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Perovskite Quantum Dots Exhibiting Strong Hole Extraction Capability for Efficient Inorganic Thin Film Solar Cells
Article
Perovskite Quantum Dots Exhibiting Strong Hole Extraction Capability for Efficient Inorganic Thin Film Solar Cells Chenhui Jiang, Jisong Yao, Peng Huang, ..., Hongbin Yao, Changfei Zhu, Tao Chen [email protected]
HIGHLIGHTS Perovskite quantum dots can serve as efficient hole-extraction materials Sb2(S,Se)3 solar cell using CsPbBr3 QDs achieved an efficiency of 7.82% Perovskite-QDs-based devices exhibit no decline in PCE over 100 days in air
Perovskite QDs serve as efficient hole-extraction material in thin-film solar cells. Jiang et al. report a surface treatment coupled with film fabrication leads to ultrathin (25 nm) perovskite QD film on the surface of planar Sb2(S,Se)3 lightharvesting material and produce a device based on perovskite QDs/Sb2(S,Se)3 heterojunction, resulting in a power conversion efficiency of 7.82%.
Jiang et al., Cell Reports Physical Science --, 100001 --, 2020 ª 2019 The Authors. https://doi.org/10.1016/j.xcrp.2019.100001
Please cite this article in press as: Jiang et al., Perovskite Quantum Dots Exhibiting Strong Hole Extraction Capability for Efficient Inorganic Thin Film Solar Cells, Cell Reports Physical Science (2019), https://doi.org/10.1016/j.xcrp.2019.100001
Article
Perovskite Quantum Dots Exhibiting Strong Hole Extraction Capability for Efficient Inorganic Thin Film Solar Cells Chenhui Jiang,1 Jisong Yao,2 Peng Huang,3 Rongfeng Tang,1 Xiaomin Wang,1 Xunyong Lei,4 Hualing Zeng,4 Shuai Chang,3 Haizheng Zhong,3 Hongbin Yao,2 Changfei Zhu,1 and Tao Chen1,5,*
SUMMARY Inorganic semiconductor Sb2(S,Se)3 possesses a suitable bandgap, environmentally benign elemental composition, and excellent stability, offering ample promise for next-generation low-cost solar cells. Here, we demonstrate that perovskite quantum dots (QDs), including CH3NH3PbBr3 and CsPbBr3, can serve as highly efficient and air-stable hole extraction materials in Sb2(S,Se)3 solar cells. Through a proper pretreatment of the colloidal QDs, a 25-nm-thick QD film can be obtained with excellent uniformity and charge transport properties. Spectroscopic and photoelectrochemical analysis show that perovskite QDs can effectively extract holes from Sb2(S,Se)3 with suppressed carrier recombination. The perovskite QDs/Sb2(S,Se)3 heterojunction also establishes an increased built-in potential so that open-circuit voltage is pronouncedly enhanced. Finally, the device based on perovskite QDs/Sb2(S,Se)3 heterojunction boosts the efficiency from 4.43% to 7.82%, setting a record value, to the best of our knowledge, in Sb2(S,Se)3 solar cells. Our research manifests another application of perovskite materials and practical strategy toward efficiency improvement of Sb2(S,Se)3 solar cells.
INTRODUCTION Halide perovskite materials have received increasing attention due to the improving power conversion efficiency (PCE) in solar cell applications. In less than 10 years, the PCE has been boosted to 25%.1 This fast growth in PCE has stimulated great interest to investigate the fundamental properties of perovskite materials and in turn found many potential applications. For instance, the high photoluminescence quantum efficiency of perovskites enables potential applications in light-emitting diodes.2,3 Due to both high extinction coefficient and large carrier mobility, the application of CsPbX3 in photodetectors has been demonstrated.4,5 In addition, the highly efficient luminescence and biexciton binding energy make perovskites promising laser materials.6,7 In this research, we demonstrate another application of perovskite quantum dots (QDs) as hole extraction materials (HEMs) in antimony selenosulfide (Sb2(S,Se)3) solar cells. Sb2(S,Se)3 is a light-harvesting material consisting of environmentally benign and earth-abundant elements. It is also stable against moisture and air, promising for practical applications.8,9 Fundamentally, it has superior photovoltaic properties, such as tunable bandgap of 1.1–1.7 eV,10,11 high absorption coefficient (>105 cm1),12,13 and mobility (42 cm2 V1 s1).14 In terms of future applications, one of the major tasks at this stage is to boost the power conversion efficiency (PCE).15,16 To date, PCE of Sb2(S,Se)3 solar cells has reached 6.6%,17,18 and a closely relevant
1CAS
Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
2Department
of Applied Chemistry, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, China
3School
of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
4International
Center for Quantum Design of Functional Materials, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China
5Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.xcrp.2019.100001
Cell Reports Physical Science --, 100001, --, 2020 ª 2019 The Authors. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Sb2Se3 solar cell delivered PCE of 9.2%;19 this achievement suggests great potential among the new materials explored for solar cell application.20,21 It is also noted that high efficiencies are generally achieved by using highly expensive organic HEMs, such as poly(3-hexylthiophene), 2,20 ,7,7’-tetrakis(N,N-di[4-methoxyphenyl] amino) 9,9’-spirobifluorene (spiro-OMeTAD), and so on. The organic HEMs, however, bring about stability issue when exposed to air and/or moisture due to the doping of inorganic ions and/or the intrinsic chemical instability. Compared to iodide perovskite, Br-based perovskite is more stable due to the reduced lattice parameter and optimized tolerance factor. With surfactant protection, the stability of Br-based perovskite QDs is further improved. Here, we introduce perovskite QDs into inorganic thin-film solar cells as low-cost HEMs. The perovskite QDs boost the device efficiency from 4.43% to 7.82%, which is the highest efficiency in all reported Sb2(S,Se)3 solar cells to the best of our knowledge. Notably, the employment of QD film as HEM displays essentially enhanced device stability when compared with conventionally applied spiro-OMeTAD. The unencapsulated devices show no PCE degradation in ambient air for 100 days. This study demonstrates the utilization of perovskite QDs as an efficient air-stable HEM in Sb2(S,Se)3 solar cells, extending the application scope of halide perovskites. The film fabrication, heterojunction characteristics, and charge transfer kinetics are extensively studied.
RESULTS Characterization of Perovskite Quantum Dots and Sb2(S,Se)3 Films We fabricated solar cells with planar heterojunction device architecture composed of fluorine-doped tin oxide (FTO)/CdS/Sb2(S,Se)3/perovskite QDs/Au, where the CdS and Sb2(S,Se)3 were synthesized by chemical bath deposition and modified hydrothermal deposition methods.22 The detailed synthetic procedure is provided in the Experimental Procedures. As a result, Sb2(S,Se)3 showed flat-surface morphology with gain size up to micrometer scale (Figure 1A). The elements of Sb, S, and Se are evenly distributed across the film (Figure S1B). During the synthesis process, the Sb2(S,Se)3 composition is tunable with regard to S/Se precursor ratios. Here, we only study the compound with Sb:S:Se atomic ratios of 1:1.25:0.19 (Figure S1A), because this composition delivers highest efficiency when compared with others. MAPbBr3 (MA = CH3NH3) or CsPbBr3 QDs were synthesized by reported methods with some refinements in which oleic acid and n-octylamine (or oleylamine) were used as surfactants.23,24 The average sizes of MAPbBr3 and CsPbBr3 QDs are 7 nm and 15 nm, respectively (Figures 1B and 1C). Both MAPbBr3 and CsPbBr3 QDs exhibit sharp photoluminescence (PL) emission with narrow full width at the half-maximum (FWHM) (Figures 1D and 1E), indicating a narrow size distribution of the QDs. In QDs, both the band gap and energy level are size dependent. The narrow size distribution permits favorable energy alignment, which in turn facilitates carrier transport. The colloidal stability of QDs originates from the surfactant tethering on the nanoparticle surface, and the surfactant usually gives rise to series resistance of films when the QDs are directly applied for film preparation.25,26 The surfactant should be partially removed or replaced by small molecules.27,28 In the current study, the treatment of MAPbBr3 and CsPbBr3 QDs is quite different because of their distinct surface nature. To partially remove the surfactant, we utilize hexane to extract MAPbBr3 QDs. In contrast, CsPbBr3 QDs are stabilized by oleic acid and oleic amine, which have long aliphatic chains. A surface ligand density control (SLDC) by using hexane and ethyl acetate mixed solvent was adopted to partially remove ligand on the CsPbBr3 QD surface. Otherwise, the utilization of QDs without above purification process generates rather poor device performance. After the treatment, a
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Figure 1. Morphological Characterization and Spectroscopic Analysis (A, H, and I) SEM images of Sb2 (S,Se) 3 film (A), MAPbBr 3 QD film (H), and CsPbBr 3 QD film (I) on Sb 2 (S,Se) 3 surface. Scale bars, 1 mm (A) and 500 nm (H and I). (B and C) TEM images of MAPbBr 3 QDs (B) and CsPbBr 3 QDs (C). Scale bars, 50 nm. (D and E) UV-Vis absorption and PL spectra of MAPbBr3 QD film (D) and CsPbBr 3 QD film (E). (F and G) EDX mappings (Br) of MAPbBr3 QD film (F) and CsPbBr3 QD film (G) deposited onto Sb 2 (S,Se) 3 surface. Scale bars, 1 mm. (J) Cross-sectional SEM image of CsPbBr3 QD film on FTO/CdS/Sb 2 (S,Se) 3 substrate. Insets are schematic crystal structures of CsPbBr 3 (top) and Sb 2 (S,Se) 3 (bottom). Scale bar, 500 nm.
spin-coating process can be adopted for the film preparation and QD films with excellent uniformity were obtained (Figures 1F–1J, S3, and S4). Figure 2A shows characteristic X-ray diffraction (XRD) patterns of pristine Sb2(S,Se)3 and Sb2(S,Se)3/perovskite QD films. The annealed Sb2(S,Se)3 film displays typical stibnite crystal structure. Obviously, the spin-coated perovskite QD HEMs did not damage the crystallinity of Sb2(S,Se)3 film. MAPbBr3 and CsPbBr3 QD HEMs deposited onto Sb2(S,Se)3 surface present two characteristic diffraction peaks at 14.9 , 29.8 and 15.3 , 30.6 , which are assigned as (100), (200) and (100), (200) crystal planes of cubic MAPbBr3 and CsPbBr3, respectively.26,29 We also observe that this film fabrication process did not influence the crystallinity of both MAPbBr3 and CsPbBr3 QDs. In this case, the perovskite QD film and Sb2(S,Se)3 substrate tend to form an abrupt interface,
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Figure 2. Structures and Spectroscopic Analysis of Absorber Film and Perovskite Quantum Dot Film (A) XRD patterns of Sb 2 (S,Se) 3 , Sb 2 (S,Se) 3 /MAPbBr 3 QD, and Sb 2 (S,Se) 3 /CsPbBr 3 QD films. The black circles, red stars, and purple rhombuses denote the diffraction peaks from FTO, MAPbBr 3 QDs, and CsPbBr 3 QDs, respectively. (B) UV-Vis absorption spectra of Sb 2 (S,Se) 3 , Sb2 (S,Se) 3 /MAPbBr 3 QD, and Sb 2 (S,Se) 3 /CsPbBr3 QD films. (C–E) The synchrotron radiation photoemission spectroscopy (SRPES) spectra of Sb 2 (S,Se) 3 (C), MAPbBr 3 QD (D), and CsPbBr 3 QD films (E). (F) Steady-state PL spectra of Sb 2 (S,Se) 3 , Sb 2 (S,Se) 3 /MAPbBr 3 QD, and Sb 2 (S,Se) 3 /CsPbBr 3 QD films. (G) Energy level diagram of the device with perovskite QD HEMs.
suggesting that there is no element interdiffusion in the formation of heterojunction. Additionally, ultraviolet-visible (UV-Vis) absorption of Sb2(S,Se)3/MAPbBr3 QD film and Sb2(S,Se)3/CsPbBr3 QD film exhibits nearly identical spectra to that of Sb2(S,Se)3 film (Figure 2B). This is understandable because MAPbBr3 QD HEM or CsPbBr3 QD HEM is too thin (25 nm) to contribute to the light absorption. The valance band maximum (VBM) of Sb3(S,Se)3 and perovskite QD films was determined by synchrotron radiation photoemission spectroscopy (SRPES), and the
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Figure 3. Device Structure and Photovoltaic Performance (A and B) Device structure. Schematic illustration (A) and cross-sectional SEM image (B) of typical device with the configuration of FTO/CdS/Sb2 (S,Se) 3 / perovskite QDs/Au are shown. Scale bar, 300 nm. (C–E) Photovoltaic performance. Current density-voltage (J-V) curves (C) and external quantum efficiency (EQE) spectra (E) of control device and those with MAPbBr 3 QDs or CsPbBr 3 QDs as HEMs. Statistical histograms (D) of control device and those with MAPbBr 3 QDs or CsPbBr 3 QDs as HEMs.
conduction band minimum was obtained by subtraction of the band gap measured by UV-Vis absorption spectroscopy (Figures 2C–2E and S2). According to the (Ahv)2 versus energy (hv) curves, the band gap (Eg) of Sb2(S,Se)3, MAPbBr3 QD, and CsPbBr3 QD films were 1.52, 2.3, and 2.33 eV, respectively. The detailed calculation process for energy levels is shown in the Supplemental Information (Figure S5 and Table S2). The band alignment is summarized in Figure 2G. We find that the perovskite QD films show similar energy levels to the corresponding bulk or film counterpart.30–33 It can be seen that both the VBM of perovskite QD films are lower than that of Sb2(S,Se)3. To examine the hole extraction efficiency, we conducted steady-state photoluminescence (PL) measurement (Figure 2F). The results show that both MAPbBr3 QDs and CsPbBr3 QDs are able to efficiently extract holes from Sb2(S,Se)3. In this case, we confirm that the barriers with regard to the VBM of Sb2(S,Se)3, 0.36 eV and 0.19 eV for MAPbBr3 QDs and CsPbBr3 QDs (Figure 2G), are so small that the hole transportation from Sb2(S,Se)3 to perovskite QD HEMs becomes feasible, thus offering the possibility of perovskite QDs as HEMs in Sb2(S,Se)3 solar cells.34,35 Device Performance and Photoelectrochemical Study The solar devices were completed by thermally evaporating Au electrode onto the perovskite QD HEMs. Figures 3A and 3B show the scheme of the device configuration and cross-sectional scanning electron microscope (SEM) image of the device, respectively. For the control device, Au was directly evaporated onto Sb2(S,Se)3 surface. The current density-voltage (J-V) curves of Sb2(S,Se)3 solar cells are presented in Figure 3C, and the corresponding photovoltaic parameters are summarized in Table 1. From Table 1, we found that the open-circuit voltage (Voc) increased
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Table 1. Photovoltaic Parameters of Control Device and Those with MAPbBr3 QDs or CsPbBr3 QDs as Hole-Extraction Layer under 100 mW cm2 Air Mass 1.5 Global Illumination Devices
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
Sb2(S,Se)3
0.51
18.02
48.15
4.43
Sb2(S,Se)3/MAPbBr3
0.58
19.05
58.24
6.42
Sb2(S,Se)3/CsPbBr3
0.62
21.50
58.55
7.82
from 0.51 V for control device to 0.58 V and 0.62 V for the devices with MAPbBr3 QDs and CsPbBr3 QDs as HEMs, respectively (Table S1). The devices based on MAPbBr3 QD HEM and CsPbBr3 QD HEM delivered enhanced fill factor (FF) by up to 10% compared with the control device. Moreover, the device with CsPbBr3 QD HEM exhibited higher short-circuit current (Jsc) improvement (3.48 mA cm2) than that of MAPbBr3 QD HEM (1.03 mA cm2). The champion device efficiency of 7.82% was achieved by using CsPbBr3 QDs as HEMs, with a Voc of 0.62 V, a Jsc of 21.50 mA cm2, and a FF of 58.55% (Figure 3C; Table 1). The statistical results of three kinds of Sb2(S,Se)3 solar cells unambiguously demonstrate that MAPbBr3 QDs and CsPbBr3 QDs could be efficient HEM for the inorganic thin-film solar cells (Figure 3D). Because the sizes of QDs are quite small, the deposition of QD films would probably fill the holes existing on the surface of Sb2(S,Se)3. In this regard, perovskite QDs act as an effective pinhole-blocking layer, which increases Voc and FF and thus improves device performance.36 For comparison, we summarized the best-reported efficiencies for planar antimony chalcogenides (Sb2S3, Sb2(S,Se)3, and Sb2Se3) solar cells in Table S3 and found the PCE in this work is the highest efficiency ever reported in Sb2S3 and Sb2(S,Se)3 solar cells and a top value in Sb2Se3 solar cells. External quantum efficiency (EQE) measurements demonstrated that devices with perovskite QD HEMs had higher photoresponse capability compared to control device, consistent with improved hole extraction and increased Jsc values (Figure 3E). In addition, perovskite QD HEM can enhance built-in electric field and increase the hole drift length, which boosts the collection of photogenerated holes near the front CdS/ Sb2(S,Se)3 heterojunction. Consequently, the strong EQE enhancement at shortwavelength region (300–500 nm) is obtained. To gain insight into the performance enhancement by perovskite QD HEMs, we characterized the devices by means of transient photocurrent and photovoltage decays. The decay time can be well fitted by an exponential equation: y = y0 + Aex=t :
(Equation 1)
It is found that the photovoltage decay time is significantly increased from 0.4 ms for the control device to 2.4 ms and 14.8 ms for those with MAPbBr3 QDs and CsPbBr3 QDs as HEMs, respectively (Figure 4A), indicating that the devices with CsPbBr3 QDs possess the longest carrier recombination lifetime. Meanwhile, the decay time of the photocurrent is reduced from 9.1 ms (control device) to 8.2 ms (MAPbBr3 QDs) and 4.4 ms (CsPbBr3 QDs; Figure 4B), suggesting a quicker charge transport time in the device with perovskite QDs. Both of the observations indicate a dramatic improvement of charge extraction and transport and reduced charge recombination rate with perovskite QD films as HEMs.37 This is consistent with the PL results, where higher level of PL quenching in Sb2(S,Se)3/CsPbBr3 QDs sample is observed when compared with Sb2(S,Se)3/MAPbBr3 QDs (Figure 2F).38 Therefore, a better performance is expected for the device based on CsPbBr3 QDs as HEM.
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Figure 4. Photoelectrochemical Characterization of the Devices (A–D) Transient photovoltage (A), transient photocurrent delay curves (B), Nyquist plots (C), and dark J-V (D) of the control device and those with MAPbBr 3 QDs or CsPbBr 3 QDs as HEMs. The inset image in (C) is the corresponding equivalent circuit. (E–G) Capacitance-voltage (C-V) characteristic curves of control device (E) and those with MAPbBr 3 QDs (F) or CsPbBr 3 QDs (G) as HEMs.
The electrical impedance spectroscopy (EIS) measurements were also carried out to investigate the interfacial charge transfer properties. Figure 4C presents the Nyquist plots of the devices measured at an applied bias of 0.5 V in the dark; the corresponding equivalent circuit is shown as inset in Figure 4C. The high-frequency component is the signature of the transfer resistance (Rs), and the low-frequency arc is corresponding to the recombination resistance (Rrec).39 Consequently, the devices with perovskite QDs exhibited larger low-frequency arc and smaller Rs when compared with the control devices (Table S4). Among the devices, the use of CsPbBr3 QDs as HEMs generates the most efficient hole transport and the slowest charge recombination rate. To further evaluate the charge carrier leakage and the recombination of free carriers, the dark J-V of these three types of devices were measured. As shown in Figure 4D, the device with perovskite QD HEMs exhibit a lower leakage current, suggesting that the perovskite QD films could reduce the current leakage pathway throughout the whole cell and retard the carrier recombination between the
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interface of Sb2(S,Se)3 and metal electrode. Thus, Voc is improved for the device with perovskite QD HEMs according to the following equation:40 AkT Jsc ln Voc = +1 ; (Equation 2) q J0 where J0 is the reverse saturation current density, q is the electron charge, A is the diode ideality factor, and k is the Boltzmann constant. To quantitatively investigate the impact of perovskite QD HEM on the charge carrier concentration and built-in potential, Mott-Schottky analysis of the capacitance versus voltage curve was conducted. The carrier concentration is inversely proportional to the slope of Mott-Schottky plot (1/C2 is plotted against the applied voltage V), and the built-in potential is estimated by the X-intercept based on the following equation:41 1 2 ðVbi V Þ; = C 2 ε0 εeA2 N
(Equation 3)
where C is capacitance, A is device area, ε0 is the vacuum dielectric constant, ε is the relative dielectric constant of the Sb2(S,Se)3 films with a value of 5, e is elementary charge, N is carrier concentration, and Vbi is the built-in potential, respectively. Capacitance-voltage (C-V) characteristic curves directly demonstrated higher built-in potentials (Vbi) of the device with perovskite QD HEMs compared with the control device. The enhanced Vbi contributes the increased Voc.42 Therefore, all the above results confirm that the perovskite QD HEMs can significantly improve the device performance by enhancing the charge extraction efficiency and suppressing the interface recombination. The most efficient improvement was achieved by using CsPbBr3 QDs as HEM. Presumably, the higher hole mobility for CsPbBr3 QDs (4,500 cm2 V1 s1)43 than that of MAPbBr3 QDs (4.3 cm2 V1 s1),44,45 along with the purification treatment for CsPbBr3 QD colloidal solution with ethyl acetate to remove excessive ligands, contribute to the best-performing CsPbBr3 QD HEM-based Sb2(S,Se)3 solar cells.28 Device Stability Examination Finally, we studied the stability of the devices, which were stored in the ambient air with relative humidity (RH) >15% and room temperature (RT) for 100 days without encapsulation. Remarkably, the devices with perovskite QD HEMs exhibited no PCE degradation, whereas the efficiency of the device with spiro-OMeTAD, a typical hole transporting material used in both Sb2(S,Se)3 and perovskite solar cells, reduced to 79% of its initial PCE (Figure 5). The outstanding stability of device with perovskite QDs can be ascribed to the natural stability of Br-based perovskite QDs. These results suggest that the perovskite QD films are stable and promising HEMs for application in Sb2(S,Se)3 solar cells.
DISCUSSION In summary, we have demonstrated that lead bromide perovskite quantum dots, MAPbBr3 and CsPbBr3, can function as efficient hole-extraction materials for inorganic thin-film solar cells. By controlling the surface functionalities and fabrication process, compact and highly uniform perovskite QD films can be formed on the surface of Sb2(S,Se)3, which effectively extract holes from Sb2(S,Se)3 light-harvesting layer and suppress the interface recombination. In addition, comparative study shows CsPbBr3 QDs lead to better-matched energy level alignment and thus higher efficiency when compared with CH3NH3PbBr3 QDs in the solar cell applications.
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Figure 5. Stability Tests Stability test for the devices with MAPbBr 3 QDs, CsPbBr 3 QDs, or Spiro-OMeTAD as HEMs without encapsulation under ambient condition (RT, >15% humidity).
Eventually, the optimized devices with CsPbBr3 QD film as HEM achieved a superior PCE of 7.82%, which is the highest reported value for antimony selenosulfide solar cells to the best of our knowledge. More importantly, the unencapsulated devices with perovskite QD HEMs exhibit no decline in PCE after being stored in ambient air with relative humidity of >15% at room temperature for over 100 days. This work expands the application field of perovskite quantum dot materials and offers a convenient strategy for improving the efficiency of Sb2(S,Se)3 solar cells.
EXPERIMENTAL PROCEDURES Chemicals Lead (II) bromide (PbBr2) (99.99%; Xi’an p-OLED); methanaminium bromide (MABr) (99%; Advanced Election Technology); N, N-dimethylformamide (DMF) (99%; J&K); N-octylamine (C8H19N) (99%; Aladdin); cesium acetate (CH3COOCs) (99.99%; Aladdin); lead acetate trihydrate (Pb(CH3COO)2$3H2O) (99.998%; Aladdin); benzoyl bromide (C6H5COBr) (98%; Aladdin); oleic acid (OA) (90%; Alfa Aesar); oleylamine (OAm) (70%; Aldrich); octadecene (ODE) (90%; Aldrich); octane (C8H18) (anhydrous 99%; Alfa Aesar); ethyl acetate (C4H8O2) (AR; Sinopharm); n-hexane (C6H14) (AR; Sinopharm); cadmium nitrate tetrahydrate (Cd(NO)2$4H2O) (AR; Sinopharm); thiourea (CH4N2S) (AR; Sinopharm); cadmium chloride hemi (pentahydrate) (CdCl2$2.5H2O) (AR; Sinopharm); potassium antimony (III) L(+)-tartrate hemihydrate (C4H4KO7Sb$0.5H2O) (AR; Sinopharm); sodium thiosulfate pentahydrate (Na2S2O3$5H2O) (AR; Sinopharm); acetone (C3H6O) (AR; Sinopharm); and all the chemicals were used as received without further purification. MAPbBr3 QD Solution Preparation MAPbBr3 QDs with average diameter of 7 nm were synthesized using the ligand-assisted reprecipitation (LARP) technique referring to the reported method.23 In brief, reagents of MABr (0.3 mmol MABr dissolved in 0.3 mL DMF) and PbBr2 (0.2 mmol PbBr2 dissolved in 0.5 mL DMF) were dropped into a mixture of hexane (10 mL), oleic acid (0.5 mL), and octylamine (20 mL). A certain amount of tert-butanol (8 mL) was used as demulsifier and dropped into the reaction system to initiate a demulsion process. The resulting suspension was precipitated from the solution by centrifugation at 7,000 rpm for 5 min. Then, the precipitates were redissolved into hexane to extract the colloidal QDs. After another centrifugation at 5,000 rpm for 5 min, a bright yellow-green colloidal solution was obtained (Figure S4). The MAPbBr3 QDs solution was filtered with 0.45-mm polyvinylidene fluoride (PVDF) filters for later use.
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CsPbBr3 QD Solution Preparation and Purification Process CsPbBr3 QDs were synthesized following the previous report with some modifications.24 CH3COOCs (19 mg; 0.1 mmol), Pb(CH3COO)2$3H2O (76 mg; 0.2 mmol), and ODE (5.0 mL) were loaded into 25 mL of 3-neck flask and dried under vacuum at 120 C for 15 min and then filled with N2 and kept under constant N2 flow for 10 min. The above process was repeated twice. Dried OA (0.3 mL) and OAm (1.0 mL) were injected at 120 C under N2 flow. About 5 min later, the flask was put into vacuum again for 15 min until the solution was no longer releasing gas. The above process was repeated twice. The temperature was increased to 170 C under N2, and 120 mL (1.0 mmol) of benzoyl bromide was swiftly injected. About 10 s later, the reaction mixture was immediately cooled down to room temperature by immersion in a cold-water bath. The obtained QDs in crude reaction solutions were precipitated by adding 5 mL of ethyl acetate and then centrifuged at 12,000 rpm for 5 min. After centrifugation, the supernatant was discarded and the QDs were redispersed in anhydrous hexane and precipitated again with the addition of ethyl acetate (hexane: ethyl acetate = 1:2 by volume). The nanocrystal dispersion was filtered through a 0.22-mm filter and diluted to 10 mg mL1 in octane before use. Device Fabrication FTO-coated glass supplied by Advanced Election Technology was patterned by laser scribing, followed by ultrasonic cleaning in deionized water, isopropanol, acetone, and ethanol for 30 min, respectively. After drying, the substrate was treated by UV ozone cleaner for 15 min. Chemical bath deposition (65 C; 15 min) was used to deposit a CdS buffer layer with the thickness of 60 nm.46 Subsequently, CdS layer was treated with 20 mg mL1 CdCl2 absolute methanol solution by spin coating at a speed of 3,000 rpm for 30 s, baked on the hotplate at 400 C for 5 min in air ambient, and then cooled down to room temperature naturally. The Sb2(S,Se)3 film was deposited onto CdS buffer layer based on the reported method with additional modifications.22 The thickness of Sb2(S,Se)3 film was controlled to be 280 nm with S/Se atomic ratio of 6.38 (Figure S3). Afterward, the perovskite QD solution was spin coated onto Sb2(S,Se)3/CdS/FTO substrate at a speed of 2,000 rpm for 30 s. Then, 50 mL ethyl acetate was quickly dropped on the perovskite QDs HEMs during the spinning process at a speed of 2,000 rpm for 60 s. Eventually, Au counter electrode was deposited through a shadow mask by a thermal evaporator. The active area of the device was defined as 0.09 cm2. Characterization Transmission electron microscope (TEM) images were taken (JEM-2100F) at an accelerated voltage of 200 kV. XRD patterns of samples were performed on a Bruker Advance D8 diffractometer equipped with Cu Ka radiation (l = 1.5416 A˚). The optical characteristics of the films were measured with a UV-visible spectrophotometer (SOLID 3700). The surface and cross-sections morphologies of the samples were examined by SEM (FE-SEM. SU 220). Band energies of Sb2(S,Se)3, MAPbBr3 QD, and CsPbBr3 QD films were performed by synchrotron radiation photoemission spectroscopy (SRPES) at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Steady-state PL was measured using a LabRamHR system with excitation at 532 nm. The J-V curves were recorded using a Keithley 2400 apparatus under solar-simulated AM 1.5 sunlight (100 mW cm2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, Japan). The solar simulator illumination intensity was calibrated by a monocrystalline silicon reference cell (Oriel P/N 91150 V, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). The external quantum efficiency (EQE) (model SPIEQ200) was measured using a single-source illumination system (halogen lamp) combined with a monochromator. Electrochemical impedance spectroscopy (EIS) measurements were performed
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using Zahner Mess Systeme PP211 electrochemical workstation at a bias potential of 0.50 V in dark with the frequency ranging from 100 Hz to 1 MHz. The capacitance-voltage (C-V) curves were obtained from Zahner Mess Systeme PP211 electrochemical workstation at room temperature in darkness at a frequency of 10 kHz, and the AC amplitude was 5 mV. DC bias voltage was changed from 0.1 to 1.2 V. The transient photovoltage delay and transient photocurrent delay (Zahner PP211 and Zahner Zennium) were generated by a microsecond pulse of a white light incident without bias light on solar cells under short circuit conditions and open circuit condition.
DATA AND CODE AVAILABILITY The authors declare that data supporting the findings of this study are available within the paper and the Supplemental Information. All other data are available from the Lead Contact upon reasonable request.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.xcrp. 2019.100001.
ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2019YFA0405600); National Natural Science Foundation of China (U1732150, 21503203, 21603012, and 21875236); and Fundamental Research Funds for the Central Universities, China under nos. WK2060140023, CX3430000001, and WK2060190085.
AUTHOR CONTRIBUTIONS T.C. conceived the idea and proposed the experimental design. C.J. performed device fabrication and data analysis. J.Y., P.H., S.C., H.Y., and H. Zhong synthesized perovskite QD solution and characterized their properties. R.T. and X.W. conducted XRD and SEM and characterized the PV performances. X.L. and H. Zeng carried out PL, transient photocurrent, and photovoltage decay measures and analyzed the results. C.J., T.C., and C.Z. co-wrote the manuscript, and all authors commented on and revised the manuscript.
DECLARATIONS OF INTEREST The authors declare no competing interests. Received: September 13, 2019 Revised: October 8, 2019 Accepted: November 13, 2019 Published: December 11, 2019
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Perovskite Quantum Dots Exhibiting Strong Hole Extraction Capability for Efficient Inorganic Thin Film Solar Cells Chenhui Jiang, Jisong Yao, Peng Huang, ..., Hongbin Yao, Changfei Zhu, Tao Chen [email protected]
HIGHLIGHTS Perovskite quantum dots can serve as efficient hole-extraction materials Sb2(S,Se)3 solar cell using CsPbBr3 QDs achieved an efficiency of 7.82% Perovskite-QDs-based devices exhibit no decline in PCE over 100 days in air
Perovskite QDs serve as efficient hole-extraction material in thin-film solar cells. Jiang et al. report a surface treatment coupled with film fabrication leads to ultrathin (25 nm) perovskite QD film on the surface of planar Sb2(S,Se)3 lightharvesting material and produce a device based on perovskite QDs/Sb2(S,Se)3 heterojunction, resulting in a power conversion efficiency of 7.82%.
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Article
Perovskite Quantum Dots Exhibiting Strong Hole Extraction Capability for Efficient Inorganic Thin Film Solar Cells Chenhui Jiang,1 Jisong Yao,2 Peng Huang,3 Rongfeng Tang,1 Xiaomin Wang,1 Xunyong Lei,4 Hualing Zeng,4 Shuai Chang,3 Haizheng Zhong,3 Hongbin Yao,2 Changfei Zhu,1 and Tao Chen1,5,*
SUMMARY Inorganic semiconductor Sb2(S,Se)3 possesses a suitable bandgap, environmentally benign elemental composition, and excellent stability, offering ample promise for next-generation low-cost solar cells. Here, we demonstrate that perovskite quantum dots (QDs), including CH3NH3PbBr3 and CsPbBr3, can serve as highly efficient and air-stable hole extraction materials in Sb2(S,Se)3 solar cells. Through a proper pretreatment of the colloidal QDs, a 25-nm-thick QD film can be obtained with excellent uniformity and charge transport properties. Spectroscopic and photoelectrochemical analysis show that perovskite QDs can effectively extract holes from Sb2(S,Se)3 with suppressed carrier recombination. The perovskite QDs/Sb2(S,Se)3 heterojunction also establishes an increased built-in potential so that open-circuit voltage is pronouncedly enhanced. Finally, the device based on perovskite QDs/Sb2(S,Se)3 heterojunction boosts the efficiency from 4.43% to 7.82%, setting a record value, to the best of our knowledge, in Sb2(S,Se)3 solar cells. Our research manifests another application of perovskite materials and practical strategy toward efficiency improvement of Sb2(S,Se)3 solar cells.
INTRODUCTION Halide perovskite materials have received increasing attention due to the improving power conversion efficiency (PCE) in solar cell applications. In less than 10 years, the PCE has been boosted to 25%.1 This fast growth in PCE has stimulated great interest to investigate the fundamental properties of perovskite materials and in turn found many potential applications. For instance, the high photoluminescence quantum efficiency of perovskites enables potential applications in light-emitting diodes.2,3 Due to both high extinction coefficient and large carrier mobility, the application of CsPbX3 in photodetectors has been demonstrated.4,5 In addition, the highly efficient luminescence and biexciton binding energy make perovskites promising laser materials.6,7 In this research, we demonstrate another application of perovskite quantum dots (QDs) as hole extraction materials (HEMs) in antimony selenosulfide (Sb2(S,Se)3) solar cells. Sb2(S,Se)3 is a light-harvesting material consisting of environmentally benign and earth-abundant elements. It is also stable against moisture and air, promising for practical applications.8,9 Fundamentally, it has superior photovoltaic properties, such as tunable bandgap of 1.1–1.7 eV,10,11 high absorption coefficient (>105 cm1),12,13 and mobility (42 cm2 V1 s1).14 In terms of future applications, one of the major tasks at this stage is to boost the power conversion efficiency (PCE).15,16 To date, PCE of Sb2(S,Se)3 solar cells has reached 6.6%,17,18 and a closely relevant
1CAS
Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
2Department
of Applied Chemistry, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230026, China
3School
of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
4International
Center for Quantum Design of Functional Materials, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China
5Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.xcrp.2019.100001
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Sb2Se3 solar cell delivered PCE of 9.2%;19 this achievement suggests great potential among the new materials explored for solar cell application.20,21 It is also noted that high efficiencies are generally achieved by using highly expensive organic HEMs, such as poly(3-hexylthiophene), 2,20 ,7,7’-tetrakis(N,N-di[4-methoxyphenyl] amino) 9,9’-spirobifluorene (spiro-OMeTAD), and so on. The organic HEMs, however, bring about stability issue when exposed to air and/or moisture due to the doping of inorganic ions and/or the intrinsic chemical instability. Compared to iodide perovskite, Br-based perovskite is more stable due to the reduced lattice parameter and optimized tolerance factor. With surfactant protection, the stability of Br-based perovskite QDs is further improved. Here, we introduce perovskite QDs into inorganic thin-film solar cells as low-cost HEMs. The perovskite QDs boost the device efficiency from 4.43% to 7.82%, which is the highest efficiency in all reported Sb2(S,Se)3 solar cells to the best of our knowledge. Notably, the employment of QD film as HEM displays essentially enhanced device stability when compared with conventionally applied spiro-OMeTAD. The unencapsulated devices show no PCE degradation in ambient air for 100 days. This study demonstrates the utilization of perovskite QDs as an efficient air-stable HEM in Sb2(S,Se)3 solar cells, extending the application scope of halide perovskites. The film fabrication, heterojunction characteristics, and charge transfer kinetics are extensively studied.
RESULTS Characterization of Perovskite Quantum Dots and Sb2(S,Se)3 Films We fabricated solar cells with planar heterojunction device architecture composed of fluorine-doped tin oxide (FTO)/CdS/Sb2(S,Se)3/perovskite QDs/Au, where the CdS and Sb2(S,Se)3 were synthesized by chemical bath deposition and modified hydrothermal deposition methods.22 The detailed synthetic procedure is provided in the Experimental Procedures. As a result, Sb2(S,Se)3 showed flat-surface morphology with gain size up to micrometer scale (Figure 1A). The elements of Sb, S, and Se are evenly distributed across the film (Figure S1B). During the synthesis process, the Sb2(S,Se)3 composition is tunable with regard to S/Se precursor ratios. Here, we only study the compound with Sb:S:Se atomic ratios of 1:1.25:0.19 (Figure S1A), because this composition delivers highest efficiency when compared with others. MAPbBr3 (MA = CH3NH3) or CsPbBr3 QDs were synthesized by reported methods with some refinements in which oleic acid and n-octylamine (or oleylamine) were used as surfactants.23,24 The average sizes of MAPbBr3 and CsPbBr3 QDs are 7 nm and 15 nm, respectively (Figures 1B and 1C). Both MAPbBr3 and CsPbBr3 QDs exhibit sharp photoluminescence (PL) emission with narrow full width at the half-maximum (FWHM) (Figures 1D and 1E), indicating a narrow size distribution of the QDs. In QDs, both the band gap and energy level are size dependent. The narrow size distribution permits favorable energy alignment, which in turn facilitates carrier transport. The colloidal stability of QDs originates from the surfactant tethering on the nanoparticle surface, and the surfactant usually gives rise to series resistance of films when the QDs are directly applied for film preparation.25,26 The surfactant should be partially removed or replaced by small molecules.27,28 In the current study, the treatment of MAPbBr3 and CsPbBr3 QDs is quite different because of their distinct surface nature. To partially remove the surfactant, we utilize hexane to extract MAPbBr3 QDs. In contrast, CsPbBr3 QDs are stabilized by oleic acid and oleic amine, which have long aliphatic chains. A surface ligand density control (SLDC) by using hexane and ethyl acetate mixed solvent was adopted to partially remove ligand on the CsPbBr3 QD surface. Otherwise, the utilization of QDs without above purification process generates rather poor device performance. After the treatment, a
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Figure 1. Morphological Characterization and Spectroscopic Analysis (A, H, and I) SEM images of Sb2 (S,Se) 3 film (A), MAPbBr 3 QD film (H), and CsPbBr 3 QD film (I) on Sb 2 (S,Se) 3 surface. Scale bars, 1 mm (A) and 500 nm (H and I). (B and C) TEM images of MAPbBr 3 QDs (B) and CsPbBr 3 QDs (C). Scale bars, 50 nm. (D and E) UV-Vis absorption and PL spectra of MAPbBr3 QD film (D) and CsPbBr 3 QD film (E). (F and G) EDX mappings (Br) of MAPbBr3 QD film (F) and CsPbBr3 QD film (G) deposited onto Sb 2 (S,Se) 3 surface. Scale bars, 1 mm. (J) Cross-sectional SEM image of CsPbBr3 QD film on FTO/CdS/Sb 2 (S,Se) 3 substrate. Insets are schematic crystal structures of CsPbBr 3 (top) and Sb 2 (S,Se) 3 (bottom). Scale bar, 500 nm.
spin-coating process can be adopted for the film preparation and QD films with excellent uniformity were obtained (Figures 1F–1J, S3, and S4). Figure 2A shows characteristic X-ray diffraction (XRD) patterns of pristine Sb2(S,Se)3 and Sb2(S,Se)3/perovskite QD films. The annealed Sb2(S,Se)3 film displays typical stibnite crystal structure. Obviously, the spin-coated perovskite QD HEMs did not damage the crystallinity of Sb2(S,Se)3 film. MAPbBr3 and CsPbBr3 QD HEMs deposited onto Sb2(S,Se)3 surface present two characteristic diffraction peaks at 14.9 , 29.8 and 15.3 , 30.6 , which are assigned as (100), (200) and (100), (200) crystal planes of cubic MAPbBr3 and CsPbBr3, respectively.26,29 We also observe that this film fabrication process did not influence the crystallinity of both MAPbBr3 and CsPbBr3 QDs. In this case, the perovskite QD film and Sb2(S,Se)3 substrate tend to form an abrupt interface,
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Figure 2. Structures and Spectroscopic Analysis of Absorber Film and Perovskite Quantum Dot Film (A) XRD patterns of Sb 2 (S,Se) 3 , Sb 2 (S,Se) 3 /MAPbBr 3 QD, and Sb 2 (S,Se) 3 /CsPbBr 3 QD films. The black circles, red stars, and purple rhombuses denote the diffraction peaks from FTO, MAPbBr 3 QDs, and CsPbBr 3 QDs, respectively. (B) UV-Vis absorption spectra of Sb 2 (S,Se) 3 , Sb2 (S,Se) 3 /MAPbBr 3 QD, and Sb 2 (S,Se) 3 /CsPbBr3 QD films. (C–E) The synchrotron radiation photoemission spectroscopy (SRPES) spectra of Sb 2 (S,Se) 3 (C), MAPbBr 3 QD (D), and CsPbBr 3 QD films (E). (F) Steady-state PL spectra of Sb 2 (S,Se) 3 , Sb 2 (S,Se) 3 /MAPbBr 3 QD, and Sb 2 (S,Se) 3 /CsPbBr 3 QD films. (G) Energy level diagram of the device with perovskite QD HEMs.
suggesting that there is no element interdiffusion in the formation of heterojunction. Additionally, ultraviolet-visible (UV-Vis) absorption of Sb2(S,Se)3/MAPbBr3 QD film and Sb2(S,Se)3/CsPbBr3 QD film exhibits nearly identical spectra to that of Sb2(S,Se)3 film (Figure 2B). This is understandable because MAPbBr3 QD HEM or CsPbBr3 QD HEM is too thin (25 nm) to contribute to the light absorption. The valance band maximum (VBM) of Sb3(S,Se)3 and perovskite QD films was determined by synchrotron radiation photoemission spectroscopy (SRPES), and the
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Figure 3. Device Structure and Photovoltaic Performance (A and B) Device structure. Schematic illustration (A) and cross-sectional SEM image (B) of typical device with the configuration of FTO/CdS/Sb2 (S,Se) 3 / perovskite QDs/Au are shown. Scale bar, 300 nm. (C–E) Photovoltaic performance. Current density-voltage (J-V) curves (C) and external quantum efficiency (EQE) spectra (E) of control device and those with MAPbBr 3 QDs or CsPbBr 3 QDs as HEMs. Statistical histograms (D) of control device and those with MAPbBr 3 QDs or CsPbBr 3 QDs as HEMs.
conduction band minimum was obtained by subtraction of the band gap measured by UV-Vis absorption spectroscopy (Figures 2C–2E and S2). According to the (Ahv)2 versus energy (hv) curves, the band gap (Eg) of Sb2(S,Se)3, MAPbBr3 QD, and CsPbBr3 QD films were 1.52, 2.3, and 2.33 eV, respectively. The detailed calculation process for energy levels is shown in the Supplemental Information (Figure S5 and Table S2). The band alignment is summarized in Figure 2G. We find that the perovskite QD films show similar energy levels to the corresponding bulk or film counterpart.30–33 It can be seen that both the VBM of perovskite QD films are lower than that of Sb2(S,Se)3. To examine the hole extraction efficiency, we conducted steady-state photoluminescence (PL) measurement (Figure 2F). The results show that both MAPbBr3 QDs and CsPbBr3 QDs are able to efficiently extract holes from Sb2(S,Se)3. In this case, we confirm that the barriers with regard to the VBM of Sb2(S,Se)3, 0.36 eV and 0.19 eV for MAPbBr3 QDs and CsPbBr3 QDs (Figure 2G), are so small that the hole transportation from Sb2(S,Se)3 to perovskite QD HEMs becomes feasible, thus offering the possibility of perovskite QDs as HEMs in Sb2(S,Se)3 solar cells.34,35 Device Performance and Photoelectrochemical Study The solar devices were completed by thermally evaporating Au electrode onto the perovskite QD HEMs. Figures 3A and 3B show the scheme of the device configuration and cross-sectional scanning electron microscope (SEM) image of the device, respectively. For the control device, Au was directly evaporated onto Sb2(S,Se)3 surface. The current density-voltage (J-V) curves of Sb2(S,Se)3 solar cells are presented in Figure 3C, and the corresponding photovoltaic parameters are summarized in Table 1. From Table 1, we found that the open-circuit voltage (Voc) increased
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Table 1. Photovoltaic Parameters of Control Device and Those with MAPbBr3 QDs or CsPbBr3 QDs as Hole-Extraction Layer under 100 mW cm2 Air Mass 1.5 Global Illumination Devices
Voc (V)
Jsc (mA cm2)
FF (%)
PCE (%)
Sb2(S,Se)3
0.51
18.02
48.15
4.43
Sb2(S,Se)3/MAPbBr3
0.58
19.05
58.24
6.42
Sb2(S,Se)3/CsPbBr3
0.62
21.50
58.55
7.82
from 0.51 V for control device to 0.58 V and 0.62 V for the devices with MAPbBr3 QDs and CsPbBr3 QDs as HEMs, respectively (Table S1). The devices based on MAPbBr3 QD HEM and CsPbBr3 QD HEM delivered enhanced fill factor (FF) by up to 10% compared with the control device. Moreover, the device with CsPbBr3 QD HEM exhibited higher short-circuit current (Jsc) improvement (3.48 mA cm2) than that of MAPbBr3 QD HEM (1.03 mA cm2). The champion device efficiency of 7.82% was achieved by using CsPbBr3 QDs as HEMs, with a Voc of 0.62 V, a Jsc of 21.50 mA cm2, and a FF of 58.55% (Figure 3C; Table 1). The statistical results of three kinds of Sb2(S,Se)3 solar cells unambiguously demonstrate that MAPbBr3 QDs and CsPbBr3 QDs could be efficient HEM for the inorganic thin-film solar cells (Figure 3D). Because the sizes of QDs are quite small, the deposition of QD films would probably fill the holes existing on the surface of Sb2(S,Se)3. In this regard, perovskite QDs act as an effective pinhole-blocking layer, which increases Voc and FF and thus improves device performance.36 For comparison, we summarized the best-reported efficiencies for planar antimony chalcogenides (Sb2S3, Sb2(S,Se)3, and Sb2Se3) solar cells in Table S3 and found the PCE in this work is the highest efficiency ever reported in Sb2S3 and Sb2(S,Se)3 solar cells and a top value in Sb2Se3 solar cells. External quantum efficiency (EQE) measurements demonstrated that devices with perovskite QD HEMs had higher photoresponse capability compared to control device, consistent with improved hole extraction and increased Jsc values (Figure 3E). In addition, perovskite QD HEM can enhance built-in electric field and increase the hole drift length, which boosts the collection of photogenerated holes near the front CdS/ Sb2(S,Se)3 heterojunction. Consequently, the strong EQE enhancement at shortwavelength region (300–500 nm) is obtained. To gain insight into the performance enhancement by perovskite QD HEMs, we characterized the devices by means of transient photocurrent and photovoltage decays. The decay time can be well fitted by an exponential equation: y = y0 + Aex=t :
(Equation 1)
It is found that the photovoltage decay time is significantly increased from 0.4 ms for the control device to 2.4 ms and 14.8 ms for those with MAPbBr3 QDs and CsPbBr3 QDs as HEMs, respectively (Figure 4A), indicating that the devices with CsPbBr3 QDs possess the longest carrier recombination lifetime. Meanwhile, the decay time of the photocurrent is reduced from 9.1 ms (control device) to 8.2 ms (MAPbBr3 QDs) and 4.4 ms (CsPbBr3 QDs; Figure 4B), suggesting a quicker charge transport time in the device with perovskite QDs. Both of the observations indicate a dramatic improvement of charge extraction and transport and reduced charge recombination rate with perovskite QD films as HEMs.37 This is consistent with the PL results, where higher level of PL quenching in Sb2(S,Se)3/CsPbBr3 QDs sample is observed when compared with Sb2(S,Se)3/MAPbBr3 QDs (Figure 2F).38 Therefore, a better performance is expected for the device based on CsPbBr3 QDs as HEM.
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Figure 4. Photoelectrochemical Characterization of the Devices (A–D) Transient photovoltage (A), transient photocurrent delay curves (B), Nyquist plots (C), and dark J-V (D) of the control device and those with MAPbBr 3 QDs or CsPbBr 3 QDs as HEMs. The inset image in (C) is the corresponding equivalent circuit. (E–G) Capacitance-voltage (C-V) characteristic curves of control device (E) and those with MAPbBr 3 QDs (F) or CsPbBr 3 QDs (G) as HEMs.
The electrical impedance spectroscopy (EIS) measurements were also carried out to investigate the interfacial charge transfer properties. Figure 4C presents the Nyquist plots of the devices measured at an applied bias of 0.5 V in the dark; the corresponding equivalent circuit is shown as inset in Figure 4C. The high-frequency component is the signature of the transfer resistance (Rs), and the low-frequency arc is corresponding to the recombination resistance (Rrec).39 Consequently, the devices with perovskite QDs exhibited larger low-frequency arc and smaller Rs when compared with the control devices (Table S4). Among the devices, the use of CsPbBr3 QDs as HEMs generates the most efficient hole transport and the slowest charge recombination rate. To further evaluate the charge carrier leakage and the recombination of free carriers, the dark J-V of these three types of devices were measured. As shown in Figure 4D, the device with perovskite QD HEMs exhibit a lower leakage current, suggesting that the perovskite QD films could reduce the current leakage pathway throughout the whole cell and retard the carrier recombination between the
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interface of Sb2(S,Se)3 and metal electrode. Thus, Voc is improved for the device with perovskite QD HEMs according to the following equation:40 AkT Jsc ln Voc = +1 ; (Equation 2) q J0 where J0 is the reverse saturation current density, q is the electron charge, A is the diode ideality factor, and k is the Boltzmann constant. To quantitatively investigate the impact of perovskite QD HEM on the charge carrier concentration and built-in potential, Mott-Schottky analysis of the capacitance versus voltage curve was conducted. The carrier concentration is inversely proportional to the slope of Mott-Schottky plot (1/C2 is plotted against the applied voltage V), and the built-in potential is estimated by the X-intercept based on the following equation:41 1 2 ðVbi V Þ; = C 2 ε0 εeA2 N
(Equation 3)
where C is capacitance, A is device area, ε0 is the vacuum dielectric constant, ε is the relative dielectric constant of the Sb2(S,Se)3 films with a value of 5, e is elementary charge, N is carrier concentration, and Vbi is the built-in potential, respectively. Capacitance-voltage (C-V) characteristic curves directly demonstrated higher built-in potentials (Vbi) of the device with perovskite QD HEMs compared with the control device. The enhanced Vbi contributes the increased Voc.42 Therefore, all the above results confirm that the perovskite QD HEMs can significantly improve the device performance by enhancing the charge extraction efficiency and suppressing the interface recombination. The most efficient improvement was achieved by using CsPbBr3 QDs as HEM. Presumably, the higher hole mobility for CsPbBr3 QDs (4,500 cm2 V1 s1)43 than that of MAPbBr3 QDs (4.3 cm2 V1 s1),44,45 along with the purification treatment for CsPbBr3 QD colloidal solution with ethyl acetate to remove excessive ligands, contribute to the best-performing CsPbBr3 QD HEM-based Sb2(S,Se)3 solar cells.28 Device Stability Examination Finally, we studied the stability of the devices, which were stored in the ambient air with relative humidity (RH) >15% and room temperature (RT) for 100 days without encapsulation. Remarkably, the devices with perovskite QD HEMs exhibited no PCE degradation, whereas the efficiency of the device with spiro-OMeTAD, a typical hole transporting material used in both Sb2(S,Se)3 and perovskite solar cells, reduced to 79% of its initial PCE (Figure 5). The outstanding stability of device with perovskite QDs can be ascribed to the natural stability of Br-based perovskite QDs. These results suggest that the perovskite QD films are stable and promising HEMs for application in Sb2(S,Se)3 solar cells.
DISCUSSION In summary, we have demonstrated that lead bromide perovskite quantum dots, MAPbBr3 and CsPbBr3, can function as efficient hole-extraction materials for inorganic thin-film solar cells. By controlling the surface functionalities and fabrication process, compact and highly uniform perovskite QD films can be formed on the surface of Sb2(S,Se)3, which effectively extract holes from Sb2(S,Se)3 light-harvesting layer and suppress the interface recombination. In addition, comparative study shows CsPbBr3 QDs lead to better-matched energy level alignment and thus higher efficiency when compared with CH3NH3PbBr3 QDs in the solar cell applications.
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Figure 5. Stability Tests Stability test for the devices with MAPbBr 3 QDs, CsPbBr 3 QDs, or Spiro-OMeTAD as HEMs without encapsulation under ambient condition (RT, >15% humidity).
Eventually, the optimized devices with CsPbBr3 QD film as HEM achieved a superior PCE of 7.82%, which is the highest reported value for antimony selenosulfide solar cells to the best of our knowledge. More importantly, the unencapsulated devices with perovskite QD HEMs exhibit no decline in PCE after being stored in ambient air with relative humidity of >15% at room temperature for over 100 days. This work expands the application field of perovskite quantum dot materials and offers a convenient strategy for improving the efficiency of Sb2(S,Se)3 solar cells.
EXPERIMENTAL PROCEDURES Chemicals Lead (II) bromide (PbBr2) (99.99%; Xi’an p-OLED); methanaminium bromide (MABr) (99%; Advanced Election Technology); N, N-dimethylformamide (DMF) (99%; J&K); N-octylamine (C8H19N) (99%; Aladdin); cesium acetate (CH3COOCs) (99.99%; Aladdin); lead acetate trihydrate (Pb(CH3COO)2$3H2O) (99.998%; Aladdin); benzoyl bromide (C6H5COBr) (98%; Aladdin); oleic acid (OA) (90%; Alfa Aesar); oleylamine (OAm) (70%; Aldrich); octadecene (ODE) (90%; Aldrich); octane (C8H18) (anhydrous 99%; Alfa Aesar); ethyl acetate (C4H8O2) (AR; Sinopharm); n-hexane (C6H14) (AR; Sinopharm); cadmium nitrate tetrahydrate (Cd(NO)2$4H2O) (AR; Sinopharm); thiourea (CH4N2S) (AR; Sinopharm); cadmium chloride hemi (pentahydrate) (CdCl2$2.5H2O) (AR; Sinopharm); potassium antimony (III) L(+)-tartrate hemihydrate (C4H4KO7Sb$0.5H2O) (AR; Sinopharm); sodium thiosulfate pentahydrate (Na2S2O3$5H2O) (AR; Sinopharm); acetone (C3H6O) (AR; Sinopharm); and all the chemicals were used as received without further purification. MAPbBr3 QD Solution Preparation MAPbBr3 QDs with average diameter of 7 nm were synthesized using the ligand-assisted reprecipitation (LARP) technique referring to the reported method.23 In brief, reagents of MABr (0.3 mmol MABr dissolved in 0.3 mL DMF) and PbBr2 (0.2 mmol PbBr2 dissolved in 0.5 mL DMF) were dropped into a mixture of hexane (10 mL), oleic acid (0.5 mL), and octylamine (20 mL). A certain amount of tert-butanol (8 mL) was used as demulsifier and dropped into the reaction system to initiate a demulsion process. The resulting suspension was precipitated from the solution by centrifugation at 7,000 rpm for 5 min. Then, the precipitates were redissolved into hexane to extract the colloidal QDs. After another centrifugation at 5,000 rpm for 5 min, a bright yellow-green colloidal solution was obtained (Figure S4). The MAPbBr3 QDs solution was filtered with 0.45-mm polyvinylidene fluoride (PVDF) filters for later use.
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CsPbBr3 QD Solution Preparation and Purification Process CsPbBr3 QDs were synthesized following the previous report with some modifications.24 CH3COOCs (19 mg; 0.1 mmol), Pb(CH3COO)2$3H2O (76 mg; 0.2 mmol), and ODE (5.0 mL) were loaded into 25 mL of 3-neck flask and dried under vacuum at 120 C for 15 min and then filled with N2 and kept under constant N2 flow for 10 min. The above process was repeated twice. Dried OA (0.3 mL) and OAm (1.0 mL) were injected at 120 C under N2 flow. About 5 min later, the flask was put into vacuum again for 15 min until the solution was no longer releasing gas. The above process was repeated twice. The temperature was increased to 170 C under N2, and 120 mL (1.0 mmol) of benzoyl bromide was swiftly injected. About 10 s later, the reaction mixture was immediately cooled down to room temperature by immersion in a cold-water bath. The obtained QDs in crude reaction solutions were precipitated by adding 5 mL of ethyl acetate and then centrifuged at 12,000 rpm for 5 min. After centrifugation, the supernatant was discarded and the QDs were redispersed in anhydrous hexane and precipitated again with the addition of ethyl acetate (hexane: ethyl acetate = 1:2 by volume). The nanocrystal dispersion was filtered through a 0.22-mm filter and diluted to 10 mg mL1 in octane before use. Device Fabrication FTO-coated glass supplied by Advanced Election Technology was patterned by laser scribing, followed by ultrasonic cleaning in deionized water, isopropanol, acetone, and ethanol for 30 min, respectively. After drying, the substrate was treated by UV ozone cleaner for 15 min. Chemical bath deposition (65 C; 15 min) was used to deposit a CdS buffer layer with the thickness of 60 nm.46 Subsequently, CdS layer was treated with 20 mg mL1 CdCl2 absolute methanol solution by spin coating at a speed of 3,000 rpm for 30 s, baked on the hotplate at 400 C for 5 min in air ambient, and then cooled down to room temperature naturally. The Sb2(S,Se)3 film was deposited onto CdS buffer layer based on the reported method with additional modifications.22 The thickness of Sb2(S,Se)3 film was controlled to be 280 nm with S/Se atomic ratio of 6.38 (Figure S3). Afterward, the perovskite QD solution was spin coated onto Sb2(S,Se)3/CdS/FTO substrate at a speed of 2,000 rpm for 30 s. Then, 50 mL ethyl acetate was quickly dropped on the perovskite QDs HEMs during the spinning process at a speed of 2,000 rpm for 60 s. Eventually, Au counter electrode was deposited through a shadow mask by a thermal evaporator. The active area of the device was defined as 0.09 cm2. Characterization Transmission electron microscope (TEM) images were taken (JEM-2100F) at an accelerated voltage of 200 kV. XRD patterns of samples were performed on a Bruker Advance D8 diffractometer equipped with Cu Ka radiation (l = 1.5416 A˚). The optical characteristics of the films were measured with a UV-visible spectrophotometer (SOLID 3700). The surface and cross-sections morphologies of the samples were examined by SEM (FE-SEM. SU 220). Band energies of Sb2(S,Se)3, MAPbBr3 QD, and CsPbBr3 QD films were performed by synchrotron radiation photoemission spectroscopy (SRPES) at the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Steady-state PL was measured using a LabRamHR system with excitation at 532 nm. The J-V curves were recorded using a Keithley 2400 apparatus under solar-simulated AM 1.5 sunlight (100 mW cm2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, Japan). The solar simulator illumination intensity was calibrated by a monocrystalline silicon reference cell (Oriel P/N 91150 V, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). The external quantum efficiency (EQE) (model SPIEQ200) was measured using a single-source illumination system (halogen lamp) combined with a monochromator. Electrochemical impedance spectroscopy (EIS) measurements were performed
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using Zahner Mess Systeme PP211 electrochemical workstation at a bias potential of 0.50 V in dark with the frequency ranging from 100 Hz to 1 MHz. The capacitance-voltage (C-V) curves were obtained from Zahner Mess Systeme PP211 electrochemical workstation at room temperature in darkness at a frequency of 10 kHz, and the AC amplitude was 5 mV. DC bias voltage was changed from 0.1 to 1.2 V. The transient photovoltage delay and transient photocurrent delay (Zahner PP211 and Zahner Zennium) were generated by a microsecond pulse of a white light incident without bias light on solar cells under short circuit conditions and open circuit condition.
DATA AND CODE AVAILABILITY The authors declare that data supporting the findings of this study are available within the paper and the Supplemental Information. All other data are available from the Lead Contact upon reasonable request.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.xcrp. 2019.100001.
ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China (2019YFA0405600); National Natural Science Foundation of China (U1732150, 21503203, 21603012, and 21875236); and Fundamental Research Funds for the Central Universities, China under nos. WK2060140023, CX3430000001, and WK2060190085.
AUTHOR CONTRIBUTIONS T.C. conceived the idea and proposed the experimental design. C.J. performed device fabrication and data analysis. J.Y., P.H., S.C., H.Y., and H. Zhong synthesized perovskite QD solution and characterized their properties. R.T. and X.W. conducted XRD and SEM and characterized the PV performances. X.L. and H. Zeng carried out PL, transient photocurrent, and photovoltage decay measures and analyzed the results. C.J., T.C., and C.Z. co-wrote the manuscript, and all authors commented on and revised the manuscript.
DECLARATIONS OF INTEREST The authors declare no competing interests. Received: September 13, 2019 Revised: October 8, 2019 Accepted: November 13, 2019 Published: December 11, 2019
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