Relationship Between Molecular Properties and Open Circuit Voltage in Organic Photovoltaic Cells

M. DOLORES PEREZ

Ventura, CA

Simposio; Renewable Energy: Solar Fuels; Gordon-Kenan Research Seminar; 2009

Gordon Research Conferences

Organic photovoltaic (OPV) cells have the potential to lower costs of solar conversion substantially, given their expected low materials and fabrication costs. In order for OPVs to contribute to our overall energy balance, the device efficiencies need to be improved significantly. The current laboratory records for OPVs are in the 5-6% range, compared to commercial solar cells giving efficiencies as high as 20% for Si based cells. A basic understanding of the processes that control and limit the operation of solar cells is crucial to the development of higher efficiency OPVs and the exploration of new materials for these devices. One of the most widely studied solar cell parameter is the origin of value for the Voc given its high impact in the photoconversion efficiency. Several reports have suggested that the Voc is determined on the energy difference between the highest occupied molecular orbital of the donor (Donor HOMO) and the lowest unoccupied orbital of the acceptor (Acceptor LUMO), Delta(EDA). While tying the Voc to Delta(EDA) in comparing devices gives a reasonable fit to experimental data in many cases, this is not universally true. Unspecific “losses” have been invoked to explain the difference between the experimental Voc and the Delta(EDA). However, a model that accounts for all the parameters that together determine the Voc has not been presented. In our study we identify those factors that govern the open circuit voltage in small molecule solar cells. First, we present a model based on the diode equation, which defines the parameters controlling the open circuit voltage. Next, data is presented for several different OPVs, in which different donor materials are coupled with the same acceptor (i.e. C60) to demonstrate the model and to illustrate the molecular properties that define the Voc. A complete understanding of those molecular aspects that determine Voc is highly relevant since it would allow the use of new materials with absorption extending into the near infrared with minimal impact in photovoltage.