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Effects of cation composition on carrier dynamics and photovoltaic performance in Cu2ZnSnSe4 monocrystal solar cells
Solar Energy Materials & Solar Cells 205 (2020) 110255
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Effects of cation composition on carrier dynamics and photovoltaic performance in Cu2ZnSnSe4 monocrystal solar cells Siming Li a, 1, Michael A. Lloyd b, 1, Brian E. McCandless b, **, Jason B. Baxter a, * a b
Drexel University, Department of Chemical and Biological Engineering, Philadelphia, PA, 19104, USA University of Delaware, Institute of Energy Conversion, Newark, DE, 19716, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Kesterite Solar cell Photovoltaic Carrier dynamics Device performance CZTSe
Understanding the relationship of doping density, carrier lifetime, and interface recombination to device per formance is critical to designing solar cells with high power conversion efficiency (PCE). In turn, it is necessary to understand how bulk material composition determines doping density and carrier lifetime. The most efficient kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells have had Cu-poor, Zn-rich compositions, while more stoichiometric compositions have lower PCEs. However, thin films are grown under highly non-equilibrium conditions, complicating fundamental studies. Here we report on a set of CZTSe monocrystals with varied cation stoichiometry, enabling correlation of bulk composition to material and device properties without the complication of grain boundaries or secondary phases. Copper-poor, zinc-rich compositions (Cu/(Zn þ Sn) ¼ 0.77–0.90 and Zn/Sn ¼ 1.17–1.25) yield bulk carrier lifetimes longer than 200 ps and PCE >5%. In contrast, near-stoichiometric compositions, with Cu/(Zn þ Sn) > 0.90 and Zn/Sn < 1.15, have carrier lifetimes shorter than 20 ps and PCE <2%. CZTSe/CdS interface recombination velocity has a similar value to the CZTSe surface recombination velocity, with values of 104–105 cm/s determined by time-resolved terahertz spectroscopy and transport-recombination modeling. Device modeling reveals the dependence of open circuit voltage (VOC ) on doping density, carrier lifetime and interface recombination. For a crystal with low doping density of 1015 cm 3, the maximum VOC is limited by the bulk lifetime. Higher VOC can be attained with higher doping density, but interface recombination becomes more significant with increased lifetime and doping density. These simulations indicate limitations and potential pathways to high performance.
1. Introduction Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising candidate for photovoltaic applications because it has a direct band gap that is tunable from 1.0 to 1.5 eV by changing the S/Se ratio [1,2], with large absorp tion coefficient (>1x104 cm 1) [3]. The current champion CZTSSe de vice (Eg ¼1.13 eV) exhibited power conversion efficiency (PCE) of 12.6% [4]. Although the stoichiometric ratio of Cu, Zn, Sn, and Se elements in the kesterite structure is 2:1:1:4, photovoltaic (PV) device performance depends strongly on the composition. Virtually all thin film CZTSSe solar cells with PCE exceeding 8% have had Cu-poor and Zn-rich composi tions, typically with Cu/(Zn þ Sn) ratio of ~0.8 and Zn/Sn ratio of ~1.2 [5–8]. These conditions are reported to minimize the concentration of deleterious point defects and cluster defects, resulting in hole
concentrations of 1015-1016 cm 3 that are within a desirable range for high PCE [8]. PCE of CZTSSe thin film PVs is primarily limited by a large open circuit voltage (VOC ) deficit (Eg =q - VOC ), which is in the range of 600–900 mV depending on bandgap [4,9–11]. Low minority carrier lifetime [9,12,13], fast interface recombination [14,15], and band tails [12,16] have all been reported as limiting factors for VOC . An absorber with high doping density, long bulk lifetime and low interface recom bination rate is desired for an efficient solar cell. Understanding the effects of photoexcited carrier lifetime, recombination mechanisms, and doping density on device performance is critical to the design of efficient solar cells. In our previous work, we applied multiple ultrafast spectroscopies to investigate carrier dynamics and their relation to solar cell performance
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B.E. McCandless), [email protected] (J.B. Baxter). 1 Authors contributed equally to this work. https://doi.org/10.1016/j.solmat.2019.110255 Received 18 August 2019; Received in revised form 16 October 2019; Accepted 25 October 2019 Available online 15 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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100 mWcm 2 via an NREL-certified Silicon standard. External quantum efficiency (EQE) was measured using an Oriel monochromator using the AC method with a lock-in amplifier. TRTS used a regeneratively amplified Ti: Sapphire laser (Coherent Libra, 800 nm, 1 kHz, 50 fs pulse duration) coupled to a home-built reflectance spectrometer. The sample was photoexcited with a pump pulse whose wavelength was selected using an optical parametric amplifier, then probed with terahertz radiation generated and detected using ZnTe nonlinear crystals. Reflection geometry was used because the large thickness and moderate conductivity of the crystal resulted in negligible terahertz transmission for most samples at room temperature. The change in reflectance upon chopped photoexcitation was normal ized by the non-photoexcited signal to determine ΔR/R, which is pro portional to photoconductivity. The reflection TRTS configuration and analysis have been described in more detail elsewhere [17,27]. Carrier transport and recombination were modeled by solving the time-dependent continuity equation. Carriers are generated instanta neously at t ¼ 0 according to Beer’s Law. Absorption coefficients of 2 � 105 cm 1 and 6 � 104 cm 1 were used for 400 nm and 800 nm photons, respectively [28]. We assume that no electric fields exist, such that transport of photoexcited electrons and holes occurs by ambipolar diffusion, resulting in the same concentration profiles for both carriers. One-dimensional numerical solar cell simulations were performed using SCAPS, software developed at the University of Gent, Belgium. SCAPS solves the Poisson equation and the continuity equations for electrons and holes, including recombination at the junction interface [29]. Our model device consists of three semiconductor layers: ZnO, CdS and CZTSe. The bulk carrier lifetime in SCAPS for SRH recombination 1 can be estimated by τ ¼ σvN , where σ is the capture cross-section area, v T
for a CZTSe monocrystal [17]. The use of monocrystals grown under quasi-equilibrium conditions eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin-film growth. For that crystal, we found that the bulk lifetime of 260 ps led to a short minority diffusion length and incomplete carrier collection, resulting in low short-circuit current density (JSC ) and likely also affecting VOC [17]. Applying this approach to a set of monocrystals with a range of cation and anion compositions can provide insight into relationships between composition, dynamics, and device performance. Phuong et al. investigated the effect of S:Se ratio in CZTSSe monocrystals [18], but no analogous studies of cation composition in monocrystals have been reported. In this work, we investigated a series of high-quality CZTSe mono crystals with different cation compositions to understand their rela tionship to device performance and ultrafast carrier dynamics. We report on the crystals with Cu/(Zn þ Sn) ratios of 0.75–1.0 and Zn/Sn ratios of 1.3–1.0, which have doping density of 1x1015 - 1x1018 cm 3, respectively. Solar cell and Hall effect devices were fabricated to analyze the crystals’ electronic properties. Carrier dynamics were measured by time-resolved terahertz spectroscopy (TRTS), which is a non-contact probe of transient photoconductivity on sub-picosecond to nanosecond time scales that are relevant for charge carrier generation, transport, and recombination [19–21]. By fitting dynamics data with a transport-recombination model, bulk lifetime and surface recombina tion velocity were determined for each crystal. Bulk lifetime increased dramatically for Cu-poor, Zn-rich compositions compared to more stoichiometric compositions, while surface recombination velocity was not sensitive to composition. Interface recombination velocity for a CZTSe crystal with a standard CdS buffer layer was similar to the surface recombination velocity. Parameters determined experimentally were utilized in a ZnO/CdS/CZTSe solar cell device simulation using SCAPS software to provide guidance on how doping density, bulk lifetime, and interface recombination velocity influence solar cell performance.
is the thermal velocity of the carriers, and NT is trap density in units of cm 3 [30]. We assume that the carrier lifetime obtained from TRTS corresponds to SRH recombination via a mid-gap single-level trap. The interface recombination velocity was calculated by IRV ¼ σ vnT , where nT is trap density at the interface. Front-surface reflection was not included in the simulations unless specifically noted.
2. Experimental Experimental procedures for growing crystals, measuring their properties, and fabricating and measuring devices were reported in detail previously [17]. Briefly, centimeter-scale ingots of CZTSe were synthesized at temperatures below the peritectic point of 790 � C [22] as described by Bishop et al. [23]. A quartz ampoule was loaded with elemental precursors of 5–6 N purity, evacuated overnight and sealed at a base pressure of 5 � 10 6 Torr. CZTSe growth and grain ripening proceeded as the ampoule was held at 750 � C for 20 days. Heating was stopped and the ampoule cooled naturally to room temperature over 36 h. Monocrystals extracted from the multicrystalline ingot were me chanically thinned and planarized. Chemical-mechanical polishing was used to remove mechanical damage induced by grinding, using a solu tion of 0.125% bromine in methanol. The crystal then was etched for 10 min in the same solution to remove a surface layer ~4 μm thick. The finished crystals studied here were ~3 mm in diameter and 0.5–1.0 mm thick. Crystal composition was measured by x-ray fluorescence spectros copy (XRF, Oxford Instruments Xstrata). Majority carrier concentration and mobility were measured by Hall effect in the van der Pauw configuration in the dark at room temperature [24]. Solar cells were fabricated as in Ref. [25]. A 60 nm thick CdS emitter layer was deposited by chemical bath deposition. ZnO/ITO was sputtered through a circular mask to create a device with area of 0.8 mm2, which is the area used when reporting current density. The crystal was mounted on a Mo-coated glass substrate with Acheson Electrodag® graphite paste to create the back contact. An active device area of 0.75 � 0.07 mm2 was measured for a representative cell by the full-width half maximum of electroluminescence (EL) signal taken along a line profile across the circular device area using the setup described in ref [26]. JV charac teristics were measured with an OAI TriSOL solar simulator calibrated to
3. Results and discussion CZTSe monocrystals were prepared with cation compositions span ning a range from Cu-poor, Zn-rich to near-stoichiometric, with Cu/(Zn þ Sn) ¼ 0.75–1.0 and Zn/Sn ¼ 1.3–1.0. Compositions of the samples are shown in Fig. 1a. Dark Hall effect measurements show that CZTSe monocrystals exhibit p-type conduction with doping density from 1x1015 to 1x1018 cm 3 depending on cation composition, and hole mobility does not show any trend with cation composition, with values ranging from 46 to 165 cm2 =Vs [31]. The high variability in hole mobility from the Hall effect measurement is attributed to variations in crystal properties owing to their particular locations in the growth matrix, as the measurements of crystal thickness and Hall resistance are repeatable to within 20%. The hole mobility in CZTSe monocrystals is higher than hole mobility of CZTSe thin films, which typically are within the range of 1–10 cm2 =Vs [27,32]. We fabricated and tested solar cells on crystals with the most promising compositions, Fig. 1b. Cells with PCE above 5% have Cu/(Zn þ Sn) of 0.77–0.90 and Zn/Sn of 1.17–1.25. In contrast, crystal D with Cu/(Zn þ Sn) ¼ 0.92 and Zn/Sn ¼ 1.12 had PCE less than 2%. Mono crystal PVs shows the same trend as in thin film PVs: devices with highest PCE have Cu-poor, Zn-rich compositions, while more stoichio metric compositions suffer from low PCE [8]. The device performance parameters of each cell are listed in Table S1. Table S2 indicates sta tistics of four devices each with Compositions A and B. Our best CZTSe monocrystal PV had JSC ¼ 29 mA/cm2, VOC ¼ 430 mV, fill factor (FF) ¼ 64%, and PCE ¼ 7.9%, which is remarkable considering the millimeter thickness of the crystal. Although the per formance is lower than the best CZTSe thin film device, which has PCE 2
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Fig. 1. (a) Cation compositions of CZTSe monocrystals studied in this work. (b) JV curves of CZTSe monocrystals with compositions shown in (a). (c) Normalized carrier dynamics of the same CZTSe monocrystals photoexcited at 1.55 eV to generate initial carrier densities of 3-9x1017 cm 3. Colors in b and c correspond to crystal compositions in a. Crystal F did not show any discernible TRTS signal. Devices were not fabricated on crystals E and F.
over 11% [33,34], the monocrystal device has a higher VOC than the record thin film device. Diode analysis (Fig. S1) for the PVs with PCE above 6% indicates an ideality factor of 1.6–1.9, which implies that the dark saturation current is limited by recombination in the depletion region [35]. Next we discuss our analysis of recombination dynamics to differentiate the relative importance of bulk recombination and surfa ce/interface recombination and their relation to device performance. We measured carrier dynamics of the CZTSe monocrystals using TRTS to determine how recombination rates and mechanisms depend on cation composition. TRTS is a noncontact probe of transient photocon ductivity with sub-picosecond temporal resolution and ~1 mm spot size, allowing carrier dynamics of CZTSe monocrystals to be measured without complications arising from the full device. Fig. 1c shows the transient photoconductivity of CZTSe monocrystals of different cation composition. Crystals were photoexcited at 1.55 eV, which is close to the bandgap of CZTSe of ~1 eV and has penetration depth of ~180 nm. Dynamics are independent of initial carrier concentration under high injection, as shown in Fig. S2, indicating that 1st order trapping and/or Shockley-Read-Hall (SRH) recombination are the dominant loss pro cesses. This conclusion is consistent with our previous results on CZTSSe thin film and CZTSe monocrystal materials [17,36]. Fig. 1c shows that the carrier lifetime is in the range of 0.1–10 ns for monocrystals with cation composition in the more Cu-poor, Zn-rich regime. These relatively long lifetimes are correlated to PCE above 5% for the corresponding PV devices. For the crystals with cation compo sition in the near-stoichiometric region, carrier lifetime is approximately 10 ps, corresponding to PCE below 2%. Short SRH carrier lifetime results from high density of recombination centers and limits the device per formance. Previous studies on kesterite thin films found that the den sities of some point defects (such as CuSn and SnZn antisites) and cluster defects ([2CuZn þ SnZn]) are higher for more stoichiometric
compositions [14]. Those defects may act as recombination centers [8, 14], although identifying the chemical nature of the recombination centers is beyond the scope of this work. To distinguish the roles of bulk SRH recombination and surface recombination, we applied the transport-recombination model devel oped in [17] to the carrier dynamics of CZTSe monocrystals photoex cited at 1.55 eV and 3.10 eV, as shown in Fig. 2 and Table 1. Photoexcitation at 3.10 eV is far above the bandgap, creating a carrier distribution closer to the surface and making surface recombination more influential than in the case of 1.55 eV photoexcitation. Crystals D and E, which have near-stoichiometric compositions have effective lifetime of ~20 ps, with carrier dynamics that are similar when photo exciting at 1.55 eV and 3.10 eV, as shown in Fig. 2a, S3a. In this case, dynamics are dominated by the short bulk carrier lifetime; surface recombination has little effect and therefore SRV cannot be determined. As bulk lifetime increases for more Cu-poor, Zn-rich crystals, surface recombination begins to contribute to the dynamics; for the crystal in Fig. 2b, the dynamic is determined by both bulk lifetime (~260 ps) and Table 1 Hole density (p) from Hall effect measurement of monocrystals, and surface/ interface recombination velocity (SRV/IRV) and bulk lifetime (τ) obtained by fitting TRTS data with a transport-recombination model using ambipolar mobility of 1–30 cm2/Vs. Samples E D B C A CdS/A
p (cm 18
3
)
~10 ~1018 8.7x1016 6.3x1016 1.8x1015
SRV/IRV (cm/s)
τ (ns)
– – 2-5x104 4-8x104 3-5x104 3-4 x104
0.015-0.017 0.013-0.018 0.22-0.27 >4 >5 >5
Fig. 2. Normalized TRTS carrier dynamics of (a) Crystal D, (b) Crystal B, and (c) Crystal A photoexcited at 3.1 eV (blue) and 1.55 eV (red). The gray regions are the simulated dynamics based on the parameters listed in Table 1. 3
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surface recombination. As effective carrier lifetime continues to increase to the multi-nanosecond scale (Fig. 2c, S3b), the large difference in dynamics with photoexcition energy indicates that surface recombina tion is dominant. SRV is 3-8 x104 cm/s while bulk lifetime is at least 4 ns. A more accurate bulk lifetime cannot be determined in this case because of the strong influence of surface recombination. While the bulk lifetime increased by more than a factor of 20 with decreasing Cu content, Table 1 indicates that SRV is in the range of 2–8 x104 cm/s and appears to be independent of cation composition. While SRV was found in the process of determining bulk lifetime of uncoated monocrystals, the CZTSe/buffer interface recombination ve locity (IRV) is actually the quantity of interest for PV devices. Multiple groups have reported that interface recombination can limit the per formance of CZTSSe thin film solar cell devices [14,15]. The spike-like band alignment within CZTSe/CdS indicates that interface recombina tion is most likely due to interface defects rather than inappropriate conduction band offsets [10]. To measure interface recombination be tween CZTSe and CdS, we coated approximately 60 nm CdS on the top of CZTSe Crystal A by chemical bath deposition, and photoexcited through the CdS side. Crystal A was chosen for this experiment because it has long bulk lifetime and dynamics limited by surface recombination. Fig. 2c shows the dynamics upon photoexciting Crystal A at 1.55 eV, revealing that dynamics of CZTSe/CdS are similar to dynamics of bare CZTSe. This result indicates that IRV is similar to SRV, which is consistent with findings of Redinger and Unold, whose measurements of PL yield also showed that the surface recombination velocity is not affected by coating with CdS buffer layer [37]. It is therefore likely that the surface recombination velocity is limited by crystal preparation prior to junction formation, which may require processing conditions dictated by the bulk composition. Furthermore, the cationic termination may affect surface recombination behavior as suggested by the device ben efits of a Zn-rich surface [10,38,39]. To obtain the value of IRV, we again studied the dependence of carrier dynamics on excitation energy, this time with a CZTSe/CdS heterostructure and using 1.55 eV and 1.03 eV photons, Fig. S3c, because CdS absorbs photons at 3.10 eV. Fitting with the transportrecombination model yielded IRV and bulk lifetime shown in Table 1. No change in bulk lifetime of CZTSe was observed upon addition of CdS; and the IRV had an upper limit of 3-4x104 cm/s, which is close to the value of SRV obtained from the same CZTSe single crystal. We use the term upper limit because we cannot distinguish between IRV and elec tron transfer into CdS followed by surface trapping. The IRV between CZTSe/CdS measured here is similar to the value of 103 -104 cm= s that was previously reported for the CZTSSe/CdS interface as determined by measurement of the pre-factor of reverse saturation current, J00 [40]. To further understand the influence of carrier dynamics on device parameters, we simulated PV device operation using SCAPS. When possible, we used parameters obtained from our measurements and previous literature results. The properties of each layer are shown in Table S3. Electron and hole mobilities were averages of the Cu-poor, Znrich crystals because mobility did not show any particular trend with composition. IRV was measured on Crystal A, as previously described. The doping density of CdS was adjusted to be 5.8x1016 cm 3 to match the EQE and JV curve of Crystal C, as shown in Fig. 3 and S4. While the aforementioned properties were assumed to be common across all de vices, individually measured lifetimes and hole densities for Crystals A – C, Table 1, were used to simulate different devices as indicated below. The bulk lifetimes listed in Table 1 are the upper limit of the minority carrier lifetime for three reasons. First, TRTS under high injection measures both the electron and hole dynamics, so the bulk lifetime (τb ) obtained from the model is τe þ τh [30]. Second, CZTSe solar cells are operated under low injection condition; our group and other showed that carrier recombination is slightly faster at lower fluence [17,41]. Third, carrier de-trapping in CZTSSe could result in an overestimate of the free carrier lifetime [42]. Fig. 3 shows the experimental and SCAPS-simulated EQEs of Crystals
B and C. Poor carrier collection for excitation wavelengths <540 nm is due to absorption by CdS and ZnO. Our previous paper and work of other groups demonstrated that the minority carrier diffusion length can limit EQE for CZTSSe [17,43]. The minority carrier diffusion length was ob tained by matching the SCAPS simulated EQE spectrum to the experi mental EQE (for λ > 600 nm), revealing minority carrier diffusion lengths of 0.18 � 0.02 μm and 1.0 � 0.2 μm for Crystals B and C, respectively, as shown in Fig. 3. The minority carrier diffusion length is pffiffiffiffiffi found from Ld ¼ Dτ. The diffusion length of Crystal B yields an elec tron mobility of 55 � 23 cm2 =Vs, which is consistent with the mobility obtained from the combination of TRTS frequency spectra and Hall ef fect [17]. However, we note that the resulting ambipolar mobility of 67 cm2 =Vs is somewhat larger than the range of ambipolar mobilities that provided the best fit for the transport-recombination model (1–30 cm2/V). This discrepancy may be due to inaccurate absorption coefficient chosen for the transport-recombination model, high varia tion in dark monocrystal Hall mobility, or injection-dependence of mobilities. Changing IRV from 1x103 to 1x106 cm/s did not significantly shift the EQE spectra, indicating that interface recombination does not limit carrier collection for the CZTSe monocrystal. While increasing lifetime will significantly improve EQE and hence JSC , the VOC deficit of CZTSSe has been reported as the main limitation to the performance of kesterite solar cells [4,9]. The VOC deficit of CZTSSe arises from several possible factors including low minority carrier life time [9,12,13], interface recombination [14,15], and band tails [12,16]. Both VOC and FF are governed by reverse saturation current in the depletion region. Increasing carrier lifetime and/or decreasing IRV could increase VOC FF, and PCE [30]. The depletion width is determined by the doping density of CZTSe, which varies based on Cu/(Zn þ Sn) composition. Doping density of our CZTSe monocrystals ranges from 1.8x1015 cm 3 for more Cu-poor, Znrich compositions to 2.5x1018 cm 3 for more stoichiometric composi tions. This trend is consistent with hole concentrations reported for thin films: 1016 -1018 cm 3 in Cu-poor region and 1017 - 1019 cm 3 in the stoichiometric region [8]. Our monocrystals with nanosecond-scale carrier lifetime have doping density of <6 x1016 cm 3. Long lifetime, low IRV and high doping density of CZTSe absorber are desired to achieve a higher maximum VOC . Fig. 4 shows the simu lated VOC as a function of bulk lifetime and interface recombination velocity for CZTSe hole densities of 1.8x1015 cm 3 and 6.3x1016 cm 3, corresponding to those measured for Crystals A and C, along with simulated band diagrams. Simulated FF and PCE follow the same trend, as shown in Fig. S5. With the lower doping density (Crystal A) and corresponding larger depletion width of >600 nm, IRV does not limit
Fig. 3. Experimental and simulated normalized quantum efficiency spectra for Crystals B (black) and C (red). 4
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Fig. 4. (a,b) Simulated band alignment of ZnO/CdS/CZTSe, and (c,d) contour plot of VOC from SCAPS simulations as a function of bulk lifetime and IRV when doping density of CZTSe is (a,c) 1.8x1015 cm 3and (b,d) 6.3x1016 cm 3. Panels c and d share the same color legend.
VOC for the range studied (IRV<106 cm/s and τ < 10 ns). Instead, VOC is controlled by the SRH lifetime. As lifetime increases from 10 ps to 10 ns, VOC increases from 170 mV to 440 mV, roughly corresponding to the dynamics and device data for the set of monocrystals shown in Fig. 1. Since the measured IRV is less than 105 cm/s, reducing interface recombination may not improve the device performance significantly, which is consistent with the conclusion found by Bar et al. that interface recombination plays a less significant role than bulk properties in CZTSSe/CdS thin film devices with S/(S þ Se) ¼ 0.1 [44]. However, the maximum VOC is below 500 mV with hole density of 1.8x1015 cm 3 even for bulk lifetime of over 10 ns. Increasing the doping density enables larger maximum VOC because of larger built-in potential, if sufficiently long lifetime and small IRV can be maintained. With doping density of 6.3x1016 cm 3, the depletion width is only 79 nm, resulting in more significant interface recombina tion. Furthermore, the smaller depletion width requires larger diffusion length to ensure collection of minority carriers. When the bulk lifetime is less than 50 ps, VOC is below 320 mV and is primarily determined by bulk lifetime. As bulk lifetime increases into the 10-ns range, interface recombination becomes the primary limitation to effective lifetime and VOC . With IRV of 5x104 cm/s measured for our monocrystal, the simu lated VOC is below 390 mV despite bulk lifetime above 5 ns. By reducing IRV from 5x104 to 1x103 cm/s, VOC could be increased from 390 mV to 550 mV for a bulk lifetime of 10 ns. Interface passivation could improve device performance in that case. While the SCAPS simulations provide important insight, we note several simplifications and sources of uncertainty in the model compared to monocrystal or thin film devices. The SCAPS simulation did not include the band-tails, series resistance (Rs ), or shunt resistance (Rsh ). Cation-based point defects and defect clusters can reduce VOC by 50–90 mV due to electrostatic potential fluctuations and band gap fluctuations as well as reduced carrier lifetime [10,45,46]. Series resis tance can be large (0.1–1.2 Ωcm2) in monocrystal devices because their thickness is nearly 1 mm, resulting in FF of 0.62–0.68. The device per formance is also sensitive to the conduction band offset, ΔEc . We used Δ Ec of 0.1 eV in this SCAPS model. Previous simulations used ΔEc of 0.10–0.34 eV for a CZTSe absorber with CdS as the buffer layer [47,48]. Choosing larger values for ΔEc in that range would increase VOC because the spike band alignment creates a large hole barrier adjacent to the
interface and suppresses interface recombination [49]. Although un certainties exist in some material and interface properties, the rela tionship of device performance to bulk lifetime, interface recombination, and doping density is robust. Although our CZTSe single crystals do not show signs of grain boundaries or secondary phases, the best PCE obtained from the CZTSe single crystal is still somewhat low, but VOC is similar to or slightly higher than the champion thin film device. Multiple possible explana tions exist. First, mechanically thinning and polishing crystals may lead to a high density of near-surface bulk defects that could reduce carrier lifetime and increase interface recombination. Second, alkali diffusion from the soda lime glass substrate into the kesterite film is a known contributor to cells with good PCE [50,51]. No alkali are present in our monocrystals, and Phuong et al. showed that intentional introduction of sodium increased lifetime in CZTS monocrystals as well [52]. 4. Conclusions We have revealed how hole density, carrier dynamics and device performance depend on cation composition for a series of CZTSe monocrystals. For crystals with more Cu-poor, Zn-rich composition, the carrier lifetime exceeds 200 ps and PCE is >5%. In contrast, crystals with more stoichiometric composition have carrier lifetimes of less than 20 ps and PCE below 2%. This trend is consistent with previous findings on CZTSe thin films. SRV is independent of cation composition, with values of 1x104 -1x105 cm/s. IRV at the CZTSe/CdS interface was similar to SRV at the CZTSe-air interface. Our experimental results informed device simulations that can pro vide insight into potentially fruitful research directions. Simulations showed that for CZTSe having low doping density on the order of 1015 cm 3, corresponding to low Cu content, device performance is limited by bulk carrier lifetime. In principle, higher VOC can be obtained with higher doping density. However, at higher Cu content and higher doping density, devices with longer bulk lifetimes become limited by interface recombination. VOC of 540 mV and PCE of 15% could be ach ieved for CZTSe with carrier lifetime of 4 ns and doping density of 6.3x1016 cm 3, corresponding to Crystal C in this work, provided the interface recombination velocity could be reduced by an order of magnitude, to 1x103 cm/s. Nonetheless, controlling bulk properties 5
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remains a key challenge going forward, especially minimizing the den sity of defects responsible for recombination centers and band tailing while holding doping at 1016 cm 3. For both monocrystals and thin films, further increases in PCE via reduction of the VOC deficit will require advances in bulk processing methodologies, including cation substitution, that further control the phase space and resulting defect chemistry.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge funding from a collaborative National Science Foundation grant, DMR-1507988 (Drexel University)/DMR1508042 (University of Delaware). The authors thank Hannes Hempel, Andrei Petsiuk, Thomas Unold, and Rainer Eichberger at Helmholtz Zentrum Berlin for assistance with TRTS experiments and useful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110255. References [1] S. Ahn, S. Jung, J. Gwak, A. Cho, K. Shin, K. Yoon, D. Park, H. Cheong, J.H. Yun, Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: on the discrepancies of reported band gap values, Appl. Phys. Lett. 97 (2010), 021905. [2] A. Walsh, S. Chen, S.-H. Wei, X.-G. Gong, Kesterite thin-film solar cells: advances in materials modelling of Cu2ZnSnS4, Adv. Energy Mater. 2 (2012) 400–409. [3] K. Ito, T. Nakazawa, Electrical and optical properties of stannite-type quaternary semiconductor thin films, Jap. J. Appl. Phys. 27 (1988) 2094–2097. [4] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater 4 (2014) 1301465. [5] H. Katagiri, K. Jimbo, M. Tahara, H. Araki, K. Oishi, The influence of the composition ratio on CZTS-based thin film solar cells, Mater. Res. Soc. Symp. Proc. 1165 (2009), 1165-M04-M01. [6] I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann, W.C. Hsu, A. Goodrich, R. Noufi, Co-evaporated Cu2ZnSnSe4 films and devices, Sol. Energy Mater. Sol. Cells 101 (2012) 154–159. [7] T.K. Todorov, K.B. Reuter, D.B. Mitzi, High-efficiency solar cell with earthabundant liquid-processed absorber, Adv. Mater. 22 (2010) E156–E159. [8] S. Chen, A. Walsh, X.G. Gong, S.H. Wei, Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers, Adv. Mater. 25 (2013) 1522–1539. [9] O. Gunawan, T. Gokmen, D.B. Mitzi, Suns-VOC characteristics of high performance kesterite solar cells, J. Appl. Phys. 116 (2014), 084504. [10] S. Bourdais, C. Chon� e, B. Delatouche, A. Jacob, G. Larramona, C. Moisan, A. Lafond, F. Donatini, G. Rey, S. Siebentritt, A. Walsh, G. Dennler, Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells? Adv. Energy Mater. 6 (2016) 1502276. [11] L. Yin, G. Cheng, Y. Feng, Z. Li, C. Yang, X. Xiao, Limitation factors for the performance of kesterite Cu2ZnSnS4 thin film solar cells studied by defect characterization, RSC Adv. 5 (2015) 40369–40374. [12] G. Brammertz, S. Oueslati, M. Buffi�ere, J. Bekaert, H.E. Anzeery, K.B. Messaoud, S. Sahayaraj, T. Nuytten, C. K€ oble, M. Meuris, J. Poortmans, Investigation of properties limiting efficiency in Cu2ZnSnSe4-based solar cells, IEEE J. Photovolt. 5 (2015) 649–655. [13] I.L. Repins, H. Moutinho, S.G. Choi, A. Kanevce, D. Kuciauskas, P. Dippo, C. L. Beall, J. Carapella, C. DeHart, B. Huang, S.H. Wei, Indications of short minoritycarrier lifetime in kesterite solar cells, J. Appl. Phys. 114 (2013), 084507. [14] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a highperformance solution-processed kesterite solar cell, Sol. Energy Mater. Sol. Cells 95 (2011) 1421–1436. [15] K. Wang, O. Gunawan, T. Todorov, B. Shin, S.J. Chey, N.A. Bojarczuk, D. Mitzi, S. Guha, Thermally evaporated Cu2ZnSnS4 solar cells, Appl. Phys. Lett. 97 (2010) 143508. [16] T. Gokmen, O. Gunawan, T.K. Todorov, D.B. Mitzi, Band tailing and efficiency limitation in kesterite solar cells, Appl, Phys. Lett. 103 (2013) 103506.
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Contents lists available at ScienceDirect
Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Effects of cation composition on carrier dynamics and photovoltaic performance in Cu2ZnSnSe4 monocrystal solar cells Siming Li a, 1, Michael A. Lloyd b, 1, Brian E. McCandless b, **, Jason B. Baxter a, * a b
Drexel University, Department of Chemical and Biological Engineering, Philadelphia, PA, 19104, USA University of Delaware, Institute of Energy Conversion, Newark, DE, 19716, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Kesterite Solar cell Photovoltaic Carrier dynamics Device performance CZTSe
Understanding the relationship of doping density, carrier lifetime, and interface recombination to device per formance is critical to designing solar cells with high power conversion efficiency (PCE). In turn, it is necessary to understand how bulk material composition determines doping density and carrier lifetime. The most efficient kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells have had Cu-poor, Zn-rich compositions, while more stoichiometric compositions have lower PCEs. However, thin films are grown under highly non-equilibrium conditions, complicating fundamental studies. Here we report on a set of CZTSe monocrystals with varied cation stoichiometry, enabling correlation of bulk composition to material and device properties without the complication of grain boundaries or secondary phases. Copper-poor, zinc-rich compositions (Cu/(Zn þ Sn) ¼ 0.77–0.90 and Zn/Sn ¼ 1.17–1.25) yield bulk carrier lifetimes longer than 200 ps and PCE >5%. In contrast, near-stoichiometric compositions, with Cu/(Zn þ Sn) > 0.90 and Zn/Sn < 1.15, have carrier lifetimes shorter than 20 ps and PCE <2%. CZTSe/CdS interface recombination velocity has a similar value to the CZTSe surface recombination velocity, with values of 104–105 cm/s determined by time-resolved terahertz spectroscopy and transport-recombination modeling. Device modeling reveals the dependence of open circuit voltage (VOC ) on doping density, carrier lifetime and interface recombination. For a crystal with low doping density of 1015 cm 3, the maximum VOC is limited by the bulk lifetime. Higher VOC can be attained with higher doping density, but interface recombination becomes more significant with increased lifetime and doping density. These simulations indicate limitations and potential pathways to high performance.
1. Introduction Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising candidate for photovoltaic applications because it has a direct band gap that is tunable from 1.0 to 1.5 eV by changing the S/Se ratio [1,2], with large absorp tion coefficient (>1x104 cm 1) [3]. The current champion CZTSSe de vice (Eg ¼1.13 eV) exhibited power conversion efficiency (PCE) of 12.6% [4]. Although the stoichiometric ratio of Cu, Zn, Sn, and Se elements in the kesterite structure is 2:1:1:4, photovoltaic (PV) device performance depends strongly on the composition. Virtually all thin film CZTSSe solar cells with PCE exceeding 8% have had Cu-poor and Zn-rich composi tions, typically with Cu/(Zn þ Sn) ratio of ~0.8 and Zn/Sn ratio of ~1.2 [5–8]. These conditions are reported to minimize the concentration of deleterious point defects and cluster defects, resulting in hole
concentrations of 1015-1016 cm 3 that are within a desirable range for high PCE [8]. PCE of CZTSSe thin film PVs is primarily limited by a large open circuit voltage (VOC ) deficit (Eg =q - VOC ), which is in the range of 600–900 mV depending on bandgap [4,9–11]. Low minority carrier lifetime [9,12,13], fast interface recombination [14,15], and band tails [12,16] have all been reported as limiting factors for VOC . An absorber with high doping density, long bulk lifetime and low interface recom bination rate is desired for an efficient solar cell. Understanding the effects of photoexcited carrier lifetime, recombination mechanisms, and doping density on device performance is critical to the design of efficient solar cells. In our previous work, we applied multiple ultrafast spectroscopies to investigate carrier dynamics and their relation to solar cell performance
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B.E. McCandless), [email protected] (J.B. Baxter). 1 Authors contributed equally to this work. https://doi.org/10.1016/j.solmat.2019.110255 Received 18 August 2019; Received in revised form 16 October 2019; Accepted 25 October 2019 Available online 15 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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100 mWcm 2 via an NREL-certified Silicon standard. External quantum efficiency (EQE) was measured using an Oriel monochromator using the AC method with a lock-in amplifier. TRTS used a regeneratively amplified Ti: Sapphire laser (Coherent Libra, 800 nm, 1 kHz, 50 fs pulse duration) coupled to a home-built reflectance spectrometer. The sample was photoexcited with a pump pulse whose wavelength was selected using an optical parametric amplifier, then probed with terahertz radiation generated and detected using ZnTe nonlinear crystals. Reflection geometry was used because the large thickness and moderate conductivity of the crystal resulted in negligible terahertz transmission for most samples at room temperature. The change in reflectance upon chopped photoexcitation was normal ized by the non-photoexcited signal to determine ΔR/R, which is pro portional to photoconductivity. The reflection TRTS configuration and analysis have been described in more detail elsewhere [17,27]. Carrier transport and recombination were modeled by solving the time-dependent continuity equation. Carriers are generated instanta neously at t ¼ 0 according to Beer’s Law. Absorption coefficients of 2 � 105 cm 1 and 6 � 104 cm 1 were used for 400 nm and 800 nm photons, respectively [28]. We assume that no electric fields exist, such that transport of photoexcited electrons and holes occurs by ambipolar diffusion, resulting in the same concentration profiles for both carriers. One-dimensional numerical solar cell simulations were performed using SCAPS, software developed at the University of Gent, Belgium. SCAPS solves the Poisson equation and the continuity equations for electrons and holes, including recombination at the junction interface [29]. Our model device consists of three semiconductor layers: ZnO, CdS and CZTSe. The bulk carrier lifetime in SCAPS for SRH recombination 1 can be estimated by τ ¼ σvN , where σ is the capture cross-section area, v T
for a CZTSe monocrystal [17]. The use of monocrystals grown under quasi-equilibrium conditions eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin-film growth. For that crystal, we found that the bulk lifetime of 260 ps led to a short minority diffusion length and incomplete carrier collection, resulting in low short-circuit current density (JSC ) and likely also affecting VOC [17]. Applying this approach to a set of monocrystals with a range of cation and anion compositions can provide insight into relationships between composition, dynamics, and device performance. Phuong et al. investigated the effect of S:Se ratio in CZTSSe monocrystals [18], but no analogous studies of cation composition in monocrystals have been reported. In this work, we investigated a series of high-quality CZTSe mono crystals with different cation compositions to understand their rela tionship to device performance and ultrafast carrier dynamics. We report on the crystals with Cu/(Zn þ Sn) ratios of 0.75–1.0 and Zn/Sn ratios of 1.3–1.0, which have doping density of 1x1015 - 1x1018 cm 3, respectively. Solar cell and Hall effect devices were fabricated to analyze the crystals’ electronic properties. Carrier dynamics were measured by time-resolved terahertz spectroscopy (TRTS), which is a non-contact probe of transient photoconductivity on sub-picosecond to nanosecond time scales that are relevant for charge carrier generation, transport, and recombination [19–21]. By fitting dynamics data with a transport-recombination model, bulk lifetime and surface recombina tion velocity were determined for each crystal. Bulk lifetime increased dramatically for Cu-poor, Zn-rich compositions compared to more stoichiometric compositions, while surface recombination velocity was not sensitive to composition. Interface recombination velocity for a CZTSe crystal with a standard CdS buffer layer was similar to the surface recombination velocity. Parameters determined experimentally were utilized in a ZnO/CdS/CZTSe solar cell device simulation using SCAPS software to provide guidance on how doping density, bulk lifetime, and interface recombination velocity influence solar cell performance.
is the thermal velocity of the carriers, and NT is trap density in units of cm 3 [30]. We assume that the carrier lifetime obtained from TRTS corresponds to SRH recombination via a mid-gap single-level trap. The interface recombination velocity was calculated by IRV ¼ σ vnT , where nT is trap density at the interface. Front-surface reflection was not included in the simulations unless specifically noted.
2. Experimental Experimental procedures for growing crystals, measuring their properties, and fabricating and measuring devices were reported in detail previously [17]. Briefly, centimeter-scale ingots of CZTSe were synthesized at temperatures below the peritectic point of 790 � C [22] as described by Bishop et al. [23]. A quartz ampoule was loaded with elemental precursors of 5–6 N purity, evacuated overnight and sealed at a base pressure of 5 � 10 6 Torr. CZTSe growth and grain ripening proceeded as the ampoule was held at 750 � C for 20 days. Heating was stopped and the ampoule cooled naturally to room temperature over 36 h. Monocrystals extracted from the multicrystalline ingot were me chanically thinned and planarized. Chemical-mechanical polishing was used to remove mechanical damage induced by grinding, using a solu tion of 0.125% bromine in methanol. The crystal then was etched for 10 min in the same solution to remove a surface layer ~4 μm thick. The finished crystals studied here were ~3 mm in diameter and 0.5–1.0 mm thick. Crystal composition was measured by x-ray fluorescence spectros copy (XRF, Oxford Instruments Xstrata). Majority carrier concentration and mobility were measured by Hall effect in the van der Pauw configuration in the dark at room temperature [24]. Solar cells were fabricated as in Ref. [25]. A 60 nm thick CdS emitter layer was deposited by chemical bath deposition. ZnO/ITO was sputtered through a circular mask to create a device with area of 0.8 mm2, which is the area used when reporting current density. The crystal was mounted on a Mo-coated glass substrate with Acheson Electrodag® graphite paste to create the back contact. An active device area of 0.75 � 0.07 mm2 was measured for a representative cell by the full-width half maximum of electroluminescence (EL) signal taken along a line profile across the circular device area using the setup described in ref [26]. JV charac teristics were measured with an OAI TriSOL solar simulator calibrated to
3. Results and discussion CZTSe monocrystals were prepared with cation compositions span ning a range from Cu-poor, Zn-rich to near-stoichiometric, with Cu/(Zn þ Sn) ¼ 0.75–1.0 and Zn/Sn ¼ 1.3–1.0. Compositions of the samples are shown in Fig. 1a. Dark Hall effect measurements show that CZTSe monocrystals exhibit p-type conduction with doping density from 1x1015 to 1x1018 cm 3 depending on cation composition, and hole mobility does not show any trend with cation composition, with values ranging from 46 to 165 cm2 =Vs [31]. The high variability in hole mobility from the Hall effect measurement is attributed to variations in crystal properties owing to their particular locations in the growth matrix, as the measurements of crystal thickness and Hall resistance are repeatable to within 20%. The hole mobility in CZTSe monocrystals is higher than hole mobility of CZTSe thin films, which typically are within the range of 1–10 cm2 =Vs [27,32]. We fabricated and tested solar cells on crystals with the most promising compositions, Fig. 1b. Cells with PCE above 5% have Cu/(Zn þ Sn) of 0.77–0.90 and Zn/Sn of 1.17–1.25. In contrast, crystal D with Cu/(Zn þ Sn) ¼ 0.92 and Zn/Sn ¼ 1.12 had PCE less than 2%. Mono crystal PVs shows the same trend as in thin film PVs: devices with highest PCE have Cu-poor, Zn-rich compositions, while more stoichio metric compositions suffer from low PCE [8]. The device performance parameters of each cell are listed in Table S1. Table S2 indicates sta tistics of four devices each with Compositions A and B. Our best CZTSe monocrystal PV had JSC ¼ 29 mA/cm2, VOC ¼ 430 mV, fill factor (FF) ¼ 64%, and PCE ¼ 7.9%, which is remarkable considering the millimeter thickness of the crystal. Although the per formance is lower than the best CZTSe thin film device, which has PCE 2
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Fig. 1. (a) Cation compositions of CZTSe monocrystals studied in this work. (b) JV curves of CZTSe monocrystals with compositions shown in (a). (c) Normalized carrier dynamics of the same CZTSe monocrystals photoexcited at 1.55 eV to generate initial carrier densities of 3-9x1017 cm 3. Colors in b and c correspond to crystal compositions in a. Crystal F did not show any discernible TRTS signal. Devices were not fabricated on crystals E and F.
over 11% [33,34], the monocrystal device has a higher VOC than the record thin film device. Diode analysis (Fig. S1) for the PVs with PCE above 6% indicates an ideality factor of 1.6–1.9, which implies that the dark saturation current is limited by recombination in the depletion region [35]. Next we discuss our analysis of recombination dynamics to differentiate the relative importance of bulk recombination and surfa ce/interface recombination and their relation to device performance. We measured carrier dynamics of the CZTSe monocrystals using TRTS to determine how recombination rates and mechanisms depend on cation composition. TRTS is a noncontact probe of transient photocon ductivity with sub-picosecond temporal resolution and ~1 mm spot size, allowing carrier dynamics of CZTSe monocrystals to be measured without complications arising from the full device. Fig. 1c shows the transient photoconductivity of CZTSe monocrystals of different cation composition. Crystals were photoexcited at 1.55 eV, which is close to the bandgap of CZTSe of ~1 eV and has penetration depth of ~180 nm. Dynamics are independent of initial carrier concentration under high injection, as shown in Fig. S2, indicating that 1st order trapping and/or Shockley-Read-Hall (SRH) recombination are the dominant loss pro cesses. This conclusion is consistent with our previous results on CZTSSe thin film and CZTSe monocrystal materials [17,36]. Fig. 1c shows that the carrier lifetime is in the range of 0.1–10 ns for monocrystals with cation composition in the more Cu-poor, Zn-rich regime. These relatively long lifetimes are correlated to PCE above 5% for the corresponding PV devices. For the crystals with cation compo sition in the near-stoichiometric region, carrier lifetime is approximately 10 ps, corresponding to PCE below 2%. Short SRH carrier lifetime results from high density of recombination centers and limits the device per formance. Previous studies on kesterite thin films found that the den sities of some point defects (such as CuSn and SnZn antisites) and cluster defects ([2CuZn þ SnZn]) are higher for more stoichiometric
compositions [14]. Those defects may act as recombination centers [8, 14], although identifying the chemical nature of the recombination centers is beyond the scope of this work. To distinguish the roles of bulk SRH recombination and surface recombination, we applied the transport-recombination model devel oped in [17] to the carrier dynamics of CZTSe monocrystals photoex cited at 1.55 eV and 3.10 eV, as shown in Fig. 2 and Table 1. Photoexcitation at 3.10 eV is far above the bandgap, creating a carrier distribution closer to the surface and making surface recombination more influential than in the case of 1.55 eV photoexcitation. Crystals D and E, which have near-stoichiometric compositions have effective lifetime of ~20 ps, with carrier dynamics that are similar when photo exciting at 1.55 eV and 3.10 eV, as shown in Fig. 2a, S3a. In this case, dynamics are dominated by the short bulk carrier lifetime; surface recombination has little effect and therefore SRV cannot be determined. As bulk lifetime increases for more Cu-poor, Zn-rich crystals, surface recombination begins to contribute to the dynamics; for the crystal in Fig. 2b, the dynamic is determined by both bulk lifetime (~260 ps) and Table 1 Hole density (p) from Hall effect measurement of monocrystals, and surface/ interface recombination velocity (SRV/IRV) and bulk lifetime (τ) obtained by fitting TRTS data with a transport-recombination model using ambipolar mobility of 1–30 cm2/Vs. Samples E D B C A CdS/A
p (cm 18
3
)
~10 ~1018 8.7x1016 6.3x1016 1.8x1015
SRV/IRV (cm/s)
τ (ns)
– – 2-5x104 4-8x104 3-5x104 3-4 x104
0.015-0.017 0.013-0.018 0.22-0.27 >4 >5 >5
Fig. 2. Normalized TRTS carrier dynamics of (a) Crystal D, (b) Crystal B, and (c) Crystal A photoexcited at 3.1 eV (blue) and 1.55 eV (red). The gray regions are the simulated dynamics based on the parameters listed in Table 1. 3
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surface recombination. As effective carrier lifetime continues to increase to the multi-nanosecond scale (Fig. 2c, S3b), the large difference in dynamics with photoexcition energy indicates that surface recombina tion is dominant. SRV is 3-8 x104 cm/s while bulk lifetime is at least 4 ns. A more accurate bulk lifetime cannot be determined in this case because of the strong influence of surface recombination. While the bulk lifetime increased by more than a factor of 20 with decreasing Cu content, Table 1 indicates that SRV is in the range of 2–8 x104 cm/s and appears to be independent of cation composition. While SRV was found in the process of determining bulk lifetime of uncoated monocrystals, the CZTSe/buffer interface recombination ve locity (IRV) is actually the quantity of interest for PV devices. Multiple groups have reported that interface recombination can limit the per formance of CZTSSe thin film solar cell devices [14,15]. The spike-like band alignment within CZTSe/CdS indicates that interface recombina tion is most likely due to interface defects rather than inappropriate conduction band offsets [10]. To measure interface recombination be tween CZTSe and CdS, we coated approximately 60 nm CdS on the top of CZTSe Crystal A by chemical bath deposition, and photoexcited through the CdS side. Crystal A was chosen for this experiment because it has long bulk lifetime and dynamics limited by surface recombination. Fig. 2c shows the dynamics upon photoexciting Crystal A at 1.55 eV, revealing that dynamics of CZTSe/CdS are similar to dynamics of bare CZTSe. This result indicates that IRV is similar to SRV, which is consistent with findings of Redinger and Unold, whose measurements of PL yield also showed that the surface recombination velocity is not affected by coating with CdS buffer layer [37]. It is therefore likely that the surface recombination velocity is limited by crystal preparation prior to junction formation, which may require processing conditions dictated by the bulk composition. Furthermore, the cationic termination may affect surface recombination behavior as suggested by the device ben efits of a Zn-rich surface [10,38,39]. To obtain the value of IRV, we again studied the dependence of carrier dynamics on excitation energy, this time with a CZTSe/CdS heterostructure and using 1.55 eV and 1.03 eV photons, Fig. S3c, because CdS absorbs photons at 3.10 eV. Fitting with the transportrecombination model yielded IRV and bulk lifetime shown in Table 1. No change in bulk lifetime of CZTSe was observed upon addition of CdS; and the IRV had an upper limit of 3-4x104 cm/s, which is close to the value of SRV obtained from the same CZTSe single crystal. We use the term upper limit because we cannot distinguish between IRV and elec tron transfer into CdS followed by surface trapping. The IRV between CZTSe/CdS measured here is similar to the value of 103 -104 cm= s that was previously reported for the CZTSSe/CdS interface as determined by measurement of the pre-factor of reverse saturation current, J00 [40]. To further understand the influence of carrier dynamics on device parameters, we simulated PV device operation using SCAPS. When possible, we used parameters obtained from our measurements and previous literature results. The properties of each layer are shown in Table S3. Electron and hole mobilities were averages of the Cu-poor, Znrich crystals because mobility did not show any particular trend with composition. IRV was measured on Crystal A, as previously described. The doping density of CdS was adjusted to be 5.8x1016 cm 3 to match the EQE and JV curve of Crystal C, as shown in Fig. 3 and S4. While the aforementioned properties were assumed to be common across all de vices, individually measured lifetimes and hole densities for Crystals A – C, Table 1, were used to simulate different devices as indicated below. The bulk lifetimes listed in Table 1 are the upper limit of the minority carrier lifetime for three reasons. First, TRTS under high injection measures both the electron and hole dynamics, so the bulk lifetime (τb ) obtained from the model is τe þ τh [30]. Second, CZTSe solar cells are operated under low injection condition; our group and other showed that carrier recombination is slightly faster at lower fluence [17,41]. Third, carrier de-trapping in CZTSSe could result in an overestimate of the free carrier lifetime [42]. Fig. 3 shows the experimental and SCAPS-simulated EQEs of Crystals
B and C. Poor carrier collection for excitation wavelengths <540 nm is due to absorption by CdS and ZnO. Our previous paper and work of other groups demonstrated that the minority carrier diffusion length can limit EQE for CZTSSe [17,43]. The minority carrier diffusion length was ob tained by matching the SCAPS simulated EQE spectrum to the experi mental EQE (for λ > 600 nm), revealing minority carrier diffusion lengths of 0.18 � 0.02 μm and 1.0 � 0.2 μm for Crystals B and C, respectively, as shown in Fig. 3. The minority carrier diffusion length is pffiffiffiffiffi found from Ld ¼ Dτ. The diffusion length of Crystal B yields an elec tron mobility of 55 � 23 cm2 =Vs, which is consistent with the mobility obtained from the combination of TRTS frequency spectra and Hall ef fect [17]. However, we note that the resulting ambipolar mobility of 67 cm2 =Vs is somewhat larger than the range of ambipolar mobilities that provided the best fit for the transport-recombination model (1–30 cm2/V). This discrepancy may be due to inaccurate absorption coefficient chosen for the transport-recombination model, high varia tion in dark monocrystal Hall mobility, or injection-dependence of mobilities. Changing IRV from 1x103 to 1x106 cm/s did not significantly shift the EQE spectra, indicating that interface recombination does not limit carrier collection for the CZTSe monocrystal. While increasing lifetime will significantly improve EQE and hence JSC , the VOC deficit of CZTSSe has been reported as the main limitation to the performance of kesterite solar cells [4,9]. The VOC deficit of CZTSSe arises from several possible factors including low minority carrier life time [9,12,13], interface recombination [14,15], and band tails [12,16]. Both VOC and FF are governed by reverse saturation current in the depletion region. Increasing carrier lifetime and/or decreasing IRV could increase VOC FF, and PCE [30]. The depletion width is determined by the doping density of CZTSe, which varies based on Cu/(Zn þ Sn) composition. Doping density of our CZTSe monocrystals ranges from 1.8x1015 cm 3 for more Cu-poor, Znrich compositions to 2.5x1018 cm 3 for more stoichiometric composi tions. This trend is consistent with hole concentrations reported for thin films: 1016 -1018 cm 3 in Cu-poor region and 1017 - 1019 cm 3 in the stoichiometric region [8]. Our monocrystals with nanosecond-scale carrier lifetime have doping density of <6 x1016 cm 3. Long lifetime, low IRV and high doping density of CZTSe absorber are desired to achieve a higher maximum VOC . Fig. 4 shows the simu lated VOC as a function of bulk lifetime and interface recombination velocity for CZTSe hole densities of 1.8x1015 cm 3 and 6.3x1016 cm 3, corresponding to those measured for Crystals A and C, along with simulated band diagrams. Simulated FF and PCE follow the same trend, as shown in Fig. S5. With the lower doping density (Crystal A) and corresponding larger depletion width of >600 nm, IRV does not limit
Fig. 3. Experimental and simulated normalized quantum efficiency spectra for Crystals B (black) and C (red). 4
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Fig. 4. (a,b) Simulated band alignment of ZnO/CdS/CZTSe, and (c,d) contour plot of VOC from SCAPS simulations as a function of bulk lifetime and IRV when doping density of CZTSe is (a,c) 1.8x1015 cm 3and (b,d) 6.3x1016 cm 3. Panels c and d share the same color legend.
VOC for the range studied (IRV<106 cm/s and τ < 10 ns). Instead, VOC is controlled by the SRH lifetime. As lifetime increases from 10 ps to 10 ns, VOC increases from 170 mV to 440 mV, roughly corresponding to the dynamics and device data for the set of monocrystals shown in Fig. 1. Since the measured IRV is less than 105 cm/s, reducing interface recombination may not improve the device performance significantly, which is consistent with the conclusion found by Bar et al. that interface recombination plays a less significant role than bulk properties in CZTSSe/CdS thin film devices with S/(S þ Se) ¼ 0.1 [44]. However, the maximum VOC is below 500 mV with hole density of 1.8x1015 cm 3 even for bulk lifetime of over 10 ns. Increasing the doping density enables larger maximum VOC because of larger built-in potential, if sufficiently long lifetime and small IRV can be maintained. With doping density of 6.3x1016 cm 3, the depletion width is only 79 nm, resulting in more significant interface recombina tion. Furthermore, the smaller depletion width requires larger diffusion length to ensure collection of minority carriers. When the bulk lifetime is less than 50 ps, VOC is below 320 mV and is primarily determined by bulk lifetime. As bulk lifetime increases into the 10-ns range, interface recombination becomes the primary limitation to effective lifetime and VOC . With IRV of 5x104 cm/s measured for our monocrystal, the simu lated VOC is below 390 mV despite bulk lifetime above 5 ns. By reducing IRV from 5x104 to 1x103 cm/s, VOC could be increased from 390 mV to 550 mV for a bulk lifetime of 10 ns. Interface passivation could improve device performance in that case. While the SCAPS simulations provide important insight, we note several simplifications and sources of uncertainty in the model compared to monocrystal or thin film devices. The SCAPS simulation did not include the band-tails, series resistance (Rs ), or shunt resistance (Rsh ). Cation-based point defects and defect clusters can reduce VOC by 50–90 mV due to electrostatic potential fluctuations and band gap fluctuations as well as reduced carrier lifetime [10,45,46]. Series resis tance can be large (0.1–1.2 Ωcm2) in monocrystal devices because their thickness is nearly 1 mm, resulting in FF of 0.62–0.68. The device per formance is also sensitive to the conduction band offset, ΔEc . We used Δ Ec of 0.1 eV in this SCAPS model. Previous simulations used ΔEc of 0.10–0.34 eV for a CZTSe absorber with CdS as the buffer layer [47,48]. Choosing larger values for ΔEc in that range would increase VOC because the spike band alignment creates a large hole barrier adjacent to the
interface and suppresses interface recombination [49]. Although un certainties exist in some material and interface properties, the rela tionship of device performance to bulk lifetime, interface recombination, and doping density is robust. Although our CZTSe single crystals do not show signs of grain boundaries or secondary phases, the best PCE obtained from the CZTSe single crystal is still somewhat low, but VOC is similar to or slightly higher than the champion thin film device. Multiple possible explana tions exist. First, mechanically thinning and polishing crystals may lead to a high density of near-surface bulk defects that could reduce carrier lifetime and increase interface recombination. Second, alkali diffusion from the soda lime glass substrate into the kesterite film is a known contributor to cells with good PCE [50,51]. No alkali are present in our monocrystals, and Phuong et al. showed that intentional introduction of sodium increased lifetime in CZTS monocrystals as well [52]. 4. Conclusions We have revealed how hole density, carrier dynamics and device performance depend on cation composition for a series of CZTSe monocrystals. For crystals with more Cu-poor, Zn-rich composition, the carrier lifetime exceeds 200 ps and PCE is >5%. In contrast, crystals with more stoichiometric composition have carrier lifetimes of less than 20 ps and PCE below 2%. This trend is consistent with previous findings on CZTSe thin films. SRV is independent of cation composition, with values of 1x104 -1x105 cm/s. IRV at the CZTSe/CdS interface was similar to SRV at the CZTSe-air interface. Our experimental results informed device simulations that can pro vide insight into potentially fruitful research directions. Simulations showed that for CZTSe having low doping density on the order of 1015 cm 3, corresponding to low Cu content, device performance is limited by bulk carrier lifetime. In principle, higher VOC can be obtained with higher doping density. However, at higher Cu content and higher doping density, devices with longer bulk lifetimes become limited by interface recombination. VOC of 540 mV and PCE of 15% could be ach ieved for CZTSe with carrier lifetime of 4 ns and doping density of 6.3x1016 cm 3, corresponding to Crystal C in this work, provided the interface recombination velocity could be reduced by an order of magnitude, to 1x103 cm/s. Nonetheless, controlling bulk properties 5
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remains a key challenge going forward, especially minimizing the den sity of defects responsible for recombination centers and band tailing while holding doping at 1016 cm 3. For both monocrystals and thin films, further increases in PCE via reduction of the VOC deficit will require advances in bulk processing methodologies, including cation substitution, that further control the phase space and resulting defect chemistry.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge funding from a collaborative National Science Foundation grant, DMR-1507988 (Drexel University)/DMR1508042 (University of Delaware). The authors thank Hannes Hempel, Andrei Petsiuk, Thomas Unold, and Rainer Eichberger at Helmholtz Zentrum Berlin for assistance with TRTS experiments and useful discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110255. References [1] S. Ahn, S. Jung, J. Gwak, A. Cho, K. Shin, K. Yoon, D. Park, H. Cheong, J.H. Yun, Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: on the discrepancies of reported band gap values, Appl. Phys. Lett. 97 (2010), 021905. [2] A. Walsh, S. Chen, S.-H. Wei, X.-G. Gong, Kesterite thin-film solar cells: advances in materials modelling of Cu2ZnSnS4, Adv. Energy Mater. 2 (2012) 400–409. [3] K. Ito, T. Nakazawa, Electrical and optical properties of stannite-type quaternary semiconductor thin films, Jap. J. Appl. Phys. 27 (1988) 2094–2097. [4] W. Wang, M.T. Winkler, O. Gunawan, T. Gokmen, T.K. Todorov, Y. Zhu, D.B. Mitzi, Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency, Adv. Energy Mater 4 (2014) 1301465. [5] H. Katagiri, K. Jimbo, M. Tahara, H. Araki, K. Oishi, The influence of the composition ratio on CZTS-based thin film solar cells, Mater. Res. Soc. Symp. Proc. 1165 (2009), 1165-M04-M01. [6] I. Repins, C. Beall, N. Vora, C. DeHart, D. Kuciauskas, P. Dippo, B. To, J. Mann, W.C. Hsu, A. Goodrich, R. Noufi, Co-evaporated Cu2ZnSnSe4 films and devices, Sol. Energy Mater. Sol. Cells 101 (2012) 154–159. [7] T.K. Todorov, K.B. Reuter, D.B. Mitzi, High-efficiency solar cell with earthabundant liquid-processed absorber, Adv. Mater. 22 (2010) E156–E159. [8] S. Chen, A. Walsh, X.G. Gong, S.H. Wei, Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers, Adv. Mater. 25 (2013) 1522–1539. [9] O. Gunawan, T. Gokmen, D.B. Mitzi, Suns-VOC characteristics of high performance kesterite solar cells, J. Appl. Phys. 116 (2014), 084504. [10] S. Bourdais, C. Chon� e, B. Delatouche, A. Jacob, G. Larramona, C. Moisan, A. Lafond, F. Donatini, G. Rey, S. Siebentritt, A. Walsh, G. Dennler, Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells? Adv. Energy Mater. 6 (2016) 1502276. [11] L. Yin, G. Cheng, Y. Feng, Z. Li, C. Yang, X. Xiao, Limitation factors for the performance of kesterite Cu2ZnSnS4 thin film solar cells studied by defect characterization, RSC Adv. 5 (2015) 40369–40374. [12] G. Brammertz, S. Oueslati, M. Buffi�ere, J. Bekaert, H.E. Anzeery, K.B. Messaoud, S. Sahayaraj, T. Nuytten, C. K€ oble, M. Meuris, J. Poortmans, Investigation of properties limiting efficiency in Cu2ZnSnSe4-based solar cells, IEEE J. Photovolt. 5 (2015) 649–655. [13] I.L. Repins, H. Moutinho, S.G. Choi, A. Kanevce, D. Kuciauskas, P. Dippo, C. L. Beall, J. Carapella, C. DeHart, B. Huang, S.H. Wei, Indications of short minoritycarrier lifetime in kesterite solar cells, J. Appl. Phys. 114 (2013), 084507. [14] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a highperformance solution-processed kesterite solar cell, Sol. Energy Mater. Sol. Cells 95 (2011) 1421–1436. [15] K. Wang, O. Gunawan, T. Todorov, B. Shin, S.J. Chey, N.A. Bojarczuk, D. Mitzi, S. Guha, Thermally evaporated Cu2ZnSnS4 solar cells, Appl. Phys. Lett. 97 (2010) 143508. [16] T. Gokmen, O. Gunawan, T.K. Todorov, D.B. Mitzi, Band tailing and efficiency limitation in kesterite solar cells, Appl, Phys. Lett. 103 (2013) 103506.
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