2D material engineering of perovskite solar cells: the emergence of MXenes
Antonio Agresti a, Sara Pescetelli a, Hanna Pazniak b, Danina Saranin b, Daniele Rossi a, Matthias Auf der Maur a, Alessia Di Vito a, Alessandro Pecchia c, Andrea Liedl d, Rosanna Larciprete d, Aldo Di Carlo a
a CHOSE - Centre for Hybrid and Organic Solar Energy, Department of Electronic Engineering, University of Rome Tor Vergata, Via del Politecnico, 1, Roma, Italy
b LASE–Laboratory for Advanced Solar Energy, National University of Science and Technology MISiS, Leninsky Avenue, 6, Moskva, Russian Federation
c Consiglio Nazionale delle Ricerche-CNR, ISMN, Rome, Italy.
d INFN-LNF - Frascati (Rome) Italy.
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Antonio Agresti, presentation 120
DOI: https://doi.org/10.29363/nanoge.hopv.2019.120
Publication date: 11th February 2019

In the context of renewable and low cost energy production, new generation photovoltaics aims to ensure high power conversion efficiency (PCE) accomplished by low cost manufacturing processes. Perovskite solar cells (PSCs) are one of the most impressive attempt in achieving such ambitious goals. In this scenario, several device architectures and materials modifications have been proposed to tune the device interface properties and the perovskite crystal morphology. Indeed, graphene, related materials and transition metal dichalcogenide such as molybdenum disulphide (MoS2) have been successful applied in PSCs and modules [1] as interface engineering by improving the device performance [2] and by enlarging the lifetime.[3,4] Recently MXenes (MX) with the general formula Mn+1XnTx, where M represents an early  transition metal, X is carbon and/or nitrogen,  and Tx stands for surface terminations (such as OH, O, and F) came out as a new class of bidimensional (2D) materials with outstanding properties. MXenes exhibit high electronic conductivity, hydrophilic surface, high surface energy, and remarkable tunability in term of work function, ranging from 1.6eV till 6.5eV.[5] In this work, we demonstrate the use of Ti3C2Tx MX for perovskite photovoltaics by fine tuning the interface optoelectronic properties in engineered mesoscopic device. In particular, we modified the photo-electrode properties by adding MX within the precursor solutions. On one hand, transient measurements revealed the addition of MX within perovskite active layer reduces the charge trapping efficiency of deep trap states by reducing the charge accumulation and eventually the hysteresis in the current-voltage (I-V) curve. On the other hand, UPS measurements showed a shift in perovskite workfunction testifying the role of MX in modifying the perovskite electronic structure. Moreover, when ETL is modified with MX, the improvement in device PCE stems mainly from VOC and FF increase. Indeed, a reduction of charge recombination rate is demonstrated at ETL+MX/perovskite+MX interface. As further confirmation, charge carrier lifetime showed an enlarged value when MX-based ETL is used in the cell. Notably, the proposed MX-perovskite cells showed superior PCE overcoming 20% with outstanding reduction of hysteresis phenomenon. Detailed DFT calculations and device simulations permitted to define the role of MX. Due to the chemical versatility of MX, this work opens the way for a further development of perovskite technology by exploiting the possibility to optimize device structures, layers and interfaces with proper material tuning.

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