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Solution-Processed CuS NPs as an Inorganic Hole-Selective Contact Material for Inverted Planar Perovskite Solar Cells Haixia Rao,†,‡ Weihai Sun,†,§ Senyun Ye,‡ Weibo Yan,‡ Yunlong Li,‡ Haitao Peng,‡ Zhiwei Liu,*,‡ Zuqiang Bian,*,‡ and Chunhui Huang‡ ‡

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, and State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing 100871, P. R. China

§

S Supporting Information *

ABSTRACT: Organic−inorganic hybrid perovskite solar cells (PSCs) have drawn worldwide intense research in recent years. Herein, we have first applied another p-type inorganic hole-selective contact material, CuS nanoparticles (CuS NPs), in an inverted planar heterojunction (PHJ) perovskite solar cell. The CuS NP-modification of indium tin oxide (ITO) has successfully tuned the surface work function from 4.9 to 5.1 eV but not affect the surface roughness and transmittance, which can effectively reduce the interfacial carrier injection barrier and facilitate high hole extraction efficiency between the perovskite and ITO layers. After optimization, the maximum power conversion efficiency (PCE) has been over 16% with low J−V hysteresis and excellent stability. Therefore, the low-cost solution-processed and stable CuS NPs would be an alternative interfacial modification material for industrial production in perovskite solar cells. KEYWORDS: perovskite, solar cell, CuS, nanoparticles, hole-selective contact



INTRODUCTION Organic−inorganic hybrid perovskites, most commonly CH3NH3PbI3, as a promising candidate for next-generation photovoltaic materials, have drawn substantial attention due to their superior photovoltaic properties including excellent light harvesting, efficient ambipolar charge transport ability and long exciton diffusion length.1−9 The perovskite materials were first introduced in the liquid electrolyte-based dye sensitized solar cells (DSSCs) in 2009, achieving a power PCE of 3.8%.1 Significantly, a solid state hole-transporting material (HTM) [2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenlamine)-9,9′-spiro-bifluorene] (spiro-MeOTAD) was employed in the PSCs in 2012, which boosted the efficiency up to ∼10%.4,5 Most recently, an inspiring PCE exceeding 20% has been obtained.9 In general, the configuration of perovskite solar cells can be categorized into the conventional planar/mesoscopic structure and the inverted planar heterojunction structure. The mesoscopic structure device is based on a mesoporous layer including n-type metal oxide semiconductor (such as TiO25,8) or insulating oxide scaffold (such as ZrO210,11 and Al2O34,12), playing a role in adsorbing and supporting the perovskite. Although a champion efficiency has been obtained in the conventional mesoscopic structure devices,9 they have suffered the issue of device processing due to the high sintering temperature for mesoporous layer. The inverted planar heterojunction (PHJ) structure, without a mesoporous layer, has been proposed and became a popular perovskite device configuration with a simple and facile device fabrication process. © 2016 American Chemical Society

For the inverted PHJ structure device, the perovskite lightabsorbing layer is sandwiched between two planar charge transport layers that selectively pass electrons and holes to the corresponding electrodes.13 To aid in photo generated hole extraction and prevent electron leakage from the perovskite layer to the ITO electrode, a hole-transporting layer (HTL) is required for hole-selective contacts. The poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is the most widely used HTM in the inverted PHJ perovskite solar cells, which has improved the PCE over 18% up to now.14,15 However, the stability of the device based on PEDOT:PSS is serious because of the hydrophilic and acidic nature of PEDOT:PSS.16−18 Compared with the organic HTMs, p-type inorganic materials seem to be a potential alternative because of their high optical transparency, high hole mobility, excellent chemical stability, simple preparation, and low cost. To date, significant efforts have been made to replace PEDOT:PSS with p-type inorganic materials,19−26 such as NiO x, 19,20 Cu-doped NiOx,21,22 Li(Mg)-doped NiO,23 and CuSCN.24 Previously, the perovskite solar cells based on sol−gel-processed NiOx19 achieved a PCE of 9.11%. Jen et al.21,22 then improved the PCE to 17.7% with Cu-doped NiOx as a hole conductor due to the improvement of the electrical conductivity. Very recently, nanostructured NiO films prepared by a relatively costly Received: December 29, 2015 Accepted: March 11, 2016 Published: March 11, 2016 7800

DOI: 10.1021/acsami.5b12776 ACS Appl. Mater. Interfaces 2016, 8, 7800−7805

Research Article

ACS Applied Materials & Interfaces

Measurement and Characterization. The current density− voltage (J−V) curves were measured under 100 mW cm−2 AM 1.5G simulated illumination by utilizing the Keithley 4200 System in cooperation with the Oriel 300 W solar simulator (Thermo Oriel 91160−1000). The IPCE spectrum was recorded on Keithley 2400 source meter under irradiation of a 150 W tungsten lamp with a 1/4 m monochromator (Spectra Product DK240). The absorption spectrum was recorded on UV-3600 Plus UV−vis−NIR spectrophotometer (Shinadzu) with a 1.0 cm optical path length quartz cuvette. The XPS data were taken on an AXIS Ultra instrument from Kratos Analytical. The work function of the ITO:CuS substrate was measured by ultraviolet photoelectron spectra (UPS) (Ac-2, Riken Keiki). The SEM and AFM images were obtained by using Hitachi S-4800 and SPA400 SPM (SeikoInstrument, Inc.), respectively. The XRD patterns were recorded on an X’Pert3 powder X-ray diffraction system with monochromated Cu Kα irradiation (λ = 1.5418 Å). The steady-state PL measurements were performed on a lifetime and steady state spectrometer (FLS980, Edinburgh Instruments Ltd.). The EIS measurements are carried out by a CHI660 electrochemical workstation (CH Intrument, Inc.) at 0.55 V bias in the dark with frequency from 100 000 to 100 Hz and amplitude (V) = 0.005, quiet time (s) = 2.

method of pulsed laser deposition have been explored in the perovskite solar cells, achieving a PCE of 17.3%.20 Chen et al.23 used heavily p-doped NiLiMgO as hole extraction layer in the planar PSCs to achieve very rapid charge extraction, leading to a PCE exceeding 18%, the doping by Li+ is an efficient way to increase the p-conductivity of NiO, while the deposition of NiO using spray pyrolysis still need the support of high temperature. Moreover, Ye et al.24 have employed the electrodeposited CuSCN films as HTL in the inverted architecture perovskite solar cells with a PCE of 16.6%. As mentioned above, the relatively complicated preparation methods or lower efficiency of the inorganic HTMs in the perovskite solar cells may restrict their practical application in industrial production. As a well-known p-type inorganic material, CuS has been widely used in the fields of gas sensor,27 nonlinear optical material28 and catalysts.29 Recently, CuS has been investigated to replace the PEDOT:PSS as HTL in organic photovoltaics (OPV),30 exhibiting a good performance compared with the devices using PEDOT:PSS. In this paper, we have introduced facile solution-processed CuS NPs as hole-selective contacts for the inverted PHJ perovskite solar cell. The CuS NPs is used to modify the surface of ITO, which can tune the surface work function and facilitate the hole collection efficiency between the perovskite and ITO layers but not affect the surface roughness and transmittance of ITO substrates. Compared with the perovskite devices without HTL, the devices employing CuS NPs as hole-selective contacts have displayed superior performance, indicating that the CuS NPs could be a potential inorganic hole-selective contact material for perovskite solar cells.





RESULTS AND DISCUSSION CuS NPs were prepared according to Zhou et al.’s work29 with small modifications (Supporting Information), which were further confirmed by the energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis (Figures S1 and S2). The CuS NPs were then spincoated on the surface of ITO substrate and served as holeselective contact in the perovskite solar cell. Figure 1a, b shows

EXPERIMENTAL SECTION

Material Preparation. All the chemicals were used as received, including PbI2 (99.9985%, Alfa Aesar), HI (in water, ≥ 45%, Sinopharm Chemical Reagent Co., Ltd.), CH3NH2 (25.0% ∼ 30.0%, Sinopharm Chemical Reagent Co., Ltd.), Na2S (≥98.0%, Xilong Chemical Co., Ltd.), Sodium Citrate (≥99.0%, Beijing Beihua Fine Chemical Co., Ltd.), CuCl2.2H2O (≥99.0%, Xilong Chemical Co., Ltd.), C60 (99.9%, Puyang Yongxin Fullerene Technology Co., Ltd.) and BCP (99.9%, Ltd. and Xi′an Polymer Light Technology Corp. (PLT)). The ITO-coated glass substrates with a sheet resistance of 24 Ω/sq were purchased from Huayulianhe Co., Ltd. Methylammonium iodide (CH3NH3I) was synthesized according to the procedure reported previously.11 The CuS NPs solution were prepared according to Zhou et al.’s work31 (1 mL of sodium sulfide solution (Na2S, 0.1 M) was slowly added into 100 mL of an aqueous solution containing CuCl2 (0.0170 g, 1 mmol) and sodium citrate (0.02 g, 0.068 mmol) which was stirred at room temperature. Upon the addition of sodium sulfide, the pale-blue CuCl2 solution turned dark-brown immediately. Five minutes later, the reaction mixture was heated to 90 °C and stirred for 15 min, then the CuS NPs (dark-green) solution was obtained. The CuS NPs solution was stored in the refrigerator.). Device Fabrication. The ITO-coated glass substrates were ultrasonically cleaned with detergent, acetone, ethyl alcohol and deionized water in sequence. Next, the CuS NPs solution was spincoated on the cleaned ITO substrate at 2000 rpm for 60 s and then dried at 140 °C for 15 min. This procedure could be repeated several times on the basis of experiment requirements. A layer of CH3NH3PbI3 was deposited on the ITO:CuS substrate by a onestep fast deposition-crystallization method32 in a nitrogen-filled glovebox (