Kesterites—a challenging material for solar cells

Abstract

Kesterite materials (Cu2ZnSn(S,Se)4) are made from non‐toxic, earth‐abundant and low‐cost raw materials. We summarise here the structural and electronic material data relevant for the solar cells. The equilibrium structure of both Cu2ZnSnS4 and Cu2ZnSnSe4 is the kesterite structure. However, the stannite structure has only a slightly lower binding energy. Because the band gap of the stannite is predicted to be about 100 meV lower than the kesterite band gap, any admixture of stannite will hurt the solar cells. The band gaps of Cu2ZnSnS4 and Cu2ZnSnSe4 are 1.5 and 1.0 eV, respectively. Hardly any experiments on defects are available. Theoretically, the CuZn antisite acceptor is predicted as the most probable defect. The existence region of the kesterite phase is smaller compared with that of chalcopyrites. This makes secondary phases a serious challenge in the development of solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

Annealing behavior of reactively sputtered precursor films for Cu2ZnSnS4 solar cells

“…The loss of excess sulfur or elimination of porosity could both lead to formation of cracks in the annealing process. The generally low density of the precursors, compared to the calculated value for CZTS (4.58 g/cm 3 derived from [19]), is not surprising in films that are sputtered at low temperature [18]. For the annealed films a lower density than the bulk value is most likely due to voids.…”

Lone conduction band in Cu2ZnSnSe4

“…These compounds are, in comparison to current high efficiency thin film materials as Cu(In,Ga)Se 2 and CdTe, composed of elements of higher abundance and less toxicity. [1][2][3] All theoretical investigations of the band structures of CZTSe and CZTS show a peculiar density of states in the conduction band: [4][5][6][7][8] The conduction band minimum is formed by a rather narrow band, formed by Sn-s and anion-p states. At higher energies, the density of states becomes zero again, forming a second band gap within the conduction band.…”

Formation of nanometer-sized Cu-Sn-Se particles in Cu2ZnSnSe4 thin-films and their effect on solar cell efficiency

“…We cannot distinguish these two phases from each other by X-ray diffraction because Cu þ and Zn 2þ are isoelectronic. Due to the smaller band gap of stannite compared to kesterite the cell efficiency can be further reduced [1]. In addition, the Cu 2-x Se phase, which is detected in the Cu-rich precursor at the surface up to the film middle and at Cu-enriched CZTSe GBs [31], is highly conductive [26].…”