Hybrid organic-inorganic perovskite solar cells are heavily researched due to their potential to offer both high conversion efficiency and low cost. However, so far, environmental device stability is a major issue. Many avenues to improve the stability of these cells are being investigated with the added constraint of retaining or reaching a high efficiency. One avenue that is seen as very promising is the use of inorganic thin films in the design of the device. ALD with its excellent control of film growth and high-quality films is seen as a key technology to this end. Mostly for research but also for pilot-production, there is a desire for flexible tools with wide processing range and both plasma and thermal capabilities. Oxford Instruments FlexAL® and OpAL® tools are ideally suited in this respect. The goal of this white paper is to give a practical overview on what ALD is and how it can be used to benefit perovskite solar cells. To illustrate these benefits we will refer to some recent publications where Oxford Instruments equipment is utilised.
Atomic Layer Deposition
In atomic layer deposition (ALD), thin films are built up in cycles in which the surface is exposed to various vaporor gas-phase species in alternating, separated doses. In each cycle, a submonolayer of material is deposited. As illustrated in Fig. 1, a typical cycle consists of four steps: (i) a precursor dosing step, where a precursor is typically an inorganic metal-organic or metal-halide (e.g. TMA); (ii) a purge and/or pump step; (iii) a co-reactant step, typically involving a small molecule (e.g. H2O or O2 plasma); and (iv) a purge and/or pump step. For the precursor, the element to be deposited is in many cases the metal center (e.g. Al), while for the reactant, it is typically a non-metal such as O. Together these then form the resulting film (e.g. Al2O3) For ALD, it is vital that the precursor and co-reactants react with the surface in a self-limiting way. The precursor molecules and co-reactants react neither with themselves nor with the surface groups that they create. In the purge and/or pump steps, the gaseous reaction products that may be generated during the surface reactions, as well as any excess precursor or co-reactant molecules, are removed from the ALD reactor. This is necessary to avoid reactions between precursor and co-reactant molecules directly in the gas phase or on the surface, as this could lead to an undesired chemical vapour deposition (CVD) component.
Figure 1. A schematic representation of the various steps in an ALD cycle consisting of two half-reactions. The exposures in the first half-cycle (precursor) and second half-cycle (co-reactant) are self-limiting such that the process stops when all available surface sites are occupied. The two half-cycles are separated by purge steps. The lower panels show the resulting coverage, or growth per cycle, as a function of exposure or time for that particular step. For sufficient exposure, saturated growth is obtained, while insufficient exposure results in incomplete saturation. For insufficient purging, a CVD component from mixing of the precursor and co-reactant is obtained.1
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