| AVS 65th International Symposium & Exhibition | |
| Thin Films Division | Monday Sessions |
| Session TF-MoA |
| Session: | IoT Session: Thin Films for Photovoltaics |
| Presenter: | Dibyashree Koushik, Eindhoven University of Technology, The Netherlands |
| Authors: | D. Koushik, Eindhoven University of Technology, The Netherlands L. Hazendonk, Eindhoven University of Technology, The Netherlands V. Zardetto, TNO-Solliance, The Netherlands W.M.M. Kessels, Eindhoven University of Technology, The Netherlands M.C. Creatore, Eindhoven University of Technology, The Netherlands |
| Correspondent: | Click to Email |
The need for environmentally stable perovskite solar cells (PSCs) has promoted the quest for moisture and thermal stress barrier layers that can adequately seal the perovskite absorber. Atomic layer deposited (ALD) metal oxides are widely acknowledged for their continuous and pinhole-free nature, derived from the self-limiting deposition process. We have recently shown that 10 cycles of ALD Al2O3 deposited directly on top of CH3NH3PbI3-xClx perovskite are effective in delivering a superior PSC performance with 18% efficiency (compared to 15% of the Al2O3-free cell) and long-term humidity-stability of more than 60 days.1,2 Motivated by these results, the present contribution focuses on the chemical modifications which the CH3NH3PbI3-xClx perovskite undergoes upon growth of ALD Al2O3. Specifically, we couple in situ infrared spectroscopy studies during film growth, with XPS analysis of the ALD Al2O3/perovskite interface. The IR-active signature of NH3+ stretch mode (asymmetric at 3179 and symmetric at 3132 cm-1) of the perovskite undergoes minimal changes upon exposure to ALD cycles, indicating no diffusion of the ALD precursors (TMA and H2O) into the bulk of the perovskite. The exclusion of H2O infiltration is evident also from the absence of two well-defined peaks at 3500 and 3450 cm-1. These conclusions are supported by Doppler broadening-positron annihilation spectroscopy studies, revealing no changes in the open volume of ‘bulk’ perovskite upon deposition of Al2O3. Analysing the differential IR spectra (Absorbance (perovskite + Al2O3)-Absorbance (pristine perovskite)) with increasing ALD cycles, the abstraction of NH3+ from the perovskite surface is discerned, with the appearance of negative N-H stretch and bend modes. In parallel, the growth of ALD Al2O3 on perovskite is witnessed by the characteristic IR-active Al-O-Al phonon and (OH)-Al=O stretch modes. Based on the XPS analysis, a plausible growth mechanism of ALD Al2O3 on top of perovskite is discussed. During the first half- cycle, TMA reacts with perovskite via the interaction of one of its methyl groups with one of the H atoms of CH3NH3+ of perovskite. This interaction weakens the hydrogen-bonds between CH3NH3+ and I- of the perovskite, resulting in breakdown of the organic moiety from the inorganic framework. CH3NH2 and CH4 are released as byproducts, leaving behind an adduct comprising of PbI3-Al(CH3)2. In the next half-cycle of H2O dosage, the H2O molecule can react with the PbI3-Al(CH3)2 adduct and generate the –OH surface sites necessary to promote the growth of Al2O3.
1. D. Koushik et al., Energy Environ. Sci, 2017, 10, 91.
2. D. Koushik et al., Adv. Mater. Inter., 2017, 4, 1700043.