Organic semiconducting single crystals: from fundamentals to advanced devices

Resume : The charge transport and photophysical properties of organic crystal depend strongly on the relative orientation of molecules within that crystal. Using our functionalization approach to crystal design, we have developed several chromophores that can be engineered to adopt an array of crystalline forms, Charge transport, photoconductivity, and singlet-fission studies on these well-defined crystals has allowed us to develop a set of general structure - function relationships for electronic properties in crystalline organic semiconductors. More recently, we have developed methods to prepare isometrically-pure anthradithiophene derivatives. In one case, the pure isomers adopt a crystal packing very different from the isomeric mix, leading to orders of magnitude difference in charge carrier mobilities. In other cases, while the crystal packing and overall mobility do not change significantly, the thermal behavior (phase transitions) of the material are substantially altered. I will discuss these findings in relation to the degree of C-H-Pi interactions in the solid state.

Resume : Single crystals of thiophene−phenelyne co-oligomers (TPCOs) have shown great potential for organic optoelectronics. These materials combine efficient charge transport properties along with very high luminescence quantum efficiency. Here we report on growth and study of novel TPCO single-crystals with exceptionally high surface quality. TPCOs with the same conjugated core 5,5’-diphenyl-2,2’-bithiophene and different various end substituents (fluorine, thrifluoromethyl, trimethylsilyl) were studied. Remarkably, we found that both vapor- and solution-grown (by using solvent-antisolvent crystallization growth method) crystals can have molecularly flat surface on the scale of tens of microns as measured by atomic force microscopy. Electrical charge transport properties were studied in organic field effect transistors (OFET) in top electrode, top gate (with parylene gate dielectric) or bottom gate (with SiO2 dielectric) configurations. We compare charge transport properties depending on oligomer end groups and device configuration. Crystals grown using different growth techniques including solvent and vapor phase are also compared. We show that molecularly flat crystals can be obtained utilizing both vapor- and solution-growth methods. These crystals demonstrate similar and repeatable electrical transport properties in both top and bottom gate configurations.

Resume : Over recent years there has been tremendous progress in developing low-temperature processible organic semiconductors that provide high charge carrier mobilities for both n-type and p-type device operation, good operational stability and other functionalities such as efficient electroluminescene, sensing or memory functions for a variety of applications. Here we are interested in understanding the charge transport physics of high mobility conjugated polymers and molecular single crystals and the relationship between molecular structure, solid-state microstructure and charge transport. In this presentation we will present our current understanding of the key factors that govern the transport physics and carrier mobilities of these materials.

Resume : The functions of pi-conjugated organic materials are generated by rational molecular design. Traditional strategy for organic semiconductor materials with high carrier mobility relies on linear shaped fused aromatic scaffolds, resulting in the poor solubility as well as thermal instability in crystal phase. To overcome such drawbacks, we newly designed chalcogen bridged V-shaped pi-conjugated materials, dinaphtho[2,3-b:2′,3′-d]chalcogenophenes (DNX-Vs). In this work, we shed light on the effect of the chalcogen atom as well as length and position of alkyl side chains on solubilities, photophysical properties, packing structures, and thermal properties. Intriguingly, as the shorter alkyl chains are substituted on the V-shaped cores, these molecules possess the higher solubility over 1.0 wt% and phase-transition temperature over 200 °C. This trend is preferable for realizing semiconductor materials with solution-processability and thermal durability. The solution crystallized FET using hexyl chain substituted DNT-V exhibits hole mobility as high as 9.5 cm2/Vs and excellent device durability under thermal stress.[1] On the other hand, oxygen-bridged congers, DNF-Vs, exhibit deep-blue luminescence with high quantum efficiency even in solid state and hole mobility over 1.0 cm2/Vs.[2] Thus, DNF-V derivatives are promising candidates for the development of blue light-emitting devices. Reference: [1] Adv. Mater. 2013, 25, 6392. [2] Chem. Commun. 2014, 50, 5342.

Resume : The nonstoichiometry phenomenon is well known for inorganic crystals. The crystal nonstoichiometry results from general thermodynamic laws at T>0 K. But in case of organic crystals it has been thought the nonstoichiometry is not valid due to a molecule structure of organic crystals. In the research we have studied the crystals behavior of tris-(8-hydroxyquinoline)aluminum (III) (Alq3) during heat treatment under ligand vapor (Pq). Alq3 is a known organic phosphor for OLED devices. The scheme of Alq3 polymorphic transitions vs temperature has been established in [1,2]. The first time it has been shown that Pq increase results to: a) reduction of photoluminescence (PL) intensity at a fixed Tannealing; b) reducing the temperatures of polymorphic transitions and melting; c) hypsochromic shift of PL maximum from 517 to 508 nm (at Tann=580 K). All changes were reversible, indicating the thermodynamic nature of the observed effects. We explained the effects by the formation of a thermodynamically equilibrium crystal structure, which depending on Pq resulted to the change of Al atom number at a constant amount of ligands in the crystal structure. The suggestion was proved by XRD analysis of the crystal lattice parameters. We found the correlation between lattice parameters vs Pq, which indicated Al-nonstoichiometry in Alq3 metal complex. 1. M. Brinkmann et al. J. Am. Chem. Soc., 2000, V. 122, N 21, 5147–5157 2. R. I. Avetisov et al. Russian Microelectronics, 2014, V. 43, N 8, 526–530

Resume : In the last ten years inkjet (IJ) printing became a very important process for creating flexible devices for electronics. This technique is very simple and at the same time it allows the deposition of all type of materials, dispersed in a specific ink with opportune chemical and physical properties. In particular, IJ printing is deeply investigated for fabrication of organic electronics applications [1-4]. Here we present first results over IJ printed OFETs based on TIPS-Pentacene (TIPS), a well-performing organic semiconductor. In order to attain IJ printed TIPS single crystals we investigated the effect of changing several different printing parameters, like i) the substrate temperature, ii) the amount of printed solution and the nature of the substrate. iii) the solution formulation. The effects of changing these parameters have been investigated over different substrates, including electrodes-incorporating ones. We will show how properly tuning the printing conditions it is possible to obtain multiple single crystals even on interdigitated electrodes realized onto flexible substrates, and that these devices are suitable for electronic applications. [1]- Minemawari et al., Japanese Journal of Applied Physics, 53, 2014, 05HC10 [2]- Paul Calvert, Chem. Mater., 13, 2001, 3299-3305 [3]- Boseok Kang et al., ACS Appl. Mater. Interfaces, 5, 2013, 2302−2315 [4]- Song Yun Cho et al. J. Mater. Chem. C, 1, 2013, 914–923

Resume : Rubrene (RUB) (5,6,11,12-tetraphenyltetracene) is, among organic semiconductors, one of the most promising materials for organic device applications thanks to its high hole mobility and its long exciton diffusion length measured in single crystals. The main limit of RUB is the difficulty in growing films with a well ordered and crystalline structure, required to have good charge transport. Here we grow RUB thin films by organic molecular beam epitaxy and we use them as active layers in field-effect transistors (FET), thanks to their crystalline structure and high degree of orientation, obtained at room temperature and without any post-growth treatments Single crystals of a properly selected water-soluble organic material, namely β-alanine, were used as substrates for exploiting organic epitaxy; in addition, the hihg β-alanine solubility in water enables the substrate removal and the RUB film transfer onto a different substrate more suitable for device fabrication. In this paper, we used Cytop as dielectric, ITO as bottom gate electrode, and gold top contacts. The extremely good FET characteristics obtained demonstrate the high quality of RUB grown by organic epitaxy and the reliability of the wet-transfer process.

Resume : Recently organic materials gave evidence to be suitable for detecting ionizing radiation as active element in direct X-ray detectors. In this work, we assess the performances of novel X-ray detectors based on a low-cost solution grown organic semiconducting single crystal (OSSC), 4-hdroxycyanobenzene (4HCB), for its direct and linear response to ionizing radiation at room temperature, air and ambient light. An investigation of the effects associated to different electrodes configurations, contact materials, charge collection areas and interaction volumes was carried out on crystals with thicknesses varying from 500 to 40 µm, demonstrating how even the thinner crystals can be effectively used to detect X-rays. A dedicated study of the electrodes geometry allowed us to maximize the charge collection efficiency and to take advantage of the anisotropy of the crystals. We investigated the performances along all crystal axes, studying both a vertical sandwich electrodes geometry and a planar one. Planar configuration presented lower X-ray signal and sensitivity than vertical, despite the mobility is three orders of magnitude higher along the planar axis. The sensitivity is 180 nC/Gy, and the signal-to-noise ratio is maximized at V<100V, assessing good OSSCs performances at low operating voltages and offering a great potential for the development of novel X-ray sensors, also using thinner crystals, paving the way for the realization of bendable detectors for flexible electronics.

Resume : Here we describe processes to transform archetypical hydrogen-bonded organic pigments into colloidal micro and nanocrystals suitable for solution processing of high-quality transistors, biosensors, and photosensors. Our methodology relies on ligand-mediated syntheses, transforming commercial colored pigment powders into stable colloidal solutions of semiconductor nanocrystals, highly suitable for electronic device developments. Tuning of process conditions can yield nanocrystals with zero, one-, two- and three-dimensional shapes, exhibiting a wide range of optical absorption and photoluminescence over spectral regions from the visible to the near infrared. The utility of such colloidal nanocrystals is demonstrated in photodetectors with responsivities up to 1 A/W, and humidity sensors operating over a dynamic range of 7 orders of magnitude and ~0.1s response. Both devices are fabricated by paint brushing or drop casting of these nanocrystals on paper substrates and outperform devices prepared from the same starting materials by vacuum deposition by several orders of magnitude. Properties can be further contolled through the use of biofunctional ligands such as riboflavin, flavin mononucleotide, and phosphonic acids. We demonstrate dedicated functionality leading to selective bioresponse. Importantly, these crystals are found to exhibit excellent operational stability in both air and various aqueous ionic environments. The semiconducting nanocrystals described here offer a cheap and nontoxic alternative to inorganic nanocrystals, as well as a new paradigm for obtaining organic semiconductor materials from low-cost and nontoxic commercial colorants.

Resume : A low-lying LUMO level is one of the most obvious requirements for highly efficient n-channel organic materials. The depth of LUMO level can be controlled by attaching strong electron-withdrawing groups to π-conjugated cores. However, such functionalization often impacts other transport parameters such as electronic coupling and the reorganization energy in an unfavorable way. By using density functional theory (DFT) calculations, we have investigated the effects of fluorination and cyanation on the LUMO level and electron reorganization energy by varying the position of substitution, number of substituents, and size of the π-conjugated core. Our results show that, unlike fluorination, cyanation always reduces the electron reorganization energy. Adding more cyano groups further lowers the reorganization energy, but the combined effect depends on relative positions; for benzene, in particular, the electron reorganization energy is down by 70 meV upon first cyanation and by additional 70 meV upon second cyanation at meta-position.

Resume : Organic Semiconducting Single Crystals (OSSCs) are interesting candidates for organic electronics applications. The practical use of OSSCs in the field of ionizing radiation has been described, showing that OSSCs can be considered as a new generation of direct electrical sensors .Solution growth technique has several positive features in view of practical application of OSSCs, namely: (i) simplicity of the method, which can be scaled up using techniques like inkjet printing or screen printing ; (ii) high degree of crystal perfection, hence good reproducibility of the device performance ; (iii) versatility in tuning the crystal size, easily achievable by changing the growth parameters . The effect of systematic changes of some growth parameters (temperature, solution concentration) on the final crystals size, yield and quality has been studied. In particular, two different compounds (i.e., 4hydroxycyanobenzene, 4HCB, and , 2,4-dinitro-1-naphthol, MY) have been used. To provide statistically reliable results, several growth batches per experiment have been prepared. Each crystallization has been hence analyzed evaluating the following data: (i) number of single-crystals, (ii) number of poly-crystals, (iii) size of the crystals. The results have been rationalized based on thermodynamics, allowing to find the best growth conditions for the analyzed compounds with respect to desired applications (like X-rays detection). M.E. Gershenson et al. – Rev. Mod. Phys. – 78 – 20 B. Fraboni et al. – Adv. Mat. - Adv. Mater.- 24 - 2012 - 2289÷2293. H. Minemawari et al.– Nature – 475 – 2011 - 364÷367. A. Fraleoni et al.– Adv. Mat. – 21 – 2009 – 1835÷1839. A. Fraleoni et al. – J. Cryst. Growth – 312 – 2010 – 3466÷3472.

Resume : We describe a novel technique to synthesize aqueous-colloidal nano- and microcrystals with promising electronic properties. Using crystal growth achieved by controlled reprecipitation of dilute organic solutions in water in the presence of various surfactant molecules, tuning of parameters such as temperature, concentration, injection speed, and especially surfactant molecules leads to a wide range of crystal- size and shape with excellent colloidal stability in water. Surfactants used here range from typical soaps, like alkyl sulfonates, to biomolecules such as flavin mononucleotide, sugars, and phosphoric acids. Such hydrophilic organic crystals can be processed into high-quality semiconducting thin film devices. Conductivity and photoconductivity are found to be highly humanity dependent. Such materials are suitable for bioelectronics applications.

Resume : Many processes in organic devices, such as charge transport, are critically influenced by the packing of the molecules, and thus ultimately influenced by the crystal structure of the semi-crystalline semiconductors. Semi-crystalline materials are quite likely to exhibit different polymorphs, depending on the structure of the side chains, the molecular weight and the processing conditions. As a consequence, designing new materials for improved efficiency is a truly multi-parameter problem and often results in much lower efficiency than expected. Being able to predict the likely crystal polymorph of a given molecule as a function of temperature, or in other words drawing its phase diagram, is therefore very interesting, however very challenging. In this work, we chose as a model system 3-hexylthiophene (3HT) oligomers of different length. We chose to represent the molecules by a modified version of the force field by Moreno et al.1 We select a number of space groups that are consistent with the likely trans-geometry of the 3HT oligomers. We build an infinite crystal using periodic boundary conditions in all three directions. To find the unit cell parameters for each space group ( stacking, lamellar stacking distances and the unit cell angles), we scan the  stacking and the lamellar stacking space using molecular dynamics simulations. In order to rank the minimised crystal structures, we calculate the Gibbs free energy unlike most of the predictive method that are ranking crystal structure likelihood based on the potential energy. Finally, when we have our ranking of crystal structures as a function of temperature, we compare with a thermodynamic study of 3HT oligomers of different size.2,3This step allows us to confidently validate our method for predicting the crystal structures of oligomers and to ultimately accept or reject the proposed crystal structures. 1M. Moreno, M. Casalegno, G. Raos, S. V. Meille, R. J. PoJ.Phys. Chem. B, 2010, 114, 1591-1602. 2F. P. V. Koch, P. Smith, M. HeeneyJ. Am. Chem. Soc. 2013, 135, 13695-13698 3 F. P. V. Koch, M. Heeney, P. Smith J. Am. Chem. Soc. 2013, 135, 13699-13709

Resume : A series of 4-pyperidinyl-1,8-naphtalimide derivatives containing at the N-position an n-methylpyridine (n = 2, 3, or 4) have been synthesized and isolated as crystalline materials. These isomers were further reacted, in the solid state, with the co-former 1,4-diiodotetrafluorobenzene (I2F4) to give three new co-crystals of general formula n2∙I2F4. All crystalline materials have been thoroughly characterized in the solid state via single-crystal and powder X-ray diffraction, infrared spectroscopy and thermal methods; in addition, the photophysical properties of all compounds have been investigated in solution and in the solid state. The absorption and emission properties of all the derivatives show a significant solvent dependence. The fluorescence features of the solids are strictly related to the crystal structure and the different arrangement of the molecules in the solid. Co-crystallization led to important changes in the emission properties of the crystalline compounds, in particular for derivatives 3 and 4, for which a 2-fold enhancement and a 2-fold decrease, respectively, of the fluorescence quantum yield with respect to the corresponding crystals was observed.

Resume : The mobility in crystalline organic semiconductors is intrinsically limited by the presence of large thermal molecular motions, which are a direct consequence of the weak Van der Waals inter-molecular bonds. These lead to a breakdown of the usual assumptions of semiclassical transport, suggesting that strongly localized carriers are a more natural starting point than free carriers to describe charge transport in these materials. In this scenario, termed "transient localization", carrier localization occurs on short time scales and is eventually destroyed by the dynamical nature of molecular fluctuations, leading to an unconventional diffusive behavior. I will explore such “transient localization” mechanism of charge transport via both microscopic model calculations and a recently developed phenomenological model which provides useful analytical formulas for the analysis of experiments. I will show that the theory can consistently explain several characteristic experimental features of organic semiconductors, such as the low values of the mobility, its power-law temperature dependence, the crossover to a thermally activated behavior in the presence of extrinsic disorder, as well as the observed non-Drude like behavior of the optical conductivity. Finally, the analogies with the properties of organic conductors and superconductors suggest that the transient localization scenario could be a general characteristic of a much broader class of organic molecular crystals.

Resume : Band-like transport of delocalized charge carriers over a few molecules has been previously observed in high quality vapor-grown crystals1. However, description of band-like transport in a solution-processed n-type semiconductor has been elusive1. The present work2 focuses on a N,N’-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide (PDIF-CN2) molecule and shows that crystalline order, attainable through solution-based self-assembly, and specific molecular orientation, resulting in reduced dipolar disorder, leads to transport via delocalized carriers, and enhanced mobility. These findings have been obtained by comparing the electrical performance of conventional spincoated organic transistors with transistors consisting of either single or multiple supramolecular fibers, while carefully considering the dependence of electrical transport on structure at multiple length scales. Despite an earlier report on apparent band-like behavior for a solution processed p-type semiconductor, we provide here the first example of measurable band-like transport from an air stable solution processed n-type semiconductor. Further, we observe that the activation energy Ea is strongly related to crystalline order, with spin-coated and fiber-based devices exhibiting two distinct transport mechanisms. References 1. N. A. Minder, S. Ono, Z. Chen, A. Facchetti, A. F. Morpurgo, Adv. Mater. 2009,19,1–9. 2. J. M. Mativetsky, E. Orgiu, I. Lieberwirth, W. Pisula, P. Samori Adv. Mater. 2014, 26, 430–435

Resume : One class of lasing materials currently attracting considerable attention is organic semiconductors. They combine the simple manufacturing of plastics with favorable optoelectronic properties such as high photoluminescence quantum yield, strong absorption/gain, and broad spectra. Therefore, there is great interest in developing an electrically-pumped organic semiconductor laser (OSL), as it would provide a new class of lasers. Although organic light-emitting diode (OLED) displays are already available in the market, OSL remains a very challenging problem for conventional OLEDs. Particularly, for electrical excitation of OSLs, extremely high current density more than 1 kA/cm2 is required. However, the maximum current density of OLEDs are typically limited to 1-10 A/cm2 due to the effect of exciton quenching and photon loss processes and, consequently, electrical excitation of OSLs has not been realized. Recently, to address this limitation, we focus on two unique organic light-emitting devices, such as organic single-crystal light-emitting transistors and organic electrochemical light-emitting cells. These light-emitting devices have p-i-n homojunction with highly conductive active area owing to electro-static or electro-chemical carrier doping, which is irrealizable for OLEDs. As the results, we demonstrated the effective light emission with extremely high current density more than 1 kA/cm2, which is the first important milestone for future electrically-pumped OSLs.

Resume : Over the last decade, the employment of organic materials in the field of X-rays detection has gained the interest of the research community. However, only few examples of organic direct detectors have been reported and the intrinsic working principle of such devices is still under debate. We recently reported [1] on an Organic Semiconducting Single Crystal (OSSC), 4-hydroxycyanobenzene (4HCB), as direct X-ray detector, with a stable and linear response with increasing dose rate. Here we will discuss on an analysis carried out on different OSSCs, i.e. 1,5-dinitronaphthalene (DNN), 1,8-naphthaleneimide (NTI), an NTI derivative (NPy), TIPS-pentacene and Rubrene, differing by electrical transport properties, band-gap, geometry, molecular packing and polarizability, with respect to their radiation detection properties. The crystals were characterized as X-ray detectors employing a Mo target X-ray tube providing a wide range of dose rates (20-120mGy/s). Our aim is to investigate how the physical and chemical properties of the crystals affect the X-ray detection in order to get a deeper insight into the photon-charge conversion mechanism. Moreover, the high sensitivity even at low bias voltages (50V at maximum) together with the response linearity and the signal stability observed for most of the investigated crystals, confirm their potential exploitation for the realization of innovative low-cost and low-power X-ray detectors. [1] B. Fraboni et al., Adv. Mater., 24, 2289 (2012)

Resume : A water-gated organic field-effect transistor (WGOFET) whose gold gate is modified with a self-assembled monolayer of porcine odorant binding proteins (OBPs) is investigated to probe weak chiral interactions occurring between OBP and the carvone enantiomers. A highly sensitive transduction of the biomolecular interaction is achieved as the transistor output current is governed by the small capacitance of the protein layer undergoing minute changes as the ligand-protein complex is formed. The binding curves of the carvone enantiomers are distinguishable down to few tens of pM concentrations, showing that chiral differential detection can be achieved at extremely low concentrations with the WGOFET. Besides, accurate determination of the free-energy balances and of the capacitance changes associated with the binding process allows ultrasensitive determination of the interaction energies down to few KJ/mol as well as of the occurrence of conformational events associated with OBP ligand binding. This approach represents a unique tool with general applicability to measure pico-molar concentration of neutral ligand and to quantitatively investigate low-energy bio-chemical interactions. The role of the OBPs in the olfaction system is still under debate and the detection of neutral odorant species at the pM level by means of a WGOFET adds relevant pieces of information to the understanding of the odor perception mechanism at the molecular level.

Resume : Organic-based devices are currently attracting great attention for applications where low-cost, large area coverage and flexibility are required. In addition, the versatility of organic synthesis allows for the preparation of materials “à la carte”. That is, since by chemically modifying their molecular structure and functionality, the solid state structure and the resulting macroscopic properties are altered, it is feasible to synthesize tailored materials for specific uses. Tetrethiafulvalenes (TTF) have shown to exhibit excellent OFET mobility (up to 6 cm2/Vs) as well as easy processability. Since the TTF chemistry is very well-known, it is possible to synthesize a large variety of very similar molecules exhibiting different solid-state organizations or different electronic structure. Thus, this family of molecules represents a suitable platform to perform correlation studies. Here we will describe our recent work related to explore the influence of crystal structure, polymorphism, electronic structure and device configuration and interfaces in TTF OFETs. Further, we demonstrate the fabrication of large area coverage devices based on solution processed blends of a TTF derivative with an inert polymer that exhibit an excellent performance and exceptionally environmental stability.

Resume : We report on an artificial synapse, an organic synapse-transistor (synapstor) working at 1 volt and with a typical response time in the range 100-200 ms. This device (also called NOMFET, Nanoparticle Organic Memory Field Effect Transistor) combines a memory and a transistor effect in a single device. We demonstrate that short-term plasticity (STP), a typical synaptic behavior, is observed when stimulating the device with input spikes of 1 volt. Both significant facilitating and depressing behaviors of this artificial synapse are observed with a relative amplitude of about 50% and a dynamic response < 200 ms. From a series of in-situ experiments, i.e. measuring the current-voltage characteristic curves in-situ and in real time, during the growth of the pentacene over a network of gold nanoparticles, we elucidate these results by analyzing the relationship between the organic film morphology (grain size) and the transport properties. This synapstor works at a low energy of about 2 nJ/spike. We discuss the implications of these results for the development of neuro-inspired computing architectures and interfacing with biological neurons.

Resume : Metal/organic interfaces have always been a key factor for the performance and efficiency of organic electronic devices. Here, we employ first-principles calculations to investigate the structural and electronic properties of prototype organic materials (i.e. P3HT and PCBM) in the proximity of Ag. We identify three stable configurations for the adsorption of PCBM on Ag, depending on the position of PCBM’s functional group. The most stable adsorption is when the C60 cage points towards Ag and the functional group is lying parallel to the interface. Furthermore, we calculate the charge reordering that takes place at the interface upon adsorption and thus the respective interfacial dipole that is formed. We use the interfacial dipole, together with the intrinsic dipole of the adsorbed molecule, to assess the work-function (Wf) of the system. For PCBM, the Wf modification is found to depend strongly on the way that it is adsorbed. Specifically, PCBM with the functional group towards Ag increases the system’s Wf while with the C60 towards Ag, decreases it. That is of great importance for achieving the desired charge transfer. For crystalline P3HT, the preferred adsorbed geometry is with the thiophene backbone parallel to Ag surface. Adsorption energy is found higher than PCBM’s and a significant decrease of the total system’s Wf is reported. Results presented here, show that by controlling the adsorption details at the metal/organic interface, Wf can be finely tuned.

Resume : Charge carrier injection and transport are crucial in the operation of electronic devices, which depend highly on the electronic structure of the semiconductor. For organic semiconductors with structural and energetic disorder, a Gaussian Disorder Model (GDM) was proposed, which establishes that all states are localized where tunnelling and trapping occurs simultaneously. However, the model focuses on the transport mechanisms, and the role of charge carrier injection is overlooked in its application to device modelling. In particular, it has been shown that a Gaussian Density of States (DOS) may lead the organic solid to behave as a degenerate semiconductor, thus resulting in reducing the Injection Barrier (IB) at the electrode. Accordingly, a further analysis of the injection process is gaining in importance, particularly for organic solar cells operating at low voltage, injection-limited, regime. In this work, we investigate the effect of the electronic structure of the semiconductor on injection process, as well as the low voltage current density-voltage J(V) characteristics of Organic Rectifying Diodes (ORDs) based on GDM. Two analytical models for the J(V) characteristics under non-degenerate and degenerate approximation are developed and fitted to the experimental J(V) characteristics of an ORD (Au/pentacene/Al). It is found that the data are better fitted by the degenerate model, with more physically-meaningful parameters i.e. variance of the DOS, IBs, and mobility.