Intermixing at the Pentacene‐Fullerene Bilayer Interface: A Molecular Dynamics Study

Abstract

Organic photovoltaics (OPVs) hold promise as a technology for low-cost, large-area power conversion. [1][2][3][4] Though conceptually straightforward, the processes required for effi cient operation of an organic solar cell -photon absorption and exciton (electron-hole pair) formation, dissociation of the exciton into separated charges, and collection of the charges -are inherently complex due to the characteristics of the π -conjugated organic materials, i.e., weak van der Waals intermolecular interactions, low dielectric constants, strong electron-electron interactions, and large electron-vibration couplings. The effective separation of the electron-hole pair requires that the active layer consist of two components, an electron-rich (hole-transport) donor material and an electron-defi cient (electron-transport) acceptor material. [ 5 ] This condition leads to a key bottleneck, both in terms of operation and basic understanding, as there exists a delicate interplay between charge-separation and (geminate and non-geminate) charge-recombination processes at the interface between the donor and acceptor materials.While the importance of the donor-acceptor interface has long been recognized, recent experimental evidence has shone new light on the morphological complexity; this is particularly the case in polymer-fullerene bulk-heterojunction (BHJ) solar cells, [ 6 , 7 ] and more recently in molecule-molecule and polymermolecule bilayers. [ 8 , 9 ] In addition to the possible miscibility of the two components, the deposition protocol (e.g., chemical vapor deposition, solution casting, or spin coating to name a few [ 2 , 10 ] ) and post-processing procedures (e.g., removal of solvent additives or solvent and thermal annealing [ 11 , 12 ] ) can impact the interface morphology. This morphological variability leads to diffi culties, then, in recognizing the underlying physical processes at the interface as the relevant electronic states and electrostatic interactions are highly dependent on the molecular packing confi gurations between the donor and acceptor molecules or chain segments. [ 3 , 13-16 , 17 ] Hence, to achieve optimal OPV performance, one must be able to understand and ultimately control the morphology of these heterojunctions. [ 12 ] OPVs based solely on small molecule donors and acceptors have recently shown considerable improvement, [ 4 , 18 ] with power conversion effi ciencies for (solution-processed) singlejunction devices reaching 6.7% [ 19 ] and (vacuum-deposited) tandem devices surpassing 10.7%. [ 20 ] Small-molecule active layers employing pentacene as the electron donor and C 60 as the electron acceptor have served as prototype systems to detail a number of key features of OPVs. [ 21 , 22 ] Due to the relatively small size and rigid structural characteristics of these molecules, the pentacene-C 60 interface has also undergone a number of theoretical investigations, ranging from electronic-structure calculations on molecular complexes to atomistic molecular dynamics (MD) simulations. [ 13-15...

Charge-Transfer States in Organic Solar Cells: Understanding the Impact of Polarization, Delocalization, and Disorder

“…35 The "edge-on" morphology of the C 60 /pentacene bilayer was generated via deposition of the C 60 molecules on top of the crystalline pentacene (001) surface. For the electronic-structure calculations, we extracted 63 clusters consisting of one C 60 and one pentacene molecule (these clusters are hereafter denoted as C 60 /P), 21 clusters consisting of one C 60 and three pentacene molecules (C 60 /3P), 10 clusters of 2C 60 /6P molecules, and 6 clusters of 3C 60 /7P molecules (see the structures of these clusters in Figures S1-S3 in the Supporting Information (SI)).…”

Polarization Energies at Organic–Organic Interfaces: Impact on the Charge Separation Barrier at Donor–Acceptor Interfaces in Organic Solar Cells

“…What was once thought to be fairly clear-cut interfaces between the donor and acceptor components, has been replaced by a complex morphological picture that includes pure domains with different extents of ordered and disordered packing as well as intermixed regions of the two materials where charge generation primarily occurs. [16][17][18][19][20][21][22][23][24][25][26][27][28] As we discussed recently, 29 the energetic landscape at a surface (i.e., organic-vacuum interface) differs significantly from the bulk of a crystalline material. One would expect, furthermore, the addition of a second organic component to further complicate the landscape.…”

Molecular dynamics simulations of graphoepitaxy of organic semiconductors, sexithiophene, and pentacene: Molecular-scale mechanisms of organic graphoepitaxy

“…There is a report that focused on the adsorption and packing of 6T molecules on Ag(111) surfaces by MD simulations, 18) but the simulations were largely different from the present case where many 6T molecules are standing on inert surfaces and form a bulklike crystal structure. In the case of pentacene, many papers on MD simulations have been published, [12][13][14][15] but the force field model used was caseby-case; the MM3 potential was used in Refs. 12 and 15, the Buckingham potential model with a Coulombic contribution for the interaction between rigid pentacene molecules (and DREIDING parameters for the interactions between pentacene molecules and silica substrates) was used in Ref.…”

“…MD simulations have already been applied to organic semiconductors, mainly to the research on pentacene (C 22 H 14 ). [12][13][14][15] However, they have not been applied to the study of "graphoepitaxy" of organic semiconductors, to the best of the author's knowledge. First, in this study, MD simulations of orientation adjustment at the groove edge were carried out.…”