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 CuGaSe 2 covered by solution combustion synthesis prepared In 2 O 3 . 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 CuGaSe 2 /In 2 O 3 into CuInSe 2 /GaO x . Incorporating this hole‐transport layer in co‐evaporated nongraded CuInSe 2 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.