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Enhanced thermochemical stability of CH3NH3PbI3 perovskite films on zinc oxides via new precursors and surface engineering Fei Qin, Wei Meng, Jiacheng Fan, Chang Ge, Bangwu Luo, Ru Ge, Lin Hu, Fangyuan Jiang, Tiefeng Liu, Youyu Jiang, and Yinhua Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07192 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017
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ACS Applied Materials & Interfaces
Enhanced thermochemical stability of CH3NH3PbI3 perovskite films on zinc oxides via new precursors and surface engineering Fei Qin†, Wei Meng†, Jiacheng Fan†, Chang Ge†, Bangwu Luo†, Ru Ge†, Lin Hu†, Fangyuan Jiang†, Tiefeng Liu†, Youyu Jiang†, and Yinhua Zhou*†‡
†
Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic Information,
Huazhong University of Science and Technology, Wuhan 430074, China ‡
Research Institute of Huazhong University of Science and Technology in Shenzhen, Shenzhen 518057,
China *Corresponding author. E-mail:
[email protected] 1
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ABSTRACT Hydroxyl groups on the surface of ZnO films lead to chemical decomposition of CH3NH3PbI3 perovskite films during thermal annealing, which limits the application of ZnO as a facile electron-transporting layer in perovskite solar cells. In this work, we report a new recipe that leads to substantially reduced hydroxyl groups on the surface of the resulting ZnO films by employing polyethylenimine (PEI) to replace generally-used ethanolamine in the precursor solutions. Films derived from the PEI-containing precursors are denoted as P-ZnO and those from the ethanoamine-containing precursors as E-ZnO. Besides the fewer hydroxyl groups that alleviate the thermochemical decomposition of CH3NH3PbI3 perovskite films, the P-ZnO also provides a template for the fixation of fullerene (PCBM) owing to its nitrogen-rich surface that can interact with the PCBM. The fullerene was used to block the direct contact between P-ZnO and CH3NH3PbI3 films, and therefore further enhance the thermochemical stability of perovskite films. As a result, perovskite solar cells based on the P-ZnO/PCBM electro-transporting layer yield an optimal power conversion efficiency (PCE) of 15.38%. We also adopt the P-ZnO as the electron transporting layer for organic solar cells that yield remarkable PCE of 10.5% based on the PBDB-T:ITIC photoactive layer.
KEYWORDS: Thermochemical stability; thermal decomposition; Zinc oxides; precursor engineering; surface physics engineering;
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INTRODUCTION Organic-inorganic halide perovskite solar cells have shown great promise as a low-cost renewable clean energy source because of the easy solution processing and high power conversion efficiency.1-4 Environmental and operational stability of perovskite solar cells is a critical issue to further push the cells into the market. The stability issue has also been attracting great attention of the community. The performance degradation of the perovskite solar cells can be induced by moisture5-8, thermal annealing9, UV illumination10, electrical field under operational conditions11, and chemical reaction among the layers (perovskite layer, charge transport layers and the electrodes) of the cells12-14. Therefore, the proper selection of the electron transport layer (ETL) and hole transport layer (ETL) is important to achieve stable perovskite solar cells.15 For the ETL, comparing to the typically used electron transport layer, titanium dioxide (TiO2)16 and tin oxide (SnO2)17, zinc oxide (ZnO) is facile to prepare films via nanoparticles or sol-gel methods, which has been widely used in organic and quantum-dots thin-film solar cells, as well as light-emitting diodes for electron injection or collection owing to its low-temperature processing, low work function and high electron mobility.18-23 However, the application of ZnO in perovskite solar cells has been limited due to the rich hydroxyl groups on the surface of ZnO films that lead to chemical decomposition of CH3NH3PbI3 perovskite films under thermal annealing shown as follows.13, 24, 25
According to the reaction, the decomposition of the CH3NH3PbI3 films mainly arises from two factors. One is the thermal annealing. To avoid the high temperature, some groups reported the two-step method to fabricate perovskite films, and the fabrication process takes place at relatively low temperature (< 100 ºC) or even room temperature.26-30 The other factor is the hydroxyl groups on the 3
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surface of ZnO. Considering this, buffer layers including organic small molecules31 and polymers24 were inserted to block the direct contact between ZnO and the CH3NH3PbI3 perovskite film.32 Besides the two strategies, it would be highly desirable that the amount of hydroxyl groups on the surface of the ZnO can be significantly reduced that would directly alleviate the thermochemical decomposition of the CH3NH3PbI3 based on the equation (1). In this work, we report a new precursor to prepare ZnO that adopts polyethylenimine (PEI)33 to replace the commonly-used ethanolamine to provide an alkaline environment in the precursors. Films derived from the PEI+Zn(Ac)2-contained precursors were named P-ZnO and from the ethanoamine+Zn(Ac)2-contained precursors were named E-ZnO (Figure 1a). By introducing the PEI in precursors, the amount of hydroxyl groups on the surface of P-ZnO was significantly reduced comparing to the E-ZnO through the X-ray photoelectron spectroscopy (XPS), which would slow down the thermochemical decomposition of CH3NH3PbI3 perovskite films. The strategy here using the PEI inside the precursors replacing the usually used ethanolamine is different from the previously reported method24 of inserting a separate layer between the ZnO and the perovskite layer to block their direct contact. The new precursor yields less hydroxyl groups on the Zinc oxide surface that would enhance the thermochemical stability even without the use of the blocking layer. What’s more, PEI chains on the surface of P-ZnO acted as a template for the fixation of fullerene (PCBM) owing to the interaction between the PEI and PCBM. The fullerene was used to block the direct contact between P-ZnO and CH3NH3PbI3 films, and therefore further enhance the thermochemical stability of perovskite films. Based on the new electron transport layer (ETL), we fabricated perovskite solar cells on P-ZnO/PCBM and organic solar cells on P-ZnO. The perovskite solar cells display a power conversion efficiency (PCE) of 15.38%. Meanwhile, the organic solar cells performed a PCE of 10.51% 4
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based
on
combination
of
a
conjugated
polymer
poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2 -thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))] (PBDB-T) and small molecular
compound
3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC).
RESULTS AND DISCUSSION An image of the CH3NH3PbI3 film deposited on the E-ZnO films after a thermal annealing at 100 ºC for 10 min was shown in Figure 1a. The film decomposed into PbI2 and displayed a yellow color which was consistent with the previous report.13 Yang et al. demonstrated that the rate of decomposition appeared to be strongly influenced by the amount of hydroxyl groups. To reduce the amount of hydroxyl groups, we employed a polymer PEI to replace the ethanolamine in the ZnO precursors. There was no hydroxyl groups in the molecular structure of PEI (unlike ethanolamine), which directly contributed to the decrease of the amount of hydroxyl groups on the surface of P-ZnO. The results were confirmed by the measurement of the X-ray photoelectron spectroscopy (XPS) in Figure 1b. The tested films of E-ZnO and P-ZnO were spin-coated at the same speed and annealed at the same temperature (150 ºC). The O 1s core level spectrum can be resolved into two main peaks located at 531.8 eV and 530.4 eV.34 The peak with lower binding energy (530.4 eV) corresponds to O atoms in ZnO matrix. Another peak, at the 531.8 eV, is attributed to the chemisorbed oxygen species such as hydroxyl groups. On the surface of E-ZnO, the ratio of O in ZnO and O in hydroxide was 1:1, while the relative ratio on the surface of P-ZnO was 2:1. There are still residues of hydroxyl groups on
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the surface of P-ZnO because the zinc oxides are produced via the hydrolysis from the Zn(CH3COO)2•2H2O precursors. The hydroxyl groups could possibly be reduced via other precursors, such as diethylzinc [Zn(C2H5)2], via atomic layer deposition method.35 At the same time, the picture of CH3NH3PbI3 film on the P-ZnO annealed at 100 ºC for 10 min was shown in Figure 1a. The decomposition rate of the CH3NH3PbI3 on P-ZnO was slower than that on E-ZnO. Up to this, we affirmed the conclusion that the application of the PEI, leading to a decline of the hydroxyl groups, could relieve the degradation of the CH3NH3PbI3. However, the thermal decomposition couldn’t be stopped completely by this strategy. The small amount of residual hydroxyl groups would also facilitate the decomposition process. To further prevent the thermal decomposition, we inserted a thin layer of organic material, PCBM, between the P-ZnO and the CH3NH3PbI3 film. What’s more, the PEI chains on the surface of P-ZnO could interact with PCBM36, 37 to reinforce its uniform coating, which would further block the contact between the ZnO and the CH3NH3PbI3 film. To verify the interaction between PEI and PCBM, conductivity along the PCBM and the PEI/PCBM films were measured using the device shown in the inset of Figure 2a.38 As shown in Figure S1, the pristine PCBM showed a very small current due to its low charge carrier density.39 However, when a thin layer PEI (
