Inverted vs standard PTB7:PC70BM organic photovoltaic devices. The benefit of highly selective and extracting contacts in device performance

In this work we compare the photovoltaic performance of different cell designs, standard and inverted, for one of the most promising systems to achieve power conversion efficiencies over 10% in polymer:fullerene single cells, namely PTB7:PC70BM. Impedance spectroscopy, charge extraction and transient photovoltage are used in order to assign the electrical losses initially observed in the current density-voltage curve and understand the main limitation of every design. While inverted devices show competitive performance in terms of charge generation, transport of carriers and also for charge collection at electrodes, standard devices present additional resistive losses that are assigned to charge transfer issues at the active layer/anode interface. This additional resistance increase the overall series resistance of devices, lowers the fill factor and it is the ultimate responsible for the observed reduced device performance of standard cells in comparison to inverted ones. In this way, devices over 7.2% are reported with ZnO and MoO3 as interlayer electrodes that act as improved highly selective and extracting contacts in comparison to standard PED-OT:PSS and Ca/Ag. Contacts are thus electrically optimized. Additional improvement of device performance must consider enhancement of intrinsic recombination properties of the blend. Lower molecular weights and/or any residual catalyst impurities with respect to other batches are the only limitation to reach record efficiencies as those shown in recent works. (C) 2014 Elsevier B.V. All rights reserved.