Limitation of Current Transport across the Heterojunction in Cu(In,Ga)Se₂Solar Cells Prepared with Alkali Fluoride Postdeposition Treatment

Abstract

Postdeposition treatments (PDTs) of chalcopyrite absorbers with alkali fluorides have contributed to improving the efficiency of corresponding solar cell devices. However, cells prepared with PDTs also tend to exhibit nonideal current–voltage (J–V) characteristics especially at low temperatures. These include blocking of the forward diode current, saturation of the open‐circuit voltage with respect to temperature, a discrepancy between dark and Jsc (Voc) characteristics, and a crossover between dark and light J–V curves. These are typical observations while measuring the temperature‐dependent J–V characteristics. Herein, the influence of electronic material parameters on the blocking of the current across the heterojunction in numerical simulations is reported. It is shown that a low‐doped ZnO window layer, acceptor defects at the CdS/ZnO interface, or a high band offset at that interface lead to similar nonideal J–V characteristics, suggesting that the carrier density in the buffer layer is a crucial parameter for the current limitation. Connections between the effects of PDT previously reported in literature and the electronic material parameters considered in the numerical model are discussed to explain the nonideal J–V characteristics caused by the PDTs.

Decay mechanisms in CdS‐buffered Cu(In,Ga)Se2 thin‐film solar cells after exposure to thermal stress: Understanding the role of Na

“…Even though acceptor states at the CdS/i‐ZnO interface show virtually the same effect, even more pronounced, they additionally lead to the formation of an injection barrier and therefore of a rollover (see Figure 9e,f). This behavior is rather similar to the case of above mentioned possible cause of strong cliff at CdS/ZnO interface for a rollover, as proposed by Villanueva‐Tovar et al 41 …”

“…However, it should be noted that these types of defects at the CdS/ZnO interface can also induce a kink anomaly resulting in lower photocurrent under forward bias and FF as suggested by Nguyen et al and Urbaniak and Igalson 39,40 . In addition to these, Villanueva‐Tovar et al have proposed that a strong cliff at the CdS/ZnO interface can also cause an injection barrier leading to a rollover anomaly 41 …”

Silver Surface Treatment of Cu(In,Ga)Se2 (CIGS) Thin Film: A New Passivation Process for the CdS/CIGS Heterojunction Interface

“…Several surface treatments, post‐deposition treatment (PDT) with an alkali metal, [ 14–21 ] the formation of a copper (Cu)‐deficient layer (CDL) on the CIGS surface, [ 5,13,22–24 ] and the interface passivation of CdS/CIGS with Al 2 O 3 , [ 25–28 ] have been used to improve the device performance of solar cells. PDT, such as potassium fluoride, modifies the CIGS surface (reduced gallium (Ga) concentration and depleted Cu), which allows the reduction of CdS thickness along with the maintenance of high efficiency.…”

Detrimental Impact of Na Upon Rb Postdeposition Treatments of Cu(In,Ga)Se2 Absorber Layers

“…We find that a kink can be simulated by applying acceptor defects at the CdS/ZnO interface, but also a thin (2 nm) barrier with lower electron affinity. [22,30] The conduction band and the electron Fermi level of the CdS layer and its interface are shown in Figure 9 for 0 and 0.7 V bias voltage. In the band diagram a step in the Fermi level at the CdS/ZnO interface is observed at 0.7 V which causes the kink in the J-V curve.…”

“…[23] Recently, the injection barrier upon alkali PDT has been shown by modeling to be at the CdS/ ZnO interface. [22,30] Losses in open-circuit voltage (V oc ) are not often observed and are attributed to the absence of an ordered defect compound (ODC) at the CIGS surface or a too low Cu/ (InþGa) ratio. [31,32] In this contribution, we apply alkali treatments on AlO x backpassivated solar cells and find that this combination deteriorates the solar cell performance.…”