Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener The performance enhancement of HTM-free ZnO nanowire-based perovskite solar cells via low-temperature TiCl 4 treatment Nanaji Islavath, Giribabu Lingamallu Polymers & Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, T.S., India ARTICLE INFO Keywords: Nanowire Low-temperature hydrolysis Charge carrier transport Solar cell ABSTRACT Charge carrier transport layer made of aligned ZnO nanowires (NW) coated with TiO 2 layer via low-temperature hydrolysis process for perovskite solar cells (PSC) applications. Aligned ZnO NWs were grown by solution ap- proach onto transparent conducting oxide substrates and treated with 25 mM of titanium tetrachloride (TiCl 4 ). TiO 2 layered ZnO NWs were composed of single crystalline, uniformly covered with amorphous TiO 2 nano- particles was characterized by scanning and transmission electron microscopy. Such TiO 2 layered ZnO NW architecture, with the unique combination of high surface-area and infiltration of perovskite more into the electrode and a maximum photocurrent density (J SC ) of 14.30 mA cm -2 and a power conversion efficiency (η) of 7.05% are demonstrated, which is higher than the ZnO NWs. Dark current density is decreased for TiO 2 layered ZnO NW-based PSCs, due to the good interface formation between the electrode/sensitizer and it has higher energy barrier to overcome required high onset potential. TiO 2 layered ZnO NWs architecture opens up a novel configuration for high-performance optoelectronic devices. 1. Introduction Organometal halide perovskites materials are one of the promising sensitizers for next-generation solar cells and it can be prepared at the ambient atmospheric condition, along with high charge carrier mobi- lity, long carrier diffusion length, and large absorption coefficient (Mitzi et al., 1994; De Wolf et al., 2014; Park, 2015; Dong et al., 2015; Stranks et al., 2014). The perovskite-based solar cells have reached 22.1% efficiency and ease to fabricated at room temperature condition and low-cost compared with p-n junction photovoltaic devices (Shin et al., 2017; Brenner et al., 2016). The first report of perovskite solar cells was demonstrated by Miyasaka and co-workers in 2009. They employed CH 3 NH 3 PbI 3 and the analogue CH 3 NH 3 PbBr 3 as sensitizers in liquid-electrolyte-based dye-sensitized solar cells, achieving a power conversion efficiency (PCE) up to 3.81% (Kojima et al., 2009). Kim and co-workers later reported a solid-state perovskite solar cell with a PCE of 9.7% (Kim et al., 2012). Up to now, these devices not only show relatively high efficiencies newcomer to the solar cell family. Con- ventionally, metal-oxide nanoparticles are usually employed as a charge carrier transport layer in dye-sensitized and perovskite solar cells (Thakur et al., 2017). Although nanoparticles offer a large specific surface-area for mooring the absorber materials (sensitizers). But na- noparticle has the poor networking and interparticle connectivity lim- iting the efficient transport of photo-generation charge carriers (e-) (Lu et al., 2015). By replacing the conventional nanoparticulate of the metal-oxide film by aligned metal-oxide nanostructures may improve the electron transport and infiltration of sensitizer (Jiang et al., 2014). Furthermore, the absence of interparticle continuity grain boundaries is expected to minimize the carrier recombination. However, such efforts always accompanying the reduction in the specific surface-area of the photoanode materials and faster electron transport. Realizing such ef- foerds by low-temperature solution base approach may surplus enhance the technical viability of PSCs (Kim et al., 2013; Qiu et al., 2013; Sun et al., 2015). In response to this problem, the article intends a zinc oxide (ZnO)-based PSC technology as an alternate for TiO 2 in solar cells (Son et al., 2014). Furthermore, ZnO has higher electron mobility than that of TiO 2 by two-to-three orders of magnitude (Law et al., 2005; Dong et al., 2014). So, ZnO is expected to reveal faster charge carrier trans- port as well as reduce the recombination rate as likened to TiO 2 . However, TiO 2 based photovoltaic device has higher efficiency than that of ZnO (Kumar et al., 2013; Islavath et al., 2017a). The main drawback of ZnO is chemically unstable and easily oxidized at normal condition. Subsequently, O'Regan et al. first reported the TiCl 4 treated NW-based solar cells have an efficiency of 0.07% which is significantly higher than the ZnO NWs (Atienzar et al., 2010). The advantage of titanium tetrachloride (TiCl 4 ) treatment is a downward movement of titania conduction band edge and a decrease recombination at the in- terface of ZnO/sensitizer (Manthina et al., 2016; Sommeling et al., 2006). Thin TiO 2 layer willing by numerous routes, such as atomic layer and chemical vapor deposition, sputtering and hydrolysis (low- https://doi.org/10.1016/j.solener.2018.05.050 Received 23 January 2018; Received in revised form 6 April 2018; Accepted 13 May 2018 E-mail addresses: islavathnanaji@gmail.com (N. Islavath), giribabu@iict.res.in (G. Lingamallu). Solar Energy 170 (2018) 158–163 0038-092X/ © 2018 Published by Elsevier Ltd. T