Decreasing exciton dissociation rates for reduced voltage losses in organic solar cells.

Enhancing the device electroluminescence quantum efficiency (EQEEL) is a critical factor in mitigating non-radiative voltage losses (VNR) and further improving the performance of organic solar cells (OSCs). While the common understanding attributes EQEEL in OSCs to the dynamics of charge transfer (CT) states, persistent efforts to manipulate these decay dynamics have yielded limited results, with the EQEEL of high-efficiency OSCs typically remaining below 10-2%. This value is considerably lower than that observed in high efficiency inorganic photovoltaic devices. Here, we report that EQEEL is also influenced by the dissociation rate constant of singlet states (kDS). Importantly, in contrast to the traditional belief that advocates maximizing kDS for superior photovoltaic quantum efficiency (EQEPV), a controlled reduction in kDS is shown to enhance EQEEL without compromising EQEPV. Consequently, a promising experimental approach to address the VNR challenge is proposed, resulting in a significant improvement in the performance of OSCs.

Interface Modification for Efficient and Stable Inverted Inorganic Perovskite Solar Cells.

Due to their excellent thermal stability and ideal bandgap, metal halide inorganic perovskite based solar cells (PSCs) with inverted structure are considered as an excellent choice for perovskite/silicon tandem solar cells. However, the power conversion efficiency (PCE) of inverted inorganic perovskite solar cells (PSCs) still lags far behind that of conventional n-i-p PSCs due to interfacial energy level mismatch and high nonradiative charge recombination. Herein, the performance of inverted PSCs is significantly improved by interfacial engineering of CsPbI3- x Brx films with 2-mercapto-1-methylimidazole (MMI). It is found that the mercapto group can preferably react with the undercoordinated Pb2+ from perovskite by forming Pb-S bonds, which appreciably reduces the surface trap density. Moreover, MMI modification results in a better energy level alignment with the electron-transporting material, promoting carrier transfer and reducing voltage deficit. The above combination results in an open-circuit voltage enhancement by 120 mV, yielding a champion PCE of 20.6% for 0.09 cm2 area and 17.3% for 1 cm2 area. Furthermore, the ambient, operational and heat stabilities of inorganic PSCs with MMI modification are also greatly improved. The work demonstrates a simple but effective approach for fabricating highly efficient and stable inverted inorganic PSCs.

A Newly Crosslinked-double Network PEDOT:PSS@PEGDMA toward Highly-Efficient and Stable Tin-Lead Perovskite Solar Cells.

Until now, poly(3,4-ethylenedioxythiophene):poly(styrensulfonate) (PEDOT:PSS) is widely used in Sn-Pb perovskite solar cells (PSCs) due to its many advantages, including high optical transparency, suitable conductivity, superior wettability, and so on. However, the acidic and hydroscopic properties of the PSS component, as well as the incongruous energy level of the hole transport layer (HTL), may lead to unsatisfying interface properties and decreased device performance. Herein, by adding polyethylene glycol dimethacrylate (PEGDMA) into PEDOT:PSS, a newly crosslinked-double-network obtain of PEDOT:PSS@PEGDMA film, which could not only optimize nucleation and crystallinity of Sn-Pb perovskite films, but also suppress defect density and optimize energy level alignment at the HTL/perovskite interface. As a result, the achieves highly efficient and stable mixed Sn-Pb PSCs with an encouraging power conversion efficiency of 20.9%. Additionally, the device can maintain good stability under N2 atmosphere.

Combined Vacuum Evaporation and Solution Process for High-Efficiency Large-Area Perovskite Solar Cells with Exceptional Reproducibility.

Organic-inorganic hybrid perovskites exhibit outstanding performances in perovskite solar cells (PSCs). However, the complex solution chemistry of perovskites precursors renders it difficult to prepare large-area devices in a reproducible way, which is a prerequisite for the technology to make an impact beyond lab scale. Vacuum processing, instead, is an established technology for large-scale coating of thin films. However, with respect to the hybrid perovskites it is highly challenging due to the high vapor pressure of the organic ammonium halide. In this work, vacuum evaporation of lead iodide and solution processing of organic ammonium halide is combined to produce large-area homogeneous perovskite films with large grains in a highly reproducible way. The resulting PSCs achieve a power conversion efficiency (PCE) of 24.3% (certified 23.9%) on small area (0.10 cm2 ), 24.0% (certified 23.7%) on large area (1 cm2 ) and 20.0% for minimodule (16 cm2 ), and maintain 90% of its initial efficiency after 1000 h 1-sun operation. The vacuum evaporation prevents advert environmental effects on lead halide formation and guarantees a reproducible fabrication of high-quality large-area perovskite films, which opens a promising way for large-scale fabrication of perovskite optoelectronics.

Double Layer Composite Electrode Strategy for Efficient Perovskite Solar Cells with Excellent Reverse-Bias Stability.

Perovskite solar cells (PSCs) have become the representatives of next generation of photovoltaics; nevertheless, their stability is insufficient for large scale deployment, particularly the reverse bias stability. Here, we propose a transparent conducting oxide (TCO) and low-cost metal composite electrode to improve the stability of PSCs without sacrificing the efficiency. The TCO can block ion migrations and chemical reactions between the metal and perovskite, while the metal greatly enhances the conductivity of the composite electrode. As a result, composite electrode-PSCs achieved a power conversion efficiency (PCE) of 23.7% (certified 23.2%) and exhibited excellent stability, maintaining 95% of the initial PCE when applying a reverse bias of 4.0 V for 60 s and over 92% of the initial PCE after 1000 h continuous light soaking. This composite electrode strategy can be extended to different combinations of TCOs and metals. It opens a new avenue for improving the stability of PSCs.

Simultaneous Lattice Engineering and Defect Control via Cadmium Incorporation for High-Performance Inorganic Perovskite Solar Cells.

Doping of all-inorganic lead halide perovskites to enhance their photovoltaic performance and stability has been reported to be effective. Up to now most studies have focused on the doping of elements in to the perovskite lattice. However, most of them cannot be doped into the perovskite lattice and the roles of these dopants are still controversial. Herein,the authors introduce CdI2 as an additive into CsPbI3-x Brx and use it as active layer to fabricate high-performance inorganic perovskite solar cells (PSCs). Cd with a smaller radius than Pb can partially substitute Pb in the perovskite lattice by up to 2 mol%. Meanwhile, the remaining Cd stays on the surface and grain boundaries (GB) of the perovskite film in the form of Cs2 CdI4-x Br-x , which is found to reduce non-radiative recombination. These effects result in prolonged charge carrier lifetime, suppressed defect formation, decreased GBs, and an upward shift of energybands in the Cd-containing film. A champion efficiency of 20.8% is achieved for Cd-incorporated PSCs, together with improved device ambient stability. This work highlights the importance of simultaneous lattice engineering, defectcontrol and atomic-level characterization in achieving high-performance inorganic PSCs with well-defined structure-property relationships.

Multifunctional succinate additive for flexible perovskite solar cells with more than 23% power-conversion efficiency.

Flexible perovskite solar cells (FPSCs) have emerged as power sources in versatile applications owing to their high-efficiency characteristics, excellent flexibility, and relatively low cost. Nevertheless, undesired strain in perovskite films greatly impacts the power-conversion efficiency (PCE) and stability of PSCs, particularly in FPSCs. Herein, a novel multifunctional organic salt, methylammonium succinate, which can alleviate strain and reinforce grain boundaries, was incorporated into the perovskite film, leading to relaxed microstrain and a lower defect concentration. As a result, a PCE of 25.4% for rigid PSCs and a record PCE of 23.6% (certified 22.5%) for FPSCs have been achieved. In addition, the corresponding FPSCs exhibited excellent bending durability, maintaining ∼85% of their initial efficiency after bending at a 6 mm radius for 10 000 cycles.

Sequential vacuum-evaporated perovskite solar cells with more than 24% efficiency.

Vacuum evaporation is promising for the high-throughput fabrication of perovskite solar cells (PSCs) because of its solvent-free characteristic, precise control of film thickness, and compatibility with large-scale production. Nevertheless, the power conversion efficiency (PCE) of PSCs fabricated by vacuum evaporation lags behind that of solution-processed PSCs. Here, we report a Cl-containing alloy-mediated sequential vacuum evaporation approach to fabricate perovskite films. The presence of Cl in the alloy facilitates organic ammonium halide diffusion and the subsequent perovskite conversion reaction, leading to homogeneous pinhole-free perovskite films with few defects. The resulting PSCs yield a PCE of 24.42%, 23.44% (certified 22.6%), and 19.87% for 0.1, 1.0, and 14.4 square centimeters (mini-module, aperture area), respectively. The unencapsulated PSCs show good stability with negligible decline in performance after storage in dry air for more than 4000 hours. Our method provides a reproducible approach for scalable fabrication of large-area, high-efficiency PSCs and other perovskite-based optoelectronics.

A Transparent Electrode Based on Solution-Processed ZnO for Organic Optoelectronic Devices.

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm-1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at the cathode/active layer interface, and the reduced parasitic absorption losses in the electrodes of the ZnO based devices.

Conformal quantum dot-SnO2 layers as electron transporters for efficient perovskite solar cells.

Improvements to perovskite solar cells (PSCs) have focused on increasing their power conversion efficiency (PCE) and operational stability and maintaining high performance upon scale-up to module sizes. We report that replacing the commonly used mesoporous-titanium dioxide electron transport layer (ETL) with a thin layer of polyacrylic acid-stabilized tin(IV) oxide quantum dots (paa-QD-SnO2) on the compact-titanium dioxide enhanced light capture and largely suppressed nonradiative recombination at the ETL-perovskite interface. The use of paa-QD-SnO2 as electron-selective contact enabled PSCs (0.08 square centimeters) with a PCE of 25.7% (certified 25.4%) and high operational stability and facilitated the scale-up of the PSCs to larger areas. PCEs of 23.3, 21.7, and 20.6% were achieved for PSCs with active areas of 1, 20, and 64 square centimeters, respectively.

Interfaces and Interfacial Layers in Inorganic Perovskite Solar Cells.

Owing to their superior thermal stability, metal halide inorganic perovskite materials continue to attract interest for photovoltaics applications. The highest reported power conversion efficiency (PCE) for solar cells based on inorganic perovskites is over 20 %. As this PCE corresponds to 73 % of the theoretical limit, there remains more room for further improving the device PCEs than for improving organic-inorganic hybrid perovskite solar cells (PSCs). The main loss is in the photovoltage, which is limited by interfaces in terms of non-radiative recombination caused by traps and energy-level mismatch. Furthermore, inefficient charge extraction at interfacial contacts reduces the photocurrent and fill factor. This Minireview summarizes the recent developments in the fundamental understanding of how the interfaces and interfacial layers influence the performance of solar cells based on inorganic perovskite absorbers. An outlook for the development of highly efficient and stable inorganic PSCs from the interface point of view is also given.

Mobile ions determine the luminescence yield of perovskite light-emitting diodes under pulsed operation.

The external quantum efficiency of perovskite light-emitting diodes (PeLEDs) has advanced quickly during the past few years. However, under pulsed operation, an operation mode which is important for display and visible light communication, the performance of PeLEDs changes a lot and requires in-depth understanding to facilitate these applications. Here, we report the response of PeLEDs under pulsed operation in the range of 10 Hz to 20 kHz. Beyond transient effects in the low frequencies, we find that for higher frequencies (>500 Hz) the transient electroluminescence intensity depends strongly on the duty cycle. This feature is much more pronounced and of different origin than that in conventional LEDs. We rationalise our experimental observations using a mathematical model and assign these features to the effect of mobile ionic charges in the perovskite. Our work also provides important implications for the operation of PeLEDs under the steady state, where accumulation of mobile ions at the interfaces could be beneficial for high electroluminescence yields but harmful for the long-term stability.

Crystal-Size-Induced Band Gap Tuning in Perovskite Films.

A comprehensive picture explaining the effect of the crystal size in metal halide perovskite films on their opto-electronic characteristics is currently lacking. We report that perovskite nanocrystallites exhibit a wider band gap due to concurrent quantum confinement and size dependent structural effects, with the latter being remarkably distinct and attributed to the perturbation from the surface of the nanocrystallites affecting the structure of their core. This phenomenon might assist in the photo-induced charge separation within the perovskite in devices employing mesoporous layers as they restrict the size of nanocrystallites present in them. We demonstrate that the crystal size effect is widely applicable as it is ubiquitous in different compositions and deposition methods employed in the fabrication of state-of-the-art perovskite solar cells. This effect is a convenient and effective way to tune the band gap of perovskites.

Vapor-assisted deposition of highly efficient, stable black-phase FAPbI3 perovskite solar cells.

Mixtures of cations or halides with FAPbI3 (where FA is formamidinium) lead to high efficiency in perovskite solar cells (PSCs) but also to blue-shifted absorption and long-term stability issues caused by loss of volatile methylammonium (MA) and phase segregation. We report a deposition method using MA thiocyanate (MASCN) or FASCN vapor treatment to convert yellow δ-FAPbI3 perovskite films to the desired pure α-phase. NMR quantifies MA incorporation into the framework. Molecular dynamics simulations show that SCN- anions promote the formation and stabilization of α-FAPbI3 below the thermodynamic phase-transition temperature. We used these low-defect-density α-FAPbI3 films to make PSCs with >23% power-conversion efficiency and long-term operational and thermal stability, as well as a low (330 millivolts) open-circuit voltage loss and a low (0.75 volt) turn-on voltage of electroluminescence.

Ba-induced phase segregation and band gap reduction in mixed-halide inorganic perovskite solar cells.

All-inorganic metal halide perovskites are showing promising development towards efficient long-term stable materials and solar cells. Element doping, especially on the lead site, has been proved to be a useful strategy to obtain the desired film quality and material phase for high efficient and stable inorganic perovskite solar cells. Here we demonstrate a function by adding barium in CsPbI2Br. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared with the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high power conversion efficiency of 14.0% and a great suppression of non-radiative recombination within the inorganic perovskite, yielding a high open-circuit voltage of 1.33 V and an external quantum efficiency of electroluminescence of 10-4.

Perovskite Solar Cells Yielding Reproducible Photovoltage of 1.20 V.

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs based on guanidinium iodide doped perovskite films.

Review on Recent Progress of All-Inorganic Metal Halide Perovskites and Solar Cells.

All-inorganic perovskites are considered to be one of the most appealing research hotspots in the field of perovskite photovoltaics in the past 3 years due to their superior thermal stability compared to their organic-inorganic hybrid counterparts. The power-conversion efficiency has reached 17.06% and the number of important publications is ever increasing. Here, the progress of inorganic perovskites is systematically highlighted, covering materials design, preparation of high-quality perovskite films, and avoidance of phase instabilities. Inorganic perovskites, nanocrystals, quantum dots, and lead-free compounds are discussed and the corresponding device performances are reviewed, which have been realized on both rigid and flexible substrates. Methods for stabilization of the cubic phase of low-bandgap inorganic perovskites are emphasized, which is a prerequisite for highly efficient and stable solar cells. In addition, energy loss mechanisms both in the bulk of the perovskite and at the interfaces of perovskite and charge selective layers are unraveled. Reported approaches to reduce these charge-carrier recombination losses are summarized and complemented by methods proposed from our side. Finally, the potential of inorganic perovskites as stable absorbers is assessed, which opens up new perspectives toward the commercialization of inorganic perovskite solar cells.

Origin of apparent light-enhanced and negative capacitance in perovskite solar cells.

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative 'capacitance' dependent on the sign of its gradient. The 'capacitance' appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon related to the hysteresis in the current-voltage curve.

Electroluminescence Dynamics in Perovskite Solar Cells Reveals Giant Overshoot Effect.

High performance of both photovoltaic and electroluminescent devices requires low nonradiative recombination losses. In perovskites, such loses strongly depend on the carrier traps related to the mobile ions and vacancies, causing I- V hysteresis of solar cells and influencing the performance of other optoelectronic devices, such as photodetectors and LEDs. To address the dynamics of the mobile ions, here we investigate electroluminescence time evolution in perovskite solar cells under constant and pulsed voltage conditions. We propose a model, accounting for the spatial ion accumulation and explaining the complex electroluminescence dynamics both on fast (microseconds) and slow (seconds) time scales. We demonstrate the appearance of a high-intensity short electroluminescence peak (overshoot pulse) immediately after termination of the electrical pulse. The generation of a giant overshoot pulse suggests a simple way to achieve high pulsed luminescence intensity with a low current density, which opens new prospects toward optical gain and implementation of electrically pumped lasers.