Morphology engineering in perovskite planar solar cells

Cristiana Figus^a, Mauro Aresti^a, Francesco Quochi^a, Andrea Mura^a, Giovanni Bongiovanni^a, Valerio Sarritzu^a, Roberto Piras^a, Nicola Sestu^a, Michele Cadelano^a, Michele Saba^a, Daniela Marongiu^a, Feipeng Chen^a

^aDipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria S.P. Monserrato-Sestu km 0.7, Monserrato , I-09042

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Feipeng Chen, 408
Publication date: 5th February 2015

Organometal trihalide perovskite materials are among the most promising candidates as absorber materials for solution-processed photovoltaic (PV) applications. Even the simplest planar architecture appears successful in terms of efficiency and photovoltaic conversion. Here we present a study of the morphology of planar perovskite solar cells obtained with different perovskite fabrication routes. Particularly we correlate the morphology of perovskite layers to cell conversion efficiencies. We prepared simple planar solar cells with the following architecture: FTO/compact TiO₂/CH₃NH₃PbI₃/P3HT/MoO₃/Ag. To get a uniform perovskite film, different methods to prepare CH₃NH₃PbI₃film were attempted, including full solution spin-casting, two steps from solution and vapor assisted deposition in two steps. The AFM measurements show us that solution spin-casting method produces needle-shaped crystals, leading to a partial surface coverage and limited conductivity, not the best morphology for a planar solar cell. The two-step from solution method was based on spin casting a PbI₂ film first, then spin cast a CH₃NH₃I solution on top of the PbI₂ layer, creating the perovskite upon reaction of the two compounds. The result is a film with smaller grains and more uniform coverage. Finally, the vapor assisted method, where a PbI₂ filmis first obtained by spin-casting from solution, then evaporation of CH₃NH₃I occurs for several hours in N₂ atmosphere. The efficiency of the solar cell from the vapor assisted method can exceed 7%. Within the vapor assisted method we study how different PbI₂ solution temperature can affect the final perovskite film morphology. We used standard 70°C PbI₂ solution and different intermediate temperature until we reached a metastable room temperature transparent solution. With the room temperature PbI₂ deposition we can achieve a perovskite film with a RMS roughness lower than 25 nm with a very low pin-hole density that can improve the cell efficiency within the planar architecture.
Figure 1| The AFM measurements of the perovskite films. A): the needle like perovskite film obtained from full solution spin-casting. B): the perovskite film fabricated by two steps from solution. C): the uniform perovskite film prepared from vapor assisted method. D): the uniform perovskite film prepared from vapor assisted method with PbI2 film deposited at room temperature.

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