Influence of TiO 2 compact layer precursor on the performance of perovskite solar cells Paola Vivo a, * , Anniina Ojanper € a a , Jan-Henrik Smått b , Simon Sand  en c , Syed Ghufran Hashmi d , Kimmo Kaunisto e , Petri Ihalainen b , Muhammad Talha Masood b , Ronald € Osterbacka c , Peter D. Lund d , Helge Lemmetyinen a a Department of Chemistry and Bioengineering, Tampere University of Technology, Finland b Laboratory of Physical Chemistry, Faculty of Science and Engineering and Center for Functional Materials, Åbo Akademi University, Finland c Physics, Faculty of Science and Engineering and Center for Functional Materials, Åbo Akademi University, Finland d New Energy Technologies Group, Department of Applied Physics, Aalto University, Finland e VTT Technical Research Centre of Finland, Finland article info Article history: Received 20 September 2016 Received in revised form 21 October 2016 Accepted 13 November 2016 Available online xxx Keywords: Perovskite solar cells TiO 2 Compact layer Hysteresis Stability TiCl 4 abstract The optimization of the hole-blocking layer in perovskite solar cells (PSC), typically based on TiO 2 , is crucial, as it strongly affects the device performance. In this work, we thoroughly characterize the thickness, roughness, and crystal structure of a set of TiO 2 compact layers produced by spin coating of different precursor sols and correlate the choice of the TiO 2 precursor to the photovoltaic performance of the PSC. By replacing the commonly used titanium isopropoxide (TTIP) blocking layer precursor with titanium tetrachloride (TiCl 4 ), a clear enhancement in the PSC performance was observed, particularly in the hysteresis behavior and stability. The results from the morphological/structural analysis and transient photoluminescence studies clarify the different behavior of the compact layers in PSCs. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Perovskite solar cells (PSC) have received tremendous interest from the scientific community, due to an incredible boost in their power conversion efficiencies (PCEs), increasing from 9% to over 22% in recent years [1e3]. Current challenges in PSC research include replacement of costly materials, enhancement of device stability, upscaling of lab-size cells to larger modules, and replacement of lead (Pb) in perovskite materials to overcome the toxicity issues [4,5]. Another issue related to the poor reproducibility of PSCs is the presence of hysteresis in the voltage-dependent photocurrent, which makes the determination of the real solar-to-power con- version efficiency of the devices difficult [6]. Among the different types of PSCs, the traditional mesoscopic PSC is the most widely adopted geometry, due to easy fabrication while still showing record efficiencies [7]. The traditional meso- scopic perovskite cell architecture utilizes a thin (30e50 nm) and compact hole blocking layer between the transparent conducting oxide (TCO) layer (mostly fluorine-doped tin oxide, FTO, coating) and a mesoporous scaffold, typically made of TiO 2 , Al 2 O 3 or ZnO. The blocking layer hinders the transportation of holes (generated in the perovskite light absorbing layer) towards the FTO, thus reducing the charge recombination and facilitating the electron transport through the FTO layer [8,9]. The optimization of the blocking layer has received increasing attention as it can strongly affect the performance of the PSCs [10-15]. Though zinc oxide (ZnO) [10,16e18] and cesium carbonate (Cs 2 CO 3 ) [19] have been adopted as alternatives for hole blocking layers, up to now TiO 2 is the most widely used compact layer. The compact TiO 2 layer can be produced either by spray pyrolysis [12,14,18e21] or atomic layer deposition (ALD) [11,22e24], thermal oxidation [22,25,26], and electro- chemical deposition [13]. Another simple method which can be also applied to produce a compact TiO 2 layer is spin coating [11,27], * Corresponding author. Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, 33100 Tampere, Finland. E-mail address: paola.vivo@tut.fi (P. Vivo). Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel http://dx.doi.org/10.1016/j.orgel.2016.11.017 1566-1199/© 2016 Elsevier B.V. All rights reserved. Organic Electronics xxx (2016) 1e7 Please cite this article in press as: P. Vivo, et al., Influence of TiO 2 compact layer precursor on the performance of perovskite solar cells, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.017