Electrodeposited, “Textured” Poly(3-hexyl-thiophene) (e-P3HT) Films for Photovoltaic Applications

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Abstract

Organic photovoltaic devices have been created on activated and modified ITO electrodes from electrodeposited poly(3-hexylthiophene) (e-P3HT) donor layers, using pulsed-potential-step (PPS) electrodeposition protocols. PPS electrodeposition uses a series of potential steps of diffusion-controlled e-P3HT deposition, alternated with rest periods where no deposition occurs and the diffusion layer region near the electrode/solution interface refills with thiophene monomer. To create the most photoactive e-P3HT films, a “carpet” layer of polymer was first deposited using dual step chronoamperometry, to create a smooth, pinhole-free film on the ITO electrode. PPS electrodeposition was subsequently used to electrodeposit additional polymer and texture the e-P3HT surface, as revealed by both AFM and SEM. The extent of doping of the polymer film was controlled by the last applied rest potential and monitored by anion incorporation into the e-P3HT film using X-ray photoelectron spectroscopy (XPS). Textured and electrochemically doped e-P3HT films were used as the donor layer in photovoltaic devices, using vacuum deposited C60 as the electron acceptor/electron transport layer: (ITO/e-P3HT/C60/BCP/Al). The performance of these ultrathin OPVs was markedly dependent upon the degree of electrochemical doping of the P3HT layers. The best OPV performance was obtained for e-P3HT films with an average doping level (ratio of oxidized to reduced thiophene units) of approximately 35%, as estimated by XPS. At 100 mW/cm2 white light illumination, optimized devices give a VOC ∼ 0.5 V and a maximum JSC ∼ 3 mA/cm2, with series resistance (RS) below 1 Ω·cm2, shunt resistance (RP) in excess of 160 kΩ·cm2, fill-factors (FF) of approximately 0.65, and an overall power conversion efficiency of approximately 1%. These results demonstrate the promise of electrochemical protocols for the creation of a variety of hybrid energy conversion materials.

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