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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 °C to 400 °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 °C and below are amorphous in nature. However, the films deposited at 300 °C and 400 °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 °C and decrease from 2.7 × 106 to 2.6 × 105 Ω cm with increasing substrate temperature from 300 to 400 °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 °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%.