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Enhanced Photovoltaic Performance of Perovskite Solar Cells Using Polymer P(VDF-TrFE) as a Processed Additive Chongyang Sun, Yiping Guo, Bijun Fang, Jiaming Yang, Ben Qin, Huanan Duan, Yujie Chen, Hua Li, and Hezhou Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05255 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016
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Enhanced Photovoltaic Performance of Perovskite Solar Cells Using Polymer P(VDF-TrFE) as a Processed Additive Chongyang Sun,† Yiping Guo,*,† Bijun Fang,‡ Jiaming Yang,† Ben Qin,† Huanan Duan,† Yujie Chen,† Hua Li† and Hezhou Liu*,†
† State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University.
‡ Jiangsu Key Laboratory for Solar Cell Materials and Technology, School of Materials Science and Engineering, Changzhou University.
Postal Address: Material Building D, Shanghai Jiao Tong University, Dongchuan Road No. 800, Minhang District, Shanghai, 200240, The People’s Republic of China.
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ABSTRACT It is known that CH3NH3PbI3 perovskite films with high crystallization and controlled morphology always show enhanced power conversion efficiency. Here we incorporate a small amount of polyvinylidene fluoride-trifluoroethylene polymer P(VDF-TrFE) into PbI2 solution to control the crystallinity and morphology of perovskite layer in a two-step deposition process. Our results show that the P(VDF-TrFE) bridges the grain boundaries and also enhances the crystallinity of perovskite layer significantly. By adjusting P(VDF-TrFE) concentration, the fabricated perovskite solar cells show improved average power conversion efficiency from 9.57±0.25% to 12.54±0.40% under a standard illumination of 100 milliwatts per square centimeter, which increases nearly by 31%. Thus, we demonstrate a new strategy to control the crystallinity and morphology of perovskite films by incorporating polymer into PbI2 film in a two-step deposition process. Besides, this method has been proved as an effective way in preparing high-performance perovskite solar cells.
INTRODUCTION
Organometallic halide perovskite solar cells based on organolead halide light harvesters (e.g., CH3NH3PbX3, X=Cl, Br, or I) are one of the most promising candidates in the new generation of photovoltaic technology, and have attracted huge attention in the past few years.1-9 Organolead halide perovskites are excellent semiconductors in solar cells due to their high light absorption coefficient (~105
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cm-1),1,10 large electron/hole diffusion lengths (over 100 nm),11 small exciton binding energy (~40 meV),11,12 and easily tunable band-gap.13,14 With the development of new deposition methods and the exploration of new solar cell architectures, the efficiency of perovskite solar cell has increased from 3.8% to more than 20%.1, 15, 16 Among all of the preparation methods, solution chemistry approaches (e.g. one-step solution deposition, and two-step sequential solution deposition) stand out due to their simple but effective characters in controlling the morphology and crystallinity of the formed perovskite thin films, which is essentially important since these factors significantly affect the photovoltaic performance of perovskite solar cells.8, 9, 17-24
Compared to the one-step deposition process, two-step sequential solution deposition process results in a more compact and uniform perovskite film. After first proposed by Grätzel M. et al.,4 the two step sequential deposition process has been repeatedly proved to be an effective method in improving the crystallinity of perovskite and its photovoltaic performance, which makes it as one of the most popular and common synthesis approaches. In the implementation of this method, PbI2 film is first deposited onto the substrate, followed by the reaction between PbI2 and MAI in the solution or solid phase to form the final CH3NH3PbI3 (MAPbI3). The growth of the perovskite and its morphology strongly depends on the deposition of the PbI2 film. However, PbI2 generally tends to form a flat and compacted structure, which is unfavorable for reaction with MAI molecule. Some strategies have been used to get a higher and faster conversion of PbI2 to MAPbI3. Recently, we have developed a solvent assisted method to accelerate the conversion of PbI2 to MAPbI3 with high
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conversion rate, which leads to the improved quality of perovskite film and the enhanced power conversion efficiencies.20 Han et al.25 also reported that the incorporation of high boiling temperature solvent (DMSO) into the PbI2 precursor can efficiently retard the crystallinity of the initial PbI2 film. Zhao et al.26 demonstrated a sequential solution process to prepare perovskite films, in which the PbI2 film, formed by the thermal decomposition of unstable PbI2-CH3NH3Cl precursor films, can rapidly convert into perovskite films without any PbI2 residue. Moreover, the interdiffusion reaction between flat PbI2 film and stacked solid MAI film in a two-step sequential deposition method has been developed by Huang et al.27,28 The interdiffusion reaction of solid precursor stacking layers (PbI2/MAI) can also eliminate the negative impact of the solvent isopropanol (IPA) in the conventional two step deposition process. Latterly, it is demonstrated that the use of additive (e.g. MAI, HCl and formamidinium iodide (FAI)) reduces the crystallinity of PbI2 and/or pre-expansion of PbI2 with adjustable morphology, and accelerates the MAI intercalation/conversion step, leading to the enhanced photovoltaic performance and stability of the resulted perovskite solar cells.8, 14, 17, 22
Polyinylidene fluoride-trifluoroethylene polymer, P(VDF-TrFE), was usually chosen as the ferroelectric material due to its chemical inertness, low fabrication temperatures, reasonable compatibility with polymer semiconducting materials. It has been incorporated into organic photovoltaic (OPV) for increasing the photovoltaic performance via enhanced exciton dissociation and reduced recombination of charges or charge transfer exciton by the induced and enhanced polarization local electric field
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of ferroelectric dipoles in the interface or bulk of semiconductors.29-32 In the field of trihalide lead perovskite solar cells, it is reported that the addition of polymer into the precursor solution in one-step deposition process can tune the perovskite morphology and enhance the photovoltaic performance.33,34 In this study, polymer P(VDF-TrFE) was first chosen as processing additive in the deposition of PbI2 precursor film in two step deposition process. Incorporating of polymer P(VDF-TrFE) into PbI2 solution can obviously retard the crystallinity of PbI2 film and promote the reaction between PbI2 and MAI. Our results show that this approach can significantly reduce charge recombination, improve carrier lifetimes and enhance photovoltaic performance of perovskite solar cells.
EXPERIMENTAL SECTION
Materials.
Lead
iodide
(PbI2,
99%)
was
from
Sigma-Aldrich.
N,N-
dimethylformamide (DMF, anhydrous, 99.8%), and isopropanol (IPA, anhydrous, 99.5%) were purchased from Aladdin Reagents. P(VDF-TrFE) copolymer (70:30 mol%)
was
from
PiezoTech..
TiO2
paste,
2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-MeOTAD) (≥99.0%),
Li-bis-(trifluoromethanesulfonyl) imide (Li-TFSI)
(≥99.5%), and 4-tert-butylpyridine came from Yingkou Optimal Choice Trade CO., LTD. (China). All of the used reagents were analytical grade, without further purification. CH3NH3I (MAI) was synthesized by the procedure stated previously.2, 20
Device Fabrication. Preparation of the mesoporous TiO2 films: Fluorine-doped tin
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oxide (FTO, Yingkou OPV Tech. China, 7 Ω sq-1) was patterned by etching with Zn powder and 2M HCl, then soaked in 5 wt% NaOH in alcohol for 16 hours and ultrasonic cleaned successively in deionized water, acetone, and ethanol, as stated in our previous work.20 A compact TiO2 layer was deposited on the patterned FTO by spray
pyrolysis
at
450
℃
using
0.2
M
Ti(IV)
bis(ethylacetoacetate)-diisopropoxiden-butanol solution, followed by annealing in air at 450℃ for 1 hour.9, 18 The mesoporous TiO2 layer was introduced from a 100ul diluted TiO2 in ethanol by spin coating at 3000 rpm for 30 s. After dried at 125℃, the TiO2 films were baked at 500℃ for 15min and then cooled to room temperature. Prior to their use, the films were dried again at 500℃ for 30min.
Preparation of CH3NH3PbI3 films: P(VDF-TrFE) copolymer (70:30 mol%) solutions with three different concentrations of 1.0 mg/ml, 2.0 mg/ml and 5.0 mg/ml were prepared in anhydrous DMF. Unless otherwise stated, 0.698 g PbI2 were dissolved in 1.5 mL three different concentration of P(VDF-TrFE) solutions at 70℃ to form three corresponding precursor solutions. For reference, a blank solution by dissolving the same amount of PbI2 in pure DMF was also prepared. The different PbI2 films were prepared by spin coating PbI2 precursor solutions with various concentrations of P(VDF-TrFE) onto the substrate with a rotation speed of 3000 rpm for 30 s at room temperature, sequentially dried at 70℃for 20 min. Then MAI (50mg/ml, dissolved in 2-propanol) solution was spin-coated on the top of the dried PbI2 layer at the same condition. The PbI2/MAI stacking films were annealed at 100℃ for 2 hours, followed by annealing at 150℃ for 2 min to crystallize the perovskite
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films.
Fabrication of perovskite solar cells: A volume of 35 µl of spiro-MeOTAD solution 4
was spin-coated onto the MAPbI3 perovskite layer at 4000 rpm for 30 s to fabricate
the hole transporter layer (HTL) in an Ar-filled glove box (H2O and O2
