ZiQi Sun

After rapid progress over the past five years, organic–inorganic perovskite solar cells (PSCs) currently exhibit photoconversion efficiencies comparable to the best commercially available photovoltaic technologies. However, instabilities in the materials and devices, primarily due to reactions with water, have kept PSCs from entering the marketplace. Here, laser beam induced current imaging is used to investigate the spatial and temporal evolution of the quantum efficiency of perovskite solar cells under controlled humidity conditions. Several interesting mechanistic aspects are revealed as the degradation proceeds along a four-stage process. Three of the four stages can be reversed, while the fourth stage leads to irreversible decomposition of the photoactive perovskite material. A series of reactions in the PbI₂-CH₃NH₃I-H₂O system explains the interplay between the interactions with water and the overall stability. Understanding of the degradation mechanisms of PSCs on a microscopic level gives insight into improving the long-term stability.

State-of-the-art perovskite solar cells are examined under accelerated aging conditions using fast laser beam induced current imaging. The results demonstrate that the degradation of perovskite solar cells in the presence of water is a four-stage process involving phase transformations of the perovskite material. The work allows a detailed understanding of the evolution and mechanisms of moisture-induced perovskite degradation on a microscopic scale.

Organic-inorganic metal halide perovskite solar cells show hysteresis in their current–voltage curve measured at a certain voltage sweep rate. Coinciding with a slow transient current response, the hysteresis is attributed to a slow voltage-driven (ionic) charge redistribution in the perovskite solar cell. Thus, the electric field profile and in turn the electron/hole collection efficiency become dependent on the biasing history. Commonly, a positive prebias is beneficial for a high power-conversion efficiency. Fill factor and open-circuit voltage increase because the prebias removes the driving force for charge to pile-up at the electrodes, which screen the electric field. Here, it is shown that the piled-up charge can also be beneficial. It increases the probability for electron extraction in case of extraction barriers due to an enhanced electric field allowing for tunneling or dipole formation at the perovskite/electrode interface. In that case, an inverted hysteresis is observed, resulting in higher performance metrics for a voltage sweep starting at low prebias. This inverted hysteresis is particularly pronounced in mixed-cation mixed-halide systems which comprise a new generation of perovskite solar cells that makes it possible to reach power-conversion efficiencies beyond 20%.

Inverted hysteresis is observed in mixed cation mixed halide perovskite solar cells, which show a power-conversion efficiency of 20%. It is attributed to charge accumulation and dipole formation at the perovskite/TiO₂ interface changing extraction barrier and recombination lifetimes in and close to the mesoporous scaffold.

On page 4653, N. Zhao, M. A. Loi, and co-workers report a giant light-induced enhancement of the photoluminescence intensity in formamidinium lead iodide perovskite thin films. They demonstrate that the “brightening” of the perovskite can be attributed to a moisture-assisted light-healing effect, which can be potentially used to increase and control the quality of hybrid perovskite thin films.

One of the factors limiting the performance of organic solar cells (OSCs) is their large energy losses (E _loss) in the conversion from photons to electrons, typically believed to be around 0.6 eV and often higher than those of inorganic solar cells. In this work, a novel low band gap polymer PIDTT-TID with a optical gap of 1.49 eV is synthesized and used as the donor combined with PC₇₁BM in solar cells. These solar cells attain a good power conversion efficiency of 6.7% with a high open-circuit voltage of 1.0 V, leading to the E _loss as low as 0.49 eV. A systematic study indicates that the driving force in this donor and acceptor system is sufficient for charge generation with the low E _loss. This work pushes the minimal E _loss of OSCs down to 0.49 eV, approaching the values of some inorganic and hybrid solar cells. It indicates the potential for further enhancement of the performance of OSCs by improving their V _oc since the E _loss can be minimized.

Polymer solar cells with minimal energy losses below 0.5 eV are realized based on a blend of a low band gap (1.49 eV) polymer PIDTT-TID and PC₇₁BM, exhibiting a high open-circuit voltage of 1.0 V and a decent efficiency of 6.7%. Solid proof shows that the good performance stems from the high-energy charge transfer state and low-energy loss, but sufficient driving force.

Power conversion efficiency (PCE) of organic photovoltaics (OPVs) lags behind of inorganic photovoltaics due to low dielectric constants (ε _r) of organic semiconductors. Although OPVs with high ε _r are attractive in theory, practical demonstration of efficient OPV devices with high-ε _r materials is in its infancy. This is largely due to the contradiction between the requirements of high ε _r and good donor:acceptor blend morphology in the bulk heterojunction. Herein, a series of fullerene acceptors is reported bearing a polar cyano moiety for both high ε _r and good donor:acceptor blend morphology. These cyano-functionalized acceptors (ε _r = 4.9) have higher ε _r than that of the widely used acceptor, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) (ε _r = 3.9). The high ε _r is realized without decrease of electron mobility and change of the lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels. Although the cyano-functionalized acceptors have increased polarity, they still exhibit good compatibility with the typical donor polymer. Polymer solar cells based on the cyano-functionalized acceptors exhibit good active layer morphology and show better device performance (PCE = 5.55%) than that of PC₆₁BM (PCE = 4.56%).

Fullerene adducts bearing a cyano moiety with large dipole moment show not only a high dielectric constant, but also good compatibility with the donor polymer for good blend morphology, and exhibit good polymer solar cell device performances.

Three acceptor–acceptor (A–A) type conjugated polymers based on isoindigo and naphthalene diimide/perylene diimide are designed and synthesized to study the effects of building blocks and alkyl chains on the polymer properties and performance of all-polymer photoresponse devices. Variation of the building blocks and alkyl chains can influence the thermal, optical, and electrochemical properties of the polymers, as indicated by thermogravimetric analysis, differential scanning calorimetry, UV–vis, cyclic voltammetry, and density functional theory calculations. Based on the A–A type conjugated polymers, the most efficient all-polymer photovoltaic cells are achieved with an efficiency of 2.68%, and the first all-polymer photodetectors are constructed with high responsivity (0.12 A W⁻¹) and detectivity (1.2 × 10¹² Jones), comparable to those of the best fullerene based organic photodetectors and inorganic photodetectors. Photoluminescence spectra, charge transport properties, and morphology of blend films are investigated to elucidate the influence of polymeric structures on device performances. This contribution demonstrates a strategy of systematically tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.

Three n-type conjugated polymers are synthesized to achieve the most efficient acceptor–acceptor type polymer-based all-polymer photovoltaic cells with an efficiency of 2.68% and the first all-polymer photodetectors with high responsivity (0.12 A W⁻¹) and detectivity (1.2 × 10¹² Jones). This contribution provides a strategy of tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.

Semiconductor micro/nano-cavities with high quality factor (Q) and small modal volume provide critical platforms for exploring strong light-matter interactions and quantum optics, enabling further development of coherent and quantum photonic devices. Constrained by exciton binding energy and thermal fluctuation, only a handful of wide-band semiconductors such as ZnO and GaN have stable excitons at room temperature. Metal halide perovskite with cubic lattice and well-controlled exciton may provide solutions. In this work, high-quality single-crystalline cesium lead halide CsPbX₃ (X = Cl, Br, I) whispering-gallery-mode (WGM) microcavities are synthesized by vapor-phase van der Waals epitaxy method. The as-grown perovskites show strong emission and stable exciton at room temperature over the whole visible spectra range. By varying the halide composition, multi-color (400–700 nm).WGM excitonic lasing is achieved at room temperature with low threshold (~ 2.0 μJ cm⁻²) and high spectra coherence (~0.14–0.15 nm). The results advocate the promise of inorganic perovskites towards development of optoelectronic devices and strong light-matter coupling in quantum optics.

High-quality cesium lead halide nanoplatelets functioning as whispering-gallery-mode microcavities are synthesized by vapor-phase van der Waals epitaxy method. Multicolor, low-threshold excitonic lasing action with a high spectra coherence of 0.14–0.15 nm is realized at room temperature. The findings are not only important for developing on-chip small lasers and high-speed exciton devices but also promising for fundamental studies in cavity quantum electrodynamics.

Effective singlet fission solar cells require both fast and efficient singlet fission as well as favorable energetics for harvesting the resulting triplet excitons. Notable progress has been made to engineer materials with rapid and efficient singlet fission, but the ability to control the energetics of these solar cells remains a challenge. Here, it is demonstrated that the interfacial charge transfer state energy of a rubrene/C₆₀ solar cell can be modified dramatically by the morphology of its constituent films. The effect is so pronounced that a crystalline system is able to dissociate and collect triplets generated through singlet fission whereas an as-deposited amorphous system is not. Furthermore, a novel technique for studying the behavior of this class of devices using external quantum efficiency (EQE) measurements in the presence of a background light is described. When this method is applied to rubrene/C₆₀ solar cells, it is shown that triplet–triplet annihilation makes significant contributions to photocurrent in the amorphous device—enhancing EQE by over 12% at relatively low intensities of background light (4 mW cm⁻²)—while detracting from photocurrent in the crystalline device. Finally, the conclusions on how the material system is affected by its morphology are strengthened by time-resolved photoluminescence experiments.

The interfacial charge transfer state energy of a crystalline rubrene/C₆₀ solar cell is over 300 meV lower than that of an amorphous rubrene/C₆₀ device. Despite that both amorphous and crystalline rubrene undergo singlet fission, this shift means that only the crystalline device can dissociate the low energy triplets generated by singlet fission and operate as a singlet fission solar cell.

On page 4464, M. J. Ko, H. J. Son, and co-workers develop a flexible perovskite solar cell by applying a novel low-temperature solution-processable polymer (PhNa-1T) as a hole-transport material. Compared with conventional PEDOT:PSS, PhNa-1T effectively improves solar cell efficiency and device stability due to the pH-neutral property of PhNa-1T and efficient charge extraction and suppressed charge recombination in solar cell devices.

S. K. Kwak, J. H. Oh, and co-workers generate multiple-patterned nanostructures by synergistically combining block-copolymer lithography with nanoimprinting lithography. As described on page 4976, these structures are used as back-reflectors for enhancing light absorption in organic optoelectronic devices including organic photovoltaics and phototransistors. The multiple-patterned electrodes significantly boost the performance of these devices, owing to the highly effective light scattering and plasmonic effects, extending the range of practical applications.

Lead-free perovskite infrared light-emitting diodes are achieved by using a halide perovskite CsSnI₃ as an emissive layer. The film shows compact micrometer-sized grains with only a few pinholes and cracks at the grain boundaries. The device exhibits maximum radiance of 40 W sr⁻¹ m⁻² at a current density of 364.3 mA cm⁻² and maximum external quantum efficiency of 3.8% at 4.5 V.

A novel non-fullerene acceptor, possessing a very low bandgap of 1.34 eV and a high-lying lowest unoccupied molecular orbital level of −3.95 eV, is designed and synthesized by introducing electron-donating alkoxy groups to the backbone of a conjugated small molecule. Impressive power conversion efficiencies of 8.4% and 10.7% are obtained for fabricated single and tandem polymer solar cells.

An ultrahigh performance MoS₂ photodetector with high photoresponsivity (1.94 × 10⁶ A W^–1) and detectivity (1.29 × 10¹² Jones) under 520 nm and 4.63 pW laser exposure is demonstrated. This photodetector is based on a methyl-ammonium lead halide perovskite/MoS₂ hybrid structure with (3-aminopropyl)triethoxysilane doping. The performance degradation caused by moisture is also minimized down to 20% by adopting a new encapsulation bilayer of octadecyltrichlorosilane/polymethyl methacrylate.

The past two decades of vigorous interdisciplinary approaches has seen tremendous breakthroughs in both scientific and technological developments of bulk-heterojunction organic solar cells (OSCs) based on nanocomposites of π-conjugated organic semiconductors. Because of their unique functionalities, the OSC field is expected to enable innovative photovoltaic applications that can be difficult to achieve using traditional inorganic solar cells: OSCs are printable, portable, wearable, disposable, biocompatible, and attachable to curved surfaces. The ultimate objective of this field is to develop cost-effective, stable, and high-performance photovoltaic modules fabricated on large-area flexible plastic substrates via high-volume/throughput roll-to-roll printing processing and thus achieve the practical implementation of OSCs. Recently, intensive research efforts into the development of organic materials, processing techniques, interface engineering, and device architectures have led to a remarkable improvement in power conversion efficiencies, exceeding 11%, which has finally brought OSCs close to commercialization. Current research interests are expanding from academic to industrial viewpoints to improve device stability and compatibility with large-scale printing processes, which must be addressed to realize viable applications. Here, both academic and industrial issues are reviewed by highlighting historically monumental research results and recent state-of-the-art progress in OSCs. Moreover, perspectives on five core technologies that affect the realization of the practical use of OSCs are presented, including device efficiency, device stability, flexible and transparent electrodes, module designs, and printing techniques.

Bulk-heterojunction organic solar cells based on solution-processable organic semiconductors enable completely new functionalities of being printable, portable, wearable, biocompatible, and attachable to any curved surfaces. The recent major advances in device efficiency and stability, flexible transparent electrodes, module design, and printing technologies for their commercialization are reviewed. The existing challenges and perspectives for these five core technologies are discussed.

The alloy acceptor (indene-C₆₀ bis-adduct (ICBA)/[6,6]-phenyl-C₇₁-butyric acid-methyl-ester (PC₇₁BM)) is employed to replace the widely used fullerene acceptor (PC₇₁BM) in organic solar cells based on five different polymer donors, which exhibit a higher efficiency and much better device stability than the PC₇₁BM counterpart.