Beyond 11% Efficient Sulfide Kesterite Cu2ZnxCd1−xSnS4 Solar Cell: Effects of Cadmium Alloying Chang Yan,†,⊥ Kaiwen Sun,†,⊥ Jialiang Huang,† Steve Johnston,‡ Fangyang Liu,† Binesh Puthen Veettil,† Kaile Sun,§ Aobo Pu,† Fangzhou Zhou,† John A. Stride,∥ Martin A. Green,† and Xiaojing Hao*,† †
Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney, NSW 2052, Australia ‡ National Renewable Energy Laboratory, Golden, Colorado 80401, United States § School of Metallurgy and Environment, Central South University, ChangSha 410083, China ∥ School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *
ABSTRACT: Kesterite Cu2ZnSnS4 (CZTS) thin-film solar cells have drawn worldwide attention because of outstanding performance and earth-abundant constituents. However, problems such as coexistence of complex secondary phases, the band tailing issue, short minority lifetime, bulk defects, and undesirable band alignment at p−n interfaces need to be addressed for further efficiency improvement. In this regard, Cd alloying shows promise for dealing with some of these problems. In this work, a beyond 11% efficient Cd-alloyed CZTS solar cell is achieved, and the effects of Cd-alloying and mechanism underpinning the performance improvement have been investigated. The introduction of Cd can significantly reduce the band tailing issue, which is confirmed by the reduction in the difference between the photoluminescence peak and optical band gap (Eg) as well as decreased Urbach energy. The microstructure, minority lifetime, and electrical properties of CZTS absorber are greatly improved by Cd alloying. Further XPS analyses show that the partial Cd alloying slightly reduces the band gap of CZTS via elevating the valence band maximum of CZTS. This suggests that there are opportunities for further efficiency improvement by engineering the absorber and the associated interface with the buffer. resistance and low Jsc for CZTS devices.13 In addition, large quantities of antisite defect clustering (Cu/Zn and/or other Sn related defect14) contribute to the band tailing issue,8,9 which shifts the photoluminescence (PL) peak to a lower energy and is responsible for the reported pronounced mismatch between the optical and the PL peak position. This mismatch is more than 100 meV for CZTS compared to 30−40 meV for CIGS.9 Moreover, a typical reported issue of CZTS materials is the short minority carrier lifetime, which is about 1−2 orders of magnitude lower than that of its CIGS counterparts.15 Last but not least, an unfavorable clifflike band alignment at the CZTS/ CdS interface is also limiting the efficiency development for sulfide kesterite.10,11,16,17 Therefore, if secondary phases are reduced or changed, the minority carrier lifetime is increased,
A
s a promising nontoxic and earth-abundant photovoltaic candidate material, kesterite Cu2ZnSnSxSe4−x (CZTSSe) based solar cell technologies have developed rapidly within the past few years.1−3 A 12.6% power conversion efficiency (PCE) for a Se-containing kesterite4 and 9.5% efficiency for pure sulfide CZTS5 have been achieved by groups at IBM and University of New South Wales, respectively. However, despite this progress, the efficiency for kesterite is still far behind that of its counterpart Cu(In, Ga)Se2 (CIGS), mainly owing to the low Voc or high Voc deficit (Eg/q − Voc) of kesterite. This is because of the problems encountered within the kesterite, such as secondary phases,6 short minority lifetime,3 band tailing and defects within the bulk,7−9 and unfavorable band alignment at the p−n interface.10,11 Usually high-efficiency kesterite devices are reported from nonstoichiometry,12 which facilitates the formation of secondary phases and antisite defects within the kesterite. For instance, large amounts of ZnS secondary phases may aggregate at the CZTS/buffer or CZTS/MoS2 interface, leading to high series © XXXX American Chemical Society
Received: February 16, 2017 Accepted: March 30, 2017
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DOI: 10.1021/acsenergylett.7b00129 ACS Energy Lett. 2017, 2, 930−936
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ACS Energy Letters
Figure 1. (a) SEM top-view image for CZCTS absorber on Mo glass. (b) TEM image of CZCTS solar cell device. (c) EDS mapping for the corresponding CZCTS devices. The top left shows the high-angle annular dark-field image of the corresponding mapping regions, and the rest are the corresponding elements’ mapping distribution images.
which is the optimal ratio of the highest-efficiency CZCTS, was adopted here. Different from the sol−gel method in which Cd is homogeneously mixed with other cation sources in the precursor,18 we used the method of sulfurizing chemical bath deposited CdS on top of cosputtered Cu/ZnS/SnS precursor to realize Cd alloying. The first step was to confirm whether CdS had been consumed thoroughly during the sulfurization process (560 °C for several mins) and if Cd atoms were distributed homogeneously within the absorber film. Figure 1a demonstrates the top-view scanning electron microscopy (SEM) image of the CZCTS film. After sulfurization, it seems that there are still some minor amounts of CdS (displayed as white tiny dots in Figure 1a)19 at the surface of the CZCTS film. Large grains with horizontal sizes of up to 4 μm can be seen, which is much larger than the previously reported CZTS grains without incorporation of Cd20,21 (see Figure S2 for comparison of grain sizes of CZCTS and CZTS). This observation indicates that Cd alloying facilitates grain growth, which is consistent with the report of Su et al.18 Figure 1b displays the bright-field transmission electron microscopy (TEM) image of CZCTS film, showing that the large grains vertically extend from the bottom directly to the top of the absorber. According to scanning transmission electron
band tailing issue is reduced, and the p-n interface band alignment is optimized, the efficiency gap between kesterite and CIGS is expected to be reduced. Recently, partial cation substitution of zinc by cadmium has shown promising effectiveness to boost the efficiency of sulfide kesterite.18 The efficiency of Cu2Zn0.6Cd0.4SnS4 (CZCTS) was significantly enhanced from 5.3% (control CZTS device) to 9.2%,18 indicating that Cd alloying exerted a hugely beneficial impact on improving the efficiency. However, the effects of Cd in CZCTS thin film are unclear, and mechanisms underpinning the associated kesterite device performance improvement need to be explored. Herein, an over 11% efficient sulfide CZCTS solar cell is fabricated in which the Cd within the absorber is sourced from chemical bath deposition (CBD)-CdS. The effects of Cd in high-efficiency CZCTS solar cells have been carefully investigated. It is found that Cd alloying exerts significant impacts on microstructure, secondary phase, band tailing, the electrical properties of the absorber, and the band alignment at the heterojunction interface. According to a previous report18 and our own study on comparing the efficiency of CZCTS devices with different Cd contentration (see Figure S1), the Cd/(Zn + Cd) ratio of 0.4, 931
DOI: 10.1021/acsenergylett.7b00129 ACS Energy Lett. 2017, 2, 930−936
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Figure 2. (a) Normalized Tauc plots created from ultraviolet−visible transmission and reflectance measurements (top), and normalized PL spectra (bottom) of CZTS and CZCTS thin films. (b) Absorption coefficients obtained from PDS measurement. The inverse of the slope from the linear part below the band gap energy provides the Urbach energy (EU).
of CZTS and CZCTS from PDS is consistent with those estimated from Tauc plots. An Urbach tail model is used to account for sub-band gap photon absorption and the characterized band tailing. Under this Urbach tail energy (EUrbach) model, in semilog plot of PDS absorption and photoenergy, the slope of the linear part of the absorption edge yields to EUrbach. A lower EUrbach indicates a more clean band edge and more order material with less band tailing.24,25 According to the Figure 2b, the EUrbach of CZTS is 65 meV, whereas that of CZCTS is 45 meV, implying a dramatic reduction in band tailing of CZTS absorber with Cd alloying. Usually, the band tailing in kesterite is reported to be related to cation disorder (CuZn + ZnCu or VCu + ZnCu or other clusters) and anionic disorder (such as S/Se disorder).26 As the only difference between CZCTS and CZTS lies in the cation, we speculated that the reduced band tailing correlates to decreased disorder resulting from I−II antisite defects.9,27 The radius of Cd is much larger than those of Cu and Zn, and the strain to accommodate I−II antisite defects is increased when Cd partially substitutes Zn, thereby lowering the disorder degree. The corresponding electrical characteristics are summarized in Table 1. Therefore, it can be concluded that the second effect of Cd alloying is reducing the band tailing and disorder of the absorber material.
microscopy (STEM) elemental mapping images (Figure 1c), the Cd is homogeneously distributed within the absorber film, except for the areas of the voids and secondary phases, confirming that Cd, initially at the surface of the precursor, diffuses well and reacts to form the bulk absorber during the high-temperature sulfurization process. Usually, the ZnS secondary phases are segregated at the top and bottom of the CZTS absorber layer.22 In contrast, herein, not only Zn and S signals but also Cd signals are homogeneously presented within the secondary phase areas both at top and bottom of the absorber (Figure 1C), suggesting that these secondary phases are ZnxCd1−xS, rather than pure ZnS. Furthermore, detailed energy-dispersive spectroscopy (EDS) line scans have been conducted along with these secondary phases (Figure S3). The atomic ratio of Cd/(Zn + Cd) is ∼0.4, identical to that in CZCTS grains and the designed value for the entire CZCTS absorber film. In addition, the fact that the diffraction peak of the (112) plane in the X-ray diffraction (XRD) patterns shifts from 28.4° (CZTS) to 28.2° (CZCTS) (Figure S4) and the Raman peak (Figure S5) red shifts from 338 to 332 cm−1, together with the above-discussed STEM elemental mapping, confirms that the Cd has homogeneously alloyed with CZTS, forming a solid-solution phase of CZCTS. The first confirmed effect of cadmium alloying is improving the kesterite microstructure, facilitating the large grain growth and changing the nature of secondary phases. To examine if Cd alloying helps to reduce the band tailing, first, we compared the optical band gap derived from Tauc plots23 (see detailed light absorption coefficienct data calculated from the transmission and reflection data of CZTS and CZCTS samples on glass shown in Figures S6 and S7) and roomtemperature steady-state PL peak, (illustrated in Figure 2a). The difference between the optical band gap and PL peak, namely, Δ(Eg − PL peak), correlated with the extent of band tailing, which is a main culprit for Voc loss in solar cells. This difference is found to decrease from ∼170 to ∼70 meV after partial Cd substitution for Zn, indicating that the band-tailing problem is significantly reduced after Cd alloying. In order to gain more information on band tailing, photothermal deflection spectroscopy (PDS), which is a direct measurement of the optical absorption and very sensitive to sub-band absorption, was used to characterize the CZTS and CZCTS films on glass substrates to exclude the interference of absorption by the Mo back contact. The results are shown in Figure 2b. The band gap
Table 1. Electrical Characteristics of the CZTS and CZCTS Samples absorber
optical Eg (eV)
PL Peak (eV)
Δ(Eg−PL peak) (mV)
EUrbach (meV)
CZTS CZCTS
1.54 1.38
1.37 1.31
∼170 ∼70
65 45
A typical reported issue of CZTS materials is the short minority carrier lifetime, which is about 1−2 orders of magnitude lower compared with CIGS counterparts.15 To further evaluate the impact of Cd-alloying on the minority carrier lifetime of the CZCTS absorber, time-resolved photoluminescence (TRPL) measurement was performed on the corresponding final devices, as shown in Figure 3a. The TRPL curve of CZCTS shows decay that is significantly slower than that of CZTS. Using a biexponential function model fitting,28 the effective minority carrier lifetimes of CZCTS and CZTS were estimated to be 10.8 and 4.1 ns, respectively, indicating 932
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Figure 3. (a) Time-resolved photoluminescence and (b) C−V and DLCP characteristic of CZTS and CZCTS solar cells.
Figure 4. (a) XPS valence band data of CZTS and CZCTS. Binding energy are measured with respect to the Fermi energy (EF). (b) Schematic of the band alignment at CZCTS/CdS interface.
minimum (CBM) or elevating the position of valence band maximum (VBM), as well as gaining more information on the band alignment of CZCTS/CdS heterojunction interface. Therefore, X-ray photoelectron spectroscopy (XPS) measurements including core-level spectroscopy and valence band (VB) spectroscopy were conducted, as shown in Figure 4a. According to the VB data, the Fermi level (EF) of CZCTS is ∼0.2 eV higher than the valence band maximum (VBM). The EF of kesterite has been constantly measured at around 0.4 eV above the VBM10,34 with free hole concentrations of ∼1016 cm−3, showing a big discrepancy compared with the ∼0.1 eV predicted by classic semiconductor theory,35 owing to a Fermi pinning problem caused by serious band tailing.36 Although the compositions of CZCTS and CZTS are quiet similar here (see the Supporting Information for compositions), CZCTS has a free carrier density that is on par or even lower than that of CZTS, but the EF and VBM positions are much closer than those of CZTS reference and reported highefficiency CZTSSe solar cells,34,36 indicating that the Fermi pinning problem caused by the severe band tailing and compensation problem in CZTS has been alleviated by Cd alloying, consistent with our aforementioned observations. By an indirect method including the measurements of XPS corelevels of different cation elements in the absorber and buffer layers as well as XPS bulk valence band spectra,37 the VBO can be readily estimated. Then the CBO can be calculated by combining the VBO and Eg, enabling the band alignment
that Cd incorporation improves the lifetime of the CZCTS absorber. This is consistent with previous observations that Cd ion diffusion into the CZTS layer increases the lifetime and efficiency of CZTS devices.29,30 It is speculated that the observed lifetime improvement should be attributed to both better microstructure due to larger grain sizes and reduced defects at the heterojunction interfaces and/or within the bulk absorber. To better understand the electrical properties, capacitance−voltage profiling (C−V) and drive level capacitance profiling (DLCP) measurements were conducted, as shown in Figure 3b. DLCP is more sensitive to the bulk rather than the interface and thus is believed to be more of a reflection on the free carrier of the absorber.31 According to the DLCP, after Cd alloying, the carrier concentration (at 0 bias) decreased from ∼4.4 × 1016 to ∼1.4 × 1016 cm−3. Admittedly the Cu/Sn ratio may also affect the doping density of CZTS and CZCTS thin films;32,33 however, because of their similar Cu/Sn ratio, we believe the lower free carrier density of CZCTS over its CZTS reference is mainly caused by Cd alloying. As a result of decreased free carrier density, the depletion width (Wd) of CZCTS has enlarged correspondingly from ∼160 to ∼400 nm. The third effect of Cd is to increase the minority lifetime and reduce the doping for absorber as well as enlarge the depletion region for the corresponding device. As the alloying of Cd reduces the band gap of CZTS, it is interesting to know whether the decreased band gap mainly comes from lowering the position of conduction band 933
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Figure 5. (a) J−V curves and (b) EQE curves of CZTS and champion CZCTS devices.
Table 2. Device Characteristics of the CZTS and CZCTS Solar Cells absorber
Voc (mV)
Jsc (mA/cm2)
FF (%)
Eff (%)
Eg/q-Voc (mV)
RS,L (Ω cm2)
Gs,L (mS cm−2)
A
J0 (A/cm2)
CZTS CZCTS
683 650
20.7 26.7
62.5 66.1
8.8 11.5
847 730
0.96 0.45
0.67 1.0
3.3 2.5
8.1 × 10−6 1.2 × 10−6
of Sites and Mauk,38 as shown in Table 2. For CZTS solar cell, highly resistive ZnS phases are usually aggregated at the top or bottom of the CZTS layer, contributing to the high Rs of the CZTS device.13,39 With Cd alloying the absorber, the highresistance ZnS is alloyed with Cd as well, forming a more conductive ZnxCd1−xS40 phase, which contributes to the reduction of the Rs correspondingly. The Voc deficit of our champion CZCTS solar cell is 729 mV, which is over 100 mV lower than that of CZTS. Bourdais et al. summarized the Voc trend for all the different types of kesterite solar cells and found that none of them achieve 60% of the maximum achievable Voc-max under 1 sun illumination.26 Here, the CZCTS has already reached 59.3% of its achievable Voc-max according to the Shockley−Queisser limit,41 based on the Voc of our champion CZCTS cell of 650 mV compared to its optical band gap of 1.38 eV. To better understand the dramatically reduced Voc deficit, we also calculated the ideality factor (A) and reverse saturation current (J0) according to the method of Sites and Mauk.38 A slightly lower ideality factor may suggest less recombination in the SCR region in the Cd-alloying CZTS cell. Besides, the J0 of the CZCTS device is decreased by almost 1 order of magnitude compared to that of CZTS, indicating a dramatically reduced recombination (despite the ∼0.15 eV band gap difference from Eg of CZTS), which can also be explained by a significant increase in minority lifetime of the CZCTS sample. As mentioned above, the reduced recombination should be attributed to the reduced band tailing problem. Our results show that another beneficial effect of Cd is the significant reduction of the Voc deficit. The large value of the ideality factor for our champion CZCTS solar cell suggests that the space-charge recombination still dominates, which is consistent with the above-mentioned unfavorable band alignment at the interface. Future work to improve the efficiency of CZCTS solar cells will focus on the optimization of the heterojunction interface. In conclusion, the Cd has positive influences in CZTS absorber quality and associated device performance. First, Cd alloying into CZTS can improve the microstructure by facilitating the growth of larger grains and change the nature of the secondary phases of the absorber. Second, alloying of Cd
schema of the CZCTS/CdS interface (Figure 4b). The CBO at the CZCTS/CdS interface is calculated to be −(0.16 ± 0.1) eV, indicating a “cliff”-like CBO of the heterojunction interface which is undesirable for efficient heterojunction solar cells. This value is quite close to the previously reported value of ∼0.2− 0.3 cliff for CZTS/CdS heterojunction interface,10,11 indicating the kesterite band gap decrease after Cd alloying is mainly attributed to a rise in the position of the VBM rather than a lowering of the position of the CBM, which does not exert a significant improvement in the heterojunction band alignments for the CZTS/CdS interface. Moreover, the temperaturedependent Voc measurement (Figure S8) on a 10% CZCTS solar cell suggests that the recombination process is still interface-dominated, consistent with the band alignment results. Therefore, the fourth effect of Cd alloying is that the reduction in band gap is mainly via elevating the VBM of the absorber. However, the band alignment of CZCTS/CdS remains the undesirable clifflike type. Figure 5a shows the J−V characteristics of the champion CZCTS (Cu2Zn0.6Cd0.4SnS4) device and its reference CZTS devices (with similar Cu/Sn composition) under standard AM1.5 illumination. The corresponding detailed device performance parameters are demonstrated in Table 2. Our best CZCTS solar cell shows active area efficiency of 11.5% (the total area efficiency of 11.0%), with the Voc of 651 mV, Jsc (active area) of 26.7 mA/cm2, and fill factor (FF) of 66.1%. Note that the active area Jsc value measured from J−V is consistent with that integrated from external quantum efficiency (EQE), shown as the red curve in Figure 2b. To the best of the authors’ knowledge, this reported efficiency of CZCTS is the highest among pure sulfide kesterite solar cells. Compared with a CZTS reference device with similar absorber composition, the improved efficiency of CZCTS device can be attributed to much lower Voc deficit (Eg/q − Voc), higher Jsc, and significantly increased FF. To be more specific, the major improvement in Jsc arises from the extended spectral absorption in the wavelength range of 820−920 nm because of decreased optical band gap of CZCTS according to EQE. The FF increment may result from reduced series resistance according to the calculated light series resistance (RS,L) using the method 934
DOI: 10.1021/acsenergylett.7b00129 ACS Energy Lett. 2017, 2, 930−936
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Film Solar Cells with 12.6% Efficiency. Adv. Energy Mater. 2014, 4, 1301465. (5) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D.; Levi, D. H.; Ho-Baillie, A. W. Y. Solar cell efficiency tables (version 49). Prog. Photovoltaics 2017, 25, 3−13. (6) Siebentritt, S. Why are Kesterite Solar Cells Not 20% Efficient? Thin Solid Films 2013, 535, 1−4. (7) Walsh, A.; Chen, S.; Wei, S.-H.; Gong, X.-G. Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4. Adv. Energy Mater. 2012, 2, 400−409. (8) Chen, S.; Walsh, A.; Gong, X.-G.; Wei, S.-H. Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 EarthAbundant Solar Cell Absorbers. Adv. Mater. 2013, 25, 1522−1539. (9) Gokmen, T.; Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Band tailing and efficiency limitation in kesterite solar cells. Appl. Phys. Lett. 2013, 103, 103506. (10) Yan, C.; Liu, F.; Song, N.; Ng, B. K.; Stride, J. A.; Tadich, A.; Hao, X. Band alignments of different buffer layers (CdS, Zn(O,S), and In2S3) on Cu2ZnSnS4. Appl. Phys. Lett. 2014, 104, 173901. (11) Bär , M.; Schubert, B.-A.; Marsen, B.; Wilks, R. G.; Pookpanratana, S.; Blum, M.; Krause, S.; Unold, T.; Yang, W.; Weinhardt, L.; Heske, C.; Schock, H.-W. Cliff-like conduction band offset and KCN-induced recombination barrier enhancement at the CdS/Cu2ZnSnS4 thin-film solar cell heterojunction. Appl. Phys. Lett. 2011, 99, 222105. (12) Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. The path towards a high-performance solution-processed kesterite solar cell. Sol. Energy Mater. Sol. Cells 2011, 95, 1421−1436. (13) Li, W.; Chen, J.; Yan, C.; Hao, X. The effect of ZnS segregation on Zn-rich CZTS thin film solar cells. J. Alloys Compd. 2015, 632, 178−184. (14) Larramona, G.; Levcenko, S.; Bourdais, S.; Jacob, A.; Choné, C.; Delatouche, B.; Moisan, C.; Just, J.; Unold, T.; Dennler, G. FineTuning the Sn Content in CZTSSe Thin Films to Achieve 10.8% Solar Cell Efficiency from Spray-Deposited Water−Ethanol-Based Colloidal Inks. Adv. Energy Mater. 2015, 5, 1501404. (15) Metzger, W. K.; Repins, I. L.; Contreras, M. A. Long lifetimes in high-efficiency Cu(In,Ga)Se2 solar cells. Appl. Phys. Lett. 2008, 93, 022110. (16) Scragg, J. J. S.; Choubrac, L.; Lafond, A.; Ericson, T.; PlatzerBjö rkman, C. A low-temperature order-disorder transition in Cu2ZnSnS4 thin films. Appl. Phys. Lett. 2014, 104, 041911. (17) Sun, K.; Yan, C.; Liu, F.; Huang, J.; Zhou, F.; Stride, J. A.; Green, M.; Hao, X. Over 9% Efficient Kesterite Cu2ZnSnS4 Solar Cell Fabricated by Using Zn1−xCdxS Buffer Layer. Adv. Energy Mater. 2016, 6, 1600046. (18) Su, Z.; Tan, J. M. R.; Li, X.; Zeng, X.; Batabyal, S. K.; Wong, L. H. Cation Substitution of Solution-Processed Cu2ZnSnS4 Thin Film Solar Cell with over 9% Efficiency. Adv. Energy Mater. 2015, 5, 1500682. (19) Witte, W.; Abou-Ras, D.; Hariskos, D. Chemical bath deposition of Zn(O,S) and CdS buffers: Influence of Cu(In,Ga)Se2 grain orientation. Appl. Phys. Lett. 2013, 102, 051607. (20) Liu, F.; Sun, K.; Li, W.; Yan, C.; Cui, H.; Jiang, L.; Hao, X.; Green, M. A. A Enhancing the Cu2ZnSnS4 solar cell efficiency by back contact modification: Inserting a thin TiB2 intermediate layer at Cu2ZnSnS4/Mo interface. Appl. Phys. Lett. 2014, 104, 051105. (21) Yan, C.; Chen, J.; Liu, F.; Song, N.; Cui, H.; Ng, B. K.; Stride, J. A.; Hao, X. Kesterite Cu2ZnSnS4 solar cell from sputtered Zn/(CuSn) metal stack precursors. J. Alloys Compd. 2014, 610, 486−491. (22) Shin, B.; Gunawan, O.; Zhu, Y.; Bojarczuk, N. A.; Chey, S. J.; Guha, S. Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber. Prog. Photovoltaics 2013, 21, 72−76. (23) Viezbicke, B. D.; Patel, S.; Davis, B. E.; Birnie, D. P. Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system. Phys. Status Solidi B 2015, 252, 1700− 1710.
in CZTS can reduce the Voc deficit and recombination by reducing the band tailing problem. Third, alloying of Cd can increase minority carrier lifetime and lowers the doping density (free carrier). Fourth, Cd incorporation elevates the VBM position of CZTS rather than lower the CBM position of CZTS, suggesting an undesirable clifflike heterojunction interface. With these positive influences of Cd, an over 11% efficient pure sulfide kesterite CZCTS solar cell has been demonstrated. Future work on the optimization of the heterojunction band alignment is expected to further boost the efficiency of Cd-alloyed kesterite solar cells, demonstrating the high-efficiency potential for developing state-of-the-art kesterite photovoltaic cells.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00129. Experimental Section and supplementary characterizations of materials and devices (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Chang Yan: 0000-0002-5568-9031 Martin A. Green: 0000-0002-8860-396X Author Contributions ⊥
C.Y and K.S contributed equally to the work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This contribution has been financially supported by the Australian Government through the Australian Renewable Energy Agency (ARENA), Australian Research Council (ARC) ,and Guodian New Energy Technology Research Institute. Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government. We acknowledge the facilities and the scientific and technical assistance of the Electron Microscope Unit (EMU) and Mark Wainwright Analytical Centre, The University of New South Wales (UNSW). The authors appreciate the use of facilities and the assistance of David Mitchell and Gilberto Casillas Garcia at the University of Wollongong (UOW) Electron Microscopy Centre. C.Y. acknowledges the insightful discussion with Dr. Zhenghua Su.
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REFERENCES
(1) Todorov, T. K.; Reuter, K. B.; Mitzi, D. B. High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber. Adv. Mater. 2010, 22, E156−E159. (2) Barkhouse, D. A. R.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Mitzi, D. B. Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se,S)4 solar cell. Prog. Photovoltaics 2012, 20, 6−11. (3) Todorov, T. K.; Tang, J.; Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Mitzi, D. B. Beyond 11% Efficiency: Characteristics of Stateof-the-Art Cu2ZnSn(S,Se)4 Solar Cells. Adv. Energy Mater. 2013, 3, 34−38. (4) Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe Thin935
DOI: 10.1021/acsenergylett.7b00129 ACS Energy Lett. 2017, 2, 930−936
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ACS Energy Letters (24) Urbach, F. The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids. Phys. Rev. 1953, 92, 1324−1324. (25) Sayakanit, V.; Glyde, H. R. Urbach tails and disorder. Comments Cond. Matter Phys. 1987, 13, 35−48. (26) Bourdais, S.; Choné, C.; Delatouche, B.; Jacob, A.; Larramona, G.; Moisan, C.; Lafond, A.; Donatini, F.; Rey, G.; Siebentritt, S.; Walsh, A.; Dennler, G. Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells? Adv. Energy Mater. 2016, 6, 1502276. (27) Scragg, J. J. S.; Larsen, J. K.; Kumar, M.; Persson, C.; Sendler, J.; Siebentritt, S.; Platzer Björkman, C. Cu−Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4: Theoretical and experimental investigations. Phys. Status Solidi B 2016, 253, 247−254. (28) Wen, X.; Yu, P.; Toh, Y.-R.; Lee, Y.-C.; Huang, K.-Y.; Huang, S.; Shrestha, S.; Conibeer, G.; Tang, J. Ultrafast electron transfer in the nanocomposite of the graphene oxide-Au nanocluster with graphene oxide as a donor. J. Mater. Chem. C 2014, 2, 3826−3834. (29) Liu, F.; Yan, C.; Huang, J.; Sun, K.; Zhou, F.; Stride, J. A.; Green, M. A.; Hao, X. Nanoscale Microstructure and Chemistry of Cu2ZnSnS4/CdS Interface in Kesterite Cu2ZnSnS4 Solar Cells. Adv. Energy Mater. 2016, 6, 1600706. (30) Tajima, S.; Umehara, M.; Hasegawa, M.; Mise, T.; Itoh, T. Cu2ZnSnS4 photovoltaic cell with improved efficiency fabricated by high-temperature annealing after CdS buffer-layer deposition. Prog. Photovoltaics 2017, 25, 14−22. (31) Heath, J. T.; Cohen, J. D.; Shafarman, W. N. Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling. J. Appl. Phys. 2004, 95, 1000−1010. (32) Sugimoto, H.; Liao, C.; Hiroi, H.; Sakai, N.; Kato, T. Lifetime improvement for high efficiency Cu2ZnSnS4 submodules. In 2013 IEEE 39th Photovoltaic Specialists Conference (PVSC), June 16−21, 2013; pp 3208−3211. DOI: 10.1109/PVSC.2013.6745135. (33) Bishop, D. M.; McCandless, B. E.; Haight, R.; Mitzi, D. B.; Birkmire, R. W. Fabrication and Electronic Properties of CZTSe Single Crystals. IEEE Journal of Photovoltaics 2015, 5, 390−394. (34) Barkhouse, D. A. R.; Haight, R.; Sakai, N.; Hiroi, H.; Sugimoto, H.; Mitzi, D. B. Cd-free buffer layer materials on Cu2ZnSn(SxSe1‑x)4: Band alignments with ZnO, ZnS, and In2S3. Appl. Phys. Lett. 2012, 100, 193904−5. (35) Gokmen, T.; Gunawan, O.; Mitzi, D. B. Semi-empirical device model for Cu2ZnSn(S,Se)4 solar cells. Appl. Phys. Lett. 2014, 105, 033903. (36) Gershon, T.; Lee, Y. S.; Antunez, P.; Mankad, R.; Singh, S.; Bishop, D.; Gunawan, O.; Hopstaken, M.; Haight, R. Photovoltaic Materials and Devices Based on the Alloyed Kesterite Absorber (AgxCu1−x)2ZnSnSe4. Adv. Energy Mater. 2016, 6, 1502468. (37) Santoni, A.; Biccari, F.; Malerba, C.; Valentini, M.; Chierchia, R.; Mittiga, A. Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy. J. Phys. D: Appl. Phys. 2013, 46, 175101. (38) Sites, J. R.; Mauk, P. H. Diode quality factor determination for thin-film solar cells. Sol. Cells 1989, 27, 411−417. (39) Redinger, A.; Siebentritt, S. Loss Mechanisms in Kesterite Solar Cells. In Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells; John Wiley & Sons Ltd: West Sussex, U.K., 2014; pp 363−386. (40) Patidar, D.; Saxena, N. S.; Sharma, T. P. Structural, optical and electrical properties of CdZnS thin films. J. Mod. Opt. 2008, 55, 79− 88. (41) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510− 519.
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DOI: 10.1021/acsenergylett.7b00129 ACS Energy Lett. 2017, 2, 930−936
