Impact of Molecular Organization on Exciton Diffusion in

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Impact of Molecular Organization on Exciton Diffusion in Photosensitive Single-Crystal Halogenated Perylenediimides Charge Transfer Interfaces Rui Montenegro Pinto, Wilson Gouveia, Ermelinda Maria Sengo Macoas, Isabel Cordeiro Santos, Sebastian Raja, Carlos Baleizão, and Helena Alves ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08523 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on December 2, 2015

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ACS Applied Materials & Interfaces

Impact of Molecular Organization on Exciton Diffusion in Photosensitive Single-Crystal Halogenated Perylenediimides Charge Transfer Interfaces Rui M. Pinto†‡, Wilson Gouveia†, Ermelinda M. S. Maçôas‡, Isabel C. Santos¤, Sebastian Raja‡, Carlos Baleizão‡, and Helena Alves†§* †

INESC-MN and IN, Rua Alves Redol 9, 1000-029 Lisboa, Portugal.

‡

CQFM and IN-Institute of Nanoscience and Nanotechnology, Instituto Superior Técnico,

University of Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. ¤

C2TN, Instituto Superior Técnico, University of Lisboa, 2695-066 Bobadela, Portugal.

§

Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal.

KEYWORDS:

organic

semiconductors,

single-crystals,

charge-transfer

interfaces,

photodetectors, perylenediimides.

ACS Paragon Plus Environment

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ABSTRACT The efficiency of organic photodetectors and optoelectronic devices is strongly limited by exciton diffusion, in particular for acceptor materials. While mechanisms for exciton diffusion are well established, its correlation to molecular organization in real systems has received far less attention. In this report, organic single-crystals interfaces were probed with wavelengthdependent photocurrent spectroscopy and their crystal structure resolved using x-ray diffraction. All systems present a dynamic photoresponse, faster than 500 ms, up to 650 nm. A relationship between molecular organization and favourable exciton diffusion in substituted butylperylenediimides (PDIB) is established. This is demonstrated by a set of PDIBs with different intra and inter stack distances and short contacts and their impact on photoresponse. Given the short packing distances between PDIs cores along the same stacking direction (3.4-3.7 Å), and across parallel stacks (2.5 Å), singlet exciton in these PDIBs can follow both Förster and Dexter exciton diffusion, with the Dexter-type mechanism assuming special relevance for interstack exciton diffusion. Yet, the response is maximized in substituted PDIBs, where a 2D percolation network is formed through strong interstack contacts, allowing for PDIBs primary excitons to reach with great efficiency the splitting interface with crystalline rubrene. The importance of short-contacts and molecular distances, which is often overlooked as a parameter to consider and optimize when choosing materials for excitonic devices, is emphasized.

1. INTRODUCTION Morphology of materials has a tremendous impact on electronic transport, magnetism, optical and mechanical properties.1-4 In organic semiconductors, molecular organization crucially affects


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materials properties and the performance of devices such as field-effect transistors, or photovoltaics.5-10 It is in single crystals that higher molecular order is achieved, leading to higher charge mobility and exciton diffusion length.11-16 These properties have been explored in a variety of single crystal devices, such as inverters, logic circuits, sensors, and field-effect transistors.17-22 Efforts from the research community have focused on exploring charge transport. However, features dependent on light-induced phenomena, like photoconductivity, light emission, or excitonic behavior have been less investigated, despite very promising achievements.23-27 Most optoelectronic applications depend on light absorption in the visible range, and organic materials (donors and acceptors alike) can present absorption in this region. In organic photovoltaics, where usually the two types of materials are combined, exciton generation can be maximized by tuning the overlap of the absorption profile with the solar spectrum, through donor and acceptor layers with complementary absorption spectra. Yet, due to the difficulty in collecting excitons generated in the acceptor material, high efficiency solar cells typically operate solely on the basis of photoinduced electron transfer from the donor to the acceptor layer. Even if recent reports indicate that excitons generated in the acceptor layer can also contribute to harvest energy, their contribution is limited due to recombination and other losses.28-30 Noteworthy is the fact that using a single crystal donor (rubrene) combined with an acceptor film (PCBM), allows for high-efficiency collection of excitons generated in the acceptor material, with an enhancement of three hundred times in the photoresponse.31 This enhancement is even more pronounced when a complete crystalline system is used, with an increase of three orders of magnitude over those using a single component material.32 Such effect results from an efficient charge-separation due to interfacial polarization which enhances


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electrical conduction towards a metallic regime,33-35 and also from a long exciton diffusion length (LD) observed in single crystals, which can reach several micrometers. Exciton diffusion occurs through energy transfer in three possible mechanisms: Coulomb interaction (Förster), electron exchange interaction (Dexter) and radiative.36 The nature of the predominant energy transfer process depends highly on the morphology and molecular system. Due to the long optical path and the short intermolecular distances observed in single crystals of organic semiconductors, all three energy transfer processes should contribute to exciton diffusion, whereas in thin films Förster is usually the dominant mechanism. In radiative energy transfer process, the initially formed exciton decays radiative and the emitted photon is used to generate an exciton elsewhere along the optical path. In single crystals with thickness of 1-2 µm, more than 90% of the emitted radiation in all directions can be re-absorbed, contributing to exciton migration over long distances. Indeed, the long diffusion lengths observed along the caxis of rubrene single-crystal (LD = 5 µm) have been recently attributed to radiative energy transfer rather than triplet diffusion.37 At intermolecular distances of 5-50 Å, Förster energy transfer usually controls exciton diffusion and due to spin conservation rules it is almost exclusive to singlet excitons with short lifetimes (on the nanosecond range).38 Dexter exchange process can be viewed as a concerted exchange of one electron and one hole between the donor and the acceptor. For that reason, it requires orbital overlap, and depends exponentially on the intermolecular distance.39 In Dexter exchange, the spin conservation rules are relaxed, and both triplet-triplet (3D* + 1A→1D + 3A*) and single-singlet (1D* + 1A→1D + 1A*) exchange are allowed. At short intermolecular distances, 1-5 Å, Dexter exchange rate approaches Förster energy transfer rate, and longer diffusion lengths can be observed for both singlet and triplet exciton diffusion. Indeed, the maximum singlet exciton diffusion length for an intermolecular


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separation of 4.5 Å has been predicted to be 230 nm, whereas the computed triplet diffusion length for different acene single-crystals varied from 1 to 14 µm, in good agreement with experimental observations.37 Even with strong theoretical and experimental evidences of the impact of molecular overlap on efficient exciton diffusion, less proofs are available on the real impact of this effect on photoresponse and its relationship with molecular stacking and intra (inter) stack contacts, specially for excitons generated in acceptor materials. Information on such relationship would provide important material design guidelines to achieve high-quality optoelectronic properties in organic semiconductor multi-layer devices. Here, we use single crystal charge transfer interfaces of rubrene and several perylenediimide derivatives (PDIB-X, X=H, 4Br, 4Cl, 4OPh) with different molecular arrangements, to probe the effect of molecular structure on the photoresponse. The selected PDIB derivatives present either planar or non-planar cores with different functional groups on the bay-area, which allow tuning of the HOMO-LUMO (highest occupied-lowest unoccupied molecular orbitals) levels and different intra- and inter-stack molecular overlap. These rubrene/PDBIs single-crystal interfaces present an absorption spectra equivalent to the sum of the donor and acceptor spectra. The sheet resistance (Rs) of all systems are of the order of 10-100 MΩ, with high photoresponse through all their absorption profile. Since the acceptors absorb in the red zone of the visible spectra, our approach allows for an extension of the photoresponse of rubrene alone and higher overall responsivity values. Electronic structure calculations of the interfacial energetics indicate that photoinduced hole-transfer is favorable for all interfaces, and that the increased response above 550 nm is related to the splitting of PDIBs excitons. By analyzing the structural properties of PDIBs single-crystals, alongside with responsivity spectra and electronic structure calculations, we show that the abundance of short inter-stack contacts and smaller molecular distance in


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PDIBs favors exciton diffusion. Our work shows that while standard guidelines for chargetransport applications (e.g., planar stacking to maximize π-overlap in OFETs) are important, they should be reconsidered when dealing with light-induced phenomena. In applications where light matters and excitons, rather than charge, are the key-players, materials should be engineered to have higher molecular orbital overlap and yield more short-contacts along different directions, giving excitons additional escape ways to diffuse towards an interface.

2. EXPERIMENTAL SECTION 2.1. Materials and Single-Crystal Growth. Rubrene (Alfa Aesar, 97%) was used as purchased. PDIB, PDIB-4Cl, PDIB-4Br and PDIB-4OPh were synthesized according to procedures described in the literature.40-42 All crystals were grown using the physical vapor transport (PVT) technique under a stream of ultrapure Ar gas.43 Sublimation temperature for rubrene was close to 310 ºC, resulting in thin (

Impact of Molecular Organization on Exciton Diffusion in

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