Effect of chemical structure of interface modifier of [formula omitted] on photovoltaic properties of poly(3-hexylthiophene)/ [formula omitted] layered solar cells

Abstract

Two classes of phosphonic acid-bearing organic molecules, 2-oligothiophene phosphonic acid and ω-(2-thienyl)alkyl phosphonic acid were adopted as interface modifiers (IMs) of the TiO 2 surface, to increase its compatibility with poly(3-hexylthiophene) (P3HT). The self-assembled monolayers of these molecules on titania surface were characterized by making contact angle measurements and X-ray photoelectron spectroscopy (XPS). Atomic force microscopic (AFM) images revealed that the adsorption of IMs effectively smoothes the TiO 2 surface. Both photoluminescence (PL) spectroscopy and PL lifetime measurements were made to investigate the photoinduced properties of the TiO 2/IM/P3HT layered-junction. The PL quenching efficiency increased with the number of thiophene rings and as the alkyl chain-length in IMs decreased. Meanwhile, the decline in the PL lifetime followed a similar trend as the PL quenching efficiency. Additionally, the power conversion efficiency (PCE) of the ITO/TiO 2/IM/P3HT/Au devices was examined by measuring their photocurrent density–applied voltage ( J – V ) curves. The experimental results indicated that the short-circuit current density ( J SC ) increased with the number of thiophene units and as the hydrocarbon chain-length in IMs decreased. However, the open-circuit voltage ( V OC ) of the devices slightly fell as the energy level of the highest occupied molecular orbital (HOMO) of IM decreased. The PCE of the device with 2-terthiophene phosphonic acid was 2.5 times that of the device with 10-(2-thienyl)decyl phosphonic acid.