Princeton University

School of Engineering & Applied Science

Spectroscopic studies of the electronic structure and excited state dynamics in novel materials for photovoltaic applications

Dr. Omer Yaffe, Energy Frontier Research Center, Columbia University
Engineering Quadrangle, B205
Thursday, October 16, 2014 - 11:00am

One of the major thrusts toward inexpensive and efficient solar cells is the development of new, solution-processable, optoelectronic materials. To date, there is a wide spectrum of emerging materials (e.g organic molecules, quantum dots and organic-inorganic perovskites) that are being studied for their photovoltaic activity. Many of the applicative studies raise fundamental questions regarding the ground and excited state electronic properties of the materials. In the talk, I will present two spectroscopic studies that address such fundamental aspects.
The first study is concerned with the electronic properties of organic-inorganic perovskite crystal (OIPC) monolayers. In the last few months, OIPC’s have emerged as an outstanding material for inexpensive and efficient solar cells. In a series of studies, various research groups have successfully demonstrated the preparation of solar cells with 10-15% power conversion efficiency. In addition to their remarkable performance in solar cell applications, OIPC’s can form layered structures that are held together by relatively weak VdW interactions. Thus, monolayers of OIPC’s can be readily separated and isolated by mechanical exfoliation, which enables controlled studies of their surface and interfacial properties. The overall goal of this ongoing project is to understand what makes OIPC’s such efficient materials for solar cell applications and to develop methods to further improve their performance.
The second study is concerned with singlet fission in hexacene. Hexacene is an aromatic molecule consisting of six linearly-fused benzene rings. It is the largest available molecules in the series of  polyacenes which are known to form the most efficient organic crystals for singlet fission.  The energy of the lowest triplet state in hexacene is less than a third of the first excited singlet state. As such, it is theoretically possible that one excited singlet state can undergo fission into three triplet states. This is beneficial for solar cell applications as a form of charge multiplication. In this study, we use transient absorption to monitor the generation and decay of both the excited singlet and triplet in polycrystalline films. Our findings indicate that 100% of the excited singlet undergoes fission and that the rate of fission is comparable to that of the extensively studied pentacene crystal.