Gregory M. Smith

Frequency‐Dependent, Alternating Current‐Driven, Field‐Induced Polymer Electroluminescent Devices with High Power Efficiency

A significant enhancement in power efficiency for alternating current-driven field-induced polymer electroluminescent devices is demonstrated by employing a high-k ferroelectric polymer dielectric through impedance matching of the device with the driving source. A peak power efficiency of 34.1 lm W(-1) at a frequency of 65 kHz is achieved, which is 2 to 12 times higher than the previous highest reports.

Emission characteristics in solution-processed asymmetric white alternating current field-induced polymer electroluminescent devices

In this work, the emission characteristics of a blue fluorophor poly(9, 9-dioctylfluorene) (PFO) combined with a red emitting dye: Bis(2-methyl-dibenzo[f,h]quinoxaline)(acetylacetonate)iridium (III) [Ir(MDQ)2(acac)], are examined in two different asymmetric white alternating current field-induced polymer electroluminescent (FIPEL) device structures. The first is a top-contact device in which the triplet transfer is observed resulting in the concentration-dependence of the emission similar to the standard organic light-emitting diode (OLED) structure. The second is a bottom-contact device which, however, exhibits concentration-independence of emission. Specifically, both dye emission and polymer emission are found for the concentrations as high as 10% by weight of the dye in the emitter. We attribute this to the significant different carrier injection characteristics of the two FIPEL devices. Our results suggest a simple and easy way to realize high-quality white emission.

Effects of electrode modification using calcium on the performance of alternating current field-induced polymer electroluminescent devices

In this work, the effects of electrode modification by calcium (Ca) on the performance of AC field induced polymer electroluminescence (FIPEL) devices are studied. The FIPEL device with Ca/Al electrode exhibits 550 cd m−2, which is 27.5 times higher than that of the device with only an Al electrode (20 cd m−2). Both holes and electrons are injected from one electrode in our FIPEL device. We found that the electron injection can be significantly enhanced by a Ca modification on the Al electrode without greatly affecting the hole injection. Therefore, the electrons and holes can be effectively recombined in the emissive layer to form more excitons under the AC voltage, leading to effective light emission. The device emitted much brighter light than other AC-based organic EL devices. This result provides an easy and effective way to improve FIPEL performance.

Charge balance and photon collection in polymer based ternary bulk heterojunction photovoltaic devices containing cadmium selenide nanoparticles

Solar cells employing a ternary bulk heterojunction active layer comprised of poly(3-hexylthiophene) (P3HT), 6,6-phenyl C61-butyric acid methyl ester (PCBM) doped with composites constructed from a combination of 2.5 nm CdSe nanoparticles (NP), and methyl viologen (MV) have been examined. It was found that the devices containing the CdSe NP/MV composite exhibit significantly more photocurrent in a region surrounding the absorption peak of the particles (560-660 nm) when compared to pristine P3HT:PCBM devices. For a low ratio of CdSe to PCBM, the photocurrent collection was accompanied by space charge build up that limited the performance of the devices. When the ratio of CdSe to PCBM was raised, the space charge dissipated and performance recovered. JV curve shape analysis suggests that charge balance was achieved; however, electrode selectivity was reduced.

Achieving High Performance in AC-Field Driven Organic Light Sources.

Charge balance in organic light emitting structures is essential to simultaneously achieving high brightness and high efficiency. In DC-driven organic light emitting devices (OLEDs), this is relatively straight forward. However, in the newly emerging, capacitive, field-activated AC-driven organic devices, charge balance can be a challenge. In this work we introduce the concept of gating the compensation charge in AC-driven organic devices and demonstrate that this can result in exceptional increases in device performance. To do this we replace the insulator layer in a typical field-activated organic light emitting device with a nanostructured, wide band gap semiconductor layer. This layer acts as a gate between the emitter layer and the voltage contact. Time resolved device characterization shows that, at high-frequencies (over 40 kHz), the semiconductor layer allows for charge accumulation in the forward bias, light generating part of the AC cycle and charge compensation in the negative, quiescent part of the AC cycle. Such gated AC organic devices can achieve a non-output coupled luminance of 25,900 cd/m2 with power efficiencies that exceed both the insulator-based AC devices and OLEDs using the same emitters. This work clearly demonstrates that by realizing balanced management of charge, AC-driven organic light emitting devices may well be able to rival today’s OLEDs in performance.

Variation of the Side Chain Branch Position Leads to Vastly Improved Molecular Weight and OPV Performance in 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene/2,1,3-benzothiadiazole Copolymers

Through manipulation of the solubilizing side chains, we were able to dramatically improve the molecular weight (𝑀𝑤) of 4,8-dialkoxybenzo[1,2-b:4,5-b′]dithiophene (BDT)/2,1,3-benzothiadiazole (BT) copolymers. When dodecyl side chains (P1) are employed at the 4- and 8-positions of the BDT unit, we obtain a chloroform-soluble copolymer fraction with 𝑀𝑤 of 6.3 kg/mol. Surprisingly, by moving to the commonly employed 2-ethylhexyl branch (P2), 𝑀𝑤 decreases to 3.4 kg/mol. This is despite numerous reports that this side chain increases solubility and 𝑀𝑤. By moving the ethyl branch in one position relative to the polymer backbone (1-ethylhexyl, P3), 𝑀𝑤 is dramatically increased to 68.8 kg/mol. As a result of this 𝑀𝑤 increase, the shape of the absorption profile is dramatically altered, with 𝜆max = 637 nm compared with 598 nm for P1 and 579 nm for P2. The hole mobility as determined by thin film transistor (TFT) measurements is improved from ∼1×10−6 cm2/Vs for P1 and P2 to 7×10−4 cm2/Vs for P3, while solar cell power conversion efficiency in increased to 2.91% for P3 relative to 0.31% and 0.19% for P1 and P2, respectively.

Synthesis and characterization of Ar-annealed zinc oxide nanostructures

Nanostructured zinc oxide samples were synthesized through CVD and annealed in argon. The samples were investigated using SEM, TEM, XRD, and UV/VIS/FTIR photoacoustic spectroscopy. The SEM/TEM images show relatively spherical particles that form elongated, connected domains post-anneal. XRD measurements indicate a typical wurtzite structure and reveal an increase in average grain size from 16.3 nm to 21.2 nm in Ar-annealed samples over pristine samples. Visible photoacoustic spectra reveal the contribution of defect levels on the absorption edge of the fundamental gap of zinc oxide. The steepness parameter of the absorption edge, which is inversely proportional to the width of the absorption edge, decreased from 0.1582 (pristine) to 0.1539 (annealed for 90 minutes) revealing increased density of defect states upon annealing. The FTIR photoacoustic spectra show an intense peak at 412 cm-1 and a shoulder at 504 cm-1 corresponding to the two transverse optical stretching modes of ZnO. These results may indic...

Determining the effect of selenium substitution on low band-gap conjugated polymer-fullerene solar cells by optoelectronic characterization

The design of new, low band-gap polymers is a major part of organic photovoltaic research. Atomic substitution in a ring structure is one way to alter the electronic properties of a polymer (such as substituting the sulfur in a thiophene ring for selenium to make a selenaphene ring). We show that, starting with a benzodithiophene donor, that substituting benzoselenadiazole for benzothiodiazole leads to a polymer with a lower band-gap as characterized by the onset of optical absorption. Photovoltaic devices, optimized for 1,2-dichlorobenzene as the solvent, are comparable except that the polymer containing benzoselenidiazole collects over 1 mA/cm2 less short circuit current than the polymer containing benzothiodiazole, a counter intuitive result. Similar fill factors suggest that film morphology is not a contributor to this discrepancy. This result is further strengthened by optical time of flight measurements that show a better hole mobility for the benzoselenidiazole polymer (at room temperature and over a range of external fields). We then conducted further test in order to find the cause of this discrepancy.