Harvesting Singlet Fission for Solar Energy Conversion: One- versus Two-Electron Transfer from the Quantum Mechanical Superposition

Singlet fission, the creation of two triplet excitons from one singlet exciton, is being explored to increase the efficiency of solar cells and photo detectors based on organic semiconductors, such as pentacene and tetracene. A key question is how to extract multiple electron–hole pairs from multiple excitons. Recent experiments in our laboratory on the pentacene/C₆₀ system (Chan, W.-L.; et al. <i>Science</i> <b>2011</b>, <i>334</i>, 1543–1547) provided preliminary evidence for the extraction of two electrons from the multiexciton (ME) state resulting from singlet fission. The efficiency of multielectron transfer is expected to depend critically on other dynamic processes available to the singlet (S₁) and the ME, but little is known about these competing channels. Here we apply time-resolved photoemission spectroscopy to the tetracene/C₆₀ interface to probe one- and two-electron transfer from S₁ and ME states, respectively. Unlike ultrafast (∼100 fs) singlet fission in pentacene where two-electron transfer from the multiexciton state resulting from singlet fission dominates, the relatively slow (∼7 ps) singlet fission in tetracene allows both one- and two-electron transfer from the S₁ and the ME states that are in a quantum mechanical superposition. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photoexcited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C₆₀, respectively. Kinetic analysis shows that ∼60% of the S₁ ⇔ ME quantum superposition transfers one electron through the S₁ state to C₆₀ while ∼40% undergoes two-electron transfer through the ME state. We discuss design principles at donor/acceptor interfaces for optimal multiple carrier extraction from singlet fission for solar energy conversion

Understanding the Interface Dipole of Copper Phthalocyanine (CuPc)/C₆₀: Theory and Experiment

Interface dipole determines the electronic energy alignment in donor/acceptor interfaces and plays an important role in organic photovoltaics. Here we present a study combining first principles density functional theory (DFT) with ultraviolet photoemission spectroscopy (UPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to investigate the interface dipole, energy level alignment, and structural properties at the interface between CuPc and C₆₀. DFT finds a sizable interface dipole for the face-on orientation, in quantitative agreement with the UPS measurement, and rules out charge transfer as the origin of the interface dipole. Using TOF-SIMS, we show that the interfacial morphology for the bilayer CuPc/C₆₀ film is characterized by molecular intermixing, containing both the face-on and the edge-on orientation. The complementary experimental and theoretical results provide both insight into the origin of the interface dipole and direct evidence for the effect of interfacial morphology on the interface dipole

The Quantum Coherent Mechanism for Singlet Fission: Experiment and Theory

The absorption of one photon by a semiconductor material usually creates one electron–hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron–hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation.In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline tetracene lattice satisfactorily accounts for the experimental observations. It also reveals the critical roles of the charge transfer states and the high dephasing rates in ensuring the ultrafast formation of multiexciton states. In addition, we address the origins of microscopic relaxation and dephasing rates, and adopt these rates in a quantum master equation description. We show the need to take the theoretical effort one step further in the near future by combining high-level electronic structure calculations with accurate quantum relaxation dynamics for large systems