Impact of MoSe ₂ Layer on Carrier Transport at the Back Contact in Cu(In,Ga)Se ₂ Solar Cells

This study focuses on the impact of MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at the Mo/Cu(In,Ga)Se<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGS) interface on back contact characteristics in CIGS solar cells. The unintentionally formed MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer has been reported to establish a quasi-ohmic contact at the Mo/CIGS interface. In this research, we construct a device model for the CIGS solar cells with the MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intermediate layer using the solar cell capacitance simulator (SCAPS-1D) considering the experimentally measured physical properties. By assuming Mo vacancies as the source of p-type conductivity of MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>, we demonstrate the reproducibility of the experimental series resistance. At the Mo/MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface, a Schottky barrier of around 0.9 eV for holes is formed due to the difference in their work functions. It is revealed that the quasi-ohmic contact is formed by recombination between holes and electrons through the defect in the MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>, despite the Schottky barrier. Since the recombination at the MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer follows the SRH model, the density of Mo vacancy significantly reduces the series resistance. Meanwhile, the decrease in the series resistance by the increase in the Mo vacancy saturates at around 0.83 Ω·cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. To further reduce series resistance, Nb doping into the MoSe<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is proposed in SCAPS-1D, enhancing p-type conductivity. It is disclosed that the Nb doping induces a transition in dominant hole transport from recombination toward tunneling, resulting in a decrease in the series resistance. If the doping density of the Nb exceeds 5 × 1019 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>, the series resistance becomes comparable to the flat band condition of the back contact.