Effect of Molecular Packing and Charge Delocalization on the Nonradiative Recombination of Charge‐Transfer States in Organic Solar Cells

Abstract

nonradiative energy loss mechanisms is highly desirable. We note that nonradiative recombination processes can also occur, for instance, because of poor contacts at the electrodes and, in the case of nongeminate recombination, not only via singlet 1 CT states but also via triplet 3 CT states [30] (a topic of future studies in our group).Up to now, the theoretical investigations of the NR recombination process in organic solar cells have been conducted under the Born-Oppenheimer (BO) approximation. [31][32][33][34][35] Thus, the possible impact of nonadiabatic vibronic coupling due to the breakdown of the BO approximation (for instance, in particular, when the energy difference between the initial and final states is small) has been neglected. We note that the nonradiative recombination via nonadiabatic coupling (NAC) was initially investigated in inorganic semiconductors in the early 1950s; there, it was found to play an important role in reducing the number of photogenerated carriers, suppressing luminescence, and decreasing the carrier lifetimes. [36][37][38] In molecular systems, the NR transition between two excited states or between an excited state and the ground state (with the same spin multiplicity) due to nonadiabatic coupling, is referred to as internal conversion. [39] In the case of organic emitters, the theoretical studies of Shuai and co-workers have demonstrated that internal conversion significantly limits the fluorescence quantum yields. [40,41] Compared to the energies (usually in the range 2.0-4.0 eV) of the first excited states in organic emitters, the 1 CT 1 -state energies are generally much lower in organic solar cells, on the order of 0.5-1.7 eV. [22,23,29,42] Thus, the nonadiabatic coupling between the 1 CT 1 state and the ground state can be expected to be large and it becomes important to evaluate the role that nonadiabatic coupling can play in the NR recombination of 1 CT 1 states at donor-acceptor interfaces. We emphasize that the NR recombination rates of interfacial 1 CT 1 states are difficult to measure experimentally since the distribution of the 1 CT 1 states in transient experiments is far from equilibrium. [43,44] Here, we have chosen the pentacene-C 60 interface as a re presentative system to study the factors determining the NR recombination rates in the context of OPV applications, since a large number of data are available from earlier experimental and theoretical studies. [31,[45][46][47][48] As the recombination process is expected to depend on the local D-A interface geometry, we have considered both edge-on and face-on interfacial orientations of the pentacene molecules relative to C 60 . Also, we consider two different packing modes of the pentacene molecules: (i) a herringbone-type packing (referred to as [P:herringbone] hereafter) directly taken from the pentacene crystal structure; [49] and (ii) a co-facial-type packing (referred to as [P:co-facial] Organic photovoltaic (OPV) [1][2][3][4][5][6][7][8] devices have a great potential to become a low-cost technology fo...

Impact of interfacial molecular orientation on radiative recombination and charge generation efficiency

“…Using the same system but flipping the donor molecular orientation, the non-radiative recombination pathway has been altered, implying it is sensitive to the molecular alignment at the donor/acceptor interface. Interestingly, these experimental results are fully consistent with a recent theoretical study: Chen et al 3 found that the higher E CT value and greater hole delocalization and migration away from a face-on pentacene/C 60 interface caused a decrease in vibronic coupling of the CT state to the GS, thus reducing the non-radiative recombination rate. While the results by Chen et al 3 refer to model molecular packings of pentacene, the similarity to our findings on p-SIDT(FBTTh 2 ) 2 /C 60 , namely a smaller E CT and more non-radiative recombination in the edge-on bilayers, is striking.…”

“…Theoretical calculations have long suggested that the nature of the donor–acceptor interface will have a large effect on the rates of charge transfer and recombination 1–3 , as well as charge delocalization 3, 4 . Other calculations have found that molecular orientation affects interfacial quadrupoles and consequently the ease of charge separation 5, 6 .…”

Open‐Circuit Voltage in Organic Solar Cells: The Impacts of Donor Semicrystallinity and Coexistence of Multiple Interfacial Charge‐Transfer Bands

“…[25,51] Considering BHJ devices for which the interfacial area is substantially greater and where the development of structural order of the donor and acceptor phases can be easily inhibited, the coexistence of different phases poses an important challenge to maximizing the V OC . In the rubrene-C 60 system, the rubrene phase that is ordered enables the formation of charge-transfer states that are as much as 380 meV lower in energy than CT states formed by the disordered phase.…”

“…Likewise, previous theoretical work [9] on tetracene/C 60 bilayers show that face-on conformation and tetracene packing, through delocalization effects, lower the E CT . [39] Thus, it can be reasonably concluded that crystalline rubrene/ C 60 interfaces will form predominantly low energy CT states and that hole delocalization [51] within the aggregated rubrene phase will in fact account for most of the measured 0.38 eV difference in E CT between a-rubrene/C 60 and t-,o-rubrene/C 60 interfaces. In situ ultraviolet photoelectron spectroscopy (UPS) measurements performed at different stages of C 60 deposition on amorphous and crystalline phases of rubrene indicate there is little or no intermixing, therefore resulting in a nominally abrupt D/A interface in BL systems ( Figure S5, Supporting Information).…”

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

“…have been recently performed 14,16,18,25,34 to investigate the effect of system size on the CT states. However, the interplay between electron delocalization and polarization effects on the nature of CT states remains to be addressed in detail.…”