High-efficiency polymer solar cells employing solution-processible and thickness-independent gallium-doped zinc oxide nanoparticles as cathode buffer layers

Solution-processible ZnO nanoparticles prepared using the sol–gel method are promising cathode interfacial materials for polymer solar cells (PSCs), but always suffer from trap-induced surface charge recombination and thickness-dependent charge collection. Herein, solution-processible gallium-doped ZnO (GZO) nanoparticles are prepared via the facile sol–gel process. The photophysical experiment discloses that Ga doping reduces the surface deep traps of the resultant GZO nanoparticles. The single-junction PSCs employing the PTB7-Th:PC71BM and PCDTBT:PC71BM blends as the active layers and the optimized GZO nanoparticles as the cathode interfacial layers have achieved PCEs of 9.83% and 7.34%, respectively, higher than those (8.69% and 6.37%) of the corresponding control devices using pure ZnO nanoparticles as the cathode interfacial layers. Moreover, the PSCs show almost unchanged photovoltaic performance with the GZO layer thickness increased up to ca. 100 nm. The enhanced photovoltaic performance originated from the suppressed interfacial charge recombination and effective charge extraction due to partial passivation of surface deep trap states in GZO nanoparticles. This is the first report using Ga-doped ZnO as the interfacial layer to prepare high-efficiency PSCs and the enhanced device performance make it a promising cathode interfacial material in practical large-scale manufacturing of PSCs.