The Roles of Photo-excited States on Charge Generation and Energetic Loss in Organic Photovoltaic Devices


Student thesis: Doctoral Thesis

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  • Zhiqiang GUAN


Awarding Institution
Award date31 Aug 2016


Organic photovoltaic (OPV) device shows great potential as next-generation solar cells due to its advantages of low cost, flexibility, light weight and easy fabrication. Recently, a power conversion efficiency (PCE) of 10% has been achieved in single-junction solar cell, which is a benchmark for commercial application. In spite of the high PCE values, the working mechanism of OPV device is still unclear. It still puzzles the community that why the internal quantum efficiency (IQE), i.e. the conversion efficiency from absorbed photon to free electrons, can be as high as near 100% in organic materials with low dielectric constant and high binding energy. The lack of the detailed understanding on fundamental principle will impede the further development of OPV both in technology and in theory.
In this thesis, we firstly investigate the energy of charge-transfer state (CTS) and its relationship with open-circuit voltage (VOC) in OPV. In donor:acceptor heterojunction OPV device, it has been proposed that the CTS energy (ECT) directly determines the VOC value. However, previous literatures show that the ECT values measured from different techniques often show considerable discrepancies. Here, we used four methods to probe the ECT values in five different bulk-heterojunction polymer:fullerene OPVs. It is found that linear relationships exist between ECT measured with different approaches and VOCs. The detailed energetic meanings of the ECT values measured from different techniques are discussed and the origin of their discrepancies are analyzed. A relationship is summarized to estimate the energetic loss of VOC by considering the state broadening of CTS, bimolecular recombination and dielectric effect. Our results provide a guideline to forecast the VOC and the energetic loss in OPV.
Secondly, we study the delocalization feature of CTS and its role in charge generation process. How CTSs assist charge separation of Coulombically bound exciton has been a hot topic in OPV. Even though it is believed that the delocalization feature of CTS plays a crucial role in the charge separation process, the delocalization of the “hot” and the “relaxed” CTSs is still under debate. In this thesis we used a novel frequency dependent charge-modulated electroabsorption spectroscopy (CMEAS) technique to probe the delocalization of particular states. The CMEAS results clearly show that both “hot” and “relaxed” CTSs are loosely bound and delocalized states, explaining why near constant IQE values can be achieved by excitation with different energies. Moreover, our results indicate no substantial delocalization gradient exists in CTSs, which is contrary to the widely used Onsager model.
Lastly, we focus on the polarization property of exciton, which is altered by inducing n-type acceptors. For long time, the properties of exciton in donor:acceptor blend films are regarded as the same with those in their pristine constituting polymers, which are closely bound hole-electron pair with large binding energy. Here, with a fitting method based on Stark effect, we extract the polarizabilities of excitons in the donor film with or without acceptor. We find that the polarization degree of exciton is dramatically enhanced after inducing acceptor. By measuring the dielectric constants of different systems, we find that the polarizability and the dielectric constant show almost the same trend, indicating that increasing the polarization of exciton improves the dielectric screening ability of active layer. Our results demonstrate that the excitons with high polarization degree also play an important role in charge generation in OPV devices.