The photoluminescence (PL) of semiconductors can be used to determine their absorption coefficient (a) using Planck's generalized law. The standard method, suitable only for self-supported thick samples, like wafers, is extended to multilayer thin films by means of the transfer-matrix method to include the effect of the substrate and optional front layers. a values measured on various thin-film solar-cell absorbers by both PL and photothermal deflection spectroscopy (PDS) show good agreement. PL measurements are extremely sensitive to the semiconductor absorption and allow us to advantageously circumvent parasitic absorption from the substrate; thus, a can be accurately determined down to very low values, allowing us to investigate deep band tails with a higher dynamic range than in any other method, including spectrophotometry and PDS.

Cu2ZnSn(S,Se)(4) (CZTS) thin film solar cells have reached efficiencies of up to 12.6% and current research is focused on understanding reasons for device limitations. At Uppsala University, a CZTS synthesis route based on compound sputtering and annealing in elemental vapors is used. Variation of chemical composition and annealing conditions is used as a tool to try to understand defect-related material- and device properties. Front and back contacts are also studied with focus on energy band matching at the hetero-interface using atomic layer deposition buffer layers and chemical stability of the back contact. In this review, we discuss recent work from our group, with reference to related work in the literature and with regards to areas for future work.

In this work, we have demonstrated the extreme radiation hardness of thin film CZTSSe solar cells. Thin film solar cells with CZTSSe, CZTS and CIGS absorber layers were irradiated with 3 MeV protons. No degradation in device parameters was observed until a displacement damage dose of 2 x 10(10) MeV/g for CZTS and CZTSSe. CIGS solar cells degraded by 13% at the same dose. For the highest proton dose both the CZTSSe and CZTS degraded by 16% while CIGS suffered from 34% degradation in efficiency. The degradation in efficiency maybe attributed to the reduction in the minority carrier lifetime due to radiation induced lattice defects. Comparisons with previously available literature show that our CZTS technology has superior radiation hardness by about two orders of magnitude compared to existing state of the art Si and GaAs technology.

Band gap grading of Cu2ZnSn(S, Se)(4) (CZTSSe) solar cells can be achieved by varying the S-r = [S]/([S]+[Se]) ratio in the absorber layer with depth. One approach is a two-step annealing process where the absorber is first sulfurized to Cu2ZnSnS4 (CZTS) followed by selenization to form CZTSSe. However, once nucleation of CZTSSe initiates, the rapid interchange of S and Se limits the control over the Sr ratio with depth. Here, we have studied incorporation of Se into CZTS and observed the behavior of Se below and up to the nucleation temperature of CZTSSe. Se diffusion at 337 and 360 degrees C is dominated by grain boundary diffusion while some increase of Se is also seen in the region from 100 to 800 nm from the surface. After selenization at 409 degrees C, recrystallization is observed and CZTSSe grains are formed. The recrystallization is more rapid for a smaller average grain size and is facilitated by diffusion of Na from the back contact. The grain boundary diffusion is identified with secondary ion mass spectrometry measurements by measuring the accumulation in the CZTS/Mo interface for three samples with different average grain size.

Cu2ZnSn(S,Se)(4) precursors are fabricated by compound cosputtering from metal sulfide and selenide targets, and annealed in mixed argon, sulfur, and selenium atmosphere at temperatures between 540 and 580 degrees C and at pressures between 24 and 47 kPa. We produce solar cell devices from these absorbers that range from 2.0% to 9.0% power conversion efficiency. We extensively characterize the morphology and elemental composition of the absorbers, and are able to closely relate the annealing conditions, precursor sulfur-selenium content, device performance, and absorber quality. We develop a qualitative model which relates the sulfur-selenium distribution in the precursor and the relative partial pressures of sulfur and selenium during the annealing process to the absorber properties. We show that selenium inclusion in the precursor allows more rapid recrystallization of the absorber at lower temperature. Alternating stacking of sulfur and selenium containing precursor material leads to differential rates of recrystallization, which allows some control over the morphology of the annealed absorber and Zn(S,Se) secondary phase segregation in that absorber. We further show that selenium containing precursors can be used to fabricate the superior devices relative to sulfur-only precursors, when the annealing phase space is subject to severe practical restrictions.

It is investigated if the performance of Cu(In,Ga)Se-2 (CIGSe) solar cells produced by co-evaporation can be improved by surface sulfurization in a postdeposition treatment. The expected benefit would be the formation of a sulfur/selenium gradient resulting in reduced interface recombination and increased open-circuit voltage. In the conditions used here it was, however, found that the reaction of the CIGSe layer in a sulfur environment results in the formation of a CuInS2 (CIS) surface phase containing no or very little selenium and gallium. At the same time, a significant pile up of gallium was observed at the CIGSe/CIS boundary. This surface structure was formed for a wide range of annealing conditions investigated in this paper. Increasing the temperature or extending the time of the dwell stage had a similar effect on the material. The gallium enrichment and CIS surface layer widens the surface bandgap and therefore increases the open-circuit voltage. At the same time, the fill factor is reduced, since the interface layer acts as an electron barrier. Due to the balance of these effects, the conversion efficiency could not be improved.

Cu-Zn disorder in Cu₂ZnSnS₄ (CZTS) may be responsible for the large open circuit voltage deficit in CZTS based solar cells. In this study, it was investigated how composition-dependent defect complexes influence the order-disorder transition. A combinatorial CZTS thin film sample was produced with a cation composition gradient across the sample area. The graded sample was exposed to various temperature treatments and the degree of order was analyzed with resonant Raman spectroscopy for various compositions ranging from E- and A-type to B-, F-, and C-type CZTS. We observe that the composition has no influence on the critical temperature of the order-disorder transition, but strongly affects the activation energy. Reduced activation energy is achieved with compositions with Cu/Sn > 2 or Cu/Sn < 1.8 suggesting an acceleration of the cation ordering in the presence of vacancies or interstitials. This is rationalized with reference to the effect of point defects on exchange mechanisms. The implications for reducing disorder in CZTS thin films are discussed in light of the new findings.

In this study, interlayers with varied thickness of TiN between Cu2ZnSnS(e)(4) (CZTS(e)) absorbers and Mo on soda-lime glass substrates are investigated for CZTS(e) thin film solar cells. Na diffusion is analyzed using Secondary Ion Mass Spectrometry and it is found that the use of thick TiN interlayers facilitates Na diffusion into the absorbers. The CZTS(e)/TiN/Mo interfaces are scrutinized using Transmission Electron Microscopy (TEM) Electron Energy Loss Spectroscopy (EELS). It is found that diffusion of chalcogens present in the precursor occurs through openings, resulting from surface roughness in the Mo, in the otherwise chemically stable TiN interlayers, forming point contacts of MoS(e)(2). It is further established that both chalcogens and Mo diffuse along the TiN interlayer grain boundaries. Solar cell performance for sulfur-annealed samples improved with increased thickness of TiN, and with a 200 nm TiN interlayer, the solar cell performance is comparable to a typical Mo reference. Pure TiN bulk contacts are investigated and shown to work, but the performance is still inferior to the TiN interlayer back contacts. The use of thick TiN interlayers offers a pathway to achieve high efficiency CZTS(e) solar cells on highly inert back contacts.

Tin monosulfide (SnS) is a promising semiconductor material for low-cost conversion of solar energy, playing the role of absorber layer in photovoltaic devices. SnS is, due to its high optical damping, also an excellent semiconductor candidate for the realization of ultrathin (nanoscale thickness) plasmonic solar cells [1].

Here, we present an important step to further control and understand SnS film properties produced using low temperature ALD with Sn(acac)2 and H2S as precursors. We show that the SnS film properties vary over a rather wide range depending on substrate temperature and reaction conditions, and that this is connected to the growth of cubic (π-SnS) and orthorhombic SnS phases. The optical properties of the two polymorphs differ significantly, as demonstrated by spectroscopic ellipsometry [2].

1. C. Hägglund, G. Zeltzer, R. Ruiz, A. Wangperawong, K. E. Roelofs, S. F. Bent, ACS Photonics 3 (3) (2016) 456–463.

2. O. V. Bilousov, Y. Ren, T. Törndahl, O. Donzel-Gargand , T. Ericson, C. Platzer-Björkman, M. Edoff, and C. Hägglund, ACS Chemistry of Materials  29 (7) (2017) 2969–2978.

Tin monosulfide (SnS) is a promising light-absorbing material with weak environmental constraints for application in thin film solar cells. In this paper, we present low-temperature atomic layer deposition (ALD) of high-purity SnS of both cubic and orthorhombic phases. Using tin(II) 2,4-pentanedionate [Sn(acac)(2)] and hydrogen sulfide (H2S) as precursors, controlled growth of the two polymorphs is achieved. Quartz crystal microbalance measurements are used to establish saturated conditions and show that the SnS ALD is self-limiting over temperatures from at least 80 to 160 degrees C. In this temperature window, a stable mass gain of 19 ng cm(-2) cycle(-1) is observed. The SnS thin film crystal structure and morphology undergo significant changes depending on the conditions. High-resolution transmission electron microscopy and X-ray diffraction demonstrate that fully saturated growth requires a large H2S dose and results in the cubic phase. Smaller H2S doses and higher temperatures favor the orthorhombic phase. The optical properties of the two polymorphs differ significantly, as demonstrated by spectroscopic ellipsometry. The orthorhombic phase displays a wide (0.3-0.4 eV) Urbach tail in the near-infrared region, ascribed to its nanoscale structural disorder and/or to sulfur vacancy-induced gap states. In contrast, the cubic phase is smooth and void-free and shows a well-defined, direct forbidden-type bandgap of 1.64 eV.