10.3% Efficient Green Cd‐Free Cu₂ZnSnS₄ Solar Cells Enabled by Liquid‐Phase Promoted Grain Growth

Abstract

efficiency (PCE) of CZTS solar cells is stagnated at 11.0%, showing a large efficiency gap compared to the Shockley-Queisser limit. [7,8] Besides the well-recognized opencircuit voltage (V OC ) deficit, E g /q-V OC , the low performance of CZTS solar cell should also be largely attributed to its low carrier collection efficiency compared to Cu(In, Ga)Se 2 (CIGS) and CdTe solar cells. [9][10][11] The low carrier collection efficiency is usually attributed to the short minority carrier diffusion length constrained by the low minority carrier lifetime. Nevertheless, our recent electron beam induced current (EBIC) and cathodoluminescence (CL) measurements have shown that: [12] i) the grain boundary recombination velocity of kesterite is relatively high; ii) the effective minority carrier lifetime is highly likely limited by the large grain boundary recombination velocity; and iii) the recombination at near-horizontal grain boundaries in the middle of kesterite thin films is the dominating carrier loss mechanism in the quasi-neutral region, leading to significant losses in short-circuit current density (J SC ). [12,13] Furthermore, the carrier recombination at nearhorizontal grain boundaries may also deteriorate the solar cell fill factor (FF). [14] Therefore, increasing grain size and reducing the number of grain boundaries (especially the near-horizontal ones) in the CZTS absorber is imperative.Small grain size and near-horizontal grain boundaries are known to be detrimental to the carrier collection efficiency and device performance of puresulfide Cu 2 ZnSnS 4 (CZTS) solar cells. However, forming large grains spanning the absorber layer while maintaining high electronic quality is challenging particularly for pure sulfide CZTS. Herein, a liquid-phase-assisted grain growth (LGG) model that enables the formation of large grains spanning across the CZTS absorber without compromising the electronic quality is demonstrated. By introducing a Ge-alloyed CZTS nanoparticle layer at the bottom of the sputtered precursor, a Cu-rich and Sn-rich liquid phase forms at the high temperature sulfurization stage, which can effectively remove the detrimental near-horizontal grain boundaries and promote grain growth, thus greatly improving the carrier collection efficiency and reducing nonradiative recombination. The remaining liquid phase layer at the rear interface shows a high work function, acting as an effective hole transport layer. The modified morphology greatly increases the short-circuit current density and fill factor, enabling 10.3% efficient green Cd-free CZTS devices. This work unlocks a grain growth mechanism, advancing the morphology control of sulfide-based kesterite solar cells.

Improved carrier collection efficiency in CZTS solar cells by Li‐enhanced liquid‐phase‐assisted grain growth

“…Li-doped CZTGS layers were fabricated by the spincoating method before the co-sputtering process as described in our reported LGG process. 17 The precursor solution was prepared by dissolving CuCl, ZnCl 2 , SnCl 4 , GeCl 4 , and thiourea into dimethylformamide in a glove box (MBraun Unilab Pro) in a nitrogen atmosphere. The (Cu þ Li)/(Sn þ Ge) ratio was ~1.90, Zn/(Sn þ Ge) ~1.05 and Ge/(Sn þ Ge) ~0.4, and the concentrations of metal elements and thiourea were 0.79 mol/L and 1.58 mol/L, respectively.…”

“…However, it is relatively difficult to apply the LGG process in pure sulfide CZTS, as the melting point of CZTS‐related metal sulfides (Cu x S, ZnS, Cu 2 SnS 3 , and SnS x ) is usually beyond the grain growth temperature region of CZTS 16 . Recently, our group reported a low‐temperature LGG process for CZTS using Na‐rich Cu‐Sn‐S liquid phase generated from controlled decomposition of a bottom CZTGS nanoparticle layer 17 . Although this method can significantly enhance the overall carrier collection efficiency in the absorber layer, there is still space for further improving the J SC as there are still considerable residual small grains lying between the large grains.…”

“…16 Recently, our group reported a low-temperature LGG process for CZTS using Na-rich Cu-Sn-S liquid phase generated from controlled decomposition of a bottom CZTGS nanoparticle layer. 17 Although this method can significantly enhance the overall carrier collection efficiency in the absorber layer, there is still space for further improving the J SC as there are still considerable residual small grains lying between the large grains.…”

Visual Color Change During Spin‐coating Guiding the Quality of Cu2ZnSn(S,Se)4 Films

“…The kesterite-structured Cu 2 ZnSn(S,Se) 4 (CZTSSe) are promising absorbers for thin-film photovoltaic devices due to their earthabundant, low-toxicity components, and high light-absorption coefficient (>10 4 cm −1 ). [1][2][3][4] The fabrication of CZTSSe through molecular ink solution is a viable processes to obtain high performance devices. Mitzi group achieved power conversion efficiency (PCE) of 12.6% by hydrazine solvent.…”

Crown Ether-Assisted Colloidal ZnO Window Layer Engineering for Efficient Kesterite (Ag,Cu)2ZnSn(S,Se)4 Solar Cells

“…In the current research stage, greater focus has been paid to the CZTSSe absorber layer itself. Researchers have continuously explored to improve the CZTSSe crystal quality through bulk phase regulation, , element doping, , selenization/sulfurization modification, , solvent engineering, , and so on. Over 14% efficiency has been achieved for kesterite solar cells by regulating the phase evolution kinetics of Ag-alloyed CZTSSe in a positive pressure .…”