Dicyano-functionalized chlorophyll derivatives with ambipolar characteristic for organic photovoltaics

Abstract

Two ambipolar chlorophyll derivatives, namely, 32,32-dicyano-pyropheophorbide-a (Chl-1) and methyl 131-deoxo-131-(dicyanomethylene) pyropheophorbide-a (Chl-2), were synthesized for use as either the electron acceptor or the electron donor in organic planar-heterojunction solar cells. Despite the higher electron mobilities of these chlorophyll derivatives compared with their hole mobilities, devices using them as the electron donor with fullerene C70 give much better photovoltaic performance than when they are used as the electron acceptor with copper phthalocyanine. In these Chl-based solar cells, the energy gap between the LUMO levels of the donor and acceptor molecules substantially affects the charge separation and resultant photocurrent and photovoltaic performance. The highest solar energy-to-electricity conversion efficiency of up to 2.3% has been achieved using the Chl-2/C70 solar cell, under AM1.5 solar illumination (100mW/cm2) after thermal annealing of the device. It was also confirmed that the electron mobility of blend films containing Chls and fullerene derivative PC70BM was determined not only by the electron mobility of PC70BM but also by that of Chls.