In this study, high efficiency perovskite solar cells (PSCs) are achieved using atomic layer deposited (ALD) TiO2 as electron transporting layer. Compared to the conventional solution processed TiO2 , at which high annealing temperature (500 degree) is required, the ALD method not only offers a much lower processing temperature (150 degree), but also provides a thin film with lower roughness and higher purity. The perovskite solar cell with ALD-TiO2 exhibit a power conversion efficiency of 16.86%, comparable to those with high temperature processed TiO2. The ALD technique shows a great potential for the wearable, flexible and lager-area planar industrial production.
To investigate how the processing condition affect the TiO2 layers and the solar cell performance, we compared two different fabrication method: solution processing (SP) and ALD. A conventional PSCs structure of Glass /FTO/TiO2/ MAPbI3(Cl)/Spiro-OMeTAD/Au is adopted. To achieve a better quality of perovskite layer, we doped a small amount (1%) of MACl into precursor. The SP-TiO2 layer was deposited by spin-coating a precursor of TiAcAc and 1-Butanol, followed by an annealing step at 500 degree; while the ALD-TiO2 was fabricated with Titanium tetrakis (dimethylamide) and water as oxidant by atomic layer deposition kept at 120 degree. The thin film morphology is measured by scanning electron microscope (SEM, ZEISS SUPRA 55) and atomic force microscopy (AFM, Bruker MultiMode 8). The thickness of FTO,TiO2 and perovskite layer were 280nm, 30nm and 400nm. ALD-TiO2 exhibited a better smoothness in surface morphology, with a RMS of 7.08nm at 150 degree and 10.2nm at 500 degree, in contrast the SP-TiO2 shows a RMS of 7.32nm and 13.6nm in order. More pin-holes could be observed in the SP-TiO2 and the crystals grow bigger along with annealing temperature, leading to a higher RMS as well. In contrast, the XRD pattern of ALD-TiO2 reveals that the as-prepared ALD-TiO2 is amorphous, with anatase phase crystal formed when it subject to post annealing treatment, even at a low temperature . Furthermore, we employed Fourier Transform Infrared Spectroscopy (FTIR, Perkinelmer FT-IR Spectrometer) to probe the organic residue and chemical bonds. In the sample by SP method, we discovered the residual Ti-OH bonds weak-peaks, H-OH bonds weak-peaks , and the Ti-O bonds strong-peaks in around 700 cm-1 area, while a residue-less condition was showed in the samples of ALD method. In performance of PSCs, the ALD-150 method based PSCs exhibited a comparable performance including PCE of 16.86%, as 14.78% by ALD-500 method,16.67% by SP-500 method, which revealed that ALD-TiO2 annealing at 150 degree had an enough crystallization and electronic transporting properties fitting the morphology analysis above.
In brief, the ALD-TiO2 layer presented a crystallization which satisfy the photovoltaic applicaiton and had a great potential for the further PSCs production especially in flexible and wearable photovoltaic devices.