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Fabrication and device characterization of potassium fluoride solution treated CZTSSe solar cell
Accepted Manuscript Fabrication and device characterization of potassium fluoride solution treated CZTSSe solar cell Tanka Raj Rana, JunHo Kim, Jun-Hyoung Sim, Kee-Jeong Yang, Dae-Hwan Kim, Jin-Kyu Kang PII:
S1567-1739(17)30199-2
DOI:
10.1016/j.cap.2017.07.007
Reference:
CAP 4545
To appear in:
Current Applied Physics
Received Date: 28 May 2017 Revised Date:
25 June 2017
Accepted Date: 10 July 2017
Please cite this article as: T.R. Rana, J. Kim, J.-H. Sim, K.-J. Yang, D.-H. Kim, J.-K. Kang, Fabrication and device characterization of potassium fluoride solution treated CZTSSe solar cell, Current Applied Physics (2017), doi: 10.1016/j.cap.2017.07.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication and Device characterization of Potassium Fluoride Solution Treated CZTSSe Solar Cell Tanka Raj Rana1, JunHo Kim1, Jun-Hyoung Sim2, Kee-Jeong Yang2, Dae-Hwan Kim2, Jin-
1
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Kyu Kang2 Department of Physics, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon
2
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22012, Republic of Korea
Convergence Research Centre for Solar Energy, DGIST, 333 Techno jungang-daero,
*JunHo Kim ([email protected]) *Kee-Jeong Yang ([email protected])
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Abstract
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Dalseong-gun, Daegu 42988, Korea
Post deposition treatment (PDT) for Cu2ZnSn(S,Se)4 (CZTSSe) was carried out by simply dipping the absorber into the KF solution at 80 oC. The dipping time of absorber in KF
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solution was found to be crucial to device parameters of CZTSSe solar cell. The K-doping improved the solar cell efficiency from 4.4 % to 7.6 % by 1 min. dipping whereas the longer than 5 min dipping solar cells showed distorted kink J-V curves. The activation energy of
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CZTSSe solar cell was increased upto 1 min KF treatment from 0.83 eV to 0.92 eV which indicates interface recombination is reduced significantly. However, the activation energies of 5 min and 10 min dipping solar cells were found to be 0.81 eV and 0.63 eV where dominant recombination was interface recombination. Furthermore, trap energies of 49 meV and 298 meV of pristine CZTSSe solar cell were modified to 33 meV and 117 meV for 1 min treated CZTSSe solar cell. Trap energies of 5 min were calculated to be 112 meV and 147 meV. The proper KF doping passivated the shallow as well as deep defects of CZTSSe solar cell which is reflected in photovoltaic performances directly.
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1. Introduction The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell is emerging as promising replacement for the Cu(In,Ga)Se2 (CIGS) solar cell which includes expensive and rare earth elements, In and
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Ga. CZTSSe, consisting of earth abundant and nontoxic elements, has suitable photovoltaic properties such as high absorption coefficient (α >104 cm-1) and tunable band gap energy 11.5 eV [1, 2]. Although CZTSSe has promising photovoltaic properties, it showed still lower
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solar cell performance than CIGS. The world champion CZTSSe solar cell fabricated by hydrazine-based process demonstrated power conversion efficiency (PCE) of 12.6%, whereas
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CIGS reached up to the best PCE of 22.6% [3, 5]. Especially, the open circuit voltage (VOC) deficit is now major limiting factor in the development of CZTSSe solar cell. Series of experiments to overcome low Voc of CZTSSe solar cell have been done, which are aimed to reduce deep point defect and secondary phases. The close ionic size of Cu, Zn and Sn is
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responsible for the cation disorder in kesterite CZTSSe, which induces deep defects in CZTSSe and results in severe band tailing and potential fluctuation. The cation disorder can be reduced by replacing Cu or Zn with larger sized atoms like Ag for Cu, Cd for Zn [6,7].
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Along with cations substitution, doping with alkali metals and antimony also has shown beneficial effects on CZTSSe improving crystallinity and charge carrier concentration [8-13].
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Recently, CIGS has shown jump in record efficiency by using the post deposition treatment (PDT) of alkali elements. At the beginning, it was believed that diffused Na from soda lime glass (SLG) to CIGS increased the PCE by improving structural properties of GIGS during the film growth. However, CIGS grown on Na-free substrate with subsequent addition of Na also showed high PCE [14-16]. This benign effect of Na on solar cell performance motivated further study of alkali doping into CIGS and CZTSSe. CIGS solar cell doped with the other
ACCEPTED MANUSCRIPT alkali elements such as Li, K, Rb and Cs also showed positive effects on photovoltaic performance. Several reasons have been proposed to explain the beneficial effects of alkali doping. Na-
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doping improves PCE of CIGS solar cell by passivating the defects in interface and grain boundaries and by increasing carrier concentrations [17-19]. KF PDT modifies the absorber surface into Cu-depleted surface containing large amounts of K. On the modified surface, thinner CdS buffer layer can grow with good quality, which results in increase of short circuit
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current density (Jsc) by reducing external quantum efficiency (EQE) loss in blue photon
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region [20,21]. The interface recombination between CdS/CIGS was also found to be significantly reduced by using KF PDT, which is explained by lowering VBM [22] with the reduction of the charge recombination, K (Na) doping increase the PCE of CZTSSe solar cell with improvement of open circuit voltage (Voc), fill factor. Alkali metals doping especially with Na and K having smaller radii compared to other heavy alkali metals resulted in larger
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CZTSSe grain sizes which enhanced carrier density by reducing carrier recombination [12]. Furthermore, K-doping provides an opportunity for optimization of n-type window layer by
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which JSC can be improved due to collection of high energy photons [12]. Alkali doping is reported to enhance p-type conductivity with increased hole density in the absorber, for
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which several mechanisms are proposed. Grain boundary (GB) mechanism states that Na is segregated at GB of CIGS with high concentration [23-25], which passivates the donor like defects at GB such as Incu, VSe resulting in the increase of net hole concentrations [23, 24, 2628]. However, Na doping is also observed to increase hole concentration of grain interior throughout CIGS layer, which has driven to propose new mechanism: Na-dopants creates new accepter by forming NaIn (NaGa) antisites or by eliminate the donor sites InCu (GaCu), which increase the hole concentration. But formation of antisites NaIn (NaGa) and reduction of donor sites InCu (GaCu) are not favorable according to the first principle calculations [29-31].
ACCEPTED MANUSCRIPT So another mechanism is proposed, where Na-dopants forms electrically inactive NaCu in CIGS due to lowest formation energy at higher temperature [32]. NaCu is unstable at low temperature so Na diffuses out leaving Cu unoccupied forming VCu in CIGS grain [33,34].The Na on absorber surface is dissolved out during rinsing process for absorber, by
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which Cu-depleted surface is produced. Thus, surface and GBs of CIGS are usually Cudepleted by alkali doping. VCu is shallow acceptor in CIGS with ionization level about 30 meV above valence band maximum (VBM) which enables the intrinsic p-type conductivity
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[20, 35]. Formation energy for K-doping is higher than that for Na-doping at the same lattice so that K is difficult to be incorporated into CIGS [32]. However, K-doping leads to the
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formation of K-enriched surface layer (Cu-free, K-In-Se compounds). Experimental results showed that large amounts of K could be doped into CIGS grains at higher temperatures [3638]. KF PDT is usually carried out after Na-doping, and then Na is replaced by K and Na may diffuse towards the surface and Mo back contact layer. VCu is increased in CIGS grain
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because of out-diffusion of Na and K after rinsing and cooling. Thus p-type conductivity is enhanced after KF PDT [32]. Besides effects mentioned above, K-doping in CZTS is
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reported to enhance (112)-preferred growth and suppress the secondary phase of ZnS [39]. Various techniques can be adopted for alkali incorporation into the absorber layer. The alkali
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elements could be doped during synthesis of absorber or by PDT of absorber layer [40-42]. The most common method till now is utilization of co-evaporation process. The introduction of alkali elements to the absorber can be controlled systematically by evaporation, which is very effective for alkali free flexible substrate. There are also different methods reported for alkali treatment by evaporation such as KF or NaF, whcih are evaporated onto CIGS absorber layer in the presence of Se at higher temperature and treated with or without subsequent selenization [20, 43, 44]. A few of solution method are reported for potassium doping into CZTS (CZTSSe), where alkali source is added to the precursor solution of CZTS (CZTSSe)
ACCEPTED MANUSCRIPT and it is spin-coated on to the film [12, 40]. But in our experiment here, we carried out the PDT process simply by dipping in the KF aqueous solution, which is similar to KCN etching process. This solution process could be attractive method for CZTSSe absorbers fabricated by using vacuum- and non-vacuum-based method, which can be applied as an alternative to
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toxic KCN etching process also. We performed KF PDT for CZTSSe and studied its effect on solar cell efficiency along with characterization of the modified surface of CZTSSe. To understand the modification of defects during KF treatment, electrical characterizations of
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solar cells such as c-f-T and J-V-T were employed.
Experimental
CZTSSe solar cells were fabricated by RF sputtering process. The devices consisted of SLG/Mo/CZTSSe/CdS/i-ZnO/AZO structure and fabrication process for each layer was same
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as previous report except KF treatment [45]. The precursor was made of Cu/SnS/ZnS/Mo and post-annealed by using RTP process at 510 o C for 20 min using only selenium source [45]. We carried out KF treatment experiment to post-selenized CZTSSe absorbers. The absorbers
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layers were dipped into the solution composed of deionized water and 4.5 wt % potassium fluoride KF at 80 oC using hot plate. We varied KF treatment time such as 30 sec, 1 min, 5
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min and 10 min. After dipping the absorbers, we cleaned the samples with di-water and further layers CdS/i-ZnO/AZO were deposited to make the completed devices. So, we fabricated five types of solar cells i.e. pure CZTSSe (reference), 30 sec KF treated CZTSSe solar cell (A), 1 min KF treated CZTSSe solar cell (B), 5 min KF treated CZTSSe solar cell (C) and 10 min KF treated CZTSSe solar cell (D).
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2.1. Characterization The surface morphologies of films were studied by using by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-7001F). The structural properties of the films were characterized by X-ray diffraction (XRD) with a high resolution XRD machine (Rigaku, phase purity
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Smart LAB) equipped with Cu-Kα source (=1.5412 Å ). To investigate
precisely, we measured Raman spectra by using a Raman scattering system (Spectro co.) equipped with an excitation laser of λ=532 nm (irradiation power on sample < 1 mW). The
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X-ray photoelectron spectroscopy (XPS) was performed by using PHI 5000 Versa Probe II. The current density−voltage (J-V) curve of the CIGS solar cell was measured by using source
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meter (Keithley 2400) under AM 1.5G filtered illumination of 1000 Wm-2 Xe lamp (Abet Technology), which was calibrated with Si reference cell. The J-V characteristics were studied in the temperature range (300 K to 90 K) under dark, white and filtered light illumination. The external quantum efficiency (EQE) was measured by using Xe light source
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and monochromator combined with light chopper and lock-in amplifier (McScience Inc.). Admittance spectroscopy (AS) was performed to analysis defect in the solar cells using LCR meter (E4980A, Agilent) which probes from 20 Hz to 2 MHz in the temperature range of
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300K-90K. All C-f scans were carried out at an ac voltage of 30 mV under the dark condition.
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The same LCR meter was used to measure capacitance-voltage (C-V) profile.
Result and discussion
3.1. Characterization of KF treated CZTSSe absorber layer The surface morphologies of bare CZTSSe and KF treated CZTSSe are shown in Fig. 1(a) and (b-e), respectively. The post-selenized CZTSSe showed compact and good crystalline texture. Figure 1(b-e) presents the SEM images of KF treated CZTSSe for 30 sec, 1 min, 5 min and 10 min. The small pores were formed on the CZTSSe surface and few white dot like
ACCEPTED MANUSCRIPT particles were observed for 30 sec and 1 min KF treated samples. The white dot like particles could be KF compounds. But for longer treated like 5 min and 10 min samples showed larger white particles attached on the CZTSSe surfaces. Therefore excessive potassium particles were remained on CZTSSe surface for longer KF treated samples even after a distilled water
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rinsing. Thus, longer potassium treatment damages surface texture which is clearly observed on the surface morphology of absorber layer. The chemical composition of one point in the absorber was observed to be Cu:Zn:Sn:S:Se=1.44:1:1.36:0.19:4.2, which was Cu-poor and
the probed position, small amount was S was detected.
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Sn-rich. The chemical ratio was a little bit fluctuated depending on the probed position. In all
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We performed XPS measurements for bare CZTSSe and 30 sec, 1 min and 5 min KF treated CZTSSe samples. Figure 2(a) shows survey spectra of all samples, where K signals are detected for the KF treated samples. Figure 2 (b) shows the magnified spectra of K 2p3/2 of CZTSSe with and without KF treatment. The increase of K 2p3/2 peak intensity with increase
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of PDT times implies that amount of K on films is also increasing with PDT time as expected. Furthermore, Cu 2p3/2 peaks of those samples are presented in Fig. 2(c), where Cu signal is observed for all samples. It is reported that for CIGS absorber KF PDT modified it into Cu-
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depleted surface. However, for sputtered CIGS solar cell Cu-depleted surface was not
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observed [46]. The formation of Cu-depleted layer might be dependent on absorber deposition process or KF PDT process. In our case, incorporation of potassium could be more followed by the interstitial than substitution process; therefore Cu-peaks of KF treated samples were not found to be reduced significantly. The Cu-peak shifts were observed randomly, peak shift towards the higher binding energy could be due to possibly oxidation of Cu, and however we cannot say clearly why binding energy is decreased after KF treatment. In order to get a better insight to surface chemistry of potassium incorporated CZTSSe by chemical PDT method requires further more systematic study. Figures (d), (e) and (f) show
ACCEPTED MANUSCRIPT spectra for Zn, Sn, and Se, respectively. It was observed that in the 5’ PDT absorber Zn is reduced more compared to other elements. Figure 3(a) represents the XRD results of the fabricated solar cells, where the dominant characteristic peak (112) is observed at 2θ of 27.16o~27.29o and additional wurtzite ZnO
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peak (002) is also seen for all types of solar cell. The XRD pattern of the solar cells showed polycrystalline kesterite crystal structure of CZTSSe which is very similar to the earlier reports [47, 48]. Figure 3(c) shows lattice constants calculated from XRD results of Fig. 3(a).
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Lattice constant is observed to be slightly decreased with increase the time of KF treatment. If the K is substituted into the Cu place, the lattice constant is expected to be increased. In
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earlier reports, reduction of diffraction angles were found to observed by increasing lattice constant associated with K-doping due to [12, 13]. The lattice constant of CZTSSe can be increased by occupying VCu or substitution of Cu with K, which are not observed in our samples as shown in Fig. 3(c). Furthermore, Raman spectroscopy of the solar cell was
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performed to characterize the crystal structure and phase. Figure 3(b) shows Raman spectra of the fabricated CZTSSe thin film solar cells. All solar cells showed typical Raman spectra of kesterite CZTSSe(CZTSe), which has Raman A1 mode at 196 cm−1 [45, 47, 48]. The peak
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at 300 cm−1 comes from the CdS buffer. Figure 3(d) shows the peak position of A1 mode and
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no noticeable differences. No peaks attributed to secondary phases were detected in Raman and XRD spectra. The KF treatment results in no noticeable structural modification on CZTSSe except morphological change. If we assume that KF PDT modifies just near the interface of CZTSSe absorber, we could not expect any noticeable changes in the results of XRD and Raman spectroscopy by the KF PDT.
3.2. Device characteristics J-V characteristics of the solar cells are shown in Fig. 4(a). The photovoltaic parameters of solar cells were summarized in Table 1. The solar cell fabricated without KF treatment
ACCEPTED MANUSCRIPT showed PCE ~4.6 % whereas the KF treatment improved VOC, F.F. and JSC until 1 min treatment. The maximum efficiency ~7.4 % is achieved by 1 min KF treatment. The solar cells showed degraded efficiency and distorted kink J-V curves for 5 min and 10 min treatment. The current blocking behavior of the longer treated solar cells could be originated
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from the barrier formed by an insulating layer during PDT or KF alters electric properties of CdS [49]. The similar pattern of J-V and EQE were absorbed as earlier reports where the distorted J-V was more pronounced with higher K-doping amount and reduced EQE [44].
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This phenomenon was not observed for CIGS solar cell. KF treatment for CIGS showed always beneficial effect by depleting Cu in absorber surface which is favorable for the Cd
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diffusion during CdS deposition and forms suitable band alignment between CIGS/CdS. The enhancement or deterioration of the device parameters by short or long time KF treatment were observed distinctly, which might be due to change of defect states of interface and bulk acting as dominant recombination center, morhphology, and surface quality etc. The reason
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of the distorted kink JV for 5’ and 10’ PDT samples could be found in defect generation and change of conduction band offset by the K-diffusion into the absorber and CdS buffer, which are modification of absorber and buffer. The EQE of the fabricated solar cells is shown in Fig.
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4(b). The more enhanced EQE for overall regions (360 nm - 1180 nm) were observed for the
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solar cell with 30 sec and 1 min treatment compared to the reference solar cell. Beyond 1 min treatment, EQE starts decreasing and shows most reduced values for 10 min PDT sample. The EQE result was found to be well consistent with J-V. The band gaps of CZTSe absorbers used in solar cell were found be ~1.1 eV, which was estimated from the EQE result by first derivative of EQE with respect to the wavelength. The enhancement of VOC was observed for 30 sec and 1 min KF treatment whereas longer than 5 min KF treatment, VOC deceased significantly. The temperature dependence of VOC was carried out in the range of temperature (300 K-90 K) to get the information about the
ACCEPTED MANUSCRIPT activation energy (Ea) of the dominant recombination mechanism in accordance with the relation.50 V =
−
ln
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Where A, J00 and JL are the diode ideality factor, reverse saturation current prefactor and photocurrent density, respectively. The intercept at 0 K of linear extrapolation of Voc versus temperature determines the activation energy. If the Ea is close to the band gap of absorber, main recombination process is the Shockley-Read-Hall recombination process, and if it is
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less than band gap of absorber, dominant recombination is the interface between absorber and
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buffer layer [50, 52]. The activation energies (Ea) were found to be 0.83 eV, 0.91 eV, 0.92 eV, 0.82 eV and 0.63 eV for the pristine, 30 sec, 1 min, 5min and 10min KF treated solar cells, respectively. This result implies that more dominant recombination is interface recombination for all samples. For the 1 min KF treated solar cell, Ea is the highest value, which ensures the reduction of the interface recombination between absorber and buffer layer significantly. We
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believe that KF treatment passivates the CZTSSe surface, adjust the proper band alignment between CZTSSe/CdS by altering the band gap of absorber near surface. Excess KF
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treatment degraded the CZTSSe surface with more surface defect densities that results in lower activation energy.
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Admittance spectroscopy (AS) and C-V profile are carried out to understand the effect of surface passivation by KF treatment on defect density. AS measurement is used to determine the energy levels of defects inside the band gap of samples. The majority carriers trapping defects for solar cells and devices can be characterized by AS. C-f measurement was performed in the dark from 20 Hz to 2 MHz in the temperature range of 300 K-90 K. The Cf spectra decayed from low frequency capacitance (Clf) to high frequency capacitance (Chf) where spectra converges beyond 1 MHz, implied that the dielectric freeze out of the free
ACCEPTED MANUSCRIPT carrier response approaching to the geometrical capacitance of the device [53]. To estimate the activation energy, the Arrhenius plot is fitted using the following equation [54].
ω = 2πϑ T exp −
where ω , ϑ and EA are the inflection frequency, the pre-exponential factor and energy level
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of defects inside the band gap, respectively. When the temperature falls beyond the certain temperature, the deep level defects with higher activation energy will be frozen out before the shallow defect. Therefore, deep level defects and shallow defects are obtained by fitting plot
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at higher temperatures and lower temperatures, respectively. Figure 6 shows the Arrhenius
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plots of characteristic frequencies derived from the admittance spectra of pure CZTSSe, 1 min and 5 min KF treated CZTSSe solar cells. The activation energies of pure CZTSSe were found to be 298 meV and 49 meV which were derived from the higher temperature region (270K-210 K) and lower temperature region (140K-90K), respectively. The deep level defect activation energy around 300 meV could be CuSn, SnCu, ZnSn, VZn, VSn, or VSe [54]. Among
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all these defects, CuSn and ZnSn were found to have the lowest formation energy. In particular, when the CZTSSe is Cu-poor and Zn-rich, the formation energy for ZnSn is significantly decreased and the concentration of [ZnSn+CuZn] defect states increase significantly.54
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Additionally, the shallow traps with the energy level of 49 meV associated with the solar cell
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could be attributed to VCu. The champion solar cell with 1 min KF treatment shows much lower deep defect level than that of pristine CZTSSe solar cell. The deep level defect activation energy level of 120–150 meV can be assigned as the dominant acceptor impurities CuZn antisites [55,56]. Therefore, deep level activation energies of 1 min and 5 min KF treated devices were found to be close to the range. The shallow defect level (~33meV) for 1 min KF treated CZTSSe is very close to the VCu which implies that the treatment modifies slightly the Cu-depleted surface. However 5 min KF treated showed 112 meV activation energy at lower temperature region, shallow defect level was found to be converted into deep
ACCEPTED MANUSCRIPT defect level by forming CuZn defect, which could be favorable formation in excess doping. Thus, the potassium doping can control the defect levels of CZTSSe solar cell which is directly reflected in solar cell performance. Moreover, the defect distribution in the absorber layer can be calculated by following equation [57],
,- . '() /0 –2 3
45#%,& % 4%
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N" #E% & =
*
'() +
where Vbi is the built-in potential, q is the fundamental charge, Eg is the band-gap energy, Eω
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is the energetic distance from the valence band corresponding to ω, and w is the width of the p side of the junction region. We plotted defect density profile using the equation for Nt (Eω)
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which is shown in Fig. 6. The integrated defect densities were 4.65x1015 cm-3 and 2.52x1015 cm-3, 1.31x1015 cm-3 and 2.14 x1015 cm-3, 4.8 x1015 cm-3 and 4.62 x1015 cm-3 corresponding to respective defect activation energies of pure CZTSSe, 1 min KF treated CZTSSe and 5 min KF treated CZTSSe solar cells, respectively. Among them, our champion cell exhibited
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4. Conclusions
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lowest value of defect densities which improves the solar cell performance.
We have fabricated CZTSSe solar cells with and without KF solution treatment of absorber
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layer. The KF solution treatment time was found to be crucial for photovoltaic performances. Excess doping of potassium showed detrimental effect on solar cell efficiency by distorting JV curve. The KF solution treatment time for best solar efficiency was 1 min, which improved power conversion efficiency from 4.4 % to 7.6 %. The enhanced VOC and JSC of solar cell are due to reduction of surface defects and improved band alignment of buffer and absorber layer by PDT. Moreover, the shallow and deep defects of CZTSSe solar cell could be also modulated by KF treatment to achieve high efficiency solar cell.
ACCEPTED MANUSCRIPT Acknowledgements This work was carried out by financial support of the National Research Foundation of Korea (NRF) funded by the Korean government (NRF-2014R1A2A1A11053109, NRF2016M1A2A2937010) and the DGIST R & D Programs of the Ministry of Science, ICT &
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Future Planning of Korea (17-BD-05).
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JSC
(V)
(mA/cm2)
Reference
0.36
25.44
30” PDT (A)
0.37
1’ PDT (B)
0.39
5’ PDT (C)
0.32
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\
Eff.
(%)
(%)
51.56
4.68
27.03
60.48
6.11
30.53
61.04
7.40
22.61
22.50
2.08
13.48
23.98
0.85
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10’ PDT (D)
F. F.
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VOC Sample
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Table. 1. Photovoltaic parameters of without and with KF PDT CZTSSE solar cells.
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Figure 1. FESEM images of the bare CZTSSe (a), 30” KF treated CZTSSe (b), 1’ KF treated
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CZTSSe (c), 5’ KF treated CZTSSe (d), 10’ KF treated CZTSSe (e) absorbers. The scale bar is 1 µm.
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0
Zn 2p3/2
1022
1018 490
1020
296
294
292
290
936
488
486
484
Binding energy (eV)
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Binding energy (eV)
934
932
930
Binding energy (eV)
Sn 3d3/2
(e) Intensity (a. u)
Intensity (a. u)
1024
298
K 2p3/2
Binding energy (eV)
Binding energy (eV)
(d)
300
Intensity (a. u.)
Intensity (a. u.) 302
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200
Se 3d Cu 3p Zn 3p Zn 3s
Se LMM2
Se LMM1,K2p
400
Se Auger
Cu 2p3/2
(c)
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600
w/o PDT (Ref.) 30'' PDT (A) 1' PDT (B) 5' PDT (C)
Intensity (a. u.)
800
(b)
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1000
O 1s Sn 3d
Sn 3p
Cu 2p
Zn 2p
Intensity (a. u.)
(a)
482
Se 3d5/2
(f)
58
56
54
52
50
Binding energy (eV)
Figure. 2. XPS survey spectra of pure CZTSSe and KF treated CZTSSe (a) and K (b) Cu (c),
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Zn (d), Sn (e) and Se peak (f) of respective samples.
1' PDT (B) 30" PDT (A)
CdS
-1
5' PDT (C)
1' PDT (B)
30'' PDT (A)
w/o PDT (Ref.)
100 150 200 250 300 350 400 450 500 -1
10 15 20 25 30 35 40 45 50 55 60 65 70
2θ (deg.)
Raman Shift (cm )
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5.75
197.0
11.40 11.35 11.30
5.65
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11.25
w/o PDT 30" PDT 1' PDT 5' PDT
10' PDT
196.5
196.0
Å
5.70
c ()
Å
(d)
-1
11.50 Lattice parameter (a) Lattice parameter (c) 11.45
(c)
Raman peak (cm )
5.80
a ()
10' PDT (D)
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w/o PDT (Ref.)
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10' PDT (D)
238 cm
-1
(400)
(316)
(220)
(b)
5' PDT (C)
5.60
172 cm -1 196 cm
(002)
* ZnO
Intensity (a.u.)
Intensity (a.u.)
(101)
(a)
(112)
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11.20
195.5
195.0
w/o PDT 30" PDT 1' PDT 5' PDT
10' PDT
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Fig. 3. XRD result of fabricated CZTSSe solar cell with different KF treatment time (a) and
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Raman spectra of respective solar cells (b). Plot of calculated lattice parameters (c) and peak position of A1 mode (d) for all CZTSSe absorbers.
100
40
10
80
0 -10 -20
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20
60 40 20
-30 -40
w/o PDT (Ref.) 30" PDT (A) 1' PDT (B) 5' PDT (C) 10' PDT (D)
(b)
w/o PDT (Ref.) 30" PDT (A) 1' PDT (B) 5' PDT (C) 10' PDT (D)
-0.2
0.0
0.2
0.4
0.6
0.8
0
400
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30
(a)
EQE (%)
2
Current density (mA/cm )
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500
600
700
800
900 1000 1100
Wavelength (nm)
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Voltage (V)
Fig. 4. J-V characteristics of the fabricated CZTSSe solar cell with different KF treatment
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time (a) and external quantum efficiency of respective solar cells (b).
1.2
(a)
w/o PDT (Ref.)
0.4
50
100
150
200
250
300
Temperature (K) 5' PDT (C)
1.0 0.8
0.6
0.6
EA= 0.82 eV
0.4 0.2
0.2
100
150
50
100
150
200
250
Temperature (K)
300
0.0 0
(e)
VOC (V)
0.6
EA= 0.92 eV
0.4
200
250
300
0.0
0
50
100
150
200
250
300
Temperature (K)
PDT 10' (D)
EA= 0.63 eV 50
100
150
200
250
300
Temperature (K)
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0.0 0
50
1.2
(d)
0.8
Temperature (K)
0.8
0.4
0.0 0
1' PDT (B)
0.2
0.2
VOC (V)
VOC (V)
1.0
EA= 0.91 eV
(c)
1.0
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VOC (V)
EA= 0.83 eV
0.2
1.2
30" PDT (A)
0.6
0.6
0.0 0
1.2
(b)
0.8
0.8
0.4
1.0
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VOC (V)
1.2
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Fig. 5. Temperature dependant open circuit voltage (VOC) and its linear extrapolation line to 0
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K of the reference (a) and KF treated CZTSSe solar cells (b-e).
(a)
8
w/o PDT (Ref.) 300 K
6
8 6 90 K
0
2
3
4
10
5
10
6
10
10
0
2
2 0
Et1= 298 meV
-2
-8 2
4
6
8
10
12
14
0 2.0
-1
1
N t1=2.52x10 0.2
0.3
E ω (eV)
0.4
15
10
12
8
1' PDT (B)
N t2 =1.31x10
0.5
0.0 0.0
0.2
15
0.3
E ω (eV)
4
5
10
10
6
10
5' PDT (C )
(f)
E t1= 147 meV
0 -4
E t2= 112 meV
-8 0
2
2.0
4
6
8
10 12 14
1/T (1000/K)
(i)
5' PDT (C)
N t2= 4.48x10
15
N t1=4.62x10
1.0
15
N t1 =2.14x10
0.1
3
10
4
1.5
0.5
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0.1
8
(h)
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15
1.0
0 0.0
6
1.5
17
-3
w/o PDT (Ref.)
N t2=4.65x10
4
2
10
Frequency (Hz)
1' PDT (B)
2
0
1/T (1000/K)
3
2
6
10
E t2 = 33 meV
1/T (1000/K)
(g)
10
E t1 = 117 meV
0
0
5
10
(e)
4
Et2= 49 meV
-4
4
10
Frequency(Hz)
w/o PDT (Ref.)
4
3
10
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ln(ω/T )
(d)
5' PDT (C)
2
Frequency (Hz) 8
(c)
4
2
2
8 6
4
4
10
Nt (x10 cm eV )
1' PDT (B)
(b)
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-8
10
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12
2
C/area (x 10 F/cm )
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15
0.5
0.4
0.5
0.0 0.0
0.1
0.2
0.3
0.4
0.5
E ω (eV)
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Fig. 6. Admittance spectra of pure CZTSSe solar cell (a) and 1min KF treated CZTSSe (b) and 5 min KF treated CZTSSe (c) solar cells. Arrhenius plot of pure CZTSSe solar cell (d) and KF treated CZTSSe solar cell for 1 min. (e) and 5 min (f). Integrated defect density profiles corresponding to defect energy levels derived from the admittance spectra (g, h, i).
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Highlights
KF-solution treated CZTSSe thin films were used as absorber layers for the solar cell fabrication.
•
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•
Proper KF treated solar cell showed enhanced VOC, JSC and FF, thus efficiency whereas
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excess doping distorted J-V curve resulting lower efficiency.
The interface recombination was found to be reduced for 1 min KF treated solar cell.
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The proper KF-treatment mitigated defect levels and reduced the defect densities of the
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CZTSSe solar cells.
S1567-1739(17)30199-2
DOI:
10.1016/j.cap.2017.07.007
Reference:
CAP 4545
To appear in:
Current Applied Physics
Received Date: 28 May 2017 Revised Date:
25 June 2017
Accepted Date: 10 July 2017
Please cite this article as: T.R. Rana, J. Kim, J.-H. Sim, K.-J. Yang, D.-H. Kim, J.-K. Kang, Fabrication and device characterization of potassium fluoride solution treated CZTSSe solar cell, Current Applied Physics (2017), doi: 10.1016/j.cap.2017.07.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Fabrication and Device characterization of Potassium Fluoride Solution Treated CZTSSe Solar Cell Tanka Raj Rana1, JunHo Kim1, Jun-Hyoung Sim2, Kee-Jeong Yang2, Dae-Hwan Kim2, Jin-
1
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Kyu Kang2 Department of Physics, Incheon National University, 119 Academy-ro, Yeonsu-gu, Incheon
2
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22012, Republic of Korea
Convergence Research Centre for Solar Energy, DGIST, 333 Techno jungang-daero,
*JunHo Kim ([email protected]) *Kee-Jeong Yang ([email protected])
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Abstract
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Dalseong-gun, Daegu 42988, Korea
Post deposition treatment (PDT) for Cu2ZnSn(S,Se)4 (CZTSSe) was carried out by simply dipping the absorber into the KF solution at 80 oC. The dipping time of absorber in KF
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solution was found to be crucial to device parameters of CZTSSe solar cell. The K-doping improved the solar cell efficiency from 4.4 % to 7.6 % by 1 min. dipping whereas the longer than 5 min dipping solar cells showed distorted kink J-V curves. The activation energy of
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CZTSSe solar cell was increased upto 1 min KF treatment from 0.83 eV to 0.92 eV which indicates interface recombination is reduced significantly. However, the activation energies of 5 min and 10 min dipping solar cells were found to be 0.81 eV and 0.63 eV where dominant recombination was interface recombination. Furthermore, trap energies of 49 meV and 298 meV of pristine CZTSSe solar cell were modified to 33 meV and 117 meV for 1 min treated CZTSSe solar cell. Trap energies of 5 min were calculated to be 112 meV and 147 meV. The proper KF doping passivated the shallow as well as deep defects of CZTSSe solar cell which is reflected in photovoltaic performances directly.
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1. Introduction The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cell is emerging as promising replacement for the Cu(In,Ga)Se2 (CIGS) solar cell which includes expensive and rare earth elements, In and
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Ga. CZTSSe, consisting of earth abundant and nontoxic elements, has suitable photovoltaic properties such as high absorption coefficient (α >104 cm-1) and tunable band gap energy 11.5 eV [1, 2]. Although CZTSSe has promising photovoltaic properties, it showed still lower
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solar cell performance than CIGS. The world champion CZTSSe solar cell fabricated by hydrazine-based process demonstrated power conversion efficiency (PCE) of 12.6%, whereas
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CIGS reached up to the best PCE of 22.6% [3, 5]. Especially, the open circuit voltage (VOC) deficit is now major limiting factor in the development of CZTSSe solar cell. Series of experiments to overcome low Voc of CZTSSe solar cell have been done, which are aimed to reduce deep point defect and secondary phases. The close ionic size of Cu, Zn and Sn is
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responsible for the cation disorder in kesterite CZTSSe, which induces deep defects in CZTSSe and results in severe band tailing and potential fluctuation. The cation disorder can be reduced by replacing Cu or Zn with larger sized atoms like Ag for Cu, Cd for Zn [6,7].
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Along with cations substitution, doping with alkali metals and antimony also has shown beneficial effects on CZTSSe improving crystallinity and charge carrier concentration [8-13].
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Recently, CIGS has shown jump in record efficiency by using the post deposition treatment (PDT) of alkali elements. At the beginning, it was believed that diffused Na from soda lime glass (SLG) to CIGS increased the PCE by improving structural properties of GIGS during the film growth. However, CIGS grown on Na-free substrate with subsequent addition of Na also showed high PCE [14-16]. This benign effect of Na on solar cell performance motivated further study of alkali doping into CIGS and CZTSSe. CIGS solar cell doped with the other
ACCEPTED MANUSCRIPT alkali elements such as Li, K, Rb and Cs also showed positive effects on photovoltaic performance. Several reasons have been proposed to explain the beneficial effects of alkali doping. Na-
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doping improves PCE of CIGS solar cell by passivating the defects in interface and grain boundaries and by increasing carrier concentrations [17-19]. KF PDT modifies the absorber surface into Cu-depleted surface containing large amounts of K. On the modified surface, thinner CdS buffer layer can grow with good quality, which results in increase of short circuit
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current density (Jsc) by reducing external quantum efficiency (EQE) loss in blue photon
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region [20,21]. The interface recombination between CdS/CIGS was also found to be significantly reduced by using KF PDT, which is explained by lowering VBM [22] with the reduction of the charge recombination, K (Na) doping increase the PCE of CZTSSe solar cell with improvement of open circuit voltage (Voc), fill factor. Alkali metals doping especially with Na and K having smaller radii compared to other heavy alkali metals resulted in larger
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CZTSSe grain sizes which enhanced carrier density by reducing carrier recombination [12]. Furthermore, K-doping provides an opportunity for optimization of n-type window layer by
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which JSC can be improved due to collection of high energy photons [12]. Alkali doping is reported to enhance p-type conductivity with increased hole density in the absorber, for
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which several mechanisms are proposed. Grain boundary (GB) mechanism states that Na is segregated at GB of CIGS with high concentration [23-25], which passivates the donor like defects at GB such as Incu, VSe resulting in the increase of net hole concentrations [23, 24, 2628]. However, Na doping is also observed to increase hole concentration of grain interior throughout CIGS layer, which has driven to propose new mechanism: Na-dopants creates new accepter by forming NaIn (NaGa) antisites or by eliminate the donor sites InCu (GaCu), which increase the hole concentration. But formation of antisites NaIn (NaGa) and reduction of donor sites InCu (GaCu) are not favorable according to the first principle calculations [29-31].
ACCEPTED MANUSCRIPT So another mechanism is proposed, where Na-dopants forms electrically inactive NaCu in CIGS due to lowest formation energy at higher temperature [32]. NaCu is unstable at low temperature so Na diffuses out leaving Cu unoccupied forming VCu in CIGS grain [33,34].The Na on absorber surface is dissolved out during rinsing process for absorber, by
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which Cu-depleted surface is produced. Thus, surface and GBs of CIGS are usually Cudepleted by alkali doping. VCu is shallow acceptor in CIGS with ionization level about 30 meV above valence band maximum (VBM) which enables the intrinsic p-type conductivity
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[20, 35]. Formation energy for K-doping is higher than that for Na-doping at the same lattice so that K is difficult to be incorporated into CIGS [32]. However, K-doping leads to the
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formation of K-enriched surface layer (Cu-free, K-In-Se compounds). Experimental results showed that large amounts of K could be doped into CIGS grains at higher temperatures [3638]. KF PDT is usually carried out after Na-doping, and then Na is replaced by K and Na may diffuse towards the surface and Mo back contact layer. VCu is increased in CIGS grain
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because of out-diffusion of Na and K after rinsing and cooling. Thus p-type conductivity is enhanced after KF PDT [32]. Besides effects mentioned above, K-doping in CZTS is
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reported to enhance (112)-preferred growth and suppress the secondary phase of ZnS [39]. Various techniques can be adopted for alkali incorporation into the absorber layer. The alkali
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elements could be doped during synthesis of absorber or by PDT of absorber layer [40-42]. The most common method till now is utilization of co-evaporation process. The introduction of alkali elements to the absorber can be controlled systematically by evaporation, which is very effective for alkali free flexible substrate. There are also different methods reported for alkali treatment by evaporation such as KF or NaF, whcih are evaporated onto CIGS absorber layer in the presence of Se at higher temperature and treated with or without subsequent selenization [20, 43, 44]. A few of solution method are reported for potassium doping into CZTS (CZTSSe), where alkali source is added to the precursor solution of CZTS (CZTSSe)
ACCEPTED MANUSCRIPT and it is spin-coated on to the film [12, 40]. But in our experiment here, we carried out the PDT process simply by dipping in the KF aqueous solution, which is similar to KCN etching process. This solution process could be attractive method for CZTSSe absorbers fabricated by using vacuum- and non-vacuum-based method, which can be applied as an alternative to
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toxic KCN etching process also. We performed KF PDT for CZTSSe and studied its effect on solar cell efficiency along with characterization of the modified surface of CZTSSe. To understand the modification of defects during KF treatment, electrical characterizations of
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solar cells such as c-f-T and J-V-T were employed.
Experimental
CZTSSe solar cells were fabricated by RF sputtering process. The devices consisted of SLG/Mo/CZTSSe/CdS/i-ZnO/AZO structure and fabrication process for each layer was same
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as previous report except KF treatment [45]. The precursor was made of Cu/SnS/ZnS/Mo and post-annealed by using RTP process at 510 o C for 20 min using only selenium source [45]. We carried out KF treatment experiment to post-selenized CZTSSe absorbers. The absorbers
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layers were dipped into the solution composed of deionized water and 4.5 wt % potassium fluoride KF at 80 oC using hot plate. We varied KF treatment time such as 30 sec, 1 min, 5
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min and 10 min. After dipping the absorbers, we cleaned the samples with di-water and further layers CdS/i-ZnO/AZO were deposited to make the completed devices. So, we fabricated five types of solar cells i.e. pure CZTSSe (reference), 30 sec KF treated CZTSSe solar cell (A), 1 min KF treated CZTSSe solar cell (B), 5 min KF treated CZTSSe solar cell (C) and 10 min KF treated CZTSSe solar cell (D).
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2.1. Characterization The surface morphologies of films were studied by using by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-7001F). The structural properties of the films were characterized by X-ray diffraction (XRD) with a high resolution XRD machine (Rigaku, phase purity
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Smart LAB) equipped with Cu-Kα source (=1.5412 Å ). To investigate
precisely, we measured Raman spectra by using a Raman scattering system (Spectro co.) equipped with an excitation laser of λ=532 nm (irradiation power on sample < 1 mW). The
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X-ray photoelectron spectroscopy (XPS) was performed by using PHI 5000 Versa Probe II. The current density−voltage (J-V) curve of the CIGS solar cell was measured by using source
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meter (Keithley 2400) under AM 1.5G filtered illumination of 1000 Wm-2 Xe lamp (Abet Technology), which was calibrated with Si reference cell. The J-V characteristics were studied in the temperature range (300 K to 90 K) under dark, white and filtered light illumination. The external quantum efficiency (EQE) was measured by using Xe light source
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and monochromator combined with light chopper and lock-in amplifier (McScience Inc.). Admittance spectroscopy (AS) was performed to analysis defect in the solar cells using LCR meter (E4980A, Agilent) which probes from 20 Hz to 2 MHz in the temperature range of
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300K-90K. All C-f scans were carried out at an ac voltage of 30 mV under the dark condition.
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The same LCR meter was used to measure capacitance-voltage (C-V) profile.
Result and discussion
3.1. Characterization of KF treated CZTSSe absorber layer The surface morphologies of bare CZTSSe and KF treated CZTSSe are shown in Fig. 1(a) and (b-e), respectively. The post-selenized CZTSSe showed compact and good crystalline texture. Figure 1(b-e) presents the SEM images of KF treated CZTSSe for 30 sec, 1 min, 5 min and 10 min. The small pores were formed on the CZTSSe surface and few white dot like
ACCEPTED MANUSCRIPT particles were observed for 30 sec and 1 min KF treated samples. The white dot like particles could be KF compounds. But for longer treated like 5 min and 10 min samples showed larger white particles attached on the CZTSSe surfaces. Therefore excessive potassium particles were remained on CZTSSe surface for longer KF treated samples even after a distilled water
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rinsing. Thus, longer potassium treatment damages surface texture which is clearly observed on the surface morphology of absorber layer. The chemical composition of one point in the absorber was observed to be Cu:Zn:Sn:S:Se=1.44:1:1.36:0.19:4.2, which was Cu-poor and
the probed position, small amount was S was detected.
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Sn-rich. The chemical ratio was a little bit fluctuated depending on the probed position. In all
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We performed XPS measurements for bare CZTSSe and 30 sec, 1 min and 5 min KF treated CZTSSe samples. Figure 2(a) shows survey spectra of all samples, where K signals are detected for the KF treated samples. Figure 2 (b) shows the magnified spectra of K 2p3/2 of CZTSSe with and without KF treatment. The increase of K 2p3/2 peak intensity with increase
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of PDT times implies that amount of K on films is also increasing with PDT time as expected. Furthermore, Cu 2p3/2 peaks of those samples are presented in Fig. 2(c), where Cu signal is observed for all samples. It is reported that for CIGS absorber KF PDT modified it into Cu-
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depleted surface. However, for sputtered CIGS solar cell Cu-depleted surface was not
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observed [46]. The formation of Cu-depleted layer might be dependent on absorber deposition process or KF PDT process. In our case, incorporation of potassium could be more followed by the interstitial than substitution process; therefore Cu-peaks of KF treated samples were not found to be reduced significantly. The Cu-peak shifts were observed randomly, peak shift towards the higher binding energy could be due to possibly oxidation of Cu, and however we cannot say clearly why binding energy is decreased after KF treatment. In order to get a better insight to surface chemistry of potassium incorporated CZTSSe by chemical PDT method requires further more systematic study. Figures (d), (e) and (f) show
ACCEPTED MANUSCRIPT spectra for Zn, Sn, and Se, respectively. It was observed that in the 5’ PDT absorber Zn is reduced more compared to other elements. Figure 3(a) represents the XRD results of the fabricated solar cells, where the dominant characteristic peak (112) is observed at 2θ of 27.16o~27.29o and additional wurtzite ZnO
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peak (002) is also seen for all types of solar cell. The XRD pattern of the solar cells showed polycrystalline kesterite crystal structure of CZTSSe which is very similar to the earlier reports [47, 48]. Figure 3(c) shows lattice constants calculated from XRD results of Fig. 3(a).
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Lattice constant is observed to be slightly decreased with increase the time of KF treatment. If the K is substituted into the Cu place, the lattice constant is expected to be increased. In
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earlier reports, reduction of diffraction angles were found to observed by increasing lattice constant associated with K-doping due to [12, 13]. The lattice constant of CZTSSe can be increased by occupying VCu or substitution of Cu with K, which are not observed in our samples as shown in Fig. 3(c). Furthermore, Raman spectroscopy of the solar cell was
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performed to characterize the crystal structure and phase. Figure 3(b) shows Raman spectra of the fabricated CZTSSe thin film solar cells. All solar cells showed typical Raman spectra of kesterite CZTSSe(CZTSe), which has Raman A1 mode at 196 cm−1 [45, 47, 48]. The peak
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at 300 cm−1 comes from the CdS buffer. Figure 3(d) shows the peak position of A1 mode and
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no noticeable differences. No peaks attributed to secondary phases were detected in Raman and XRD spectra. The KF treatment results in no noticeable structural modification on CZTSSe except morphological change. If we assume that KF PDT modifies just near the interface of CZTSSe absorber, we could not expect any noticeable changes in the results of XRD and Raman spectroscopy by the KF PDT.
3.2. Device characteristics J-V characteristics of the solar cells are shown in Fig. 4(a). The photovoltaic parameters of solar cells were summarized in Table 1. The solar cell fabricated without KF treatment
ACCEPTED MANUSCRIPT showed PCE ~4.6 % whereas the KF treatment improved VOC, F.F. and JSC until 1 min treatment. The maximum efficiency ~7.4 % is achieved by 1 min KF treatment. The solar cells showed degraded efficiency and distorted kink J-V curves for 5 min and 10 min treatment. The current blocking behavior of the longer treated solar cells could be originated
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from the barrier formed by an insulating layer during PDT or KF alters electric properties of CdS [49]. The similar pattern of J-V and EQE were absorbed as earlier reports where the distorted J-V was more pronounced with higher K-doping amount and reduced EQE [44].
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This phenomenon was not observed for CIGS solar cell. KF treatment for CIGS showed always beneficial effect by depleting Cu in absorber surface which is favorable for the Cd
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diffusion during CdS deposition and forms suitable band alignment between CIGS/CdS. The enhancement or deterioration of the device parameters by short or long time KF treatment were observed distinctly, which might be due to change of defect states of interface and bulk acting as dominant recombination center, morhphology, and surface quality etc. The reason
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of the distorted kink JV for 5’ and 10’ PDT samples could be found in defect generation and change of conduction band offset by the K-diffusion into the absorber and CdS buffer, which are modification of absorber and buffer. The EQE of the fabricated solar cells is shown in Fig.
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4(b). The more enhanced EQE for overall regions (360 nm - 1180 nm) were observed for the
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solar cell with 30 sec and 1 min treatment compared to the reference solar cell. Beyond 1 min treatment, EQE starts decreasing and shows most reduced values for 10 min PDT sample. The EQE result was found to be well consistent with J-V. The band gaps of CZTSe absorbers used in solar cell were found be ~1.1 eV, which was estimated from the EQE result by first derivative of EQE with respect to the wavelength. The enhancement of VOC was observed for 30 sec and 1 min KF treatment whereas longer than 5 min KF treatment, VOC deceased significantly. The temperature dependence of VOC was carried out in the range of temperature (300 K-90 K) to get the information about the
ACCEPTED MANUSCRIPT activation energy (Ea) of the dominant recombination mechanism in accordance with the relation.50 V =
−
ln
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Where A, J00 and JL are the diode ideality factor, reverse saturation current prefactor and photocurrent density, respectively. The intercept at 0 K of linear extrapolation of Voc versus temperature determines the activation energy. If the Ea is close to the band gap of absorber, main recombination process is the Shockley-Read-Hall recombination process, and if it is
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less than band gap of absorber, dominant recombination is the interface between absorber and
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buffer layer [50, 52]. The activation energies (Ea) were found to be 0.83 eV, 0.91 eV, 0.92 eV, 0.82 eV and 0.63 eV for the pristine, 30 sec, 1 min, 5min and 10min KF treated solar cells, respectively. This result implies that more dominant recombination is interface recombination for all samples. For the 1 min KF treated solar cell, Ea is the highest value, which ensures the reduction of the interface recombination between absorber and buffer layer significantly. We
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believe that KF treatment passivates the CZTSSe surface, adjust the proper band alignment between CZTSSe/CdS by altering the band gap of absorber near surface. Excess KF
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treatment degraded the CZTSSe surface with more surface defect densities that results in lower activation energy.
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Admittance spectroscopy (AS) and C-V profile are carried out to understand the effect of surface passivation by KF treatment on defect density. AS measurement is used to determine the energy levels of defects inside the band gap of samples. The majority carriers trapping defects for solar cells and devices can be characterized by AS. C-f measurement was performed in the dark from 20 Hz to 2 MHz in the temperature range of 300 K-90 K. The Cf spectra decayed from low frequency capacitance (Clf) to high frequency capacitance (Chf) where spectra converges beyond 1 MHz, implied that the dielectric freeze out of the free
ACCEPTED MANUSCRIPT carrier response approaching to the geometrical capacitance of the device [53]. To estimate the activation energy, the Arrhenius plot is fitted using the following equation [54].
ω = 2πϑ T exp −
where ω , ϑ and EA are the inflection frequency, the pre-exponential factor and energy level
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of defects inside the band gap, respectively. When the temperature falls beyond the certain temperature, the deep level defects with higher activation energy will be frozen out before the shallow defect. Therefore, deep level defects and shallow defects are obtained by fitting plot
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at higher temperatures and lower temperatures, respectively. Figure 6 shows the Arrhenius
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plots of characteristic frequencies derived from the admittance spectra of pure CZTSSe, 1 min and 5 min KF treated CZTSSe solar cells. The activation energies of pure CZTSSe were found to be 298 meV and 49 meV which were derived from the higher temperature region (270K-210 K) and lower temperature region (140K-90K), respectively. The deep level defect activation energy around 300 meV could be CuSn, SnCu, ZnSn, VZn, VSn, or VSe [54]. Among
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all these defects, CuSn and ZnSn were found to have the lowest formation energy. In particular, when the CZTSSe is Cu-poor and Zn-rich, the formation energy for ZnSn is significantly decreased and the concentration of [ZnSn+CuZn] defect states increase significantly.54
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Additionally, the shallow traps with the energy level of 49 meV associated with the solar cell
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could be attributed to VCu. The champion solar cell with 1 min KF treatment shows much lower deep defect level than that of pristine CZTSSe solar cell. The deep level defect activation energy level of 120–150 meV can be assigned as the dominant acceptor impurities CuZn antisites [55,56]. Therefore, deep level activation energies of 1 min and 5 min KF treated devices were found to be close to the range. The shallow defect level (~33meV) for 1 min KF treated CZTSSe is very close to the VCu which implies that the treatment modifies slightly the Cu-depleted surface. However 5 min KF treated showed 112 meV activation energy at lower temperature region, shallow defect level was found to be converted into deep
ACCEPTED MANUSCRIPT defect level by forming CuZn defect, which could be favorable formation in excess doping. Thus, the potassium doping can control the defect levels of CZTSSe solar cell which is directly reflected in solar cell performance. Moreover, the defect distribution in the absorber layer can be calculated by following equation [57],
,- . '() /0 –2 3
45#%,& % 4%
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N" #E% & =
*
'() +
where Vbi is the built-in potential, q is the fundamental charge, Eg is the band-gap energy, Eω
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is the energetic distance from the valence band corresponding to ω, and w is the width of the p side of the junction region. We plotted defect density profile using the equation for Nt (Eω)
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which is shown in Fig. 6. The integrated defect densities were 4.65x1015 cm-3 and 2.52x1015 cm-3, 1.31x1015 cm-3 and 2.14 x1015 cm-3, 4.8 x1015 cm-3 and 4.62 x1015 cm-3 corresponding to respective defect activation energies of pure CZTSSe, 1 min KF treated CZTSSe and 5 min KF treated CZTSSe solar cells, respectively. Among them, our champion cell exhibited
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4. Conclusions
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lowest value of defect densities which improves the solar cell performance.
We have fabricated CZTSSe solar cells with and without KF solution treatment of absorber
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layer. The KF solution treatment time was found to be crucial for photovoltaic performances. Excess doping of potassium showed detrimental effect on solar cell efficiency by distorting JV curve. The KF solution treatment time for best solar efficiency was 1 min, which improved power conversion efficiency from 4.4 % to 7.6 %. The enhanced VOC and JSC of solar cell are due to reduction of surface defects and improved band alignment of buffer and absorber layer by PDT. Moreover, the shallow and deep defects of CZTSSe solar cell could be also modulated by KF treatment to achieve high efficiency solar cell.
ACCEPTED MANUSCRIPT Acknowledgements This work was carried out by financial support of the National Research Foundation of Korea (NRF) funded by the Korean government (NRF-2014R1A2A1A11053109, NRF2016M1A2A2937010) and the DGIST R & D Programs of the Ministry of Science, ICT &
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Future Planning of Korea (17-BD-05).
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JSC
(V)
(mA/cm2)
Reference
0.36
25.44
30” PDT (A)
0.37
1’ PDT (B)
0.39
5’ PDT (C)
0.32
AC C
\
Eff.
(%)
(%)
51.56
4.68
27.03
60.48
6.11
30.53
61.04
7.40
22.61
22.50
2.08
13.48
23.98
0.85
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10’ PDT (D)
F. F.
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VOC Sample
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Table. 1. Photovoltaic parameters of without and with KF PDT CZTSSE solar cells.
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Figure 1. FESEM images of the bare CZTSSe (a), 30” KF treated CZTSSe (b), 1’ KF treated
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CZTSSe (c), 5’ KF treated CZTSSe (d), 10’ KF treated CZTSSe (e) absorbers. The scale bar is 1 µm.
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0
Zn 2p3/2
1022
1018 490
1020
296
294
292
290
936
488
486
484
Binding energy (eV)
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Binding energy (eV)
934
932
930
Binding energy (eV)
Sn 3d3/2
(e) Intensity (a. u)
Intensity (a. u)
1024
298
K 2p3/2
Binding energy (eV)
Binding energy (eV)
(d)
300
Intensity (a. u.)
Intensity (a. u.) 302
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200
Se 3d Cu 3p Zn 3p Zn 3s
Se LMM2
Se LMM1,K2p
400
Se Auger
Cu 2p3/2
(c)
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600
w/o PDT (Ref.) 30'' PDT (A) 1' PDT (B) 5' PDT (C)
Intensity (a. u.)
800
(b)
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1000
O 1s Sn 3d
Sn 3p
Cu 2p
Zn 2p
Intensity (a. u.)
(a)
482
Se 3d5/2
(f)
58
56
54
52
50
Binding energy (eV)
Figure. 2. XPS survey spectra of pure CZTSSe and KF treated CZTSSe (a) and K (b) Cu (c),
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Zn (d), Sn (e) and Se peak (f) of respective samples.
1' PDT (B) 30" PDT (A)
CdS
-1
5' PDT (C)
1' PDT (B)
30'' PDT (A)
w/o PDT (Ref.)
100 150 200 250 300 350 400 450 500 -1
10 15 20 25 30 35 40 45 50 55 60 65 70
2θ (deg.)
Raman Shift (cm )
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5.75
197.0
11.40 11.35 11.30
5.65
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11.25
w/o PDT 30" PDT 1' PDT 5' PDT
10' PDT
196.5
196.0
Å
5.70
c ()
Å
(d)
-1
11.50 Lattice parameter (a) Lattice parameter (c) 11.45
(c)
Raman peak (cm )
5.80
a ()
10' PDT (D)
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w/o PDT (Ref.)
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10' PDT (D)
238 cm
-1
(400)
(316)
(220)
(b)
5' PDT (C)
5.60
172 cm -1 196 cm
(002)
* ZnO
Intensity (a.u.)
Intensity (a.u.)
(101)
(a)
(112)
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11.20
195.5
195.0
w/o PDT 30" PDT 1' PDT 5' PDT
10' PDT
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Fig. 3. XRD result of fabricated CZTSSe solar cell with different KF treatment time (a) and
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Raman spectra of respective solar cells (b). Plot of calculated lattice parameters (c) and peak position of A1 mode (d) for all CZTSSe absorbers.
100
40
10
80
0 -10 -20
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20
60 40 20
-30 -40
w/o PDT (Ref.) 30" PDT (A) 1' PDT (B) 5' PDT (C) 10' PDT (D)
(b)
w/o PDT (Ref.) 30" PDT (A) 1' PDT (B) 5' PDT (C) 10' PDT (D)
-0.2
0.0
0.2
0.4
0.6
0.8
0
400
SC
30
(a)
EQE (%)
2
Current density (mA/cm )
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500
600
700
800
900 1000 1100
Wavelength (nm)
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Voltage (V)
Fig. 4. J-V characteristics of the fabricated CZTSSe solar cell with different KF treatment
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time (a) and external quantum efficiency of respective solar cells (b).
1.2
(a)
w/o PDT (Ref.)
0.4
50
100
150
200
250
300
Temperature (K) 5' PDT (C)
1.0 0.8
0.6
0.6
EA= 0.82 eV
0.4 0.2
0.2
100
150
50
100
150
200
250
Temperature (K)
300
0.0 0
(e)
VOC (V)
0.6
EA= 0.92 eV
0.4
200
250
300
0.0
0
50
100
150
200
250
300
Temperature (K)
PDT 10' (D)
EA= 0.63 eV 50
100
150
200
250
300
Temperature (K)
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0.0 0
50
1.2
(d)
0.8
Temperature (K)
0.8
0.4
0.0 0
1' PDT (B)
0.2
0.2
VOC (V)
VOC (V)
1.0
EA= 0.91 eV
(c)
1.0
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VOC (V)
EA= 0.83 eV
0.2
1.2
30" PDT (A)
0.6
0.6
0.0 0
1.2
(b)
0.8
0.8
0.4
1.0
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1.0
VOC (V)
1.2
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Fig. 5. Temperature dependant open circuit voltage (VOC) and its linear extrapolation line to 0
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K of the reference (a) and KF treated CZTSSe solar cells (b-e).
(a)
8
w/o PDT (Ref.) 300 K
6
8 6 90 K
0
2
3
4
10
5
10
6
10
10
0
2
2 0
Et1= 298 meV
-2
-8 2
4
6
8
10
12
14
0 2.0
-1
1
N t1=2.52x10 0.2
0.3
E ω (eV)
0.4
15
10
12
8
1' PDT (B)
N t2 =1.31x10
0.5
0.0 0.0
0.2
15
0.3
E ω (eV)
4
5
10
10
6
10
5' PDT (C )
(f)
E t1= 147 meV
0 -4
E t2= 112 meV
-8 0
2
2.0
4
6
8
10 12 14
1/T (1000/K)
(i)
5' PDT (C)
N t2= 4.48x10
15
N t1=4.62x10
1.0
15
N t1 =2.14x10
0.1
3
10
4
1.5
0.5
EP
0.1
8
(h)
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15
1.0
0 0.0
6
1.5
17
-3
w/o PDT (Ref.)
N t2=4.65x10
4
2
10
Frequency (Hz)
1' PDT (B)
2
0
1/T (1000/K)
3
2
6
10
E t2 = 33 meV
1/T (1000/K)
(g)
10
E t1 = 117 meV
0
0
5
10
(e)
4
Et2= 49 meV
-4
4
10
Frequency(Hz)
w/o PDT (Ref.)
4
3
10
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2
ln(ω/T )
(d)
5' PDT (C)
2
Frequency (Hz) 8
(c)
4
2
2
8 6
4
4
10
Nt (x10 cm eV )
1' PDT (B)
(b)
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-8
10
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12
2
C/area (x 10 F/cm )
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15
0.5
0.4
0.5
0.0 0.0
0.1
0.2
0.3
0.4
0.5
E ω (eV)
AC C
Fig. 6. Admittance spectra of pure CZTSSe solar cell (a) and 1min KF treated CZTSSe (b) and 5 min KF treated CZTSSe (c) solar cells. Arrhenius plot of pure CZTSSe solar cell (d) and KF treated CZTSSe solar cell for 1 min. (e) and 5 min (f). Integrated defect density profiles corresponding to defect energy levels derived from the admittance spectra (g, h, i).
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Highlights
KF-solution treated CZTSSe thin films were used as absorber layers for the solar cell fabrication.
•
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•
Proper KF treated solar cell showed enhanced VOC, JSC and FF, thus efficiency whereas
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excess doping distorted J-V curve resulting lower efficiency.
The interface recombination was found to be reduced for 1 min KF treated solar cell.
•
The proper KF-treatment mitigated defect levels and reduced the defect densities of the
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CZTSSe solar cells.