Singlet fission, a process that converts a singlet exciton into two triplet excitons, has the potential to achieve a high-efficiency solar cell that exceeds the Shockley-Queisser limit ^[1] . Singlet fission has been previously employed to enhance the photovoltaic efficiency of organic nanostructured solar cells ^[2] , photodetectors ^[3] , and fission-sensitized quantum dot solar cells ^[4] . To obtain optimum efficiency from singlet fission, we need to understand the molecular factors that control its rate. Since singlet fission involves two neighboring chromophores, their intermolecular coupling is expected to play an essential role in determining the single fission rate. Here, we investigate how intermolecular interactions control singlet fission in pentacene, an archetypal molecule exhibiting singlet fission.

In this project, we examine the rate of singlet fission while modulating the intermolecular coupling by altering the side group of pentacene derivatives. We study unsubstituted pentacene and four other pentacene derivatives in thin-film states, 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), 6,13-diphenylpentacene (DP-pentacene), 6,13-di-biphenyl-4-yl-pentacene (DB-pentacene), and 6,13-di(2’-thienyl)pentacene (thienyl pentacene); see Figure 1 for their crystal structures. We characterize the intermolecular coupling by monitoring the ref shift and peak broadening in their absorption spectra when the molecules in solutions become solid-state thin films. Then, we compared the intermolecular coupling with the rate of singlet fission measured using femtosecond photoinduced absorption spectroscopy; see Figure 2. We also perform density-functional calculations to estimate the coupling between a singlet and two neighboring triplets. We expect our study to contribute to better understanding of the mechanism of singlet fission and rational designs of singlet-fission-based photovoltaic devices.

1. M. B. Smith and J. Michl, “Singlet fission,” Chemical Reviews, vol. 110, pp. 6891-6936, Nov. 2010.
2. P. J. Jadhav, A. Mohanty, J. Sussman, J. Lee, and M. A. Baldo, “Singlet exciton fission in nanostructured organic solar cells,” Nano Letters, vol. 11, pp. 1495-1498, Feb. 2011.
3. J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Applied Physics Letters, vol. 95, p. 033301, 2009.
4. B. Ehrler, M. W. B. Wilson, A. Rao, R. H. Friend, and N. C. Greenham, “Singlet exciton fission-sensitized infrared quantum dot solar cells,” Nano Letters, vol. 12, pp. 1053-1057, Jan. 2012.

Organic solar cells and photodetectors that feature singlet exciton fission materials have two additional exciton processes that traditional organic solar cells do not: singlet fission and triplet doublet annihilation. To maximize the usable power of the photovoltaic cell, we must understand how to optimize the gain from singlet fission and minimize the loss from doublet annihilation. We focus on the former here.

An organic photodetector is composed of thin layers of pentacene and PTCBI stacked repeatedly, as in Figure 1. This device structure is designed to enhance exciton dissociation at the donor/acceptor interface. The rapid dissociation of the singlet exciton in the photodetector competes with the singlet fission process, which is the formation of two triplet excitons from one singlet, and has been shown to be very fast and efficient ^{[1] [2]} . Modulation of the singlet fission rate by application of an external magnetic field changes the photocurrent by reducing the singlet fission rate relative to the rate of singlet dissociation into a charge.  The high field asymptotic value of the change in photocurrent is proportional to the fission yield ^[2] .

We built photodetectors that utilize variably thick layers of pentacene to modulate the rate competing with singlet fission. The maximum the internal quantum efficiency (IQE) of 130% occurs for an 8-nm-thick pentacene layer. The trend in IQE is matched by the trend of an increasing change in photocurrent with applied magnetic field, as Figure 2 shows. We conclude that there is less competition between the singlet for performing fission and dissociating at the donor/acceptor interface.

The gain in IQE from the singlet fission process is largest for pentacene layers of 8 nm. The change in photocurrent suggests that the fission efficiency is even larger for thicker layers; however, we observe loss in IQE, which could be due to exciton diffusion.

1. M. W. B. Wilson, A. Rao, J. Clark, R. S. S. Kumar, D. Brida, G. Cerullo, and R. H. Friend, “Ultrafast dynamics of exciton fission in polycrystalline pentacene,” Journal of the American Chemical Society, vol. 133, no. 31, pp. 11830–11833, 2011
2. J. Lee, P. Jadhav, and M. A. Baldo, “High efficiency organic multilayer photodetectors based on singlet exciton fission,” Applied Physics Letters, vol. 95, p. 033301, 2009.