Hopping Conduction Observed in Thermal Admittance Spectroscopy

We observe variable-range hopping conduction in thermal admittance spectroscopy and develop a method to evaluate the signal under this condition. As a relevant example of demonstration we employ Cu(In,Ga)(Se, S)(2) thin-film solar cells and show that the fundamental N1 signal, which has been discussed for more than a decade in terms of minority carrier traps, does not display trap parameters, but is generated by the freezing-out of carrier mobility with decreasing temperature when hopping conduction prevails. This effect offers a new approach to carrier hopping and to semiconductors suffering from small mobility.