High efficiency planar junction perovskite solar cells processed on electron beam deposited TiOx films

David Cheyns^a, Ulrich W. Paetzold^a, Robert Gehlhaar^a, Weiming Qiu^a^,^b, Paul Heremans^a^,^c, Jef Poortmans^a^,^c, Ludo Froyen^b, Weimin Zhang^d, Nielsen Christian^d, Iain McCulloch^d

^aIMEC, Belgium, Kapeldreef, 75, Leuven, Belgium

^bMTM, KU Leuven, Kasteelpark Arenberg 44, Heverlee, 3001, Belgium

^cESAT, KU Leuven, Kasteelpark Arenberg 10, LEUVEN, B-3001, Belgium

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, Weiming Qiu, 100
Publication date: 5th February 2015

In the past few years, the research on organic lead halide perovskite solar cells has resulted in unprecedented certified power conversion efficiencies with a current record of 20.1%. In most high efficiency perovskite devices, high temperature (>400^oC) sol-gel based TiO_x layers are required as a compact electron transport layer or as a porous scaffold. The preparation of these TiO_x layers, however, can be time-consuming, incompatible with flexible substrates, and may increase the costs. In this contribution, electron beam deposited TiO_x films are used as electron transport layer as an alternative to the sol-gel prepared TiO_x. The vapor deposition combines a simple TiO_x layer fabrication process with an applicability to a variety of substrates. Moreover, the thickness of the TiO_x layer can be precisely controlled. High efficiency planar junction CH₃NH₃PbI_xCl_3-x based perovskite solar cells are presented in a device structure of ITO/TiO_x/perovskite/doped PTAA/Au. Our best devices show an open-circuit voltage of 0.91 V, a short-circuit current density of 20.5 mA/cm², and a fill-factor of 78%, resulting in a PCE value of 14.6%. The devices show a stabilized efficiency of 12.8% after 10 min operation around the maximum power point. In our experiments, we observe a dependence of the active layer morphology on the TiO_x thickness. An increase of the TiO_x layer thickness from 15 nm to 60 nm reduces the surface roughness of the CH₃NH₃PbI_xCl_3-x films and results in higher device performance. In the case of a 60nm TiO_x film, a flat and homogenous CH₃NH₃PbI_xCl_3-x layer is formed, requiring only a thin doped PTAA hole transport layer of 50-60 nm thickness to achieve a full coverage. This will benefit the hole collection at the anode and therefore result in very high fill factors of the devices.
Figure 1. (a) Scanning electron microscopy image of a cross-section of a planer junction perovskite device. The cross section was prepared by focused ion beam. (b) The current density-voltage curve of the best device, and the inset is the power conversion efficiency (PCE) over time, measured at the maximum power point. (c) The External quantum efficiency curve of the best device.

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