Fig. 1. Best laboratory certified efficiencies for CIGS solar cells ^［19］

Fig. 2. （a，b）Schematics of the device structure of the small-area cell and the monolithically-connected module structure，respectively.（c）Schematics of the typical band profile in the CIGS absorber layer. E_C and E_V represent the energetic positions of the conduction band minimum and the valence band maximum，respectively. Front and back correspond to the buffer/CIGS and the CIGS/Mo interfaces，respectively^［20］

Fig. 3. Schematic illustration of different co-evaporation processes:（a）single stage process,（b）bilayer or Boeing process in which the first layer was deposited at lower substrate temperature, and the second layer was deposited at a higher substrate temperature,（c）three stage process in which In and Ga are deposited in the first and third stage, whereas Cu was deposited in the second stage（after Ref. [33]）

Fig. 4. Fabrication process of Solar Frontier’s baseline CIGS solar cell ^[20]

Fig. 5. （a）Schematics of the profiles of composition and bandgap in the CIGS absorber layer to explain the condition of the device simulation.（b）Device simulation results with varying surface [S]/（[S]+[Se]）composition and position of the bandgap minimum（xE_g,min）（after Ref. [20]）

Fig. 7. After Na incorporation into the CIGS film，the following changes were observed：（a）enhanced carrier density and grain boundary passivation，（b）gallium segregation，and（c）changes in crystallographic orientation（after Ref.［45］）

Fig. 8. Surface chemical analysis（a）Schematic view of the three investigated absorbers. The purple layer on the KF absorber indicates the modified surface composition，（b-e）XPS peak of Cu 2p_3/2，In 3d_5/2，Ga 2p_3/2 and Se 3s，respectively，obtained from the surface of CIGS absorbers with no alkali evaporation（no PDT），only NaF addition and only KF addition，（f）Sputtering of the CIGS absorber subjected to KF-PDT shows the appearance of the Cu 2p_3/2 peak within the first approximately 20 nm with similar intensity as in the case of no PDT，（g）K is clearly measurable at the surface up to a depth of approximately 20 nm，（h）Schematic view of two absorbers measured after sputtering through the CdS layer，（i）XPS peak of Cu 2p_3/2，In 3d_5/2，Ga 2p_3/2 and Se 3s，respectively，at the CdS/CIGS interface with only NaF addition and only KF addition，（m）XPS spectra of K at different sputtering depths（after Ref.［48］）

Fig. 9. Schematic drawing of the NaF PDT and the NaF&KF PDT applied on low-temperature coevaporated CIGS thin films ^［49］

Fig. 10. External quantum efficiency for CIGS cells with different buffer materials. All cells with anti-reflection coatings. The shaded areas below the curves represent the current gain relative to the corresponding CdS reference when available（after Ref.［53］）

Fig. 11. （a）Comparison of the optical properties of AZO and BZO with comparable sheet resistance. The high transmission in the near-infrared region for BZO stems from the reduced carrier density（AZO n=4.4*10²⁰ cm^-3，BZO n=9.2*10¹⁹ cm^-3）（after Ref.［64］）.（b）Damp heat stability（85 °C，85% r.h.）of different，nonencapsulated TCO materials（after Ref.［70-72］）

Fig. 12. Photovoltaic application products such as flexible CIGS thin film solar cell template and building integration^[73]

Fig. 13. （a）Schematic of the 4-T perovskite/CIGS tandem solar cell.（b）J-V curves of the CIGS cell with and without filtering，and reverse and forward scanning curves（scanning rate of 50 mV s^-1）and steady-state efficiency of the semi-transparent perovskite.（c）Transmittance and absorption spectra and EQE of the transparent perovskite cells，and EQE of the standalone CIGS and that placed under a filter（after Ref.［89］）

Fig. 14. Performance of the perovskite/CIGS tandem cells.（a）Schematic and cross-sectional SEM image of the monolithic perovskite/CIGS tandem solar cell.（b）J-V curve and efficiency at the maximum power point（inset）of the champion tandem device.（c）EQE spectra for the subcells of the monolithic perovskite/CIGS tandem solar cell.（d）Stability test of the monolithic perovskite/CIGS tandem solar cell. The unencapsulation device maintained 88% of their initial PCE after 500 hours of aging under continuous 1-sun illumination and maximum power point tracking at 30 °C ambient environment. The inset shows that the device can recover 93% of its initial performance after a 12-hour resting period without load and illumination（after Ref.［90］）

Table 1. List of various growth methods used for the preparation of CIGS films, and their advantages and disadvantages

Table 2. The influence of different alkali metal combinations on the photovoltaic parameters of CIGS solar cells

Table 3. Summary of best performing small-area CIGS cells with different buffer layers and respective deposition methods