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.

Copper-zinc-tin-chalcogenide kesterites, Cu(2)ZnSnS(4) and Cu(2)ZnSnSe(4) (CZTS(e)) are ideal candidates for the production of thin film solar cells on large scales due to the high natural abundance of all constituents, a tunable direct band gap ranging from 1.0 to 1.5 eV, a large absorption coefficient, and demonstrated power conversion efficiencies close to 10%. However, Sn losses through desorption of SnS(e) from CZTS(e) at elevated temperatures (above 400 °C) impede the thorough control of film composition and film homogeneity. No robust and feasible fabrication process is currently available. Here we show that understanding the formation reaction of the kesterite absorber is the key to control the growth process and to drastically improve the solar cell efficiency. Furthermore, we demonstrate that this knowledge can be used to simplify the four-dimensional parameter space (spanned by the four different elements) to an easy and robust two-dimensional process. Sufficiently high partial pressures of SnS(e) and S(e) (a) prevent the decomposition reaction of the CZTS(e) at elevated temperatures and (b) introduce any missing Sn into a Sn-deficient film. This finding enables us to simplify the precursor to a film containing only Cu and Zn, whereas Sn and S(e) are introduced from the gas phase by a self-regulating process.

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Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

Although kesterite solar cells show the same range of band gaps as the related chalcopyrites, their efficiencies have so far reached only 10%, compared to 20% for the chalcopyrites. A review of the present literature indicates that several non-ideal recombination channels pose the main problem: (i) recombination at the interface between the kesterite and the CdS buffer. This is very likely due to an unfavourable clifflike band alignment between the absorber and the buffer. However, for pure selenide absorbers, this recombination path is not dominating, which could be due to a spike-like band alignment at the absorber-buffer interface. (ii) A second major recombination becomes obvious in a photoluminescence maximum well below the band gap, even in record efficiency absorbers. This is either due to a very high density of defects, comparable to the density of states in the band, or to stannite inclusions. In view of the phase diagram, secondary phases are not likely the source of the low energy emission.Only in sulphide kesterite a non-stoichiometric SnS phase could also cause this low energy radiative recombination.

This work reports the in-depth resolved Raman scattering analysis with different excitation wavelengths of Cu2ZnSnS4 layers. Secondary phases constitute a central problem in this material, particularly since they cannot be distinguished by x-ray diffraction. Raman spectra measured with 325 nm excitation light after sputtering the layers to different depths show peaks that are not detectable by excitation in the visible. These are identified with Cu3SnS4 modes at the surface region while spectra measured close to the back region show peaks from ZnS and MoS2. Observation of ZnS is enhanced by resonant excitation conditions achieved when working with UV excitation.

Phone: þ46 (0) 18 471 3382, Fax: þ46 (0) 18 555 095Cu 2 ZnSn(S,Se) 4 (CZTS(e)) solar cells suffer from low-opencircuit voltages that have been blamed on the existence of band gap fluctuations, with different possible origins. In this paper, we show from both theoretical and experimental standpoints that disorder of Cu and Zn atoms is in all probability the primary cause of these fluctuations. First, quantification of Cu-Zn disorder in CZTS thin films is presented. The results indicate that disorder is prevalent in the majority of practical samples used for solar cells. Then, ab initio calculations for different arrangements and densities of disorder-induced [Cu Zn þ Zn Cu ] defect pairs are presented and it is shown that spatial variations in band gap of the order of 200 meV can easily be caused by Cu-Zn disorder, which would cause large voltage losses in solar cells. Experiments using Raman spectroscopy and room temperature photoluminescence combined with in situ heat-treatments show that a shift in the energy of the dominant band-to-band recombination pathway correlates perfectly to the order-disorder transition, which clearly implicates Cu-Zn disorder as the cause of band gap fluctuations in CZTS. Our results suggest that elimination or passivation of Cu-Zn disorder could be very important for future improvements in the efficiency of CZTS(e)-based solar cells.