Effect of chemical structure of interface modifier of TiO2 on photovoltaic properties of poly(3-hexylthiophene)/TiO2 layered solar cells

Two classes of phosphonic acid-bearing organic molecules, 2-oligothiophene phosphonic acid and omega-(2-thienyl)alkyl phosphonic acid were adopted as interface modifiers (IMs) of the TiO2 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 TiO2 Surface. Both photoluminescence (PL) spectroscopy and PL lifetime measurements were made to investigate the photoinduced properties of the TiO2/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/TiO2/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)) increase 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. (c) 2008 Elsevier Inc. All rights reserved.