Transparent conducting oxide top contacts for organic electronics

Abstract

A versatile method for the deposition of transparent conducting oxide (TCO) layers directly onto conjugated polymer thin film substrates is presented. Using pulsed laser deposition (PLD) we identify a narrow window of growth conditions that permit the deposition of highly transparent, low sheet resistance aluminium-doped zinc oxide (AZO) without degradation of the polymer film. Deposition on conjugated polymers mandates the use of low growth temperatures (<200 °C), here we deposit AZO onto poly-3-hexylthiophene (P3HT) thin films at 150 °C, and investigate the microstructural and electrical properties of the AZO as the oxygen pressure in the PLD chamber is varied (5–75 mTorr). The low oxygen pressure conditions previously optimized for AZO deposition on rigid substrates are shown to be unsuitable, resulting in catastrophic damage of the polymer films. By increasing the oxygen pressure, thus reducing the energy of the ablated species, we identify conditions that allow direct deposition of continuous, transparent AZO films without P3HT degradation. We find that uptake of oxygen into the AZO films reduces the intrinsic charge carriers and AZO films with a measured sheet resistance of approximately 500 Ω □⁻¹ can be prepared. To significantly reduce this value we identify a novel process in which AZO is deposited over a range of oxygen pressures – enabling the deposition of highly transparent AZO with sheet resistances below 50 Ω □⁻¹ directly onto P3HT. We propose these low resistivity films are widely applicable as transparent top-contacts in a range of optoelectronic devices and highlight this by demonstrating the operation of a semi-transparent photovoltaic device.