Large photocurrents in single layer graphene thin films: effects of diffusion and drift

This paper reports large photocurrents in air-assisted depositions of single layer graphene (derived from reduced single layer graphene oxide) upon illumination with near-infrared (NIR) light. NIR-induced charge carrier generation and subsequent separation at the metal-graphene interface resulted in photocurrent generation. Varying bias voltages were applied to test samples and allowed for evaluating photoresponses in either diffusion- or drift-dominated regions. In the diffusion-dominated region, position-dependent effects of photoconductivity were demonstrated. The photocurrent exhibited increase when the positive electrode was illuminated, decrease when the negative electrode was illuminated, and negligible response when the area between the electrodes was illuminated. At a 100 μV bias voltage, a per cent change in current from ∼150% (40 mW NIR) to ∼1800% (335 mW NIR) is reported. Such large photocurrent responses result from built-in electric fields and optically generated temperature gradients (maximum NIR-induced temperature rise ∼70 °C). The per cent photocurrent change was observed to depend on both annealing temperature and NIR power, but not resistance value. In the drift-dominated realm, a Gaussian photocurrent profile was obtained, signaling drift of charge carriers with increase in localized electric field, akin to the classic Haynes-Shockley experiment. A minority carrier mobility value of μ ∼700 cm² V⁻¹ s⁻¹ is reported. The simple low cost graphene devices presented in this paper were fabricated without lithographic processing and are ideal candidates for assorted infrared imaging applications.