Efficient planar antimony sulfide thin film photovoltaics with large grain and preferential growth
Graphical abstract
Introduction
The demand for low cost, high-efficiency solar cells are always the driving force for thin film photovoltaics. The metal chalcogenide semiconductor absorbers which can meet above characteristics have attracted intense research interest and obtained great progress in device performances. The power conversion efficiencies (PCE) of the typical chalcogenide solar cells, such as cadmium telluride (CdTe) and copper (indium, gallium) (diselenide, disulfide) (CIGS) solar cells have reached 21% and 21.1% [1], respectively. The scarce elements of indium and gallium as well as the toxicity of cadmium in CdTe excite wide exploration of new metal chalcogenides.
Among those new metal chalcogenides, antimony sulfide (Sb2S3) is particularly appealing as promising absorber due to its suitable bandgap (~1.7 eV), strong light extinction coefficient (1.8×105 cm−1 in the visible region), earth-abundant and environment-friendly characters [2]. The superior light absorber properties triggered the Sb2S3 applications in sensitized solar cells [3]. The Sb2S3 sensitizer was commonly synthesized by aqueous based chemical bath deposition (CBD) [4], [5]. Ito et al. had further systematically studied the doping effects in Sb2S3 absorber by CBD, and the full-inorganic solar cell based on optimized Ti-doped Sb2S3 absorber reached a PCE of 5.7% [6]. The antimony oxide, byproduct from CBD, induced deep traps which led to the backward recombination of photocarriers [7]. In order to eliminate the deep oxide defects, efficient thioacetamide (TA) treatment was developed, and the PCE reached 7.5% [8]. Recently, a more simple and reproducible sensitizer synthesis method, thermal pyrolysis of antimony based thiourea complex, was implemented and the PCE reached 6.4% [9]. In addition to above solution methods, atomic layer deposition (ALD) with precise thickness deposition was applied in planar Sb2S3 structure solar cells and a novel 5.7% efficiency was achieved [10]. Although Sb2S3 sensitized solar cells had obtained dramatic progress, the main obstacles such as the impure phase, metal organic utilization and time consuming synthesis process were still left as challenges. By contrast, the traditional vacuum deposition method can overcome the deep oxide defect and the related work was limited in Sb2S3 thin film deposition. The high saturated vapor pressure and low melting points of Sb2S3 enabled it to be deposited by vacuum method. The reported Sb2S3 planar heterojunction device by thermal evaporation had obtained a PCE of 1.2% [11]. Comparing with the higher efficiency (7.5%) from solution method, the PCE of vacuum method (1.2%) for Sb2S3 solar cells contains much room to improve.
In present study, we utilized rapid thermal evaporation (RTE) method [12] to deposit Sb2S3 film for the first time. In contrast to direct sublimation from solid state in close-space sublimation(CSS) [13], the source of RTE method firstly melted and then was evaporated at liquid phase. Furthermore, the deposition rate could reach ~2 µm min−1, much faster than that of regular thermal evaporation or sputtering method and comparable to CSS deposition rate. The CSS technology has obtained high manufacturing throughput and commercial success of CdTe solar cells. Similarly, the present RTE method was expected to fabricate Sb2S3 film with high quality, throughput and reproducibility.
The common device structure of Sb2S3 sensitized solar cells contained an ultrathin layer absorbers (~100 nm) sandwiched by electron transport layer (ETL) and hole transport layer (HTL) [14], [15]. The built-in electric field by ETLs and HTLs could efficiently extract the photocarriers to transport to corresponding electrodes. Such physical separation device structure significantly suppressed the recombination and relaxed the quality requirements of the device constituent materials [3]. However, there were two key stability issues for the introduction of ETL and HTL in Sb2S3 solar cells. One was the discoloration effect of Sb2S3 originated from direct contact with the oxide ETLs (TiO2 or ZnO) [16]. The other was the utilization of organic HTLs such as poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) [3], [17], [18], [19]. For the long term operation of thin film solar cells, the degraded HTLs seriously led to unstable output. Therefore, we adopted CdS as ETL rather than oxide semiconductor, and also avoided the utilization of HTL by growing high quality Sb2S3 thin film [20], [21].
On the other hand, the recent report of non-cubic Sb2Se3 absorbers had obtained great breakthrough in carrier transport mechanism [12]. One-dimensional ribbon like Sb2Se3 chains with aligned orientation could greatly improve the transport capabilities as well as the thin film solar cell performances. In addition, the oriented ribbons held benign grain boundaries, distinct to silicon based cubic absorbers. Similarly, Sb2S3 held the same crystal structure as Sb2Se3 with one dimensional ribbon like structure (Fig. 1a) [22]. The vertical or quasi-vertical of ribbon orientation was expected to obtain high performance planar solar cells without HTL assistance.
Herein, compact Sb2S3 thin film was deposited by RTE (~μm/min). Planar solar cells based on ITO/CdS/Sb2S3/Au were implemented without metal oxide and HTL utilization. The device performances were optimized by crystallization and orientation controlling. And the PCE of corresponding planar Sb2S3 thin film solar cells was almost three times of the one by thermal evaporation [11]. The device showed little hysteresis and degradation after 100 d storing under ambient condition, which demonstrated the high stability of present device.
Section snippets
Sb2S3 thin film deposition
The Sb2S3 thin films were prepared by RTE from Sb2S3 powder (98%, Aladdin Industrial Corporation) in a tube furnace with a high ramp rate (~50 °C/s). The source was placed on the AlN plate located on the quartz boat, and the indium-doped SnO2 (ITO)/CdS substrate was placed on top of the quartz boat (0.8 cm distance from source) with the CdS side facing down. When the pressure was reduced to 18 mtorr, the heating process was started. The thin film deposition mainly contained two steps: the first
Results and discussion
Antimony sulfide has a low melting point (550 °C) and high saturated vapor pressure, which enabled us to deposit film using RTE method in high rate. The temperature-dependent vapor pressure curves of elemental S, elemental Sb and Sb2S3 were plotted in Fig. 1b according to Antoine Equation [24]. The Sb2S3 and Sb vapor pressure follow Eq. (1):where for Sb2S3, A is 13.96, B is 10,490; for elementary Sb, A is 9.051, B is 9871; T is the absolute temperature in kelvin. The elementary S vapor
Conclusions
In summary, we developed a planar Sb2S3 heterojunction solar cell by a new rapid thermal evaporation method. The obtained large-grain and compact film combing with CdS layer were fabricated into planar heterojunction solar cells without HTL and metal oxide layer. The device performances were gradually optimized by relatively high crystallization temperature and the suppression of (200). The champion device obtained a PCE of 3.5% with the Jsc of 10.8 mA/cm2, Voc of 0.71 V, and FF of 45.5%. The
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (61306137), the Research Fund for the Doctoral Program of Higher Education (20130142120075), the Fundamental Research Funds for the Central Universities (HUST: 2014QN014). Z. B. He acknowledged the support of the start-up fund of SUSTC, internal fund of SUSTC (Grant No. FRG-SUSTC1501A-67) and the Foundation of Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20150529152146471). The authors
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