When the perovskite layers are thicker, however, the device is less tolerant to variation. Our results show that close matching of bandgap pairs is not essential for the fabrication of high-performance tandems. These findings should allow manufacturing efforts to proceed without the need for precise compositional control during formation of the absorber layers.

Perovskite nanocrystals and SWCNTs join forces in optical synapses with attractive switching metrics and low-energy operation., Long-lived photon-stimulated conductance changes in solid-state materials can enable optical memory and brain-inspired neuromorphic information processing. It remains challenging to realize optical switching with low-energy consumption, and new mechanisms and design principles giving rise to persistent photoconductivity (PPC) can help overcome an important technological hurdle. Here, we demonstrate versatile heterojunctions between metal-halide perovskite nanocrystals and semiconducting single-walled carbon nanotubes that enable room-temperature, long-lived (thousands of seconds), writable, and erasable PPC. Optical switching and basic neuromorphic functions can be stimulated at low operating voltages with femto- to pico-joule energies per spiking event, and detailed analysis demonstrates that PPC in this nanoscale interface arises from field-assisted control of ion migration within the nanocrystal array. Contactless optical measurements also suggest these systems as potential candidates for photonic synapses that are stimulated and read in the optical domain. The tunability of PPC shown here holds promise for neuromorphic computing and other technologies that use optical memory.

Recently, lead chloride perovskites represented by $\mathrm{CsPb}{\mathrm{Cl}}_{3}$ have been theoretically predicted to be ideal $p$-type transparent conductors and hence have attracted a lot of attention. However, experimentally, these materials have long been known to be insulators that can hardly be converted to $p$-type conductors by extrinsic doping. In this work, we systematically reevaluate the $p$-type dopability of lead chloride perovskites by density functional theory calculations. We find that the previously predicted dopability is due to an overestimation caused by the functional employed that gives an unreasonable high-lying valence band maximum. The hybrid functional with an optimized mixing parameter and the inclusion of spin-orbit coupling gives a suitable description of the band edge positions and thus a better assessment of the dopability. Our defect calculations suggest that lead chloride perovskites are intrinsically insulating and can hardly be converted to $p$-type conductors due to the lack of effective dopants, in agreement with the experimental observations. Our results highlight the importance of the suitable description of band edge positions on the prediction of defect properties and dopability of semiconductors.

Copper (Cu) doping is a crucial process to improve the hole concentration in CdTe absorber layer and reduce the back-barrier height for efficient CdTe solar cells. However, excess Cu creates detrimental defects and deteriorates device stability due to its high diffusivity. Here, we introduce a wet chemical method to incorporate Cu in CdTe using a CuCl solution. This approach allows a better control of Cu doping than the conventional method using evaporated Cu. Our investigation shows that the CuCl treatment can significantly improve the overall device performances due to an increased carrier concentration in the CuCl treated CdTe absorber layer. Furthermore, the CuCl treated devices show a longer minority carrier lifetime than the conventional Cu treated devices.

Sputtering is an efficient way to fabricate CdTe solar cells. However, the open-circuit voltage (Voc) of a sputtered device is low compared to close space sublimation or vapor deposited CdTe devices. Here, we report an outstanding Voc > 840 mV and fill factor (FF) > 71.0% for all-sputtered CdS/CdTe solar cells. These results were achieved by solution-based copper (Cu) doping of CdTe by copper (II) chloride (CuCl 2 ) in deionized water. Compared with evaporated Cu, solution-based doping increased the Voc and FF, and improved the optoelectronic properties of the sputtered CdTe thin films. Capacitance-voltage measurements indicated that the free hole concentration increased by two-fold for CuCl 2 treated devices.

In recent years, significant efforts have been paid to develop Pb-free halide perovskite and perovskite derivative absorber materials for solar cell applications. However, so far, solar cells based on these materials showed significantly inferior performances as compared to their Pb halide perovskite counterparts. Here, we provide a theoretical perspective on lead-free metal halide perovskites and derivatives for solar cell applications. We first show a theoretical understanding of the origins of the superior photovoltaic properties of Pb halide perovskites. We then show that Pb-replacing leads to reduced structural and electronic dimensionality, degrading the superior photovoltaic properties in Pb-free metal halide perovskites and perovskite derivatives.

Cu doping of CdTe has enabled reasonable device performance, but has some significant drawbacks, including limited achievable carrier concentration and high mobility in the CdTe films. Group V elements, on the other hand, are expected to facilitate higher carrier concentration and expected to have low mobility. Consequently, incorporate of As in CdTe is of great interest to the CdTe community. Here, we deposited CdTe from the CdTe:As feedstock to complete devices. We demonstrate that the As diffuses through the CdTe absorber layer and accumulates at the front and back interfaces with Cl activation. Current density-voltage and capacitance-voltage measurements indicate that the devices have low carrier concentration, even though the concentration of As is high. These results indicate that the process employed here was insufficient to activate the As doping.

We report a space tandem solar cell enabling an enhanced power conversion efficiency with radiation tolerance in low radiation environments for space photovoltaic (PV) applications. The presented space perovskite/Si 4-terminal tandem cell features an efficient semitransparent perovskite top cell with the bandgap of 1.61 eV on a thin Si space solar cell specifically designed for high radiation tolerance in space. Under a simulated air mass zero (AM0) spectrum (136.6 mW/cm2), the space tandem solar cell by combining the perovskite solar cell with a thin Si space solar cell results in a power conversion efficiency of 21.1% surpasses a limited efficiency of ~17% in a single junction Si space solar cell. Considering the potential for developing more efficient, low-cost perovskite PV technology, our approach of using the thin Si space solar for the space tandem cells makes it possible to realize cost-effective PV power generation in space.

As the efficiency of single junction devices improve, bifacial thin film (BFT) devices are being increasingly investigated as a way to increase energy yield per unit area. At the same time, increasing doping density of thin film devices are also being developed to increase the energy yield per unit area. Here, we investigate how critical parameters to improving backside illuminated devices and increasing the absorber doping density affect backside illuminated device performance. We show that reducing recombination at the back of the device through increasing initial Fermi level offset (IFLO) or decreasing back surface recombination velocity (BSRV) is far more important than increasing absorber doping density. We also show that there is a wide range of the IFLO-BSRV parameter space that can lead to high backside illumination device efficiency for highly doped thin film devices.

Multi-junction all-perovskite tandem solar cells are a promising choice for next-generation solar cells with high efficiency and low fabrication cost. However, the lack of high-quality low-bandgap perovskite absorber layers seriously hampers the development of efficient and stable two-terminal monolithic all-perovskite tandem solar cells. Here, we report a bulk-passivation strategy via incorporation of chlorine, to enlarge grains and reduce electronic disorder in mixed tin–lead low-bandgap (~1.25 eV) perovskite absorber layers. This enables the fabrication of efficient low-bandgap perovskite solar cells using thick absorber layers (~750 nm), which is a requisite for efficient tandem solar cells. Such improvement enables the fabrication of two-terminal all-perovskite tandem solar cells with a champion power conversion efficiency of 21% and steady-state efficiency of 20.7%. The efficiency is retained to 85% of its initial performance after 80 h of operation under continuous illumination. Two-terminal monolithic all-perovskite tandem solar cells are attractive due to their flexible nature and low-cost fabrication. Here the authors develop a process to obtain high-quality Sn–Pb perovskite thin films by incorporating chlorine. Such layers are employed to fabricate 20.7%-efficient tandem cells with 80 h operational stability.

Halide semiconductors are undergoing a period of intense interest, buoyed by the outstanding optoelectronic properties of hybrid perovskites. It is worthwhile to consider whether related materials sharing unique aspects of the perovskite chemistry, such as the incorporation of metals that provide lone pair s electrons when integrated within the halide lattice, might yield semiconductors with comparable properties. One such metal that has not been widely studied in the context of photovoltaics is monovalent indium. In this work, we investigate a method of depositing films of indium(I) iodide, coupled with optoelectronic characterization and incorporation of these films into thin-film solar cells. We find that, although indium(I) iodide exhibits a photovoltaic effect, it is likely to be compromised by difficult-to-remove defects, e.g., iodine vacancies, which are expected to introduce recombination centers deep within the band gap.