Abstract
Monte Carlo simulations are used to examine charge-transfer (CT) state recombination dynamics considering the effects of energetic disorder and bulk heterojunction morphology. Strongly biexponential recombination kinetics were observed, in agreement with spectroscopy. Data over a range of electric fields 106 ≤ F ≤ 108 V m–1 suggest that the slow component of recombination is due to energetic and spatial trapping of charges, as increasing the field reduces the magnitude of the slow decay. This behavior could not be described using a simple Onsager–Braun type model; hence, an alternative kinetic framework including an intermediate “quasi-free” state between the CT state and free charges is proposed and subsequently shown to fit the MC data very well. The predictive capability of the modified model was then tested by repeating MC simulations with an altered recombination rate. It is shown that more than just the recombination rate had to be changed in the modified kinetic model to retrieve good agreement with MC simulations. This suggests that the derived rates from the modified kinetic model do not have exact correspondence with physical processes in organic photovoltaic blends. We attribute the difficulty in fitting kinetic models to CT recombination data to the dispersive nature of hopping transport.
Highlights
Organic photovoltaic devices (OPVs) are an attractive alternative to their inorganic counterparts because of the capability to tune their absorption to the solar spectrum[1] and the availability of scalable manufacturing processes.[2,3] unlike inorganic photovoltaics, photoabsorption in OPVs results in an exciton with a large binding energy.[4]
We note that Monte Carlo (MC) simulations of this type have been successful in obtaining quantitative agreement with all-polymer[30,31] and inefficient polymer−fullerene[20] OPVs
We have used Monte Carlo simulations to examine CT separation dynamics in an OPV including the effects of energetic disorder and bulk heterojunction morphology
Summary
Organic photovoltaic devices (OPVs) are an attractive alternative to their inorganic counterparts because of the capability to tune their absorption to the solar spectrum[1] and the availability of scalable manufacturing processes.[2,3] unlike inorganic photovoltaics, photoabsorption in OPVs results in an exciton with a large binding energy.[4] Efficient charge generation in OPVs is instead achieved using a pair of materials, the donor and acceptor, in which the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs respectively) form a type-II heterojunction. The efficiency with which CT states are converted into free charges (η) is vital in determining the power conversion efficiency of OPVs
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