The path towards a high-performance solution-processed kesterite solar cell ☆

The kesterite-structured semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe2 and CuInSe2 . Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor CuZn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn +SnZn ], [VCu +ZnCu ] and [ZnSn +2ZnCu ] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn +SnZn ] clusters, especially in Cu2ZnSnS4. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S composition is high.

Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers.

The structural and electronic properties of Cu2ZnSnS4 and Cu2ZnSnSe4 are studied using first-principles calculations. We find that the low energy crystal structure obeys the octet rule and is the kesterite (KS) structure. However, the stannite or partially disordered KS structures can also exist in synthesized samples due to the small energy cost. We find that the dependence of the band structure on the (Cu,Zn) cation ordering is weak and predict that the band gap of Cu2ZnSnSe4 should be on the order of 1.0 eV and not 1.5 eV as was reported in previous absorption measurements.

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Crystal and electronic band structure of Cu2ZnSnX4 (X=S and Se) photovoltaic absorbers: First-principles insights

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

The electronic structure as well as the optical response of kesterite and stannite structures of Cu2ZnSnS4 and Cu2ZnSnSe4 are analyzed by a relativistic full-potential linearized augmented plane wa ...

Electronic and optical properties of Cu2ZnSnS4 and Cu2ZnSnSe4

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Thin films of Cu 2 ZnSnS 4 (CZTS), a potential candidate for absorber layer in thin film heterojunction solar cell, have been deposited by spray pyrolysis technique onto soda-lime glass substrates held at a substrate temperature (T s ) of 643 K. The effect of copper salt and thiourea concentrations on the formation of Cu 2 ZnSnS 4 thin films is investigated. CZTS films formed under optimized conditions are found to be polycrystalline in nature with kesterite structure. The lattice parameters are found to be a = 0.543 nm and c = 1.086 nm. The optical band gap of these films is found to be 1.43 eV. It is found to increase with decrease in copper salt concentration in the solution.

Effect of copper salt and thiourea concentrations on the formation of Cu2ZnSnS4 thin films by spray pyrolysis

Effect of Cu/(Zn+Sn) ratio on the properties of co-evaporated Cu2ZnSnSe4 thin films

Cu2ZnSnS4 (CZTS) thin films, a potential solar-cell absorber material, have been successfully deposited onto soda lime glass held at a substrate temperature of 643 K. The effect of starting-solution pH on the properties of deposited films is investigated. The solution pH was varied in the range 3.0 to 5.5. The films were characterized by studying their composition, structural, optical and electrical properties. X-ray diffraction studies revealed that polycrystalline CZTS films with kesterite structure could be obtained from a solution with pH = 3.0. However, the films are nonstoichiometric and may contain binary phases in the amorphous form. Films deposited from a solution with pH = 4.5 contain binary phases in addition to CZTS. The lattice parameters of CZTS films are found to be a = 0.542 nm and c = 1.085 nm. The optical bandgap of these films is 1.45 eV and the optical absorption coefficient is >104 cm–1. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of starting‐solution pH on the growth of Cu2ZnSnS4 thin films deposited by spray pyrolysis

The effect of substrate temperature and post-deposition annealing on the growth and properties of Cu2ZnSnSe4 thin films, a potential candidate for a solar cell absorber layer, is investigated. The substrate temperature (Ts) is chosen to be in the range 523–673 K and the annealing temperature (Tpa) is kept at 723 K. Powder x-ray diffraction (XRD) patterns of as-deposited films revealed that the films deposited at Ts = 523 K and 573 K contain Cu2−xSe as a secondary phase. Single phase, polycrystalline Cu2ZnSnSe4 films are obtained at Ts = 623 K and films deposited at Ts = 673 K have ZnSe as a secondary phase along with Cu2ZnSnSe4. Direct band gap of as-deposited CZTSe films is found to lie between 1.40 eV and 1.65 eV depending on Ts. XRD patterns of post-deposition annealed films revealed that the films deposited at Ts = 523–623 K are single phase CZTSe and films deposited at Ts = 673 K still contain ZnSe secondary phase. CZTSe films are found to exhibit kesterite structure with the lattice parameters a = 0.568 nm and c = 1.136 nm. Optical absorption studies of post-deposition annealed films show that there is a slight increase in the band gap on annealing, due to decrease in the Cu content. Electrical resistivity of the films is found to lie in the range 0.02–2.6 Ω cm depending on Ts.

https://iopscience.iop.org/article/10.1088/0268-1242/23/8/085023/pdf

Y.B. Kishore Kumar

Papers

Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films

Effect of copper salt and thiourea concentrations on the formation of Cu2ZnSnS4 thin films by spray pyrolysis

Effect of Cu/(Zn+Sn) ratio on the properties of co-evaporated Cu2ZnSnSe4 thin films

Effect of starting‐solution pH on the growth of Cu2ZnSnS4 thin films deposited by spray pyrolysis

Effect of post-deposition annealing on the growth of Cu2ZnSnSe4 thin films for a solar cell absorber layer