Photocurrent, Photovoltage, and Rectification in Large‐Area Bilayer Molecular Electronic Junctions

Abstract

between two electrode contacts, referred to as a "molecular junction (MJ)". [1] A MJ may employ a single organic substituent or repeating units (e.g., an oligomer) to achieve variations in the electronic response depending on structure and molecular layer thickness. [2] Alternatively, MJs may contain two or more molecular layers differing in structure and orbital energies to create multilayered MJs with novel electronic behaviors. As examples, bilayer MJs integrate electron donor and acceptor layers in a single oligomer to achieve an electronic rectifier, [2e,3] and trilayers that further incorporate mobile Li + ions can act as a redox active memory or energy storage device with total thickness <15 nm. [4] Understanding the relationship between the structure, orbital energies, and electronic behavior of both single and multilayer MJs is paramount to engineering new ME devices for a potentially broad range of electronic functions.In addition to optical spectroscopy used to verify MJ structure, illumination of MJs can produce an electronic response, [5] and current through a MJ can result in light emission. [6] The single-component MJs studied to date show relatively weak photoeffects such as internal photoemission (IPE) [5a,e,f ] and conductance changes, [5b] due in part to weak optical absorption by the thin molecular layer. We recently reported PCs at zero bias for eight single-component carbon/molecule/carbon MJs with varying molecular structure. While the PCs correlated with transport properties across a range of structures, the external quantum efficiencies were small, in the range of 10 −5 to 10 −4 photoelectrons/incident photon. [7] In contrast to symmetric current density versus bias voltage (JV) behavior observed with single-component layers, bilayer MJs with an electron accepting layer that is subsequently overlaid with an electron donating layer between symmetric sp 2 carbon contacts exhibit nonlinear, asymmetric JV responses with rectification ratios (RR) of 10-290 for different bilayer combinations. [3] The bilayer introduced asymmetry into a previously symmetric single-layer device, and the rectification direction could be reversed by changing the order of the molecular layers. The current investigation examines photoeffects in molecular bilayers (10-15 nm total thickness) with two main motivations. First, the photoresponse of the MJs should reveal relationships between internal energy levels of both the molecules and the associated contacts in completed, functioning MJs. Second, sensitivity to light is a potentially useful function in ME, and bilayer devices may be good candidates for photodetectors. Unlike the common semiconductor photodetectors, Bilayer donor-acceptor molecular junctions are successfully fabricated with active layer thickness <15 nm to study the mechanism of photo-induced charge transport. The bilayer devices exhibit lifetimes greater than 1 h with laser irradiation, and a strong dependence of the sign and magnitude of the photovoltage and photocurrent on structure and order ...

Light‐Stimulated Charge Transport in Bilayer Molecular Junctions for Photodetection

“…We noted previously that increasing the thickness of the AQ layer in BTB/AQ bilayers decreased the observed PC yield, by ≈85% when the AQ layer was increased from 5.4 to 16 nm . This thickness dependence was examined here for two additional examples, AQ/FL and FL/NAB, shown in Figure .…”

“…Device fabrication and PC measurements were conducted as described previously,11a,15 and all structures had identical electrodes using eC, denoted as Au 30 /eC 10 /molecular layer/eC 10 /Au 20 . The subscripts indicate layer thicknesses in nm and the molecular layers consisted of one or two of the components listed in Table 1 .…”

“…For bottom electrode, Cr (4 nm), Au (30 nm), and eC (10 nm) were deposited using electron beam evaporation (Kurt J. Lesker PVD75) through a shadow mask following a previously established procedure 11a. The molecular layers on the bottom electrode were deposited sequentially by electrochemical reduction of diazonium reagents, as described previously . The grafting parameters are provided in Table S1 in the Supporting Information.…”

Electrochemical Potential‐Driven High‐Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

“…[96] This technique is also extremely effective in creating heterostructures with the desired layer thickness of two or more discrete and different molecules. [97,98] Ac onventional electrochemical cell equipped with three electrodes,t he working,r eference,a nd counter electrodes, which was used for the reduction of aryl diazonium salts is schematically shown in Figure 4. To generate aryl radicals of the corresponding diazonium salt, an appropriate bias between the working and reference electrodes must be…”

Electrochemical Potential‐Driven High‐Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

“…[96] This technique is also extremely effective in creating heterostructures with the desired layer thickness of two or more discrete and different molecules. [97,98] Ac onventional electrochemical cell equipped with three electrodes,t he working,r eference,a nd counter electrodes, which was used for the reduction of aryl diazonium salts is schematically shown in Figure 4. To generate aryl radicals of the corresponding diazonium salt, an appropriate bias between the working and reference electrodes must be applied.…”