Physics of contacts to organic solar cells
16:20 - 16:50
Auditorium between Building 4 and 5
The rapid rise in performance of organic solar cells over the last decade derived in large part from advances in materials chemistry and processing morphology control of the donor–acceptor photoactive active layers. As a consequence, much of the work in device physics of organic solar cells has focused on the fundamental limits imposed by the photoactive layer, assuming that contacts are non-limiting through the use of MoOx and ZnO2 as hole- and electron-collection layers, with sufficiently high and low work functions, respectively. However, even so, contacts still do limit the performance of organic solar cells.
In this talk, I will explain recent work that shows how device energy-level alignment leads to inevitable losses that limit open-circuit voltage, fill factor, short-circuit current density and thus power conversion efficiency, and how contact behavior transits from the under-optimized regime to optimal to the over-optimized regime as the effective workfunction is swept across the Fermi-level pinning threshold, revealing an unexpected barrier to carrier extraction. As a consequence, there is still a lot of room to improve solar cell performance significantly by contacts engineering, especially in the development of technologically viable contacts, such as those formed by solution processing. I will demonstrate what can now be achieved in this space based on the self-compensated doped polymer charge-collection layers with ultrahigh and ultralow work functions.