Monte Carlo modeling of geminate recombination in polymer-polymer photovoltaic devices

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Abstract
A Monte Carlo model is used to examine geminate pair dissociation in polymer-polymer photovoltaic devices. It is found that increasing one or both carrier mobilities aids geminate separation yield η GS particularly at low fields. This, in turn, leads to improved maximum power output from polymer-polymer blend photovoltaics, even when carrier mobilities are unbalanced by a factor of 10. The dynamic behaviors of geminate charges that eventually separate and recombine are examined for the first time. It is shown that geminate pairs in a bilayer become effectively free when separated by ∼ 4 nm , which is far smaller than the thermal capture radius of 16 nm here. This may lead one to expect that η GS would not be limited by the separation allowed by the morphology once the domain size has increased above 4 nm . In fact it is found that η GS in a blend improves continuously as the average domain size increases from 4 to 16 nm . We show that although a small degree of separation may be available in a blend, the limited number of possible routes to further separation makes charge pairs in blends more susceptible to recombination than charge pairs in a bilayer.