lizhendong

Perovskite solar cells (PSCs) are of great interest in current photovoltaic research due to their extraordinary power conversion efficiency of ≈20% and boundless potentialities. The high efficiency has been mostly obtained from TiO₂-based PSCs, where TiO₂ is utilized as a hole-blocking, mesoporous layer. However, trapped charges and the light-induced photocatalytic effect of TiO₂ seriously degrade the perovskite and preclude PSCs from being immediately commercialized. Herein, a simplified PSC is successfully fabricated by eliminating the problematic TiO₂ layers, using instead a fluorine-doped tin oxide (FTO)/perovskite/hole–conductor/Au design. Simultaneously, the sluggish charge extraction at the FTO/perovskite interface is overcome by modifying the surface of the FTO to a porous structure using electrochemical etching. This surface engineering enables a substantial increase in the photocurrent density and mitigation of the hysteretic behavior of the pristine FTO-based PSC; a remarkable 19.22% efficiency with a low level of hysteresis is obtained. This performance is closely approaching that of conventional PSCs and may facilitate their commercialization due to improved convenience, lower cost, greater stability, and potentially more efficient mass production.

Electrochemically etched fluorine-doped tin oxide (FTO) provides large surfacial area compared with commercial FTO and quickly extracts photoexcited electrons at the FTO/perovskite interface. Accordingly, the photocurrent density and performance of hole-blocking layer-free planar-type perovskite solar cell are improved, where the remarkable power conversion efficiency of 19.22% is achieved.

A new donor (D)–acceptor (A) conjugate, benzodithiophene-rhodanine–[6,6]-phenyl-C₆₁ butyric acid methyl ester (BDTRh–PCBM) comprising three covalently linked blocks, one of p-type oligothiophene containing BDTRh moieties and two of n-type PCBM, is designed and synthesized. A single component organic solar cell (SCOSC) fabricated from BDTRh–PCBM exhibits the power conversion efficiency (PCE) of 2.44% and maximum external quantum efficiency of 46%, which are the highest among the reported efficiencies so far. The SCOSC device shows efficient charge transfer (CT, ≈300 fs) and smaller CT energy loss, resulting in the higher open-circuit voltage of 0.97 V, compared to the binary blend (BDTRh:PCBM). Because of the integration of the donor and acceptor in a single molecule, BDTRh-PCBM has a specific D–A arrangement with less energetic disorder and reorganization energy than blend systems. In addition, the SCOSC device shows excellent device and morphological stabilities, showing no degradation of PCE at 80 °C for 100 h. The SCOSC approach may suggest a great way to suppress the large phase segregation of donor and acceptor domains with better morphological stability compared to the blend device.

Integration of donor and acceptor in a single molecule by a covalent linkage is a promising approach to overcome unfavorably large-phase separation in bulk heterojunction blend solar cells. A new all-in-one system forms a specific molecular arrangement which decreases energetic disorder and facilitates ultrafast charge separation.

Conjugated polymers came to an unprecedented epoch that the charge transport is limited only by small disorder within aggregated domains. Accurate evaluation of transport performance is thus vital to optimizing further molecule design. Yet, the routine method by means of the conventional field-effect transistors may not satisfy such a requirement. Here, it is shown that the extrinsic effects of Schottky barrier, access transport through semiconductor bulk, and concurrent ambipolar conduction seriously influence transport analysis. The planar transistors incorporating ohmic contacts free of access and ambipolar conduction afford an ideal access to charge transport. It is found, however, that only the planar transistors operating in low-field regime are reliable to explore the inherent transport properties due to the energetic disorder lowering by the lateral field induced by high drain voltage. This work opens up a robust approach to comprehend the delicate charge transport in conjugated polymers so as to develop high-performance semiconducting polymers for promising plastic electronics.

Conventional and planar polymer transistors are compared to explore the charge transport in conjugated polymers. The former is found to suffer from extrinsic effects of Schottky barrier, access transport, and ambipolar conduction. The latter is free from such issues so is more reliable for transport exploration. However, only those operating in the low-field regime are reliable to explore the charge transport.

Organic–inorganic halide perovskites are promising photodetector materials due to their strong absorption, large carrier mobility, and easily tunable bandgap. Up to now, perovskite photodetectors are mainly based on polycrystalline thin films, which have some undesired properties such as large defective grain boundaries hindering the further improvement of the detector performance. Here, perovskite thin-single-crystal (TSC) photodetectors are fabricated with a vertical p–i–n structure. Due to the absence of grain-boundaries, the trap densities of TSCs are 10–100 folds lower than that of polycrystalline thin films. The photodetectors based on CH₃NH₃PbBr₃ and CH₃NH₃PbI₃ TSCs show low noise of 1–2 fA Hz^−1/2, yielding a high specific detectivity of 1.5 × 10¹³ cm Hz^1/2 W⁻¹. The absence of grain boundaries reduces charge recombination and enables a linear response under strong light, superior to polycrystalline photodetectors. The CH₃NH₃PbBr₃ photodetectors show a linear response to green light from 0.35 pW cm⁻² to 2.1 W cm⁻², corresponding to a linear dynamic range of 256 dB.

Photodetectors based on organic–inorganic halide perovskite thin single crystals (TSCs) are fabricated. Due to the absence of grain-boundaries, very low trap densities, and small thickness (≈10 µm) of the TSCs, the TSC photodetectors with vertical p–i–n structure show low noise (1–2 fA Hz^−1/2), high specific detectivity (≈1.5 × 10¹³ cm Hz^1/2 W⁻¹), and large linear-dynamic-range (256 dB).

In organic solar cells continuous donor and acceptor networks are considered necessary for charge extraction, whereas discontinuous neat phases and molecularly mixed donor–acceptor phases are generally regarded as detrimental. However, the impact of different levels of domain continuity, purity, and donor–acceptor mixing on charge transport remains only semiquantitatively described. Here, cosublimed donor–acceptor mixtures, where the distance between the donor sites is varied in a controlled manner from homogeneously diluted donor sites to a continuous donor network are studied. Using transient measurements, spanning from sub-picoseconds to microseconds photogenerated charge motion is measured in complete photovoltaic devices, to show that even highly diluted donor sites (5.7%–10% molar) in a buckminsterfullerene matrix enable hole transport. Hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules, over distances of up to ≈4 nm. Hence, these results question the relevance of “pristine” phases and whether a continuous interpenetrating donor–acceptor network is the ideal morphology for charge transport.

Transient measurements reveal that in organic solar cells a continuous donor network is not strictly necessary for hole transport. Hole hopping between isolated donor sites can occur by long-range hole tunneling through several buckminsterfullerene molecules (4 nm). This often disregarded mechanism questions the importance of pristine phases and whether a continuous donor–acceptor network is the ideal morphology for charge transport.

A comparison of the efficiency, stability, and photophysics of organic solar cells employing poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′″-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′″-quaterthiophen-5,5′″-diyl)] (PffBT4T-2OD) as a donor polymer blended with either the nonfullerene acceptor EH-IDTBR or the fullerene derivative, [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) as electron acceptors is reported. Inverted PffBT4T-2OD:EH-IDTBR blend solar cell fabricated without any processing additive achieves power conversion efficiencies (PCEs) of 9.5 ± 0.2%. The devices exhibit a high open circuit voltage of 1.08 ± 0.01 V, attributed to the high lowest unoccupied molecular orbital (LUMO) level of EH-IDTBR. Photoluminescence quenching and transient absorption data are employed to elucidate the ultrafast kinetics and efficiencies of charge separation in both blends, with PffBT4T-2OD exciton diffusion kinetics within polymer domains, and geminate recombination losses following exciton separation being identified as key factors determining the efficiency of photocurrent generation. Remarkably, while encapsulated PffBT4T-2OD:PC₇₁BM solar cells show significant efficiency loss under simulated solar irradiation (“burn in” degradation) due to the trap-assisted recombination through increased photoinduced trap states, PffBT4T-2OD:EH-IDTBR solar cell shows negligible burn in efficiency loss. Furthermore, PffBT4T-2OD:EH-IDTBR solar cells are found to be substantially more stable under 85 °C thermal stress than PffBT4T-2OD:PC₇₁BM devices.

A high efficiency, burn-in-free nonfullerene-based PffBT4T-2OD:EH-IDTBR solar cell is reported, fabricated without processing additives. Transient absorption and optoelectronic analyses elucidate the causes of this high efficiency and stability, with the superior stability compared to PC₇₁BM devices being correlated with increased crystallinity and reduced photogeneration of trap states.

The recent meteoric rise in the field of photovoltaics with the discovery of highly efficient solar-cell devices is inspired by solution-processed organic–inorganic lead halide perovskites that exhibit unprecedented light-to-electricity conversion efficiencies. The stunning performance of perovskites is attributed to their strong photoresponsive properties that are thoroughly utilized in designing excellent perovskite solar cells, light-emitting diodes, infrared lasers, and ultrafast photodetectors. However, optoelectronic application of halide perovskites in realizing highly efficient subwavelength photonic devices has remained a challenge. Here, the remarkable photoconductivity of organic–inorganic lead halide perovskites is exploited to demonstrate a hybrid perovskite–metamaterial device that shows extremely low power photoswitching of the metamaterial resonances in the terahertz part of the electromagnetic spectrum. Furthermore, a signature of a coupled phonon–metamaterial resonance is observed at higher pump powers, where the Fano resonance amplitude is extremely weak. In addition, a low threshold, dynamic control of the highly confined electric field intensity is also observed in the system, which could tremendously benefit the new generation of subwavelength photonic devices as active sensors, low threshold optically controlled lasers, and active nonlinear devices with enhanced functionalities in the infrared, optical, and the terahertz parts of the electromagnetic spectrum.

Remarkable photoconductivity of solution-processed ultrathin perovskite film at terahertz frequencies is exploited for low-power, high-speed photoactive switching of the Fano resonance in a hybrid perovskite–metamaterial device. At higher pump fluences, a mode-splitting behavior is observed, which manifests a phonon–metamaterial resonance coupling in the proposed system.

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH₂)₂PbI₃) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.

Low-temperature measurements of the photoluminescence from HC(NH₂)₂PbI₃ thin films are presented. The emission exhibits a power-law intensity decay with time after excitation, and an additional low-energy peak displaying an anomalous Stokes shift. These phenomena demonstrate that charge–carrier recombination in this perovskite is mediated by a band tail with characteristic energy 3 meV, determined from a rate-equation model and Monte Carlo simulation.

High-efficiency small-molecule-based organic photovoltaics (SM-OPVs) using two electron donors (p-DTS(FBTTh₂)₂ and ZnP) with distinctively different absorption and structural features are reported. Such a combination works well and synergically improves device short-circuit current density (J_sc) to 17.99 mA cm⁻² and fill factor (FF) to 77.19%, yielding a milestone efficiency of 11%. To the best of our knowledge, this is the highest power conversion efficiency reported for SM-OPVs to date and the first time to combine high J_sc over 17 mA cm⁻² and high FF over 77% into one SM-OPV. The strategy of using multicomponent materials, with a selecting role of balancing varied electronic and structural necessities can be an important route to further developing higher performance devices. This development is important, which broadens the dimension and versatility of existing materials without much chemistry input.

High-efficiency all-small-molecule organic solar cells using two electron donors with distinctively different absorption and structural features are reported. Such a combination works well and synergically improves device current and fill factor, yielding a milestone efficiency of 10.97%.

In article number 1606909, Ming-Fai Lo, Chun-Sing Lee, and co-workers report unusual free-carrier generation in some bipolar organic materials (SubNc and SubPc) upon photoexcitation. Single-layer devices with SubNc or SubPc sandwiched between two electrodes can give power conversion efficiencies 30 times higher than previously reported single-layer devices. Interestingly, these photoactive layers can be stacked onto each other with any sequence to generate a photocurrent in OPV devices.

Organic–inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low-temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high-quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin-hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.

A different precursor ratio is found to be essential for the formation of high-quality perovskite films on flexible substrates compared to those formed on rigid substrates. A high efficiency of 18.1% is achieved for flexible perovskite solar-cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low-temperature (≤100 °C) solution process.

Organic–inorganic halide perovskite (OHP) materials, for example, CH₃NH₃PbI₃ (MAPbI₃), have attracted significant interest for applications such as solar cells, photodectors, light-emitting diodes, and lasers. Previous studies have shown that charged defects can migrate in perovskites under an electric field and/or light illumination, potentially preventing these devices from practical applications. Understanding and control of the defect generation and movement will not only lead to more stable devices but also new device concepts. Here, it is shown that the formation/annihilation of iodine vacancies (V_I's) in MAPbI₃ films, driven by electric fields and light illumination, can induce pronounced resistive switching effects. Due to a low diffusion energy barrier (≈0.17 eV), the V_I's can readily drift under an electric field, and spontaneously diffuse with a concentration gradient. It is shown that the V_I diffusion process can be suppressed by controlling the affinity of the contact electrode material to I⁻ ions, or by light illumination. An electrical-write and optical-erase memory element is further demonstrated by coupling ion migration with electric fields and light illumination. These results provide guidance toward improved stability and performance of perovskite-based optoelectronic systems, and can lead to the development of solid-state devices that couple ionics, electronics, and optics.

Electric field and light illumination controlled iodine vacancy (V_I) redistribution and resistive switching effects are demonstrated in organic–inorganic halide perovskite films. The diffusion energy barrier of V_I is ≈0.17 eV. The V_I diffusion dynamics can be modulated through engineering the anode material and controlling illumination conditions. An electrical-write and optical-erase memory element is demonstrated.