An Efficient Bulk-Heterojunction Photovoltaic Cell Based on Energy Transfer in Graded-Bandgap Polymers

It is demonstrated that the energy transfer from low- conjugated (LC) poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylene vinylene] (MEHPPV) to high-conjugated (HC) MEHPPV, coupled with significant electron transfer from HC- MEHPPV to an acceptor species, offers a viable route for an efficient photodiode over a wide spectral range. An enhanced incident-photon-tocurrent conversion efficiency (IPCE) of 19% over a wide spectral range and a power- onversion efficiency $(\eta_P)$ of 1% (under monochromatic illumination at $\lambda \sim 530$ nm and a power density of ca. $1 mWcm^{-2})$ are achieved in a ternary polymer-blend film that consists of HC-MEHPPV (low bandgap), LC-MEHPPV (high bandgap), and an acceptor polymer, cyanoPPV (CNPPV), in an optimized ratio. The decisive role of the morphology that emerges during phase separation in the polymer blend is demonstrated by wide-field photocurrent imaging