Enhanced Electronic Properties of SnO2 via Electron Transfer from Graphene Quantum Dots for Efficient Perovskite Solar Cells Jiangsheng Xie,† Kun Huang,† Xuegong Yu,*,† Zhengrui Yang,† Ke Xiao,‡ Yaping Qiang,‡ Xiaodong Zhu,† Lingbo Xu,‡ Peng Wang,‡ Can Cui,‡ and Deren Yang*,† †
State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Center for Optoelectronics Materials and Devices, Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China S Supporting Information *
ABSTRACT: Tin dioxide (SnO2) has been demonstrated as an effective electron-transporting layer (ETL) for attaining high-performance perovskite solar cells (PSCs). However, the numerous trap states in low-temperature solution processed SnO2 will reduce the PSCs performance and result in serious hysteresis. Here, we report a strategy to improve the electronic properties in SnO2 through a facile treatment of the films with adding a small amount of graphene quantum dots (GQDs). We demonstrate that the photogenerated electrons in GQDs can transfer to the conduction band of SnO2. The transferred electrons from the GQDs will effectively fill the electron traps as well as improve the conductivity of SnO2, which is beneficial for improving the electron extraction efficiency and reducing the recombination at the ETLs/perovskite interface. The device fabricated with SnO2:GQDs could reach an average power conversion efficiency (PCE) of 19.2 ± 1.0% and a highest steady-state PCE of 20.23% with very little hysteresis. Our study provides an effective way to enhance the performance of perovskite solar cells through improving the electronic properties of SnO2. KEYWORDS: perovskite solar cells, tin dioxide, graphene quantum dots, electron transfer, electron traps ince researchers in Japan first found the hybrid organic− inorganic perovskite as a photovoltaic material in 2009, it has become a promising candidate for the next generation of solar cells because of its excellent photovoltaic properties and low material costs.1,2 The efficiency of perovskite solar cells (PSCs) has risen from 3.8% to 22.1% via morphology control, device architecture optimization, and interface engineering.3−5 Most reported power conversion efficiency (PCE) values of over 20% are achieved using a mesoporous TiO2 as ETL in PSCs which needs a hightemperature-sintering process.6,7 This high-temperature process makes manufacturing more complex and higher energy consumption, which is not favorable on flexible substrates such as polyethylene terephthalate (PET). Thus, large efforts have focused on employing solution processed ETLs including organic and inorganic semiconductors to realize low-temperature processed N−I−P PSCs.8−11 Recently, the planar PSCs based on low-temperature-processed SnO2 and TiO2 nanocrystallines have achieved a certified efficiency of >20%, which is closed to the state-of-the-art PCE.8,9
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© 2017 American Chemical Society
Compared with TiO2, bulk SnO2 has several advantages such as higher mobility (240 cm2/(V·s)), wider bandgap (3.8 eV), and deeper conduction band, which is a potential candidate for attaining high-performance N−I−P PSCs.8,12,13 The better mobility and deeper conduction band means a more efficient electron transfer from the CH3NH3PbI3 to SnO2, which is beneficial for improving the performance of PSCs.13 Lowtemperature SnO2 films have been prepared by various methods including spin-coating solution,14−17 chemical-bath deposition,18 and atomic layer deposition (ALD).13 Unfortunately, most of the devices based on low-temperature solution processed SnO2 suffer from serious hysteresis which makes it difficult to determine their real PCEs.14−17 Although ionic movement changing the electric field in the device is now widely accepted as the main mechanism that causes hysteresis in the current−voltage curves,19−21 it has been demonstrated that the hysteresis could be alleviated or eliminated through Received: June 11, 2017 Accepted: August 31, 2017 Published: August 31, 2017 9176
DOI: 10.1021/acsnano.7b04070 ACS Nano 2017, 11, 9176−9182
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Figure 1. (a) TEM micrograph of the GQDs. (b) UV−vis absorption spectrum of the GQDs. The inset image is the photograph of the water solution of GQDs. (c) PL spectra of GQDs in water excited by different wavelengths.
Figure 2. (a) XPS spectra of N 1s peaks of GQDs, SnO2, and SnO2:GQDs. (b) Transmission spectra of SnO2 and SnO2:GQDs. The inset image is the subtracted spectrum of these two transmission spectra. (c) J−V characteristics of the device ITO/SnO2:GQDs/Ag under dark and a simulated sunlight (AM 1.5 G). (d) Current responses for the device ITO/SnO2:GQDs/Ag under dark and a simulated sunlight (AM 1.5 G). A voltage of 0.1 V is biased.
then annealing in air at a low temperature (180 °C). The 0D GQDs possess a tunable bandgap associated with the strong quantum confinement and edge effects, making them distinct from conventional 2D graphene.27 We demonstrate that the photogenerated electrons in GQDs can transport to the conduction band of SnO2, which can effectively fill the electron trap states, and improve the Fermi level and conductivity in SnO2. As a result, the planar PSCs based on SnO2:GQDs ETLs can reach a maximum steady-state efficiency of 20.23%, with very little hysteresis.
improving the electron extraction efficiency between the ETLs and perovskite.8,22 Once perovskite films with good quality are achieved, the performance including efficiency and hysteresis effect of PSCs is mainly determined by the ETLs. Thus, we reasoned that the hysteresis of device based on low-temperature solution processed SnO2 is caused by the numerous trap states in SnO2 originated from oxygen vacancies.23,24 These trap states can capture electrons and deteriorate the electronic properties in SnO2, leading to poor charge transport and serious recombination at the ETLs/perovskite interface.23 It is worthy to note that the imperfect interface not only leads to hysteresis behavior but also decreases the device efficiency. Doping low-concentration metal ions in SnO2 is a common method to enhance the conductivity for enhancing the performance and reducing the hysteresis, for example, lithium (Li), yttrium (Y), and stibium (Sb) dopants have been successfully applied.23,25,26 Here, we adopt a strategy to attain high performance and very little hysteresis N−I−P planar PSCs using SnO2:GQDs as ETLs. The SnO2:GQDs ETLs were attained via a simple method through spin-coating the SnCl2 solution by adding a small amount of zero dimentional (0D) GQDs on ITO and
RESULTS AND DISCUSSION Figure 1a shows the high-resolution transmission electron microscopy (TEM) micrograph of the GQDs with a diameter of 5−10 nm. The GQDs are highly crystallized with a lattice spacing of 0.21 nm, which is in good agreement with that in the (1100) plane of graphite.28 The thickness of the GQDs layer is 20.23% with very little hysteresis. The electron transfer from the GQDs to SnO2 can fill the electron trap and improve the electron density in the conduction band, which account for the reduction of work function and the improvement of conductivity in SnO2. The higher conductivity and lower electron traps in ETLs can improve electron extraction efficiency and reduce recombination in the devices, which are beneficial to increase the PCE and reduce the J−V hysteresis. Our results demonstrate a simple and effective way to enhance the device efficiency by improving the electronic properties of ETLs.
ASSOCIATED CONTENT
S Supporting Information *
EXPERIMENTAL DETAILS
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04070. Figures S1−S17 and Table S1 give more details on characterization of the GQDs, SnO2, SnO2:GQDs, and the device analysis data. AFM of GQDs, SEM images of films, conduction of SnO2 and SnO2GQDs, UPS and XPS analysis, PL and PL decay, trap density and mobility, absorption spectra, dark current and stability of device (PDF)
Device Fabrication. The ITO was cleaned sequentially in deionized water, cleaning fluid, acetone, and ethanol under sonication for 5 min, respectively. Then the ITO was dried by nitrogen gas and treated with a UV-ozone machine for 20 min. SnO2 ETLs were prepared by spin-coating precursor solutions of SnCl2·2H2O in ethanol with 23 mg/mL of the clean ITO substrates at 3000 rpm for 30 s. The GQDs were dispersed in water with ∼1 or 5 mg/mL concentration. The SnO2:GQDs ETLs were attained through spincoating the SnCl2·2H2O ethanol solution (23 mg/mL) with adding different amounts of GQDs dispersion on the ITO. The SnO2 with 0.5% and 1% (weight percentage) GQDs were achieved by adding 115 and 230 μL of 1 mg/mL GQDs dispersion in SnCl2·2H2O ethanol solution, respectively. The SnO2 with 2% and 5% GQDs were achieved by adding 92 μL and 230 μL of 5 mg/mL GQDs dispersion in SnCl2· 2H2O ethanol solution, respectively. These samples were annealed in air at 180 °C and then treated with a UV-ozone machine for 5 min. The 461 mg of PbI2, 159 mg of CH3NH3I, and 78 mg of DMSO (molar ratio 1:1:1) were mixed in 600 mg of DMF solution. The prepared solution was dropped on the ITO/SnO2: or SnO2:GQDs substrates and then rapidly spin-coated at 1000 rpm for 10 s and 5000 rpm for another 20 s. 0.6 mL of diethyl ether was drop-casted quickly 15 s before the 5000 rpm spin-coating ended.36,37 The perovskite films were heated at 70 °C for 1 min and 100 °C for 10 min on a hot plate, respectively. After several minutes, a hole-transport material was spincoated on the top of perovskite film at the rotation speed of 3000 rpm for 30 s in glovebox. The hole-transport solution was prepared by dissolving 72.3 mg Spiro-MeOTAD, 17.5 μL solution of 520 mg mL−1 lithium bis (trifluoromethylsulfonyl) imide in acetonitrile, and 28.8 μL 4-tert-butylpyridine in 1 mL chlorobenzene. At last, 100 nm-thick Au electrodes were deposited under a high vacuum ( 175 μm in Solutiongrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (35) Shao, Y.; Yuan, Y.; Huang, J. Correlation of Energy Disorder and Open-circuit Voltage in Hybrid Perovskite Solar Cells. Nat. Energy 2016, 1, 15001. (36) Xie, J.; Yu, X.; Sun, X.; Huang, J.; Zhang, Y.; Lei, M.; Huang, K.; Xu, D.; Tang, Z.; Cui, C.; Yang, D. Improved Performance and Air Stability of Planar Perovskite Solar Cells via Interfacial Engineering Using a Fullerene Amine Interlayer. Nano Energy 2016, 28, 330−337. (37) Zhang, Y.; Wang, P.; Yu, X.; Xie, J.; Sun, X.; Wang, H.; Huang, J.; Xu, L.; Cui, C.; Lei, M.; Yang, D. Enhanced Performance and Light Soaking Stability of Planar Perovskite Solar Cells Using an Aminebased Fullerene Interfacial Modifier. J. Mater. Chem. A 2016, 4, 18509−18515. (38) Li, Y.; Zhao, Y.; Chen, Q.; Yang, Y.; Liu, Y.; Hong, Z.; Liu, Z.; Yao, H.; Lei, M.; Li, Y.; Yang, Y. Multifunctional Fullerene Derivative for Interface Engineering in Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 15540−15547.
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DOI: 10.1021/acsnano.7b04070 ACS Nano 2017, 11, 9176−9182
