Interface Engineering of Layer‐by‐Layer Stacked Graphene Anodes for High‐Performance Organic Solar Cells

Abstract

The major efforts in solar energy research are currently directed at developing cost-effective systems for energy conversion and storage. [1][2][3] The high cost of materials and preparation methods that are required for the fabrication of inorganic solar cells prevent their widespread deployment. Seeking a low-cost alternative in the form of solution-processable or roll-to-roll printable organic solar cells features prominently in the energy research roadmap. The conventional anode of choice for organic solar cells has been indium tin oxide (ITO), which consumes as much as 30% of the fabrication cost in solar cells. High quality ITO is expensive due to the dwindling supplies of indium. ITO also suffers from drawbacks like brittleness, sensitivity to acids and bases during processing, and reactive interface formation with copper indium sulfi de during high-temperature sintering. Graphene fi lms have been proposed as the new generation of multifunctional, transparent, and conducting electrodes. The attractiveness of graphene arises from their low cost, transparency, high electrical conductivity, chemical robustness, and fl exibility, as opposed to the rising cost and brittleness of ITO. [4][5][6] In particular, the 2D graphene sheet is emerging as a possible substitute for ITO in fl exible displays, touch screens, and solar cells. [7][8][9] The sheet resistance of graphene is given by Rs = (F 2D N) −1 where F 2D is the 2D sheet conductivity and N is the number of layers. The intrinsic sheet resistance of single layer graphene is calculated to be ≈ 6 k Ω and is inferior to that of ITO (10-20 Ω sq − 1 ). [ 10 ] In principle, increasing the thickness (increasing N ) of graphene using layer-by-layer stacking and doping the graphene (increasing F 2D by increasing the carrier concentration) can allow the extrinsic sheet resistance values to be reduced to as low as 20 Ω sq − 1 although it is not trivial to reach this limit at present. [ 10 ] Many are excited about the transparency and conductivity of multilayered graphene fi lms, however the ultimate performance of graphene in solar cells may be limited by other factors. First, the interfacial energy offset between graphene and the photoactive materials has to be tuned in order to optimize charge transfer. Second, the planarity and hydrophilic character of multilayer graphene has to be improved to allow for spin-coating with hole-transporting layers such as poly(3,4-ethylenedioythiophene):poly (styrenesulfonate) (PEDOT:PSS). Third, the preparation of the graphene fi lms has to be improved in terms of achieving large grain crystal growth, as well as reducing contaminations from organic residues in the transfer process. The organic residues have a deleterious effect on the conductivity, transparency, and roughness of graphene fi lms. To date, there have been no reports that graphene can exhibit a power conversion effi ciency (PCE) higher than ITO when replacing the latter in solar cell devices. [ 11 ] This underscores the diffi culty in optimizing any factors in the fabric...

Giant enhancement in vertical conductivity of stacked CVD graphene sheets by self-assembled molecular layers

“…CVD graphene has been deployed as diffusion barriers in electrode modifiers, transparent conductors in touch screen panels, organic light-emitting diodes and solar cells [1][2][3][4][5][6][7] . However, the conductivity of pristine graphene is limited by the amount of carriers, and further modification such as chemical or electrostatic doping is needed to improve the conductivity.…”

“…However, the conductivity of pristine graphene is limited by the amount of carriers, and further modification such as chemical or electrostatic doping is needed to improve the conductivity. One way to improve the conductivity is to stack single-layer CVD graphene in a layer-by-layer (LBL) manner to form multilayers 2,4,5,7 . Although the overall conductivity (inplane or out-of-plane) improves with layer thickness, weak electronic coupling between the layers prevents a linear scaling of the conductivity with thickness of the stacked layers.…”

Synthesis and applications of graphene electrodes

“…It functions as an alternate anode material that eliminates the limitations of the existing material. For example, graphene can replace ITO in organic solar cells or fluorine tin oxide in dye-sensitized solar cells [68,69]. Graphene provides high transparency, a smoother contact surface and high thermal stability.…”

Graphene-based hybrid materials and their applications in energy storage and conversion

“…and fluorine tin oxide in optoelectronic devices owing to its high electron mobility, high thermal stability, and flexibility [4,5]. Graphene films are fabricated because of the solution processable operation of (functionalized) graphene sheets, such as the spinning coating methods [55,56].…”