1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Energy Technology & Environmental Science The Use of Rutile- and Anatase-Titania Layers towards Back Light Scattering in Dye-Sensitized Solar Cells Niaz Muhammad, [a] Kassio Papi Silva Zanoni, [b] Neyde Yukie Murakami Iha, [b] and Safeer Ahmed* [a] Dye-sensitized solar cells (DSCs) with nanocrystalline TiO 2 (NCT) particles (ca. 20 nm) as foundation layer and mesoporous anatase phase microspheres (AMT) or rutile phase microspheres (RMT), as the back light-scattering layer, were fabricated. The phase-pure AMT, 0.42 μm or RMT particles, 0.50 μm were obtained at 500 o C and 900 o C, respectively. The DSCs employ- ing NCT/AMT and NCT/RMT bi-layered films depicted power conversion efficiencies of 5.9% and 6.3%, respectively, com- pared to 4.0% of DSCs with only NCT films. The efficiencies were improved up to 9.0% after depositing compact nano-TiO 2 layers beneath the NCT film. The optical and electrochemical investigations of the DSCs quantitatively elucidated, through longer electron life time and enhanced back light scattering, the substantial improvement in power conversion efficiency to the texture tuning of titania films, in particular by the rutile phase. 1. Introduction In the context of global energy demand, dye-sensitized solar cells (DSCs) have attracted intense research interest due to their low cost, easy fabrication processes and environment friendly technologies. [1–5] Extensive research has been devoted to optimize the structural, morphological and optical properties of TiO 2 -based DSCs to improve the power conversion efficiency, which is still challenged by many crucial limiting factors. [6–7] For well performing photoanodes, for example, the confluence of high internal surface area, enhanced solar-light harvesting efficiency and fast electron transport in a single morphology is still a pressing need. [5] DSCs based on anatase TiO 2 nanocrystalline films (with particles having ca. 20 nm in diameter) are highly porous, providing sufficient internal surface area for sensitizer uptake with certified power conversion efficiency of 11% and overall 13% so far. [8,9] However, the high transparency and negligible light scattering ability of such electrodes usually result in loss (non-harvesting) of near infra-red photons, hardly absorbed by the sensitizer layer. [10,11] Extensive work has been dedicated to boost the power conversion efficiency by developing new photoanode architectures that can integrate both the benefits of high internal surface area and sufficient light scattering in a single morphology. [12,13] Anatase TiO 2 particles with bigger dimensions (ca. 200 400 nm range) have been widely employed to enhance the back light scattering ability of photoanodes. [14,15] The main strategies are i) mixing both small and large TiO 2 particles in a single foundation film or ii) using bi-/multi-layered film structures comprising a top scattering layer of large TiO 2 particles and a nanocrystalline TiO 2 underlayer. [16,17] Rutile TiO 2 , on the other hand, has higher refractive index as well as larger white light scattering ability and is chemically more stable than anatase TiO 2 . [18] However, it has gained very little attention as a competent photoanode candidate in DSCs research due to its low internal surface area hence low dye uptake and slow electron transport, resulting in lower short-circuit currents (J SC ) and open-circuit potentials (V OC ) than anatase TiO 2 . [19,20] Few works report suitable performances by DSCs fabricated with photoanodes based on hierarchical-structured rutile titania as top scattering layer. [21–22] Till date, no DSC device with semi- conductor other than TiO 2 has challenged the certified power conversion efficiency, showing a stressing need to even finer tuning of the TiO 2 particle size, crystal phase, film porosity and developing new morphologies. [23–26] In the present investigation, we report quantitatively the back light scattering effect of anatase and rutile crystal phase of TiO 2 submicrospheres on the power conversion efficiency of DSCs. TiO 2 submicrospheres and nanoparticles were synthe- sized by glycol mediated and hydrothermal methods, respec- tively. The hydrothermal synthesis was selected because it is a facile method and does not need any seed, catalyst, harmful and expensive surfactant or template and is promising for large scale and low-cost production with high-quality crystals. Further it provides very good control over the rate and uniformity of the nucleation, growth and aging of the particles. The size of the titanium glycolates can easily be tuned by varying the precursor concentration and are highly susceptible to aqueous medium and conveniently be converted into porous microspherical TiO 2 simply by refluxing for one hour [a] N. Muhammad, Dr. S. Ahmed Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Is- lamabad, Pakistan E-mail: safeerad@qau.edu.pk [b] K. P. S. Zanoni, Prof. N. Y. M. Iha Laboratory of Photochemistry and Energy Conversion, Departamento de Química Fundamental, Instituto de Quí o Paulo - USP, Av. -o Paulo, SP, Brazil Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201801569 Full Papers DOI: 10.1002/slct.201801569 10475 ChemistrySelect 2018, 3, 10475–10482 © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim