Monochromated Electron Energy-Loss Spectroscopy Spectrum Imaging of Organic Photovoltaic Devices

Bulk heterojunction (BHJ) organic photovoltaic (OPV) based solar cells hold the promise of achieving the highly desirable objective of low cost, clean renewable electrical power via solar energy collection in a flexible device architecture. A very active OPV research and development effort is being driven by the potential to produce large active areas on flexible substrates at low cost and high throughput [1, 2]. Recent developments in polymeric OPV device technology have increased power conversion efficiencies (PCE) into the 10% range [3], pushing OPV devices closer to being viable low-cost, environmentally friendly alternative to contemporary inorganic based solar cells. Extending OPV performance significantly beyond the 10% PCE barrier is a critical challenge. The active layer of the BHJ OPV device studied here consists of a bi-continuous network of electron donor and electron acceptor constituents, in this case poly(3hexylthiophene) (P3HT) and [6,6]phenyl C61butyric acid methylester (PCBM), respectively. Only photo-generatedexcitons that diffuse to the interface between these electron donor and acceptor materials can dissociate into holes and electrons. Factors such as the amount of light absorption, efficiency of photo-generation of electrons and holes, and their collection efficiency at the respective electrodes must be optimized in order to improve the device PCE.