Jonathan J. Scragg

A Detrimental Reaction at the Molybdenum Back Contact in Cu2ZnSn(S,Se)4 Thin-Film Solar Cells

Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell material Cu(2)ZnSn(S,Se)(4) (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS(2) and secondary phases at the CZTS|Mo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.

A low-temperature order-disorder transition in Cu2ZnSnS4 thin films

Cu2ZnSnS4 (CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limited by the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, a phenomenon which is difficult to detect by standard techniques. We show that this issue can be overcome using near-resonant Raman scattering, which lets us estimate a critical temperature of 533 ± 10 K for the transition between ordered and disordered CZTS. These findings have deep significance for the synthesis of high-quality material, and pave the way for quantitative investigation of the impact of disorder on the performance of CZTS-based solar cells.

Thermodynamic aspects of the synthesis of thin-film materials for solar cells.

A simple and useful thermodynamic approach to the prediction of reactions taking place during thermal treatment of layers of multinary semiconductor compounds on different substrates has been developed. The method, which uses the extensive information for the possible binary compounds to assess the stability of multinary phases, is illustrated with the examples of Cu(In,Ga)Se(2) and Cu(2)ZnSnSe(4) as well as other less-studied ternary and quaternary semiconductors that have the potential for use as absorbers in photovoltaic devices.

Zn(O, S) Buffer Layers and Thickness Variations of CdS Buffer for Cu

To improve the conduction band alignment and explore the influence of the buffer-absorber interface, we here investigate an alternative buffer for Cu2ZnSnS4 (CZTS) solar cells. The Zn(O, S) system was chosen since the optimum conduction band alignment with CZTS is predicted to be achievable, by varying oxygen to sulfur ratio. Several sulfur to oxygen ratios were evaluated to find an appropriate conduction band offset. There is a clear trend in open-circuit voltage (Voc), with the highest values for the most sulfur rich buffer, before going to the blocking ZnS, whereas the fill factor peaks at a lower S content. The best alternative buffer cell in this series had an efficiency of 4.6% and the best CdS reference gave 7.3%. Extrapolating Voc values to 0 K gave activation energies well below the expected bandgap of 1.5 eV for CZTS, which indicate that recombination at the interface is dominating. However, it is clear that the values are affected by the change of buffer composition and that increasing sulfur content of the Zn(O, S) increases the activation energy for recombination. A series with varying CdS buffer thickness showed the expected behavior for short wavelengths in quantum efficiency measurements but the final variation in efficiency was small.

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Cu2ZnSnS4 (CZTS) solar cells typically include a CdS buffer layer in between the CZTS and ZnO front contact. For sulfide CZTS, with a bandgap around 1.5 eV, the band alignment between CZTS and CdS ...

ZnSnS

The best literature reported kesterite sulfide Cu<inf>2</inf>ZnSnS<inf>4</inf> and selenide Cu<inf>2</inf>ZnSnS<inf>4</inf> thin film solar cell device performance and photocurrent spectra have been compared with their analogous chalcopyrite CulnS<inf>2</inf> and CulnSe<inf>2</inf> devices. Reasons for the kesterites worse performance have been analyzed in terms of (i) phase composition and morphology (ii) opto-electronic properties and (iii) layer interfaces. A new electrodepositon and annealing process is presented for producing Cu<inf>2</inf>ZnSnS<inf>4</inf> absorber layers that gave a highest power conversion efficiency of over 3.2 %. Furthermore photoluminescence spectra of Cu<inf>2</inf>ZnSnS<inf>4</inf> single crystals showing defect related is presented for the first time.

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Photoluminescence (PL) is commonly used for investigations of Cu2ZnSnS(e)4 [CZTS(e)] and Cu(In,Ga)Se2 (CIGS) thin film solar cells. The influence of interference effects on these measurements is, however, largely overlooked in the community. Here, it is demonstrated that PL spectra of typical CZTS absorbers on Mo/glass substrates can be heavily distorted by interference effects. One reason for the pronounced interference in CZTS is the low reabsorption of the PL emission that typically occurs below the band gap. A similar situation occurs in band gap graded CIGS where the PL emission originates predominantly from the band gap minimum located at the notch region. Based on an optical model for interference effects of PL emitted from a thin film, several approaches to reduce the fringing are identified and tested experimentally. These approaches include the use of measured reflectance data, a calculated interference function, use of high angles of incidence during PL measurements as well as the measurement of polarized light near the Brewster angle.

Solar Cells

Solar cells based on Cu2ZnSn(S,Se)(4) absorber layers have received a growing amount of interest. Typically a Mo(S,Se)(2) layer is formed at the Cu2ZnSn(S,Se)(4)/Mo interface during processing. Tra ...

Reduced interface recombination in Cu2ZnSnS4 solar cells with atomic layer deposition Zn1−xSnxOy buffer layers

B-type Cu2ZnSnS4 (CZTS) thin films with varying degrees of cation order were produced and examined with resonant Raman spectroscopy. Simulations based on Vineyards theory of order allowed kinetic analysis of the final degree of order after the applied thermal treatments. Combining the results from the simulations and the resonant Raman spectra, the kinetic parameters within the Vineyard model for the order-disorder transition in B-type CZTS were determined, as well as a method which allows quantification of the degree of order based on resonant Raman spectra. The knowledge gained about the order-disorder transition in B-type CZTS allowed the prediction of a best practice thermal treatment for high ordering. This further leads to awareness about practical limits of thermal treatments regarding the cation ordering in B-type CZTS, and suggests that such treatments are not able to produce the high cation order necessary to sufficiently reduce detrimental potential fluctuations.

A review of the challenges facing kesterite based thin film solar cells

The implementation of terawatt photovoltaic solar energy conversion will place new demands on materials supply and environmental impact. As a consequence, the search for sustainable photovoltaic materials that combine low cost with low toxicity and low energy manufacturing processes is becoming increasingly important. This paper examines the preparation and properties of two emerging indium-free photovoltaic absorber materials, Cu2ZnSnS4 and Cu3BiS3. Electrochemical routes to fabrication of absorber layers are considered, and characterization methods based on photoelectrochemistry and electrolyte contacts are discussed.