Opportunities of Atomic Layer Deposition for Perovskite Solar Cells
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 (VOC = 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 (JSC, VOCand 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, we fabricated also the first flexible perovskite module (8 cm2) with an overall performance of 3.1%.
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