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 TiO₂ 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 TiO₂ deposited on ITO/PET substrates for a hybrid halide (CH₃NH₃PbI_3-xCl_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)(NMe₂)₃ step alternated with an O₂ 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 TiO₂ 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<0). The introduction of the ultrathin ALD layers brought to an increment in all the photovoltaic parameters (J_SC, V_OCand FF), with the saturation of the efficiency for layers thicker than 5.5 nm. Above this value, we observed a reduction of both the exchange current and the dark reverse current up to three orders of magnitude, pointing out that an effective blocking behaviour can be achieved already with ultrathin film (over 5.5 nm and up to 44 nm).

The high quality of the ALD layer led to a maximum performance of 9.2% with 11 nm (200 ALD cycles), overcoming the 4% achieved with a conventional sol gel TiO₂ compact layer (20 nm thick). Interestingly, when the same layer works also as electron transport material (without the mesoporous TiO₂) in the planar configuration, the efficiency of the device decreased up to 1.3% due to the ineffective electron injection at the interface between the perovskite and the ALD TiO₂ compact layer. Finally, with an accurate upscaling of the materials and techniques,^[4] we fabricated also the first flexible perovskite module (8 cm²) with an overall performance of  3.1%.

[1] Y.Wu, et al, Applied Physics Express 7, (2014), 052301; .               

[2] A.K. Chandiran et al, Adv. Mater., 26, (2014), 26, 4309;

[3] W.M.M. Kessels et al, ECS Transactions, 3,(2007), 183;

[3] M. Grätzel, Nature Materials, 13, 838–842 (2014).

[4] F. Di Giacomo and V. Zardetto, et al, Adv. Energy. Mater. (2015), DOI: 10.1002/aenm.201401808.