| Abstract [eng] |
Simonas Driukas ORGANIC HOLE TRANSPORT MONOLAYERS FOR PEROVSKITE SOLAR CELLS The main objective of this work was to understand how organic hole-transporting monolayers MeO-2PACz and 2PACz affect the performance and charge carrier motion of inverted perovskite solar cells compared to the commonly used conductive polymer PEDOT:PSS. Because the perovskite in this device structure is formed on top of the monolayers or PEDOT:PSS, substantial structural and morphological changes may occur during material processing. Based on SEM and XRD measurements, it was found that the changes in crystal structure or morphology were minimal for all samples. Subsequently, the current voltage characteristics of complete perovskite solar cells with the above mentioned hole transport materials (HTMs) were measured. Significant differences were observed in the Voc values of the different PSCs, with the monolayers performing better than PEDOT:PSS. In addition, 2PACz performed slightly better than its counterpart MeO-2PACz. The differences in Voc values were attributed to better energy alignment and lower surface defect concentration. The PL decay kinetics of perovskite films deposited on different HTMs and quartz substrate were measured to evaluate hole extraction from perovskite to HTMs. All samples with HTMs showed a fast initial PL decay component, which was attributed to hole extraction and/or surface defects. To separate these two processes, PL decay kinetics of complete PSCs were measured with the addition of external voltage for carrier extraction. PSCs with monolayers showed faster PL quenching compared to simple thin films, which was attributed to charge extraction by internal electric field. The PL decays behaved similarly with increasing reverse bias voltage, showing faster PL quenching, but the PL lifetimes were longest for the 2PACz sample regardless of the applied voltage, followed by MeO-2PACz and finally PEDOT:PSS. In addition, considerable internal field screening was observed by applying a forward voltage and measuring the PL decay at 0 V pulses. Integrated transient photocurrent measurements were also performed to reveal carrier extraction dynamics. Two dynamic components were identified – an initial fast extraction and a subsequent slower one. Combined with the PL decay data, the fast component was attributed to direct charge extraction and slower – to carrier extraction from shallow traps. In conclusion, it was determined that the HTMs did not affect the crystallinity and morphology of the perovskite layer. Concerning the performance of the different HTMs, it was found that monolayers have a slower hole extraction rate than PEDOT: PSS, but lead to a better interface quality, especially in the case of 2PACz. |
