发表文章毕业论文201703-zhangchengxi-ScienceDirect-High efficient Planar-heterojunction perovskite solar cell based on two-step deposition processs

1876-6102 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi: 10.1016/j.egypro.2017.03.391 Energy Procedia 105 ( 2017 ) 793 798 ScienceDirect The 8th International Conference on Applied Energy ICAE2016 High efficient Planar-heterojunction perovskite solar cell based on two-step deposition process Chengxi Zhanga Weiling Luana,* ,Yuhang Yina a Key Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering,East China University of Science and Technology, Shanghai 200237,P.R.China Abstract Recently, the perovskite solar cells have attracted widely attentions due to their high photoelectric conversion efficiency and simple preparing processes. Uniformity and high crystalline perovskite film was synthesized via the two-steps deposition process. In comparison with the CH3NH3PbBr3 perovskite films, the CH3NH3PbI3 perovskite film showed the better absorbance of visible light and near-infrared. The preparing process play an important role in forming high quality perovskite films. The two-steps deposition process could get the high quality CH3NH3PbI3 perovskite films. Meanwhile, scanning electron microscopy measurement indicated that the perovskite films with good compactness and no obvious grain cracks. The TiO2 was introduced to act as the electron selective layer and result in efficient electrons extraction and holes blocking. As a result, the photoelectric conversion efficiency of 13.53% was achieved with good reproducible. 2016 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of ICAE Keywords: perovskite solar cells, deposition process, perovskite films, reproducible 1. Introduction As one of the most promising renewable energy, solar energy has received increasing attention and supposed to be a great potential candidate in the past decade. Photovoltaic devices have been considered to be the most promising technology due to their advantages, such as reliability, safety and sustainability 1. However, the high materials cost, complex manufacturing processes, rigid substrates limit the silicon cells and the inorganic thin-film solar cells application 2. To meet the market demand and reduce the cost of photovoltaic device, the emerging photovoltaic, including dye-sensitized solar cells (DSSCs) 3, * Corresponding author. Tel/fax: +86-21-64253513. E-mail address: luan Available online at 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. 794 Chengxi Zhang et al. / Energy Procedia 105 ( 2017 ) 793 798 quantum dot-sensitized solar cells (QDSSCs) 4, organic solar cells (OSCs) 5, and perovskite solar cells (PSCs) 6. Nevertheless, DSSCs and QDSSCs showed poor stability and electrolyte leakage defect, the materials of PSCs with low carrier mobility and a short diffusion length, which result in low photoelectric conversion efficiency (PCE). In terms of superior optical electric properties, the organic- inorganic hybrid perovskite materials have attracted large attention due to their adjustable bandage, strong absorption, and long carrier diffusion length 7-8. The PCE of perovskite solar cells have increased from 3.8% to 20.1% in the past 5 years 9-10. According to the literatures, the quality of each layers, especially the perovskite layer, is extremely important for the device. Some research methods, such as sequential deposition of inorganic and organic precursor 11, co-evaporation of the precursors 12, single step solution method 13-15, and vapor assistant solution process have been used to form perovskite films 16. One-step method produced a shapeless morphology and defects, these were not conducive to the photoelectric conversion. Thermal evaporation could form uniformity of the film and much better than that of the solution-processed, but it requires high vacuum, which restricts cost effectiveness and mass production. In this study, the ternary inverted perovskite solar cells with structure: FTO/TiO2/PbI2: CH3NH3I/spiro- OMeTAD /Ag were fabricated. The scanning electron microscope (SEM) results show that two-steps method could prepare high crystallization perovskite film, and there is no obvious grain crack. In particular, high-quality perovskite film could greatly promote the photoelectric conversion efficiency and stability of devices. The TiO2 and spiro-OMeTAD layer are introduced to act as the electron selective layer and hole selective layer, respectively. Finally, on the basis of planar structure and two-steps method, the highest PCE of 13.53% is obtained. 2. Experimental 2.1 Materials and Solutions preparation CH3NH3I (MAI) was synthesized following the solvent reaction process described as follows: 24 mL of methylamine (33wt% in ethanol) and 10 mL of hydroiodic acid (57 wt% in water, Aladdin reagent, Shanghai, China), were reacted in a 100 mL round-bottomed flask at 0 oC for 3 h with continuous stirring under nitrogen protection. The white crystal of CH3NH3I was obtained through rotary evaporation at 50 oC for several minutes and the white power recrystallized by using diethyl ether three times, the white CH3NH3I powder was finally collected and dried at 50 oC in a vacuum oven overnight. The CH3NH3I precursor solution with the concentration of 20 mg/ml in isopropyl alcohol. PbI2 power was purchased from Sigma-Aldrich, and the PbI2 precursor solution was prepared by dissolving 500mg power in 1 ml of N, N-dimethylformamide and magnetic stirring under 90 oC. The spiro-OMeTAD-based hole-transfer solution was prepared by dissolving 72.3 mg of spiro-OMeTAD in 1 ml of chlorobenzene, to which 28.8 l of 4-tert-butylpyridine and 17.5 l of lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg Li-TFSI in 1 ml acetonitrile, Xian Polymer Light Technology Corp, 99.8%) were added. All prepared solution was placed for 1 h at room temperature and were filtered with a 0.45 m PTFE filter before using. 2.2 Experimental setup and Device fabrication The perovskite solar cells were prepared with an inverted structure, which was shown in Fig. 1. Where TiO2 and spiro-OMeTAD acted as the electron selective layer and hole selective layer, respectively. The preparation of perovskite solar cells processes as, firstly, the glass substrate pre-coated with fluorine tin oxide (FTO) (with size of 2 cm 2 cm, 15 /square) was cleaned in an ultrasonic bath with deionized Chengxi Zhang et al. / Energy Procedia 105 ( 2017 ) 793 798 795 water, acetone and isopropyl alcohol for 15 min, respectively. Then it was dried in a drying oven. Secondly, TiO2 solution was spin-coated with 5000 rpm onto the FTO substrate, and sintered at 500 oC for 1 hour. Thirdly, the PbI2 film was obtain by spin-coating the PbI2 precursor solution on the top of TiO2 layer at the speed of 2500rpm for 35s. The blend films were placed on a hot plate and annealed at the temperature of 90 oC for several mins. Subsequently, the CH3NH3I precursor solution was spin-coated on the PbI2 layer and annealed at 120 oC for 1 h. Finally, the spiro-OMeTAD-based hole-transfer solution was spin-coated at the speed of 6000 rpm and the Ag electrode was prepared by thermal evaporation as the counter electrode with an active area of 0.1cm2 through shadow mask. Fig. 1 Schematic architecture of the perovskite solar cell 2.3 Characterization The current density versus voltage (J-V) characteristic of the devices were measured with a Keithley 2400 source measurement unit and a solar simulator (Abet Technologies, USA) under AM 1.5 (1000 W/m2) irradiation, which was calibrated with a standard silicon solar cell. Surface morphology of films was characterized by using scanning electron microscopy (SEM, Hitachi S-4800). The absorption properties of films were acquired by Cary500 UV-vis-NIR spectrophotometer. 3. Result and discussion Fig. 2 Schematic band energy diagram of planar MAPbI3 perovskite hybrid solar cells with inverted structures 796 Chengxi Zhang et al. / Energy Procedia 105 ( 2017 ) 793 798 Interface engineering between different layers is important to achieve high photoelectric conversion efficiency. The optimization of the interface barrier and minimum resistance could realize an ideal interface. To understand the interface engineering of this hybrid composite contact, surface work function and surface morphology were investigated. Fig. 2 shows the band energy diagram of planar MAPbI3 perovskite hybrid solar cells with inverted structures. The work function of FTO was -4.5eV, the LUMO value of TiO2 and MAPbI3 were -4.3ev and -3.88 eV, respectively. The electrons could easy to transfer from the MAPbI3 layer to FTO electrode. The work function of Ag was -4.26eV, the HOMO value of spiro-OMeTAD and MAPbI3 were -5.22ev and -5.43 eV, respectively. Holes transmit between different layers has a smaller potential barrier. It is obvious that orderly energy level alignment could lead to efficient carrier extraction and transportation without excessive interface recombination. Fig. 4 (left) shows the UV-vis absorption spectrum of different films with different spin speed. The TiO2 layer have lower absorbance in 300 nm to 1000 nm, which is benefit for the perovskite layer to absorb more photons. Thin film of absorbance decreased with the increase of spin-coated speed, and the absorbance of the film is relating to the thickness of it. Therefore, we further improved the thickness of Fig. 3 SEM images of MAPbI3 film with the PbI2 spin speed of (a) 1500rpm, (b) 2500rpm, (c) MAPbBr3 film with the PbI2 spin speed of 2500rpm (d) cross-sectional view of FTO/TiO2/PbI2: CH3NH3I films to increase the absorbance. The absorption edge of CH3NH3Br3 is 660nm, but the CH3NH3I3 is about 800nm, this indicate that the CH3NH3I3 perovskite material could absorb more photon from visible and near-infrared. In this study, the CH3NH3I3 perovskite material was taken into consideration to act as the photosensitive layer. It was reported that high crystallization of the perovskite film was benefit for realizing high Chengxi Zhang et al. / Energy Procedia 105 ( 2017 ) 793 798 797 performance of device, the different materials and deposition processes are investigated to optimize the Fig. 4 UV-vis absorption spectrum of different films and J-V curves of the perovskite solar cells, crystallization of perovskite film. From the SEM images of Fig. 3a and b, under the different spin speed, the MAI and PbI2 could react to perovskite materials, but, the more uniform films could be formed with the higher spin speed of PbI2. Comparing with the MAI material, the MABr could get better perovskite film under the same deposition process. But, the MAPbBr3 film has some crack defects, which are not conducive to photoelectric conversion due to increase leakage current (see Fig. 3c). Fig. 3d shows the cross-sectional view of different films. Using the two-step deposition process to prepare the perovskite film, the ternary inverted perovskite solar cells with structure: FTO/TiO2/perovskite/spiro-OMeTAD /Ag were fabricated, and the J-V curves are shown in Fig. 4. The best device exhibited a short circuit current density (Jsc) of 21.75 mA/cm2, an open circuit voltage (Voc) of 1.023V, a fill factor (FF) of 60.7%, and photoelectric conversion efficiency (PCE) of 13.53%. The performance of device was largely increased due to the quality of the film. With uniform and high crystallinity perovskite films, the Jsc increase from 16.7mA/cm2 to 21.75 mA/cm2. It is worthwhile to consider that systematic optimize process parameters and materials can be further improved the performance of the device. 4. Conclusions In summary, we have demonstrated that systematic optimize process parameters could enhance the performance of solar cells. The anatase TiO2 and spiro-OMeTAD materials were introduced to act as the electron selective layer and hole selective layer, respectively. By the energy level diagram, the orderly energy level alignment could lead to efficient carrier extraction and transportation without excessive interface barrier. The uniformity and high crystalline perovskite films were prepared via two-steps deposition process. In particular, high-quality perovskite film could greatly promote the photoelectric conversion efficiency and stability. The UV-vis absorption spectrum of different films indicate that the CH3NH3PbI3 show better performance of light absorption in the visible light and near-infrared. The ternary inverted perovskite solar cells with structure: FTO/TiO2/perovskite/spiro-OMeTAD /Ag were fabricated, as a results, the PEC of 13.53% was achieved with good reproducible. Acknowledgements 798 Chengxi Zhang et al. / Energy Procedia 105 ( 2017 ) 793 798 The authors gratefully acknowledge the financial support from the National Nature Science Fundation of China (51475166). References 1 Rong YG, Ku ZL, Mei AY, Liu TF, Xu M, Ko SG, Li X, Han HW. Hole-conductor-free mesoscopic TiO2/CH3NH3PbI3 heterojunction solar cells based on anatase nanosheets and carbon counter electrodes. J. Phys. Chem. Lett. 2014, 5, 21604. 2 Wang X, Kulkarni SA, Li Z, Xu WJ, Batabyal SK, Zhang S, Cao AY, Wong LH. Wire-shaped perovskite solar cell based on TiO2 nanotubes. Nanotechnology, 2016, 27(20):20LT01. 3 Ttreault N, Grtzel M, Novel nanostructures for next generation dye-sensitized solar cells, Energy Environ. Sci., 2012, 5, 8506-16. 4 Pan ZX, Zhao K, Zhong XH. 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