Chlorine Incorporation for Enhanced Performance of Planar

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Chlorine Incorporation for Enhanced Performance of Planar Perovskite Solar Cell Based on Lead Acetate Precursor Jian Qing, Hrisheekesh Thachoth Chandran, Yuan-Hang Cheng, XiaoKe Liu, Ho-Wa Li, Sai Wing Tsang, Ming-Fai Lo, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06819 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 8, 2015

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ACS Applied Materials & Interfaces

Chlorine Incorporation for Enhanced Performance of Planar Perovskite Solar Cell Based on Lead Acetate Precursor Jian Qing,†,‡ Hrisheekesh-Thachoth Chandran,†,‡ Yuan-Hang Cheng,‡ Xiao-Ke Liu,†,‡ Ho-Wa Li,‡ Sai-Wing Tsang,‡ Ming-Fai Lo*,†,‡,ǁ and Chun-Sing Lee*,†,‡,ǁ †

Center of Super-Diamond and Advanced Films (COSDAF), City University of Hong

Kong, Hong Kong SAR, P.R. China ‡

Department of Physics and Materials Science, City University of Hong Kong, Hong

Kong SAR, P.R. China ǁ

City University of Hong Kong Shenzhen Research Institute, Shenzhen, P.R. China

ABSTRACT: We show the effects of chlorine incorporation in the crystallization process

of perovskite film based on a lead acetate precursor. We demonstrate a fabrication process for fast grain growth with highly preferred {110} orientation upon only 5 minutes annealing at low temperatures. By studying the correlation between precursor composition and morphology, growth dynamic of perovskite film in the current system is discussed. In particular, we found that both lead acetate precursor and Cl incorporation

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are beneficial to perovskite growth. While lead acetate allows fast crystallization process, Cl improves perovskite crystallinity. Planar perovskite solar cells with optimized parameter deliver a best power conversion efficiency of 15.0% and average efficiency of 14.0% with remarkable reproducibility and good stability.

KEYWORDS: perovskite solar cells, lead acetate, chlorine incorporation, grain size,

crystallinity, solvent annealing

1. INTRODUCTION

Methylammonium lead halide perovskites are emerging as a novel class of absorbers for efficient perovskite solar cells (PVSCs). This is because they exhibit many advantages, such as inherent broad spectral absorption, ambipolar transport properties, long carrier transport length and facile solution processability.1-4 Remarkable progress has been achieved in recent years with power conversion efficiency (PCE) increased from 3.8% to the current record of 20.1%.5,6 High performance has been realized in both mesoporous and planar heterojunction structures with various deposition techniques, including single step, two-step, and vapor deposition methods.7-12 Nevertheless, the use of metal oxides (TiO2, Al2O3) as electron-transporting layers or scaffolds generally requires high-temperature (> 450

o

C) processing.13 Therefore planar structures employing low-temperature

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solution-processed materials are becoming attractive as they offer much better processability and potential capability for large-area, high throughput production.4,14 In particular,

PVSCs

with

perovskite

film

Poly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate)

sandwiched (PEDOT:PSS)

between and

phenyl-C61-butyric acid methyl ester (PCBM) are widely studied.15-18 It has been broadly observed that performance of a PVSC is closely related to the morphology and chemical composition of the perovskite absorbers.19 Much research has been carried out to increase quality of perovskite films by varying processing conditions, incorporating additives and solvent engineering.20-23 For absorption materials, the most widely studied perovskites are methylamonium lead triiodide perovskite (MAPbI3), fabricated from lead iodide (PbI2) and methylammonium iodide (MAI), and its mixed chloride-iodide analogue (MAPbI3−xClx), obtained by replacing PbI2 precursor with lead chloride (PbCl2).10,12 Even though the amount of Cl in the final MAPbI3−xClx films is negligible, the Cl in precursor solution can enhance crystallinity of the resulting perovskite films with preferred {110} orientation.24,25 In addition, improved carrier transport has been observed in MAPbI3−xClx films with carrier diffusion length exceeding 1 µm, which is one order of magnitude larger than that of MAPbI3.2 Although the role of Cl is still under debate,26-28 many studies have introduced Cl in the fabrication of perovskite to enhance crystal formation and improve morphology.29-37

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Recently, lead acetate (Pb(Ac)2) has been reported as a promising precursor for efficient PVSCs.38,39 Facile removal of CH3NH3Ac (MAAc) accelerates perovskite crystal growth. Smooth and pinhole-free perovskite films can be obtained by a simple one-step solution coating with short annealing duration. Based on Pb(Ac)2 precursor, Normal device configuration with perovskite deposited on TiO2 layer has been reported by Zhang et al.38 However, the device requires high temperature process for TiO2 and shows pronounced hysteresis. Soon after, Forgács et al. reported inverted device structure with perovskite sandwiched between PEDOT:PSS and PCBM, which can be fabricated at low temperature and shows negligible hysteresis.39 Nevertheless, the device shows relatively lower performance with a highest PCE of 12.5% compared to those from other precursors with the same device configuration.8 The main reason is that the perovskite film from Pb(Ac)2 consists of small crystals owning to fast crystallization, which could increase defect and trap densities.15,40 It is thus of interest to explore possible solutions for further improving the crystallization and film forming processes in perovskite films prepared with Pb(Ac)2. In this work, we investigate the effects of Cl incorporation on the crystallization of perovskite film by blending different amount of PbCl2 into the Pb(Ac)2 precursor solution. With an optimal blend ratio, high quality perovskite film with a strong {110} texture and large sized grains (average size ~ 1 micron) can be obtained with short annealing duration, leading to significant improvement in device performance. 4


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Employing inverted device structure, efficient PVSCs with a champion PCE of 15.0% can be fabricated in a simple one-step, low-temperature and fast-annealing solution process.

2. EXPERIMENTAL SECTION

2.1 Preparation of perovskite precursor solutions Pb(Ac)2 was first prepared by dehydration of Pb(Ac)2·3H2O (Sigma-Aldrich) under flowing nitrogen at 80 oC. Two stock solutions were first prepared by mixing 1 mmol Pb(Ac)2 and PbCl2 (Sigma-Aldrich) respectively with 3 mmol of MAI (Lumtech) in 1 mL anhydrous DMF (RCI Labscan). The solutions were then stirred at room temperature overnight. The two stock solutions (Pb(Ac)2:MAI and PbCl2:MAI) were mixed in different ratios for preparing perovskite films with different extent of Cl incorporation.

2.2 Device fabrication The devices have a simple structure of ITO/PEDOT:PSS (40 nm)/MAPbI3 (250 nm)/PCBM (60 nm)/BCP (10 nm)/Ag (70 nm). Indium tin oxide (ITO) coated glass substrates were first routinely cleaned.41 PEDOT:PSS (Baytron P-VP AI4083) was then spin-coated onto the ITO substrates at 3000 rpm followed by drying at 140 oC for 10 min in air. Then the samples were transferred into a N2-filled glove box (

Chlorine Incorporation for Enhanced Performance of Planar

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