Change in Tetracene Polymorphism Facilitates Triplet Transfer in Singlet Fission-Sensitized Silicon Solar Cells

Revealing the Singlet Fission Mechanism for a Silane-Bridged Thienotetracene Dimer

Tetraceno[2,3-b]thiophene is regarded as a strong candidate for singlet fission-based solar cell applications due to its mixed characteristics of tetracene and pentacene that balance exothermicity and triplet energy. An electronically weakly coupled tetraceno[2,3-b]thiophene dimer (Et2Si(TIPSTT)2) with a single silicon atom bridge has been synthesized, providing a new platform to investigate the singlet fission mechanism involving the two acene chromophores. We study the excited state dynamics of Et2Si(TIPSTT)2 by monitoring the evolution of multiexciton coupled triplet states, 1TT to 5TT to 3TT to T1 + S0, upon photoexcitation with transient absorption, temperature-dependent transient absorption, and transient/pulsed electron paramagnetic resonance spectroscopies. We find that the photoexcited singlet lifetime is 107 ps, with 90% evolving to form the TT state, and the complicated evolution between the multiexciton states is unraveled, which can be an important reference for future efforts toward tetraceno[2,3-b]thiophene-based singlet fission solar cells.

Computational Discovery of Intermolecular Singlet Fission Materials Using Many-Body Perturbation Theory

Intermolecular singlet fission (SF) is the conversion of a photogenerated singlet exciton into two triplet excitons residing on different molecules. SF has the potential to enhance the conversion efficiency of solar cells by harvesting two charge carriers from one high-energy photon, whose surplus energy would otherwise be lost to heat. The development of commercial SF-augmented modules is hindered by the limited selection of molecular crystals that exhibit intermolecular SF in the solid state. Computational exploration may accelerate the discovery of new SF materials. The GW approximation and Bethe-Salpeter equation (GW+BSE) within the framework of many-body perturbation theory is the current state-of-the-art method for calculating the excited-state properties of molecular crystals with periodic boundary conditions. In this Review, we discuss the usage of GW+BSE to assess candidate SF materials as well as its combination with low-cost physical or machine learned models in materials discovery workflows. We demonstrate three successful strategies for the discovery of new SF materials: (i) functionalization of known materials to tune their properties, (ii) finding potential polymorphs with improved crystal packing, and (iii) exploring new classes of materials. In addition, three new candidate SF materials are proposed here, which have not been published previously.

Defect-Assisted Exciton Transfer across the Tetracene-Si(111):H Interface

Exciton transfers are ubiquitous and extremely important processes, but often poorly understood. A recent example is the triplet exciton transfer in tetracene sensitized silicon solar cells exploited for harvesting high-energy photons. The present ab initio molecular dynamics calculations for tetracene-Si(111):H interfaces show that Si dangling bonds, intuitively expected to hinder the exciton transfer, actually foster it. This suggests that defects and structural imperfections at interfaces may be exploited for excitation transfer.

Methylation of Si(111) Modulates Molecular Orientation in Perylenediimide Thin Films

Singlet fission produces a pair of low-energy spin-triplet excitons from a single high-energy spin-singlet exciton. While this process offers the potential to enhance the efficiency of silicon solar cells by ∼30%, meeting this goal requires overlayer materials that can efficiently transport triplet excitons to an underlying silicon substrate. Herein, we demonstrate that the chemical functionalization of silicon surfaces controls the structure of vapor-deposited thin films of perylenediimide (PDI) dyes, which are prototypical singlet fission materials. Using a combination of atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS), we find terminating Si(111) with either a thin, polar oxide layer (SiOx) or with hydrophobic methyl groups (Si-CH3) alters the structures of the resulting PDI films. While PDI films grown on SiOx are comprised of small crystalline grains that largely adopt an "edge-on" orientation with respect to the silicon surface, films grown on Si-CH3 contain large grains that prefer to align in a "face-on" manner with respect to the substrate. This "face-on" orientation is expected to enhance exciton transport to silicon. Interestingly, we find that the preferred mode of growth for different PDIs correlates with the space group associated with bulk crystals of these compounds. While PDIs that inhabit a monoclinic (P21/c) space group nucleate films by forming tall and sparse crystalline columns, PDIs that inhabit triclinic (P1̅) space groups afford films comprised of uniform, lamellar PDI domains. The results highlight that silicon surface functionalization profoundly impacts PDI thin film growth, and rational selection of a hydrophobic surface that promotes "face-on" adsorption may improve energy transfer to silicon.

Developments and Challenges Involving Triplet Transfer across Organic/Inorganic Heterojunctions for Singlet Fission and Photon Upconversion

In this Perspective, we provide an overview of recent advances in harvesting triplets for photovoltaic and photon upconversion applications from two angles. In singlet fission-sensitized solar cells, the triplets are harvested through a low band gap semiconductor such as Si. Recent literature has shown how a thin interlayer or orientation of the singlet fission molecule can successfully lead to triplet transfer. On the other hand, the integration of transition metal dichalcogenides (TMDCs) with suitable organic molecules has shown triplet-triplet annihilation upconversion (TTA-UC) of near-infrared photons. We consider the theoretical aspect of the triplet transfer process between a TMDC and organic semiconductors. We discuss possible bottlenecks that can limit the harvesting of energy from triplets and perspectives to overcome these.

Effect of Chalcogenophenes on Chiroptical Activity of Twisted Tetracenes: Computational Analysis, Synthesis and Crystal Structure Thereof

The synthesis of multiply substituted acenes is still a relevant research problem, considering their applications and future potential. Here we present an elegant synthetic protocol to afford tetra-peri-substituted naphthalene and tetracene from their tetrahalo derivatives by a Pd(0)-catalyzed C-C cross-coupling method in a single step. The newly synthesized tetracenes were characterized by NMR, HRMS, UV-vis spectrophotometry, and single-crystal X-ray diffraction (SCXRD). In addition, the first systematic computational study of the effect of chalcogenophenyl substitutions on the chiroptical properties of twistacenes was reported here. The gas phase computational studies using density functional theory (DFT) on a series of chalcogenophene-substituted tetracenes revealed that their chiroptical activity could be systematically increased via the atomistic tuning of peripheral substituents.

Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein

Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes provided that the conjugated backbone is twisted out of plane. However, this hypothesis has been forwarded only in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a "minimal" one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in a trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission despite the canthaxanthin twist in OCPo.

Inverse Design of Tetracene Polymorphs with Enhanced Singlet Fission Performance by Property-Based Genetic Algorithm Optimization

The efficiency of solar cells may be improved by using singlet fission (SF), in which one singlet exciton splits into two triplet excitons. SF occurs in molecular crystals. A molecule may crystallize in more than one form, a phenomenon known as polymorphism. Crystal structure may affect SF performance. In the common form of tetracene, SF is experimentally known to be slightly endoergic. A second, metastable polymorph of tetracene has been found to exhibit better SF performance. Here, we conduct inverse design of the crystal packing of tetracene using a genetic algorithm (GA) with a fitness function tailored to simultaneously optimize the SF rate and the lattice energy. The property-based GA successfully generates more structures predicted to have higher SF rates and provides insight into packing motifs associated with improved SF performance. We find a putative polymorph predicted to have superior SF performance to the two forms of tetracene, whose structures have been determined experimentally. The putative structure has a lattice energy within 1.5 kJ/mol of the most stable common form of tetracene.

Exciton Modulation in Perylene-Based Molecular Crystals Upon Formation of a Metal-Organic Interface From Many-Body Perturbation Theory

Excited-state processes at organic-inorganic interfaces consisting of molecular crystals are essential in energy conversion applications. While advances in experimental methods allow direct observation and detection of exciton transfer across such junctions, a detailed understanding of the underlying excitonic properties due to crystal packing and interface structure is still largely lacking. In this work, we use many-body perturbation theory to study structure-property relations of excitons in molecular crystals upon adsorption on a gold surface. We explore the case of the experimentally-studied octyl perylene diimide (C8-PDI) as a prototypical system, and use the GW and Bethe-Salpeter equation (BSE) approach to quantify the change in quasiparticle and exciton properties due to intermolecular and substrate screening. Our findings provide a close inspection of both local and environmental structural effects dominating the excitation energies and the exciton binding and nature, as well as their modulation upon the metal-organic interface composition.

Realistic Efficiency Limits for Singlet-Fission Silicon Solar Cells

Singlet fission is a carrier multiplication mechanism that could make silicon solar cells much more efficient. The singlet-fission process splits one high-energy spin-singlet exciton into two lower-energy spin-triplet excitons. We calculated the efficiency potential of three technologically relevant singlet-fission silicon solar cell implementations. We assume realistic but optimistic parameters for the singlet-fission material and investigate the effect of singlet energy and entropic gain. If the transfer of triplet excitons occurs via charge transfer, the maximum efficiency is 34.6% at a surprisingly small singlet energy of 1.85 eV. For the Dexter-type triplet energy transfer, the maximum efficiency is 32.9% at a singlet energy of 2.15 eV. For Förster resonance energy transfer (FRET), the triplet excitons are first transferred into a quantum dot, from which they then undergo FRET into silicon. For this transfer mechanism, the maximum efficiency is 28.% at a singlet energy of 2.33 eV. We show that the efficiency gain from singlet fission is larger the more efficient the silicon base cell is, which stands in contrast to tandem perovskite-silicon solar cells.

Generating Triplets in Organic Semiconductor Tetracene upon Photoexcitation of Transition Metal Dichalcogenide ReS2

We studied the dynamics of transfer of photoexcited electronic states in a bilayer of the two-dimensional transition metal dichalcogenide ReS₂ and tetracene, with the aim to produce triplets in the latter. This material combination was used as the band gap of ReS₂ (1.5 eV) is slightly larger than the triplet energy of tetracene (1.25 eV). Using time-resolved optical absorption spectroscopy, transfer of photoexcited states from ReS₂ to triplet states in tetracene was found to occur within 5 ps with an efficiency near 38%. This result opens up new possibilities for heterostructure design of two-dimensional materials with suitable organics to produce long-lived triplets. Triplets are of interest as sensitizers in a wide variety of applications including optoelectronics, photovoltaics, photocatalysis, and photon upconversion.

Elucidating the Influence of Sulfur Vacancies on Nonradiative Recombination Dynamics in Cu2ZnSnS4 Solar Absorbers

We report a time-domain ab initio simulation of charge carrier trapping and relaxation dynamics in pristine and defect-containing kesterite Cu₂ZnSnS₄ (CZTS) structures. Our simulations show that introduction of a neutral sulfur vacancy in the CZTS system leads to a decrease of the charge recombination rate by a factor of ∼4, and the doubly positively charged sulfur vacancy results in a minor decrease of carrier lifetime, as compared to the pristine CZTS system. The neutral sulfur vacancy weakens the nonadiabatic (NA) electron-phonon coupling by moderately localizing charge density and accelerates the pure dephasing process, extending charge carrier lifetime. Therefore, the neutral sulfur vacancy is electrically benign. The doubly positively charged sulfur vacancy introduces a subgap state which is hardly populated, and recombination of the electron and hole bypassing the trap state dominates. As a result, the recombination rate decreases in the doubly charged sulfur vacancy structure. The reported results identified the key role of the sulfur-related vacancy on charge carrier trapping and relaxation of CZTS materials, carrying important implications for further optimization of CZTS and other thin-film solar cell materials.