基于非极性晶面保护策略的近红外PbS量子点光活性层制备及光伏应用

Lead sulfide quantum dots (PbS QDs) have been recognized as key materials for near-infrared (NIR) photo-to-electric conversion due to their merits such as tunable bandgap in the broad spectral range from IR to visible, and solution-based film-forming technology. However, the NIR responsive PbS QDs contain a large proportion of nonpolar (100) exposed facet, interacting weakly with surface ligands, which makes the QDs easy to be oxidized and form aggregate in solution, and in turn renders a poor control over the microstructure and defects of QD film and the relatively low power conversion efficiency of NIR-QD solar cells. Being about to tackle this critical issue, we propose in the present project a strategy of rational protecting the nonpolar (100) exposed facet of NIR responsive PbS QDs for the purpose of obtaining QD photoactive layer with low defect density and fabricating NIR QD solar cell with high efficiency. The main research contents include: 1) exploring liquid-epitaxy methods to grow ultrathin protection layer of metal halides on the nonpolar (100) exposed facets of PbS QDs; 2) exploring recipes to optimize the composition of ionized surface ligand layer during phase transfer process for stable QD ink; 3) exploring the conditions of atomic layer deposition to post-encapsulate PbS QD films with inert oxides for lowering the defect density of QD film further; 4) investigating the influence of the above processing procedures on the physics of QD solar cell including the behavior of carrier transportation, injection and recombination, and optimizing the fabrication technology of QD solar cell based on the above investigation to obtain high-efficiency QD solar cell with threshold wavelength longer than 1400 nm. The above research will advance the knowledge of science community on NIR responsive QD photovoltaic materials, and impact positively on the research and development of NIR solar cells.

PbS量子点的带隙宽谱可调、可溶液成膜等优势使其成为近红外区太阳能光电转换的关键材料。但是近红外响应型PbS量子点的大比例、非极性（100）暴露晶面缺乏稳定的表面配体，致使量子点薄膜的微结构和缺陷态难以控制，严重影响量子点太阳能电池性能提升。针对这一关键问题，本项目提出基于PbS量子点（100）暴露晶面保护策略制备低缺陷密度量子点光活性层及高效近红外响应型太阳能电池的研究构想。通过在近红外PbS量子点（100）暴露晶面液相外延生长卤化物超薄保护层，优化相转移过程中量子点表面电离配体层提升其稳定性，原子层沉积惰性氧化物后包覆量子点薄膜等手段制备低缺陷密度的PbS量子点光活性层。深入考察非极性面保护方法对器件载流子输运、注入、复合等过程的影响，优化器件制备工艺，获得阈值波长长于1400nm的高效量子点太阳能电池，推进近红外太阳能光电转换的研究进程。

本项目聚焦带隙宽谱可调、可溶液成膜的近红外响应型PbS量子点光伏材料，通过PbS量子点（100）暴露晶面保护策略，破解非极性（100）晶面与配体结合力弱导致PbS量子点缺陷态难以控制的难题。从PbS量子点合成和溶液配体交换的不同过程、实现PbS量子点光伏材料的表面配体和晶面结构的可控调节，揭示了量子点表面结构、光/电物性和光伏性能三者间的构效关系；从界面工程、新型电子传输层材料和器件光电模拟等多个角度，优化近红外量子点太阳能界面能带排列和光生载流子收集/复合过程；在此基础上，将量子点太阳能电池器件构型从异质结型向量子结型拓展，实现量子点光伏器件光响应范围和稳定性的大幅突破。取得如下重要结果：（1）建立了以乙酰丙酮铅（Pb(acac)2）为前驱体的量子点合成新方法，大幅减小合成过程中水副产物对量子点表面的不良影响，实现量子点表面配体的精确调控和优化。（2）系统考察乙酸铵、乙酰丙酮铕、醋酸锌、氢碘酸胍等多种添加剂和辅助配体对量子点溶液配体交换过程及其材料光电特性的影响，并提出溶液后退火方法，获得高稳定性、低缺陷量子点墨水。（3）发展了原子层沉积Al2O3超薄界面层钝化p型PbS量子点-Au背电极，压力后处理增强PbS量子点电子耦合等界面工程策略，显著提升了器件性能。（4）发展了多种适用于量子点光伏的ZnO-PEI、SnO2等新型电子传输层材料，利用浸渍提拉制备的SnO2电子传输层实现了器件效率和工作稳定性的同时提升。（5）研制高效率无电子传输层“量子结”光伏器件，以3 nm- SnOx为界面缓冲层，将量子结型PbS量子点太阳能电池的光响应范围突破至1400 nm，光电转换效率成功突破至11.5%；该效率为同类型光伏器件文献报道的最高值，较同类型器件效率提升35%。相关研究结果对于深刻理解量子点表面性质对其光电性质的影响机制具有重要的参考价值，为后续研发包括太阳能电池、光探测器、场效应晶体管等量子点光电器件提供了科学依据和技术基础。