Silver Alloying in Highly Efficient CuGaSe₂Solar Cells with Different Buffer Layers

Abstract

The chalcopyrite semiconductor CuGaSe 2 exhibits a bandgap energy (E G ) of 1.6-1.7 eV [1][2][3] and is thus a promising absorber material for top solar cells in two-junction tandem devices. [4,5] However, the bulk quality of CuGaSe 2 is comparatively poor, [6,7] with a low electron lifetime resulting from a high Shockley-Read-Hall (SRH) recombination rate. Different origins, such as energetically deep defects (e.g., via Ga Cu ), a high density of defects, and/or detrimental, Cu-rich grain boundaries, are discussed. [8][9][10][11][12][13][14][15][16] In addition to losses directly related to the CuGaSe 2 layer itself, front interface recombination is supposed to be pronounced when a standard CdS buffer is used. This is due to a negative conduction band offset (CBO) at the CdS/CuGaSe 2 interface (ΔE C %À0.35 eV), leading to a very minor (or absent) type inversion and a high recombination rate at the interface. [17][18][19] The highest efficiencies of CuGaSe 2 solar cells reported so far are 11.9% for a sample with a (Zn 1Ày ,Sn y )O z (ZTO) buffer layer (our lab, noncertified) [2] and 11.0% with a standard CdS buffer layer (certified). [20] The corresponding photovoltaic parameters are open-circuit voltages (V OC ) of 1.017 and 0.901 V, short-circuit current densities ( J SC ) of 17.5 and 17.1 mA cm À2 , and fill factors (FF) of 67.0 and 71.3%, respectively. Obviously, the main improvement when exchanging CdS by an alternative ZTO buffer layer is the higher V OC , resulting from a reduced (or canceled out) interface recombination, as a negative CBO can be avoided when using ZTO. [21,22] It has to be mentioned that in order to use these wide-gap devices in a tandem configuration, a transparent back contact (TBC) needs to be incorporated. So far, the highest efficiencies of CuGaSe 2 solar cells on TBCs are much lower, at about 5%. [23][24][25][26] Recently, research on the topic of TBCs has intensified and significant progress was observed, lifting the efficiency (η) of chalcopyrite solar cells with TBCs close to the level of those with standard Mo back contacts. [27][28][29][30][31][32][33] Thus, further improvements may be expected in the near future.The best V OC values for CdS-buffered samples are achieved after a postannealing at 200 °C, reaching 971 mV for a polycrystalline [34] and 946 mV for a single-crystal absorber. [35] However, usually the postannealing results in a decrease in FF and the best V OC values for a sample without postannealing are about 920 mV [20,36] or 960 mV when a Cu-deficient surface layer was deliberately introduced and the CdS thickness increased. [37] In our lab, running a three-stage deposition process at a maximum temperature of 550 °C, the reported V OC values for CuGaSe 2 cells with CdS buffer layers are typically in the range of 730-830 mV. [2,38,39] Differences in device parameters between research groups are explainable by details in the sample processing, such as the temperature, [2] metal evaporation profiles, [40] and Se flux [20] during absorber deposition, variations in t...

Sn1–xGexOy and Zn1–xGexOy by Atomic Layer Deposition─Growth Dynamics, Film Properties, and Compositional Tuning for Charge Selective Transport in (Ag,Cu)(In,Ga)Se2 Solar Cells

“…As previously shown, it is not trivial to use ALD-based ESLs on alkali halide-treated ACIGS surfaces, and they do, in general, come short of chemical bath-based alternatives, e.g., CdS, because of this . However, previous studies also showed that the ability of the ALD ZTO ESL to shift its E c up enables a higher V oc compared to CdS for absorbers where the E c resides higher, e.g., ACIGS with high Ga content, Cu­(In,Ga)­S 2 , and CZTS. − Even with the ZTO ESL, these solar cells still showed large V oc deficits, which was partially believed to be due to the ZTO not fully being able to push its E c all the way up to match the absorber E c . Therefore, it is very encouraging to see that ZGO, with its ability to shift E c even further up, comes so close in performance to ZTO in this initial study.…”

“…In comparison to the common low-Ag content (10–20%) and low-Ga content (20–30%) ACIGS, several of the more recently emerging solar cell absorbers, such as halide perovskites, kesterites, or higher-band-gap chalcopyrites, have higher E c placements. , Initial results show that especially ZTO can clearly improve the open-circuit voltage ( V oc ) of these devices by shifting its E c up. − However, since the E c shift is limited in ZTO, it cannot match most of these absorbers’ E c fully; therefore, there is still room to further improve V oc by shifting the ESL E c further up . Thus, there has been a need to find other ESL options with this possibility.…”

Ag‐Dependent Behavior Threshold and Metastability in Wide‐Gap (Ag,Cu)(In,Ga)Se2Solar Cells

“…[9] Second, crystallization and grain growth are enhanced for even small [Ag]/ ([Ag] þ [Cu]) ratios (AAC), allowing the fabrication of absorber material with fewer and larger grains at relatively low temperatures, reducing defect formation, and enabling application in tandem device structures. [5,[10][11][12] Indeed, our group has successfully produced high-efficiency (up to 16.3%) wide-gap ACIGS devices [13][14][15] and Yang et al produced high-efficiency bifacial narrow-gap CIGS devices at low temperatures through Ag alloying. [10] Despite these promising results, we have previously discovered that high-Ga (GGI ≈ 0.75), high-Ag (AAC ≥ 0.60) ACIGS with bandgaps in the region of 1.4-1.5 eV exhibit a large degree of performance instability after dark storage, dark annealing at 85 °C, and light soaking under standard test conditions.…”