Deposition temperature effect of RF magnetron sputtered molybdenum oxide films on the power conversion efficiency of bulk-heterojunction solar cells

We have reported efficient bulk-heterojunction (regioregular poly (3-hexylthiophene) : (6,6)-phenyl C61 butyric acid methyl ester (P3HT : PCBM)) solar cells with MoO3 as a hole-selective layer deposited at different substrate temperatures from 100 deg. C to 400 deg. C by radio-frequency magnetron sputtering. The structure, morphology, optical and electrical properties of the MoO3 films deposited at different substrate temperatures are also investigated. MoO3 thin films deposited at 200 deg. C and below are amorphous in nature. However, the films deposited at 300 deg. C and 400 deg. C exhibit the presence of monoclinic Mo9O26 and orthorhombic MoO3, respectively. The electrical resistivity values of the MoO3 thin films are close to each other from 100 to 300 deg. C and decrease from 2.7 x 106 to 2.6 x 105 Ω cm with increasing substrate temperature from 300 to 400 deg. C. X-ray photoelectron spectroscopy core level analysis reveals the presence of Mo6+ oxidation state only in the films. We found that the optical band gap of MoO3 has reduced from 3.82 to 3.67 eV with decreasing substrate temperature from 400 to 100 deg. C. This decrease in band gap reduces the potential barrier between FTO and P3HT : PCBM, leading to an increase in the short circuit photocurrent density from 8.51 mA cm-2 to 9.50 mA cm-2 and an increase in efficiency of ∼20.7%.