有机-无机杂化钙钛矿复合多层结构的制备、能带调控及光电转换性能研究

Hybrid organic/inorganic solar cells based on organometal halide perovskites are currently among the most competitive candidates for photovoltaic applications. During the past 5 years, hybrid perovskite solar cells have been reported to achieve remarkably high photoelectronic conversion efficiency of ~15%. In such devices, organometal halide perovskites can assume both the role of light absorber and photogenerated carrier (electron and hole) transporters, representing the core of the devices. Thus, the properties of the perovskites, such as energy band structure, light harvesting efficiency, carrier mobilities and diffusion length determine the device performances. In this proposal, we will first synthesize a group of organometal halide perovskite materials with different crystal lattice size via tuning the component of the perovskite crystal lattice, and then investigate the energy band structure, light absorbing abilities and photogenerated carrier transport properties by experimental characterizations (XRD, XPS, UPS, TRPL, absorption spectra and electrochemical impedance analysis, etc.) and theoretical calculation. Based on the results mentioned above, we will demonstrate a strategy to construct an oxide semiconductor/organometal halide perovskite multi-layer composite heterojunction solar cell without containing any organic hole transport materials (HTMs) which are typically employed as hole extractor in the current hybrid perovskite solar cells. In this heterojunction solar cell, the component (halogen) of the perovskite multi-layer composite is in situ gradiently changed to make their valance band (VB) positions gradually increase to the Femi level of the metal counter electrode, giving rise to the better hole transport properties in perovskite composite and better hole transfer efficiency from perovskite composite to the metal electrode. We believe this research would provide a valuable theoretical and experimental reference for design and synthesis of hybrid perovskite heterojunction solar cells with high performance.

基于有机-无机杂化钙钛矿的固态太阳能电池已获得超过15%的光电转换效率，是极具发展潜力的新一代光伏技术。作为这种器件的核心，有机-无机杂化钙钛矿材料的性能，如能级结构、光吸收性能、载流子迁移率和扩散长度，直接影响着器件的效率，对这些因素能否进行有效调控也就成为设计构建电池和提高器件性能的关键。在前期工作基础上，本项目拟通过调控材料的组成元素（卤素），合成出具有不同晶格尺寸的杂化钙钛矿材料，并结合理论计算和实验结果，分析由晶格组成元素改变所引起的材料能级结构、光吸收和载流子输运性能的变化。以这些结果为基础，在制备过程中原位梯度调控钙钛矿晶体中的元素（卤素）组成，合成出价带顶位置阶梯式升高的复合多层钙钛矿材料，从而改善光生空穴在其中的输运效率，制备出不含有机空穴传输层的“氧化物半导体/复合多层钙钛矿/金属电极”异质结太阳能电池，为该种电池的设计和性能提高提供有价值的理论和实验依据。

经过近几年发展，以含卤有机-无机杂化钙钛矿材料为核心的光伏电池转换效率已突破20%，具有极佳的民用前景。研究表明，钙钛矿太阳能电池的性能主要取决于器件中钙钛矿薄膜的质量，而能否制备致密性高、孔洞少、光电转换性能优良的薄膜也就成为制备性能优良钙钛矿光伏电池的关键。本课题以此为背景，首先探索高质量MAPbI3型钙钛矿薄膜的制备方法，开发出一种两步旋涂-中间滴加反极性溶剂的方法制备蓝紫色MAPbI3薄膜。与传统方法相比，我们所开发的方法改进了旋涂过程中滴加的反极性溶剂与前驱体中间相膜的浸润过程，消除了薄膜中未转化的中间相材料，提高了薄膜红光区的吸收能力，薄膜因此呈现蓝紫色。以这种膜为核心的光伏电池的能量转换效率比传统方法提高近10%，总光电转换效率达16%。以此为基础，通过在前驱体溶液中引入N-甲基吡咯烷酮（NMP）作为路易斯碱添加剂，我们研制出一种稳定性高、不含delta相的蓝黑色FAPbI3型钙钛矿薄膜的制备工艺。结果表明，NMP可与前驱体PbI2、FAI作用形成稳定中间体，促进其形核但抑制其在低温下生长，最终抑制热处理后薄膜中delta相FAPbI3的形成。与采用DMSO等传统添加剂的方法相比，我们合成的FAPbI3薄膜具有更佳优异的光电转换性能，以其为核心的光伏电池的能量转换效率达到16.6%。此外，通过研究Br在卤素混合型钙钛矿材料（MAPb(I1-xBrx)3）中的掺杂行为，我们发现MAPb(I1-xBrx)3材料晶体结构发生相转换的精确掺杂量：当Br元素的掺杂量（x值）在1/6时，材料的晶体结构由四方结构向立方/准立方结构转换。结果表明，随着Br掺杂量的增加，MAPb(I1-xBrx)3材料的禁带宽度逐渐增加，相变温度逐渐降低，MAPb(I5/6Br1/6)3）材料在25~100 ℃温度范围内不发生相变，为稳定的立方/准立方结构。与传统MAPbI3材料相比，MAPb(I5/6Br1/6)3材料的稳定性和光电转换性能均得到改善，其能量转换效率在16.4%左右。本课题的研究结果对于高质量钙钛矿薄膜制备、光电转换性能优化和稳定性提高提供了重要的理论和实验依据，对这类电池性能的改善具有重要参考价值。