Opportunities of Atomic Layer Deposition for Perovskite Solar Cells

The recent outbreak of organo-metal halide perovskite absorber has catalyzed the interest in the photovoltaic (PV) community due to the remarkable increase in the device performance during the last 3 years and the easy solution manufacturing steps. Atomic Layer Deposition (ALD) is well known nowadays to be adopted in different photovoltaic (PV) technologies due to the accurate control in film thickness and composition, the high deposition conformality on structures with high aspect ratio. Thermal ALD processes have been already applied in glass based perovskite solar cells, 1,2 for the deposition of a blocking TiO 2 layer. This film is required to avoid the recombination process at the interface between the transparent conductive oxide film (TCO) and the perovskite and/or the hole transport layer (HTL). Compared to the thermal approach, plasma-assisted ALD enables the fabrication of higher quality films in terms of density and materials properties, 3 extending the processing window down to temperatures compatible also with (conductive) polymer substrates,suitable for roll to roll manufacturing process. In this work we investigate the role of plasma-assisted ALD compact TiO 2 deposited on ITO/PET substrates for a hybrid halide (CH 3 NH 3 PbI 3-x Cl x ) perovskite solar cells, demonstrating also the fabrication of an efficient large area flexible module. The layers were prepared in a remote plasma reactor (FlexAL TM ) at 150 °C using an heteroleptic alkylamido precursor Ti(CpMe)(NMe 2 ) 3 step alternated with an O 2 inductively coupled plasma exposure. The back reaction at the interface TCO/perovskite or HTL is extremely detrimental when the device is fabricated on a conductive polymer (ITO-PET). Very low open circuit voltage (V OC = 50mV) and efficiency (η = 0.01%) have been measured without the compact TiO 2 layer. The analysis of JV dark current revealed the lack of rectifying behavior at this interface and consequently a high value of the exchange current (7 mA∙cm -2 ) as well as a high current under reverse bias (V