Low temperature solution processing of compact TiO2 layers for organo-lead halide photovoltaic devices

Matthew Carnie^a, Cecile Charbonneau^a, Jenny Baker^a, Katherine Hooper^a, David Worsley^a, Trystan Watson^a, Joel Troughton^a, Matthew Davies^a, Peter Holliman^b

^aSPECIFIC, College of Engineering Swansea University, SPECIFIC, Baglan Bay Innovation Centre, Central Avenue, Baglan, Port Talbot, SA12 7AX, United Kingdom

^bBangor University, School of Chemistry, United Kingdom, Bangor LL57 2UW, Reino Unido, United Kingdom

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Cecile Charbonneau, 241
Publication date: 5th February 2015

One very attractive aspect of organolead-halide perovskite light absorbers relates to their low temperature processability, with the crystallization typically carried out below 150 °C [1]. From a product form perspective, this is particularly interesting because it opens access to a large range of conductive polymer-based substrates such as flexible ITO-coated PET [2] offering the combined advantages of transparency, light weight and flexibility. Hence, alongside the efforts associated with optimizing the solution processing of these perovskites, there has been a recent regain of interest in tackling the processing issues related to the deposition of the compact TiO₂ layer. This thin layer (approx. 50 nm) sits at the interface between the substrate and perovskite layer and usually prepared by hydrolysis of a spin coated wet film of an organo-titanate precursor, followed by a half hour heating stage at 450-500 °C. However, spin coating is not transferrable to large scale manufacturing and the high temperature step required to achieve sufficient crystallization of the TiO₂ compact layer [3,4] is not compatible with the use of plastic substrates. To overcome this, research groups have started to introduce the use of crystalline TiO₂ nanoparticle colloidal precursors [1,5,6], the high surface energy of these allowing for physical particle necking at relatively low temperature. Here, we present our latest contributions on low temperature solution processed TiO₂ compact layers. This work has been done using FTO glass and ITO-coated PET substrates using two techniques: i) an aqueous solution of TiCl₄ has been coated using an automated temperature controlled K-bar coating instrument at 80 °C; or ii) an aqueous colloidal suspension of TiO₂ nanoparticles (3-5 nm, Figure 1) [7] has been sprayed (1-5 pass) onto a hot plate heated substrate at 120 °C. In the second case we have chemically engineered and optimized the dispersion and wetting properties of the colloid though the use of oxalic acid surface adsorbates [8]. The properties of the resulting films have been characterized on scales ranging from nm to cm using cyclic voltammetry, electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. We present J-V, impedance spectroscopy and transient electron data from perovskite devices mounted on these low-T electron collection layers. We will laso discuss the impact of the surface chemistry and the morphological, topographical and structural features of our low-T processed TiO₂ compact layers on the wetting and crystallization of the perovskite precursor.
Transmission electron microscope image of anatase TiO2 nanoparticles synthesized by hydrolysis of a TiCl4 aqueous precursor (80 °C, 30mins, Patm)
[1] Wojciechowski, K.; Saliba, M.; Leijtens, T.; Abate, A.; Snaith, H. J. Sub-150 °C Processed Meso-superstructured Perovskite Solar Cells with Enhanced Efficiency. Energy and Environmental Science 2014, 7, 1142-1147. [2] Di Giacomo, F.; Zardetto, V.; D'Epifanio, A.; Pescetelli, S.; Matteocci, F.; Razza, S.; Di Carlo, A.; Licoccia, S.; Kessels, W. M. M.; Creatore, M.; Brown, T. M. Flexible Perovskite Photovoltaic Modules and Solar Cells Based on Atomic Layer Deposited Compact Layers and UV-Irradiated TiO2 Scaffolds on Plastic Substrates. Adv. Energy Mater. 2015, 1401808-1401816. [3] Eperon, G. E.; Burlakov, V. M; Docampo, P.; Goriely, A. and Snaith H. J. Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24, 151-157. [4] Wochnik, A. S.; Handloser, M.; Durach, D.; Hartschuh, A. and Sheu C. Increasing Crystallinity for Improved Electrical Conductivity of TiO2 Blocking Layers. ACS Appl. Mater. Interfaces 2013, 5 (12), 5696-5696. [5] Wang, J. T.-W.; Ball, J. M.; Barea, E. M.; Abate, A.; Alexander-Webber, J. A.; Huang, J.; Saliba, M.; Mora-Sero, I.; Bisquert, J.; Snaith, H. J.; Nicholas, R. J. Low-Temperature Processed Electron Collection Layers of Graphene/TiO2 Nanocomposites in Thin Film Perovskite Solar Cells. Nano Letters 2014, 14 (2), 724-730. [6] Conings, B.; Baeten, L.; Jacobs, T.; Dera, R.; D’Haen, J.; Manca, J.; Boyen, H.-G. An Easy-to-Fabricate Low-Temperature TiO2 Electron Collection Layer for High Efficiency Planar Heterojunction Perovskite Solar Cells. APL Mat. 2 2014, 081505, 1-7. [7] Charbonneau, C,; Gauvin, R.; Demopoulos, G. P. Aqueous Solution Synthesis of Crystalline Anatase Nanocolloids for the Fabrication of DSC Photoanodes. Journal of the Electrochemical Society 2011, 158 (3), H224-H231. [8] Charbonneau, C.; Holliman, P.; Davies, M. L.; Watson, T. M.; Worsley. D. A. Facile self-assembly and stabilization of metal oxide nanoparticles. J Colloid Interface Sci. 2015, 15 (442), 110-117.

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