 The researchers found that the higher the HOMO energy level of the whole transport layer material, the lower the open circuit voltage of the solar cell. This is because the higher the HOMO energy level, the more difficult it is for electrons to reach the perovskite layer, leading to reduced efficiency. The researchers also noted that the higher the HOMO energy level, the greater the potential for non-radiative recombination losses. Therefore, it is important to select the whole transport layer material with a low HOMO energy level to maximize the efficiency of the solar cell. This article was authored by Zhongyao Jiang, Tian Du, Qi Ting Lin, and others.