Reliable Annealing of CH3NH3PbI3 Films Deposited on ZnO

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Reliable Annealing of CHNHPbI Films Deposited on ZnO Christopher Manspeaker, Patrick Scruggs, Jonathan Preiss, Dmitry A. Lyashenko, and Alex Zakhidov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00364 • Publication Date (Web): 09 Mar 2016 Downloaded from http://pubs.acs.org on March 9, 2016

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The Journal of Physical Chemistry

Reliable Annealing of CH3NH3PbI3 Films Deposited on ZnO Christopher Manspeaker†, Patrick Scruggs‡, Jonathan Preiss‡, Dmitry A. Lyashenko† and Alex A. Zakhidov*,†,‡ †

Materials Science, Engineering, and Commercialization, Texas State University, San Marcos, TX 78666, USA. ‡

Department of Physics, Texas State University San Marcos, TX 78666, USA.

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ABSTRACT: CH3NH3PbI3 deposited on a ZnO electron transport layer is chemically unstable and decomposes at >80°C, leaving PbI2 on the substrate along with traces of iodine. We found that this decomposition was reversible and could be prevented if a restricted volume solvent annealing procedure was applied. We also found that decomposition requires the presence of a certain amount of the processing solvent within the film. Finally, we developed a reliable annealing protocol for depositing perovskite film on ZnO, which resulted in the generation of repeatable solar cell devices.


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1. INTRODUCTION Zinc oxide (ZnO) is a promising semiconducting material that can serve as an electron transport layer (ETL) for solar cell devices based on organo-halide lead perovskites.1-10 A power conversion efficiency (PCE) of >15% was demonstrated for a methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) absorber with a ZnO electron transporter.4,11 ZnO ETLs for perovskite photovoltaics has a combination of attractive electronic and optical properties: i) the electron affinity of ZnO is well aligned with the valence band edge of MAPbI3, ii) the electron mobility of ZnO is >1 cm2/(V·s), which is a few orders of magnitude higher than that of TiO2 (another popular choice for ETLs in perovskite photovoltaic devices), and iii) ZnO has a large band gap of 3.3 eV, which ensures optical transparency and a large barrier for hole injection. Moreover, ZnO nanostructures can be printed on flexible substrates at room temperature in a cost effective manner.12 However, it was recently found that organic perovskites deposited on ZnO are unstable and readily decompose at >90˚C.13,14 Yang et al. performed a density functional theory (DFT) study of a ZnO/ MAPbI3 interface and found that methylammonium cations in the perovskite film were prone to the deprotonation with the release of methylamine.15 While this explains the degradation of the ZnO/perovskite interface, the mechanism of the decomposition of bulk perovskite films remains unknown. The inability to anneal MAPbI3 films due to its rapid decomposition on the ZnO interface can be a limiting factor for the fabrication of perovskite solar cell devices. Several papers have suggested that substrate temperatures of over >100˚C during and after perovskite film formation is necessary to form a high quality perovskite films.16,17 Similarly, annealing is often required to subsequently deposit hole transport layers (HTL).18 Moreover, if the annealing step is omitted, the perovskite film and HTL (if deposited from solution) remain wet with unknown amounts of the remaining solvents trapped within the


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device. This can in turn cause problems with the reproducibility and reliability of device fabrication. In this paper, we further investigate the mechanism of decomposition of MAPbI3 films deposited onto ZnO and reveal the role of the solvent in the film during the annealing process. We also develop a restricted volume solvent annealing (RVSA) process for post annealing of the perovskite film on ZnO without decomposition. We demonstrate that RVSA enables reliable perovskite solar cell fabrication with an efficiency of 13.7%. 2. MATERIALS AND METHODS 2.1 RVSA VS OVA PROCESSING In this work, we used a sequential deposition method19 to form MAPbI3 perovskite on a ZnO film. First, a PbI2 film was deposited from a dimethylformamide (DMF) solution. Then, CH3NH3I dissolved in 2-propanol (IPA) was applied to complete the perovskite film (Fig. 1A). Both solutions were prepared with anhydrous solvents and kept under an N2 atmosphere. Film deposition and post-processing were also conducted in an N2 environment. Fig. 1B, C show the two different annealing processes employed in this work: open volume annealing (OVA) and RVSA. In the latter process, the central part of the sample was enclosed by a Viton O-ring and a glass slide placed on top. This simple schema enabled the containment of volatile components (including the processing solvent) and kept them in close proximity with the perovskite film. Both annealing procedures were conducted on a hot plate in an N2-filled glovebox to prevent possible degradation effects due to the reaction of MAPbI3 with water vapor.20,21


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Figure 1. (A) Perovskite film on ZnO post applied thermal annealing flowchart. (C) Open volume annealing (OVA) and (B) restricted volume solvent annealing (RVSA) result in (D) complete and (E) partial decomposition of the perovskite film. Photographs of the samples were taken after 30 min annealing at 100oC in N2 atmosphere. 2.2 RVSA CHARACTERIZATION To further investigate the decomposition mechanism, Fourier transform infrared spectroscopy (FTIR) was used to record real time gas generation from the sample. Attenuated total reflection (ATR) schema was used to identify the volatile components of the decomposition in situ. The sample apparatus retained the same design, but the system was inverted such that the FTIR detector served as the collection slide and the O-ring diameter was reduced to put the sample as close to the FTIR sample window as possible. A separate ITO glass substrate was used as a heater by applying current to the unpatterned substrate. For the RVSA heated sample, the central


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part of the sample and FTIR detector were completely enclosed with an O-ring. For the OVA process, the apparatus was slightly modified in that ~20% of the O-ring was removed, allowing for a path for the gas to escape the surface. The OVA process resulted in a visible color change of the perovskite film after 10 min of annealing.

2.3 DEVICE FABRICATION The perovskites films were prepared using ITO coated glass samples (Kintec) with a 120 nm thick ITO layer and a sheet resistance of 15 Ω/sq as substrates. Each film was cleaned using a 10% solution of Deconex OP121 in an ultra-sonic tank for 20 minutes. The samples were then rinsed three times with DI water. Samples were blown dry with CO2 and baked at 200˚ C for an additional 10 minutes. The samples were then placed in an O2 plasma cleaner (Harrick, PDC32G, 18 W) and plasma cleaned for 10 minutes. A ZnO film was deposited on the clean substrates following a recipe reported in the literature.22 Alternatively, 12 wt. \% water dispersion of ZnO nanoparticles (Sigma-Aldrich,

Reliable Annealing of CH3NH3PbI3 Films Deposited on ZnO

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