Degradation Mechanism in Cu(In,Ga)Se2 Material and Solar Cells Due to Moisture and Heat Treatment of the Absorber Layer

The impact of moisture and heat treatment on the microstructural, chemical, and electrical properties of Cu(In,Ga)Se₂ films and their collective effect on the solar cell device performance was studied. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy measurements show that water exposure causes surface modification and alters the alkali metal distribution, while no composition or structural effect was observed. Deep level transient and optical spectroscopies revealed that the trap densities (N_T) for both the E_V + 0.65 eV and E_V + 0.98 eV traps increase after water exposure, while the majority carrier concentration (N_A) decreases. Time-resolved photoluminescence (PL) and steady-state PL measurements indicated the presence of static, not dynamic, quenching. Reduction of open-circuit voltage (V_OC) and fill factor (FF) was observed for the devices but was not associated with a change of recombination mechanism, which remains in the absorber space charge region. A small increase in series resistance and shunt conductance accounts for most of the FF change, while the modification in both N_A and N_T yield most of the change in V_OC. A gradient of majority carrier concentration, related to the alkali profile, also yields a small voltage-dependent current collection after moisture and heat treatment.