Structure and Disorder in Squaraine–C₆₀ Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor–Acceptor Interface

Abstract

1 www.MaterialsViews.com wileyonlinelibrary.com design, processing procedures, and new device architectures. [9][10][11][12][13][14][15][16][17][18][19][20] Although these results are encouraging, there remains limited comprehensive understanding of the factors impacting the underlying physical processes that take place in OPV, and in particular how the myriad morphologies available in the active layer infl uence the fi nal PCEs.Large absorption coeffi cients, especially in the red and near-infrared spectral regions, and the ability to tune the solidstate molecular packing have led to recent interest in the use of squaraine dyes as the primary absorbing and hole-transport (electron-donor) material in organic solar cells. [ 18,[21][22][23][24][25][26][27] For instance, Thompson, Forrest, and co-workers have exploited 2,4-bis[4-( N,N -diphenylamino)-2,6-dihydroxyphenyl] squaraine, DPSQ, see Figure 1 , as it has good absorption in the red portion of the visible spectrum (>650 nm), while thermal and solvent-vapor annealing (SVA) techniques can be used to manipulate the relative crystallinity/order of DPSQ-C 60 active layers. [ 21,23,28 ] In particular, fi ne control of the bulk and interfacial morphologies of DPSQ-C 60 bilayers lead to a 30% increase in PCE (3.4% to 4.8%). [ 23 ] The results of these studies, in conjunction with the theoretical underpinnings discussed by Giebink and co-workers, [ 29,30 ] suggest that an optimized bilayer OPV should maintain considerable order within the bulk to minimize cell resistance and improve exciton diffusion effi ciency while the heterojunction with the acceptor should be rough/disordered to limit the intermolecular electronic coupling (i.e., molecular orbital overlap) between the donor and acceptor to reduce geminate charge (polaron-pair) recombination. [ 23 ] In addition, Würthner and co-workers reported on the aggregation-dependent photovoltaic properties of squaraine-PC 61 BM (phenyl-C 61 -butyric acid methyl ester) bulk heterojunctions with both H-and J-aggregates in the mixture; [ 31 ] the formation of J-aggregates, which red-shift the absorption maximum, results in increased device effi ciency.Since morphological variability is seen to impact significantly the electronic processes within the bulk and interfacial regions of the active layer, [32][33][34][35][36][37][38][39][40] it is important to develop insight into the materials behavior at the molecular level. However, it remains very diffi cult to obtain precise, molecular-scale experimental details concerning the nature of the molecular Structure and Disorder in Squaraine-C 60 Organic Solar Cells: A Theoretical Description of Molecular Packing and Electronic Coupling at the Donor-Acceptor Interface Yao-Tsung Fu , Demetrio A. da Silva Filho , Gjergji Sini , Abdullah M. Asiri , Saadullah Gary Aziz , Chad Risko , * and Jean-Luc Brédas * Organic solar cells based on the combination of squaraine dyes (as electron donors) and fullerenes (as electron acceptors) have recently garnered much attention. Here, molecular dyn...

Symmetry-Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage Organic Photovoltaics

“…25 It has been shown in polymer:fullerene BHJs and bilayer small-molecule/fullerene devices that increased crystallinity 15 of the active layer correlates with a shift in E CT to lower energies, in agreement with our findings for C 60 and ZCl. 46 Additionally, it has been suggested that steric effects at the D/A interface can increase the energy of the CT state by increasing separation between the donor and acceptor, resulting in a higher V OC . 40 Differing molecular orientation will similarly lead to changes in the electronic coupling between the donor and acceptor as different conformations may exist at the donor/acceptor interface.…”

Molecular-Scale Understanding of Cohesion and Fracture in P3HT:Fullerene Blends

“…Although there is increasing interest in the mechanical properties of BHJ active layers, [1][2][3][4][5][6][7][8][9] and such studies are common for traditional polymers, [10][11] the efforts in the OPV arena remain limited when compared to performance efficiency studies. [12][13][14][15][16] Along with the electronic processes involved in photocurrent generation, the active-layer mechanical properties inherently depend on the multiphase morphology of the BHJ thin films [17][18][19][20][21][22] and therefore are contingent on the thin-film processing protocols and the nature and strength of non-covalent, solid-state intermolecular interactions. However, a molecular-scale understanding of these properties, which is key to the design of more mechanically and electronically robust active layers, remains elusive.…”

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“…Their simulations indicate that even at 300K, the thermal motions of the molecules at the interface can be quite profound and the degree of disorder inherent around the interface can greatly affect the formation of electron/hole pairs. 28 Finally, we recently presented a fully quantum dynamical model of photo-induced charge-fission at a polymeric type-II heterojunction interface. 29 Our model supposes that the energy level fluctuations due to bulk atomic motions create resonant conditions for coherent separation of electrons and holes via longrange tunnelling.…”