Covalently Connecting Crystal Grains with Polyvinylammonium

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Covalently Connecting Crystal Grains with Polyvinylammonium Carbochain Backbone to Suppress Grain Boundaries for Long-Term Stable Perovskite Solar Cells Han Li, Chao Liang, Yingliang Liu, Yiqiang Zhang, Jincheng Tong, Weiwei Zuo, Shengang Xu, Guosheng Shao, and Shaokui Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15434 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Applied Materials & Interfaces

Covalently Connecting Crystal Grains with Polyvinylammonium Carbochain Backbone to Suppress Grain Boundaries for Long-Term Stable Perovskite Solar Cells

Han Li,1,

†

Chao Liang,1,2,

†

Yingliang Liu,1,* Yiqiang Zhang,1,2,* Jincheng Tong,1 Weiwei

Zuo,1 Shengang Xu,1 Guosheng Shao1,2 and Shaokui Cao1,*

1. School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P.R. China 2. State Centre for International Cooperation on Designer Low-Carbon and Environmental Material (SCICDLCEM), Zhengzhou University, Zhengzhou 450001, P. R. China

ABSTRACT: Grain boundaries act as rapid pathways for nonradiative carrier recombination, anion migration and water corrosion, leading to low efficiency and poor stability of organometal halide perovskite solar cells (PSCs). In this work, the strategy suppressing the crystal grain boundaries is applied to improve the photovoltaic performance, especially moisture-resistant stability, with polyvinylammonium carbochain backbone covalently connecting the perovskite crystal grains. This cationic polyelectrolyte additive serves as nucleation sites and template for crystal growth of MAPbI3 and afterwards the immobilized adjacent crystal grains grow into the continuous compact, pinhole-free perovskite layer. As a result, the unsealed PSC devices, which are fabricated under low-temperature fabrication protocol with a proper content of polymer additive PVAmHI, currently exhibit the maximum efficiency of 16.3%. Remarkably, these unsealed devices follow up an “outside-in” corrosion mechanism and respectively retain 92% and 80% of initial PCE value after being exposed under ambient environment for 50 days and 100 days, indicating the superiority of carbochain polymer additives in solving the long-term stability problem of PSCs.

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KEYWORDS: polyvinylammonium, covalently connecting, suppressed boundary, perovskite solar cell, long-term stability

INTRODUCTION 3D solution-processable organometal halide (MAPbX3, X=Cl, Br or I) perovskites have attracted tremendous attention owing to their potential applications in solar cells,1,2 lightemitting diodes,3 photodetectors,4 and nanowire lasers,5 etc. In particular, since the first solidstate perovskite-based solar cells (PSCs) with a power conversion efficiency (PCE) of 9.7% was reported in 2012,6 PSCs have been extensively focused as a new kind of photovoltaic technology.7-14 Nowaday, the highest certified PCE has reached up to 22.1% for PSCs.15 Several favorable optoelectronic properties, such as high absorption coefficients, tunable composition/structure and long carrier diffusion length, guarantee MAPbX3 to be an efficient photovoltaic material.16,17 In the past several years, the rapid rise in device efficiency is mainly originated from the development of fabrication protocols capable for producing continuous, pinhole-free perovskite films with large grains in mesoporous, bilayer and planar structure PSCs.18 The precise control of synthesis and deposit conditions via various processing methods, including substrates,19 solvent,20 compositions of raw materials,21,22 and temperature,11 produces better morphologies, which in turn improves the optoelectronic property of perovskite films. Although grain boundaries in the perovskite films are active channels for carrier seperation and collection,23,24 they also serve as the rapid pathways for nonradiative carrier recombination25 and dominate the ion migration,26 which may hinder the further PCE improvements of PSCs. In contrast, the boundary-suppressed single crystals of MAPbI3 show ultra-long carrier diffusion length in the range of 1.8 µm~8 mm and low trap density of 3.3 × 1010 cm–3. However, it is unfortunately that their relevant growing processes are not suitable for the fabrication of PSCs.26-29 Passivation strategy using PCBM,30 Cl-,31 pyridine,25 PbI2,32 2 ACS Paragon Plus Environment

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and MAI33 has been demonstrated to be effective in mitigating the negative effect of the trap states at/near grain boundaries. The adjoint problems from grain boundaries are the energetical unstability and the easy water corrosion, especially under moisture attack.10 The perovskite instability hampers the down-to-earth application of PSCs. So far, except for decreasing the area of grain boundaries by obtaining large crystal grains, adjusting other components in the device architecture34 and using encapsulation technology,10,34 the introduction of proper additives to modify the perovskite layer seems to be effective in softening the impact of unstable grain boundaries and stablizing the crystal grains, which facilitates the stability improvement of PSCs. Recently, alkylphosphonic acid ammonium additive, such as 4-ABPACl, is chosen as a crosslinking agent between neighbouring grains through P–OH…I− hydrogen bonding and strong immobilization effect derived from the insertion of NH3+ into A sites in the perovskite structure, which ultimately enhances the moisture-resistant stability of PSCs.10 Besides, the insulting polymers such as PMMA14, PVP,35 PEG36 and PEIHI37,38 are introduced as stabilizer, scaffold or nucleation/crystal growth template into perovskite layer. In these cases, the modified PSCs show much superior stability to the pristine devices, suggesting the capacity of functional polymeric additives in retarding the decomposition of perovskites in ambient environment. It is well known that carbon homochain polymers is much more stable than small molecules or heterochain polymers,39,40 Therefore, it is still an attractive scientific subject to prepare boundarysuppressed stable perovskite layers using well-designed carbon homochain polymers with high content of functional groups to link the neighbouring grains under common fabrication protocols. Herein we report an attempt to fabricate an immobilized boundary-suppressed perovskite layer using long carbochain polyvinylammonium, which has the highest content of primary ammonium groups of any polymer, derived from polyvinylamine hydriodide (PVAmHI) as a potent crosslinking agent.20 A low-temperature chemical bath deposition produces a 3 ACS Paragon Plus Environment


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nanocrystalline rutile TiO2 layer (nr-TiO2)41,42 serving as electron transport layer (ETL), which is fully embedded into the continuous perovskite layer firmly being connected with 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9-spirobifluorene

layer

(spiro-

MeOTAD) as hole transport layer (HTL) in the planar heterojunction PSCs. We propose that the large content of primary ammonium functional groups in PVAmHI serve as nucleation sites and template for crystal growth of MAPbI3 and afterwards the immobilized adjacent crystal grains grow into a continuous compact perovskite layer, affording a pinhole-free photovoltaic film. The addition of PVAmHI also strengthens the adhesion of perovskite layer with both nr-TiO2 ETL and spiro-MeOTAD HTL, reducing the interfacial defects between active perovskite layer and ETL/HTL. As a result, the low-temperature ( 175 µm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. (29)

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Covalently Connecting Crystal Grains with Polyvinylammonium

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