A primary source of energy loss within semiconductor photovoltaics is charge carrier thermalization, wherein carriers produced by high-energy photons relax to the semiconductor's band edge, giving up their excess energy as heat. In commercial silicon photovoltaics, this process accounts for nearly 50% of the energy lost by these cells. One strategy to combat this loss is to use a material that undergoes singlet exciton fission (SF) to capture high energy photons. SF, a process that occurs in select organic semiconductors, generates two spin-triplet excitons from a singlet photo-generated spin-singlet excitation. These triplet excitons can each transfer to a silicon layer resulting in a reduction of thermalization losses. However, efforts to couple SF materials with silicon have largely been hindered by inefficient triplet energy transfer across their junction. Currently, the physical basis underlying this result is unclear, and may be tied to poor orbital overlap as well as band edge mismatch. Differentiating between these scenarios and others is central to the design of SF-based photovoltaics. Herein, we use electronic sum frequency generation (ESFG) to probe how the electronic structure of perylenediimides, a family of promising SF materials, is altered at buried interfaces. ESFG measurements of perylenediimide thin films suggest that strain relaxation within crystalline grains can lead to exciton energy shifts of ~150 meV, affecting both their ability to undergo SF near interfaces and transfer triplet excitons to silicon. Our measurements suggest careful control of the arrangement of SF-materials at a semiconductor interface is critical to effecting energy transfer between them.
Aaron P Moon, Ravindra Pandey, Aaron K. Le, Jon A. Bender, Daniel E. Cotton, and Sean T Roberts, "Measuring the electronic structure of buried organic semiconductor interfaces (Conference Presentation)," Proc. SPIE 10348, Physical Chemistry of Semiconductor Materials and Interfaces XVI, 103480Q (Presented at SPIE Nanoscience + Engineering: August 08, 2017; Published: 11 October 2017); https://doi.org/10.1117/12.2273557.5606303912001.
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