Behavior of Na and RbF‐Treated CdS/Cu(In,Ga)Se₂ Solar Cells with Stress Testing under Heat, Light, and Junction Bias

Abstract

The effects of Na and RbF alkali treatment on the metastability behavior of CdS/Cu(In,Ga)Se2 solar cells are investigated with stress factors of heat, junction bias, and illumination. Four device types with and without Na or RbF treatments are subjected to heat‐ and light‐soaking under open‐ and short‐circuit (OC, SC) junction bias. Low‐Na devices show a higher bandgap due to increased minimum Ga content, higher recombination current, and lower open‐circuit voltage (VOC). Devices with RbF post‐deposition treatment (PDT) show an improvement in net doping density ≈1016 cm−3, VOC, and efficiency. Heat‐ and light‐soaking under OC junction bias provokes an increase in net carrier concentration and VOC irrespective of the alkali treatments. After SC stress, a decrease in VOC and net carrier concentration is observed, which can be stabilized by RbF‐PDT. An increase in Na and oxygen concentration in CIGS is observed for baseline and low‐Na devices, respectively, after OC stress. The oxygen concentration in CdS decreases after heat‐ and light‐soaking for devices without RbF‐PDT, whereas it remains unchanged for devices with RbF‐PDT. The atomic concentration profiles in CIGS significantly stabilize as a function of stress with the addition of RbF‐PDT.

Heat-light soaking treatments for high-performance CIGS solar cells on flexible substrates

“…Experimental time-resolved photoluminescence (TRPL) lifetimes tend to decrease, which in turn may limit the diffusion lengths and exacerbate the J SC degradation for devices suffering from collection issues 10 . The magnitude of the LS effects appears to depend on extrinsic elements such as Na [11][12][13] and other heavier alkali 3,14 . The doping increase tends to be most effective when the device is maintained in open-circuit condition during treatment, and to a lower extent in short-circuit 12,14 .…”

“…The magnitude of the LS effects appears to depend on extrinsic elements such as Na [11][12][13] and other heavier alkali 3,14 . The doping increase tends to be most effective when the device is maintained in open-circuit condition during treatment, and to a lower extent in short-circuit 12,14 . Also, changes similar to LS can be triggered by forward voltage biasing 12,15 , suggesting that the position of the Fermi level plays an important role.…”

Ag‐Dependent Behavior Threshold and Metastability in Wide‐Gap (Ag,Cu)(In,Ga)Se2 Solar Cells

“…On the contrary, excessive Na incorporation can lead to negative effects, with reports of minority carrier diffusion length reduction, tentatively attributed to deep defect formation (Na Se or Cu In/Ga ). [ 28 ] With regard to doping responsivity, our past results indicate very minimal depletion width variations in 0.75GGI CIGS and although literature details the response of doping to annealing and light soaking, the magnitude of the change in depletion width is generally much less than observed in Type‐2 devices. [ 17,28–30,32 ] CIGS is known to have high tolerance of off‐stoichiometry and the best devices are grown Cu poor to avoid CuSe secondary phases that compromise the interface for stoichiometric and over‐stoichiometric compositions.…”

“…Light soaking-induced doping increases can reduce J SC , especially in devices with low diffusion lengths. [20,28,29] Dark storage or a short dark anneal at relatively low temperatures (often 85 °C) is used to return devices to the relaxed state. [19] One of the most commonly invoked theories to explain this metastable behavior is the (V Se -V Cu ) divacancy model proposed by Lany and Zunger.…”

“…[30,31] Aside from the possible role of the (V Se -V Cu ) divacancy complex, it is widely reported that light soaking and heatlight soaking almost always result in a migration of Na toward the CIGS/buffer interface, which can also increase the Na concentration within the bulk of the CIGS layer. [28,[32][33][34] Indeed in comparisons between devices with low/no Na (achieved through the use of a barrier layer between the glass substrate and Mo) and those with NaF postdeposition treatments, it can be seen that the low-Na devices exhibit a much reduced response to light soaking. [32,34] It is possible that the increase of Na in the bulk CIGS passivates the In Cu donor defect, leading to the observed increases in net doping.…”