Article pubs.acs.org/cm

Hydrazine-Processed Ge-Substituted CZTSe Solar Cells Santanu Bag, Oki Gunawan, Tayfun Gokmen, Yu Zhu, and David B. Mitzi* IBM T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, United States S Supporting Information *

ABSTRACT: The p-type Cu2ZnSn(SxSe1−x)4 (with x ≈ 0; CZTSe) thin-film solar cell absorber, made from earthabundant elements, was substituted with Ge using a hydrazine-based mixed particle-solution approach for the film deposition. The crystallographic unit cell parameters of the absorber layer decrease with gradual incorporation of Ge. A solar cell fabricated from a 40% Ge-substituted absorber showed a 9.1% power conversion efficiency, a higher opencircuit voltage, and a wider band gap compared with the device based on the unsubstituted absorber layer. This result shows the possibility of substituting, using the hydrazine-processing approach, the metal site of CZTSe with Ge for further device optimization. One area for further improvement in the substituted absorber layer devices includes reduction of a ZnSe secondary phase, which was apparent in the higher-Ge-content films. KEYWORDS: CZTS, CZTGSe, germanium, kesterite, thin film, solar cell



limiting the device efficiency.9 Substitution of Sn by the group IV element Ge in the upper row of the periodic table might be attractive due to its lower propensity toward the +II oxidation state. Here, we report a synthesis of the Cu2Zn(Sn1−yGey)(SxSe1−x)4 (with x ≈ 0) absorber layer by a hydrazine-based deposition process and fabrication of a solar cell that exhibits a power conversion efficiency of as high as 9.1%. We observe that, by substituting 40% Sn with Ge in CZTSe, the band gap of the resulting absorber layer increases from 1.08 to 1.15 eV.

INTRODUCTION Solar cells with absorber layers made from earth-abundant metals, such as Cu2ZnSn(SxSe1−x)4 (CZTSSe), are of great interest due to their reduced content of heavy metals and expected lower cost compared with commercially available CdTe and Cu(In,Ga)(SxSe1−x)2 (CIGSSe) devices.1 Using a nonvacuum, hydrazine-based deposition process, devices with power conversion efficiencies of over 10% have already been demonstrated.2−4 The CZTSSe absorber layer in these devices possesses a direct band gap in the range of ∼1.0 to ∼1.5 eV, depending upon the [S]/[S + Se] ratio.1 On the basis of firstprinciples calculations, it has been shown that the band gap of the CZTSSe alloy increases linearly with an increase in S content, consistent with experimental results.5,6 However, controlling the band gap by varying the [S]/[S + Se] ratio has limitations, due to the high volatility of these elements and resulting difficulty in controlling the film stoichiometry. To date, indium-based CuInSe2 chalcopyrite devices are shown to offer the highest power conversion efficiency among thin-film photovoltaic technologies, and a record power conversion efficiency (20.3%) was obtained by substituting In with Ga and then fine-tuning the Ga/[Ga + In] ratio.7 Substitution of In with the lighter group III element, Ga, widens the band gap as the unit cell volume decreases. Similar band-gap tuning using metal substitutions in CZTSSe devices, that is, substitution of Sn with the lower atomic number and isoelectronic Ge, is also expected to be promising, and Gedoped CZTSSe devices with 8.4% efficiency have been recently demonstrated using a nanoparticle-based deposition approach.8 Moreover, because of the multivalent nature of Sn (i.e., +II and +IV states), the presence of the +II oxidation state in the CZTSSe absorber layer is predicted to create deep recombination centers for the photoexcited electrons and holes, thereby © XXXX American Chemical Society



EXPERIMENTAL SECTION

CZTSe and Ge-substituted CZTSe absorber layers were deposited on Mo-coated soda lime glass substrates using a hybrid solution-particle approach in hydrazine as previously described.2−4 All commercially available chemicals (99.998% S from Sigma Aldrich, 99.9% Zn from Strem Chemical, 99.999% SnSe from Alfa Aesar, 99.999% Se from Alfa Aesar, and 99.8% hydrazine from Arch Chemicals) were used here as received, except for Cu2S and GeSe2, which were synthesized in-house. All elemental metal and binary metal chalcogenides were dissolved or dispersed in hydrazine before spin-coating onto Mo-coated soda lime glass slides (1 in. × 1 in. area) in an oxygen- and water-free nitrogenfilled glovebox. Caution! Hydrazine is highly toxic and hazardous and must be handled with extreme care at all times using appropriate protective equipment. In a typical preparation, solution A (1.2 M Cu2S-S) was made by dissolving Cu2S and S in hydrazine. Another SnSe (SnSe and GeSe2 for Ge-substituted samples) solution was prepared by dissolving Se and Sn/Ge selenides in hydrazine to form solution B (0.57 M Sn/ GeSe-Se), with addition of stoichiometric amounts of elemental Zn powder (in a real sense, this is a slurry since Zn powder was not Received: September 6, 2012 Revised: November 9, 2012

A

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Figure 1. PXRD of thin films on Mo-coated soda lime glass substrates, prepared from pure CZTSe and 20% and 40% Ge-substituted CZTSe. The simulated ZnSe and CZTSe PXRD patterns (calculated from the known structures of these materials) are shown as a reference. The peaks due to the Mo layer on the substrate are marked. Also evident in the thin-film diffraction profiles are weak peaks due to the MoSe2 interfacial layer at 32° and 56°. Insets show the magnified (112) and (220) peaks. soluble). Solutions A and B were mixed together after all constituents (except Zn) had fully dissolved. This mixture had the following final metal composition: Cu/(Zn + Sn) or Cu/(Zn + Sn + Ge) = 0.8 and Zn/Sn or Zn/(Sn + Ge) = 1.20, with a nominal kesterite Cu2−pZn1+qSnSe4 or Cu2−pZn1+q(Sn1−yGey)Se4 concentration of approximately 0.4 M. Thin-film absorber layers with a final thickness of ∼2−2.5 μm were prepared by spin-coating this mixture over four to seven consecutive layers (depending on the concentration of mixture), followed by annealing on a hot plate (set point at ∼540 °C) in the inert nitrogen atmosphere. During heat treatment of the sample, almost all sulfur used in the solution preparation evaporates, leaving the essentially pure CZTSe (or CZTGSe) phase with a negligible sulfur content (∼2 at. % as estimated from TEM EDS). The CdS buffer, ZnO window, and indium-doped tin oxide (ITO) layers were subsequently deposited by chemical bath deposition and RF magnetron sputtering, respectively, giving a standard CZTS device structure with a device area of approximately 0.45 cm2, as defined by mechanical scribing. A Ni/Al collection grid was deposited on top of the device by electron-beam evaporation using a shadow mask. On two top performing devices (one pure CZTSe and one 40% Ge-substituted sample), an ∼110 nm thick MgF2 antireflection coating was deposited by electron-beam evaporation. Samples for TEM/STEM (scanning transmission electron microscopy) analysis were prepared using the FEI Helios 400 S DB-FIB. TEM images were taken using a JEOL 3000F TEM operated at 300 kV. Compositional profiles were acquired by STEM/EDX (energydispersive X-ray spectroscopy). The electrical characterization was done using a Xe-based light source solar simulator to provide an AM1.5 G spectrum. The system is equipped with a light stabilization system to provide stable 1 sun illumination. A small liquid nitrogen flow cryostat was used for temperature-dependent measurement, which could be inserted under the solar simulator. Measurements are automated by a MATLAB-based computer program. The quantum efficiency measurement was performed using a Protoflex system equipped with a xenon light source and a monochromator with a

chopper running at 270 Hz. The time-resolved photoluminscence (TRPL) measurement was performed on a finished cell using a Hamamatsu time-correlated single-photon counting system with a 532 nm solid-state laser, a pulse width of less than 1 ns, and a repetition rate of 15 kHz.



RESULTS AND DISCUSSION The partially Ge-substituted CZTGSe absorber layer was deposited by a modified hybrid solution-particle approach reported earlier using hydrazine as a solvent.2 GeSe2 was used as a source for Ge. For a comparative study, three different sets of samples were prepared: pure CZTSe (no Ge substitution) and 20% and 40% Ge-substituted CZTGSe. By changing the solution compositions of the starting elemental metal and binary metal−chalcogenides, CZTSe/CZTGSe films of desired stoichiometry were obtained, for example, Cu1.5ZnSn0.5Ge0.4Se4 for 40% Ge substitution (analyzed by Rutherford backscattering spectrometry (RBS)). In each case, Cu-poor and Zn-rich compositions (Cu/Zn + Sn = 0.8 and Zn/Sn = 1.2) were targeted in accordance with record efficiency devices.2−4 For a better comparison, all samples were prepared under the same deposition/annealing conditions. However, deviations from the targeted stoichiometry in the final films may occur because of the volatility of S, Se, and the tin and germanium chalcogenides during heat treatment. In Figure 1, powder X-ray diffraction (PXRD) data for each film are consistent with a kesterite phase, with the main (112) peak shifted toward higher 2θ (smaller unit cell size) going from pure CZTSe to 40% Ge-substituted CZTGSe. A similar shift is also observed for other high-angle diffraction peaks. However, in the case of the high Ge content sample, peak broadening generally occurs. As an example, the (220) peak for B

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Figure 2. Cross-sectional SEM images of (a) pure CZTSe and (b) 20% and (c) 40% Ge-substituted CZTSe full devices. (d) TEM image of a 40% Ge-substituted CZTSe sample. The MoSe2 layer is evident at the bottom of the 40% Ge-substituted sample.

Figure 3. (a) Cross-sectional TEM image of a 40% Ge-substituted CZTGSe full device and (b) STEM EDS line scan at a position far away from the phase-segregated region, showing an almost uniform composition. (c) Cross-sectional TEM image and (d) STEM-EDS line scan of a 40% Gesubstituted CZTSe full device showing phase segregation near the back contact layer.

the 40% Ge-substituted sample broadens and exhibits a shoulder. This suggests impurity phase formation in the higher Ge sample, with the diffraction data suggesting that the impurity phase is ZnSe.

In Figure 2, scanning electron microscopy (SEM) images of the full devices derived from the corresponding pure CZTSe and 20% and 40% Ge-substituted CZTGSe absorber layers demonstrate large grains and a comparable film thickness of C

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Figure 4. (a) J−V characteristics (under AM1.5 illumination) of pure CZTSe and 40% Ge-substituted CZTSe devices (after MgF2 coating). (b) External quantum efficiency (EQE) data and the bias ratio plot EQE(−1 V)/EQE(0 V). The band gaps of the absorber layers are also indicated by arrows.

Figure 5. Temperature dependence data of the 40% Ge-substituted sample: (a) Voc and Jsc. (b) Series resistance in the dark and under light.

∼2.0 μm in all cases. A cross-sectional TEM image of a device fabricated from a 40% Ge-substituted CZTGSe absorber layer in Figure 2d shows mostly single-phase large grains (∼2 to 2.6 μm), but, in some regions, also phase segregation near the CZTGSe/MoSe2 back interface. Compositional profiling across the absorber layer where no phase segregation is detected demonstrates almost homogeneous metal distribution, except for a slightly less Ge content at the top surface than at the bottom, which may be due to the volatility of GeSe2 (Figure 3a,b). In regions where a secondary phase is observed, the secondary phase is ZnSe, as revealed by the EDX line scans employing the STEM mode in Figure 3c,d (consistent with the X-ray diffraction profile in Figure 1). An ∼165−210 nm thick MoSe2 layer was grown during the high-temperature reaction between the Mo substrate and extra Se from the CZTGSe layer (Figure S1, Supporting Information). Cu diffusion into the MoSe2 may trigger a compositional change in the CZTGSe layer, perhaps contributing to the observed phase segregation of ZnSe.10 Characterization of device electrical properties for a 40% Gesubstituted CZTGSe sample in Figure 4a shows a power conversion efficiency of 9.1% (after MgF2 coating) under AM1.5 G simulated illumination. A device made during a control run under identical conditions using pure CZTSe reveals 9.1% efficiency. A small shift to a larger band gap, as deduced from the inflection point of the external quantum efficiency (EQE) data, has been noted in comparing the pure Sn sample with a mixed Ge/Sn sample (e.g., 1.08 and 1.15 eV for a z = 0 and z = 0.4 sample, respectively), as shown in Figure 4b. This shift in band gap contributes to an increase in opencircuit voltage (Voc) (e.g., 423 mV for pure CZTSe to 476 mV

for 40% Ge-substituted CZTGSe) in the resulting device. In two separate runs, more than 16 solar cells with efficiencies close to 7% (without MgF2 antireflection coating; Figures S2 and S3, Supporting Information) have been fabricated by Ge substitution, apart from the “champion device” example shown in Figure 4. In the Ge-substituted samples, the overall power conversion efficiency is limited by higher series resistance, resulting in a substantial loss in fill factor. However, a clear trend of increase in open-circuit voltage is observed. While the Ge-substituted cell has a higher band gap, it still suffers from the Voc deficit issue, as indicated by the large difference with its band gap: Eg/q − Voc = 0.674 V.11 The Voc deficit becomes worse at higher Ge content (larger band gap). That is, at z = 0 and 0.2, Eg/q − Voc = 0.657, and at z = 0.4, Eg/ q − Voc = 0.674. As a comparison, a top-performing CIGS chalcogenide solar cell has a voltage deficit of less than 500 mV.12 This open-circuit voltage deficit issue is associated with the low activation energy, EA, of the main recombination mechanism, as determined from the Voc vs T plot of Figure 5a. EA, which is the 0 K intercept of the Voc vs T plot, is less than the band-gap value in this case. This deficiency in EA has been observed in all hydrazine- and vacuum-processed CZTSSe cells so far and is suspected to arise from dominant interface recombination in the CZTSSe devices.1,11 However, we note that a similar deficiency in EA could be observed in the case of dominant bulk recombination if recombination is mediated by the tails states due to the inhomogeneous absorber layer.13 Another factor that affects the Voc is the minority carrier lifetime, τ, which is measured by the time-resolved photoluminescence data (Figure 6). It is clear that the TRPL data do not follow a single-exponential decay, suggesting a varying D

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nanoparticle approach showed a power conversion efficiency of 6.8% and was also limited by high series resistance.8 In that case, the band gap increased from 1.11 eV for 0% Gecontaining CZTSe to 1.40 eV for a 70% Ge-substituted sample, with a corresponding improvement of the open-circuit voltage from 430 to 640 mV.



CONCLUSION In conclusion, we show that the band gap of hydrazineprocessed CZTSSe solar cells can be tuned by substitution on the metal sitethat is, Sn with Gerather than by using anion substitution. Power conversion efficiencies of as high as 9.1% are obtained in the Ge-substituted samples, with an increase in open-circuit voltage also being noted. However, the gradient of the Ge content across the CZTGSe layer, a phase segregation issue near the back interface for higher-Ge-content samples, and high Eg/q − Voc values in this system demonstrate areas where further optimization is necessary. Nevertheless, the initial results are promising and indicate an alternative way to tailor the band gap of the CZTSSe absorber layer and an additional option for targeting higher efficiency in hydrazine-processed kesterite devices.

Figure 6. Time-resolved PL data of the 40% Ge-substituted sample. Inset: PL spectrum, with the arrow indicating where the TRPL trace is taken at 1181 nm.

lifetime as the time scale changes. In our Ge-substituted cell, although we observe τ as high as 17.4 ns for longer time scales, it is only 5 ns for shorter time scales. Such a nonlinear behavior in the TRPL data could be attributed to inhomogeneity in the sample, with the TRPL signal having individual contributions from different regions with varying lifetimes. We also note that the nonlinear behavior in the TRPL data is less pronounced in the pure CZTSSe device (data not shown). Therefore, we speculate that the observed increased nonlinearity in our Gesubstituted sample may be the result of inhomogeneity introduced by Ge substitution, which may also partially account for the increased Voc deficit for samples with a higher Ge content. Moreover, the peak of the PL spectrum is at 1.05 eV (see Figure 6 inset), which is less than the band gap of the absorber layer (1.15 eV) as determined from the EQE data. This red shift of the PL peak relative to the band gap also suggests a defective and inhomogeneous absorber layer.14,15 In Figure 5a, we also note that the short-circuit current drops at low temperature. This occurs concomitantly with the collapse in fill factor stemming from increasing series resistance at low temperature, as shown in Figure 5b. This increase in series resistance is associated with a carrier freeze-out effect due to the lack of a shallow acceptor level in the CZTSSe absorber layer.16 At low temperature and in the dark, the free hole density is very low and could be comparable to the photogenerated carrier density (when under illumination), and thus, under light illumination, the series resistance is reduced, as shown in Figure 5b. Additionally, a photodoping effect that modulates the barrier potential in the CdS layer could contribute to the smaller series resistance under light.17 As shown earlier in Figure 4b, the solar cell also exhibits a severe voltage-dependent current collection, as apparent in the EQE bias ratio at −1 and 0 V, which is larger than one and is increasing toward the long-wavelength regime. This suggests that the minority carrier diffusion length is significantly shorter than the thickness of the absorber layer and, combined with the observation of a reasonable minority carrier lifetime, points toward a low minority carrier mobility value. This low mobility value in the CZTSSe devices could also be responsible for the relatively high series resistance encountered in most CZTSSe and CZTGSSe cells (an exception is the pure selenide CZTSe device fabricated using a vacuum-based approach).18 Recently, a 70% Ge-substituted CZTGSe solar cell prepared from the



ASSOCIATED CONTENT

S Supporting Information *

Cross-sectional TEM images and comparison of device characteristics (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (+1)914-945-4176. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was conducted as part of a joint development project between Tokyo Ohka Kogyo Co., Ltd., DelSolar Co., Ltd., Solar Frontier K. K., and IBM Corporation. We thank Andrew Kellock for RBS measurements, S. Thiruvengadam for glasssubstrate preparation, S. Jay Chey for deposition of ZnO and ITO, R. Ferlita for Ni/Al evaporation, and Sunit Mahajan for area measurements.



REFERENCES

(1) Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. Sol. Energy Mater. Sol. Cells 2011, 95, 1421. (2) Todorov, T. K.; Reuter, K. B.; Mitzi, D. B. Adv. Mater. 2010, 22, E156. (3) Barkhouse, D. A. R.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Mitzi, D. B. Prog. Photovoltaics 2012, 20, 6. (4) Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Todorov, T. K.; Mitzi, D. B. Energy Environ. Sci. 2012, 5, 7060. (5) Chen, S.; Walsh, A.; Yang, J. H.; Gong, X. G.; Sun, L.; Yang, P. X.; Chu, J. H.; Wei, S. H. Phys. Rev. B 2011, 83, 125201. (6) Haight, R.; Barkhouse, A.; Gunawan, O.; Shin, B.; Copel, M.; Hopstaken, M.; Mitzi, D. B. Appl. Phys. Lett. 2011, 98, 253502. (7) Jackson, P.; Hariskos, D.; Lotter, E.; Paetel, S.; Wuerz, R.; Menner, R.; Wischmann, W.; Powalla, M. Prog. Photovoltaics 2011, 19, 894. (8) (a) Ford, G. M.; Guo, Q.; Agrawal, R.; Hillhouse, H. W. Chem. Mater. 2011, 23, 2626−2629. (b) Guo, Q.; Ford, G. M.; Hillhouse, H. W.; Agrawal, R. Proceedings of the 2011 37th IEEE Photovoltaics Specialists Conference, Seattle, WA, Jun 19−24, 2011; IEEE: New York, E

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2011; pp 003522−003526. (c) Guo, Q.; Ford, G. M.; Yang, W.-C.; Hages, C. J.; Hillhouse, H. W.; Agrawal, R. Sol. Energy Mater. Sol. Cells 2012, 105, 132. (9) Biswas, K.; Lany, S.; Zunger, A. Appl. Phys. Lett. 2010, 96, 201902. (10) Wang, K.; Shin, B.; Reuter, K. B.; Todorov, T.; Mitzi, D. B.; Guha, S. Appl. Phys. Lett. 2011, 98, 051912. (11) Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Appl. Phys. Lett. 2010, 97, 233506. (12) Nadenau, V.; Rau, U.; Jasenek, A.; Schock, H. W. J. Appl. Phys. 2000, 87, 584. (13) Rau, U.; Werner, J. H. Appl. Phys. Lett. 2004, 84, 3735. (14) Levanyuk, A. P.; Osipov, V. V. Sov. Phys.-Usp. 1981, 24, 187. (15) Mattheis, J.; Rau, U.; Werner, J. H. J. Appl. Phys. 2007, 101, 113519. (16) Gunawan, O.; Gokmen, T.; Warren, C. W.; Cohen, J. D.; Todorov, T. K.; Barkhouse, D. A. R.; Bag, S.; Tang, J.; Shin, B.; Mitzi, D. B. Appl. Phys. Lett. 2012, 100, 253905. (17) Pudov, A. O.; Kanevce, A.; Al-Thani, H. A.; Sites, J. R.; Hasoon, F. S. J. Appl. Phys. 2005, 97, 64901. (18) Repins, I.; Beall, C.; Vora, N.; DeHart, C.; Kuciauskas, D.; Dippo, P.; To, B.; Mann, J.; Hsu, W.-C.; Goodrich, A.; Noufi, R. Sol. Energy Mater. Sol. Cells 2012, 101, 154.

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