Orb Acton

Development of New Conjugated Polymers with Donor−π-Bridge−Acceptor Side Chains for High Performance Solar Cells

Two new conjugated polymers have been designed and synthesized for polymer solar cells. Both of them exhibit excellent photovoltaic properties with a power conversion efficiency as high as 4.74%. Different from the traditional linear donor-acceptor (D-A) type conjugated polymers, these newly designed polymers have a two-dimensional conjugated structure with their tunable acceptors located at the end of D-A side chains and connected with the donors on the main chain through an efficient pi-bridge. This approach provides great flexibility in fine-tuning the absorption spectra and energy levels of the resultant polymers for achieving high device performance.

Interfacial modification to improve inverted polymer solar cells

We report improved device performance of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl C61butyric acid methyl ester (PCBM)-based inverted bulk-heterojunction (BHJ) solar cells through the modified interface of the TiO2/BHJ with a series of carboxylic acid functionalized self-assembled monolayers (SAMs). The SAMs reduce the series resistance and improve the shunt resistance of the cell leading to increased fill factor and photocurrent density. Different aspects of device improvement can be affected depending on the nature of the SAMs. Modification with a C60-SAM shows the largest enhancement leading to a 35% improvement (η = 3.78%) over unmodified inverted devices (η = 2.80%). This SAM serves multiple functions to affect the photoinduced charge transfer at the interface to reduce the recombination of charges, passivation of inorganic surface trap states, improve the exciton dissociation efficiency at the polymer/TiO2 interface as well as a template to influence the overlayer BHJ distribution of phases, morphology and crystallinity leading to better charge selectivity and improved solar cell performance.

Surface Doping of Conjugated Polymers by Graphene Oxide and Its Application for Organic Electronic Devices

Conjugated polymers are a novel class of solution-processable semiconducting materials with intriguing optoelectronic properties. [ 1 ] They have received great attention as active components in organic electronic devices such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and organic fi eld-effect transistors (OFETs) due to their light weight, facile tuning of electronic properties through molecular engineering, and ease of processing. The performance and lifetime of conjugated polymer-based electronic devices are critically dependent on the bulk properties of the active materials and the interfacial properties of electrode/polymer contacts. [ 2–4 ] In these devices, the electrode(s) either inject charge into or extract charges from the organic semiconductor layer(s). Mismatch of the work functions between metal or metal oxide electrodes and molecular orbital energy levels of organic semiconductors can lead to high contact resistance, which decreases the charge injection and extraction effi ciency. Therefore, it is essential to minimize contact resistance at the electrode/organic semiconductor interface. To improve charge injection/extraction across the electrode/ organic semiconductor interface, several strategies have been developed. One is to tune the interfacial dipole across the electrode/semiconductor interface to reduce the injection/collection energy barrier. This can be achieved by modifying the electrode surface with self-assembled dipolar molecules to tune the energy level alignment at the semiconductor/electrode interface. [ 5–7 ] Alternatively, the introduction of a thin layer of polymer surfactant that contains polar side chains between the conjugate polymer/electrode interface can also be used to improve the interfacial properties. The polar side chains can provide not

Multifunctional phosphonic acid self-assembled monolayers on metal oxides as dielectrics, interface modification layers and semiconductors for low-voltage high-performance organic field-effect transistors

Insulating and semiconducting molecular phosphonic acid (PA) self-assembled monolayers (SAMs) have been developed for applications in organic field-effect transistors (OFETs) for low-power, low-cost flexible electronics. Multifunctional SAMs on ultrathin metal oxides, such as hafnium oxide and aluminum oxide, are shown to enable (1) low-voltage (sub 2 V) OFETs through dielectric and interface engineering on rigid and plastic substrates, (2) simultaneous one-component modification of source-drain and dielectric surfaces in bottom-contact OFETs, and (3) SAM-FETs based on molecular monolayer semiconductors. The combination of excellent dielectric and interfacial properties results in high-performance OFETs with low-subthreshold slopes down to 75 mV dec(-1), high I(on)/I(off) ratios of 10(5)-10(7), contact resistance down to 700 Ω cm, charge carrier mobilities of 0.1-4.6 cm(2) V(-1) s(-1), and general applicability to solution-processed and vacuum-deposited n-type and p-type organic and polymer semiconductors.

Conjugated polymers based on C, Si and N-bridged dithiophene and thienopyrroledione units: synthesis, field-effect transistors and bulk heterojunction polymer solar cells

A series of low band-gap conjugated polymers (PDTC, PDTSi and PDTP) containing electron-rich C-, Si-, and N-bridged bithiophene and electron-deficient thienopyrroledione units were synthesized viaStille coupling polymerization. All these polymers possess a low-lying energy level for the highest occupied molecular orbital (HOMO) (as low as −5.44 eV). As a result, photovoltaic devices derived from these polymers show high open circuit voltage (Voc as high as 0.91 V). These rigid polymers also possess respectable hole mobilities of 1.50 × 10−3, 6.0 × 10−4, and 3.9 × 10−4 cm2 V−1s−1 for PDTC, PDTSi, and PDTP, respectively. The combined high Voc and good hole mobility enable bulk hetero-junction photovoltaic cells to be fabricated with relatively high power conversion efficiency (PCE as high as 3.74% for the PDTC-based device).

Graphene oxide nanosheets based organic field effect transistor for nonvolatile memory applications

Reversible switching characteristics of organic nonvolatile memory transistors (ONVMTs) using chemically synthesized graphene oxide (GO) nanosheets as a charge-trapping layer are reported. The transfer curves of GO based ONVMTs showed large gate bias dependent hysteresis with threshold voltage shifts over 20 V. After writing and erasing, stored data were well maintained showing more than two orders of ON/OFF ratio (ION/IOFF=∼102) for 104 s. These results suggest that GO nanosheets are one potential candidate as the charge-trapping layer in ONVMTs.Reversible switching characteristics of organic nonvolatile memory transistors (ONVMTs) using chemically synthesized graphene oxide (GO) nanosheets as a charge-trapping layer are reported. The transfer curves of GO based ONVMTs showed large gate bias dependent hysteresis with threshold voltage shifts over 20 V. After writing and erasing, stored data were well maintained showing more than two orders of ON/OFF ratio (ION/IOFF=∼102) for 104 s. These results suggest that GO nanosheets are one potential candidate as the charge-trapping layer in ONVMTs.

Low-voltage organic thin-film transistors with π-σ-phosphonic acid molecular dielectric monolayers

Pentacene-based organic thin-film transistors (OTFTs) have been fabricated using π-σ-phosphonic acid self-assembled monolayers (SAMs) on top of aluminum oxide as the gate dielectrics. With ultrathin dielectrics, high capacitances up to 760nF∕cm2 and low leakage current densities of 10−8A∕cm2 at 2V could be obtained, allowing operation of OTFTs within −3V. Vast improvements in the gate leakage current (∼2 orders), on/off current ratio (1 order), and subthreshold slope down to 85mV∕decade are achieved compared to control devices without SAMs. The OTFTs with pentacene vapor deposited at room temperature on SAM dielectrics-modified substrates exhibit mobilities of 0.14–0.30cm2∕Vs, on/off current ratios of 105, and threshold voltages of −(1.3–1.5)V.

Dielectric surface-controlled low-voltage organic transistors via n-alkyl phosphonic acid self-assembled monolayers on high-k metal oxide.

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO(2)) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO(2) with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO(2) hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 x 10(6), threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec(-1), and field-effect mobilities as high as 1.8 cm(2) V(-1) s(-1).

All‐Organic Photopatterned One Diode‐One Resistor Cell Array for Advanced Organic Nonvolatile Memory Applications

ONVM is one emerging technology that has been explored as the next generation data storage media. In literature, there are numerous reports regarding the switching mechanisms for ONVM devices, [ 15–18 ] as well as the development of new organic materials [ 15 , 18–21 ] and device architectures [ 22–27 ] for ONVM applications. Research on novel device architectures can potentially generate more reliable organic memory devices with higher density and improved performance, while simultaneously mitigating misreading (cross-talk) issues. For these reasons, organic one diode-one resistor (1D–1R) type devices have been demonstrated. [ 23,24 ] However, these devices exhibit irreversible switching behavior and are nonpatternable, which strongly limits their application in systems that require both an array architecture and rewritable memory capability. Recently, we have reported a hybrid type 1T–1R and 1D–1R device architecture, which is composed of a hybrid ONVM device with Si-based transistors or Si schottky diodes. [ 25,26 ]

Spin-cast and patterned organophosphonate self-assembled monolayer dielectrics on metal-oxide-activated Si.

An efficient process is developed for modifying Si with self-assembled monolayers (SAMs) through in situ metal oxide surface activation and microcontact printing or spin-coating of phosphonic-acid-based molecules. The utility of this process is demonstrated by fabricating self-organized and solution-processed low-voltage organic thin-film transistors enabled by patterned and spin-cast phosphonate SAM/metal oxide hybrid dielectrics.