From 20.9 to 22.3% Cu(In,Ga)(S,Se)₂solar cell: Reduced recombination rate at the heterojunction and the depletion region due to K-treatment

Abstract

Certified efficiency of 22.3% has been achieved for Cu(In,Ga)(Se,S) 2 solar cell. Compared to our previous record cell with 20.9% efficiency, the major breakthrough is due to the increased V oc , benefited from potassium treatment. A lower reverse saturation current and a longer carrier collection length deduced from electron-beam induced current indicate that the degree of carrier recombination at the heterojunction and depletion region for the 22.3% cell is lower. Further characterizations (capacitance-voltage profiling, temperature-dependent V oc , Suns-V oc ) and analysis indicate that the recombination coefficients at all regions were reduced, especially for the interface and depletion regions. Device simulation was performed assuming varying defect densities to model the current-voltage curve for the 22.3% cell. The best model was also used to estimate the achievable V oc if defect densities were further reduced. Furthermore, by using higher bandgap Cd-free buffer layers, a higher J sc was achieved which gives an in-house solar cell efficiency of 22.8%. Recombination analysis on the 22.8% cell indicates that the interface recombination is further reduced, but the recombination coefficients at the depletion region was higher, pointing out that further improvement on the depletion region recombination could help to achieve a higher V oc and therefore an efficiency beyond 23%.

Effects of Ammonia‐Induced Surface Modification of Cu(In,Ga)Se2 on High‐Efficiency Zn(O,S)‐Based Cu(In,Ga)Se2 Solar Cells

“…Another type of approach to noticeably improve the performance of Zn(O,S) based CIGS solar cells is to apply post‐treatment to the CIGS surface before the buffer layer is deposited. Using alkali element treatment, including sodium and potassium, on the prior formed CIGSSe thin film absorber, Solar Frontier has recently fabricated Zn(O,S) based CIGSSe solar cells with conversion efficiency consistently beyond 22%, which is comparable with the world record of the CIGS solar cell with CdS buffer of 22.6% by ZSW in 2016 and greatly encourages research enthusiasm on Zn(O,S) buffer layer. The alkali treatment, which results in better junction properties, is believed to improve the solar cell performance by various plausible mechanisms such as reduced recombination rate at the heterojunction, increased net acceptor concentration, reduced concentration of compensating donors in the bulk and grain boundaries .…”

“…Thin film solar cells are promising for the reduction of production cost due to the effective use of the raw materials . Chalcopyrite Cu(In,Ga)Se 2 (CIGS) and Cu(In,Ga)(S,Se) 2 (CIGSS)‐based thin film solar cells are intriguing because their energy conversion efficiencies above 22% with small area have been routinely reported with long‐term reliability and their modules have already been produced on large scales . Normally, high efficiency CIGS‐based thin film solar cells are produced with chemical bath deposited (CBD) CdS buffer layers .…”

Proton Irradiation on Cesium‐Fluoride‐Free and Cesium‐Fluoride‐Treated Cu(In,Ga)Se2 Solar Cells and Annealing Effects under Illumination

“…For the CIGS absorber, two defect energy levels, E V + 0.80 eV (neutral‐type, N 1 ) and E V + 0.13 eV (acceptor‐type, N 2 ), were set. The electron and hole capture cross sections for N 1 (N 2 ) are set to be 3.3 × 10 −13 (1.0 × 10 −19 ) cm −2 and 2.2 × 10 −13 (2.33 × 10 −17 ) cm −2 , respectively . The defect density was linearly increased from the surface to the bulk region from 5 × 10 14 to 7 × 10 18 cm −3 .…”

“…The electron and hole capture cross sections for N 1 (N 2 ) are set to be 3.3 Â 10 À13 (1.0 Â 10 À19 ) cm À2 and 2.2 Â 10 À13 (2.33 Â 10 À17 ) cm À2 , respectively. [37] The defect density was linearly increased from the surface to the bulk region from 5 Â 10 14 to 7 Â 10 18 cm À3 . The defect depth profile is given in Figure 6a, and the simulated EQE curves are given in Figure 6b.…”

Giant Voc Boost of Low‐Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers

“…Historical CIGS panels contain a CdS buffer layer that is toxic and does absorb part of the solar radiation without contributing to charge carrier generation. Therefore, reducing the thickness of this layer and finding wider‐gap replacements have been successful strategies for record efficiency devices . Alternative buffers, such as In x S y , (Mg,Zn)O and Zn(O,S), yield promising device performance, in some cases even outperforming CdS‐based devices .…”