Doping effect of electron transport layer on nanoscale phase separation and charge transport in bulk heterojunction solar cells

We investigated the influence of the Ga doping in the ZnO interlayer as an electron transport layer (ZnO ETL) on the nanoscale phase separation in the bulk heterjunction (BHJ) layer coated on the ETL as well as the morphological and electrical properties of a low temperature sol-gel-derived pristine ZnO ETL (P-ETL) and Ga-doped ZnO ETL (G-ETL), which affect the performance of inverted organic solar cells (IOSCs). X-ray photoelectron spectroscopy (XPS) confirms the successful incorporation of the element Ga in the ZnO ETL. The short circuit current densities (J_SC) of IOSCs fabricated using a G-ETL were significantly improved from those of IOSCs fabricated with a P-ETL. The maximum J_SC was obtained at 2 at. % Ga doping. The IOSCs fabricated with a 2 at. % G-ETL demonstrated power conversion efficiencies of 3.51% (P3HT:PC ₆₀BM) and 5.43% (PCDTBT:PC₇₀BM), which were higher than the power conversion efficiencies of 2.88% (P3HT:PC₆₀BM) and 4.90% (PCDTBT:PC₇₀BM) of the IOSCs fabricated with a P-ETL under simulated air mass 1.5 global full-sun illumination. The better performance was attributed to the improved electrical properties of the G-ETL and the enhanced nanoscale phase separation in the BHJ active layer.