Full length articleEffect of TiO2 particle size and layer thickness on mesoscopic perovskite solar cells
Introduction
Perovskite solar cells (PSCs), based on organic–inorganic halide perovskite light-absorbing materials, are one of the most promising photo-energy conversion devices, having shown a rapid development in power conversion efficiency (PCE) of up to 22.1% during the last several years [1]. A great deal of effort has been concentrated on PSCs to improve the device performance and/or understand the underlying principles. Focus has been placed on their varied chemistries, such as doping, substitution, and interfacial treatments; physical analyses, such as charge carrier generation, separation, recombination, and transport; as well as device engineering, such as adopting mesoscopic/planar- and normal/inverted-type structures [2]. The highest PCEs have been achieved with mesoscopic PSCs [3], [4], [5], in which a mesoporous metal oxide layer plays important roles as the charge-transport channel, scaffold for loading the light-absorbing materials, and electron–hole separator [6].
TiO2 is one of the most studied mesoporous layer (mp-layer) materials due to its suitable electronic band levels and ease of nanoparticle (NP) fabrication [7], [8]. Moreover, it is known that the n-type semiconducting nature of a TiO2 layer aids electron injection and transport in PSCs, as such layers help the generated charge carriers in the perovskite to be well separated [9]. Therefore, the photovoltaic performance of mesoscopic PSCs might be largely influenced by electron injection at the perovskite/mp-TiO2 interface and the electron transport in the mp-TiO2 layer. The charge injection from the perovskite to the TiO2 layer can be improved by increasing the perovskite/mp-TiO2 interfacial area, that is, the surface area of the TiO2 NPs, which is expected to be beneficial to the PSC performance. However, enlarging the surface area does not always give rise to an improvement in PSC performance, because electron transport should be considered. For example, decreasing the TiO2 NP size would increase the TiO2/TiO2 interfacial area, which could diminish the electron transport ability of the mp-layer.
In this study, we investigate the charge-injection and transport properties of mp-TiO2 layers in PSCs by varying the size of the TiO2 NPs and the thickness of the mp-TiO2 layer. The particle size and layer thickness are directly related to the surface area (i.e., perovskite/TiO2 interface) and the area of the TiO2/TiO2 interfaces, which can affect the electron injection and transport properties, respectively. We compared the electron injection, electron transport, and interfacial resistance of mp-TiO2 layers in PSCs, with the variation of the layer thickness (150 nm, 250 nm, and 400 nm) and NP diameter (25 nm and 41 nm) of TiO2.
Section snippets
Device fabrication
All the devices were fabricated via a similar process to that given in our previous report [10]. Fluorine-doped tin oxide (FTO) substrates (Pilkington TEC15) were cleaned sequentially using acetone, ethanol, and deionized water in an ultrasonic bath for 15 min. A compact TiO2 (c-TiO2) layer was spin-coated (3000 rpm, 20 s) on the cleaned FTO substrate using 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt.% in isopropanol, Sigma-Aldrich) in anhydrous 1-butanol (Tokyo Chemical
Results and discussion
To investigate the effect of particle size on the photovoltaic properties of PSCs, we used two TiO2 pastes with NP sizes of 25 nm and 41 nm. Fig. 1a shows the TEM images of these two kinds of TiO2 NPs. The diameters were measured from the TEM images and the diameter distributions are shown in Fig. S1 (Supplementary materials). The mean diameters of the smaller and the larger NPs are 21.7 nm and 45.4 nm. The XRD patterns (Fig. 1b) of the NPs confirm that both pastes contain anatase-phase TiO2
Conclusion
Mesoporous TiO2 layers as an electron transport layer are known to be very important components of PSCs. However, their various effects, for example, on electron injection and transport, have not been clearly identified despite recent intensive studies. We investigated the electron injection and transport properties of mp-TiO2 layers by varying the layer thickness and the size of the TiO2 NPs in the layer. For our PSC structure comprised of a thick (∼600 nm) perovskite layer and a thin
Acknowledgements
This work was supported by the Technology Development Program to Solve Climate Changes (NRF-2015M1A2A2056827), and the Global Frontier R&D Program of the Center for Multiscale Energy System (NRF-2012M3A6A7054855). This research was also supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B2013087 and 2017R1A4A1015022).
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Equally contributed to this work.