The electronic transport in organic bulk heterojunction solar cells is typically described in terms of polaron motion, since it is well-known that organic materials have a strong electron-phonon coupling. Polaron effects lead to activated hopping transport and broadening of the optical transitions. However, organic semiconductors are also disordered materials and hence expected to have a band tail of localized states, which also gives activated carrier transport and broadened optical absorption. We explore the electronic structure to determine whether polaron or band tail effects are dominant. Transient photoconductivity (time-of-flight) measurements reveals dispersive transport with the characteristic power law time dependence of the current, from which the carrier mobility is obtained. The temperature dependence of the dispersion agrees with the exponential band tail model. The spectral response of the cell photocurrent also reveals a low energy exponential tail below the interface band gap, having the same slope as found by the transport measurements. The data therefore show that the band tail localized state distribution dominates over the polaron effects in both electrical and optical measurements. We compare the BHJ cells with thin film transistors made from similar materials which also support the band tail model with comparable slope. The data and theoretical modeling allow us to develop the approximate density of states distribution for the solar cell, which is important for an understanding of the cell properties.
Street, R. A. Band tails and polarons in organic solar cells. Organic Microelectronics & Optoelectronics Workshop VII; 2011 July 18-20; San Francisco, CA.