Measurement of defect densities and Urbach energies of tail states in PTB7 solar cells

Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) is an important material for solar cells. We report on measurement of bulk and interfacial defects in PTB7 devices and measurement of Urbach energies of tail states near the HOMO and LUMO levels. The bulk defects and Urbach energies were measured using sub-gap quantum efficiency techniques and the donor/acceptor interfacial defects using capacitance techniques. Interfacial defects were found to peak at ∼0.7 eV above the HOMO level. Dark current-voltage curve indicated both interfacial and bulk recombination. Urbach energy for tail states near the HOMO level of PTB7 is 33 meV and near the LUMO level 55 meV.