Boosting Photon Harvesting in Organic Solar Cells with Highly Oriented Molecular Crystals via Graphene–Organic Heterointerface

Abstract

Photon harvesting in organic solar cells is highly dependent on the anisotropic nature of the optoelectronic properties of photoactive materials. Here, we demonstrate an efficient approach to dramatically enhance photon harvesting in planar heterojunction solar cells by using a graphene-organic heterointerface. A large area, residue-free monolayer graphene is inserted at anode interface to serve as an atomically thin epitaxial template for growing highly orientated pentacene crystals with lying-down orientation. This anisotropic orientation enhances the overall optoelectronic properties, including light absorption, charge carrier lifetime, interfacial energetics, and especially the exciton diffusion length. Spectroscopic and crystallographic analysis reveal that the lying-down orientation persists until a thickness of 110 nm, which, along with increased exciton diffusion length up to nearly 100 nm, allows the device optimum thickness to be doubled to yield significantly enhanced light absorption within the photoactive layers. The resultant photovoltaic performance shows simultaneous increment in Voc, Jsc, and FF, and consequently a 5 times increment in the maximum power conversion efficiency than the equivalent devices without a graphene layer. The present findings indicate that controlling organic-graphene heterointerface could provide a design strategy of organic solar cell architecture for boosting photon harvesting.

Mixed-dimensional van der Waals heterostructures

“…Graphene has played a similar role for electrical contacts, wherein the highly delocalized graphene electronic structure leads to preferential faceon stacking for most π-conjugated organic small molecules ( Figure 3b). 48,49 Both graphene [50][51][52] and graphene-coated metals 53 have been used in pentacene FETs, each resulting in mobility increases ranging from 10-fold to 100-fold due to improved carrier injection. Solution-phase ordered assembly of small organic molecules on graphene has also been achieved 52,54,55 and has been successfully used for high-yield liquid-phase exfoliation of layered crystals.…”

Effect of donor–acceptor molecular orientation on charge photogeneration in organic solar cells

“…5c), and the face-on-rich film yielded L D = 11.1 nm, in agreement with the values reported in previous reports 30 . We hypothesized that this increase in L D in the face-on-rich film occurred because the face-on orientation featured a short π-π stacking distance that facilitated exciton diffusion in the vertical direction 5,15 . Fӧrster energy transfer, which describes singlet exciton diffusion in an organic semiconductor, defines L D as inversely proportional to the square of the distance between chromophores 31 .…”

“…Conjugated organic molecules oriented parallel to the substrate plane provide good charge transport in vertically structured OSC devices 8 . Controlling the molecular orientations can affect the dipole interactions and energy levels of the molecules and can therefore improve other properties, such as the light harvesting of the photoactive layer and the energetics of the device [13][14][15][16] . The donor and acceptor molecular orientations can also affect photocurrent generation [4][5][6][7][9][10][11] .…”

From flat to tilted: gradual interfaces in organic thin film growth

“…This growth continues usually for a few layers, but can persist, e.g, for pentacene adsorbed on thermally treated graphene, until a film thickness of 110 nm. 52 In general, after reaching a critical thickness in the film, a fast transition into the bulk structure occurs. This transition can induce a restructuring of the flat layer, as proposed for para-sexiphenyl (6P) on Ag(100).…”