Facile Silver-Incorporated Method of Tuning the Back Gradient of Cu(In,Ga)Se₂ Films

An appropriate gallium (Ga) back gradient in Cu(In,Ga)Se2 (CIGS) films is vital to achieve solar cells with good device performances. As a steeper Ga back gradient can induce an enhanced electric potential variation or affinity (conduction band) variation, it can make the electrons stay away from the rear contact. In addition, significant lattice distortions can be found at the back gradient of CIGS films, which increases carrier recombination and impedes carrier transport. In this study, by employing an Ag–Se precursor before the CIGS process, silver (Ag) is purposefully doped to the back gradient of CIGS films to reduce the lattice distortions. A comparison of the structure and device characteristics of CIGS films with different Ag–Se deposition durations is carried out. This study suggests that the addition of an Ag–Se precursor can enhance the quality of the CIGS film and modify the back gradient as well as enhance recrystallization, which improves the device performance of solar cells. Simultaneously, the enhanced grain sizes achieved by the Ag-treated process are helpful for the improvement of carrier mobility and the reduction of boundary recombination in CIGS films. Furthermore, the accumulation of indium caused by the Ag–Se precursor film in the first stage should contribute to the formation of a modified back gradient. This work offers an easy route for the control of the Ga back gradient in CIGS thin film solar cells.