ZiQi Sun

A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V _OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.

A new acceptor–donor–acceptor-structured nonfullerene acceptor, ITCC, is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V _OC of over 1 V is recorded in photovoltaic devices, suggesting great potential for applications in tandem organic solar cells.

Metal halide perovskites have been brought to the forefront of research focus in solution-processable photovoltaics, with the device efficiency swiftly surging to over 22% over the past few years. The state-of-the-art metal halide perovskites that have been intensively investigated include toxic lead, which potentially hampers their commercialization process. To address this toxicity issue, intensive recent research effort has been devoted to developing low-toxic metal halide perovskites and their derivatives for photovoltaic applications. Herein, the recent research progress achieved so far in addressing the toxicity issue of lead halide perovskites in photovoltaics is summarized. By comparing the merits and drawbacks of different low-toxic metal halide systems, the current challenges and opportunities in the photovoltaic field are highlighted. Potential low-toxic metal halide perovskites and their derivatives are also discussed from the perspective of theoretical calculations. Furthermore, promising applications of low-toxic metal halide perovskites beyond the photovoltaic sector are briefly discussed.

Recent progress in low toxic metal halide perovskites/derivatives is reviewed with a main focus on photovoltaic applications. The merits and drawbacks of the emerging alternatives in replacement of the state-of-the-art lead halide perovskites are discussed and their future challenges and research opportunities in both photovoltaics and other potential applications are highlighted.

PEDOT:PSS suffers from its poor structural homogeneity. In article number 1601499, Yuan Li, Xueqing Qiu, Hin-Lap Yip, and co-workers report a novel PEDOT:MNSF with enhanced structural homogeneity and promising performance in organic electronic devices. The branched structure of PEDOT:MNSF contributes to its hole extract property and the herringbone image arranged by ladybug indicates that this design concept will move PEDOT-based materials toward a brilliant future.

High-performance nonfullerene polymer solar cells (PSCs) are developed by integrating the nonfullerene electron-accepting material 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophne) (ITIC) with a wide-bandgap electron-donating polymer PTzBI or PTzBI-DT, which consists of an imide functionalized benzotriazole (TzBI) building block. Detailed investigations reveal that the extension of conjugation can affect the optical and electronic properties, molecular aggregation properties, charge separation in the bulk-heterojunction films, and thus the overall photovoltaic performances. Single-junction PSCs based on PTzBI:ITIC and PTzBI-DT:ITIC exhibit remarkable power conversion efficiencies (PCEs) of 10.24% and 9.43%, respectively. To our knowledge, these PCEs are the highest efficiency values obtained based on electron-donating conjugated polymers consisting of imide-functionalized electron-withdrawing building blocks. Of particular interest is that the resulting device based on PTzBI exhibits remarkable PCE of 7% with the thickness of active layer of 300 nm, which is among the highest values of nonfullerene PSCs utilizing thick photoactive layer. Additionally, the device based on PTzBI:ITIC exhibits prominent stability, for which the PCE remains as 9.34% after thermal annealing at 130 °C for 120 min. These findings demonstrate the great promise of using this series of wide-bandgap conjugated polymers as electron-donating materials for high-performance nonfullerene solar cells toward high-throughput roll-to-roll processing technology.

High-performance nonfullerene polymer solar cells with power conversion efficiencies of around 10% are achieved by integrating the wide-bandgap polymers PTzBI or PTzBI-DT with a nonfullerene acceptor ITIC. The extension of conjugation can affect the optical and electronic properties, molecular aggregation properties, and charge separation in the bulk-heterojunction films, and thus the overall photovoltaic performances.

In article number 1605290, Hao-Wu Lin and co-workers report efficient all-vacuum-deposited inorganic cesium lead halide perovskite solar cells of which the stoichiometric ratios of the precursors were carefully calibrated by ellipsometry. The incorporation of bromine was exploited to further enhance the device performance and stability. The results provide a paragon for the use of inorganic precursors en route to efficient vacuum-deposited perovskite devices.

2D photonic crystals (2D-PCs) are directly patterned into methylammonium lead iodide perovskite layers by thermal nanoimprint lithography (NIL) at moderate temperatures of only 100 °C, as described in article number 1605003 by Thomas Riedl and co-workers. The imprinted layers are significantly smoothened and surface defects are eliminated upon thermal imprint. 2D-PCs afford lasing with ultra-low lasing thresholds of 3.8 μJ/cm² at room temperature, which is indicative of excellent material quality of the perovskite after imprint.

Inspired by the remarkable promotion of power conversion efficiency (PCE), commercial applications of organic photovoltaics (OPVs) can be foreseen in near future. One of the most promising applications is semitransparent (ST) solar cells that can be utilized in value-added applications such as energy-harvesting windows. However, the single-junction STOPVs utilizing fullerene acceptors show relatively low PCEs of 4%–6% due to the limited sunlight absorption because it is a dilemma that more photons need to be harvested in UV–vis–near-infrared (NIR) region to generate high photocurrent, which leads to the significant reduction of device transparency. This study describes the development of a new small-bandgap electron-acceptor material ATT-2, which shows a strong NIR absorption between 600 and 940 nm with an E_g^opt of 1.32 eV. By combining with PTB7-Th, the as-cast OPVs yield PCEs of up to 9.58% with a fill factor of 0.63, an open-circuit voltage of 0.73 V, and a very high short-circuit current of 20.75 mA cm⁻². Owing to the favorable complementary absorption of low-bangap PTB7-Th and small-bandgap ATT-2 in NIR region, the proof-of-concept STOPVs show the highest PCE of 7.7% so far reported for single-junction STOPVs with a high transparency of 37%.

A small-bandgap electron acceptor, ATT-2, is designed and synthesized. By combining PTB7-Th donor, the power conversion efficiencies reach 9.58% and 7.74% for opaque and semitransparent devices, respectively. The highest PCE among single-junction STOPVs can be attributed to the beneficial complementary near-infrared absorption of the low-bandgap donor and small-bandgap acceptor. Non-fullerene acceptors are thus very promising for the development of high-performance STOPVs.