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

Abstract

We report evidence for the idendification of the capacitance transients detected at room temperature for thin-film photovoltaic cells with CIGS absorbers as an additional non-ohmic contact in the structure with a time constant larger than that of the solar cell pn-jucntion. The N1 signal was recently interpreted as a back contact barrier for which the RC-like time constant is smaller than the time constant of the junction. In this work we unite these experimental observations in one model. Since for a Mo/CIGS/CdS/ZnO solar cell several interfaces are connected in series, we introduce the idea of modeling capacitance spectroscopy signals based on RC-circuits in series for each interface. It is shown that the distinct features observed in capacitance spectroscopy of CIGS solar cells can be mimicked using this circuit as an electrical model. The differential equation for this structure as a function of time is solved numerically. It is inherent to the model that the transients in such a structure are voltage transients over each of the interfaces and that the transients are coupled. These findings question the practical use of capacitance spectroscopy for direct measurement of defects in the absorber layer.

Alkali treatments of Cu(In,Ga)Se2 thin‐film absorbers and their impact on transport barriers

“…Lauwaert et al showed that DLTS signals recorded for different pulse directions are not compatible with standard models of carrier capture and emission by defect states in the device. Instead, R‐C‐ like non‐ideal contacts or interlayers in series with the main junction were shown to result in capacitance steps and DLTS peaks which could explain the N1 signature . With the main junction located at the front CIGS/buffer interface in the device, the R‐C ‐like barrier or counter‐diode in series with the main junction has been attributed to the back contact in these studies.…”

Buffer Layers, Defects, and the Capacitance Step in the Admittance Spectrum of a Thin-Film Solar Cell

“…Primarily, it has been attributed to a defect, although conflicting evidence exists concerning where these defects are located within the device [9,11,12]. This standard interpretation was recently challenged by an increasing number of publications which suggest alternative explanations for the N1 level, mostly linked to the transport characteristics of the solar-cell absorber [13,14] or to a transport barrier [7,[15][16][17][18][19][20][21] within the device.…”

Size and grain-boundary effects on the performance of polycrystalline CIGS-based solar cells

“…This is due to its high absorption coefficient, appropriate band gap and outstanding electro-optical properties [1][2][3]. CIGSbased solar cells with x = 0.3 which corresponds to a band-gap energy range of 1.1-1.2 eV yields the best efficiency both in laboratory and commercial solar cells [3].…”