Development of Wide-Bandgap Cu(In,Ga)Se₂ Solar Cells by In-Line Co-Evaporation

Chalcopyrite-type thin-film solar cells with bandgap energy (E_g) above 1.5 eV are ideal candidates for application as stable inorganic top cells in tandem devices. In recent years, the power conversion efficiency (PCE) could be increased for sulfide-based Cu(In,Ga)S₂ to 16.0% [1] and for pure CuGaSe₂ to 11.9% [2]. The implementation of a state-of-the-art post-deposition treatment (PDT) with heavy alkali elements like Rb or Cs resulted in an increased open-circuit voltage (V_OC) on wide-bandgap Cu(In,Ga)Se₂ (CIGS) with Ga/(Ga+In) (GGI) ratios above 0.7 [3]. Furthermore, the addition of Ag to wide-bandgap CIGS was demonstrated as a measure to reach higher V_OC values [4]. All these results were achieved with laboratory deposition coaters in a static mode. However, from an industrial point of view a fast and scalable in-line deposition process would be preferable for the growth of the wide-bandgap CIGS absorber by co-evaporation.<br/>In this study, we present our results for wide-bandgap selenide-based CIGS solar cells deposited with an industrially relevant 30 × 30 cm² multi-stage co-evaporation system. So far, best cells with E_g &gt; 1.5 eV achieved efficiencies up to 6% without RbF-PDT. After implementation of an in-line RbF-PDT the typical boost in V_OC and power conversion efficiency (PCE) could be obtained and best cells resulted in PCEs above 9%.<br/>The solar cell performance strongly depends on the substrate temperature during CIGS growth and the choice of subsequent buffer and high-resistive layers. By exchanging the standard CdS/i-ZnO buffer layers with CBD-Zn(O,S)/(Zn,Mg)O, which is a good option for CIGS [5] and Cu(In,Ga)(S,Se)₂ [6] with E_g in the range of 1.0 – 1.2 eV, the V_OC gap between CdS- and Zn(O,S)-buffered wide-bandgap solar cells increased even further. Nevertheless, (Zn,Mg)O is a good partner for CdS and with this buffer combination we achieved a high V_OC of 899 mV for a CIGS cell with E_g = 1.54 eV (GGI = 0.83), resulting in an open-circuit voltage deficit (ΔV_OC) of 640 mV.<br/>Moreover, we observe a gain in PCE as a result of slight Ag addition with low Ag/(Ag+Cu) ratios &lt; 0.1 for CIGS absorbers with GGI &gt; 0.6. Ag was incorporated by an additional evaporation source and these wide-bandgap (Ag,Cu)(In,Ga)Se₂ (ACIGS) solar cells exhibit increased FF values compared to the Ag-free CIGS reference samples. This observation was also made for our ACIGS cells compared to CIGS with standard GGI = 0.3, i.e. higher FF for Ag-containing samples. For high GGI values above 0.8, the Ag incorporation shifted E_g to lower values which resulted in an increased short-circuit current density and finally in an increased PCE which was accompanied by a slight increase in FF. We observe an increasing Na content in the bulk of the absorber with increasing AAC at GGI = 0.8, possibly due to altered absorber growth during deposition when more Ag is provided. Our best fully in-line prepared cell with a wide-bandgap ACIGS (E_g <b>≈</b> 1.5 eV) achieved a PCE of 11.4% (without ARC). The corresponding V_OC and ΔV_OC values were 829 mV and 680 mV, respectively.<br/>A minor reduction of E_g of about 0.2 eV to 1.34 eV (corresponding to GGI = 0.61), i.e. near the 2^nd theoretical PCE maximum, resulted in a Ag-free CIGS cell with 18.2% PCE (with ARC) and a good V_OC of 856 mV with ΔV_OC = 480 mV.<br/><br/>[1] N. Barreau et al., EPJ Photovolt. <b>13</b> (2022) 17<br/>[2] F. Larsson et al., Prog. Photovolt. Res. Appl. <b>25</b> (2017) 755<br/>[3] S. Zahedi-Azad et al., Thin Solid Films <b>669</b> (2019) 629<br/>[4] J. Keller et al., Sol. RRL <b>5</b> (2021) 2100403<br/>[5] D. Hariskos et al., IEEE J. Photovolt. <b>6</b> (2016) 1321<br/>[6] M. Nakamura et al., IEEE J. Photovolt. <b>9</b> (2019) 1863