Metal oxide management in perovskite-based photovoltaic cells and photodiodes

Masashi Ikegami^a, Yohei Numata^a, Tsutomu Miyasaka^a, Atsushi Kogo^a, Hsin-Wei Chen^a^,^b, Kuo-Chuan Ho^b

^aToin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, 2258503, Japan

^bNational Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China

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

Oral, Tsutomu Miyasaka, presentation 049
Publication date: 5th February 2015

Photovoltaic performance of organo-lead triiodide perovskite photovoltaic cells is highly affected by the material and morphology (porosity and thickness) of metal oxide layers that regulate unidirectional carrier transfers between the perovskite and TCO substrate. Large influence of metal oxide layers generally appears in open-circuit voltage (V_oc) and fill factor (FF) as the results of recombination and influence of carrier transport resistance. Metal oxide compact layer (TiO₂, Nb₂O₅, etc.) especially plays an important role in regulating not only photovoltaic efficiency but also photodiode-like performance of perovskite. First, we have compared the charge blocking ability of TiO₂ compact layers prepared by different coating processes (spray-coat, spin-coat, dip-coat, etc.) which give difference in film morphology and denseness. These compact layers showed large difference in the photodiode gain of the perovskite in terms of photocurrent amplification under external bias. Spin-coated TiO₂ compact layer in combination of planar perovskite layer (MAPbI₃) enabled the highest photodiode performance while spray-coated one contributed most to enhancement of photovoltaic performance. This effect was rationalized by pinhole and defect character of the compact layer. As for mesoporous metal oxide structures, Al₂O₃ as a scaffold of perovskite crystal contributes to high V_oc generation in comparison with TiO₂. This improvement is assumed to be due to suppression by the insulation Al₂O₃ layer of the contact of the compact TiO₂ layer and perovskite. The internal resistance of perovskite/Al₂O₃ layer, however, tends to increase, leading to reduction of FF and conversion efficiency. To solve this, we have prepared Al₂O₃/TiO₂ bilayer composite structure as the scaffold of MAPbI₃. The composite layer could improve FF by maintaining high V_oc of 1.1V. Architecture of the metal oxide layers also alters the hysteretic I-V performance of perovskite. Combination of dip-coated compact TiO₂ layer and mesoporous TiO₂ scaffold achieved non-hysteretic performance with efficiency more than 13% with perfect matching of forward and back scan curves. Our analysis on hysteretic performance also showed that there is influence of ferroelectric dipole polarization on the I-V characteristics. This influence becomes minimal in the cell of mesoporous TiO₂ scaffold without capping perovskite layer while it becomes largest in the meso structure-free planar cell.¹ Our metal oxide engineering enabled cell fabrication for 17% efficiency with relatively small hysteresis. In conclusion, perovskite cell of non-hysteretic and high efficiency (high voltage) performance can be obtained with perovskite prepared on an Al₂O₃/TiO₂ composite scaffold and dense TiO₂ compact layer.

1. Chen, H.-W.; Sakai, N.; Ikegami, M.; Miyasaka, T. Emergence of hysteresis and transient ferroelectric response in organo-lead halide perovskite solar cells. J. Phys. Chem. Lett., 6, 164–169(2015).

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