MoS₂ Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH₃NH₃PbI₃ Perovskite Solar Cell with an Efficiency of over 20%

Abstract

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related twodimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of hybrids between molybdenum disulfide (MoS2) and reduced graphene oxide (RGO) as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based PSC. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by liquid phase exfoliated MoS2 flakes, provide both holeextraction and electron-blocking properties. In fact, on the one hand, intrinsic n-type doping-induced intra-band gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS2 (from 1.4 eV for the flakes to > 3.2 for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of conduction band of MAPbI3 (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional (2D) nature of RGO effectively plug the pinholes of the MoS2 QDs films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS2 QD/graphene hybrids enable MAPbI3-based PSCs to achieve PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs. Figure 1. (a) Sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. (b) Scheme of the energy band edge positions of the materials used in the different components of the assembled mesoscopic MAPbI3-based PSC. The energy band edge positions of MoS2 flakes and MoS2 QDs were determined from OAS and UPS measurements detailed along the text, while those of the other materials were taken from literature: FTO, 52 TiO2, 52 MAPbI3, 134-139 spiro-OMeTAD 52 and Au 52 . (c) State-of-the-art and predicted PCE evolution for PSCs, highlighting the synergistic potential of GIE and the formulation of advanced perovskite chemistries. The RGO flakes are effective to plug the pinholes MoS2 QDs films, thus to homogenize the HTL. The choice of the functionalization for RGO relies on the bifunctional r...

Solution‐Processed GaSe Nanoflake‐Based Films for Photoelectrochemical Water Splitting and Photoelectrochemical‐Type Photodetectors

“…The estimated E g is 1.9 eV, as the one measured for GaSe crystals . Ultraviolet photoelectron spectroscopy (UPS) measurements were performed to determine the Fermi level energy (E F ), i.e., the WF, and the E VBM , of the GaSe flakes . Figure k shows that secondary electron cut‐off (threshold) energies of the He I (21.22 eV) UPS spectrum of GaSe flakes is ≈16.5 eV, corresponding to a WF of 4.7 eV.…”

“…The E g of the GaSe nanoflakes was determined using Kubelka‐Munk theory of R phenomenon, i.e., analyzing the ( F ( R ) hν ) n versus hν (Tauc plot) (inset to Figure i) using the Tauc relation ( F ( R ) hν ) n = Y ( hν − E g ), in which F ( R ) is the Kubelka‐Munk function (defined as F ( R ) = (1 − R ) 2 /2 R ), h is Planck's constant, ν is the photon's frequency, and Y is a proportionality constant . The value of the exponent denotes the nature of the electronic transition, discriminating between direct‐allowed transition ( n = 2) and indirect‐allowed transition ( n = 0.5) . Due to the pseudodirect gap behavior of GaSe, n was set equal to 2.…”

Beneficial impact of materials with reduced dimensionality on the stability of perovskite-based photovoltaics

“…Another interesting family of 2D materials suitable for charge transport applications are transition metal dichalcogenides assembled by a strong covalent bonding between metal and chalcogen (Se, Te or S) atoms. Notable examples of such materials are Mo and W disulfides [31,34], well-known for their high charge mobility and adjustable direct bandgap. Although the inorganic nature of 2D dichalcogenides has a positive impact on the stability of perovskite solar cells, the main hindrance preventing mass application of these materials is a lack of standardized, low-cost fabrication protocol able to produce high-quality layers without unwanted vacancies, dislocations and out-of-plane bonds.…”

Supramolecular Coordination of Pb2+ Defects in Hybrid Lead Halide Perovskite Films Using Truxene Derivatives as Lewis Base Interlayers

“…[9] Recently, in hybrid lead halide perovskite materials, many research groups have started to study the effect of molecules, as additives, to reduce the presence of surface defects . [12][13][14] Organic molecules with truxene structure (see Scheme 1, 1a) have been explored [15] in organic solar cells, [16][17][18] organic light emitting diodes, [19] and as a hole transport material in perovskite solar cells [20][21][22] due to their good optical and semiconductor properties. [12][13][14] Organic molecules with truxene structure (see Scheme 1, 1a) have been explored [15] in organic solar cells, [16][17][18] organic light emitting diodes, [19] and as a hole transport material in perovskite solar cells [20][21][22] due to their good optical and semiconductor properties.…”

“…[2,10,11] As an example, the recent work of Di Carlo's group shows that the use of graphene oxide leads to an increase in the solar to energy conversion efficiency due to the suppression of carrier recombination centres associated to surface defects. [12][13][14] Organic molecules with truxene structure (see Scheme 1, 1a) have been explored [15] in organic solar cells, [16][17][18] organic light emitting diodes, [19] and as a hole transport material in perovskite solar cells [20][21][22] due to their good optical and semiconductor properties. In addition, truxene molecules have good solubility in most common organic solvents, which improves film morphology.…”