Hopping Conduction Observed in Thermal Admittance Spectroscopy

Abstract

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.

Electrical Characterization of Defects in Cu-Rich Grown CuInSe2Solar Cells

“…Therefore it is probably neither the response of a barrier or a mobility freeze-out due to variablerange hopping [19], because this should be characterized by a linear T − 1 4 -plot. Since the measured energies are much lower than for step 1 it is probably also lower than the Fermi-energy, allowing for this step to be due to the freeze-out of a doping defect.…”

“…Both evaluation methods assume the capacitance response to originate from bulk defects within the p-type absorber. Obviously the complex structure of a solar cell leads to other phenomena, which can show a similar contribution, namely localized defects at the absorber-buffer interface [16], the response of the equivalent circuit with a thermally activated series resistance R S [17], a back-contact barrier [18] or mobility freeze-out [19]. The TAS measurement gives further insight into the character of the signals.…”

“…However, it can still contain information about defects, if the thermally activated behaviour of R S is due to carrier freeze-out, or about barriers, if the activated R S behaviour is due to a transport barrier. A mobility freezeout leads to a non-linear behaviour of the Arrhenius plot, which can be straightened out by plotting ln ωT [19] if the low temperature mobility is dominated by variable range hopping. Also it should be the last step in the admittance spectrum and go down to the geometric capacitance C geo (lowest dashed blue line in fig.…”

Assignment of capacitance spectroscopy signals of CIGS solar cells to effects of non-ohmic contacts

“…Besides an assignment to electrically active defect levels also alternative explanations have been given for N1 like temperature dependent carrier hopping [18] or Maxwell-Wagner polarization of the absorber [19] based on AS. A second signal appears at higher temperatures (lower frequencies) in AS (see Fig.…”

Deep level transient spectroscopy measurements on Mo/Cu(In,Ga)Se 2 /metal structure

“…The first signal, often labelled N1, appears at low temperature (T < 150 K) and its DTLS signal after pulses with V r , V p < 0 (V r reverse bias, V p pulse bias) appears with a sign opposite to that expected for majority carrier traps in a bulk semiconductor. Its peculiar 20 properties in DLTS and admittance spectroscopy have been the source of a long debate about its origin [3,4,5,6,7,8,9]. The signal has been interpreted as a bulk acceptor, interface defects, hopping conduction freeze out and non-ideality at the back contact.…”