Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS‐Pentacene

Abstract

order provide an opportunity to understand many fundamental physical properties relevant to solar energy conversion. Additionally, organic crystals are promising in their own right due to the effi cient carrier and energy transport properties associated with their long-range order.In particular, crystalline and polycrystalline fi lms of pentacene (PEN) and its derivatives have high carrier mobility for charge transport (≈1−10 cm 2 /Vs hole mobility) [ 1 ] and signifi cant photoconductivity. [ 2,3 ] Moreover, PEN and many of its derivatives display a propensity for singlet fi ssion (SF), [ 4,5 ] a phenomenon that results in greater than 100% internal quantum effi ciency in organic photovoltaics. Numerous possible molecular functionalizations may modify solid-state structural and optoelectronic properties. [ 6 ] Therefore, elucidating the structure-property relation is important for the design of new functional molecular materials. 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN), [ 7 ] shown in Figure 1 , has recently attracted much interest. The bulky groups functionalizing PEN enable solubility in organic solvents and allow for solution-processing of polycrystalline TIPS-PEN thin fi lms with high carrier mobility and photoconductivity. [ 2,3,7 ] While TIPS-PEN apparently retains optical properties similar to PEN, its enhanced carrier mobility [ 8 ] and Relating the Physical Structure and Optoelectronic Function of Crystalline TIPS-PentaceneSahar Sharifzadeh , * Cathy Y. Wong , Hao Wu , Benjamin L. Cotts , Leeor Kronik , Naomi S. Ginsberg , and Jeffrey B. Neaton * Theory and experiment are combined to investigate the nature of low-energy excitons within ordered domains of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-PEN) polycrystalline thin fi lms. First-principles density functional theory and many-body perturbation theory calculations, along with polarizationdependent optical absorption spectro-microscopy on ordered domains, show multiple low-energy absorption peaks that are composed of excitonic states delocalized over several molecules. While the fi rst absorption peak is composed of a single excitonic transition and retains the polarization-dependent behavior of the molecule, higher energy peaks are composed of multiple transitions with optical properties that can not be described by those of the molecule. The predicted structure-dependence of polarization-dependent absorption reveals the exact inter-grain orientation within the TIPS-PEN fi lm. Additionally, the degree of exciton delocalization can be signifi cantly tuned by modest changes in the solid-state structure and the spatial extent of the excitations along a given direction is correlated with the degree of electronic dispersion along the same direction. These fi ndings pave the way for tailoring the singlet fi ssion effi ciency of organic crystals by solid-state structure.

Low-lying excited states in crystalline perylene

“…Consistent with previous studies (6,29,33), the extent and direction of exciton delocalization is strongly correlated with the extent and direction of band dispersion, which facilitates the delocalization. While exciton delocalization is not directly accessible in our experimental setup, we can test this prediction by probing exciton dynamics instead.…”

“…Data at negative times for the α-phase reflect the long exciton lifetimes, which exceed the inverse repetition rate of our exciting laser. intermolecular interactions along this direction, which is expected to lead to broader dispersion in general (6,29,32,33). The calculated 2.87-eV fundamental gap is indirect, from Y to Γ.…”

Structural and excited-state properties of oligoacene crystals from first principles

“…For large acenes, notably pentacene, singlet states have been shown to extend over several molecules [19,24,150,161]. The degree of delocalization is larger for smaller unit-cell volumes, an effect related to the increased intermolecular hybridization [162]. Delocalization decreases the excitation energy, which is therefore larger the smaller the unit cell is.…”

“…For short acenes, this is a much smaller effect and indeed no significant structure dependence is observed. Triplets, however, are always predominantly localized on a single monomer [161,162], explaining their weak dependence on the geometry. Owing to this negligible delocalization, triplet energies calculated in the gas and solid state are within 0.2 eV (compare with Table I), i.e., the triplet is largely independent of the solid-state environment.…”

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

“…Excitonic properties of organic crystals are substantially different from those of isolated molecules, owing to excitonic couplings, near-field interactions between electronic transitions [17][18][19][20]. Interactions with a charge-transfer (CT) statea state in which the electron and hole are located on spatially separated regions-can also affect the optical properties of an exciton, as has been demonstrated by several theoretical studies [21][22][23][24][25][26]. Experimentally, the degree of CT character has been studied by momentum-dependent electron-loss spectroscopy [27][28][29] or electroabsorption spectroscopy [30][31][32].…”