(604c) Charge Carrier Transport Modeling in Organic Photovoltaic Devices with Active Layers of P3HT/PCBM Self-Assembled Nanoparticles

Authors: 
Maroudas, D., University of Massachusetts Amherst
Han, X., University of Massachusetts Amherst

Conjugate semiconducting
polymer nanoparticles are fabricated in aqueous dispersions to reduce the usage
of chloroarene solvents for health and safety reasons
and have potentially significant benefits in organic solar cell manufacturing.  In this presentation, we study the performance
of organic photovoltaic (OPV) devices with active layers consisting of P3HT and
PCBM self-assembled nanoparticles.  The
two types of active layer morphologies examined are P3HT/PCBM blend
nanoparticle assemblies, where individual nanoparticles contain both donor and
acceptor polymer materials, and separate nanoparticle assemblies, where individual
nanoparticles contain either the donor or the acceptor material only.

We present a systematic
analysis of charge carrier transport in these OPV devices based on deterministic
charge carrier transport models.   The models account for transient drift
and diffusional transport of electrons and holes together with charge carrier
trapping and detrapping kinetics in the active layer,
coupled self-consistently with Poisson's equation for the electric field in the
layer.  Dynamics of free charge
carrier bimolecular recombination, as well as charge pair dissociation and geminate
recombination processes also are accounted for.  Photocurrent-voltage (I-V) relations at
steady state are generated for the analysis of OPV device performance.  We demonstrate the improvement in power
conversion efficiency (PCE) by the introduction of interlayers between the
active layer and the electrodes and by the reduction of the energy barriers for
charge extraction.  Fitting the
modeling predictions to experimental measurements of I-V device characteristics
determines key kinetic parameters such as exciton
generation rate, zero-field electron and hole mobilities,
electron-hole distance in bounded charge pairs, as well as geminate and
bimolecular recombination rates.  We
analyze the effects on PCE of nanoparticle size in both types of P3HT/PCBM nanoparticle
assemblies and of P3HT:PCBM nanoparticle ratio in
separate nanoparticle assemblies.  The
model predictions provide valuable input toward developing strategies for synthesis
of active layers with prescribed morphology that optimizes OPV device
performance.