Sequential deposition as a route to high performance perovskite sensitized solar cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Poster, Julian Burschka, 372
Publication date: 1st March 2014

Mesoscopic sensitized solar cells are one of the most promising third-generation photovoltaic technologies. Dye-sensitized solar cells (DSCs) were the first photovoltaic devices to utilize a mesoscopic heterojunction for the conversion of solar irradiation into electrical power. Solid state dye-sensitized solar cells (ssDSCs), that employ an organic hole-transporting material (HTM) in place of a liquid redox electrolyte, have evolved as viable contenders to conventional liquid DSCs. A typical ssDSC is composed of a mesostructured wide-bandgap metal oxide semiconductor—such as titanium dioxide—that is sensitized with a lightabsorbing chromophor and infiltrated with the HTM, usually by solution processing.

Organic-inorganic hybrid perovskites, such as CH₃NH₃PbX₃ (X = Cl, Br, I) have recently attracted great attention as light harvesters for mesoscopic solar cells and have been porposed to replace the molecular sensitizer in a typical ssDSC configuration. So far, the perovskite pigment has been deposited onto mesoporous metal oxide films from a solution of PbI₂ and CH₃NH₃I, using a single step process. However, the uncontrolled precipitation of the perovskite pigment during the solution processing produces large morphological variations, resulting in a wide spread of photovoltaic performance and hampering the prospect of such systems for practical applications. Recently, we reported on the use of a sequential deposition method (Figure 1) to realize the perovskite nanomorphology [1, 2]. PbI₂ is first applied onto a mesoporous film of TiO₂ by spin-coating. The TiO₂/PbI₂ nanocomposit is subsequently exposed to a solution of CH₃NH₃I in 2-propanol. The desired perovskite absorber forms as soon as the two precursors are brought into contact. Within the mesoporous TiO₂ scaffold the size of the PbI₂ crystals is constrained by the pore size of the host. An important finding of our work is that confining the PbI₂ crystals to such a small size drastically enhances their rate of conversion to perovskite: the reaction is complete within a few seconds of their coming into contact with the CH₃NH₃I solution.

Employing this technique for the fabrication of solid-state mesoscopic solar cells greatly improves the reproducibility of their performance and allows us to achieve a power conversion efficiency of up to 15% percent, measured under standard solar test conditions (AM 1.5 G, 100 mW cm^-2). Our findings open up new opportunities to realize solution-processed photovoltaics with unprecedented power conversion efficiencies and high stability matching or even exceeding those of today’s best thin film photovoltaic devices.

Figure 1. Scheme that illustrates the formation of the organic-inorganic hybrid perovskite using a sequential deposition method. Lead(II)iodide is first applied onto a mesoporous film of titanium dioxide by spin-coating a solution of lead(II)iodide in N,N-dimethylformamide (DMF) (Step 1). The resulting nanocomposit is subsequently exposed to a solution of methylammonium iodide in 2-propanol (Step 2), which leads to the formation of the desired hybrid perovskite.
[1] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Nature 2013, 499, 316–319. [2]Burschka, J.; Thesis N° 6006, Ecole Polytechnique Fédérale de Lausanne, 2013, DOI:10.5075/epfl-thesis-6006.

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