(6t) Charge Carrier Dynamics in Thin Film Solid-State Solar Cells

Authors: 
Christians, J. A., University of Notre Dame

Extremely thin absorber (ETA) solar cells are a promising solar cell architecture because they can be made using inexpensive materials and low temperature bench top fabrication. In these solar cells, contrary to traditional p-n junction photovoltaics, charge separation is achieved by injection of photogenerated electrons and holes into electron and hole acceptors, respectively. Thus, these charge transfer processes are integral in the charge separation and overall power conversion efficiency. I examined the hole transfer between Sb2S3 and a CuSCN hole conductor, and was able to elucidate the two-step trap and transfer mechanism of this hole transfer process using femtosecond transient absorption spectroscopy. Further, by varying the thickness of the Sb2S3 layer I was able to extract quantitative hole diffusion and transfer parameters which show a very strong correlation to corresponding solar cell performance. In this way I was able to derive a clear connection between spectroscopic studies and the performance of real-world photovoltaics.

Another promising solution-processable photovoltaic absorber is methylammonium lead iodide perovskite. Despite their recent entry onto the scene, these so-called perovskite solar cells have already surpassed 16% efficiency. The organic polymer hole conductors, such as spiro-OMeTAD, traditionally used for these solar cells pose potential stability problems and are very expensive. In an effort to reduce cost and improve stability, I was able to report the first perovskite solar cell featuring an inorganic hole conductor, viz. copper iodide. These cells showed improved stability in ambient conditions, and CuI exhibits approximately 2 orders of magnitude higher conductivity than spiro-OMeTAD. This opens up an interesting new avenue for the further optimization of perovskite solar cells. I am currently working on further optimization of perovskite solar cells with inorganic hole conductors, elucidating the interactions between the perovskite and hole conductor, and studying the stability of these devices.

In my previous research I have gained an expertise in the fabrication of solid-state solution-processed semiconductor photovoltaics. I have been able to correlate the dynamics observed with transient spectroscopy with device performance, and I employed electrochemical characterization techniques to provide insight into the role of the various device components. My research program will focus on the fabrication and characterization of solution-processed optoelectronic devices. For example, the broad tunability and high emission intensity of layer-type organometal halide perovskites makes them promising candidates for light emitting diodes. My knowledge of solution-based device fabrication, transient spectroscopy, and electrochemistry will allow me to construct, characterize, and optimize these devices.

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