Phillip J. Dale

The Consequences of Kesterite Equilibria for Efficient Solar Cells

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.

In-depth resolved Raman scattering analysis for the identification of secondary phases: Characterization of Cu2ZnSnS4 layers for solar cell applications

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.

Raman analysis of monoclinic Cu2SnS3 thin films

Secondary phases like Cu2SnS3 are major obstacles for kesterite thin film solar cell applications. We prepare Cu2SnS3 using identical annealing conditions as used for the kesterite films. By x-ray diffraction, the crystal structure of Cu2SnS3 was identified as monoclinic. Polarization-dependent Raman investigations allowed the identification of the dominant peaks at 290 cm−1 and 352 cm−1 with the main A′ symmetry vibrational modes from the monoclinic Cu2SnS3 phase. Furthermore, micro-resolved Raman investigations revealed local variations in the spectra that are attributed to a secondary phase (possibly Cu2Sn3S7). This exemplifies the abilities of micro-resolved Raman measurements in the detection of secondary phases.

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.

Route Toward High-Efficiency Single-Phase Cu

Thin-film chalcogenide kesterites Cu2ZnSnS4 and Cu2 ZnSnSe4 (CZTSSe) are promising candidates for the next-generation thin-film solar cells. They exhibit a high natural abundance of Cu, Zn, Sn and S2, a high absorption coefficient, and a tunable direct bandgap between 1.0 and 1.5 eV. A prerequisite for the use of CZTSSe as absorber layers in photovoltaic applications on large scales is a detailed knowledge of the formation reaction. Recently, we have shown that a decomposition/formation equilibrium governs the formation reaction. The presence of Sn(S,Se) during the high-temperature preparation steps is essential to prevent decomposition. This improves the solar cell efficiency from 0.02% to 6.1%. In this paper, we show that the decomposition is universal. Absorbers produced by high-temperature coevaporation and samples produced by low-temperature precursor fabrication followed by annealing in a tube furnace in S or Se atmosphere are compared in order to elucidate that in all cases, the loss of Sn(S,Se) forms a degraded surface region. We demonstrate that the degraded surface of CZTSe absorbers contains grains of ZnSe. These new insights can be used to explain why some of the synthesis routines described in the literature yield much better efficiencies than others.

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CuInSe2-based absorber layers for photovoltaic devices have been fabricated using two different scalable processes, electrodeposition and sputtering, both followed by thermal annealing. The structural properties of the absorber layers were studied by SEM, XRD and MiniSIMS. Sputtered absorber layers exhibit larger grain sizes than electrodeposited layers, but both types of film consist of randomly orientated crystallites. Electrodeposited layers appear to have a uniform composition with evidence of a MoSe2 layer at the back contact, whilst sputtered layers show no evidence for a MoSe2 layer. The external quantum efficiency spectrum of films and completed devices was measured, and the band gap and broadening parameters were obtained using electroreflectance spectroscopy. A device based on electrodeposited CuInSe2 achieved an AM 1.5 efficiency of 6.6%, whilst a device based on sputtered CuInSe2 had an efficiency of 8.3%. Impedance measurements were used to calculate doping densities of 2 ? 1016 and 4 ? 1015?cm?3 for the electrodeposited and sputtered devices, respectively.

ZnSn(S,Se)

Secondary phases, such as ZnSe, occur in Cu2ZnSnSe4 and can be detrimental to the resulting solar cell performance. Therefore, it is important to have simple tools to detect them. We introduce subband gap defect excitation room temperature photoluminescence of ZnSe as a practical and non-destructive method to discern the ZnSe secondary phase in the solar cell absorber. The PL is excited by the green emission of an Ar ion laser and is detected in the energy range of 1.2–1.3 eV. A clear spatial correlation with the ZnSe Raman signal confirms this attribution.

_{\bf 4}

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.

Thin-Film Solar Cells: Model Experiments and Literature Review

We present a high-temperature Cu

Characterization of CuInSe2 material and devices: comparison of thermal and electrochemically prepared absorber layers

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