The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices.

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We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Förster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.

One of the most fundamental properties of both organic and inorganic semiconductors is charge mobility. It has been unambiguously shown that the mobility in both of these materials systems is strongly linked to the degree of long range order-that is, more extended crystallinity leads to a larger charge mobility, which ultimately determines such extrinsic properties as series resistance and response to current and optical pulses. An equally fundamental property for organic semiconductors is the molecular excited state-, or exciton-, diffusion length which characterizes energy transport within these more correlated solids. While it has been predicted that exciton transport should also be linked to the extent of crystalline order, to our knowledge no such dependence has yet been established. Here, we accurately measure the exciton diffusion length of the archetypal organic semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and clearly show its relationship to thin-film crystal morphology. As in the case of charge mobility, we show that the exciton transport diffusion length is a monotonic function of the extent of crystalline order. This study provides insight into the control and ultimately the tunability of the exciton diffusion length in organic systems, which is crucial for the management of energy transport in a wide range of important organic electronic devices.The exciton diffusion length (L D ) and charge mobility in organic semiconductors are central parameters for the optimal design of organic thin film electronic devices. [1][2][3][4] For example, charge mobility, and hence material conductivity, can vary by several orders of magnitude depending on the degree of crystalline order, [5][6][7] where the largest values have been found for single crystals. [5,8] Analogously, exciton diffusion has also been suggested to depend on structural order. [9][10][11][12] For example, the triplet exciton transfer rate may be described by the product of electron and hole transfer rates [12] which connects the diffusion of excitons to charge carrier mobility. Yet, the correlation between L D and crystalline order remain ambiguous. While the triplet diffusivities of amorphous [13] and crystalline [14][15][16][17] tetracene have been measured, no systematic dependence on crystallinity has been observed (i.e., reported values of exciton diffusivity in amorphous tetracene are both greater than and less than that of a single crystal). Other studies have explored possible connections between crystalline order and the exciton diffusion length in substituted phthalocyanines, [10,18] porphyrins, [18] perylene derivatives, [9,19] and polymers. [20] However, it is unclear from these studies whether the variations in the exciton diffusion length stem primarily from changes in the crystalline texture, the different electronic couplings of the various molecules, or from energetic disorder-induced transport percolation pathways. [21] Thus, despite the measurement of the exciton diffusion length for a large range of mate...