The Impact of Grain Alignment of the Electron Transporting Layer on the Performance of Inverted Bulk Heterojunction Solar Cells

This report presents a new strategy for improving solar cell power conversion efficiencies (PCEs) through grain alignment and morphology control of the ZnO electron transport layer (ETL) prepared by radio frequency (RF) magnetron sputtering. The systematic control over the ETL's grain alignment and thickness is shown, by varying the deposition pressure and operating substrate temperature during the deposition. Notably, a high PCE of 6.9%, short circuit current density (J_sc) of 12.8 mA cm^-2, open circuit voltage (V_oc) of 910 mV, and fill factor of 59% are demonstrated using the poly(benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione):[6,6]-phenyl-C₇₁-butyric acid methyl ester polymer blend with ETLs prepared at room temperature exhibiting oriented and aligned rod-like ZnO grains. Increasing the deposition temperature during the ZnO sputtering induces morphological cleavage of the rod-like ZnO grains and therefore reduced conductivity from 7.2 × 10^-13 to ≈1.7 × 10^-14 S m^-1 and PCE from 6.9% to 4.28%. An investigation of the charge carrier dynamics by femtosecond (fs) transient absorption spectroscopy with broadband capability reveals clear evidence of faster carrier recombination for a ZnO layer deposited at higher temperature, which is consistent with the conductivity and device performance. An approach to fabricate a high-efficiency bulk heterojunction solar cell by engineering the ZnO electron transport layer (ETL) prepared by radio frequency sputtering is demonstrated. Active layers from poly(benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione) and [6,6]-phenyl-C₇₁-butyric acid methyl ester show a power conversion efficiency of 6.9%, exceeding those obtained in the literature with sol-gel, layer-by-layer, and sputtering processed ETLs.