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Gearing towards cheaper perovskite solar cells: A breakthrough in solution technique

Organo metallic perovskite (MAPbX3) materials are an emerging class of light absorbers with tremendous potential in harvesting the solar power comparable to currently used silicon solar cells (20%). Organo metallic perovskites are a blend of inorganic (Pb and X=halogens = F, Cl, Br or I) and organic (MA=CH3NH3) components that form a layered structure as graphite. A thin layer (300 nm) of perovskites on a partially conducting substrate (titanium dioxide, TiO2) can collect most of the light falling on it, but the quality – uniformity, crystallinity – of the film dictates how much of absorbed light will be converted to electricity. Currently perovskite films prepared by cheap solution processes have lower film quality – i.e. lower power conversion efficiency, 5% (1), – and are not suitable for commercialization. In this paper, Nam Joong Jeon and coworkers demonstrate a five-step solution engineering to prepare organo metallic perovskite films without penalizing their film quality (2). Their technique produced perovskite films with an efficiency of 16.5 % comparable to best quality films prepared by expensive techniques (3). The authors proposed a mechanism for the formation of high quality film and suggest choosing appropriate thicknesses for perovskite and TiO2 substrate films to further improve the efficiency.

The authors aimed for perovskite films with a composition, CH3NH3Pb(I(1-x)Brx)3, where x= 0.1-0.15, due to its better stability. The five steps followed are as below:
1. A mixture of starting compounds MAI, MABr, PbI2, PbBr2 and two solvents, y-buterolactone (GBL) and N,N – Dimethyl formamide (DMSO) in the appropriate ratio is spread over the entire substrate (TiO2).
2. The substrate loaded with the mixture is placed on a spin-coater, which spins the substrate – for 10 sec (at 1000 rpm) and 20 sec (5000 rpm) – to spread the constituents uniformly and to evaporate the solvents simultaneously.
3. A third solvent, toluene, that is miscible with other two solvents but immiscible with starting compounds, is dripped on to the substrate while spinning.
4. Any residual DMSO is removed by spinning while all other constituents are frozen in a uniform layer to form a intermediate complex
5. Substrate was heated to 100oC for 10 minutes to convert the intermediate complex to final composition, CH3NH3Pb(I(1-x)Brx)3.
The resulting perovskite film is crystalline, extremely uniform and transparent with low roughness – minimal variation in grain sizes.

The authors proposed a mechanism that led to high quality films, which they tested on a simple perovskite compound, MAPbI3. The choice of appropriate solvents and toluene dripping does the magic. During the second step, MAI and PbI2 in the mixture do not react to form perovskite compound as they would, if DMSO was absent – no color change of the film but only the solvent GBL evaporated. On adding toluene, DMSO again prevents reaction between MAI and PbI2 by forming a complex MAI-PbI2-DMSO; DMSO molecules and MAI insert themselves between the layers of PbI2; the excess DMSO evaporates. The authors confirmed the complex with characteristic new peaks observed in X-ray diffraction (XRD) experiment; insertion of MAI and DMSO between the layers of PbI2 increases the interlayer distance in PbI2 reflected in XRD; infrared spectroscopy helped identify the presence of DMSO and MAI in the complex. The complex molecules distributed uniformly over the substrate surface are they key. Upon heating the substrate to 100oC to evaporate DMSO, MAI and PbI2 in the complex react to form perovskite crystals of uniform grain size.

The authors report 16.5% as highest efficiency of solar cells fabricated with solution engineered perovskite films/TiO2. However, they found discrepancies in the measured efficiencies upon varying the thickness of the TiO2 substrate film. The electrical charges, created in the perovskite film by sunlight, move through the film slowly to reach the TiO2 substrate, after which they are collected to power up a device. This slow movement caused the discrepancies in measured efficiencies. Therefore, thinner perovskite layers with larger perovskite/TiO2 interface are required for efficient charge collection. While a moderate thickness of perovskite films is required for efficient light absorption, the ratio of perovskite and substrate thicknesses must be optimized to make the best device.

The five-step solvent engineering method and a appropriate ratio of perovskite film to perovskite-TiO2 interface area should lead to cost effective solar cells. But this ratio will depend on the composition of the perovskite film. These strategies will help perovskite solar cells to become better competitors to crystalline silicon solar cells.

1. Ball, J. M., Lee, M. M., Hey, A. & Snaith, H. J., Energy Environ. Sci., 6, (2013) 1739
2. Nam Joong Jeon et al., Nature Materials 13 (2014) 897
3. Burschka, J. et al., Nature 499, (2013) 316