Effects of solvent-vapor annealing on bulk-heterojunction morphology of photoactive layers prepared by electrostatic spray deposition

• P3HT:PCBM bulk-heterojunction photoactive layers were prepared by electrospray.
• The effects of solvent-vapor annealing on solar cell performances were studied.
• Enhanced power conversion efficiencies were achieved due to modified ESD condition.
• Excessive SVA treatments resulted in a decrease in open-circuit voltage (Voc).
• Introducing LiF at the cathode interface prevented a decrease in Voc.

We studied the effects of solvent-vapor annealing (SVA) on the device performance of bulk-heterojunction organic photovoltaic solar cells. Electrostatic spray deposition (ESD) was used to prepare photoactive layers composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend. While the surface of the photoactive layer could be flattened by SVA, excessive treatment resulted in a decrease in open-circuit voltage. We attribute this to too large vertical phase separation within the P3HT:PCBM blend and accumulation of P3HT on the surface of the photoactive layer, which can induce recombination of photogenerated charges at the cathode/photoactive layer interface. A decrease in the open-circuit voltage could be prevented to some extent by depositing the photoactive layers using smaller droplets produced in the ESD process; this was probably due to the slower progress of vertical phase separation. Furthermore, introducing LiF as a hole-blocking layer between the cathode and the photoactive layer significantly reduced recombination-related leakage current, thereby preventing a decrease in open-circuit voltage.