Shifting the Paradigm: A Functional Hole-Selective Transport Layer for Chalcopyrite Solar Cells

High-efficiency Cu(In,Ga)Se-2 solar cells rely on Ga grading to mitigate back surface recombination. However, the inhomogeneous absorber has drawbacks, including increased non-radiative loss and inadequate absorption. Therefore, literatures demand a paradigm shift of using a hole-transport layer to passivate the back surface. Herein, a functional hole-transport layer is demonstrated as an alternative to Ga grading. The novel hole-transport layer is prepared as a double-layer: co-evaporated CuGaSe2 covered by solution combustion synthesis prepared In2O3. As demonstrated by micrographs, elemental mapping, and photoluminescence spectroscopy, the oxide layer improves thermal stability and prevents Ga diffusion. However, during the absorber deposition, a complete ion exchange of In and Ga converts CuGaSe2/In2O3 into CuInSe2/GaOx. Incorporating this hole-transport layer in co-evaporated nongraded CuInSe2 solar cells leads to significantly increased minority carrier lifetime from 5 to 113 ns, yielding an 80 meV improvement in quasi-Fermi-level splitting. The devices exhibit improved open-circuit voltage, as well as a promising fill factor of over 71%, indicating good hole-transport properties. In these results, the passivation effect and good hole-transport properties of the hole-transport layer are experimentally demonstrated. Thus, high-efficiency solar cells can be achieved by using a functional hole-transport layer without relying on Ga grading.