“…On the efficiency side, however, they have not yet treated as viable competitors for Si, CdTe or Cu(In,Ga)Se2 photovoltaic materials ,which is due to the high concentration of deep intrinsic defects, leading in particular to a low minority carrier lifetime, electrostatic potential fluctuations and band tail states [3][4][5]. Recently, the band-gap engineering has been explored by alloying Cu2ZnSnSe4 or Cu2ZnGeSe4 with S [6][7][8][9][10] or Sn [11][12][13][14][15][16][17][18][19], respectively. This has led to improved electronic properties of the absorber layer and an increase in conversion efficiency of the Cu2ZnSnxGe1-xS(Se)4 solar cells up to 13 % [12,13,[20][21][22]23].…”