The transport properties of two photovoltaic polymers,
poly(3-hexylthiophene) (P3HT) and poly(2,7-carbazole)
derivative (PCDTBT), and their polymer:fullerene bulk heterojunction (BHJ) are studied by space-charge-limited current
(SCLC), dark-injection space-charge-limited current (DI-SCLC), and admittance spectroscopy(AS). For a nominally
hole-only device, electrons leakage occurs. This results in a current larger than the theoretical SCLC and ill-defined
DI-SCLC and AS signals. In order to prevent electron leakage, a
hole-transporting but electron blocking/trapping thin
layer is added between active layer and Au. The layer composed of copper phthalocyanine (CuPc) doped into an
amine-based small molecule. Using this interlayer, well-defined carrier transit time can be obtained for mobility
extraction. With a suitable interlayer to suppress undesirable carrier injection and transport, these techniques should find
broad applications in the transport characterization of narrow gap photovoltaic polymers and BHJ blends.
We describe how to use the thin-film transistor (TFT) technique to quantify carrier transport of amorphous organic semiconductors relevant to organic electronic devices. We have chosen several amorphous materials, including arylamine compounds, 4,4′-N,N′-dicarbazole-biphenyl (CBP), and a phosphorescent dye molecule [Ir(ppy)3] for investigations. Generally, the field effect (FE) mobility was found to be about one order of magnitude smaller than that obtained from an independent time-of-flight (TOF) technique. For N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and N,N′-Bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene (spiro-TPD), the FE mobilities were found to be 1.7×10−5 and 1.3 ×10−5cm2/Vs, respectively. Temperature-dependent measurements were carried out to study the FE mobility. It was found that the energetic disorder increased in the neighborhood of a gate dielectric layer. This factor is one of the origins causing the discrepancy between TFT and TOF mobilities. We also examined how the hole transport of CBP is affected by Ir(ppy)3 when it is doped into CBP.
The charge transport properties of organic electronic materials are of critical importance for the understanding of the operation of organic photonic devices. We show in this study that admittance spectroscopy (AS), which is a measure of the frequency dependent capacitances of organic electronic materials, can be used to determine charge carrier mobilities and transport parameters in organic photovoltaic materials. A plot of the negative differential susceptance versus frequency yields a maximum at a frequency. The position of the maximum is related to the average carrier transit time. Using AS, we study the hole transport properties of thin films of poly(3-hexylthiophene) (P3HT) based materials for organic photovoltaic applications, including pristine P3HT, and P3HT doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We demonstrate that AS can be used to extract hole mobilities of P3HT and P3HT:PCBM with a thickness of ∼350 nm. For P3HT:PCBM, copper phthalocyanine is used as an electron trapping layer to reduce the effect of negative capacitance.
We use dark-injection space-charge-limited current (DI-SCLC) and admittance spectroscopy (AS) to study charge
injection and transport properties of spiro-linked arylamine compounds. Examples are spiro-linked TPD and spiro-linked
NPB which are both important amorphous hole transporters for organic light-emitting diodes (OLEDs). With
PEDOT:PSS as the hole-injection layer, the contact is demonstrated to be generally Ohmic. Both techniques can be used
to measure the carrier mobilities of thick films (>μm) of
spiro-compounds. DI-SCLC can be viewed as a pulse technique
whereas AS can be treated as an ac technique. For the hole mobilities evaluation of the spiro-compounds, both
techniques are in general agreement to each other. However, for the case of the thinner films of less than 1μm, AS is
superior over the DI-SCLC technique. The advantages of AS will be highlighted. It appears that AS is broadly applicable
to characterize transport properties of organic thin films.
This contribution examines in details the effects of dopants on the hole transporting properties of N,N'-
diphenyl-N,N'-bis (1-naphthyl) (1,1'-biphenyl)- 4,4'diamine (NPB). Dopants for NPB are copper phthalocyanine
(CuPc),
4-(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryle)-4H-pyran (DCM1), 4-dicyanomethylene-2-methyl-
6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij]
quinolizin-8-yl)vinyl]-4H-pyran (DCM2), 2-(4-biphenyl0-5-(4-
tertbutylphenyl)-1,3,4-oxadiazole (tBu-PBD) and 2,9-dimethyl-4,
7-diphenyl-1,10-phenanthroline (BCP). The effects of
these dopants on the hole transport of NPB will be presented. Generally, the dopant molecules behave like hole traps or
scatterers. Their detailed behaviors are determined by their highest occupied molecular orbitals relative to that of NPB.
Traps are found to induce significant reduction in hole mobility. However, hole scatterers only alter the mobility slightly.
Two different underlying charge transport mechanisms are proposed and then it is further examined by temperature
dependent measurements.
We demonstrate that organic thin film transistors (OTFTs) can act as an alternative tool for carrier mobility evaluation in
amorphous organic electronic materials. OTFT is a three terminal device which can be operated with an active layer of
film thickness thinner than 100 nm. The materials under investigation are phenylamine-based (PA) compounds, which
are amorphous hole transporting materials widely used in organic light emitting diodes (OLEDs). The field effect (FE)
mobilities of PA compounds (hole) were determined in a TFT configuration. For the case of
N,N'-diphenyl-N,N'-bis(1-naphthyl)
(1,1'biphenyl)-4,4'diamine (NPB), the FE mobility was found to be 2 × 10-5 cm2/Vs. It is about one order of
magnitude smaller than that obtained from independent time-of-flight (TOF) technique (2 x 10-4 cm2/Vs) using a thick
film of ~ 5 μm. Temperature dependent measurement was performed under temperature ranging from 235 to 360 K. The
extracted energetic disorder by means of the Gaussian Disorder Model from OTFT was 85meV, which was larger than
that of TOF (~74 meV). Similar observations were found in other PA compounds. The increase in the extracted disorder
parameter in TFT configuration was one of the origins of the discrepancy between the FE and TOF mobility. OTFTs can
be regarded as a useful tool for carrier mobility evaluation with little material consumption.
We studied the carrier injection and transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-
4,4'diamine (NPB), a common hole transporter for organic light-emitting diodes (OLEDs). NPB was found to possess
significant electron mobility from time-of-flight (TOF) measurement. With bipolar transporting ability, NPB was used to
fabricate single-layer devices with a configuration of ITO/ PEDOT:PSS/ NPB/ Ca/ Ag. PEDOT:PSS was demonstrated
to form a quasi-Ohmic contact to NPB by admittance spectroscopy (AS) and dark-injection space-charge-limited current
(DISCLC) measurements. From current-voltage (JV) characteristics, single-layer NPB devices exhibited a bulk-limited
hole current in low-voltage region. Electron injection was clearly observed at a turn-on voltage of about 4V, which
coincided with the luminance-voltage measurement. In order to confine the recombination zone, dye-doped NPB layer
was inserted into single-layer devices. This intentional doping technique made a notable improvement in current
efficiency. The mechanisms of the doped devices were also addressed.
We demonstrate that poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) can act as an
excellent hole injection material for small organic charge transporters. With PEDOT:PSS as a conducting anode, it is
possible to achieve nearly Ohmic hole injection contacts to phenlyamine-based materials with HOMO values of up to
5.5 eV. In current-voltage experiment, the PEDOT:PSS anode can achieve nearly Ohmic hole injection to NPB (N,N'-
diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine), and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl) (1,1'-
biphenyl)-4,4'diamine). Meanwhile, dark-injection space charge limited current (DI-SCLC) transients are clearly
observed and are used to evaluate the charge-carrier mobility of these phenylamine compounds. The carrier mobilities
extracted by DI-SCLC are in excellent agreement with independent time-of-flight (TOF) technique. It is conceivable that
PEDOT:PSS can be used as a general conducting anode for the electrical characterizations of organic materials that
require Ohmic hole contacts.
A fluorescent star-shaped oligomer with a nitrogen atom as a core and
both a hole transporting arylamine and an electron transporting
1,3,4-oxadiazole moiety, tri(4-(5-phenyl-1, 3, 4-oxadiazol-2-yl)phenyl)amine
(TPOPA), has been designed and synthesized using a 5-step reaction procedure. The
synthesized compound was characterized by elemental analysis, 1H-NMR and mass
spectroscopy. Thermogravimetric (TG) and differential scanning calorimetry (DSC)
analysis show that TPOPA exhibits high thermal stability (Td, 373°C) and high
glass-transition temperature (Tg, 116 °C). Photoluminescence measurements indicate
that the star-shaped oligomer shows intense blue emission peaked at 445 nm with a
high quantum yield of 0.68 under near UV light excitation. The HOMO value of
TPOPA is -5.64 eV and the LUMO is -2.58 eV based on the electro-chemical
determinations. Reversible anodic oxidation results suggest that the hole-transporting
is predominant for TPOPA. Two single layered devices were fabricated by vacuum
evaporation with configurations of ITO / CuPC (15 nm) / TPOPA (95 nm) / Ca(30 nm)
/ Al(100 nm) (device 1) and ITO / CuPC(15 nm) / TPOPA (175 nm) / Ca(30 nm) /
Al(100 nm) (device 2), where TPOPA was used both as emitter and carrier -
transporting material, CuPC as a hole-injection and electron block material. The
devices show blue wide-band emission peaked at 438 nm with a maximum luminance
of 650 cd/m2 and 512 cd/m2 under an operating voltage of 12 V, respectively.
We show that admittance spectroscopy (AS) can be used to determine charge carrier mobilities and transport parameters in materials relevant to organic light-emitting diodes (OLEDs). Via computer simulation, we found that a plot of the negative differential susceptance vs frequency yields a maximum at a frequency τr-1. The position of the maximum τr-1 is related to the average carrier transit time τdc by τdc = 0.56 τr. Thus knowledge of τr can be used to determine the carrier mobility in the material. Devices with the structure anode/phenylamines/Ag have been designed to evaluate their mobilities. The extracted hole mobility data from AS in pristine and doped material systems are in excellent agreement with those independently extracted from time-of-flight (TOF) technique. In addition, materials with different energy levels of highest occupied molecular orbital (HOMO), are further examined in order to study the effects of injection barrier on the extracted mobility by AS. In the case of an Ohmic hole contact (e.g. ITO or Au /m-MTDATA), the mobility data is good agreement with TOF results. However, for a non-Ohmic contact, the extracted mobility appears to be smaller. Thus AS can be used a means of evaluating the quality of electric contact between the injection electrode and the organic material.
Electroluminescent bipolar small molecules have been attracted with great interests recently. They are found to exhibit many interesting features such as (i) reducing the structural complexity of organic light emitting diodes (OLEDs) from multilayer heterojunction to monolayer homojunction devices; (ii) offering molecular p/n junction, and (iii) minimizing the formation of exciplexes. In this paper, the optical and electrical properties of novel oxadiazole-triphenylamine derivatives will be investigated. The derivatives are N-phenyl-N-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)phenylamine (POT) and N-phenyl-N-(4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)phenyl)phenylamine (m-POT). The absolute absorption coefficient and refractive index have been investigated by ellipsometry and modeling. The electron mobility of POT at room temperature has been studied. The results show that the derivatives have bipolar characteristics. The electron-transporting properties of POT is better than that of m-POT. The EL emission peaks of POT and m-POT are the same at 435nm which match with their photoluminescent (PL) peaks.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.