Revealing the Role of Potassium Treatment in CZTSSe Thin Film

Article pubs.acs.org/cm

Revealing the Role of Potassium Treatment in CZTSSe Thin Film Solar Cells Wenjie Li,† Zhenghua Su,§ Joel Ming Rui Tan,†,‡ Sing Yang Chiam,∥ Hwee Leng Seng,∥ Shlomo Magdassi,⊥ and Lydia Helena Wong*,†,‡ †

Campus of Research Excellence and Technological Enterprise, Energy Research Institute @ NTU (ERI@N), Nanyang Technological University, 637553 Singapore ‡ School of Materials Science & Engineering, Nanyang Technological University, 639798 Singapore § Physics Department, South University of Science and Technology of China, Shenzhen 518055, China ∥ Institute of Materials Research and Engineering, A*STAR (Agency for ScienceTechnology and Research), 2 Fusionopolis Way, Innovis, 138634 Singapore ⊥ The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: Potassium (K) post-treatment on CIGSSe has been shown to yield the highest efficiency reported to date. However, very little is known on the effect of K doping in CZTSSe and the mechanism behind the efficiency improvement. Here we reveal the mechanism by which K enhances the charge separation in CZTSSe. We show that K accumulates at the CdS/ CZTSSe, passivating the recombination at the front interface and improving carrier collection. K is also found to accumulate at the CZTSSe/Mo interface and facilitates the diffusion of Cd into the absorber which affects the morphology and grain growth of CZTSSe. As revealed by the C−V, external quantum efficiency, and color J−V test, K doping significantly increases the carrier density, improves carrier collection, and passivates the front interface and grain boundaries, leading to the enhancement of Voc and Jsc. The average power conversion efficiency has been promoted from 5% to above 7%, and the best 7.78% efficiency has been achieved for the 1.5 mol % K-doped CZTSSe device. This work offers some new insights into the K doping effects on CZTSSe via solution-based approach and demonstrates the potential of facile control of K doping for further improvement of CZTSSe thin film solar cells.

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INTRODUCTION Quaternary earth-abundant semiconductor Cu 2 ZnSnS 4 (CZTS) has always been regarded as one of the most promising materials for thin film solar cells. By incorporating selenium (Se) into CZTS, a tunable bandgap from 1.0 to 1.5 eV could be achieved for its alloy Cu2ZnSn(S,Se)4 (CZTSSe), of which the highest efficiency of 12.7% has been obtained by using a hydrazine-based solution approach.1,2 Unlike CIGS technology, CZTSSe fabricated with solution-based approaches showed better performance compared with those by high-cost vacuum approaches. And recently, several groups have reached high power conversion efficiency (PCE) from 7% to 11% using © 2017 American Chemical Society

non-hydrazine solvents to form precursor solution, followed by spin or spray coating method,3−10 showing great potential for low-cost, large-scale, and more environmentally friendly PV manufacturing in the future. Alkali elemental doping, such as Na and K doping, is wellknown to exert beneficial effects on both CIGS and CZTS devices. For Na doping in CIGS and CZTS solar cells, its positive effects, such as promoting grain growth, increasing hole Received: January 31, 2017 Revised: April 27, 2017 Published: April 28, 2017 4273

DOI: 10.1021/acs.chemmater.7b00418 Chem. Mater. 2017, 29, 4273−4281

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Chemistry of Materials Table 1. Chemical Composition of CZTSSe Thin Films with Different K Doping Concentration K concn (mol %) 0 0.5 1.0 1.5

Cu (%)

Zn (%)

Sn (%)

Se (%)

S (%)

Cu/(Zn + Sn)

Zn/Sn

22.80 22.89 22.59 22.51

13.31 13.67 12.69 13.11

12.48 12.47 12.39 12.33

36.93 36.78 38.05 38.24

14.48 14.12 14.15 13.65

0.88 0.88 0.90 0.88

1.07 1.10 1.02 1.06

(CH3COO)2·H2O (1.55 mol/L), Zn(CH3COO)2·2H2O (1.0 mol/ L), SnCl2·H2O (0.8 mol/L), and thiourea (6.6 mol/L) directly into the 2-methoxyethanol while stirring at 50 °C for 2 h to get a dark yellow solution.42 The metal cation ratio of Cu/(Zn + Sn) is around 0.86 and of Zn/Sn is 1.25. The potassium chloride was used as potassium source, which was also mixed in the precursor solution. Four types of precursor solutions with atomic ratios K/Cu = 0, 0.5, 1.0, and 1.5 mol % were prepared, respectively. The as-prepared solution was diluted to one-third, and then proper triethanolamine (TEA) and monoethanolamine (MEA) were added to stabilize and adjust the adhesion of the solution. All chemical reagents were purchased from Sigma-Aldrich Co. The precursor solution was spin coated onto a molybdenum coated soda-lime glass (SLG/Mo) substrates at 3000 rpm for 30 s, followed by preheating at 280 °C for 2 min on a hot plate in the air. This coating step was repeated 12 times to get enough thick films. Next, the precursor films were annealed in a sealed furnace with selenium/Ar atmosphere at a total pressure of 400 mbar during selenization. The sealed furnace was heated to 560 °C in 10 min and then kept for 10−40 min. To fabricate the device, a 50 nm thick CdS buffer layer was deposited on the obtained absorber layer by chemical bath method (CBD), and then 50 nm i-ZnO and 700 nm AZO (ZnO:Al) layers were deposited by RF and DC magnetron sputtering sequentially. Finally, top contact fingers were formed with silver glue printed on an AZO layer. Each device has a total area of approximately 0.15 cm2 defined by mechanical scribing. Characterization Techniques. The surface and cross-section morphology of thin films were studied by field emission SEM (FESEM; JEOL JSM-6340F). The X-ray diffraction (XRD) patterns and Raman spectra were collected by using Bruker D8 Advance and Renishaw Raman systems (532 nm excitation wavelength), respectively. The chemical composition and vertical elemental distribution of CZTSSe thin films were determined from energy dispersive X-ray analysis (EDX). To further study the elemental distribution, the depth profiling of each element including K dopants in CdS/CZTSSe thin films was studied by SIMS using Ar+ sputtering ions with 3 keV ion energy and 36 nA ion current. A slower sputtering rate with 1 keV ion energy and 10 nA ion current was also used to obtain precise information at the CdS/CZTSSe interface. The capacitance−voltage measurement was performed using HP 4284A and Keithley 4200 analyzers. Current density−voltage curves were performed using a Keithley 2612A source meter in the dark and under simulated 1 sun AM 1.5G illumination (100 mW/cm2) from a Xe-based light source solar simulator (VS-0852). The incident power was calibrated with a standard Si reference cell. Color J−V measurement was performed under white light (no filter), blue light (600 nm short pass filter), and red light illumination (600 nm long pass filter). The EQE was measured by PVE300 (Bentham) IPCE instrument with calibration by Si/Ge reference detectors.

concentration, passivating nonradiative defects, and GBs, have been investigated thoroughly in recent years.11−17 While in the case of potassium, some different positive effects were found on CIGS, i.e., facilitating higher Ga content at CIGS/CdS interface and boosting the efficiency of CIGS up to 20.8%.18 Further optical and buffer layer optimization has pushed it up to 21.7%.19 Meanwhile, Tiwari’s group carefully examined the role of K doping effects on flexible CIGS devices by the same postdeposition treatment (PDT) method.20−23 They concluded that postdeposition of KF led to Cu and Ga depletion in the near surface of CIGS, facilitating Cd diffusion into Cu-depleted surface and shift of conduction band toward the Fermi level, thus suppressing interfacial recombination and improving heterojunction quality and thinning CdS layer.20,21 Despite great progress made in CIGS solar cells, only a few studies of Na and K doping in CZTS/CZTSSe solar cells have been reported so far.24−27 Aydil’s group studied the diffusion effects of alkali elements on the grain growth in CZTS in detail and demonstrated an approach for precisely controlling the alkali metal amounts to facilitate large grain growth.43 Another study found that K doping enhances the crystallinity of CZTS film and suppresses secondary phases such as ZnS.24 Yang et al. recently demonstrated that metallic dopants could impact the morphology of CZTSSe due to the akali metal−Se binary compounds and smaller alkali metals are beneficial for increasing carrier concentration.25 However, besides these, there is still a lack of comprehensive study of the relationship between the alkali elements distribution especially K and accordingly solar cell performance improvement. Therefore, further understanding of the doping effects and precise control of the alkali metal dopants is needed for tailoring the microstructure of thin film absorbers and developing a highperformance device. In this work, we performed K doping on CZTSSe thin film deposited by simply incorporating KCl into the precursor solution for the absorber. With K doping, PCE of the CZTSSe device has been improved from around 5% to above 7% with optimization of selenization duration. The best efficiency of 7.78% has been achieved with addition of 1.5 mol % K and selenization duration of 30 min. More importantly, we have systematically investigated and identified the effects of K doping on the morphological, chemical properties of CZTSSe thin film and electrical performance of CZTSSe solar cell with scanning electron microscopy (SEM), secondary mass ion spectroscopy (SIMS), capacitance−voltage (C−V) measurement, external quantum efficiency (EQE), and color current density and voltage (J−V) test. It is concluded that K doping facilitates Cd diffusion into the absorber layer, increases the carrier density, improves the carrier collection, and passivates the front interface and grain boundaries, thus leading to the enhancement of Voc, Jsc, and solar cell performance.

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RESULTS AND DISCUSSION CZTSSe thin films of ∼1.5 μm thickness containing a fixed amount of K doping (0, 0.5, 1.0, and 1.5 mol %), were fabricated via spin coating and selenization at 560 °C for 10 min. Characterizations of selenized thin film containing various amounts of K were carried out with XRD, Raman, and EDX methods. Chemical compositions of the selenized films are estimated to be (Table 1) 0.28 for S/(S + Se), 0.88−0.9 for Cu/(Zn + Sn), and 1.02−1.10 for Zn/Sn, which are suitable for high-efficiency solar cells.1−6 XRD patterns of selenized thin films show that all the films crystallize in kesterite phase with no

EXPERIMENTAL SECTION

Materials Preparation and Device Fabrication. The CZTS sol−gel precursor solution was prepared by dissolving Cu4274


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Chemistry of Materials

Figure 1. (a) XRD patterns and (b) Raman spectra of the undoped and K-doped CZTSSe thin films selenized at 560 °C for 10 min. The Raman spectra were taken using 532 nm laser excitation wavelength.

Figure 2. Plan-view SEM images of CZTSSe thin films with (a) 0, (c) 0.5, (e) 1.0, and (g) 1.5 mol % K doping and cross-section images of CZTSSe devices with (b) 0, (d) 0.5, (f) 1.0, and (h) 1.5 mol % K doping. All the thin films and devices are taken at 560 °C for 30 min.

different measurement depth of XRD (bulk) and Raman (surface), the results indicate that the amount of incorporated K may not be sufficient to induce lattice distortion in the bulk; however, the atomic vibrations of the surface region in the Kdoped sample have been varied slightly. In addition, no other peaks were detected for both undoped and K-doped CZTSSe thin films. To understand the influence of selenization conditions on the grain size and device performance, the duration of the annealing process has been systemically studied here (Supporting Information Table S2). It is measured that devices made from CZTSSe thin films selenized for 30 min show the best average performance, with a 10% increment in FF. Therefore, further characterizations are made on this set to understand the factors improving the solar cell efficiencies. From SEM images (Figure 2), it is found that K doping has a synergistic effect with Se. First of all, undoped CZTSSe films exhibited much smoother and larger grains of 0.5−2 μm, while K-doped films exhibited faceted and smaller grains of 0.5−1 μm. This difference in grain distribution is attributed to the different rate of grain growth affected by the alkali−Se compounds during selenization process, while the presence of faceted grains is probably due to reaction controlled grain growth under high Se partial pressure.28,29,32 The wider spread of grain size for the undoped thin film can be explained by the abnormal grain growth, which may be due to the nonuniformed alkali diffusion from SLG substrate. The introduction of K

secondary phases (Figure 1a). In general, three major sharp peaks at 27.48°, 45.68°, and 54.18° correspond to the characteristic peaks of CZTSSe (JCPDS No. 00-026-0575 and JCPDS No. 00-052-0868). During selenization, S is replaced by larger Se causing an increase in lattice constant. This is also revealed in our diffractograms where the characteristic peak at 27.48° shifts toward lower angles indicative of enlargement of the lattice spacing.28 During the comparison of the diffractograms of various K-doped CZTSSe thin films, it is observed that there is no clear peak shift for the K-doped films with respect to the undoped one. Since some binary and ternary secondary phases such as Zn(S,Se) and Cu2Sn(S,Se)3 have diffraction patterns similar to that of CZTSSe, additional Raman spectroscopy is required for better phase purity analysis (Figure 1b).28−30 From the Raman spectrum obtained, all selenized films show Raman peaks at ∼200 and ∼235 cm−1, which is characteristic of A1 vibration mode peaks of CZTSSe. The shift from ∼338 cm−1 (CZTS) is caused by the partial replacement of S by Se atoms (CZTSSe), owing to the mass difference and different vibration frequencies between these two anion atoms.28 The peak at 325 cm−1 is assigned to the bimodal behavior of CZTSSe with an intermediate S/(S + Se) ratio.28 Compared with undoped samples, the K-doped films show a slight peak shift toward higher wavenumbers. This phenomenon could be explained by the incorporation of potassium into the crystal lattice, leading to the minor variation in atomic vibrations.31 Considering the 4275


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Figure 3. SIMS spectra of (a) undoped and (b) 1.5 mol % K-doped CdS/CZTSSe thin film (CdS/CZTSSe/Mo) and SIMS slow scan spectra for (c) undoped and (d) 1.5 mol % K-doped CdS/CZTSSe interface (CdS/CZTSSe).

Figure 4. Box charts showing the device parameters of undoped and K-doped CZTSSe solar cells selenized at 560 °C for 30 min: efficiency, opencircuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF), for the best eight devices fabricated from each type of device.

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Figure 5. (a) C−V curves, (b) standard C−V profiles, (c) space charge density, and (d) depletion width of undoped and K-doped devices selenized at 560 °C for 30 min.

accelerates the normal grain growth and results in a more uniform grain distribution. Second, the cross-section film shows a bilayer structure, where the top layer consists of large densely packed columnar grains while the bottom layer consists of sparsely packed smaller grains. Even though this bilayer structure is common for solution processed CZTSSe thin film, there is still a debate on whether it is detrimental to the device, as some CZTSSe devices with the existence of fine grain layers also show good performance.4−6,33 We also note that external addition of K could influence the growth of a fine grain layer. Because the fine grain layer is usually associated with carbon from the precursors, we speculate that the likely source of carbon is the residual high boiling point organic additives TEA (335 °C) and MEA (170 °C) trapped within the film prior to the annealing. As a higher K doping usually requires more additives to stabilize and a slightly higher amount of TEA and MEA, this leads to the formation of a thicker fine grain layer. It is interesting to note that the thicker fine grain layer does not show much negative effects on device performance. We therefore conclude that K may accumulate and also passivate the grain boundaries of these fine grains. To further explore the chemical composition distribution and elemental migration behaviors both at the bulk and interface of CZTSSe, SIMS measurement (Figure 3) has been conducted for CZTSSe thin films after CdS deposition (CdS/CZTSSe). It is shown that Cu, Zn, Sn, and Se distribute uniformly along the thickness of absorber layer for both undoped and K-doped samples. By comparing the spectra in Figure 3a,b, it is clear that the K-doped sample shows a much higher intensity of K profile than the undoped one. The weak K intensity in the undoped samples is ascribed to the K diffused form SLG substrate. At the

same time, the higher concentration of K at the back interface is noticed from SIMS, which means that K also tends to accumulate at the bottom layers of CZTSSe. Because alkali elements usually segregate at the grain boundaries,16 we suspect that K segregates at the grain boundaries of the fine grain layers at the CZTSSe/Mo interface. Another remarkable difference between these two samples is that the Cd and S profiles increase dramatically toward the back contact in the K-doped sample. This seems to indicate that the presence of K enhances the diffusion of Cd and S into the CZTSSe bulk. Besides, the CdS/CZTSSe interface was also investigated with slower sputtering rate to obtain precise information at the interface (Figure 3c,d). As shown in Figure 3d, there is a “bump” at the CdS/CZTSSe interface, which shows the accumulation of K at the interface. The same bump is absent in the sample without intentional K doping and thus can be attributed to the K doping effect. Therefore, we suspect that K at the absorber surface diffused and accumulated at the CdS/CZTSSe interface during the chemical bath deposition (CBD) process. The measured solar cell characteristics of the CZTSSe device with various K doping concentrations under AM 1.5 illumination are shown in Figure 4. All doped devices show the beneficial effect of potassium incorporation, as reflected by higher average and median power conversion efficiency than the undoped device. The KCl addition (0.5−1.5 mol %) enhances the average device efficiency from 6% to above 7%, while further increase of K concentration beyond 1.5 mol % decreases the efficiency. We believe that this is related to the formation of some deep defect level. From C−f measurement (Figure S7), we thought that some deep defects may be created with K doping,23 as revealed by the increased capacitance in the 4277


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Figure 6. (a) EQE spectra of undoped and K-doped CZTSSe device selenized at 560 °C for 30 min and (b) bandgap of undoped and K-doped CZTSSe device selenized at 560 °C for 30 min.

C is the measured capacitance for each DC bias, while ε0, εr, q, and A stand for the vacuum dielectric constant, dielectric constant for CZTSSe devices, elementary charge, and device area, respectively. The space charge density and depletion width (at zero voltage bias) for the CZTSSe devices with various doping concentrations are depicted in Figure 5c,d as well. With the increase of K doping concentration, the charge density increases from 1 × 1016 to 6.6 × 1016 cm−3. Similar to the beneficial effects of Na in increasing p-type hole concentration, the improvement in Voc caused by K doping can be estimated approximately using the following equation:22

low-frequency region for the K-doped device. This phenomenon becomes much more severe with higher doping concentration above 1.5 mol %, so we proposed that this may be the threshold value in our case and devices suffered from significant degradation with excess dopants. As a result, the 1.5 mol % K-doped device yields the highest efficiency of 7.78% with a Voc of 405 mV, a Jsc of 30.90 mA/cm2, and a FF of 62.2% (Figure S5), which is comparable to other non-hydrazine solution processed CZTSSe thin film solar cells.3−6 In general, the trend variation of PCE is mainly dominated by the variation of Voc and Jsc, as the addition of K increases the Voc as well as the Jsc. As for the Voc, an average increase of 10 mV was obtained with 0.5 mol % K doping, while the presence of 1.0 and 1.5 mol % KCl further results in an increase of about 30 mV. A similar trend has been observed for the Jsc. The graph shows that the average Jsc values are in the range of 26−29 mA/ cm2 for the undoped devices, while the Jsc values are between 30 and 32 mA/cm2 with a smaller spread for the K-doped devices. The higher Voc and Jsc may be attributed to the increase of carrier concentration due to the suitable incorporation of external potassium, which will be characterized in detail by C− V measurement next. Finally, the average and median FF values show similar values of ∼60% for all the devices, with a slightly wider spread range in undoped devices. Therefore, the Voc and Jsc of the CZTSSe device are improved by K doping treatment. In order to get more insight into the effects of potassium on the electronic transport properties and the nature of the defects at the p−n junction, capacitance−voltage measurement was carried out. The C−V sweep was performed under reverse bias voltage condition to make the junction capacitance more dominant, in the range from 0 to −5 V. Panels a and b of Figure 5 display the C−V sweep and space charge density profiling (NC−V) as a function of the distance, ⟨x⟩, to the p−n junction interface, respectively. It is clearly noted that the capacitances for all the CZTSSe devices decrease monotonously with the increase of reverse bias voltage, meaning that increased reverse bias widens the depletion width. The specific NC−V and ⟨x⟩ can be calculated as follows.34,35 NC − V =

⟨x⟩ =

−1 C 3 ⎛⎜ dC ⎞⎟ qε0εrA2 ⎝ dV ⎠

Aε0εr C

ΔVoc =

K doping ⎫ kT ⎧ ND ln⎨ no doping ⎬ q ⎩ ND ⎭ ⎪

⎪

⎪

⎪

(3)

doping where NKD doping and Nno are carrier concentrations in KD doped and undoped CZTSSe thin films, k is Boltzmann’s constant, and q is electronic charge. The calculated Voc improvement for 1.5 mol % K-doped devices is around 50 mV, which is comparable with the measured values (Table S2), where the Voc increment is between 30 and 50 mV. As shown in Figure 5d, the higher charge density is usually accompanied by a trend of shorter depletion width in CZTSSe as the depletion width is a reciprocal function of the doping concentration.1,30,36 Despite the fact that shorter depletion width may not be good for charge separation, the carrier collection width consists of depletion width and diffusion length, and the diffusion length could play a more important role.1 Because of the enhanced performance for the K-doped device, we speculate that the carrier collection becomes better and K doping may increase the minority carrier diffusion length in a way. Figure 6a displays the external quantum efficiency spectra of the undoped and K-doped CZTSSe solar cells. The EQE for Kdoped device has been improved from 400 to 1100 nm, and the maximum quantum efficiency exceeding 80% is obtained for a photon wavelength of 580 nm, indicating the enhancement of light absorption in the absorber layer. The EQE enhancement in the almost entire-wavelength range could be attributed to the effect of Cd diffusion from the CdS into the CZTSSe layer as shown by SIMS. The Cd diffusion into the CZTSSe layer may improve the charge separation at the grain boundaries, thereby enhancing EQE and improving charge collection in all wavelength ranges.37,38 The bandgap values of CZTSSe devices annealed at 560 °C for 30 min are shown in Figure 6b,

(1)

(2) 4278


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Figure 7. J−V characteristics of (a) undoped and (b) 1.5% K-doped CZTSSe solar cell under white, red, and blue light illumination.

determined by fitting a plot of [E ln(1 − EQE)]2 vs photon energy graph near the band edge. The external K doping does not change the bandgap of as-selenized CZTSSe, and the bandgap values for all the CZTSSe thin films are around 1.05 eV. The influence of K doping on the diode characteristics of CZTSSe solar cells was checked by the color J−V measurement (Figure 7). By using different optical band-pass filters with a cut off wavelength at 600 nm, the location of photon absorption into either the absorber layer or n-type top layers could be controlled. When using the blue light band-pass filter (600 nm). Here the color J−V characteristics of the best 1.5 mol % Kdoped CZTSSe and undoped devices were measured under dark conditions, white light conditions, filtered red light, and blue light conditions. The undoped CZTSSe device shows an efficiency of 6.5% under white light conditions with a Voc of 371.6 mV, a Jsc of 30.1 mA/cm2, and a fill factor of 58.3% (Figure 7a). This solar cell exhibits a crossover effect between the white light and dark light J−V curves at a voltage around 396 mV. After incorporation of K, the best CZTSSe solar cell shows a higher Voc of 405 mV, a higher Jsc of 30.5 mA/cm2, and a higher fill factor of 62.2% (Figure 7b). For this K-doped device, there is no obvious crossover effect between white and dark J−V curves even beyond 500 mV, which is usually related to the improvement of a secondary barrier at the CdS/CZTSSe front interface. Spectral J−V measurements are then performed to acquire more information. Due to the reduced light intensity under red and blue light, both devices exhibited lower Voc and Jsc values compared with white light illumination. At both low-energy and high-energy illumination conditions, there is no visible kink on the J−V curves for all devices. The J−V curves have shapes similar to the ones measured under white light. When using red light bandpass, a slight decrease of fill factor for all the devices could be observed while the fill factor values are higher when the highenergy blue light illumination condition is utilized. The decrease of FF is correlated with the absence of photons absorbed in the CdS buffer layer and increased series resistance. The crossover effect is observed for both devices under blue light illumination, even though it is more severe for the undoped one. Therefore, we assume that K doping in CZTSSe

solar cells can relieve the crossover effects due to the improvement of carrier collection and passivation of front contact.39−41 To sum up, we systematically investigated the effects of K doping on the CZTSSe absorber formation and electrical properties of CZTSSe thin film solar cells. First of all, it was shown that all three potassium doping concentrations (0.5, 1.0, and 1.5 mol %) lead to an improvement of device performance from 5% to above 7% after optimization with selenization duration, which is mainly ascribed to the enhancement of Voc and Jsc. The best device with 7.78% efficiency is achieved with the 1.5 mol % K-doped CZTSSe device with selenization at 560 °C for 30 min. Second, based on FESEM and SIMS, we observed that K is likely to locate at the bottom fine grain layer passivating the grain boundaries, which may influence the grain growth and morphology. The passivation of grain boundary defects by K could explain why the device efficiency of 1.5 mol % K-doped CZTSSe is high despite the small grain size. K is also found to facilitate the Cd diffusion from the CdS layer into the CZTSSe layer, which could be the reason for improved charge separation and enhancement of EQE and Jsc. In addition, the diffusion and accumulation of K at the CdS/CZTSSe interface could be the reason for the improved collection at the front interface as shown by color J−V measurement. Finally from C−V measurement, we concluded that the increased Voc is mainly originated from the increase of free carrier concentration, while improved Jsc is due to the passivation of the front interface and enhanced carrier collection. However, in our study, the collection at high wavelength is still the bottleneck for the device, although K-doped devices show much more improvement in the long-wavelength region. The incomplete collection at high wavelength could be affected by the electrical loss and recombination loss, which may relate to our selenization process and thick fine grain layers. In this way, making absorber thinner may further help reduce recombination while thicker films may help light absorption. The thickness of thin films therefore should be examined and optimized carefully in different conditions.

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CONCLUSIONS

In this work, the role of K doping on the CZTSSe grain growth mechanism and device performance have been investigated. The importance and positive effects of K doping for the performance improvement of solution processed CZTSSe thin 4279


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Chemistry of Materials film solar cell is demonstrated. Through the facile control of K doping concentration, solar cell performance has been increased from 5% to above 7%, with the best efficiency of 7.78%. The role of K is summarized in four aspects: (1) K accumulates at CdS/CZTSSe passivating the front interface and (2) K facilitates the Cd diffusion from the CdS layer into the CZTSSe layer and improves the charge separation, (3) K accumulation at the bottom may relate to the formation of fine grain layers, passivating the grain boundaries of small grains and leading to reduced recombination and enhanced carrier collection, and (4) K doping in the CZTSSe absorber increases the carrier concentration and may also increase the short carrier diffusion length, thus enhancing the Voc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00418. Additional experimental information and results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].. ORCID

Shlomo Magdassi: 0000-0002-6794-0553 Lydia Helena Wong: 0000-0001-9059-1745 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the funding support from NTU-COE Industry Research Collaboration Award 2015; and CREATE Programme under the Campus for Research Excellence and Technological Enterprise (CREATE), which is supported by the National Research Foundation, Prime Minister’s Office, Singapore.

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REFERENCES

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Revealing the Role of Potassium Treatment in CZTSSe Thin Film

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