Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)/fullerene-based heterojunction polymer solar cells

Abstract

Electrical and optical properties of poly(3-hexylthiophene-2,5diyl) (P3HT-2,5diyl) used as the main component in a bulk heterojunction polymer/fullerene solar cell were investigated. The HOMO level of the polymer was estimated at about 4.7–5.1 eV, from the observed space charge limited current (SCLC) studies in ITO/P3HT-2,5diyl/Au hole-only devices, which confirmed the formation of ohmic contacts between the polymer and the Au and ITO electrodes. The values calculated for hole mobility and density range from 1.4×10 −6 cm 2/(V s) and 5.3×10 14 cm −3 at 150 K to 8.5×10 −5 cm 2/(V s) and 1.1×10 15 cm −3 at 250 K, respectively. A HOMO–LUMO gap of 2.14 eV was estimated from an absorption spectrum of the polymer. Photoinduced charge transfer from polymer to PCBM was evidenced by strong photoluminiscence quenching, which was observed when the polymer was mixed with [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM). Charge carrier transport properties of the polymer/fullerene solar cells were studied by analysing the dependence of J– V characteristics on temperature and illumination intensity. A linear decrease of the open-circuit voltage with increasing temperature, with a local maximum around 320 K, was observed. The short-circuit current density increased with temperature, having a maximum around 300 K and decreased thereafter. Efficiency and fill factor presented maxima around 3 mW/cm 2 white light intensity, and this was attributed to the poor bulk transport properties of the active layer. Typical values recorded for the solar cell at 300 K under white light of 100 mW/cm 2 intensity were: open-circuit voltage 0.48 V, and current density 1.28 mA/cm 2, with an efficiency of 0.2% and fill factor of 30.6%.