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

Cu(In,Ga)Se2 solar cells with high efficiency rely on Ga grading to mitigate back surface recombination and achieve optimal performance. However, the inhomogeneous absorber has drawbacks, including increased non‐radiative loss and inadequate absorption. Therefore, a paradigm change has been demanded in the literatures, to passivate the back contact by introducing a hole selective transport layer. Here, we demonstrate a functional hole transport structure as an alternative to Ga grading. The novel hole transport structure is prepared as a double layer: co‐evaporated CuGaSe2, covered by In2O3, made by solution combustion synthesis. As demonstrated by micrographs, elemental mapping and photoluminescence spectroscopy, the oxide layer improves thermal stability and prevents Ga diffusion. However, during the absorber deposition process, a complete ion exchange of In and Ga takes place between the two layers. Incorporating this hole‐selective transport layer in co‐evaporated non‐graded CuInSe2 solar cells leads to significantly increased minority carrier lifetime, measured by time resolved photoluminescence from 5 ns to 113 ns, yielding an 80 meV improvement in quasi‐Fermi level splitting, as determined by absolute photoluminescence spectroscopy. The devices are finished with standard buffer/window layers and exhibit improved open circuit voltage, as well as a promising fill factor of over 71%, indicating good hole transport properties. These results experimentally demonstrate the passivation effect of the new hole transport layer as well as its good transport properties. Thus, high‐efficiency solar cells can be achieved without relying on Ga grading, enabled by a functional hole transport layer.This article is protected by copyright. All rights reserved.