有机光伏太阳能电池由于其半透明、低成本和环境友好的特性，是下一代新能源材料的重点研究领域之一。对有机光伏太阳能电池物理和化学反应过程的深入理解，有利于其光电转化效率的提高。为此本文利用自行搭建的时间分辨多维光谱系统，深入研究了有机光伏材料中的能量转化过程，为新型有机光伏异质结的设计提供了方向。本文主要完成了以下几方面的研究工作：
（1）基于非线性激光技术，完成了光学参量放大器、非共线光学参量放大器、可见瞬态吸收系统和傅里叶变换二维可见光谱系统的搭建工作。通过LabView编写的控制软件协同各个仪器间的工作，实现了系统的软硬件一体化设计。开发了可用于各种不同光谱系统采集的具有普遍适用性的控制软件。通过对强散射环境下二维光谱系统中信号构成的理论分析，开发了一种shot to shot的多维光谱噪声移除方案，使得在强散射情况下测量的多维光谱信号可以具备和无散射情况下一样的信号质量。
（2）使用线性和非线性红外光谱法研究了模型化合物异氰酸苄酯的光谱和动力学性质。发现异氰酸酯官能团的不对称拉伸振动带具有显著的消光系数，并且位于红外光谱的未拥挤区域。虽然该振动带的频率仅显示出对溶剂极性的适度依赖，但其动力学对溶剂极性相当敏感。这些结果表明，异氰酸酯基团可用作局域环境的位点标记特异性红外探针。
（3）使用瞬态吸收光谱和傅里叶变换二维可见光谱研究了PBDB-T:O-4TBC-2F异质结及其单体的能量转移动力学过程。通过瞬态吸收光谱构建了PBDB-T:O-4TBC-2F异质结中动力学-形貌-性能三者间的联系。研究发现在异质结中形成了O-4TBC-2F的两种具有不同的动力学行为的聚集体：弱聚集和J聚集，它们对热退火的动力学响应差异非常明显。对它们的动力学分析很好地说明了热退火优化可以提高器件性能的原因，填补了有机光伏形貌与性能之间的动力学研究机制的空白；研究结果表明在通过形貌控制来进一步优化有机太阳能电池性能的过程中，瞬态吸收光谱可以作为评估形态影响并平衡其对性能的双刃效应的有效方法。傅里叶变换二维可见光谱研究发现在PBDB-T中存在着三个不同激子模式，而O-4TBC-2F中仅存在一个激发态模式。由于PBDB-T中存在多激发态模式的原因，在PBDB-T:O-4TBC-2F异质结的二维光谱中观察到了三个电子转移路径和两个空穴转移路径；这些转移路径间的效率有着很大的区别。工作证明了有机光伏异质结中多通道能量转移路径的存在；同时指出通过优化异质结中的激子模式来优化能量转移路径，关闭低效率的能量转移通道是进一步提升有机光伏太阳能电池光电转化效率的方向。这对于非稠合环太阳能电池光电转化效率的提升有着重要意义。
（4）使用傅里叶变换二维可见光谱深入研究了D18分子的时间分辨光谱动力学性质。实验结果表明在给体材料D18中存在着三种具备不同能量的激子模式，通过瞬态吸收光谱的寿命分析发现他们之间的激发态寿命有着很大的差异。对D18溶液和薄膜的傅里叶变换二维光谱分析发现D18分子中不同寿命的激发态粒子在两种状态下的比重不同。薄膜状态下长寿命激发态粒子数的减少阻碍了基于D18给体材料的有机光伏太阳能电池效率的提升。采取有效的优化策略提升形成薄膜后D18分子中长寿命激子所占的比重是提高D18太阳能电池效率的关键。我们的工作证明了在D18分子中存在着多种激发态模式的作用，同时指出了D18分子薄膜状态下长寿命激子粒子数的显著减少是阻碍其效率提升的关键。该研究为基于D18的有机光伏太阳能电池效率的进一步提升提供了方向。

Organic photovoltaic solar cells are one of the key research areas of the next generation of new energy materials due to their excellent solution processability, high translucency, low cost and environmental friendliness. The further improvement of its photoelectric conversion efficiency depends on the basic understanding of the physical and chemical reaction process. Through time-resolved multi-dimensional spectroscopy technology, we systematically studied the energy conversion process of organic photovoltaic materials, and pointed out the direction for the design of new organic photovoltaic heterojunctions. This thesis mainly completed the following research work:
(1) Based on nonlinear laser technology, completed the construction of optical parametric amplifier, nonlinear optical parametric amplifier, visible transient absorption system, and Fourier transform two-dimensional visible spectrum system. The control software written by LabView is used to coordinate the work between various instruments. The integrated design of software and hardware of the system is realized, and the control software with universal applicability that can be used for the collection of various spectroscopy systems has been developed. At the same time, based on the theoretical analysis of the signal composition in the two-dimensional spectrum system in a strong scattering environment, a shot to shot multi-dimensional spectral noise removal scheme has been developed. With the new noise suppression scheme, the two-dimensional spectral signal under strong scattering situations can be as good as no scattering situations.
(2) We used linear and nonlinear infrared spectroscopy to study the spectral and kinetic properties of the model compound benzyl isocyanate. It is found that the asymmetric stretching vibration band of isocyanate functional groups has a significant extinction coefficient and is located in the uncrowded region of the infrared spectrum. Although the frequency of the vibration band only shows a moderate dependence on the polarity of the solvent, its kinetics is quite sensitive to the solvent. These results indicate that isocyanate groups can be used as site-specific infrared probes for local environments.
(3) The transient absorption spectroscopy and fourier transform two-dimensional visible spectroscopy were used to study the energy transfer dynamics of PBDB-T:O-4TBC-2F heterojunction and its monomer. Transient absorption spectroscopy established the relationship between dynamics, morphology and performance in the PBDB-T:O-4TBC-2F heterojunction. The study found that two kinds of aggregates of O-4TBC-2F with different kinetic behaviors were formed in the heterojunction: weak aggregation and J aggregation. Their kinetic analysis well explained the reason why thermal annealing optimization can improve the performance of the device, filling the gap in the kinetic mechanism between morphology and performance. The research results also show that in the process of further optimizing the performance of organic solar cells through morphology, transient absorption spectroscopy can be used as an effective method to evaluate the impact of morphology and balance its double-edged effect on performance. The fourier transform two-dimensional visible spectroscopy found that there are three different excitonic modes in PBDB-T, but there is only one excited state mode in O-4TBC-2F. Due to the multiple excited state modes in PBDB-T, we found that there are three electron transfer paths and two hole transfer paths in PBDB-T:O-4TBC-2F. There is a big difference in the efficiency of these transfer channels. Our work proves the existence of multi-channel energy transfer paths in organic photovoltaic heterojunctions. At the same time, it is pointed out that optimizing the exciton mode in the heterojunction to close the low-efficiency energy transfer path is one of the directions to further improve the photoelectric conversion efficiency of organic photovoltaic solar cells. 
(4) Fourier transform two-dimensional visible spectroscopy is used to deeply study the time-resolved spectrodynamic properties of D18 molecule. The experimental results show that there are three exciton modes with different energies in the donor material D18. Through the lifetime analysis of the transient absorption spectrum, it is found that the excited state lifetimes between them are quite different. The Fourier transform two-dimensional spectrum analysis of D18 solution and film found that the specific gravity of various excited particles in D18 molecule is different in the two states. The decrease in the number of long-lived excited state particles in the thin film state hinders the improvement of the efficiency of organic photovoltaic solar cells based on D18 donor materials. Taking an effective optimization strategy to increase the proportion of long-lived excitons in the D18 film is the key to improving the efficiency of the D18 solar cell. Our work proved that there are multiple excited state modes in the D18 molecule, and pointed out that the significant reduction in the number of long-lived excitons in the D18 film is the key to hindering its efficiency improvement. This research provide a direction for the further improvement of the efficiency of organic photovoltaic solar cells based on D18.