Diode Factor in Solar Cells with Metastable Defects and Back Contact Recombination

摘要

To achieve a high fill factor, a small diode factor close to 1 is essential. The opticaldiode factor determined by photoluminescence is the diode factor from theneutral zone of the solar cell and thus a lower bound for the diode factor. Due tometastable defects transitions, the optical diode factor is higher than 1 even atlow excitation. Here, the influence of the backside recombination and the dopinglevel on the optical diode factor are studied. First, photoluminescence and solarcell capacitance simulator (SCAPS) simulations are used to determine the backsurface recombination velocity of Cu(In, Ga)Se_2 with various back contacts anddifferent doping levels. Then, experimental results and simulations show thatboth back surface recombination and high doping density reduce the opticaldiode factor. The back surface recombination reduces the optical diode factorwith undesirable extra nonradiative recombination. The smaller value achievedby higher doping can increase quasi-Fermi level splitting at the same time. Thesimulations show that the back surface recombination reduces the optical diodefactor due to an illumination-dependent recombination rate. In addition, a highermajority carrier doping reduces the influence of majority carrier gain from metastabledefect transitions, thus reducing the optical diode factor.