Advanced Energy Materials(IF>29.698)》Cadmium Alloying Effectively Suppresses Defects and Improves CZTS Solar Cell Efficiency to 12.3%

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Highlights

Cadmium (Cd) alloying can effectively suppress defects in CZTS solar cells and improve conversion efficiency.
The efficiency of CZTS solar cells with 35% cadmium content can reach 12.3%, an increase of over 70% compared to pure CZTS.
Cadmium alloyed CZTS has the lowest open-circuit voltage loss, demonstrating the importance of cadmium alloying strategy.

Background

Kesterite solar cells are composed of copper, zinc, tin and sulfur, which is copper zinc tin sulfide (CZTS). CZTS has low cost, low toxicity, and is considered an ideal absorber material for next-generation thin film solar cells. However, CZTS suffers from serious defect issues, resulting in low conversion efficiency. Cadmium alloying of CZTS has been theoretically proven to inhibit defects and improve conversion efficiency, but the efficiency of cadmium alloyed CZTS solar cells needs further improvement.

Results

A research team led by Feng Zhao from University of Colorado Boulder reported a study on cadmium alloyed CZTS (Cu2(Zn,Cd)SnS4, CZCTS). CZCTS absorber layers were prepared via dimethyl sulfoxide (DMSO) solution processing, and the influence of cadmium content on the reaction path, grain growth and electronic properties of CZCTS absorbers was investigated. The results showed that cadmium can directly participate in the phase transformation and grain growth of CZTS, effectively suppressing band tailing. High quality CZCTS absorber films and efficient solar cells can be fabricated over a wide range of cadmium concentrations. A champion CZCTS device with 12.3% power conversion efficiency was achieved at 35% cadmium content without post heat treatment, improved by over 70% compared to 7.0% of CZTS. This device exhibits the lowest open-circuit voltage loss, demonstrating the importance of cadmium alloying strategy.

Methods

The study prepared cadmium alloyed CZTS films using a solution processing method. First, metal precursors were dissolved in DMSO solutions with varied Cd contents, then spin-coated to deposit precursor films. Next, thermal annealing was applied to the precursor films for sulfurization, forming CZCTS absorber layers. Characterizations of CZCTS films including X-ray diffraction, Raman scattering, electron probe microanalysis etc. were used to determine film compositions and structures. Finally, ZnS/Al2O3 layers were deposited by evaporator to complete the solar cell devices. The results show that the solution method enables controlled Cd incorporation, and thermal treatment drives Cd to participate in CZTS phase change, suppressing defects and improving device performance.

Conclusion

This study demonstrates that cadmium alloying can be effectively achieved via solution processing, and significantly increase CZTS solar cell efficiency. Cadmium can directly participate in the phase change and grain growth of CZTS, inhibiting defect formation. The 12.3% efficiency CZCTS solar cell with 35% cadmium content has the minimum open-circuit voltage loss. The research provides an important pathway to realize high efficiency kesterite solar cells.

Figure S3. The XRD patterns of the absorber films fabricated from the DMSO solutions with different Cd concentration of 0 (CZTS), n% (CZCTS-n), and 100% (CCTS).
The standard XRD patterns of CZTS (#26-0575) and CCTS (#29-0537) are also shown forcomparison.
The enlargement of the (200), (004), (220), (204), (002), (101) diffraction peaks of films, which could reflect the difference of kesterite and stannite are shown in Figure S3 right.

Figure S4. The dependence of band gaps (left) and VOC/VOCSQ (right) with the ratio of Cd/(Zn+Cd) for CZTS and CZCTS-n and CCTS absorber films.
The VOC SQ is the

Figure S5. a) The current density–voltage (J–V) and b) the external quantum efficiency(EQE) curves of best performing CZCTS-35 solar cells at different stages ofsulfurization.
c-f) Statistical device parameters of CZCTS-35 solar cells at differentstages of sulfurization. The statistical data are based on 18 solar cells.