Morphological Tuning of the Energetics in Singlet Fission Organic Solar Cells

Abstract

6489wileyonlinelibrary.com conversion effi ciency (PCE) of a single junction solar cell is 41.9%-a signifi cant improvement over the so-called Shockley-Queisser limit of 32% for conventional devices. [ 6,7 ] The theoretical maximum PCE can be boosted even further if a long wavelength absorber is incorporated to take advantage of solar photons with energies between the singlet and triplet levels of the singlet fi ssion material. [ 1,3,8 ] Because of these exciting prospects, singlet fi ssion solar cells have attracted signifi cant research interest in the last several years.Ultimately, the functionality of this class of solar cells hinges on two requirements: (1) the ability to undergo rapid and effi cient singlet fi ssion, and (2) the ability to dissociate and extract the triplets that are generated by singlet fi ssion. Regarding the fi rst criterion, signifi cant efforts have been made to identify and engineer materials that readily undergo singlet fi ssion. The polyacenes (e.g., tetracene, pentacene) stand out in terms of usage, and have been incorporated in singlet-fi ssion-based solar cells [ 1,2,5 ] and photodetectors with great success. [ 3,9 ] Rubrene, a tetracene derivative, is capable of fast singlet fi ssion [ 10,11 ] but has not experienced the same levels of usage as other acenes, perhaps on the basis of its morphological variability and inferior performance in solar cells. As for the second requirement, little effort has been made to gain control over triplet extraction, yet it remains fundamental to the functionality of singlet fi ssion solar cells. In particular, only charge transfer processes from the triplet, and not the singlet, of the singlet fi ssion material are able to contribute to the effi ciency gains outlined above.We demonstrate that the tunable morphology of rubrene can be used to substantially change the behavior of devices in favor of more effi cient photocurrent generation. Specifically, the charge transfer (CT) state of a rubrene/C 60 solar cell can be shifted by over 300 meV simply by crystallizing the asdeposited amorphous fi lms. This shift in CT state energy is able to transform the amorphous device from one that blocks charge transfer of rubrene triplets into one that readily dissociates rubrene triplets. We conduct a comprehensive study of the impact of morphology on exciton dynamics in rubrene/C 60 systems and analyze its implications on device performance. Ultimately, the ability to morphologically fi ne-tune the interfacial energetics and the singlet fi ssion kinetics in such devices opens Effective singlet fi ssion solar cells require both fast and effi cient singlet fi ssion as well as favorable energetics for harvesting the resulting triplet excitons. Notable progress has been made to engineer materials with rapid and effi cient singlet fi ssion, but the ability to control the energetics of these solar cells remains a challenge. Here, it is demonstrated that the interfacial charge transfer state energy of a rubrene/C 60 solar cell can be modifi ed dramatically by the mo...

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

“…[9,24,25] In their aggregated states, donor or acceptor domains typically have lower ionization potentials and increased electron affinities than the nonaggregated phases owing to polarization and delocalization effects. [15,28] The presence of such structural heterogeneities at the proximity of D-A interfaces will thus invariably influence the V OC . [15,28] The presence of such structural heterogeneities at the proximity of D-A interfaces will thus invariably influence the V OC .…”

Charge-transfer excited states in the donor/acceptor interface from large-scale GW calculations

“…Moreover, the energy of the CT state correlates with the open-circuit voltage value in an organic solar cell. 3,4 Therefore, understanding the energetics and nature of CT states and correlating them with interfacial morphologies 5 are essential. Theoretical investigations on organic/organic interfaces have provided microscopic information regarding the energies and wave function properties of the CT states.…”

High‐Voltage Photogeneration Exclusively via Aggregation‐Induced Triplet States in a Heavy‐Atom‐Free Nonplanar Organic Semiconductor

“…[19] But TAS literature has shown that intermolecular interactions that result upon solidification can alter singlet and triplet energy levels and ISC kinetics. [2,[25][26][27] To assess, we turn to the grazing-incidence X-ray diffraction (GIXD) pattern of as-deposited TBF films, which shows a broad amorphous halo centered about q = 0.5 Å -1 , indicating weak sub-nanometer nearest-neighbor correlations. [19,28] These nearest-neighbor interactions could indicate local TBF aggregation, which would result in low-order triplet excimers when TBF neat films are photoexcited.…”

“…Figure 7 proposes a Jablonski diagram that is consistent with TBF neat films having broad, energetically-overlapping singlet-and triplet-excimer states in equilibrium via an energetically-diverse range of singlet states given band splitting. The singlet and triplet excimer bands in Figure 7 Figure S13) by sensitive EQE spectroscopy [2] reveal the CT states of TBF/C 60 and TBF/12Cl to have energies centered at 1.24 and 1.77 eV, respectively, with the CT band of TBF/12Cl only subtly visible due to the optical response of the neat constituents. The CT band of TBF/8Cl is expected to be ≥ 0.3 eV higher than that of the CT band of TBF/12Cl, and cannot be resolved from the bulk TBF/8Cl EQE response ( Figure S13c).…”

“…Additionally, the thin-film structure that organic semiconductors adopt represents a second degree of freedom for tuning the resulting macroscopic properties and photophysical processes critical to device functions. [1,2] For instance, the behavior of triplet exciton states -populated via intersystem crossing (ISC) -depends on molecular structure and intermolecular interactions. [3,4] Because triplet exciton lifetimes and characteristic diffusion lengths can extend orders of magnitude beyond those of their singlet counterparts, efficient ISC can enhance exciton harvesting in organic optoelectronic devices.…”