周期量级激光脉冲对有机太阳能电池中超快载流子行为的研究

Recently, organic solar cells based on D-A copolymers have attracted a lot of interest both in scientific and commercial field. Compared with the inorganic solar cells, organic solar cells are very promising because of the low cost, ease of processing, thin film flexibility, etc. However, the power conversion efficiencies of the organic solar cells are usually low in contrast to the inorganic ones. Therefore, how to enhance the efficiencies of the organic solar cells is the focus of studies in the relative field. Currently, scientists have realized that understanding the photoinduced charge generation dynamics in the bulk heterojunction (BHJ) blends is especially important to improve the light-to-current conversion. Even though it has been studied for several years, there is still no universal consensus about the precise mechanisms. In our project, we will apply few-cycle laser pulse in the ultrafast transient absorption spectroscopy to study the charge generation and recombination processed in the organic solar cells with D-A copolymers as the donor. Since most of the laser spectroscopy in bulk heterojunction are studied by visible laser pulses, the photoinduced mechanism under even shorter wavelength has been rarely studied. At first, we will use ultrashort visible laser pulse with duration of 6 fs to clarify the primary photoexcitations is exciton or free charge carriers. Also, we will study the intramolecular charge transfer character in the PBDTTT copolymers using the laser pulse centered at 400nm with duration of 9 fs for the first time to our knowledge, and investigate the possible influence on the charge separation in the organic solar cells. Furthermore, also for the first time, we investigate the charge photogeneration by excite the donor to its second absorption band using the laser excitation center at 400nm with pulse duration shorter than 10 fs. We will focus our attention on the study on the photoinduced excited-state delocalization and its contribution to the full dissociation of electron and hole. We hope the results achieved in our project could be use for designing new low-bandgap organic semiconductors and to improve energy conversion at photon energies largely exceeding the bandgap.

有机太阳能电池相关材料和器件是近年来国内外科学研究和尖端工业技术的前沿和热点。围绕如何提高光伏电池转换效率的核心问题，越来越多的研究小组采用飞秒激光脉冲研究有机太阳能电池的工作机理。本项目将主要进行以下几方面的研究：采用可见波段周期量级激光脉冲,实时观测光生载流子的超快演变过程，探索解决有机太阳能电池中光激发初始产物存在的争议；率先采用400nm周期量级激光脉冲，研究多余光子能量对PBDTTT类窄带隙共聚物/富勒烯衍生物混膜构成的有机太阳能电池中光生载流子的超快产生、分离及传输机制的影响，探索PBDTTT分子内电荷迁移过程对有机太阳能电池转换效率的潜在催动作用。这些研究有助于将人们对有机太阳能工作机理的认识推向到更广的波长范围，有望填补目前太阳能电池的电荷分离和传递研究中的诸多空白，从而进一步对聚合物太阳能电池中器件性能进行优化，最终有望解决有机太阳能电池低能量转换效率的难题。

能源问题是当今世界面临的最重要问题之一。太阳能电池是将清洁无污染、取之不尽、用之不竭的太阳能转化为电能的有效途径，有望根本解决人类的能源问题。有机太阳电池相关材料和器件具有纤薄轻便、柔韧性好、成本低廉、工艺简单等突出优点，是近年来国内外科学研究和尖端工业技术的前沿和热点。围绕如何提高光伏电池转换效率的核心问题，越来越多的研究小组采用飞秒激光脉冲研究有机太阳能电池的工作机理。我们通过超快光谱实验的手段对有机太阳能电池材料体系内电荷产生以及分离等重要过程进行了实验追踪，研究了有机太阳能电池中光激发产物和多余光子能量对有机太阳能电池中光生载流子的超快产生、分离及传输机制的影响等，以及退火、添加剂等处理方法对异质结内电荷迁移过程及有机太阳能电池转换效率的潜在催动作用。对于富勒烯受体的有机太阳能电池体系，我们主要研究了溶剂添加剂对此类富勒烯有机太阳能器件中激子的传输以及解离等超快过程的影响。对比发现，添加剂的加入能有效的调控激子解离寿命以及载流子收集效率，最终提高此类器件的光电转换效率，这对于提高基于富勒烯类有机太阳能电池材料光电转换效率提供了重要基础支持。对于小分子体系的有机太阳能电池材料，通过研究退火过程对于ITIC 类非富勒烯有机太阳能器件中激子的传输以及解离过程的影响，发现退火过程能有效控制活性层的共混薄膜形貌，使得体异质结表面附近的激子解离寿命得到降低，并且使远离体异质结表面激子扩散速度迅速提高，激子分离产生载流子的效率增加，进一步提高了有机太阳能电池器件载流子的收集效率。这些超快光谱实验得到的结果将有助于提升人们对有机太阳能工作机理的微观认识以及优化有机太阳电池太阳能电池材料结构以及光伏电池器件界面，进一步提高有机太阳能电池的能量转换效率，从而有望进一步推动有机太阳能电池商品化的步伐。