Qifan Xue

Highly efficient fullerene/perovskite planar heterojunction solar cells via cathode modification with an amino-functionalized polymer interlayer

A new amino-functionalized polymer, PN4N, was developed and applied as an efficient interlayer to improve the cathode interface of fullerene/perovskite (CH3NH3PbIxCl3−x) planar heterojunction solar cells. The PN4N polymer is soluble in IPA and n-BuOH, which are orthogonal solvents to the metallohalide perovskite films, and therefore they can be spuncast on the heterojunction layer before the deposition of the metal cathode. This simple modification of the cathode interface showed a remarkable enhancement of power conversion efficiency (PCE) from 12.4% to 15.0% and also reduced the hysteresis of photocurrent. We also found that conventional water–methanol-soluble polymer interlayer, such as PFN, was incompatible with the perovskite films because of the small molecular size of aprotic solvent such as MeOH, which could decompose the perovskite films to PbI2, resulting in considerably lower solar cell performance. This study provides new design guidelines for efficient interfacial materials and also demonstrates that interface engineering could be a key strategy to improve perovskite solar cells.

Decomposition of Organometal Halide Perovskite Films on Zinc Oxide Nanoparticles

Solution processed zinc oxide (ZnO) nanoparticles (NPs) with excellent electron transport properties and a low-temperature process is a viable candidate to replace titanium dioxide (TiO2) as electron transport layer to develop high-efficiency perovskite solar cells on flexible substrates. However, the number of reported high-performance perovskite solar cells using ZnO-NPs is still limited. Here we report a detailed investigation on the chemistry and crystal growth of CH3NH3PbI3 perovskite on ZnO-NP thin films. We find that the perovskite films would severely decompose into PbI2 upon thermal annealing on the bare ZnO-NP surface. X-ray photoelectron spectroscopy (XPS) results show that the hydroxide groups on the ZnO-NP surface accelerate the decomposition of the perovskite films. To reduce the decomposition, we introduce a buffer layer in between the ZnO-NPs and perovskite layers. We find that a commonly used buffer layer with small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can slow down but cannot completely avoid the decomposition. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. The power conversion efficiencies of perovskite photovoltaic cells are significantly increased from 6.4% to 10.2% by replacing PC61BM with PEI as the buffer layer.

Phosphonium Halides as Both Processing Additives and Interfacial Modifiers for High Performance Planar-Heterojunction Perovskite Solar Cells

Organic halide salts are successfully incorporated in perovskite-based planar-heterojunction solar cells as both the processing additive and interfacial modifier to improve the morphology of the perovskite light-absorbing layer and the charge collecting property of the cathode. As a result, perovskite solar cells exhibit a significant improvement in power conversion efficiency (PCE) from 10% of the reference device to 13% of the modified devices.

Metallohalide perovskite–polymer composite film for hybrid planar heterojunction solar cells

A polymer with tailored chemical functionality was introduced as a processing additive to control the film formation of the CH3NH3PbI3 perovskite structure, leading to enhanced photovoltaic performance in a PEDOT:PSS/perovskite/PCBM-based planar heterojunction solar cell under optimized conditions. By adjusting the polymer doping content and the processing solvent, the grain size, film coverage and the optical properties of the perovskite films can be effectively tuned. At optimized conditions, the planar heterojunction solar cell composed of a thin layer of perovskite–polymer film (∼50 nm) exhibits an average PCE of 6.16% with a Voc of 1.04 V, a Jsc of 8.85 mA cm−2 and a FF of 0.65, which are much higher than those of the control device with a pristine perovskite film. The higher performance was attributed to improved morphology and interfaces of the perovskite–polymer films, which reduced the undesired contact between PEDOT:PSS and PCBM and minimized the shunting paths in the device. In addition, since the fabrication process for the perovskite solar cells can be performed at low temperature, flexible cells built on plastic substrates can therefore be realized with a PCE of 4.35%.

High-Performance Color-Tunable Perovskite Light Emitting Devices through Structural Modulation from Bulk to Layered Film

Adding 2-phenoxyethylamine (POEA) into a CH3 NH3 PbBr3 precursor solution can modulate the organic-inorganic hybrid perovskite structure from bulk to layered, with a photoluminescence and electroluminescence shift from green to blue. Meanwhile, POEA can passivate the CH3 NH3 PbBr3 surface and help to obtain a pure CH3 NH3 PbBr3 phase, leading to an improvement of the external quantum efficiency to nearly 3% in CH3 NH3 PbBr3 LED.

A PCBM Electron Transport Layer Containing Small Amounts of Dual Polymer Additives that Enables Enhanced Perovskite Solar Cell Performance

A polymer/PCBM hybrid electron transport layer is reported that enables high‐performance perovskite solar cells with a high power conversion efficiency of 16.2% and with negligible hysteresis. Unlike previous approaches of reducing hysteresis by thermal annealing or fullerene passivation, the success of our approach can be mainly attributed to the doping of the PCBM layer using an insulating polymer (polystyrene) and an amine‐containing polymeric semiconductor named PFNOX.

Synthesis of donor–acceptor copolymers based on anthracene derivatives for polymer solar cells

A series of narrow band gap donor–acceptor type conjugated copolymers based on 2,6-linked anthracene derivatives are synthesized via Suzuki copolymerization. The resulting copolymers typically exhibit dual absorption characteristics in both solution and as thin films with optical band gaps in the range of 1.85–2.13 eV. By varying the substitutions from alkyloxy to aromatic thienyl and phenyl groups in the 9,10-positions of the anthracene unit, the constructed two-dimensional 2,6-linked anthracene structures lead to broader absorption, lower-lying highest occupied molecular orbitals, as well as improved charge carrier mobilities of the resulting copolymers relative to the alkyloxy side chain substituted counterparts. Additionally, it was found that the sizes of the substitutions in the benzo[c][1,2,5]thiadiazole acceptor also play an important role in the optoelectronic properties of these anthracene based conjugated copolymers. The best polymer solar cell device with a power conversion efficiency of 4.34% and a high open circuit voltage of 0.98 V was realized based on the resulting materials. Our results indicate that substantial optimization in the sizes and patterns of substitutions of both the 2,6-linked anthracene donor and benzo[c][1,2,5]thiadiazole acceptor may potentially lead to high performance narrow band gap copolymers for solar cell applications.

Recent advances in semi-transparent polymer and perovskite solar cells for power generating window applications

Semi-transparent photovoltaic (ST-PV) technologies can be applied to replace facades and roofs in conventional buildings and coatings on vehicles to produce energy from sunlight. Current ST-PV technology is Si-based, but although Si achieves adequate efficiencies, it compromises on aesthetic appeal; its color is intrinsically difficult to tune. However, this presents an opportunity for semi-transparent polymer and perovskite-based PVs, the optical properties of which can be modulated easily by tuning their material compositions. In this review article, we summarize recent progress made in the material selection, optical engineering and device architecture design for high-performance, semi-transparent polymer and perovskite solar cells and discuss challenges for the commercialization of these semi-transparent solar cells for power-generating applications in windows.

The fabrication of color-tunable organic light-emitting diode displays via solution processing

Electroluminescent devices based on organic semiconductors have attracted significant attention owing to their promising applications in flat-panel displays. The conventional display pixel consisting of side-by-side arrayed red, green and blue subpixels represents the mature technology but bears an intrinsic deficiency of a low pixel density. Constructing an individual color-tunable pixel that comprises vertically stacked subpixels is considered an advanced technology. Although color-tunable organic light-emitting diodes (OLEDs) have been fabricated using the vacuum deposition of small molecules, the solution processing of conjugated polymers would enable a much simpler and inexpensive manufacturing process. Here we present the all-solution processing of color-tunable OLEDs comprising two vertically stacked polymer emitters. A thin layer of highly conducting and transparent silver nanowires is introduced as the intermediate charge injection contact, which allows the emission spectrum and intensity of the tandem devices to be seamlessly manipulated. To demonstrate a viable application of this technology, a 4-by-4 pixelated matrix color-tunable display was fabricated.

Interface Engineering for All‐Inorganic CsPbI2Br Perovskite Solar Cells with Efficiency over 14%

In this work, a SnO2 /ZnO bilayered electron transporting layer (ETL) aimed to achieve low energy loss and large open-circuit voltage (Voc ) for high-efficiency all-inorganic CsPbI2 Br perovskite solar cells (PVSCs) is introduced. The high-quality CsPbI2 Br film with regular crystal grains and full coverage can be realized on the SnO2 /ZnO surface. The higher-lying conduction band minimum of ZnO facilitates desirable cascade energy level alignment between the perovskite and SnO2 /ZnO bilayered ETL with superior electron extraction capability, resulting in a suppressed interfacial trap-assisted recombination with lower charge recombination rate and greater charge extraction efficiency. The as-optimized all-inorganic PVSC delivers a high Voc of 1.23 V and power conversion efficiency (PCE) of 14.6%, which is one of the best efficiencies reported for the Cs-based all-inorganic PVSCs to date. More importantly, decent thermal stability with only 20% PCE loss is demonstrated for the SnO2 /ZnO-based CsPbI2 Br PVSCs after being heated at 85 °C for 300 h. These findings provide important interface design insights that will be crucial to further improve the efficiency of all-inorganic PVSCs in the future.