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Enhancement of Open Circuit Voltage of Solution Processed CuZnSnS Solar Cell with 7.2% Efficiency by Incorporation of Silver 2

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Asim Guchhait, Zhenghua Su, Ying Fan Tay, Sudhanshu Shukla, Wenjie Li, Shin Woei Leow, Joel Ming Rui Tan, Stener Lie, Oki Gunawan, and Lydia H. Wong ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00509 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 16, 2016

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ACS Energy Letters

Enhancement of Open Circuit Voltage of Solution Processed Cu2ZnSnS4 Solar Cell with 7.2% Efficiency by Incorporation of Silver Asim Guchhait,┴ Zhenghua Su,§ Ying Fan Tay,§ Sudhanshu Shukla,§ Wenjie Li,§ Shin Woei Leow,┴ Joel Ming Rui Tan,┴ Stener Lie,§ Oki Gunawan,Ɨ and Lydia Helena Wong*,┴§ ┴

Campus of Research Excellence and Technological Enterprise, Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, 637553, Singapore §

School of Materials Science & Engineering, Nanyang Technological University, 639798, Singapore Ɨ

IBM T.J. Watson Research Center, Yorktown Heights, NY 10598, USA.

Corresponding Author *[email protected]

ABSTRACT Recently, considerable attention in the development of Cu2ZnSnS4 (CZTS)-based thin film solar cells has been given to the reduction of antisite defects via cation substitution. In this paper, we report the substitution of copper atoms by silver, incorporated into the crystal lattice through a solution processable method. We observe an increment in open circuit voltage (VOC) by 50 mV and an accompanying rise in device efficiency from 4.9% to 7.2%. The

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incorporation of Ag is found to improve the grain size, enhance the depletion width of pnjunction and reduce antisite defect states concentration. This work shows the promising role of Ag in reducing the VOC deficit of Cu-kesterite thin film solar cells.

TOC GRAPHICS

In the last decade, Cu2ZnSnS4 (CZTS) has garnered much attention as a solar light harvester due to its favourable optoelectronic properties with high absorption coefficient and its earth abundant constituents.1-3 Among the different techniques employed for the fabrication of CZTS based solar devices, a solution processable approach offers a high potential towards the realization of low cost large scale photovoltaic cells due to the ease of absorber stoichiometry control and dopant introduction. At present, the best reported efficiency of 8.4%, for CZTS photovoltaic cells markedly lags theoretically predicated values.4-5 This is due to problems encountered in CZTS films such as secondary phases, antisite defects, shorter carrier lifetime among other things.6-8 Current literature emphasizes film stoichiometric control and elemental doping in order


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to overcome these limitations and still now the highest reported efficiency through solution processing is ~ 5.5%. 9-11 It has been reported that adjusting the Cu/(Cu+Zn) ratio increased cell efficiency which was attributed to the reduction of antisite defects present in these absorbers.6, 12 However, samples exhibiting non-stoichiometry could introduce secondary phases and more intrinsic defects such as vacancies and interstitials.6 CuZn antisite defects where Cu atoms occupy the Zn lattice sites are best avoided as they could produce band tailing, deep surface/bulk defects and limit the device open-circuit voltage.13-14 CuZn antisite defects are easily produced in high concentrations due to the similar ionic radius of Cu and Zn which results in low formation energy.6, 15 It has been reported that the substitution of cations with much larger size than Cu can increase the formation energy of antisite defects.13 Previous forays into cation substitution with foreign atoms in compound semiconductors, such as Cu2ZnSn(S,Se)4, Cu(In,Ga)Se2, uncovered many exciting properties, for example changes in carrier concentration, crystal grain size and bandgap.16-19 In fact, this idea of cation substitution has been previously explored with the substitution of Sn and Zn for Ge and Cd to adjust the band gap.20-21 More recently, our group demonstrated band gap tuning and improved crystallinity of pure Cu2ZnSnS4 thin films, by replacing Zn with Cd to form Cu2Zn1-xCdxSnS4 (CCZTS). Power conversion efficiency improved significantly from 5.30% to 9.24%,21 demonstrating cation substitution possesses great potential for enhanced device performance. Nonetheless, we observed that these devices still suffered from low open circuit voltage (VOC) for the given material bandgap. In this regard, theoretical calculation suggested that Ag substitution of Cu in the form of (Cu,Ag)2ZnSnS4(ACZTS) alloys can improve the VOC deficit due to the high formation energy of AgZn antisite defects as Ag is 16% larger than both Cu and Zn.13 Recently, it was reported that


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Ag doping in CZT(S,Se) can improve the VOC of the device and the power conversion efficiency due to enhanced grain growth, improved carrier lifetime, reduction in carrier density/antisite defects and band-tailing.22-24 Previous theoretical calculation revealed that the CuZn antisite defect concentration is higher at the surface of the absorber. So by controlling the antisite defects at the ACZTS-CdS interface, device performance may be improved.13 Here, we demonstrated this theoretical prediction experimentally by depositing pure CZTS at the bottom and ACZTS layer at the top. With this approach, we demonstrate for the first time, Ag substitution in CZTS which produces a VOC improvement of 50mV and an absolute efficiency increase of 2.4%.

In Fig.1a, the XRD patterns of CZTS and ACZTS films agree well with that of tetragonal kesterite phase (JCPDS No. 026-0575), which shows a preferred orientation along the (112) plane at 28.53o. The peak position is shifted towards the lower angle by 0.12o indicating an increase in the lattice parameter. (Fig. 1b) These XRD results support the hypothesis that Ag atoms substitute Cu in the CZTS crystal especially since we have purposely kept the ratio of (Cu+Ag)/(Zn+Sn) the same. Previously it was reported that pure Ag2ZnSnS4 has a stannite type crystal structure,13,

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but in our case, no peaks corresponding to the stannite structure was

observed, possibly due to the very low content of Ag in the CZTS absorber. Fig. 1c shows the Raman spectra of the CZTS and ACZTS films with an excitation wavelength of 532 nm. Raman spectroscopy analysis is important because some secondary phases like Cu2SnS3 and SnS2, ZnS cannot be distinguished from CZTS in XRD analysis. From the Raman spectra in Fig.1c, both CZTS and ACZTS thin films show similar Raman peaks. The peaks appearing at 287 cm-1 and 338 cm-1 and 372cm-1 correspond to pure CZTS phase,26-27. There are no significant peaks corresponding to secondary phases can be observed except a little trend of ZnS (See Figure S2).


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It is worth noting that the Ag incorporation does not result in any Raman shifts possibly due to the low content of Ag in CZTS. Fig 1d shows the absorbance spectra of the CZTS and ACZTS films. We noticed the absorption edge of the both CZTS and ACZTS absorber are very close which indicates bandgap of the absorbers is similar. Another important observation after Ag doping in CZTS is the increase in grain size. This is evident from the FESEM thin film cross section images (Fig. 1e and f) and FESEM images of thin film surfaces (Fig. S3a and b) which show that the average grain size increases from 450 nm to 750 nm after Ag doping. Incorporation of the Ag in CZTS absorber is confirmed by SEM-EDX as shown in Supporting Information (Figure S1).

Figure 1. (a) X-ray diffraction (XRD) patterns of the CZTS and ACZTS thin films, (b) Zoom in XRD spectra of the CZTS and ACZTS thin film to verify the shift in peak position at 28.53o, (c)


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Raman spectroscopy of the CZTS and ACZTS thin film with an excitation wavelength of 532 nm, (d) optical absorbance spectra of the thin films, cross-section FESEM images of the CZTS based and ACZTS based devices have been shown in (e) and (f) respectively (scale bar is 1 micron).

Figure 2. (a) Current-voltage characteristics of the CZTS and ACZTS devices, the device schematic structure has been shown in inset, (b) external quantum efficiency (EQE) of CZTS and ACZTS device, (c) derivative of EQE w.r.t. hν , d(EQE)/d(hν) vs bandgap energy (hν) plot and the peak position corresponding to the band gap of the absorbers, (d) ln(-ln(1-EQE)) vs energy (hν) plot to verify the band-tails states. The dashed line corresponding to the band gap of the


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absorbers. The inverse of slope from linear portion after bandgap energy in lower energy side provides the Urbach energy (EU) of the devices, lower in EU reflects less in antisites defects and (e) tabulated form of all device parameters of CZTS and ACZTS device. The inset of Fig.2a shows a schematic of our device structure, in which the absorber layer is composed of CZTS and ACZTS. Since after sulfurization Ag atoms may diffuse throughout the absorber, we will call it only as ACZTS. Fig.2a shows the current-voltage (I-V) characteristics of the CZTS and ACZTS devices. All device parameters like JSC, VOC and fill-factor corresponding to the ACZTS absorber is much higher compared to the pure CZTS absorber and yields a power conversion efficiency of 7.24% under 1 Sun condition. From I-V curves, the series resistance (RS) is calculated to be ~ 5.2 Ω cm2 and shunt resistance (RSH) is ~ 251.2 Ω cm2 in the ACZTS device whereas for CZTS device RS~ 11.5 Ω cm2 and RSH ~ 184 Ω cm2. The decrease in series resistances and increase of shunt resistance could be a result of increased grain size and reduction of defect density in the ACZTS device. These improvements in series and shunt resistance results in the improved current density and fill factor. We also observed a significant improvement in VOC of about 50mV similar to a recent publication.22 This is mainly due to the reduction of CuZn antisite defects which could reduce band-tailing and yield high band bending at the interface of the absorber and buffer layer. We did repeat measurements of 8 best devices from different batches of devices and we found a similar trend of efficiency improvement as shown in Table S1. The device which is composed of only ACZTS layer showed poor performance compared than single CZTS, CZTS -ACZTS bi-layer (Fig. S4). This is because of amount of Ag content in the absorber is higher than optimum value which may affect the proper band alignment and could induce other defects/scattering centers which is reducing the carrier transport properties.


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Fig.2b shows the external quantum efficiency (EQE) spectra of the devices. EQE spectra shows an overall improvement in whole absorbing region with a significant improvement in the 550 nm to 720 nm range from ACZTS device. This EQE improvement is consistent with the increase in JSC of ACZTS device. Fig. 2c shows the plot between the derivative of EQE (d(EQE/d(hν)) with respect to bandgap energy. The peak positions in this graph marks the corresponding bandgap of the absorber. From the plot, the bandgaps of both CZTS and ACZTS absorbers are aligned closely suggesting that low content of Ag in CZTS does not change the bandgap significantly.2223

Additionally we can observe that the value of the derivative of d(EQE)/d(hν) is much higher

for the Ag substituted absorber compared to pure CZTS. This sharper transition indicates better band-edge absorption of photons which can be achieved only by reducing the band-tailing. To characterize the band-tailing for these absorbers, an Urbach tail model analysis was reported before.23 Under this Urbach model, a plot of ln(-ln(1-EQE)) vs the bandgap energy was done to estimate the Urbach tail energy(EU), with a lower value indicating less band-tailing. Fig.2(d) shows the Urbach band-tail analysis. An estimate of EU is obtained from taking an inverse of the slope of the linear region below the bandgap. We found the value of EU for CZTS device is ~48 meV and for ACZTS device ~38 meV. This implies a significant reduction in bandtailing/antisite defects with Ag substitution. The combination of these analyses seems to suggest that Ag substitution in the CZTS absorber improves VOC by the reduction of CuZn antisites effect and unlikely due to changes in bandgap. To analyze the quality of the p-n junction, capacitance-voltage measurements were performed with 0 to −3 V reverse bias. The capacitance in this work is measured under reverse bias condition so the junction capacitance is more dominant. Figure 3a and b show the C–V sweep, C–V derived space-charge density and depletion width for the devices respectively according to


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relevant relation described in literature.21,

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The junction capacitance at zero bias can be

expressed as ε0εA/C, where C is the measured capacitance for each DC bias, A is the device area and ε is the dielectric constants for the absorber. (the ε is fixed at 6.7 in this work based on the assumption that the dielectric constant is more affected by anion).29 So the depletion width (xd) can be determined by using ε0εA/C at zero bias. For both CZTS and ACZTS devices, the capacitances decrease monotonously with the increase in reverse bias voltage, indicating that reverse bias increases the depletion layer width in these devices. Furthermore, Ag substitution in CZTS can apparently change the xd and charge density of the devices as shown in Fig.3b. With doping, the xd value increases from 0.19 µm (CZTS) to 0.29 µm (ACZTS), while the C-V charge density of CZTS device(2×1016cm-3) remains higher compared to the ACZTS device (1.6×1016cm-3). The CZTS device with high charge density generally show small depletion width, consistent with previous reports.28,

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It should also be noted that the wider depletion

region with Ag doping could replace more effective charge separation between electron-hole pairs generated at surface as well as at the bulk of the device and consequently an improvement in short-circuit current. We note that the C-V charge density is sensitive to free carriers (bulk carrier) density, defect density and interface defects at the depletion edge xd.28 To better assess the charge density we performed drive level capacitance profiling (DLCP)28 on the ACZTS device as shown in Fig.3b. The charge density due to DLCP is generally lower than C-V as the former is not sensitive to interface defects and serves as better estimate for free carrier density. We observed that away from the buffer/absorber interface (at xd = 0.28µm) the ACZTS DLCP yield charge density as low as 2 ×1014cm-3-this is consistent with the free major carrier density determined from Hall measurement on the exfoliated absorber layer as discussed next.


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To verify the free carrier density independently, Hall measurements of similar films were performed using exfoliation technique and Van derPauw device as described previously.31 Due to low mobility (that leads to small Hall signal/noise ratio) we employ rotating parallel dipole line/ac Hall system as detailed in Ref.32-33 From these measurements we obtain the resistivity for CZTS(ACZTS) to be 6.4×108 (8.6×108) Ωcm, carrier both p-type with density: (1.7±1.2)x1014 ( (1.1±1.0) x1014) cm-3 and mobility of 0.5±0.4 (0.6 ±0.5) cm2/Vs. The measurement is consistent with decreasing carrier density and increasing depletion width as suggested in the C-V and DLCP study. 4.8

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Figure3. (a) Capacitance-voltage characteristics (C-V) of CZTS and ACZTS based devices. (b) Standard C–V profiles for CZTS and ACZTS thin film solar cells.The C-V is performed using 50 mV and 100 kHz AC signal with DC bias from 0 to−3 V at 300 K. The charge density from the drive level capacitance profiling (DLCP) is shown as red star points. In summary, we have demonstrated in this report the effect of Ag substitution in CZTS solar cell. We observed that with very low amount of Ag content, (< 7 mol% of the Cu content) the device efficiency enhanced from 4.88% to 7.24% by improving VOC by 50 mV, JSC by 3mA/cm2 and FF from 51.97% to 59.7%. From FESEM, band-tail model and C-V measurements of the absorber and the device we found that the benefit of Ag substitution originates from an increased grain


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size, enhanced depletion width and reduced antisite defect states concentration. This yielded an enhancement in VOC, JSC and fill factor of the device. Thus Ag substitution improves the overall performance of CZTS solar cell and should be pursued as a solution to the Voc deficit issue in the Cu-kesterite thin film solar cells.

EXPERIMENTAL METHODS The precursor solution for the ACZTS device fabrication was prepared by mixing Cu(CH3COO)2·H2O + Ag(NO3)2 (0.38 mol L−1), Zn(CH3COO)2·2H2O (0.25 mol L−1), SnCl2·2H2O (0.2 mol L−1), and SC(NH2)2 (1.6 mol L−1) into 2-methoxyethanol then stirring at 50 °C for 2.5 h to obtain a dark brown solution. For pure CZTS device fabrication we took Cu(CH3COO)2·H2O of 0.38 mol L-1 keeping rest the same. All chemical reagents were purchased from Sigma-Aldrich Company. The ratio of (Cu+Ag)/(Zn+Sn) is about 0.86 and Zn/Sn is 1.25. We maintained the molar ratio of 7% of Ag to Cu. Then the prepared solution was spin coated on molybdenum glass substrates at 3000 rpm for 30 s followed by preheating at 280 °C for 2 min on a hot plate in air. This spin coating was repeated ten times and then the as deposited thin films were annealed at 600°C for 30 minute in sulfur atmosphere to obtain CZTS and ACZTS thin films. A CdS buffer layer with ~60 nm thickness was deposited on CZTS and ACZTS thin film by chemical bath deposition (CBD). Next, 50 nm i-ZnO followed by 600 nm AZO or 300 nm ITO layer was deposited by DC magnetron sputtering. After ITO deposition, the devices were annealed at 300oC for 10 min under Argon atmosphere. Finally, silver glue was printed onto the ITO or AZO surface to form top contact fingers. The thin films and the devices were characterized by X-ray diffraction (Bruker D8 Advance), UV-vis absorbance spectroscopy (Shimadzu UV-3600), surface morphology, and cross section


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by FESEM (JEOL, JSM-7600F). Current density–voltage (J–V) curves for solar cells were performed using a Xe-based light source solar simulator (VS-0852 and KEITHLEY 2612A) to provide simulated 1 Sun AM 1.5G illumination. The system was calibrated with a standard Si reference cell. Capacitance-Voltage (C–V) characterizations were performed using HP 4284A and Keithley 4200 analyzers.

ASSOCIATED CONTENT Supporting Information. SEM-EDX of cross-section view of CZTS and ACZTS thin film, Raman peak fitting of CZTS and ACZTS thin films, FESEM images of CZTS and ACZTS thin film surface, current-voltage characteristics of the device composed only ACZTS single film, device parameters for the 8 best devices fabricated at different batches. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected],

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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Authors acknowledges the funding support from Energy Innovation Research Programme grant number NRF2011EWT-CERP001-019, NTU-COE Industry Research Collaboration Award, Singapore -Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), that is supported by the National Research Foundation, Prime Minister’s Office, Singapore.

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(7) Polizzotti, A.; Repins, I. L.; Noufi, R.; Wei, S.-H.; Mitzi, D. B. The State and Future Prospects of Kesterite Photovoltaics. Energy Environ. Sci. 2013, 6, 3171-3182. (8) Yu, K.; Carter, E. A. A Strategy to Stabilize Kesterite CZTS for High-Performance Solar Cells. Chem Mater. 2015, 27, 2920-2927. (9) Tan, J. M. R.; Lee, Y. H.; Pedireddy, S.; Baikie, T.; Ling, X. Y.; Wong, L. H. Understanding the Synthetic Pathway of a Single-Phase Quarternary Semiconductor Using Surface-Enhanced Raman Scattering: A Case of Wurtzite Cu2ZnSnS4 Nanoparticles. JACS 2014, 136, 6684-6692. (10) Hsieh, Y.-T.; Han, Q.; Jiang, C.; Song, T.-B.; Chen, H.; Meng, L.; Zhou, H.; Yang, Y. Efficiency Enhancement of Cu2ZnSn(S,Se)4 Solar Cells via Alkali Metals Doping. Adv. Energy Mater. 2016, 6, 1502386. (11) Tiwari, D.; Koehler, T.; Lin, X.; Harniman, R.; Griffiths, I.; Wang, L.; Cherns, D.; Klenk, R.; Fermin, D. J. Cu2ZnSnS4 Thin Films Generated from a Single Solution Based Precursor: The Effect of Na and Sb Doping. Chem. Mater. 2016, 28, 4991-4997. (12) Chantana, J.; Hironiwa, D.; Watanabe, T.; Teraji, S.; Minemoto, T. Flexible Cu(In,Ga)Se2 Solar Cell on Stainless Steel Substrate Deposited by Multi-Layer Precursor Method: its Photovoltaic Performance and Deep-Level Defects. Prog. Photovoltaics: Res. Appl. 2016, 24, 990-1000. (13) Yuan, Z.-K.; Chen, S.; Xiang, H.; Gong, X.-G.; Walsh, A.; Park, J.-S.; Repins, I.; Wei, S.H. Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu- and Ag-Based Kesterite Compounds. Adv. Funct. Mater. 2015, 25, 6733-6743. (14) 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.


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(15) Chen, S.; Gong, X. G.; Walsh, A.; Wei, S.-H. Defect Physics of the Kesterite Thin-Film Solar Cell Absorber Cu2ZnSnS4. Appl. Phys. Lett. 2010, 96, 021902. (16) Wei, S.-H.; Zhang, S. B.; Zunger, A. Effects of Na on the Electrical and Structural Properties of CuInSe2. J. Appl. Phys. 1999, 85, 7214-7218. (17) Cho, D.-H.; Lee, K.-S.; Chung, Y.-D.; Kim, J.-H.; Park, S.-J.; Kim, J. Electronic Effect of Na on Cu(In,Ga)Se2 solar cells. Appl. Phys. Lett. 2012, 101, 023901. (18) Sutter-Fella, C. M.; Stückelberger, J. A.; Hagendorfer, H.; La Mattina, F.; Kranz, L.; Nishiwaki, S.; Uhl, A. R.; Romanyuk, Y. E.; Tiwari, A. N. Sodium Assisted Sintering of Chalcogenides and Its Application to Solution Processed Cu2ZnSn(S,Se)4 Thin Film Solar Cells. Chem. Mater. 2014, 26, 1420-1425. (19) Wang, C.; Chen, S.; Yang, J.-H.; Lang, L.; Xiang, H.-J.; Gong, X.-G.; Walsh, A.; Wei, S.H. Design of I2–II–IV–VI4 Semiconductors through Element Substitution: The Thermodynamic Stability Limit and Chemical Trend. Chem. Mater. 2014, 26, 3411-3417. (20) Kim, S.; Kim, K. M.; Tampo, H.; Shibata, H.; Matsubara, K.; Niki, S. Ge-Incorporated Cu2ZnSnSe4 Thin-Film Solar Cells with Efficiency greater than 10%. Sol. Energy Mater. Sol. Cells 2016, 144, 488-492. (21) 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. (22) 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.


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(23) Hages, C. J.; Koeper, M. J.; Agrawal, R. Optoelectronic and Material Properties of Nanocrystal-based CZTSe Absorbers with Ag-alloying. Sol. Energy Mater. Sol. Cells 2016, 145, Part 3, 342-348. (24) Cui, H.; Lee, C.-Y.; Li, W.; Liu, X.; Wen, X.; Hao, X. Improving Efficiency of Evaporated Cu2ZnSnS4 Thin Film Solar Cells by a Thin Ag Intermediate Layer between Absorber and Back Contact. Int. J. Photoenergy 2015, 2015, 170507. (25) Yeh, L.-Y.; Cheng, K.-W. Preparation of the Ag–Zn–Sn–S Quaternary Photoelectrodes using Chemical Bath Deposition for Photoelectrochemical Applications. Thin Solid Films 2014, 558, 289-293. (26) Fontané, X.; Calvo-Barrio, L.; Izquierdo-Roca, V.; Saucedo, E.; Pérez-Rodriguez, A.; Morante, J. R.; Berg, D. M.; Dale, P. J.; Siebentritt, S. In-depth Resolved Raman Scattering Analysis for the Identification of Secondary Phases: Characterization of Cu2ZnSnS4 Layers for Solar Cell Applications. Appl. Physics Lett. 2011, 98, 181905. (27) Ahmed, S.; Reuter, K. B.; Gunawan, O.; Guo, L.; Romankiw, L. T.; Deligianni, H. A High Efficiency Electrodeposited Cu2ZnSnS4 Solar Cell. Adv. Energy Mater. 2012, 2, 253-259. (28) 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. (29) Persson, C. Electronic and Optical Properties of Cu2ZnSnS4 and Cu2ZnSnSe4. J. Appl. Phys. 2010, 107, 053710. (30) Duan, H.-S.; Yang, W.; Bob, B.; Hsu, C.-J.; Lei, B.; Yang, Y. The Role of Sulfur in Solution-Processed Cu2ZnSn(S,Se)4 and its Effect on Defect Properties. Adv. Functional Mater. 2013, 23, 1466-1471.


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ACS Energy Letters

(31) Tai, K. F.; Gunawan, O.; Kuwahara, M.; Chen, S.; Mhaisalkar, S. G.; Huan, C. H. A.; Mitzi, D. B. Fill Factor Losses in Cu2ZnSn(SxSe1−x)4 Solar Cells: Insights from Physical and Electrical Characterization of Devices and Exfoliated Films. Adv. Energy Mater. 2016, 6, 1501609. (32) Gunawan, O.; Virgus, Y.; Tai, K. F. A Parallel Dipole Line System. Appl. Phys. Lett. 2015, 106, 062407. (33) Gunawan, O.; pereira, M. Rotating Magnetic Field Hall Measurement System. US 14/682696 US 14/826934 2016.


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Letter - ACS Publications - American Chemical Society

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