Hello, I would like to invite you all to my prelim talk. Date: Feb.7.2014 Time: 3:30pm Location: BRK 1001 Title: Performance limits of solar cells Abstract: The solar cell device concepts can be broadly classified into two groups: (i) high efficiency crystalline cells with efficiencies approaching the thermodynamic limit, and (ii) lower cost thin film alternatives that offer improved cost-efficiency tradeoffs. For the first group, the challenge is to develop a meticulous understanding of cell operation so that the remaining efficiency gap can be closed. For the second group, the challenge is to develop a material-specific understanding of cell operation to improve the efficiency and reliability of the cell. First, in an effort to explore the limits of cell operation for both types of technologies, we have studied the thermodynamics limit of PV operation from a new perspective of a 2-level solar cell. Our approach provides qualitative and intuitive understanding of the loss components in an ideal PV device. The analysis framework also shows how the existence of heterojunction (HJ) in organic photovoltaics (OPVs) limits the upper bound of performance even for perfect absorption and 100% carrier extraction. Second, we considered the detailed balance limit in presence of non-ideal optical response (incomplete absorption and existence of non-radiative recombination) for a given PV material. This study demonstrates that the thermodynamic efficiency limit can be asymptotically reached with imperfect absorption-however, designs towards ultra-high efficiency systems need to consider non-radiative recombination effect. Third and finally, we considered two major topics of practical systems: (i) addressing poor carrier collection (for OPVs), and (ii) general light absorption schemes. We have proposed design perspectives and new structures to address poor carrier transport in OPVs. These ideas involve providing shorter route for carrier to contact paths. Our nanowire (NW) based photon absorption scheme has separate tuning handles for reflection and absorption-unlike previous NW schemes where there was a trade-off between reflection and absorption. In our latest work, we have proposed a light management scheme that can perfectly trap the photons that are coupled into the structure. This approach has the potential to break the geometric light trapping limit, and also can be easily integrated into a planar thin film PV device. Regards, M. Ryyan Khan, PhD RA, Prof M. A. Alam, ECE, Purdue University http://web.ics.purdue.edu/~khan23/<http://web.ics.purdue.edu/%7Ekhan23/>