Quantum efficiency of ambipolar light-emitting polymer field-effect transistors

This thesis describes the study of the development and device physics of organic field-effect transistors and organic light-emitting field-effect transistors. Using chemical and physical properties of organic semiconductors such as light emission, multi-functional devices, or “Active Channel Field-Effect Transistors” can be created. This thesis first describes the creation of basic ambipolar FETs before building on this knowledge to develop ambipolar as well as hole-dominated light emitting field-effect transistors. Ambipolar FETs using a diketopyrrolopyrrole-based copolymer were first investigated in this thesis. When gold electrodes were used, the devices could transport only holes. The charge injection was altered by changing the contacts to aluminium. The devices then were able to conduct both electrons and holes with maximum electron and hole mobilities of order 10−4 cm2 V−1 s−1 and 10−3 cm2 V−1 s−1 respectively. Light-emitting field-effect transistors were first studied using the model emissive polymer Super Yellow. Charge injection was studied by altering the electron-injecting contact. Materials used included the low work function metals Ca, Ba and Sm, the inorganic salt Cs2CO3 and the organic material TPBi. Ambipolar devices were created using a neat Super Yellow layer. Electron and hole mobilities in these devices were both of the order of 10−4 cm2 V−1 s−1. The maximum external quantum efficiency was 1.2 ± 0.2 % with a Sm electron-injecting contact, however this occurred at a brightness of less than 1 cd m−2. A maximum brightness of 43 ± 9 cd m−2 was obtained using a Cs2CO3-Ag electron-injecting contact in electron accumulation mode. At this brightness, the external quantum efficiency was 0.19 ± 0.04 %. Unipolar devices were also fabricated by adding high mobility charge transport layer of PBTTT underneath the Super Yellow. Since PBTTT had a hole mobility that was orders of magnitude greater than the charge carrier mobilities of Super Yellow, charges were transported in the PBTTT, and recombined within the Super Yellow layer. Super Yellow acted as an emissive layer only. Using this bilayer device architecture, a maximum brightness of 100 ± 4 cd m−2 was obtained with a Ca electron-injecting contact. An external quantum efficiency of 0.04 ± 0.01 % was achieved at the maximum brightness. In order to improve the external quantum efficiency of LEFETs, phosphorescent materials were chosen for use as the emissive layer. A co-evaporated layer of the iridium complex Ir(ppy)3 in a host of CBP was initially used in the devices. Charge injection was studied by using either a Ba or a TPBi/Ba electron-injecting contact. Devices were first fabricated using PBTTT as a hole-dominated charge transport layer. Using a TPBi/Ba electron-injecting contact, the maximum brightness achieved was 200 ± 40 cd m−2 with an external quantum efficiency of 0.18 ± 0.01 % at the maximum brightness. Devices were then fabricated using a charge transport layer of DPP-DTT to create ambipolar devices. Electron and hole mobilities in these devices were both of order 10−2 cm2 V−1 s−1. The maximum external quantum efficiency was 0.4 ± 0.2 % at the maximum brightness of 500 ± 200 cd m−2 in hole accumulation mode using an electron-injecting contact of TPBi/Ba. In order to avoid using a co-deposited emissive layer, a solution-processed iridium-cored dendrimer layer that could be deposited without a host material was used. Like in the previous work, a charge transport layer of PBTTT was deposited underneath the emissive layer. Charge injection in the devices was altered using a change in the device geometry. Non-planar source and drain contacts were used, where one contact was deposited directly on top of the charge transport layer and the other contact on top of the emissive layer. Planar contacts were also used as control devices. There was clear evidence for transport of both holes and electrons when planar contacts were used, with light emission occurring at both contacts. The maximum mobility, brightness and external quantum efficiency was achieved using the non-planar contact geometry. Holes could be directly injected into the charge transport layer, rather than via the emissive layer. The hole mobility in these devices was 3.5 × 10−3 ± 6 × 10−4 cm2 V−1 s−1. The maximum brightness was 140 ± 50 cd m−2 with an external quantum efficiency of 0.11 ± 0.02 % at the maximum brightness. This thesis describes work on the study of charge injection into organic field-effect transistors by changing either the material used for the electron-injecting contact, or by changing the device geometry. This was achieved in devices that were either dominated by holes, or showed evidence of both electron and hole transport within the device. By changing the charge injection into the device, it was possible to improve the charge carrier mobilities, the brightness or the external quantum efficiency. Improving one or more of these things brings light emitting transistors towards becoming a viable technology.

We have investigated the electronic structure and carrier mobility of monolayer black phosphorus nanoribbons (BPNRs) using density functional theory combined with Boltzmann transport method with relaxation time approximation. It is shown that the calculated ultrahigh electron mobility can even reach the order of 10(3) to 10(7) cm(2) V(-1) s(-1) at room temperature. Owing to the electron mobility being higher than the hole mobility, armchair and diagonal BPNRs behave like n-type semiconductors. Comparing with the bare BPNRs, the difference between the hole and electronic mobilities can be enhanced in ribbons with the edges terminated by H atoms. Moreover, because the hole mobility is about two orders of magnitude larger than the electron mobility, zigzag BPNRs with H termination behave like p-type semiconductors. Our results indicate that BPNRs can be considered as a new kind of nanomaterial for applications in optoelectronics, nanoelectronic devices owing to the intrinsic band gap and ultrahigh charge mobility.

Recently there has been much interest in combining the fields of organic electronics and spintronics. This has been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes. Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new phenomena which must be explained with novel physics, here we explore one of these phenomena, organic magnetoresistance (OMAR). OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a large change in resistance (up to 25%) at low magnetic fields ( 20mT). OMAR represents a scientific puzzle since no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of the sign change since understanding this unique property should be a major step in unraveling the microscopic origin of OMAR. We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule and polymer devices, in which we observed sign changes as functions of voltage and temperature. These devices showed a strong correlation between the sign change and the onset of minority charge carrier injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign. From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR. To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons. The models show, counter-intuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. We show that these models describe the observed MR(V) behavior very well, and if one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature. The device model also predicts how different device parameters affect the observed MR, to test its predictions we performed experiments in which we increased the charge recombination by dye doping the organic active layer, we also observed how changing the charge injection by altering the organic semiconductor/ metal contacts experimentally compared with the device model. The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR.

Achieving simultaneous enhancement of electron and hole mobility in organic semiconductors (OSCs) using a single additive remains a significant challenge. In this study, we present a unique additive strategy that enables concurrent improvement of both n- and p-type transport in an n-dominant ambipolar polymer incorporating para-azaquinodimethane and diketopyrrolopyrrole. By individually applying various additives including ionic, p-type, and n-type compounds, we achieve unprecedented enhancements of both electron (∼400%) and hole (∼100%) mobility. Such parallel improvement is unattainable via conventional electron transfer mechanism. We ascribe this effect to optimized film morphology, reduced activation energy, and lowered contact resistance. To further elucidate additive-induced variations in electronic structure and guide future molecular design, density functional theory calculations reveal that incorporating para-azaquinodimethane into the polymer backbone facilitates strong orbital coupling with additives. This coupling introduces additional charge transport pathways between polymer segments, enhancing both electron and hole transport along lamellar and π-π stacking directions. Our findings suggest promoting robust orbital coupling between host polymers and additives offers a promising strategy to concurrently boost electron and hole mobility in a single OSC, effectively circumventing traditional limitations associated with separate p- and n-type additives/dopants.

In this dissertation, a series of polythiophene copolymers have been synthesized to study to photovoltaic characteristics. First of all, we have used Grignard metathesis polymerization to successfully synthesize a series of regioregular polythiophene copolymers that contain electron-withdrawing and conjugated phenanthrenyl-imidazole moieties as side chains. The introduction of the phenanthrenyl-imidazole moieties onto the side chains of the regioregular polythiophenes increased their conjugation lengths and thermal stabilities and altered their band gap structures. The band gap energies, determined from the onset of optical absorption, could be tuned from 1.89 eV to 1.77 eV by controlling the number of phenanthrenyl-imidazole moieties in the copolymers. Moreover, the observed quenching in the photoluminescence of these copolymers increases with the number of phenanthrenyl-imidazole moieties in the copolymers, owing to the fast deactivation of the excited state by the electron-transfer reaction. Both the lowered bandgap and fast charge transfer contribute to the much higher external quantum efficiency of the poly(3-octylthiophene)-side-chain-tethered phenanthrenyl-imidazole than that of pure poly(3-octylthiophene), leading to much higher short circuit current density. In particular, the short circuit current densities of the device containing the copolymer having 80 mol % phenanthrenyl-imidazole, P82, improved to 14.2 mA/cm2 from 8.7 mA/cm2 for the device of pure poly(3-octylthiophene), P00, an increase of 62%. In addition, the maximum power conversion efficiency improves to 2.80% for P82 from 1.22% for P00 (pure P3OT). Second, intramolecular donor–acceptor structures prepared by binding conjugated octylphenanthrenyl-imidazole moieties covalently onto the side chains of regioregular poly(3-hexylthiophene)s exhibit lowered bandgaps and enhanced electron transfer. For instance, conjugating 90 mol% octylphenanthrenyl-imidazole moieties onto poly(3-hexylthiophene) chains reduced the optical bandgap from 1.91 to 1.80 eV, and the electron transfer probability was at least twice than that of pure poly(3-hexylthiophene) when blended with [6,6]-phenyl-C61-butyric acid methyl ester. The lowered bandgap and the fast charge transfer both contribute to the much higher external quantum efficiencies—and, thus, much higher short-circuit current densities—for the copolymers presenting octylphenanthrenyl-imidazole moieties, relative to those of pure poly(3-hexylthiophene)s. In particular, the short-circuit current density of a device containing the copolymer presenting 90 mol% octylphenanthrenyl-imidazole moieties improved to 13.7 mA/cm2 from 8.3 mA/cm2 for the device containing pure poly(3-hexylthiophene)—an increase of 65%. In addition, the maximum power conversion efficiency was 3.45% for the copolymer presenting 90 mol% octylphenanthrenyl-imidazole moieties. Finally, PHPIT, a new kind of intramolecular D–A side-chain-tethered hexylphenanthrenyl-imidazole polythiophene has been synthesized. The visible light absorption of the PHPIT/PCBM blend is enhanced by the presence of the electron-withdrawing hexylphenanthrenyl-imidazole. The EQE of the device was maximized when the PHPIT/PCBM blend experienced annealing at 120 °C for 30 min. The more-balanced electron and hole mobilities and the enhanced visible and internal light absorptions in the devices consisting of annealed PHPIT/PCBM blends both contributed to a much higher short-circuit current density, which in turn led to a power conversion efficiency as high as 4.1%, despite the fact that PHPIT is only comprised of ca. 20 repeating units.

Grain Boundary Controlled Electron Mobility in Polycrystalline Titanium Dioxide

The device physics of inverted all-polymer solar cells based on a blend of the polymers poly(3-hexylthiophene) and poly(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)2,1,3-benzothiadiazole]-2′,2″-diyl) is investigated. In particular, the influence of active layer thickness on device parameters is investigated and compared to that of devices with a standard geometry. Increasing the active layer thickness from 70 to 480 nm is found to increase the open circuit voltage from 0.1 to 0.71 V and the external quantum efficiency (EQE) from 7% to 24%. In contrast, an optimum EQE of about 25% for the standard geometry is found for a film thickness of 70 nm, which decreases sharply with increasing active layer thickness. The shape of the EQE spectra of standard geometry devices also become severely distorted with increasing active layer thickness, with a minimum in EQE coinciding with the wavelength corresponding to maximum light absorption. In contrast, the shape of the EQE spectra of inverted devices remains essentially unchanged with increasing active layer thickness. Optical simulations of light absorption in the active layer have also been performed and demonstrate that the distortion in the EQE spectra of thicker standard geometry devices is consistent with photoexcitations created in the back half of these devices being more efficiently harvested than those in the first 100 nm of the inverted device. Furthermore, the fact that the EQE spectrum of inverted devices does not significantly broaden with increasing thickness suggests that harvesting of photoexcitations remains efficient in the front half of the device where most of the light is absorbed. Device modeling is employed to demonstrate that the lower mobility of electrons (and electron trapping) causes a favorable redistribution of the internal electric field in the inverted device with electric field increasing near the transparent electrode coinciding with the region of maximum light absorption. In contrast, in the standard device the internal electric significantly decreases near the transparent electrode causing a reduction in field-dependent charge separation and increased bimolecular recombination. Our results demonstrate that inverted devices may be an effective way to overcome losses in organic solar cells where electron mobility is typically lower than hole mobility.

We report on the ambipolar charge transport modulation of organic semiconductors (OSCs) in field-effect transistors (FETs) based on joint experimental and theoretical studies. We investigated pentacene, one of the most widely studied OSCs, as an example to achieve efficient and balanced ambipolar FETs (i.e., electron mobility is as fast as hole mobility) via fine control of the interfacial properties of the active channel. Through elimination of the interfacial tension of the active channel, ultrahigh performance pentacene-based ambipolar FETs were achieved, with electron and hole mobilities both near 3.0 cm2 V−1 s−1 in the same channel. Theoretical investigation of the pentacene molecule showed that the electron density is mainly located on the terminal ends but the hole density is on the centered three aromatic rings, a finding that provides insight into the difficulty of observing high electron mobility levels compared with hole mobility in FETs. Analysis of theoretical vibrational reorganization energy supports that electron transport between pentacene molecules suffers from more serious interfacial effects than hole transport. With the help of theoretical calculations, Raman spectroscopy can be used to characterize pentacene films at an early stage of growth at the nanometer scale to assess the ambipolar transport potential in FETs. The results demonstrate an excellent connection between ambipolar characteristics in actual organic FETs and microscopic molecular-level properties, thereby advancing the design and synthesis of new OSCs for electronic and photonic applications.

This paper will review strain-Si and strain-Ge formation by ion implantation and advanced annealing to form localized tensile and compressive strain channel for improved electron and hole mobility in Group IV materials. Carbon implant was used to reduce the surface Si lattice constant forming Si+C material for localized compressive strain-Si channel. Ge implant was used to increase the surface Si lattice constant forming Si+Ge material for localized tensile strain-Si channel and Sn implant to increase the surface Ge lattice constant forming Ge+Sn material for localized tensile strain-Ge channel. Use of ion implant and advanced annealing for strain formation resulted in tensile strain-Si+Ge improving hole mobility by 4x and electron mobility by 1.5x while tensile strain-Ge+Sn improved electron mobility by 2-2.5x.

We present a study of electron and hole mobility in blends of the solution-processed small molecule (DTS(PTTh2)2):fullerene as a function of the fullerene content. The mobilities are measured using the Field–Effect Transistor device configuration with the blend as the material in the channel. The hole mobility of the small molecule increases when about 10% of [6-6]-phenyl C71 butyric acid methyl ester (PC70BM) is added and then decreases at higher fullerene content. The electron mobility always increases as the fullerene content increases. Balanced charge transport occurred at 70:30 and 60:40 donor to acceptor ratios in the DTS(PTTh2)2:PC70BM system. The observation of balanced charge mobility agrees well with the reported solar cell performance.

Electronic structures and charge carrier mobilities of boron-graphdiyne sheet and nanoribbons

We examine the electron mobility and hole mobility at the Si/buried oxide (BOX) interface at which the valley splitting of the electron system is strongly enhanced, and compare the values observed to those at a standard Si/thermal oxide (T-SiO2) interface in the same silicon-on-insulator device. In contrast to the electron mobility, which is lower at the Si/BOX interface, the hole mobility at the Si/BOX interface is found to be slightly higher than that at the Si/T-SiO2 interface.

The mobility of electrons and holes has been deduced in the temperature range −100 to 60 °C from transit time measurements. The electron mobility in solid benzene can be described by μ ∝ T−2, which may be explained by interaction with acoustic phonons. The hole mobility in solid benzene rises exponentially with increasing temperature. The mobility is controlled by interaction with traps depending on the purity of the samples. Approaching the melting point there is a reduction of the electron and hole mobility. This may be explained by thermally produced lattice defects which show up already some ten degrees below the melting point. In liquid benzene the mobilities are three orders of magnitude smaller rising slightly upon temperature. In the liquid polaron‐like states are formed, where the charge carriers propagate by an intermolecular hopping mechanism.

Gate-all-around (GAA) silicon (Si) nanowires (NW) field effect transistor is today considered as a valuable alternative to FinFET architecture for advanced CMOS devices. Recent advances in characterization and integration have enabled the fabrication of nanowires with diameter below 10 nm. Although nanowires transistors with excellent electrostatic control can be achieved, the nanoscale dimensions induce quantum confinement effects and confer to nanowires electrical properties which differ from bulk Si. Then, low-field electron and hole mobility in Si NW transistors fabricated on (100) silicon-on-insulator (SoI) substrates have been measured over a wide range of NW width (8 nm ≤ W ≤ 220 nm) and height (6 nm ≤ Hfin ≤ 24 nm). A significant increase of hole effective mobility in thick and narrow Si NWs is evidenced along the [110] direction. In contrast, a decrease of NW width results in an electron mobility reduction. An electron mobility enhancement in n-FET NWs is achieved through the use of tensile strained-SoI (sSoI) substrates. As compared to unstrained-Si, +85% electron mobility enhancement is observed in uniaxially tensile strained-Si NW transistors. We have otherwise studied dual-channel CMOS co-integration, with Si channel n-FET and compressively-strained SiGe channel p-FET NWs. For p-FETs, the efficiency of compressive SiGe channels is evidenced for improving hole mobility in nanowires thanks to a Ge enrichment process. As compared to Si channel, the hole mobility is enhanced by 135% in p-FET NWs. Efficiency of compressive SiGe channel is evidenced for ultra-scaled p-FET NWs (LG = 15 nm) with +90% ION current improvement compared to Si. [110]-oriented NWs are shown to be the best candidates to improve electron and hole mobility under uniaxial tensile and compressive strain, respectively. Finally, in order to increase still further the drive current per layout footprint, GAA stacked-nanowire/nanosheet (NS) MOSFET are discussed in this paper. We will present intrinsic performance and design considerations of GAA structures that will allow to make this architecture a competitive solution for futures technology nodes. The key technological challenges (such as 3D integration process including inner spacer and strain engineering) will be discussed in relation to recent research results. We will show that more flexibility can be achieved with stacked-nanosheet transistors in order to manage power and performance optimization.

A solution processed n‐channel zinc oxide (ZnO) field effect transistor (FET) was fabricated by simple dip coating and subsequent heat treatment of a zinc acetate film. The field effect mobility of electrons depends on ZnO grain size, controlled by changing the number of coatings and zinc acetate solution concentration. The highest electron mobility achieved by this method is 7.2 cm2 V−1 s−1 with On/Off ratio of 70. This electron mobility is higher than for the most recently reported solution processed ZnO transistor. We also fabricated bilayer transistors where the first layer is ZnO, and the second layer is pentacene, a p‐channel organic which is deposited by thermal evaporation. By changing the ZnO grain size (or thickness) this type of bilayer transistor shows p‐channel, ambipolar and n‐channel behavior. For the ambipolar transistor, well balanced electron and hole mobilities are 7.6 × 10−3 and 6.3 × 10−3 cm2 V−1 s−1 respectively. When the ZnO layer is very thin, the transistor shows p‐channel behavior with very high reversible hysteresis. The nonvolatile tuning function of this transistor was investigated.