Chen Weijie

In article number 1601882, Keyou Yan, Jianbin Xu, and co-workers report a simple intermediate halide exchange reaction between nonstoichiometric FAI and unstable intermediate HPbI₂Br to produce high-quality FAPbI_3-xBr_x thin films with crystal domain up to 2-3 μm, eliminating the use of antisolvent dripping process. And it demonstrates outstanding opto-electronic performances as well as enhanced stability under moisture and heat stress.

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.

A cross-linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino-functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n-type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7-Th:PC₇₁BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.

Cross-linkable dual functional hybrid electron transport layers are developed and can work as both cathode interlayer and light harvesting layer in polymer solar cells, which enhance electron collection and contribute to photocurrent production in the resulting devices. These results may provide new directions for the design of multifunctional interface materials and novel device architectures.

This paper presents a systematic study of the influence of electron-transport materials on the operation stability of the inverted perovskite solar cells under both laboratory indoor and the natural outdoor conditions in the Negev desert. It is shown that all devices incorporating a Phenyl C₆₁ Butyric Acid Methyl ester ([60]PCBM) layer undergo rapid degradation under illumination without exposure to oxygen and moisture. Time-of-flight secondary ion mass spectrometry depth profiling reveals that volatile products from the decomposition of methylammonium lead iodide (MAPbI₃) films diffuse through the [60]PCBM layer, go all the way toward the top metal electrode, and induce its severe corrosion with the formation of an interfacial AgI layer. On the contrary, alternative electron-transport material based on the perylendiimide derivative provides good isolation for the MAPbI₃ films preventing their decomposition and resulting in significantly improved device operation stability. The obtained results strongly suggest that the current approach to design inverted perovskite solar cells should evolve with respect to the replacement of the commonly used fullerene-based electron-transport layers with other types of materials (e.g., functionalized perylene diimides). It is believed that these findings pave a way toward substantial improvements in the stability of the perovskite solar cells, which are essential for successful commercialization of this photovoltaic technology.

Diffusion of CH₃NH₃I and other volatile products of photodegradation of CH₃NH₃PbI₃ into the [60]PCBM electron-transport layer represents the key failure mechanism of inverted hybrid perovskite solar cells.

The photovoltaic performance of polymer solar cells can be dramatically improved upon the incorporation of a nonfullerene electron acceptor (NFA) as the third component. In article number 1602127, Lei Ying, Fei Huang, and co-workers develop a new NFA that can lead to obviously improved performance by virtue of cascade energy transfer. This cover describes a frame of such ternary device, where the three components are highlighted in different colors.

A wide-bandgap polymer, (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,5-(methyl thiophene carboxylate))]) (3MT-Th), is synthesized to obtain a complementary broad range absorption when harmonized with 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′]dithiophene (ITIC). The synthesized regiorandom 3MT-Th polymer shows good solubility in nonhalogenated solvents. A film of 3MT-Th:ITIC can be employed for forming an active layer in a polymer solar cell (PSC), with the blend solution containing toluene with 0.25% diphenylether as a nonhalogenated additive. The corresponding PSC devices display a power conversion efficiency of 9.73%. Moreover, the 3MT-Th-based PSCs exhibit excellent shelf-life time of over 1000 h and are operationally stable under continuous light illumination. Therefore, methyl thiophene-3-carboxylate in 3MT-Th is a promising new accepting unit for constructing p-type polymers used for high-performance nonfullerene-type PSCs.

A wide-bandgap polymer, (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,5-(methyl thiophene carboxylate))]) (3MT-Th), displays a high efficiency of 9.73% with 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′]dithiophene acceptor using toluene as a nonhalogenated solvent. The 3MT-Th-based PSCs exhibit excellent shelf-life stability of over 1000 h and operationally stable under continuous light illumination compared with the PTB7-Th-based PSCs.

Perovskite solar cells have emerged as a promising technique for low-cost, light weight, and highly efficient photovoltaics. However, they still largely rely on 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) to serve as hole-transporting materials (HTMs). Here, a series of HTMs with small molecular weight is designed, which are constructed on a spiro core involving phenylpyrazole and a second heteroaromatics, i.e., xanthene (O atom), thioxanthene (S atom), and acridine (N atom). Through varying from phenylpyrazole substituted xanthene (PPyra-XA), thioxanthene (PPyra-TXA), to acridine (PPyra-ACD), their optical and electrochemical properties, hole mobilities, and the photovoltaic performance are optimized. As a consequence, PPyra-TXA based device exhibits the highest power conversion efficiency (PCE) of 18.06%, outperforming that of Spiro-OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene. In addition, the dopant-free device shows PCE of 11.7%. These results open a new direction for designing spiro-HTMs by simple modification of chemical structures.

Perovskite solar cells bearing spiro-phenylpyrazole-9,9′-thioxanthene exhibit power conversion efficiency of 18.06%, outperforming that of Spiro-OMeTAD (16.15%), which could be attributed to the enhancement of hole mobility exerted by the thioxanthene.

The halide perovskite (PVSK) materials (with ABX₃ formulation) have emerged as “dream materials” for photovoltaic (PV) applications due to their remarkable physical properties such as high optical absorption coefficient, carrier mobility, long carrier diffusion lengths, etc. These properties have enabled the PV devices to reach higher than 20% power conversion efficiencies (PCE) in record time. The further pursuit of higher PCE and improved stability brings forth increasing interests in so-called “mixed composition” PVSK materials, consisting of partial substitution of the A, B, and/or X-sites with alternative elements/molecules of similar size. Herein, we highlight the recent advances in developing mixed PVSK for PVs and their relevant optoelectronic properties. We mainly focus on mixed PVSK materials in the form of polycrystalline thin films, but also discuss nanostructured and two-dimensional (2D) PVSK materials due to the increasing interest of broad readership. Efforts are exerted to elucidate the design principles of mixed PVSK and fabrication techniques for high performance optoelectronic devices, which help deepen our fundamental understanding of mixed PVSK systems. We hope this review will shed light onto the design and synthesis of mixed PVSK materials to further the progress of PVSK photovoltaics towards higher efficiencies and longer lifetimes.

Perovskite solar cells (PSCs) employing metal halide perovskites with mixed composition as light absorbers generally show high device performance. The recent progress of PSCs based on mixed perovskites is reviewed, trying to shed light onto the design and synthesis of mixed perovskite materials to further improve the device performance on both efficiency and stability.

Organic–inorganic hybrid perovskite solar cells have resulted in tremendous interest in developing next generation photovoltaics due to high record efficiency exceeding 22%. For inverted structure perovskite solar cells, the hole extraction layers play a significant role in achieving efficient and stable perovskite solar cell by modifying charge extraction, interfacial recombination losses, and band alignment. Here, cesium doped NiO_x is selected as a hole extraction layer to study the impact of Cs dopant on the optoelectronic properties of NiO_x and the photovoltaic performance. Cs doped NiO_x films are prepared by a simple solution-based method. Both doped and undoped NiO_x films are smooth and highly transparent, while the Cs doped NiO_x exhibits better electron conductivity and higher work function. Therefore, Cs doping results in a significant improvement in the performance of NiO_x-based inverted planar perovskite solar cells. The best efficiency of Cs doped NiO_x devices is 19.35%, and those devices show high stability as well. The improved efficiency in devices with Cs:NiO_x is attributed to a significant improvement in the hole extraction and better band alignment compared to undoped NiO_x. This work reveals that Cs doped NiO_x is very promising hole extraction material for high and stable inverted perovskite solar cells.

Cesium doping of NiO_x enhances the conductivity of the oxide film and the hole extraction from the perovskite film in inverted planar perovskite solar cells. Significantly improved photovoltaic performance is obtained with the best efficiencies of 16.04% and 19.35% for NiO_x and Cs:NiO_x, respectively. The devices exhibit negligible hysteresis and good stability.

This paper reports highly bright and efficient CsPbBr₃ perovskite light-emitting diodes (PeLEDs) fabricated by simple one-step spin-coating of uniform CsPbBr₃ polycrystalline layers on a self-organized buffer hole injection layer and stoichiometry-controlled CsPbBr₃ precursor solutions with an optimized concentration. The PeLEDs have maximum current efficiency of 5.39 cd A⁻¹ and maximum luminance of 13752 cd m⁻². This paper also investigates the origin of current hysteresis, which can be ascribed to migration of Br⁻ anions. Temperature dependence of the electroluminescence (EL) spectrum is measured and the origins of decreased spectrum area, spectral blue-shift, and linewidth broadening are analyzed systematically with the activation energies, and are related with Br⁻ anion migration, thermal dissociation of excitons, thermal expansion, and electron–phonon interaction. This work provides simple ways to improve the efficiency and brightness of all-inorganic polycrystalline PeLEDs and improves understanding of temperature-dependent ion migration and EL properties in inorganic PeLEDs.

Efficient and bright CsPbBr₃ perovskite light-emitting diodes are achieved using a one-step fabrication of uniform CsPbBr₃ polycrystalline layers on a self-organized buffer hole injection layer without synthesis of quantum dots. A study of the temperature dependence of current hysteresis and electroluminescence spectrum provides understanding of ion migration, nonradiative pathways, and electron–phonon interaction in the CsPbBr₃ perovskite light-emitting diodes.

Ultrafast solar-blind UV detection, enabled by a hybrid inorganic perovskite CsPbX₃ quantum-dot radial junction architecture, achieves rise/fall response times of 0.48/1.03 ms and a high responsivity of 54 mA W⁻¹ at 200 nm without an external power supply, as described by Linwei Yu, Xiangxing Xu, and co-workers in article number 1700400. These results pave the way toward large-area manufacturing of high-performance Sibased perovskite UV detectors in a low-cost procedure.

Upon cooling and heating, organic–inorganic perovskite nanoparticles are reversibly decomposed and formed through the lower-critical-solution-temperature phenomenon. Akinori Saeki and co-workers demonstrate an aurora-like photoemissive object in article number 1700047, and reveal the roles of the involved chemicals (methylammonium cation, lead, halogen, and molecular additives). The cloud temperature is varied from ca. 30 to 80 °C by controlling the concentration, and multi-luminescent-color thermoresponsive solutions are achieved through compositional engineering of the halogen.