Organic solar cell optimizations

This paper presents recent experimental and theoretical approaches for optimizing organic solar cell efficiencies in both space and energy/time domains. Specifically, in spatial domain, a ‘tertiary’ block copolymer supra-molecular nano structure has been designed, and a series of –DBAB- type of block copolymers, where D is a conjugated donor block, A is a conjugated acceptor block, and B is a non-conjugated and flexible bridge unit, have been synthesized and preliminarily examined for target photovoltaic functions. For instance, in comparison to simple donor/acceptor (D/A) blend film, a corresponding –DBAB- block copolymer film exhibited much better photoluminescence (PL) quenching (from less then 70% to over 90%), biased conductivity (2–3 orders of magnitude better), and photo conductivity (100% increase). These are attributed mainly to spatial domain improvement for charge carrier generation and transportation. In energy level domain, the photo induced charge separation appears most efficient when the donor/acceptor frontier orbital energy offset is close to the sum of two major energy costs: the charge separation reorganization energy and the exciton binding energy. Other donor/acceptor frontier orbital energy offsets are also identified where the charge recombination becomes most severe, and where the charge separation rate constant over charge recombination rate constant become largest. These energy offset values are very critical for designing high efficiency organic solar cells.