Energetics of Electron–Hole Separation at P3HT/PCBM Heterojunctions

Abstract

The energetics of electron−hole separation at the prototypical donor−acceptor interface P3HT/PCBM is investigated by means of a combination of molecular dynamics simulations, quantum-chemical methods, and classical microelectrostatic calculations. After validation against semiempirical Valence Bond/Hartree−Fock results, microelectrostatic calculations on a large number of electron−hole (e-h) pairs allowed a statistical study of charge separation energetics in realistic morphologies. Results show that charge separation is an energetically favorable process for about 50% of interfacial e-h pairs, which provides a rationale for the high internal quantum efficiencies reported for P3HT/PCBM heterojunctions. Three effects contribute to overcome the Coulomb attraction between electron and hole: (i) favorable electrostatic landscape across the interface, (ii) electronic polarization, and (iii) interface-induced torsional disorder in P3HT chains. Moreover, the energetic disorder due to the PCBM polar group is shown to play a key role in increasing the dissociation probability.

Modulating the Exciton Dissociation Rate by up to More than Two Orders of Magnitude by Controlling the Alignment of LUMO + 1 in Organic Photovoltaics

“…[1][2][3][4][5][6][7][8] With the rapidly growing interests in OPVs in both scientific and industrial communities, theoretical modeling of the electronic structure of excitons and their accompanying dynamics has been widely implemented and now plays an important role in interpreting the elementary processes or designing new materials. [5][6][7][8][9][10][11][12][13][14][15][16][17] Due to the large size of the system, the theoretical investigations are mainly based on simplified models with the common assumption that only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of each molecule participate in the exciton dynamics within the donor (D)/acceptor (A) heterojunction, i.e. one initially photo-excited Frenkel exciton (FE) can convert to a charge transfer (CT) excited state where the hole occupies the HOMO of the donor (or a linear combination of HOMOs of many donors) and the electron occupies the LUMO of the acceptor (or a linear combination of LUMOs in case of many acceptors).…”

From Chiral Islands to Smectic Layers: A Computational Journey Across Sexithiophene Morphologies on C60

“…If this process can successfully compete with charge recombination, the separated charges can travel towards their respective electrodes performing a necessary step towards achieving conversion of solar light in electricity. The evaluation of the different competing electronic processes taking place at the DA interface is far from obvious [3][4][5][6] and its results can even be counterintuitive [7][8][9] because they strongly depend on the detailed knowledge of the donor and acceptor molecular organization, [9][10][11][12] as can now be achieved on C 60 . [ 27 ] We have chosen as donor α-sexithiophene (T6) [ 28 ] and as acceptor fullerene (C 60 ) that have the advantage of being relatively simple but also well studied experimentally.…”

Dynamics of charge separation at an organic donor-acceptor interface

“…Dielectric constant, molecular packing structure, and molecular multipole moments will vary near the interface. It has been argued that this results in a strong effective screening and would therefore facilitate charge separation at the D/A interface [8][9][10][11][12][13]. A third explanation which has * zhesu@ifm.liu.se † svens@ifm.liu.se been discussed in the context of charge separation is related to the character of the electronic states involved.…”

“…It should be noted that we have not performed investigations on the relevance of the values of the screening. The standard Ohno value of β = 3.4 is used for bulk materials and since interface charges have been shown to increase screening [8][9][10][11][12][13] it is likely that the value of β is larger than the value used by Miranda [20]. Furthermore, depending on the structure at the interface, there is most likely a varying strength of the screening which might result in different tendencies for charge separation at different positions along the interface.…”