Fabrication of solar cells using Ge–Sn–S thin film prepared by co-evaporation

Tin and germanium monosulfide (SnS and GeS, respectively) are two of the well-known simple binary-compound semiconductors in the Ge–Sn–S system. These layered compounds are stable in the ambient atmosphere, non-toxic, and contain elements abundant in the Earth's crust. Therefore, these semiconductors are cheaper, safer to use, and less polluting than copper indium gallium selenide (CIGS) and cadmium telluride (CdTe), which are commonly used as absorber materials for solar cells.

Research has recently focused on investigating the physical properties of SnS and its application to solar cells. ^1,2) SnS can be produced as both n-type and p-type. It has a band gap (E_g) of 1.3 eV ^3–9) and a high optical absorption coefficient (over 10⁴ cm⁻¹), which makes it an ideal material for the fabrication of thin film solar cells. ^4,6,7) In particular, solar cells utilizing homojunction with n-type SnS thin films are expected to become the target of future research due to their excellent properties. ¹⁰⁾

SnS thin films have already been fabricated by various methods, namely, sputtering, ¹¹⁾ spin-coating, ¹²⁾ chemical spray pyrolysis, ¹³⁾ atomic layer deposition, ^6,13) vapor transport deposition, ^14,15) CVD, ¹⁶⁾ sulfurization of evaporated precursors, ^8,9) close space sublimation, ⁴⁾ and co-evaporation. ^3,5) Furthermore, SnS/CdS-based thin film solar cells have been reported to exhibit a power conversion efficiency (PCE) of 4.225%. ¹⁴⁾ Yun et al. reported a maximum PCE of 4.8% with nanostructured TiO₂/SnS heterojunctions. ¹²⁾

GeS exhibits p-type conduction, with an E_g of 1.6–1.7 eV (direct transition) and a high optical absorption coefficient (over 10⁴ cm⁻¹). ^17–19) Although there have been only a few reports on the fabrication of thin-film using GeS as absorbing layers, Feng et al. have reported that solar cells with a GeS/CdS thin-film heterostructure exhibit a PCE of 1.36%. ¹⁹⁾

Considering the application of thin films to multi-junction solar cells to further improve the conversion efficiency, it is preferable to have one with a tunable E_g. Even though CIGS has a tunable band gap, ²⁰⁾ it is not preferred because of the toxicity of the constituent elements, high cost, and difficulty in controlling the composition of the quaternary system. It has been reported that the band gap of SnS can be tuned in the range 0.9–1.2 eV and 1.2–1.6 eV by solid solutions of Se and Ge, respectively. ²¹⁾ However, while the use of Sn(S, Se) alloys has been reported in nanocrystal and thin films, ^22,23) the use of (Ge, Sn)S alloys has only been reported in nanocrystal materials. ²¹⁾

Therefore, the aim of this study was to fabricate different Ge–Sn–S thin films by varying the substrate and Knudsen-cell temperatures of Ge, Sn, and S during co-evaporation and using them to build solar cells. The photovoltaic properties of these solar cells were subsequently characterized.

Ge–Sn–S thin films were fabricated by depositing Ge, Sn, and S on 0.8 μm thick Mo-coated Eagle XG substrates (Eagle XG/Mo) via co-evaporation in a high-vacuum chamber. To obtain a highly reactive flux, vaporized sulfur at 150 °C was thermally cracked at 800 °C into smaller molecular weight S₈ molecules using a valved cracker effusion cell. ²⁴⁾

Different Ge-containing SnS thin films were fabricated by varying the amount of Ge (batch A) and substrate temperature (batch B) while suppressing the re-evaporation of GeS from the thin film during deposition with high vapor pressure. Both batches of A and B thin films were prepared by depositing Sn, Ge, and S simultaneously. For batch A thin films, the Sn Knudsen-cell, and the substrate temperatures were 1020 °C and 150 °C, respectively. For batch A, the amount of Ge in the thin film was varied by adjusting the Ge Knudsen-cell temperature to 1150 °C or 1175 °C. For the batch B thin films, substrate temperatures of 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, and 300 °C were used. For batch B, the Sn and Ge cell temperatures were maintained at 1010 °C and 1200 °C, respectively. A thin film with only Sn and S was also prepared for comparison. Table I lists the details of the deposition conditions.

The obtained thin films were analyzed using X-ray fluorescence spectroscopy (XRF, ZSX Primus Ⅵ, Rigaku), X-ray diffraction (XRD, MiniFlex, Rigaku), Raman spectroscopy (RMP-510, JASCO) with an excitation wavelength of 532 nm, and scanning electron microscopy (SEM, JSM-6060LV, JEOL). Subsequently, a CdS layer (n-type buffer, ∼90 nm) was deposited on the Ge–Sn–S thin film using the chemical bath deposition method. ²⁵⁾ After deposition, the obtained Eagle XG/Mo/Ge–Sn–S/CdS thin films were annealed in the air for 30 min at 200 °C. An Al-doped ZnO window layer and Al top electrode were deposited on the CdS by RF sputtering and thermal evaporation, respectively, to fabricate Eagle XG/Mo/Ge–Sn–S/CdS/ZnO:Al/Al structure cells. The film was then scribed into several small 4.4 mm × 4.4 mm (aperture area ∼0.16 cm²) cells. The photovoltaic properties of the final Ge–Sn–S cells were measured under AM1.5 illumination at 100 mW cm⁻² irradiation using a solar simulator (SX-UI 500XQ, Ushio). The external quantum efficiency (EQE) of the solar cells was also evaluated using a solar cell evaluation system (SML-250J, Bunkoukeiki).

The thin films obtained after co-evaporation were analyzed using XRF. The elemental compositions thus obtained are listed in Table II. The ratio of Sn to S in the SnS thin film was smaller than the stoichiometric ratio of 1:1. Since SnS was deposited at a substrate temperature of 300 °C, which is higher than that of the sample in batch A (150 °C), thinner films were obtained due to the increased re-evaporation of SnS. The Ge–Sn–S thin films (batch A) were prepared at Ge Knudsen-cell temperatures of 1150 °C and 1175 °C. The ratios of Ge to Ge + Sn were 0.18 and 0.27, respectively, confirming that higher Knudsen-cell temperatures increase Ge uptake. The reason for the higher Ge content than in batch B (discussed later) is probably that the lower substrate temperature suppressed the re-evaporation of GeS from the deposited thin film.

The thickness of batch B films, fabricated at different substrate temperatures, ranged from 0.63 to 0.65 μm; the estimated Ge/(Ge + Sn) composition ratio varied from 0.02 to 0.07. The thinner thickness and lower Ge content of these films than those of batch A can be attributed to the re-evaporation from the deposited film due to the high vapor pressure of GeS. Figure 1(a) shows the Raman spectra of the SnS and batch A thin films. For the SnS thin film, peaks attributed to orthorhombic-SnS (ORT. SnS) ^26,27) were observed in the spectrum at 94, 160, 190, and 219 cm⁻¹. Batch A films showed broader spectra, with peaks at higher wavenumbers than in the SnS film. This peak broadening could be due to a decrease in crystallinity resulting from the substitution of Ge with Sn and inhomogeneity in the chemical composition of Ge and Sn. In fact, in a similar chalcogen compound, Cu₂(Sn, Ge)S₃, the same process has been reported to shift Raman peaks to higher frequencies. ²⁸⁾ This further supports the idea that, in our study, part of Sn was substituted with Ge to form Ge_x Sn_1−x S. In addition, since no peaks attributed to GeS ²⁹⁾ were observed in the Raman spectra, it can be concluded that no segregation of GeS occurred near the surface.

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Figure 1(b) shows the Raman spectra of Ge–Sn–S in batch B and SnS thin films. Herein, the peaks of the Ge–Sn–S thin films prepared at all substrate temperatures can be attributed to orthorhombic SnS, which is believed to be because of the very low Ge content in these thin films. However, the peaks were broader than those of SnS [note that the Raman spectra of the SnS in both Figs. 1(b) and 1(a) are the same]. This suggests that the Ge–Sn–S thin films were less crystalline than the SnS thin film.

Figure 2 shows the SEM images of the SnS and Ge–Sn–S thin film surfaces prepared at varying substrate temperatures (i.e. batch B). Herein, it can be observed that the former consists of approximately 1 μm block-shaped grains. This morphology was similar to those prepared by our group in a previous study. ⁵⁾ Furthermore, the grain size in the latter slightly increased with substrate temperature. A similar observation was reported in a previous study, wherein SnS thin films were prepared by co-evaporation. ³⁾ However, the grain size of the Ge–Sn–S thin film was smaller than that of the SnS thin film fabricated at a substrate temperature of 300 °C, which could be due to the inhibition of crystal growth resulting from the re-evaporation of Ge from the deposited film.

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Table III and Fig. 3(a) show the photovoltaic characteristics of the solar cells with different concentrations of Ge. The solar cell fabricated using the SnS thin film exhibited an open circuit voltage (V_oc) of 0.053 V, short circuit current density (J_sc) of 9.00 mA cm⁻², fill factor (FF) of 0.315, and a PCE of 0.15%. The value of PCE was of the same order of magnitude as that of the SnS thin film solar cells previously fabricated by our group using co-evaporation. ³⁾ The PCEs of the solar cells fabricated using Ge_x Sn_1−x S (x = 0.18) and Ge_x Sn_1−x S (x = 0.27) in batch A were 0.023% and 0.036%, respectively. In particular, the V_oc of the Ge_x Sn_1−x S (x = 0.27) thin film was more than three times that of the SnS samples, and their FFs were comparable. This increase in V_oc could be due to the widening of the band gap of the light absorption layer, which also suggests the formation of a (Ge, Sn)S solid solution. Another possibility is an improvement in grain boundary passivation due to the higher Ge composition. In a previous study, this process was reported for the case of the analogous compound Cu₂(Ge, Sn)S₃, wherein increasing the Ge composition at grain boundaries suppressed carrier recombination. ³⁰⁾

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Figure 3(b) shows the J–V curves of the solar cells fabricated with the Ge–Sn–S thin films of batch B. Compared to the solar cell fabricated with SnS, those fabricated at substrate temperatures of 250 °C–270 °C can be observed to have a higher V_oc. As discussed in the case of batch A, this may also be due to the suppression of carrier recombination due to the presence of Ge at the grain boundaries. In contrast, V_oc decreased when the substrate temperature was above 280 °C. From Table II, it can be observed that the (Ge + Sn)/S composition ratio was greater than 1.06 when the substrate temperature was above 280 °C. This ratio is over the stoichiometric ratio significantly and could occur slight segregation of Sn.

Figure 4 shows the EQE and [E ln(1-EQE)]² versus E plots of the solar cells at each deposition condition. The EQE of the SnS thin film solar cell exhibited an absorption bandwidth of 350–1000 nm with an estimated band gap of 1.27 eV. This result is consistent with those reported by previous studies. ^3–9) The Ge_x Sn_1−x S (x = 0.18) and Ge_x Sn_1−x S (x = 0.27) thin film solar cells in batch A showed an absorption edge near 850 nm and had band gaps of 1.53 and 1.57 eV, respectively.

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A higher value of E_g compared to the SnS thin film suggests the formation of a (Ge, Sn)S solid solution. However, since the EQEs formed a tail at the absorption edge, the effective E_g might be lower than the values reported above. These values are higher than those reported for a Ge_x Sn_1−x S sample, ²¹⁾ which may be due to the inhomogeneity in the composition of Ge in the thin films. Furthermore, the Ge–Sn–S thin film solar cells fabricated at different substrate temperatures exhibited an EQE absorption edge of approximately 950 nm and an estimated band gap of 1.31 eV [see Fig. 4(d)], which implies that their band gap was slightly wider than that of the SnS film. However, the Ge content was low, and their band gaps were close to those of SnS film.

Thin-film solar cells using co-evaporated Ge–Sn–S thin film as the light-absorbing layer were fabricated. Ge_x Sn_1−x S (x = 0.27) thin film solar cell fabricated at a substrate temperature of 150 °C by co-evaporation exhibited an open circuit voltage (V_oc) of 0.169 V, short circuit current density (J_sc) of 0.66 mA cm⁻², FF of 0.324, and a PCE of 0.036%. Interestingly, V_oc values of the films fabricated at 300 °C were approximately three times higher than that of the SnS solar cell. The formation of (Ge, Sn)S solid solution and estimation of the band gap were investigated using Raman spectroscopy and EQE, respectively. The band gap of the Ge_x Sn_1−x S (x = 0.27) thin film estimated by extrapolating the absorption edge of the EQE was 1.57 eV, which is larger than that of the SnS thin film. This suggests that Sn (in SnS) is partially replaced by Ge to form a solid solution, thus widening the band gap. However, the solar cell properties of the Ge–Sn–S thin films fabricated on substrates with temperatures varying from 250 °C to 300 °C showed no significant change compared to those of the SnS thin film. This might be attributable to the high substrate temperature that prevented Ge incorporation into the film.

This work was supported by the Japan Society for the Promotion of Science (KAKENHI, Grant No. JP19H02663).