The presence of chloride in mixed halide perovskites for high performing solar cell and its implications on the material growth

Aurora Rizzo^a, SIlvia Colella^a, Valentino Guerra^a, Giuseppe Gigli^a, Andrea Listorti^a, Giovanna Pellegrino^c, Alessandra Alberti^c, Filippo De Angelis^d, Edoardo Mosconi^d, Cinzia Giannini^f, Davide Altamura^f, Guglielmo Condorelli^g

^aNational Nanotechnology Laboratory of CNR-NANO, Via Arnesano, Lecce, 73100, Italy

^bCNR-IMM, Istituto per la Microelettronica e Microsistemi, Strada VIII n°5, 95121 Catania, Italy

^cComputational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM, Via Elce di Sotto 8, I-06123, Perugia, Italy

^dCenter for Biomolecular Nanotechnologies (CBN), Fondazione Istituto Italiano di Tecnologia (IIT), Lecce, IT, Via Barsanti 1, Arnesano, 73010, Lecce, Italy

^eIstituto di Cristallografia, CNR-IC, V. Amendola 122/O, 70126-Bari, Italy

^fUniversità degli studi di Catania and INSTM UdR Catania, Viale Andrea Doria 6, 95125, Catania, Italy

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

Oral, SIlvia Colella, presentation 220
Publication date: 5th February 2015

The field of solid-state solution processable photovoltaics has experienced in the recent years a deep transformation thanks to the exploitation of the emergent class of self-assembling lead-halide hybrid perovskites, which have pushed the efficiency up to 19.3%.[1] The leading players in this evolution, are methylamonium tri-iodide lead perovskite (MAPbI₃) and its mixed iodide-chloride analogue (MAPbI_3-xCl_x). The two materials possess the same basic crystalline structure and optical band-gap, however they significantly differ in terms of film morphology and as charge diffusion length and recombination. Despite the conspicuous number of works, fundamental questions remain regarding the reaction mechanism and the role of Cl in the growth. We have demonstrated, by a combined experimental and theoretical approach, the presence of chloride anions in mixed MAPbI_3-xCl_x perovskite films and their preferential concentration at the interface with titanium dioxide. [2, 3] Our achievement was accomplished by means of a high-sensitive and non-destructive angle resolved XPS techniqueperformed on extremely thin perovskite films deposited onto flat and smooth TiO₂. DFT calculations confirm a favorable chloride-TiO₂ interaction, and indicate a chloride-induced bending of the perovskite conduction band at the interface with titania, which could facilitate the electron extraction/collection efficiency of the solar cell. We propose that the preferential location of chloride on the TiO₂ surface, besides modifying the energetics of the interface, also influences the chemical properties of the substrate having a strong influence in the perovskite growth. In this frame, we exploit TiO₂ surface functionalization as specific way to discontinue the TiO₂-chloride interaction and modulate the crystallization process of MAPbI_3-xCl_x resulting in extremely different polycrystalline order of the films, from highly to randomly oriented crystals. [4] Notably, we find that the crystalline order of the film does not affect the photovoltaic performances of the perovskite-based devices, whose efficiency remains mostly unchanged, in average, from randomly oriented to highly oriented perovskite films. This work represents a step towards the control of the mixed halide perovskites crystallization process and the comprehension of the film peculiarities influencing the photovoltaic efficiency.

[1] Zhou, H. et al. Interface engineering of highly efficient perovskite solar cells. Science 2014, 45, 542. [2] Colella, S. et al. Elusive Presence of Chloride in Mixed Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 3532-3538. [3] Colella, S. et al. MAPbl(3-x)Clx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25, 4613-4618. [4] Guerra V.L.P. Et al. Implications of TiO2 Surface Functionalization on Perovskite Thin Film Crystal Growth and Photovoltaic Performances, submitted.

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