Article Cite This: ACS Appl. Energy Mater. 2019, 2, 3822−3829
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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 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, É cole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ∥ University of Chinese Academy of Sciences, Beijing 100049, China ⊥ Laboratory of Photomolecular Science, É cole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ‡
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S Supporting Information *
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 toward 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 oleylamine (OAm), oleic acid (OA), and trioctylphosphine (TOP) ligands are prepared through the traditional nonaqueous 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 towards 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-p-methoxyphenylamine)-9,9′-spirobifluorene (spiroOMeTAD)-based control device, the SnS-based perovskite solar cell presents a better air stability, showing unaltered device performance after 1000 h storage under ambient conditions. 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 top-performing PSC has reached the certificated efficiency of 24.2% in 2019,7 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 perovskite,10 have been intensively developed recently toward 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 CuCaO2,15 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 © 2019 American Chemical Society
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 type II 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. Because SnS powder is insoluble in common solvents, binary solvent mixtures of 1,2-ethanedithiol (EDT) and 1,2-ethylenediamine (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 n−i−p device structure. Employing the oil-soluble SnS nanocrystals or quantum dots (QDs) forebodes well to avoid this caveat. 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 Received: March 8, 2019 Accepted: May 3, 2019 Published: May 3, 2019 3822
DOI: 10.1021/acsaem.9b00510 ACS Appl. Energy Mater. 2019, 2, 3822−3829
Article
ACS Applied Energy Materials
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 the HRTEM image.
Figure 2. Top-view SEM images of (a) the perovskite film and (b) that covered with the SnS QD layer. (c) Steady-state 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 and >99% for SnS) was calculated by comparing the integrals of PL spectra with or without HTMs.
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.
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RESULTS AND DISCUSSION
Our SnS QDs were prepared by the conventional one-pot hotinjection method in nonaqueous octadecene (ODE) at an injection temperature of 180 °C.35 SnCl2 and thioacetamide were utilized as the two sources for the synthesis of SnS QDs 3823
DOI: 10.1021/acsaem.9b00510 ACS Appl. Energy Mater. 2019, 2, 3822−3829
Article
ACS Applied Energy Materials
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 °C 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) 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 fabricated with one-layer SnS.
The hole extraction property of SnS QDs was investigated from thermodynamic and kinetic 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 photogenerated electrons and holes. Significant PL quenching after introducing the hole transporters signified charge transfer occurring at the energy offset of perovskite/HTM interfaces. Compared with the spiroOMeTAD 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 IPL = A0 + A1 exp(−t/τ1) + A2 exp(−t/τ2), and the fitting parameters are 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.
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 average dot sizes of 7 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 antisolvent assisted 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. 3824
DOI: 10.1021/acsaem.9b00510 ACS Appl. Energy Mater. 2019, 2, 3822−3829
Article
ACS Applied Energy Materials
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.
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 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 all collected after 15 day aging under the storage conditions of dry air (relative humidity
