Organo-lead halide perovskite solar cells prepared on low-temperature processed brookite-type TiO2 layers

Tsutomu Miyasaka^a, Atsushi Kogo^a, Masashi Ikegami^a

^aGraduate School of Engineering, Toin University of Yokohama, Kurogane-cho 1614, Aoba-ku, Yokohama, 225-8503

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Atsushi Kogo, 158
Publication date: 5th February 2015

Current studies of organo-lead-halide perovskite solar cells corroborated high conversion efficiency (>20%) using sintered TiO₂ compact and mesoporous layers on glass substrates. Notable advantage of perovkite cells is that device fabrication meets low temperature processes in which the perovskite absorber is prepared at around 100 °C. The low temperature process enables fabrication of flexible devices on plastic substrates. In this study, we employed brookite-type TiO₂ mesoporous layer (MPL) for low temperature fabrication (<150 °C) of TiO₂ layers to make efficient printable perovskite solar cells. For cell fabrication, FTO substrate was coated with a compact TiO₂ hole-blocking layer by spray pyrolysis method. A viscous mesoscopic dispersion of brookite TiO₂ was prepared and spin-coated on the compact layer. The brookite TiO₂ layer was sintered at 150 °C to form a MPL composed of fine crystalline grains (<10 nm). A perovskite (CH₃NH₃PbI_3-xCl_x) layer and spiro-OMeTAD (hole transport material) were spun on the TiO₂ MPL and Au was deposited on the top as counter electrode. Photocurrent-voltage (J-V) curves of the perovskite solar cells with brookite TiO₂ MPL are shown in Figure 1. As reference, J-V curves of cells with MPLs of anatase-type TiO₂ and Al₂O₃, both sintered at 500 °C, were also measured and compared in Fig. 1. The perovskite cell with brookite TiO₂ MPL exhibited a high open-circuit voltage (V_OC) of 0.99 V with fill factor (FF) of 0.68. Power conversion efficiency reached more than 14%, which was higher than those of the reference cells using sintered anatase TiO₂ and Al₂O₃ of equivalent thickness. Morphology observation by SEM showed that the brookite TiO₂ MPL forms a very flat surface being covered by perovskite layer, effectively preventing the contact of MPL with spiro-OMeTAD that causes recombination. Electrochemical impedance spectroscopy also supports that recombination from brookite TiO₂ MPL to spiro-OMeTAD was effectively suppressed. A similar effect was obtained for the cell with Al₂O₃ MPL which separates compact TiO₂ and spiro-OMeTAD as an insulator and exhibited high V_OC of 1.03 V. However, high internal resistance resulted in low FF (0.52) accompanied by a strong hysteresis in J-V curve. With brookite TiO₂ MPL, on the other hand, high V_OC (0.99 V) and FF (0.68) was realized without accompaniment of significant hysteresis. These results suggest that the brookite TiO₂ MPL efficiently collects electrons from perovskite layer and reduces interfacial resistance and capacity, which can lead to hysteretic performance. In the presentation, we will also discuss the fabrication of a flexible solar cell on a plastic ITO-PEN film.
Figure 1. Photocurrent density-voltage curves for perovskite solar cells with anatase TiO2, brookite TiO2, and Al2O3 mesoporous layers.

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