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Post-surface selenization for high performance Sb2S3 planar thin film solar cells Shengjie Yuan, Hui Deng, Xiaokun Yang, Chao Hu, Jahangeer Khan, Wanneng Ye, Jiang Tang, and Haisheng Song ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00858 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017
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Post-surface selenization for high performance Sb2S3 planar thin film solar cells Shengjie Yuan,1,
3
Hui Deng,1,
3
Xiaokun Yang,
1
Chao Hu,
1
Jahangeer Khan1,
Wanneng Ye, 2 Jiang Tang, 1 Haisheng Song1, 3* 1
Wuhan National Laboratory for Optoelectronics (WNLO) and School of Optical and
Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, 430074, Wuhan, Hubei, P. R. China 2
Laboratory of Fiber Materials and Modern Textiles, Growing Base for State Key
Laboratory, Qingdao University, Qingdao 266071, Shandong, P. R. China 3
Shenzhen R&D Center of Huazhong University of Science and Technology Shenzhen
518000, P. R. China ABSTRACT Sb2S3 has attracted great research interest very recently as a promising absorber material for thin film photovoltaics because of their unique optical and electrical properties, binary compound and easy synthesis. Sb2S3 planar solar cells from evaporation method without hole-transport layer (HTM) assistance suffer from sulfur deficit vacancy and high back contact barrier. Herein, we developed a post-surface selenization treatment to Sb2S3 thin film in order to improve the device performance. The XRD, Raman and UV-vis spectra indicated the treated film kept the typical characters of Sb2S3. TEM/EELS mapping of treated Sb2S3 film revealed that only surface adjacent section was partly selenized and formed Sb2(SxSe1-x)3 alloy. In addition, XPS results further unfolded that there was trace selenium doping in the bulk of Sb2S3 film. The treated HTM-free Sb2S3 based solar cells were fabricated and an improved efficiency of 4.17 % was obtained. The obtained VOC of 0.714 V was the highest and the power conversion efficiency also reached the top value among HTMfree planar Sb2S3 solar cells. The non-encapsulated device exhibited high stability. After stored in ambient air for up to 100 days, the device could maintain 90% efficiency. Systematic materials and device characterizations were implemented to investigate the improvement mechanism for post-surface selenization. The back alloying could suppress the rear contact barrier to improve the fill factor and carrier extraction capability. The bulk Se-doping helped to passivate the interface and bulk
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defect so as to improve the CdS/Sb2S3 heterojunction quality and enhance the longwavelength photon quantum yield. The robust treatment method with multifunctional effect holds great potential for new chalcogenide thin film solar cell optimization. KEYWORDS: antimony sulfide; post-surface selenization; rear contact; defect passivation; planar heterojunction
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The urgent need for high-efficiency and low-cost photovoltaics drives the sustained research on new absorber materials for thin-film photovoltaics. Copper indium gallium selenide (CIGS), Cadmium Telluride (CdTe) and the organicinorganic hybrid perovskite, the two leading players have respectively reached 22.6%1, 21%2 and 21.6%3 power conversion efficiencies. However, for the CIGS system, it contains the scarcity materials of indium and gallium. And for hybrid perovskite system, it contains toxic lead in a highly soluble state and exhibits insufficient device stability, which might precludes their potential commercialization. Thus, a new stable and lead-free alternative would benefit sustainable use of solar energy and it is of great research importance. Antimony sulfide (Sb2S3) is appealing as a promising absorber due to its suitable bandgap (~1.7 eV), strong light extinction coefficient (105 cm-1), earthabundant and environment-friendly characters.4-5 Intensive studies have been exerted to improve the performance of Sb2S3 solar cells. And the Sb2S3 solar cells had achieved fast development in last few years. To date, the highest efficiency of 7.5%5 was obtained in mesoporous device structure with organic hole-transporting material (HTM) assistance. And most of the studies focused on the Sb2S3 preparation condition,6 metal ion doping Sb2S3,7 HTM optimising8 and midwifing electron transporting materials (ETL).9 Comparing with traditional chemical bath deposition (CBD) method or atomic layer deposition (ALD) method,10 the present rapid thermal evaporation (RTE) method avoided the impurity phase introduction and demonstrated more efficient deposition capability. Nevertheless, due to the high vapour pressure of sulfur, there was S loss during the evaporation deposition process. It was inevitably to form S vacancies in Sb2S3. Efforts had been dedicated to develop solutions for defect passivation in Sb2S3 absorber.5 The defect treatment of nanostructured Sb2S3 with thioacetamide (TA) had been proved to be a successful solution for the extinction of trap sites. However, the TA treatment was not efficient for the highly crystallized Sb2S3 film by RTE method. Thus, RTE method based Sb2S3 solar cells are highly required for a facile technique to passivate its defects. In Sb2Se3 thin film solar cell, post-selenization has been demonstrated to be effective and beneficial in attenuating VSe related recombination loss in Sb2Se3 absorbers, resulting in an improved device performance.11 Similar doping was also applied in Sb2Se3 thin film with oxygen to passivate bulk and interfacial defects.12
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Hence, a post-selenization or post-sulfurization was expected to passivate the sulfur vacancy and suppress the recombination loss, further improve the heterojunction quality and device performance. On the other hand, there was a high rear barrier between Sb2S3 and Au due to their band offsets.13 In contrary, Sb2Se3 film could obtain good ohmic contact between Sb2Se3 and Au due to the band alignment.14 Therefore, the sulfur atoms on Sb2S3 surface partly replaced by Se atoms was expected to greatly improve the back contact. And the improved contact could also avoid the utilization of HTM. In CIGS and CZTS solar cells, H2S and H2Se gas were commonly utilized for sulfurization and selenization under 450-550 °C.15-16 At this high temperature, the selenization would react with the entire absorber film rather than the back surface, which greatly decreased open circuit voltage (VOC) value. By decreasing the post-surface selenization temperature, selenium may only react with back surface due to the selflimitation effect and diffuse little into the bulk. Therefore, the post-surface selenization at proper reaction temperature was expected to synergistically improve the rear contact and maintain the high VOC of Sb2S3 solar cells. In this work, we demonstrated a simple and efficient approach to modify the Sb2S3 surface and passivate the bulk defects in Sb2S3 by post-surface selenization. Utilizing such strategy, our superstrate CdS/Sb2S3 solar cells exhibited significantly improved VOC, short circuit current density (JSC), and fill factor (FF) and achieved a high power conversion efficiency (PCE) of 4.17 %, which was highest among the HTM-free planar devices. And the VOC of 0.714 V was also the top value in Sb2S3 solar cells. Systematic characterizations were employed to reveal the post-surface selenization affections on the Sb2S3 device performance. The selenium was mainly alloyed with Sb2S3 on the surface of Sb2S3 to improve the back contact. And a small amount of Se atoms ~1 at. % diffused into bulk to passivate the interface and bulk defects. RESULTS AND DISCUSSION Three kinds of annealing atmosphere were applied for as-synthesized Sb2S3 film in post-annealing treatment. First, the As-Sb2S3 film was annealed in N2 for 30 min at 300 °C. As shown in Figure S2a, there were pinholes on the Sb2S3 surface. Comparing with our previous work,17 the corresponding device (Figure S2b)
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exhibited a lower efficiency of 2.37 %, with a VOC of 0.61 V, a JSC of 9.76 mA/cm2 and a FF of 39 %. Then, a post-surface sulfurization was utilized to treat the assynthesized Sb2S3 thin film. After post-surface sulfurization, the Sb2S3 thin film surface was compact and pinhole-free (Figure S3a). However, the efficiency of 3.13 % (Figure S3b) was still lower than the average efficiency of 3.2 % in our previous work. 17
Since the annealing in N2 and post-surface sulfurization processes were not
effective to improve the device performance, both of them were not further investigated. We thus focused on the post-surface selenization to study the device performance evolution in present work. Different post-surface selenization temperatures were investigated to obtain optimal device performances. The results were shown in Figure S4 and Table S1. The as-prepared Sb2S3 based device was labeled as As-Sb2S3, the As-Sb2S3 film underwent the post-surface selenization process was labeled as Se-Sb2S3 and the Se-Sb2S3 film was further annealed in N2 atomosphere was labeled as SeA-Sb2S3. Figure 1a showed the current density-voltage (J-V) curves for above three typical devices, and their typical photovoltaic parameters were summarized in Table 1. The corresponding external quantum efficiency (EQE) curves were plotted in Figure1b. As shown in Figure 1a and Table 1, compared with As-Sb2S3 device, the Se-Sb2S3 device obtained higher VOC and FF. However, the JSC drastically decreased from 10.23 mA/cm2 to 7.34 mA/cm2. The overall PCE was reduced from 3.32 % to 2.53 %. The decreased JSC may be attributed to the resistive selenium residue on the Sb2S3 surface, which was commonly observed in postselenization of CIGS thin film solar cells.18 For the SeA-Sb2S3 device, the VOC, JSC and FF were 0.714 V, 11.39 mA/cm2 and 51.22 %, respectively. All the solar cell parameters obtained obvious improvement. Eventually, the SeA-Sb2S3 device obtained a champion PCE of 4.17 %. The histogram of 25 SeA-Sb2S3 devices was shown in Figure S5. Their VOC, JSC, FF and PCE distributed in a narrow range, which demonstrated a reliable improvement comparing with other two kinds of devices. In terms of JSC change, we carefully checked the EQE as shown in Figure 1b. All the EQE spectra were not extended, which indicated that the post-surface selenization did not change the main absorbance characters of Sb2S3 film. The SeA-Sb2S3 solar cell exhibited a higher EQE from 540 nm to 750 nm compared with the As-Sb2S3 solar cells. The above long wavelength spectra conversion corresponded to bulk film
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absorption. The higher quantum yield of SeA-Sb2S3 demonstrated better carrier collection efficiency. In the spectra region below 540 nm, the EQE showed a steep decay, indicating the light loss due to the absorption of CdS buffer layer.19 To analyze the device performance evolution mechanism, we first investigated the morphology variation by SEM. As shown in Figure 2a, the As-Sb2S3 obtained large grain size indicating high crystallization. The surface morphology of SeA-Sb2S3 was changed from non-uniform grains into fine facets and uniform grains as shown in Figure 2c, which was similar to CIGSSe sample with surface sulfurization.20 The cross-section TEM images (Figure 2d-f) showed that all samples had large grains throughout the Sb2S3 film and the film thickness kept constant ~ 1 µm. Subsequently, the phase evolution was also characterized by XRD as shown in Figure 3a. The XRD patterns of As-Sb2S3, Se-Sb2S3 and SeA-Sb2S3 film could be indexed and assigned as orthorhombic Sb2S3 phase (JCPDS 74-1046).21 The enlarged characteristic peaks (120) plane (Figure 3b) matched well with standard diffraction peaks. While compared with As-Sb2S3 pattern, the XRD diffraction peaks of Se-Sb2S3 and SeA-Sb2S3 showed asymmetrical shapes. This was ascribed to the superposition of multi-peaks of the Se-treated films. The peak located at 17.23 degree indicated that there was Sb2(SxSe1-x)3 alloy phase in Sb2S3 film.22 Raman curves for three types of Sb2S3 film were shown in Figure 3c. The peaks at ∼280 and ∼310 cm−1 could be assigned to anti-symmetric Sb-S stretch and symmetric Sb-S stretch in Sb2S3.23 In SeSb2S3 Raman spectra, the weak peak located at 252 cm-1 manifested the existence of Se8 rings on back surface,24 which may be the Se annealing residue during the cooling process. In contrary, for SeA-Sb2S3 Raman spectra, the Se8 rings signal disappeared once the Se-Sb2S3 was further annealed in N2. As an absorber, the Sb2S3 absorption characters played the key role in energy conversion. The UV-visible absorption spectra of As-Sb2S3, Se-Sb2S3 and SeA-Sb2S3 films were shown in Figure 3d. Although their XRD spectra of Se-Sb2S3 and SeA-Sb2S3 film contained Sb2(SxSe1-x)3 alloy phase, all the absorption characters was similar to As-Sb2S3 with ultra-low absorption value in extended spectra. Therefore, the XRD, Raman and UV-visible absorption demonstrated that the post-surface selenization maintained the main characters of pure Sb2S3 with little Sb2(SxSe1-x)3 alloy signal.
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To further reveal the selenium distribution in Sb2S3, Transmission Electron Microscopy/Electron Energy-Loss Spectrometry (TEM/EELS) characterization was performed for SeA-Sb2S3. As shown in Figure 4a, selenium distributed on the top surface layer, the depth was ~ 100 nm. Because of the post-selenium treatment, Se content might diffuse into bulk film due to the measurement limitation of EELS. XPS characterization was further utilized to probe the high resolution Se distribution in bulk film. Figure 4b showed the XPS results of SeA-Sb2S3 surface. The peaks located at 162.3, 161.1 and 159.6 eV were assigned to 2p3/2 and 2p1/2 of sulfur and 3p3/2 of selenium, respectively.25-26 The calculated atomic ratio of Se/S was 0.2. To investigate the selenium distribution in interior Sb2S3 film, a chemical etching method was adopted by a solution of ethanediamine and thioglycolic acid (volume ratio, 10:1). In the depth of 500 nm, the 159.6 eV peak of selenium was not detected (Figure 4c). By high-resolution scanning, there was a weak peak located at 53.2 eV (inset of Figure 4c), which was attributed to 3d5/2 of selenium, suggesting that the valence states of Se ions was -2.27 The weak signal of 3d5/2 orbit of selenium revealed that there was small amount of selenium in the interior Sb2S3 film. And the calculated atomic percentage of Se/S was 1at. %. The small amount of selenium introduction could not change the chemistry environment of Sb2S3.28 Combining the EELS mapping and XPS results, it was certain that most selenium distributed at the surface and there was weak signal in bulk Sb2S3. To investigate the more detailed Se distribution along the thickness, the depth information of the SeA-Sb2S3 film was obtained by HR-TEM (Figure 5a). For the top surface of Position I, the space of lattice line was measured as 0.317 nm (Figure 5b). While the values far from the surface (Position II and III, Figure 5c, d) were the same ~ 0.312 nm corresponding to the (230) planes of Sb2S3. The increase of d-spacing in the top area was ascribed to the larger Se atom substitution. According to above complementary characterizations, we could extract the Se roles as the alloying element on the top and extrinsic doping element in bulk. In CIGSe thin film solar cells, surface sulfurization could lower the VBM of CIGSe which could greatly improve the device VOC due to the partly replacement by sulfur atoms.29 Similarly, for our Sb2S3 thin film solar cells, post-surface selenization led to partial substitution of S atoms by Se. Hence, the VBM of treated Sb2S3 film surface was expected to be elevated. The illustrative energy level diagrams of AsSb2S3 and SeA-Sb2S3 devices were schematically shown in Figure 6. In As-Sb2S3
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device (Figure 6a), there was a high rear barrier for Sb2S3/Au interface,13 which greatly decreased the hole extraction capability. After post-surface selenization, the VBM on the surface of SeA-Sb2S3 was elevated (Figure 6b), which had better band alignment for Sb2S3/Au interface, and favoured the hole transfer from Sb2S3 to Au. Room temperature Current–voltage (I–V) curve comparison for the As-Sb2S3, SeSb2S3 and SeA-Sb2S3 were shown in Figure S6. SeA-Sb2S3 exhibited higher current, indicating the improved ohmic contact with Au. To get more insight of post-surface selenization affections, we employed physical analysis to the As-Sb2S3, Se-Sb2S3 and SeA-Sb2S3 devices. It was well known that device performance strongly depends on diode ideal factor (A) and the saturation current density (J0). As shown in Figure 1a, the dark J-V could be modeled according to Shockley’s diode equation: J = [exp
− 1]
Where J0 was the saturation current density, q was the elementary charge, A was the diode ideality factor, and k was the Boltzmann constant. Table 1 showed the fitted parameters of J0 and A extracted from the dark J-V curves according to Sites’s method.30 Compared with the As-Sb2S3 device, the SeA-Sb2S3 device showed reduced A and J0 values, suggesting the heterojunction quality of the CdS/Sb2S3 was improved after post-surface selenization. Since the VOC was strongly correlated to J0 by the following equation: =
ln + 1# !
The suppression of reverse saturation current density would naturally increase the VOC value. Because A and J0 values could reflect the recombination degree induced by interface and bulk defect. The decreased values were mainly ascribed to the bulk Se doping which could suppress the bulk defect concentration.12 The bulk defect passivation was schematically described in Figure 6b. Admittance spectrum (AS) measurements were primarily used to determine the characteristics of majority carrier trapping defects in solar cells. The AS technique had been widely applied to CIGS and CZTS based solar cells,31-32 thus we employed this method to gain physical insight for the defects of Sb2S3 based solar cells. It
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measured the current responses to small AC bias voltage modulation at different frequency and temperature. Such responses were assumed to occur mostly as a result of the capture and emission of electrically active defects.33 Figure 7a showed the admittance spectra of As-Sb2S3, Se-Sb2S3 and SeA-Sb2S3 devices, respectively. The capacitance of the As-Sb2S3 device revealed more serious frequency dependence than the Se-Sb2S3 and SeA-Sb2S3 devices, and the SeA-Sb2S3 device showed much weaker dependence. The weak frequency dependence of SeA-Sb2S3 device demonstrated the lower trap densities in the absorber.32-33 C-V profiling was also applied to explore the charge carrier concentration and built-in potential (Vbi) for the devices. The abrupt p-n heterojunction capacitance could be approximately described utilizing parallel-plate capacitor model. The plots of 1/C2-V were shown in Figure 7b. The charge carrier concentration and Vbi were calculated according to the following equations: $,& = .(0/23) .5
'()*,+ ,-,+
)*,+ ,-,6 ,-,+ 3 ,! 7(,-,6
2( :; − ) 1 = ( (
