Inverted vs Standard PTB7:PC70BM Organic Photovoltaic Devices. The benefit of highly selective and extracting contacts in device performance

摘要

In this work we compare the photovoltaic performance of different cell designs, standardand inverted, for one of the most promising systems to achieve power conversion efficienciesover 10% in polymer:fullerene single cells, namely PTB7:PC70BM. Impedance spectroscopy,charge extraction and transient photovoltage are used in order to assign the electricallosses initially observed in the current density–voltage curve and understand the main limitationof every design. While inverted devices show competitive performance in terms ofcharge generation, transport of carriers and also for charge collection at electrodes, standarddevices present additional resistive losses that are assigned to charge transfer issuesat the active layer/anode interface. This additional resistance increase the overall seriesresistance of devices, lowers the fill factor and it is the ultimate responsible for theobserved 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 thatact as improved highly selective and extracting contacts in comparison to standard PEDOT:PSSand Ca/Ag. Contacts are thus electrically optimized. Additional improvement ofdevice performance must consider enhancement of intrinsic recombination properties ofthe blend. Lower molecular weights and/or any residual catalyst impurities with respectto other batches are the only limitation to reach record efficiencies as those shown inrecent works.