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Kesterite Cu2ZnSnS4 as a Low-Cost Inorganic Hole-Transporting Material for High-Efficiency Perovskite Solar Cells Qiliang Wu,† Cong Xue,† Yi Li,‡ Pengcheng Zhou,† Weifeng Liu,† Jun Zhu,‡ Songyuan Dai,‡,§ Changfei Zhu,† and Shangfeng Yang*,† †
Hefei National Laboratory for Physical Sciences at Microscale, Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, China ‡ Key Laboratory of Novel Thin Film Solar Cells, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China § Beijing Key Lab of Novel Thin Film Solar Cells, North China Electric Power University, Beijing 102206, China S Supporting Information *
ABSTRACT: Kesterite-structured quaternary semiconductor Cu 2 ZnSnS 4 (CZTS) has been commonly used as light absorber in thin film solar cells on the basis of its optimal bandgap of 1.5 eV, high absorption coefficient, and earthabundant elemental constituents. Herein we applied CZTS nanoparticles as a novel inorganic hole transporting material (HTM) for organo-lead halide perovskite solar cells (PSCs) for the first time, achieving a power conversion efficiency (PCE) of 12.75%, which is the highest PCE for PSCs with Cu-based inorganic HTMs reported up to now, and quite comparable to that obtained for PSCs based on commonly used organic HTM such as 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD). The size of CZTS nanoparticles and its incorporation condition as HTM were optimized, and the effects of CZTS HTM on the optical absorption, crystallinity, morphology of the perovskite film and the interface between the perovskite layer and the Au electrode were investigated and compared with the case of spiro-MeOTAD HTM, revealing the role of CZTS in efficient hole transporting from the perovskite layer to the top Au electrode as confirmed by the prohibited charge recombination at the perovskite/Au electrode interface. On the basis of the effectiveness of CZTS as a lowcost HTM competitive to spiro-MeOTAD in PSCs, we demonstrate the new role of CZTS in photovoltaics as a hole conductor beyond the traditional light absorber. KEYWORDS: perovskite solar cells, hole-transport material, CZTS, light absorber, interface
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INTRODUCTION Thin film solar cells as a promising renewable energy source have been developed toward the cost-effective competitor to the commercialized Si solar cells.1 Among different types of thin film solar cells developed so far, solar cells based on kesterite-structured quaternary semiconductors Cu2ZnSnS4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe), and Cu 2 ZnSn(S,Se) 4 (CZTSSe) light absorbers appear most promising because of their optimal bandgap (e.g., 1.5 eV for CZTS), high absorption coefficients (e.g., ∼1 × 104 cm−1 for CZTS), and earthabundant elemental constituents.2−6 Up to now the highest power conversion efficiency (PCE) of 12.6% was achieved for the CZTSSe light absorber prepared by the hydrazine puresolution approach.7 It has been commonly recognized that CZTS-like kesterites act as outstanding light absorbers in photovoltaic since 1988.8 As an emerging thin film solar cell technology, recently organo-lead halide perovskite solar cells (PSCs) employing CH3NH3PbX3 (X = I, Br, Cl) as light absorbers have been © 2015 American Chemical Society
attracting great attention since 2009 because of their everincreasing power conversion efficiencies (PCEs) exceeding 20% already.9−21 Organo-lead halide PSCs are advantageous in terms of simple fabrication, large absorption coefficients, tunable bandgaps, high carrier mobility, and especially long charge carrier diffusion lengths.10−12 Noteworthy, for the high efficiency state-of-the-art PSCs, organic hole transporting materials (HTMs) such as 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) have been popularly used, which are crucial for efficient hole extraction from perovskite layer to the metal electrode resulted from prohibited charge recombination.9,22−25 In particular, because the first application in solid-state PSC in 2012, spiroMeOTAD has been extensively used as HTM in PSCs. For instance, in 2014 Yang et al. reported planar heterojunction Received: October 9, 2015 Accepted: December 8, 2015 Published: December 8, 2015 28466
DOI: 10.1021/acsami.5b09572 ACS Appl. Mater. Interfaces 2015, 7, 28466−28473
Research Article
ACS Applied Materials & Interfaces
chloride dihydrate (SnCl2·2H2O) were added into 10 mL of oleylamine in a 100 mL of three-neck flask connected to a Schlenk line. The mixture was vacuumed under room temperature for 30 min to remove moisture and other gases dissolved in oleylamine, and then heated to 160 °C. After the mixture gradually turned into brown, the temperature was raised to 225 °C, then 3 mL of 1 M solution of sulfur dissolved in oleylamine was injected into the reaction mixture immediately. The reaction was maintained for 30 min and the mixture turned into black. After cooled to 80 °C, the precipitate was obtained and then washed by toluene and isopropanol so as to remove oleylamine from CZTS nanoparticles. Device Fabrication. The FTO-coated glass substrate was etched with Zn powder and 2 M HCl diluted in water, then ultrasonicated in detergent, deionized water, acetone and isopropanol for 15 min every time, and subsequently dried in a vacuum oven at 60 °C overnight. A compact TiO2 layer was deposited onto FTO by spin-coating a mixture solution of 350 μL of titanium isopropoxide, 5 mL of ethanol, and 65 μL of HCl (2 mol dm−3) at 2000 rpm, followed by annealing at 500 °C for 60 min. CH3NH3PbI3 perovskite layer was fabricated by a two-step method reported in literatures.40 A PbI2 solution (dissolved in dimethyl sulfoxide with a concentration of 460 mg/mL) was then spin-coated on top of the compact TiO2 layer at 4500 rpm for 30 s. The coated substrate was dipped into a solution of CH3NH3I in isopropanol (10 mg/mL) for 10 min, and then washed by isopropanol and dried by spinning at 3000 rpm. Subsequently, the as-prepared substrate was heated at 100 °C for 10 min. After the substrate was cooled down to room temperature, a CZTS dispersion (200 mg/mL in 1-hexanethiol) was deposited on top of the pervoskite layer by spincoating at variable spin-coating speeds (3000, 4000, 5000, and 6000 rpm), followed by annealing for variable time (5, 10, and 15 min) at 100 °C. For comparison, reference PSC devices based on spiroMeOTAD HTM were also fabricated following the procedure reported in literatures.13 In brief, 73.2 mg of spiro-MeOTAD, 29 μL of 4-tert-butylpyridine (tBP), and 18 μL of lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg LiTFSI in 1 mL acetonitrile) were mixed and dissolved in 1 mL of chlorobenzene. Spiro-MeOTAD layer was deposited on top of the pervoskite layer by spin-coating the mixture solution at 3000 rpm. Finally, the device was transferred into a vacuum chamber (∼1 × 10−6 Torr), and a Au electrode (ca. 100 nm thick) were thermally deposited through a shadow mask to define the effective active area of the devices (0.10 cm2). All device fabrication procedures were carried out in a N2-purged glovebox (
