Chen Weijie

The front cover illustrates a flexible small-molecule organic solar cell employing a semitransparent electrode with and without surface modifying layer, as described by D. S. Ghosh and K. Leo in article number 1600518. Without the modifying layer, the silver film grows in an island form, making it highly resistive. Together with better electrical properties, the semitransparent electrode contributes to light trapping inside the cell due to the microcavity effect, leading to enhanced efficiencies.

Thin-film formation by solution-shearing technique is examined for layered-crystalline donor–acceptor charge-transfer compounds composed of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene as a donor and optionally fluorinated derivatives of tetracyanoquinodimethane (n = 0, 2, 4) as acceptors. Polycrystalline thin films of the compounds whose crystalline size is over 2 mm along the blade-scan axis are successfully fabricated, and the formed films demonstrate anisotropic alignment of the crystalline grains where the crystal a-axis with the largest transfer integral is parallel to the blade-scan axis. Such anisotropic alignment of large crystalline grains affords air-stable n-type field-effect operation with a mobility as high as 0.61 cm² V⁻¹ s⁻¹ which is comparable to that of the single-crystal devices.

Unidirectionally crystallized thin-film formation of donor–acceptor charge-transfer complexes, (diC₈BTBT)(F _n TCNQ) [n = 0, 2, 4], is achieved using a solution-shearing technique. Donor–acceptor stacking axis (or crystallographic a-axis) with the largest transfer integral was aligned parallel to blade-scan axis. Field-effect transistors based on the films afford air-stable n-type characteristics with mobility as high as 0.6 cm² V⁻¹ s⁻¹.

Perovskite photodetectors are fabricated via structuring a perovskite/organic heterojunction with CH₃NH₃PbI₃ and a high-mobility and stable organic semiconductor dioctylbenzothieno [2,3-b] benzothiophene (C8BTBT), which possess broad range photoresponse from ultraviolet to near-infrared, fast response, and excellent stability. The CH₃NH₃PbI₃/C8BTBT heterojunction photodetectors exhibit an excellent ratio of photocurrent to dark current, I_light/I_dark, as high as 2.4 × 10⁴, a high responsivity up to 24.8 AW⁻¹, and a fast response of about 4.0 ms. Meanwhile, the photodetectors can maintain 90% performance even exposed in ambient condition without encapsulation for 20 d. In addition, the detailed mechanism is disclosed based on ultraviolet photoemission spectroscopy, steady-state photoluminescence (PL) spectra, and PL lifetime experiments. The C8BTBT layer acts as an efficient hole-extraction layer to let the holes quickly transport to the electrodes due to its perfect filling in the gaps between perovskite grains, as well as its intrinsic high mobility and the energy-level match with the CH₃NH₃PbI₃. The stable C8BTBT layer can well play as a waterproof layer as well to prevent the perovskite CH₃NH₃PbI₃ from the degradation. The research provides an excellent method for fabricating high-performance and stable perovskite photodetectors using perovskite/organic heterojunction.

Perovskite photodetectors are fabricated by structuring a perovskite/organic heterojunction with CH₃NH₃PbI₃ and organic semiconductor dioctylbenzothieno [2,3-b] benzothiophene (C8BTBT). The CH₃NH₃PbI₃/C8BTBT heterojunction photodetectors exhibit an excellent ratio of photocurrent to dark current, I_light/I_dark, as high as 2.4 × 10⁴, a high responsivity up to 24.8 AW⁻¹, a fast response of about 4.0 ms, and good stability.

Through detailed device characterization using cross-sectional Kelvin probe force microscopy (KPFM) and trap density of states measurements, we identify that the J–V hysteresis seen in planar organic–inorganic hybrid perovskite solar cells (PVSCs) using SnO₂ electron selective layers (ESLs) synthesized by low-temperature plasma-enhanced atomic-layer deposition (PEALD) method is mainly caused by the imbalanced charge transportation between the ESL/perovskite and the hole selective layer/perovskite interfaces. We find that this charge transportation imbalance is originated from the poor electrical conductivity of the low-temperature PEALD SnO₂ ESL. We further discover that a facile low-temperature thermal annealing of SnO₂ ESLs can effectively improve the electrical mobility of low-temperature PEALD SnO₂ ESLs and consequently significantly reduce or even eliminate the J–V hysteresis. With the reduction of J–V hysteresis and optimization of deposition process, planar PVSCs with stabilized output powers up to 20.3% are achieved. The results of this study provide insights for further enhancing the efficiency of planar PVSCs.

Through detailed characterizations, it is identified that the current density-voltage hysteresis of planar perovskite solar cells using low-temperature atomic-layer deposited SnO₂ electron selective layers originates from the poor-electrical conductivity of the SnO₂ layers. A facile low-temperature thermal annealing in ambient air can effectively reduce the degrees of the hysteresis and improve the power conversion efficiency of planar perovskite solar cells.

This study reports a scalable and room-temperature solid-state redox functionalization process for single-walled carbon nanotubes (SWNTs) with instant efficacy and high stability. By drop-casting/spin-coating CuCl₂/Cu(OH)₂ colloidal ethanol solution onto SWNT films, the sheet resistance of the SWNT films achieves 69.4 Ω sq⁻¹ at 90% transparency without noticeable increase for more than 12 months. The charge transfer mechanism between the redox and the SWNTs is revealed by Raman and X-ray photoelectron spectroscopies. The SWNT/silicon solar cells are utilized as a benchmark to evaluate the effectiveness of the redox functionalization process and its compatibility for device integration. The power conversion efficiency of the SWNT/Si solar cell increases by 115% after redox functionalization, reaching the value of 14.09% without degradation in the ambient for over 12 months. Temperature-dependent operation characteristics of the redox functionalized SWNT/Si solar cells demonstrate that the Fermi level unpinning and enhanced tunneling of the charge carriers contribute to the significant improvement of the photovoltage and fill factor. The CuCl₂/Cu(OH)₂ redox also serves as an antireflection layer, resulting in a 20% increase of the photocurrent. The proposed redox functionalized SWNTs are promising as multifunctional transparent conductive films for wide-range solar cell applications.

A scalable and room-temperature solid-state redox functionalization process for single-walled carbon nanotubes with instant efficacy and high stability is reported, with the sheet resistance reaching 69.4 Ω sq⁻¹ at 90% transparency without noticeable increase over 12 months. The redox functionalized films also serve as an antireflective layer for Si heterojunction solar cells, achieving the record-high air-stable power conversion efficiency of 14.09%.

Despite their excellent power conversion efficiency, MAPbI₃ solar cells exhibit strong hysteresis that hinders reliable device operation. Herein it is shown that ionic motion is the dominant mechanism underlying hysteresis of MAPbI₃ solar cells by studying the effects of electrical poling in different temperature ranges. Complete suppression of the hysteresis below 170 K is consistent with temperature activated diffusion of I⁻ anions and/or the motion of the MA⁺ cations. Ionic motion has important effect on the overall efficiency of the MAPbI₃ solar cells: the initial decrease of the power conversion efficiency while lowering the operating temperature is recovered and even enhanced up to 20% of its original value by applying an electrical poling. The open circuit voltage significantly increases and the current density fully recovers due to the reduction of the electron extraction barrier at the TiO₂/MAPbI₃ interface driven by the charge accumulation at the interface. Moreover, beside TiO₂/MAPbI₃ interfacial charge transfer, charge transport in TiO₂ strongly affects the photovoltaic performance, as revealed by MAPbI₃/ms-TiO₂ field effect transistors. These results establish the basis to develop effective strategies to mitigate operational instability of perovskites solar cells.

It is proven that ionic motion is the dominant mechanism behind MAPbI₃ solar cell hysteresis by investigating electrical poling effects in a wide temperature range. The power conversion efficiency reduces at low temperature, but then recovers and improves up to 20% of its original value under electrical bias. This effect is attributed to the electron extraction barrier reduction at the TiO₂/MAPbI₃ interface.

Two hole-extraction materials (HEMs), TPP-OMeTAD and TPP-SMeTAD, have been developed to facilitate the fabrication of efficient p-i-n perovskite solar cells (PVSCs). By replacing the oxygen atom on HEM with sulfur (from TPP-OMeTAD to TPP-SMeTAD), it effectively lowers the highest occupied molecular orbital of the molecule and provides stronger PbS interaction with perovskites, leading to efficient charge extraction and surface traps passivation. The TPP-SMeTAD-based PVSCs exhibit both improved photovoltaic performance and reduced hysteresis in p-i-n PVSCs over those based on TPP-OMeTAD. This work not only provides new insights on creating perovskite-HEM heterojunction but also helps in designing new HEM to enable efficient organic–inorganic hybrid PVSCs.

Two hole-extraction materials (HEMs), TPP-OMeTAD and TPP-SMeTAD, are developed for the construction of p-i-n perovskite solar cells (PVSCs). Through replacing the oxygen atom with sulfur at the arylamine terminal substituents, TPP-SMeTAD exhibits superior energetics, charge extraction and trap passivation capabilities to perovskite, over those of TPP-OMeTAD. It leads to improved photovoltaic performance and reduced hysteresis in TPP-SMeTAD based p-i-n PVSCs.

Although perovskite solar cells (PSCs) have emerged as a promising alternative to widely used fossil fuels, the involved high-temperature preparation of metal oxides as a charge transport layer in most state-of-the-art PSCs has been becoming a big stumbling block for future low-temperature and large-scale R2R manufacturing process. Such an issue strongly encourages scientists to find new type of materials to replace metal oxides. Except for expensive PC₆₁BM with unmanageable morphology and electrical properties, the past investigation on the development of low-temperature-processed and highly efficient electron transport layers (ETLs) has met some mixed success. In order to further enhance the performance of all-solution-processed PSCs, we propose a novel n-type sulfur-containing small molecule hexaazatrinaphtho[2,3-c][1,2,5]thiadiazole (HATNT) with high electron mobility up to 1.73 × 10⁻² cm² V⁻¹ s⁻¹ as an ETL in planar heterojunction PSCs. A high power conversion efficiency of 18.1% is achieved, which is fully comparable with the efficiency from the control device fabricated with PC₆₁BM as ETL. This superior performance mainly attributes from more effective suppression of charge recombination at the perovskite/HATNT interface than that between the perovskite and PC₆₁ BM. Moreover, high electron mobility and strong interfacial interaction via SI or SPb bonding should be also positive factors. Significantly, our results undoubtedly enable new guidelines in exploring n-type organic small molecules for high-performance PSCs.

A new sulfur-containing n-type organic small molecule hexaazatrinaphtho[2,3-c][1,2,5]thiadiazole (HATNT) is proposed for perovskite solar cells. In comparison with traditional PC₆₁BM, benefitting from much more significant suppression of charge recombination at the MAPbI₃/HATNT interface and strong interfacial interaction between the MAPbI₃ and HATNT via SI or SPb bonding, solution-processed high-performance HATNT-based perovskite solar cells are demonstrated with an optimized efficiency up to 18.1%.

The meteoric rise of perovskite single-junction solar cells has been accompanied by similar stunning developments in perovskite tandem solar cells. Debuting with efficiencies less than 14% in 2014, silicon–perovskite solar cells are now above 25% and will soon surpass record silicon single-junction efficiencies. Unconstrained by the Shockley–Quiesser single-junction limit, perovskite tandems suggest a real possibility of true third-generation thin-film photovoltaics; monolithic all-perovskite tandems have reached 18% efficiency and will likely pass perovskite single-junction efficiencies within the next 5 years. Inorganic–organic metal–halide perovskites are ideal candidates for inclusion in tandem solar cells due to their high radiative recombination efficiencies, excellent absorption, long-range charge-transport, and broad ability to tune the bandgap. In this progress report, the development of perovskite tandem cells is reviewed, with presentation of their key motivations and challenges. In detail, it presents an overview of recombination layer materials, bandgap-tuneability, transparent contact architectures, and perovskite compounds for use in tandems. Theoretical estimates of efficiency for future tandem and triple-junction perovskite cells are presented, outlining roadmaps for future focused research.

The remarkable progress of perovskite tandem solar cells, now above 25% efficiency for a silicon–perovskite four-terminal tandem and 18% for monolithic all-perovskite tandems, is reviewed. In detail, the candidate materials, contact layers, and device challenges are examined, outlining a roadmap toward a future of true third-generation thin-film photovoltaics comprising high-efficiency at low cost.

Pinhole-free, dense perovskite films with large grains, which can be achieved by control of the nucleation and crystal growth, are essential for high-performance perovskite solar cells. These perovskite-solar-cell systems are discussed by Fuzhi Huang, Yi-Bing Cheng, and co-workers in article number 1601715.

Solution-processed perovskite (PSC) solar cells have achieved extremely high power conversion efficiencies (PCEs) over 20%, but practical application of this photovoltaic technology requires further advancements on both long-term stability and large-area device demonstration. Here, an additive-engineering strategy is developed to realize a facile and convenient fabrication method of large-area uniform perovskite films composed of large crystal size and low density of defects. The high crystalline quality of the perovskite is found to simultaneously enhance the PCE and the durability of PSCs. By using the simple and widely used methylammonium lead iodide (MAPbI₃), a certified PCE of 19.19% is achieved for devices with an aperture area of 1.025 cm², and the high-performing devices can sustain over 80% of the initial PCE after 500 h of thermal aging at 85 °C, which are among the best results of MAPbI₃-based PSCs so far.

By enhancing the crystalline quality of the simple and widely used MAPbI₃ perovskite through additive engineering, unprecedented photovoltaic performance and thermal stability are achieved. A certified efficiency of 19.19% is obtained for devices with active area over 1 cm², and the high-performing devices show unprecedented durability, maintaining >80% of the initial efficiency after 500 h of thermal aging at 85 °C.

Photodetectors with ultrafast response are explored using inorganic/organic hybrid perovskites. High responsivity and fast optoelectronic response are achieved due to the exceptional semiconducting properties of perovskite materials. However, most of the perovskite-based photodetectors exploited to date are centered on Pb-based perovskites, which only afford spectral response across the visible spectrum. This study demonstrates a high-performance near-IR (NIR) photodetector using a stable low-bandgap Sn-containing perovskite, (CH₃NH₃)_0.5(NH₂CHNH₂)_0.5Pb_0.5Sn_0.5I₃ (MA_0.5FA_0.5Pb_0.5Sn_0.5I₃), which is processed with an antioxidant additive, ascorbic acid (AA). The addition of AA effectively strengthens the stability of Sn-containing perovskite against oxygen, thereby significantly inhibiting the leakage current. Consequently, the derived photodetector shows high responsivity with a detectivity of over 10¹² Jones ranging from 800 to 970 nm. Such low-cost, solution processable NIR photodetectors with high performance show promising potential for future optoelectronic applications.

A high-performance NIR photodetector derived from a stable low optical bandgap (E _g) Sn-containing perovskite, MA_0.5FA_0.5Pb_0.5Sn_0.5I₃, is introduced. Ascorbic acid is used as an effective antioxidant additive to enhance the performance of the photodiode. Finally, a high detectivity of over 10¹² Jones between 800 and 970 nm with a high response rate is achieved.

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃ perovskite films deposited under different yet optimized conditions using X-ray scattering and cathodoluminescence spectroscopy, respectively. X-ray scattering shows that a CH₃NH₃PbI₃ film involving spin coating of CH₃NH₃I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH₃NH₃PbI₃ film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH₃NH₃PbBr₃, while the crystallinity decreases when the HBr solution is used in a single-step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH₂)₂PbBr₃ devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.

A comparative analysis of the local structural, morphological, and emission characteristics of different perovskite films rationally justifies the higher photovoltage yielded by the better performing devices.

Recent research on fabricating scaffold-type perovskite solar cells on plastic substrates has reported noteworthy progress in replacing the high-temperature processing of TiO₂ scaffolds and compact layers with various low-temperature processes. Herein, recent progress in the laboratory is reported regarding the development of electrodeposited TiO_x compact layers and brookite TiO₂ scaffolds, both of which can be processed under 150 °C without greatly sacrificing their photovoltaic performance. Through systematic characterization of device properties and careful optimization of the fabrication conditions, a record-high 15.76% power conversion efficiency of a plastic TiO₂ scaffold-type perovskite solar cell is demonstrated. In addition, bending durability and preliminary stability tests on this plastic perovskite solar cell show promising results and indicate clear directions for future improvement.

An efficient plastic perovskite solar cell with a mesoporous scaffold structure is fabricated by using a low-temperature processable electrodeposited TiO_x compact layer and spin-coated brookite TiO₂ scaffold. The electrodeposition TiO_x compact layer exhibits suitable morphology, favorable band position, and an extraordinary hole-blocking effect, while brookite TiO₂ offers the capability to form a tightly packed scaffold without the need of sintering. Bending and dry-storage stability assessments are taken and showed promising results and clear direction of future improvements.

All-inorganic perovskite CsPbX₃ (X = Cl, Br, I) and related materials are promising candidates for potential solar cells, light emitting diodes, and photodetectors. Here, a novel architecture made of CsPbX₃/ZnS quantum dot heterodimers synthesized via a facile solution-phase process is reported. Microscopic measurements show that CsPbX₃/ZnS heterodimer has high crystalline quality with enhanced chemical stability, as also evidenced by systematic density functional theory based first-principles calculations. Remarkably, depending on the interface structure, ZnS induces either n-type or p-type doping in CsPbX₃ and both type-I and type-II heterojunctions can be achieved, leading to rich electronic properties. Photoluminescence measurement results show a strong blue-shift and decrease of recombination lifetime with increasing sulfurization, which is beneficial for charge diffusion in solar cells and photovoltaic applications. These findings are expected to shed light on further understanding and design of novel perovskite heterostructures for stable, tunable optoelectronic devices.

A novel architecture made of perovskite CsPbBr_3−xI_x/ZnS quantum dot heterodimers is reported by material synthesis, characterization, optical measurements, and systematic first-principles calculations. It is found that CsPbBr_3−xI_x/ZnS heterostructures exhibit high crystal quality, enhanced photostability, and tunable electronic properties, which may provide an exciting playground for future understanding and design of perovskite based nanostructures for high-performance optoelectronic devices.

The ease of processing hybrid organic–inorganic perovskite (HOIPs) films, belonging to a material class with composition ABX₃, from solution and at mild temperatures promises their use in deformable technologies, including flexible photovoltaic devices, sensors, and displays. To successfully apply these materials in deformable devices, knowledge of their mechanical response to dynamic strain is necessary. The authors elucidate the time- and rate-dependent mechanical properties of HOIPs and an inorganic perovskite (IP) single crystal by measuring nanoindentation creep and stress relaxation. The observation of pop-in events and slip bands on the surface of the indented crystals demonstrate dislocation-mediated plastic deformation. The magnitudes of creep and relaxation of both HOIPs and IPs are similar, negating prior hypothesis that the presence of organic A-site cations alters the mechanical response of these materials. Moreover, these samples exhibit a pronounced increase in creep, and stress relaxation as a function of indentation rate whose magnitudes reflect differences in the rates of nucleation and propagation of dislocations within the crystal structures of HOIPs and IP. This contribution provides understanding that is critical for designing perovskite devices capable of withstanding mechanical deformations.

Dynamic mechanical response of hybrid organic–inorganic and inorganic perovskite crystals suggests that the time-dependent mechanical properties of lead–halide perovskites are independent of the chemical character of the A-site cation. Moreover, significant viscoplastic behavior is revealed through creep and stress-relaxation measurements. These phenomena are interpreted as direct results of the crystal structures and how dislocations propagate within them.