Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Physical properties of quasi-solid-state polymer electrolytes for dye- sensitised solar cells: A characterisation review Ahmad Azmin Mohamad School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia ARTICLE INFO Keywords: Quasi-solid state electrolytes Gel polymer electrolytes Polymer electrolytes Dye-sensitised solar cells Characterisations ABSTRACT The electrolyte is a critical component of dye-sensitised solar cells (DSSCs). Among them, quasi-solid state (QSS) electrolytes are considered as one of the most prospective substitutes for liquid electrolytes to DSSCs. To entirely understand and optimise the performance and stability of QSS, comprehensive characterisation is needed to design new materials, comprehend and optimise the QSS. This review summarises the characterisations of QSS beginning from 2010. Emphasis is placed on the physical characterisations such as viscosity, morphology, thermal, infrared spectroscopy, nuclear magnetic resonance, Raman, ultraviolet-visible, and X-ray diffraction. All parts of the characterisation process are divided into the introduction, the objective of testing, main outcomes and finally, the selected examples. A summary of recent and important measurements on QSS are mentioned at the end. 1. Introduction Dye-sensitised solar cells (DSSCs) have attracted immense interest and attention due to their low fabrication cost, eco-friendly and as one of the potential renewable energy sources. Currently, DSSCs can achieve a conversion efficiency of 14.3% (Kakiage et al., 2015). Al- though to extend this efficiency, it relies on many factors. One of the most crucial factors is an electrolyte. In general, electrolytes can be classified into several groups; liquid electrolytes and solid-state electrolytes, and between them are quasi-solid state (QSS) electrolytes. Indeed, some researchers have reported it as gel polymer electrolytes (GPEs) (Wanninayake et al., 2016; Fan et al., 2014; Lan et al., 2012; Jin et al., 2018; Yoon et al., 2012). The volatilisation issues associated with the long-term operation of DSSCs results in leakage and/or the evaporation of liquid electrolyte, which are considered to be one of the vital factors that shorten the lifetime of these devices. Even, the inconvenience of handling liquid electrolyte during the fabrication process has made the work of https://doi.org/10.1016/j.solener.2019.08.016 Received 28 June 2019; Received in revised form 5 August 2019; Accepted 7 August 2019 Abbreviations: ACN, acetonitrile; AIN, aluminium nitride; ATR, attenuated total reflection; BDI, 1-buty-2,3-dimethylimidazolium iodide; C 3 PyI-C 7 PyI, 1-alkyl-2- methylpyrazolium iodides; C 6 ImI, 1-hexyl-3-methylimidazolium iodide; CH 2 , methylene; CH 3 CN, acetonitrile; CTAB, cetyltrimethyl ammonium bromide; CuI, copper (I) iodide; DMBS, gelator of d-sorbitol and 3,4-dimethyl-benzaldehyde; DMPII, 1,2-dimethyl-3-propylimidazolium iodide; DSC, differential scanning calorimetric; DSSC, dye-sensitised solar cells; DTA, differential thermal analysis; EC, ethylene carbonate; EDOT, ethylenedioxythiophene; EDX, energy-dispersive X-ray analysis; EG, ethylene glycol; EGA, evolved gas analysis; EMITFSI, 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imid; FE-SEM, field-emission scanning electron microscopy; FTIR, fourier transform infrared spectroscopy; FTO, fluorine-doped tin oxide; GNS, graphene nanosheet; GO, graphene oxide; GOS, graphene oxide sponge; GPE, gel polymer electrolytes; GR, glycerin; HDT, poly(hexamethylene diisocyanate tripolymer); HEMA, poly(hydroxyethyl hethacrylate); HFP, hexa- fluoropropyle; I 2 , iodine; IL, ionic liquid; IR, infrared spectroscopy; KBr, potassium bromide; KI, potassium iodide; LiClO 4 , lithium perchlorate; LiI, lithium iodide; LMOG, low molecular mass organogelator; MOGE, metal-organic gel electrolyte; MPN, methoxypropionitrile; MSNs, mesoporous silica nanoparticles; MWCNT, multi- wall carbon nanotubes; NaI, sodium iodide; NH 4 I, ammonium iodide; NMBI, N-methyl-benzimida- zole; NMP, N-methyl-2- pyrrolidone; NMR, nuclear magnetic resonance; NPs, nanoparticles; P 13 I, 1-propyl-3-methylimidazolium iodide; PAA, polyacrylic acid; PANi, polyaniline; PBVIm Br, poly(1-butyl-3-vinylimidazolium bromide); PBVIm TFSI, poly(1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide); PC, polycarbonates; PEAA, poly(ethylene-co-acrylic acid); PEDOT, poly(3,4-ethylenedioxythiophene); PEG, poly(ethylene glycol); PEI, poly-ethyleneimine; PEO, polyethylene oxide; PGA, poly(glycidyl acrylate); PHEA, poly(hy- droxyethyl acrylate); PMII, 1-propyl-3-methyl imidazolium iodide; PMMA, poly(methyl methacrylate); PMMA-EA, poly(methyl methacrylate-co-ethyl acrylate); POM, polarised optical microscopy; PPy, poly(pyrrole); PSS, poly(styrenesulfonate); Pt, platinum; PVAc, polyvinyl acetate; PVA-EL, poly(vinyl alcohol-co-ethylene); PVDF, polyvinylidene fluoride; PVP, polyvinylpyrrolidone; QSS, quasi-solid state; SAN, styrene-acrylonitrile; SAXS, small-angle X-ray scattering; SEM, scanning electron microscopy; SEOS, poly(styrene-block-ethyleneoxide-block-styrene); SiO 2 , silica; TBP, 4-tertbutylpyridine; TFSI, bis(trifluoromethanesulfonyl)imide; TGA, thermogravimetric analysis; T gel , transition temperature; T sg , solution-to-gel transition temperature; TiO 2 , titanium oxide; UV–vis, ultraviolet–visible; WAXD, wide- angle X-ray diffraction; WAXS, wide-angle X-ray diffraction scattering; XRD, X-ray diffraction; β-CD, β-cyclodextrin E-mail address: aam@usm.my. Solar Energy 190 (2019) 434–452 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T