Ultrafast Charge Generation in an Organic Bilayer Film

Abstract

The dynamics of charge generation in a high performing molecular photovoltaic system, p-SIDT(FBTTh2)2 (see Figure 1 ) is studied with transient absorption. The optimized bulk heterojunction material shows behavior observed in many other systems; the majority of charges are generated at short time scales (<150 fs), and a slower contribution from incoherently diffusing excitons is observed at low pump fluence. In a separate experiment, the role of bulk heterojunction material morphology on the process of ultrafast charge generation was investigated with bilayers made with solution processed donor molecules on a photopolymerized C60 layer. The majority of carriers are again produced at short time scales, ruling out the idea that subpicosecond charge generation can be understood wholly in terms of localized excitons. We evaluate possible causes of this behavior and propose that the excited state is highly delocalized on short time scales, providing ample probability density at the charge generating interface.

Exciton delocalization incorporated drift-diffusion model for bulk-heterojunction organic solar cells

“…The ratio of 70%-80% is demonstrated to be an appropriate approximation for high-performance organic materials. 6,9 The current-voltage curves of numerical and experimental results are shown in Figure 3, which confirms the validity of our model.…”

“…Different from the exciton diffusiondissociation process, the exciton delocalization in an ultrafast timescale has been observed recently. [2][3][4][5][6][7][8][9] The delocalized excitons have a great influence on the charge separation and device performance of BHJ organic solar cells.…”

“…14 However, in contrast to the traditional view of charge separation from the exciton diffusion and dissociation, the existence of exciton delocalization has been confirmed by several groups recently. 2,3,[5][6][7][8][9][10][11] Through the quantum mechanics/ molecular simulation and experiments, researchers found an ultrafast charge transfer (generation) process at the donoracceptor interfaces in a very short timescale (<100 fs) after the light absorption, as depicted in Figure 1(a). As described in step I in Figure 1(a), the excited exciton states have a large spatial delocalization, which means a large exciton radius.…”

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.…”

Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

“…Although the degree of CT character in excited states has been reported as described in the earlier paragraph, its role in excitation dy- * Electronic address: t.fujita@kuchem.kyoto-u.ac.jp † Electronic address: aspuru@chemistry.harvard.edu namics has remained unclear. Furthermore, there has been growing interest in the effects of excited-state delocalizations on charge photogeneration processes [35,[50][51][52][53][54][55]. Therefore, we studied the excited-state dynamics in DNTT as a model system to provide insight into the role of the CT states in OSCs.…”