SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells

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SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells Yang Li, Zaiwei Wang, Dan Ren, Yuhang Liu, Aibin Zheng, Shaik M. Zakeeruddin, Xiandui Dong, Anders Hagfeldt, Michael Grätzel, and Peng Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00510 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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ACS Applied Energy Materials

SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells Yang Li,†,‡,§,‖ Zaiwei Wang,┴ Dan Ren,§ Yuhang Liu,§ Aibin Zheng,† Shaik Mohammed Zakeeruddin,§ Xiandui Dong,‡ Anders Hagfeldt,┴ Michael Grätzel,§ and Peng Wang*,† †Department

of Chemistry, Zhejiang University, Hangzhou 310028, China

‡Changchun

Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,

China §Laboratory

of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne, CH-1015

Lausanne, Switzerland ‖University

of Chinese Academy of Sciences, Beijing 100049, China

┴Laboratory

of Photomolecular Science, École Polytechnique Fédérale de Lausanne, CH-1015

Lausanne, Switzerland

ACS Paragon Plus Environment

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ABSTRACT: Perovskite solar cells have achieved comparable power conversion efficiencies as commercial silicon cells, making the issue of long-term operation stability as the most critical scientific factor towards industrial realization. In this work, we introduce SnS quantum dots (QDs) as a new inorganic hole transporting material (HTM) to perovskite solar cells. The SnS QDs decorated with the oleyamine (OAm), oleic acid (OA) and trioctylphosphine (TOP) ligands are prepared through the traditional non-aqueous solvothermal method. Therefore, the as-synthesized SnS QDs can be orthogonally processed onto the top of a triple cation perovskite film, exhibiting a good surface coverage and an excellent hole extraction ability. With careful device engineering on film thickness, annealing procedure and ligand exchange on the SnS layer, we have obtained a power conversion efficiency (PCE) of 13.7%. Compared with the 2,2’,7,7’-tetrakis(N,N-di-pmethoxyphenylamine)9,9’-spirobifluorene (spiro-OMeTAD) based control device, the SnS based perovskite solar cell presents a better air stability, showing unaltered device performance after 1000 h storage under the ambient condition.

KEYWORDS: Hole transporting materials, quantum dots, perovskite solar cells, stability, ligand exchange


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INTRODUCTION Organic‒inorganic halide perovskite solar cells (PSCs) have attracted tremendous research interest in the past decade with swift growth of power conversion efficiency (PCE).1‒6 Since the topperforming PSC has reached the certificated efficiency of 23.7% in 2018,7 the issue of device stability becomes the limiting factor for future commercialization.8 More robust perovskite components, i.e. methylammonium-free organic‒inorganic hybrid perovskite9 and all inorganic perovskite10 have been intensively developed recently towards stable PSCs. Besides the instability of the perovskite layer, the PSCs are also suffering from poor air and thermal stability due to the widely used organic hole transporting material, spiro-OMeTAD.11 Compared with the organic components, the inorganic HTMs intrinsically show a great potential in terms of stability, and many inorganic hole transporters such as CuSCN,12 NiOx,13 CuI,14 CuCaO215 and CuCrO216 have been reported. However, the PCEs of PSCs based on inorganic HTMs are still not satisfactory. Tin monosulfide (SnS) is a promising photovoltaic material because of the earth abundant elemental composition and appropriate bandgap of bulk material (direct bandgap of 1.3 eV and indirect bandgap of 1.0 eV).17 Moreover, SnS is an intrinsic p-type semiconductor with a carrier concentration of ≈1014‒1017 cm-3 and hole mobility of 90 cm2 v1 s1.18,19 It could also form a typeII staggered band alignment with halide perovskite semiconductors.20,21 These advantages indicate the potential of SnS as the HTM of PSCs, which so far has not been explored. Since SnS powder is insoluble in common solvents, binary solvent mixtures of 1,2-ethanedithiol (EDT) and 1,2ethylenediamine (en) have been used involving the “dissolve and recover” process22,23 to produce the SnS ink. Nevertheless, both EDT and en will strongly attack the perovskite materials in the ni-p device structure. Employing the oil-soluble SnS nanocrystals or quantum dots (QDs) forebodes well to avoid this caveat.


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Long chain surfactants are typically used for QD synthesis in the common solvothermal method to realize the precise control of QD size and shape by regulating the nucleation and growth of nanocrystals.24,25 When these QDs are assembled into a solid thin film for device integration, the long insulating ligands on the surface of QDs inhibit hole transfer between neighboring dots, resulting in a poorly conductive QD film. As reported,26 the carrier mobility decreases exponentially with the increase of ligand length. Hence, ligand exchange in the solution phases or on thin films have both been explored to improve carrier mobility of QDs. Several small organic (i.e. dithiols, amines and acids)27‒30 and inorganic (i.e. metal chalcogenides, halides and thiocyanate)31‒34 compounds have been introduced as the ligand exchange agents. In this paper, we will synthesize SnS QDs and assemble them into a hole transporting thin film, which will be further exchanged with a short ligand of EDT. We finally demonstrate the excellent air and operational stability of perovskite solar cells based on the SnS hole transporter layer. RESULTS AND DISSCUSSION Our SnS QDs were prepared by the conventional one-pot hot-injection method in non-aqueous octadecene (ODE) at an injection temperature of 180 oC.35 SnCl2 and thioacetamide were utilized as the two sources for SnS QDs encapsulated by the bulky ligands of OAm, OA and TOP, resulting in a good dispersity in chlorobenzene. The XRD pattern of the as-synthesized SnS QDs (Figure 1a) displayed the (110), (021), (101), (111), (131), (210), (141), (002), (211), (151), (122), and (042) diffraction peaks, which could be indexed to the orthorhombic herzenbergite phase (JCPDS no. 00-039-0354). No impurity phases from SnS2 and SnO2 were recognized, indicating the pure phase of SnS QDs. Figure 1b illustrates the transmission electron microscopy (TEM) image of the as-synthesized SnS QDs, exhibiting the irregular polygon shape with an average dot size of 8 nm (Figure 1c). By adjusting injection temperature of synthesis, we also obtained SnS QDs with


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average dot sizes of 7 nm and 4 nm (Figure S1). The high-resolution transmission electron microscopy (HRTEM) image (Figure 1d) of our SnS QDs revealed a highly crystalline structure with continuous lattice fringes, and a lattice spacing of 2.8 Å could be ascribed to the (111) plane of orthorhombic SnS. Fast Fourier transform (FFT) on the corresponding HRTEM image confirmed the single crystal nature of a single SnS dot. We further investigated the morphology of SnS films through spin-coating the chlorobenzene solution of SnS QDs onto the perovskite substrate. Herein, the triple cation perovskite (CsPbI3)0.05(FAPbI3)0.79(MAPbI3)0.16 with 3% excess lead was employed as the photoactive layer, based on the traditional anti-solvent assisting one-step deposition method.2,36 Refer to the experimental section for details about film processing. As depicted in Figure 2a, the surface morphology of the triple cation perovskite manifested an excellent film quality, featuring a granular structure with grain sizes around 500 nm. After deposition of SnS QDs, a compact and dense film stacked by numerous small crystals (Figure 2b) could be observed, and no features related to the perovskite film were visible from the SEM image, suggesting the good coverage of SnS QDs on top of the perovskite substrate. The hole extraction property of SnS QDs was investigated from thermodynamics and kinetics aspects. We derived the energy levels of our SnS and perovskite samples by means of UPS spectra and Tau plots (Figure S2), confirming the type-II energy alignment. The valence band of SnS (~5.3 eV) is higher than that of the triple cation perovskite (~5.5 eV), suggesting that it should be energetically feasible to extract holes by our SnS QDs from the perovskite layer. We then evaluated the hole extraction ability through measuring the photoluminescence (PL) spectra (Figure 2c) of the perovskite films with or without hole transporter coating. PL signals of the pure perovskite film mainly originated from the bulk recombination of photo-generated electrons and holes.


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Significant PL quenching after introducing the hole transporters signified charge transfer occurring at the energy-offset of perovskite/HTM interfaces. Compared with the spiro-OMeTAD reference, the higher PL quenching efficiency of the SnS hole transporter suggests more efficient hole extraction, which was further confirmed by time-resolved photoluminescence (TRPL) experiments. PL decays with or without hole extraction layers (Figure 2d) were fitted with two exponential decay functions I PL  A0  A1 exp( t /  1 )  A2 exp( t /  2 ) , and the fitting parameters were tabulated in Table S1. The pure perovskite sample possessed a long average lifetime of 1075 ns with a fast decay time constant over 100 ns implying a high quality perovskite film with a low trap density.37,38 After covering the perovskite film with the hole transporters, a considerable fast PL decay can obviously be recognized, followed by a slow decay originating from the existence of some unquenched carriers in the bulk of perovskite films because of the slow diffusion of charge carriers.39,40 Both the fast decay time constant at the initial stage and the average lifetime were smaller for the perovskite/SnS film than the perovskite/spiro-OMeTAD reference, confirming the faster hole extraction kinetics of the SnS QD film. Next, we fabricated the perovskite solar cells employing SnS QDs as the hole transporter. We adopted the most efficient n-i-p cell structure based on FTO/compact TiO2/mesoporous TiO2/perovskite/HTM/Au. Cross-sectional SEM image (Figure 3a) of the device revealed the well-defined interfaces between the neighboring layers, which can be clearly distinguished. In order to get a better performance of the SnS based perovskite solar cells, we firstly optimized the solution concentration of SnS in chlorobenzene and the annealing procedure on the HTM film. The optimal processing concentration and the annealing procedure were identified as 20 mg/ml (Figure 3b), and 70 oC annealing for 5 min (Figure S3 and Table S2), respectively, generating a ~40 nm thick SnS layer (Figure 3a). Layer-by-layer deposition with the short chain EDT as the


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ligand exchange agent was further exploited to improve the conductivity of the QD film. In order to minimize the damage of perovskite film by EDT, we employed the EDT and iso-propanol mixture with a very low EDT volume percentage (0.02%) to conduct the ligand exchange. The EDT-containing iso-propanol solution was dropped on the SnS film for 15 s before starting the spin-coating program to achieve the exchange of bulky ligand with EDT. Nevertheless, no significant efficiency improvement (Figure S4a) was obtained with the double layer of SnS film. As illustrated in Figure S4b, the increased mobility due to the reduced interdot spacing cannot compensate the resistance by the increase of film thickness. Moreover, continually increasing the deposition circle (Figure S4a) leads to the deterioration of device performance. The perovskite solar cells based on different SnS sizes were also fabricated, and the device performance increased as the enlargement of dot size (Figure S5 and Table S3). We reasoned that the QD size dependent device performance mainly comes from the improved crystallinity41 of SnS QDs with the increased injection temperature. Figure 3c depicts the forward and reverse current density‒voltage (J‒V) curves of the champion device based on the SnS hole transporter after optimization of SnS QD films, displaying a relatively small hysteresis. The cell was measured under AM 1.5 G irradiation of 100 mW cm-2 with the cell area of 0.16 cm-2. The champion cell possessed a short‒circuit photocurrent (JSC) of 22.96 mA cm-2, open‒circuit photovoltage (VOC) of 0.944 V, and fill factor of 0.633, resulting in an efficiency of 13.72% in the reverse scan. Maximum power point (MPP) tracking with the champion cell revealed a stable power output of 13.11% (Figure 3d). The efficiency histogram chart sampled from over 30 devices is also included in Figure 3e, generating an average efficiency of 12.65%. The integrated photocurrent densities calculated from the IPCE spectrum (Figure 3f) were comparable with the JSC derived from the J‒V curves, indicating that the spectrum of our


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solar simulator matches well with the AM 1.5 G standard solar irradiation. It is worth mentioning that the device performance continuously improved (Figure S6 and Table S4) from its initial value over the course of 15 days under dry air, similar to the beneficial aging effect reported in other QD related photovoltaic devices.42,43 Thus, our device performance data were collected after 15-day aging under the storage conditions of dry air (relative humidity < 2%) unless otherwise stated. The spiro-OMeTAD based devices were also fabricated as the control showing an efficiency of 19.67% with negligible hysteresis (Figure S7). Compared with the control device, the combined effect of poorer VOC and FF of the SnS based devices resulted in the lower efficiency. Subsequently, we resorted to the impedance spectroscopy to diagnose the origin of the lower VOC and FF of the SnS based devices. The impedance spectra were measured at a series of forward biases in the dark for the devices based on different hole transporters. Figure 4a illustrates the selected Nyquist plots at the forward bias of 0.9 V, featuring two distinct impedance arcs. Generally, the high frequency arc representing the fast kinetic process is related to the hole transportation in the HTMs, whereas the low frequency arc is related to the charge carrier recombination in the devices.44 The hole transportation and charge recombination resistance were then obtained through fitting the Nyquist plots with the simplified equivalent circuit depicted as the inset in Figure 4a. According to the equation σ=L/(ARHTM), where L is the film thickness of the hole transporter, A is the active area of the device, and RHTM is the hole transportation resistance, we further acquired the conductivity (Figure 4b) of the hole transporter films. The conductivity of the spiro-OMeTAD film is around one order of magnitude higher than that of the SnS QD film at all the forward biases. Moreover, the device based on the SnS hole transporter possessed a lower recombination resistance (Figure 4c). This can also be attributed to the lower conductivity of the SnS QD film, resulting in an enrichment of charge carriers at the


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perovskite/HTM interface. Hence, we reasoned that the poor conductivity of the SnS QD film causes the lower VOC and FF. Ligand exchange with EDT indeed improved the conductivity of the SnS QD film as shown in Figure S8, which is, however, still not as good as the spiro-OMeTAD film. Further efforts towards increasing the conductivity of SnS QD film is vital to improve the performance of SnS based perovskite solar cells. Finally, we compared the stability of PSCs based on SnS and spiro-OMeTAD as HTMs. The un-encapsulated devices were stored in ambient environment with the relative humidity of 30%‒50%. Figure 5a illustrates the time evolution of the photovoltaic parameters obtained by periodically recording the J‒V curves under AM 1.5 G irradiation of 100 mW cm-2. The device based on SnS as HTM retained 99% of the initial efficiency after 1000 h storage in air (Figure S9) while the spiro-OMeTAD based control device showed only 75% retention. The water contact angles of hole transporting films were measured to figure out the difference of hydrophobicity between SnS and spiro-OMeTAD. As depicted in Figure 5b, c, the wettability of the SnS film by the water droplet is lower than that of spiro-OMeTAD, the contact angle increasing from 76 ° for spiro-OMeTAD to 96 ° for SnS. Hence, we attribute the superior air stability of SnS based perovskite solar cells to hydrophobic character of the SnS QD film. The operation stability (Figure S10) of the cell based on SnS hole transporter was also surveyed at MPP tracking under continuous one sun illumination in nitrogen atmosphere at 25 oC. After 500 h aging, the device achieved 75% of the initial efficiency, which was comparable with that of the spiro-OMeTAD based device. However, there was 15% loss of the device efficiency at the first 20 hours, and this burn-in degradation has been ascribed to the perovskite compositional redistribution45 at the initial stage during the device aging. Hence, it is still necessary to develop


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more robust perovskite photoactive layer in the future towards the operation stability of perovskite solar cells. CONCLUSIONS In summary, we synthesized the SnS QDs with the average size of 8 nm, and applied them as HTM in n-i-p structured PSCs achieving a PCE of 13.7%. The SnS based PSC showed superior air and operational stability, demonstrating the robustness and practical potential of the SnS hole transporter. Further work on enhancing the conductivity of SnS for high-performance perovskite solar cells are underway. EXPERIMENTAL SECTION Materials and device fabrication Synthesis of SnS QDs. 5 ml of ODE (Sigma-Aldrich, 90%), 3 ml of TOP (Sigma-Aldrich, 90%), 4.5 ml of OA (Fisher chemical, 99%) and 0.383 g of SnCl2 (Alfa Aesar, 99%) were loaded into a 100 ml three-necked flask, degassed at 120 oC for 1 h under Argon flow. Afterwards, the mixture was heated to injection temperatures (180 oC, 150 oC and 120 oC) in 10 min and kept for 30 min. Then 10 ml OAm (Arcos, 80-90%), 3 ml TOP and 0.075g thioacetamide (Alfa Aesar, 99%) were quickly injected into this mixture. After 5 min, the reaction was quenched by water bath. The solution was then dissolved in hexane, precipitated by acetone, and separated via centrifugation at 5000 rpm for 10 min. The volume ratio of mother solution, hexane and acetone is 1:1:3. The purification procedure was repeated once more. The resultant QDs were dried under vacuum at 50 oC

for 3 h, and then transferred to the argon-filled glove box for further use.

Deposition of electron transporting layer. An etched fluorine doped tin oxide (FTO) conducting glass was ultrasonically cleaned with Hellmanex (2%, ionized water), acetone and ethanol in turn for 10 min, followed by 20 min treatment under UV-ozone. A TiO2 compact layer (～30 nm) was


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then prepared by spray pyrolysis with O2 as the carrying gas at 450 oC from a precursor solution of 0.6 ml titanium diisopropoxide bis(acetylacetonate) (Sigma-Aldrich) and 0.4 ml acetylacetone (Sigma-Aldrich) in 9 ml anhydrous ethanol. Next, a mesoporous TiO2 layer ( ～ 150 nm) was deposited by spin-coating for 20 s at 5000 rpm with a ramp rate of 2000 rpm s-1, using the commercial TiO2 paste (30NRD, Dyesol) diluted with anhydrous ethanol at a weight ratio of 1:6. After drying at 80 oC for 5 min, the TiO2 film was sintered at 450 oC for 30 min under dry air flow to remove organic components. Deposition of perovskite layer. PbI2 (TCI), FAI (Dyesol), CsI (ABCR GmbH), PbBr2 (TCI) and MABr (Dyesol) were dissolved into the DMSO/DMF (Arcos, v/v, 1/4) mixture to make the 1.47 M perovskite precursor with the stoichiometry of [(CsPbI3)0.05(FAPbI3)0.79(MAPbI3)0.16](PbI2)0.03. CsI was added in the form of 1.5 M CsI/DMSO stock solution. The perovskite film was deposited by a consecutive two-step spin-coating process at 2000 rpm for 10 s (ramp rate 200 rpm s1) and 6000 rpm for 30 s (ramp rate 2000 rpm s1) on TiO2 substrate. During the spin-coating, 100 μl of chlorobenzene was dripped in the center of the substrate at 15 s prior to the program end. Subsequently, the film was annealed at 100 oC for 1 h. Deposition of hole transporting layer and gold electrode. For the Spiro-OMeTAD layer, 90 mg ml1 spiro-OMeTAD (Merck) in chlorobenzene (Arcos) was spin-coated atop the annealed perovskite film at 4000 rpm for 20 s. Spiro-OMeTAD was doped with LiTFSI (Sigma-Aldrich) and t-BP (Sigma-Aldrich) in a molar ratio of 0.5 and 3.3, respectively. For single layer SnS film, 20 mg ml1 SnS in chlorobenzene was spin-coated at 4000 rpm for 20 s. For multilayer, the SnS film was layer-by-layer deposited, and the procedure of each SnS layer is the same with that of single layer. Between each layer, EDT was used as the agent for the ligand exchange. For ligand exchange procedure, 2.4 mM EDT/isopropanol (0.02% vol.) solution was covered on the SnS film


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for 15 s, and then spin-coated at 4000 rpm for 20 s. The resulting SnS film was annealed at 70 oC for 5 min. Finally, the gold layer with a thickness of 80 nm was thermally evaporated on top of the device. Note that, both the deposition of perovskite layer and hole transporting layer were carried out in a dry air filled glove box with the relative humidity < 2%. Characterization XRD patterns were recorded with a PANalytical Empyrean diffractometer in the transmissionreflection mode, using the Cu Kα radiation and the Ni β-filter. TEM images were acquired on Talos (FEI) with the acceleration voltage of 300 kV. The QD solution was dropped onto a Cu grid coated with lacey carbon film (300 mesh, Ted Pella, Inc). The grid was left dry before being transferred to the grid box. SEM images were recorded on a Zeiss Merlin microscope with an in-lens detector at 5 kV acceleration voltage and 150 pA probe current. The working distance was 4.0 mm and no tilt was used. Transmission and reflection spectra of perovskite and SnS films were acquired using a Varian Cary5 UV–visible spectrophotometer equipped with an integrate sphere. UPS spectra were recorded on the AXIS Supra instrument (Kratos). The perovskite film was deposited on the FTO substrate. Steady-state PL and TRPL measurements were performed with a Fluorolog 322 spectrometer (Horiba Jobin Yvon iHr320 and a CCD) by exciting the films at a wavelength of 630 nm with a 5 nm band pass. The time-correlated single-photon counting mode was used for the PL decay measurements with a sub-nanosecond time resolution. JV curves of PSCs were measured without device preconditioning under the simulated sunlight using a 450 W xenon lamp (Oriel, USA) equipped with a Schott K113 Tempax sunlight filter


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(Praezisions Glas and Optik GmbH). The light intensity was calibrated with a Si reference diode, and the data were recorded with the Keithley model 2400 digital source meter. The cell area was 0.16 cm2 determined by a non-reflective metal mask. The scan rate was 50 mV s–1 with a 5 mV voltage step. MPP tracking measurement was performed with the digital source meter driven by an algorithm implemented in the IgorPro software. IPCE spectra were recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). The light source was a 300-W xenon lamp (ILC Technology) and focused through a Gemini-180 double monochromator (Jobin Yvon Ltd). Impedance spectroscopy measurements were performed with a potentiostat (BioLogic MPG2) in the dark by applying a voltage perturbation of 20 mV over a series of forward bias, with the frequency range from 0.1 Hz to 1 MHz. The impedance spectra were fitted using the Z-view software (v2.80, Scribner Associates Inc.). Air stability measurements were performed by measuring the JV curves under the solar simulator at certain time intervals. The un-encapsulated PSCs were stored in ambient air with the relativity humidity 30%50% at ambient temperature in the dark. The humidity was recorded with the Hygrometer Testo 608 H1. Water contact angle measurements were performed with a drop shape analyzer (KRÜSS, DSA100) at ambient temperature. Operation stability measurements were conducted by MPP tracking of PSCs with a Biologic SP 300 potentiostat under continuous illumination of a white 1 sun equivalent LED lamp. The MPP data were updated every 60 s using a standard perturb and observe method. The devices were kept at 25 oC under nitrogen flow. Peltier elements were directly contacted with the devices to precisely control the temperatures, and the temperatures were recorded by surface thermometers inserted


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between the Peltier element and PSCs. Before MPP tracking, the devices were stabilized through aging under one sun illumination for 20 min. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on ACS Publications website. at DOI: 10.1021/acsaem.xb0xxxx. SEM images, energy level diagram, Tauc plots, UPS spectra, J‒V curves, TRPL fitting parameters, photovoltaic parameters, σ‒V curves, and operational stability. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: 0086-571-88273217. ORCID Michael Grätzel: 0000-0002-0068-0195 Peng Wang: 0000-0002-6018-1515 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT P.W. expresses sincere gratitude to the National 973 Program (2015CB932204), the National Science Foundation of China (No. 51673165 and No. 91733302), the Key Technology R&D Program (BE2014147-1) of Science and Technology Department of Jiangsu Province, and H.


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Glass for financial support. We thank Amita Ummadisingu (EPFL) and Brian Carlsen (EPFL) for the help with measurements of TRPL and transmission-reflection spectra, respectively. REFERENCES (1) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316–319. (2) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. Il. Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897– 903. (3) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. Il. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476– 480. (4) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. Il. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234–1237. (5) Bi, D.; Yi, C.; Luo, J.; Décoppet, J.-D.; Zhang, F.; Zakeeruddin, S. M.; Li, X.; Hagfeldt, A.; Grätzel, M. Polymer-Templated Nucleation and Crystal Growth of Perovskite Films for Solar Cells with Efficiency Greater than 21%. Nat. Energy 2016, 1, 16142. (6) Jeon, N. J.; Na, H.; Jung, E. H.; Yang, T.-Y.; Lee, Y. G.; Kim, G.; Shin, H.-W.; Seok, S. Il.; Lee, J.; Seo, J. A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells. Nat. Energy 2018, 3, 682–689. (7) National Renewable Energy Laboratory (NREL) Best Research Cell Efficiency Chart; https://www.nrel.gov/pv/assets/pdfs/best-reserch-cell-efficiencies.20190411.pdf.


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Figure 1. (a) XRD pattern of the SnS QD powder. (b) TEM image of the as-synthesized SnS QDs. (c) Size histogram of SnS QDs. (d) HRTEM image of an as-synthesized SnS QD. The inset image shows the fast FFT of HRTEM image.


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Figure 2. Top-view SEM images of (a) the perovskite film and (b) that covered with the SnS QD layer. (c) Static PL spectra and (d) TRPL of the perovskite films covered with or without hole transporters. Note that the steady state PL spectra were normalized with respect to the PL peak value of the pure perovskite film. The PL quenching efficiency (98% for spiro-OMeTAD, > 99 % for SnS) was calculated by comparing the integral of PL spectra with or without HTMs.


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Figure 3. (a) Cross-sectional SEM image of device based on SnS hole transporter (FTO/c-mp TiO2/perovskite/SnS/Au) generating the film thickness of 30 nm for compact-TiO2, 150 nm for mesoporous-TiO2, 500 nm for perovskite and 40 nm for SnS. (b) Plots of device efficiency versus the concentration of SnS QDs dispersed in chlorobenzene under 70 oC annealing procedure. (c) Forward and reverse J‒V curves of the champion perovskite solar cells based on SnS hole transporter. The inset shows the photovoltaic parameters obtained from the corresponding J‒V scans. (d) MPP tracking of the champion device for 300 s. (e) The PCE histogram chart of the devices based on SnS hole transporter. (f) IPCE spectrum of the champion device. Note that the devices shown here were all employed one-layer SnS.


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Figure 4. (a) Nyquist plots at 0.9 V forward bias measured in the dark. The inset shows the simplified equivalent circuit for the impedance spectroscopy analysis. (b) Plots of conductivity versus forward bias. (c) Recombination resistance of devices based on different hole transporters under a series of voltages.


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Figure 5. (a) Evolution of photovoltaic parameters over time of the un-encapsulated PSCs based on SnS hole transporter, stored in the ambient air (~30%‒50% humidity) at room temperature. The curves were normalized to the initial values of the corresponding photovoltaic parameters. Water contact angel images of the (b) SnS and (c) spiro-OMeTAD films deposited on the perovskite substrate.


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SnS Quantum Dots as Hole Transporter of Perovskite Solar Cells

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