Extended thermal admittance spectroscopy for the investigation of composition-dependent meta-stability behaviours in wide-gap (Ag,Cu)(In,Ga)Se ₂ solar cells

Abstract In this contribution, voltage- and temperature-dependent admittance spectroscopy is used in concert with other electrical characterisation techniques to gain insight into composition-dependent variations in the meta-stable behaviour of the (Ag,Cu)(In,Ga)Se 2 system. The extended thermal admittance spectroscopy techniques are powerful tools for the evaluation of meta-stabilities, providing multiple approaches for the determination of the likely origins of observed capacitance features. These capabilities, which enable cross-referencing and verification of capacitance feature labelling, enhance the confidence in conclusions deduced from the often ambiguous and complex admittance spectroscopy methods. Our results indicate that high levels of Ag alloying lead to charge transport barriers active at low temperatures (below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>220</mml:mn> </mml:mrow> </mml:math> K) and, for the off-stoichiometric case, a shallow acceptor state <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>150</mml:mn> <mml:mstyle scriptlevel="0"/> </mml:mrow> </mml:math> meV above the valence band edge. Lightsoaking (LS)-induced increases in the activation energy and, consequently, decreases in the occupation of this acceptor state closely reflect the decrease in net doping concentration induced by LS in the off-stoichiometric high-Ag device. The presence of the acceptor state also correlates with the large difference in net doping concentration between the close- and off-stoichiometric high-Ag devices. For moderate Ag-alloying and Ag-free devices, it is shown that off-stoichiometry introduces secondary capacitance features, alongside a common feature observed in both close- and off-stoichiometric devices. These features are determined to originate from a combination of deep defects and charge transport barriers.