Efficiency Improvement of Near‐Stoichiometric CuInSe₂ Solar Cells for Application in Tandem Devices

Abstract

the formation of defect complexes and possibly even phases of ordered defect compounds. [9][10][11][12] Increasing the Cu content toward stoi chiometry can considerably reduce the pre sence of such defects and defect complexes. It has been shown that stoichiometric Cu concentrations can be beneficial for absorber crystallinity, [13] defect density, [7,13] mobility, [14,15] and doping density. [14][15][16] Despite this, solar cells based on Cu stoichiometric CuInSe 2 (CIS) have never reached efficiencies above 13.5%, [7,[17][18][19] while Cudeficient CIS yielded efficiencies only up to 15.0%. [20] CIS cells with high CGI are mainly limited by a low open circuit voltage (V OC ), and to a lesser extent, by a reduced fill factor (FF) and current density. However, the implementation of a Cudeficient surface layer has shown to recover the V OC loss in those high CGI cells, while a decrease in current density remains attributed to tun neling recombination. [17,21] Similar improvements of the absorber surface have been achieved with alkali treatments after etching of the secondary phases in Curich samples, [22,23] although most recent results indicate that this treatment may passivates defects that were at least partly generated by the etching in the first place. [24] This suggests that the front absorber surface limits the efficiency of Cu stoichiometric devices. Therefore, it is important to reduce the front interface recombination, besides any other, in such devices.In an earlier publication, we have shown that ungraded CIS solar cells are constrained by recombination at the back interface, [25] which limits the solar cell efficiency. This back sur face recombination can be effectively suppressed by the imple mentation of a single bandgap grading achieved by adding Ga close to the Mo back contact, leading to improved efficiencies up to 16.1% with a bandgap of 1.0 eV absorber [25] and later to 18.0% with application of a RbF postdeposition treatment (PDT). [26] Those solar cells were processed with Cudeficient absorbers (CGI: 0.85-0.90). CIS solar cells processed with high Cu content show lower V OC , and therefore lower efficiency. Believing in recombination as the root cause, we hypothesize that the application of heavy alkali PDT, for example with RbF, may suppress the recombination observed for absorbers with stoichiometric Cu concentrations. To investigate this, we processed and analyzed CISbased solar cells with various Cu concentrations and amounts of RbF during PDT treatment.Here we report a large improvement in efficiency of solar cells processed with Cu composition close to stoichiometry. In State-of-the-art Cu(In,Ga)Se 2 (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe 2 sola...

Challenges and opportunities for an efficiency boost of next generation Cu(In,Ga)Se2 solar cells: prospects for a paradigm shift

“…RbF). 17 Additionally, due to the implications of composition gradients for the generation of strain and defects, 18 their replacement also emerges as a plausible pathway to enhance the properties of the absorber.…”

Near‐Infrared‐Transparent Perovskite Solar Cells and Perovskite‐Based Tandem Photovoltaics

“…Thus, to achieve TCOs with higher conductivity and an excellent transmission at the same time, it is desirable to develop TCOs with high carrier mobility instead of high carrier concentration. [ 163,180,195 ] In recent years, hydrogen doped indium oxide (IO:H), [ 83,190 ] zirconium doped indium oxide (IZrO), [ 199,200 ] and indium doped zinc oxide (IZO) [ 177,197 ] electrodes have gained increasing research interest due to their high carrier mobility. Taking advantage of the low free carrier absorption, these TCO electrodes offer superior NIR transparency compared to the FTO electrode.…”

“…[ 163 ] These TCOs are well‐suited for tandem architecture and have already been demonstrated by numerous research works with excellent electrical and optical characteristics. [ 84,197,198,200,211 ] Further research progress on developing high mobility transparent electrode could prove to be extremely beneficial in minimizing the parasitic losses incurred in the TSCs.…”

Recent Progress in Metal Halide Perovskite‐Based Tandem Solar Cells

“…As a narrow-bandgap light absorber, a CIS thin film with a Cu content close to stoichiometry was prepared by adding RbF postdeposition treatment, and it was employed in a 4T MHP/CIS tandem solar cell to achieve better efficiency due to the lower recombination of the CIS subcell. [147] Solution processable QDs are advantageous in the design of perovskite-based tandem solar cells because their bandgap is easily tunable based on their size and composition. Manekkathodi et al demonstrated a PCE of 20.2% for 4T MHP/colloidal QD tandem cells using an MoO 3 /Au/Ag/MoO 3 OMO TCE, which improved IR transmittance.…”

“…As a narrow‐bandgap light absorber, a CIS thin film with a Cu content close to stoichiometry was prepared by adding RbF postdeposition treatment, and it was employed in a 4T MHP/CIS tandem solar cell to achieve better efficiency due to the lower recombination of the CIS subcell. [ 147 ]…”