Zhicheng Hu

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.

High-Performance Polymer Solar Cells with Electrostatic Layer-by-Layer Self-Assembled Conjugated Polyelectrolytes as the Cathode Interlayer.

An easy and efficient approach to achieve a large-area cathode interlayer with controlled film composition, uniformity, and thickness under a nanometer scale is reported by using an electrostatic layer-by-layer (eLbL) self-assembly process. The eLbL films provide a new means for preparing efficient interlayers for polymer solar cells (PSCs) and also represent a potential candidate for use in high-performance large-area PSC modules in the future.

Towards a bright future: polymer solar cells with power conversion efficiencies over 10%

Remarkable progress in high-performance polymer solar cells demonstrates their great potential for practical applications in the near future. Indeed, the power conversion efficiencies over 10% have been reported by many research groups, which are achieved through rational optimization of light-harvesting materials, interfaces and device processing technologies. In this mini review, we summarized the recent progress of highly efficient polymer solar cells, with specifically concern on successful strategies of rational molecular design of electron-donating and electron-accepting materials, elaborative interfacial engineering, and reasonable device architectures.

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%.

Quaternisation-polymerized N-type polyelectrolytes: synthesis, characterisation and application in high-performance polymer solar cells

Perylene diimide (PDI) based semiconductors with high mobility are promising electron-transporting materials (ETMs), which are used to fabricate polymer solar cells (PSCs) using roll-to-roll (R2R) processing. However, PDI-based molecular semiconductors have a strong tendency to aggregate, which hinders their use as ETMs in the fabrication of high-performance thin-film devices. Additionally, multi-layer organic opto-electronic devices require that the materials for different layers should possess orthogonal solubility. Here, we develop an in situ polymerisation method to successfully prepare ion-containing PDI-based polyelectrolytes with good water/alcohol solubility, which can enable high-performance PSCs. The doping behaviour, self-assembling and charge-transporting properties of these polyelectrolytes can be fine-tuned by their anions, which allows the fabrication of high-quality and high-mobility electron-transporting thin films for PSCs. PSCs with these polyelectrolytes can maintain high power conversion efficiencies of over 8% when the thickness of the polyelectrolyte is up to 50 nm, which offers a remarkable processing window for the mass-fabrication of PSCs using R2R techniques. Our findings on the structure–property–performance relationships of these polyelectrolytes provide insights and guidelines for the design of high-performance n-type polyelectrolytes for organic opto-electronic devices.

Impact of the Electron‐Transport Layer on the Performance of Solution‐Processed Small‐Molecule Organic Solar Cells

Although the performance of polymer solar cells has been improved significantly recently through careful optimization with different interlayers for the same materials, more improvement is needed in this respect for small-molecule-based solar cells, particularly for the electron-transport layers (ETLs). In this work, three different solution-processed ETLs, PFN, ZnO nanoparticles, and LiF, were investigated and compared in the performance of small-molecule-based devices, and power conversion efficiencies (PCEs) of 8.32, 7.30, and 7.38% were achieved, respectively. The mechanism for the ETL-induced enhancement has been studied, and different ETLs have a significantly different impact on the device performance. The clearly improved performance of PFN is attributed to the combination of reduced bimolecular recombination and increased effective photon absorption in the active layer.

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.

Cross-Linkable and Dual Functional Hybrid Polymeric Electron Transporting Layer for High-Performance Inverted Polymer Solar Cells

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:PC71 BM 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.

The influence of amino group on PCDTBT-based and P3HT-based polymer solar cells: Hole trapping processes

Polymer solar cells (PSCs) based on aliphatic-amino-functionalized materials presented low performance with negligibly small efficiency, the prime mechanism of which is found to be hole trapping induced by the amine end groups. We propose that such hole trapping behavior depends on the relative energetic position of the hole transport states and the trapping states. Herein, we comparatively study the photovoltaic properties of PSCs based on amino-functionalized fullerene derivative blended with poly [N-9′-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′, 1′, 3′-benzothiadiazole)] (PCDTBT) or poly (3-hexylthiophene) (P3HT). The former polymer has a lower-positioning highest occupied molecular orbital (HOMO) level, whereas the latter has a comparable HOMO level relative to the ionization state of tertiary aliphatic amine in energy. Our investigation confirms our proposition, revealing an ultrafast trapping process in PCDTBT:amino-group-functionalized fullerene derivative film, which seriously crippled hole transport, consequently results in very poor device performance. In contrast, trapping process is almost negligible in P3HT systems.