Photoconductivity: A probe of defect states in carbon aerogels

The photoconductivity and dark conductivity of carbon aerogels with various heat-treatment temperatures, ${\mathit{T}}_{\mathrm{HT}}$, and densities are studied. Unlike the trend for as-prepared samples, the magnitude of the photoconductivity for heat-treated (${\mathit{T}}_{\mathrm{HT}}$>1050 °C) carbon aerogel samples decreaes as the sample density and heat-treatment temperature increase. We propose a model which quantitatively explains the observed temperature dependence of the photoconductivity data. Based on this model, we attribute the observed photoconductivity to photoholes present in the system. The number of defect states and their corresponding energy decrease with increasing ${\mathit{T}}_{\mathrm{HT}}$ and increasing density. The decrease in the energy is attributed to a decrease in the screening effect of the defects on trapped charges. The effect of increasing the heat-treatment temperature causes the structure of the carbon aerogel to become more graphitic, especially for aerogel samples with higher mass density.