Solar Energy 208 (2020) 212–219 Available online 5 August 2020 0038-092X/© 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. Effect of UV-ozone exposure on the dye-sensitized solar cells performance Chandan Dawo a , Mohammad Adil Afroz b , Parameswar Krishnan Iyer b, c , Harsh Chaturvedi a, * a Centre for Energy, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India b Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India c Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India A R T I C L E INFO Keywords: Electron transport layer Ultraviolet-ozone Dye sensitized solar cell Organic contaminants Hydrophilicity ABSTRACT The surface states of the active TiO 2 layer is crucial while fabricating an efficient solar cell. This work experi- mentally analyses the effect of exposing TiO 2 based electron transport layer (ETL) to the ultraviolet-ozone (UV- O 3 ) and optimizes the exposure time for improving power conversion efficiency (PCE) of fabricated dye- sensitized solar cells (DSSCs). These results demonstrate that the performance of DSSCs can be improved significantly by UV-O 3 exposure of sintered TiO 2 photoanode surface, with the duration of exposure being a critical parameter. Fabricated devices show 33.01% increase in PCE for the optimum exposure. Nevertheless, overexposure of the sample beyond the optimum time decreases the efficiency of the fabricated solar cells. The device with optimum exposure exhibits the highest PCE of 8.34% with short circuit current density (J sc ) of 15.15 mA/cm 2 , open circuit voltage (V oc ) of 756 mV and Fill factor (FF) of 71.10%. This increase in efficiency is attributed to the enhanced crystallization and reduction in the organic contaminants C–C/C–H from 57.90 to 52.40% as shown by the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The XPS result further indicates an increase in oxygen vacancy from 12.40 to 23.40% for O 1s state and from 9.30 to 14.30% for Ti 2p state of Ti 3+ . Results from the Atomic Force Microscope (AFM) also confirms the minimized surface roughness of 16.36 nm for the optimally exposed TiO 2 film, and increase in hydrophilicity leading to improved efficiency of the solar cells which were optimally exposed to UV-O 3 . 1. Introduction Gr¨ atzel and O’Reagan developed dye-sensitized solar cells (DSSCs) in 1991 with mesoporous TiO 2 film as photoanode (O’Regan et al., 1991). Since then, DSSCs have drawn the attention of the scientific community because of its low materials cost, easy fabrication process, lightweight, flexibility, tunable optical properties, and raw materials availability. DSSCs are generally composed of mesoporous TiO 2 photoanode to up- take dye photosensitizer, liquid electrolytes (I  /I 3 ), and platinum (Pt) counter electrode. So far, 14.3% efficiency has been reported for DSSCs based on TiO 2 photoanode (Kakiage et al., 2015; Kavan, 2017). The most preferred photoanode material for DSSC is nanocrystalline TiO 2 due to its electro-optical characteristics i.e., favourable valence band and conduction band position for dye and FTO, chemical stability, high refractive index (2.45), cost-effectiveness, strong oxidizing power, mesoporous nature and low toxicity (Hadi et al., 2018; Mor et al., 2006; Castro et al., 2015). TiO 2 based photoanode plays multiple roles for efficient charge generation, charge separation, and charge transfer from dye molecule to transparent conducting oxide (TCO) substrate. The conversion of light to electricity consists of multiple dynamic processes in DSSCs. Several undesired recombination processes are also known to occur at the photoanode, significantly reducing the PCE. Minimizing such unwanted processes at the active layer of TiO 2 is required for the efficient performance of the DSSCs. Hence, the surface states and the structural parameters of the TiO 2 plays an important role in the fabri- cation of an efficient solar cell (Raju et al., 2017; Wu et al., 2013; Zukalov´ a et al., 2008; Afroz et al., 2017; Hagfeldt et al., 2010; Gr¨ atzel, 2003). Several modifications of the TiO 2 photoanode have been reported to increase the efficiency of DSSCs, such as compact or blocking layer (Sangiorgi et al., 2014; G´ oes et al., 2012; Tanvi et al., 2017), scattering layer (Garmaroudi et al., 2016), atomic doping (Huo et al., 2017; Thogiti et al., 2018), surface treatment (Jun et al., 2006), etc. The surface treatment of TiO 2 electron transport layer (ETL) has become an essential step for improving the performance of DSSCs. Surface treatment of the photoanode affects dye loading capacity, electron transport, and elec- tron recombination processes. TiCl 4 treatment on the TiO 2 ETL has been the most commonly applied method to increase efficiency of DSSCs * Corresponding author. E-mail address: harshc@iitg.ac.in (H. Chaturvedi). Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener https://doi.org/10.1016/j.solener.2020.07.064 Received 12 March 2020; Received in revised form 17 July 2020; Accepted 20 July 2020