Organic Electronics 2011 Volume 12 №05

Elsevier. — 149 p. — ISSN: 1566-1199.«Organic Electronics» is a journal whose primary interdisciplinary focus is on materials and phenomena related to organic devices such as light emitting diodes, thin film transistors, photovoltaic cells, sensors, memories, etc.Papers suitable for publication in this journal cover such topics as photoconductive and electronic properties of organic materials, thin film structures and characterization in the context of organic devices, charge and exciton transport, organic electronic and optoelectronic devices.«Organic Electronics» provides the forum for applied, fundamental and interdisciplinary contributions spanning the wide range of electronic properties and applications of organic materials. A Letters section is included for rapid publication of short articles announcing significant and highly original results.725-730
Kyoung Soo Yook, Jun Yeob Lee. Multilevel luminance control in solution processed tandem organic multistable light-emitting diode fabricated by a stamp transfer printing method
Abstract:
A solution processed organic multistable light-emitting diode was developed by devising a tandem device structure combining the organic memory unit and organic light-emitting diode unit using a metal interlayer. A transfer printing method was used to fabricate the organic memory unit of the tandem structure and a well-defined organic layer structure could be obtained using the stamp transfer printing process. The organic multistable light-emitting diode showed the nonvolatile multistable current state of the organic memory and corresponding nonvolatile multistable luminance state. The luminance could be simply managed just simply changing the writing voltage of the organic multistable lightemitting diodes without changing the operating voltage of the device and multilevel luminance control of the organic multistable light-emitting diode was realized. A high on/off ratio over 1000, luminance control over 300 cd/m² and four level luminance control could be stably achieved at 7 V.731-735
Michael Kraus, Simon Haug, Wolfgang Brütting, Andreas Opitz. Achievement of balanced electron and hole mobility in copper-phthalocyanine field-effect transistors by using a crystalline aliphatic passivation layer
Abstract:
Ambipolar charge carrier transport of copper phthalocyanine (CuPc) field-effect transistors with an aliphatic and insulating tetratetracontane (TTC) interlayer are investigated systematically. The TTC interlayer provides a dielectric-semiconductor interface that is free of electron traps. The growth mechanisms of TTC on SiO₂ and CuPc on TTC are studied and charge carrier transport properties are analyzed in dependence of the TTC morphology. Different growth regimes for as-grown and temperature-annealed TTC layers are identified and correlated to the behavior of the respective charge carrier mobilities. Additionally, the thickness of the TTC passivation layer has turned out to influence the grain size of the polycrystalline CuPc layer significantly. Hole and electron transport are affected differently. This behavior can be explained qualitatively with the help of simulations taking into account a difference in grain boundary trap density for electrons and holes. By optimizing the TTC film, balanced charge carrier mobilities can be achieved with μ ≈ 3 x 10^-2 cm²/Vs, which are record values for electron transport in CuPc.736-744
Eszter Voroshazi, Bregt Verreet, Andrea Buri, Robert Müller, Daniele Di Nuzzo, Paul Heremans. Influence of cathode oxidation via the hole extraction layer in polymer:fullerene solar cells
Abstract:
In this paper, we elucidate the role of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the degradation of polymer:PCBM ((6,6)-phenyl C₆₁-butyric acid methyl ester) solar cells. The study is done on unencapsulated cells exposed to ambient conditions in dark. The cell degradation results from reduced carrier extraction, and an investigation of the various interfaces within the cell allows us to correlate this to oxidation of the low work function metal cathode. We further show that this oxidation is caused by water vapor diffusion from the edges through the hygroscopic PEDOT:PSS layer. We demonstrate that only the hygroscopic nature of PEDOT:PSS, and not its acidity, has a detrimental impact. The oxidation of the cathode progresses in synchrony with the water ingress into the PEDOT:PSS layer from the edges of the device towards the central part, and results in a progressive constriction of the active area. When the PEDOT:PSS layer is replaced by an evaporated layer of MoO₃, the device lifetime is improved considerably even with highly reactive metal cathodes. Finally, we provide a quantitative relationship between device lifetime and the level of humidity in the ambient, thus establishing a suitable accelerated shelf-life test for organic solar cells and their encapsulation.745-750
Hyokyun Hama, Jongwoon Park, Dongchan Shin, Junghwan Park. Fabrication of OLEDs without photolithography patterning
Abstract:
We report on a new cost-cutting fabrication process for organic light-emitting diodes (OLEDs), requiring no photolithography patterning for indium-tin-oxide (ITO). We formed the patterned ITO electrodes using a sputtering deposition system. However, sputter-patterned ITO-based OLEDs caused leakage current on the slope of the ITO edges due to spikelike surface. To suppress it, an organic molecule was thermally evaporated as an insulating layer that covers the sputter-patterned ITO edges. Such an insulating organic molecule demands low carrier mobility. We found that the reverse current density of OLEDs based on this scheme was comparable to that of photolithography-patterned ITO-based OLEDs.751-755
Yu-Chiang Chao, Shih-De Yeh, Hsiao-Wen Zan, Gao-Fong Chang, Hsin-Fei Meng, Chen-Hsiung Hung, Tzu-Ching Meng. Integrated semiconductor optoelectronic devices for real-time and indicator-free detection of aqueous nitric oxide
Abstract:
A real-time and indicator-free sensor specific to nitric oxide (NO) is realized for the first time by integrating a sensing hydrogel film, a top-emitting polymer light-emitting diode (PLED), and a silicon photodetector. The top-emitting PLED is utilized to excite the sensing film specific to NO, and the photoluminescence of the sensing film is transformed into electric signal by the photodetector. The influence of the excitation light on the out-put photocurrent of the photodetector is diminished by two delicately selected organic filters based on their absorption spectra. Lead phthalocyanine is deposited on a top-emitting PLED as a filter for generating an excitation light without overlapping the emission maximum of the sensing hydrogel film. Poly(3-hexylthiophene) is placed in front of the photodetector as another filter to remove the excitation light. This sensor demonstrated a fast photocurrent response to NO with response time within 5 min.756-765
Yi-Hsin Lan, Chih-Hung Hsiao, Pei-Yu Lee, Yi-Chi Bai, Chung-Chieh Lee, Chih-Chiang Yang, Man-Kit Leung, Mao-Kuo Wei, Tien-Lung Chiu, Jiun-Haw Lee. Dopant effects in phosphorescent white organic light-emitting device with double-emitting layer
Abstract:
In this paper, we investigated the effects of dopants on the electrical and optical characteristics of dichromatic and white organic light-emitting devices (OLEDs) with a double emitting layer (DEML). Such a DEML consisted of two host materials with bipolar transport characteristics doped with two distinct types of phosphorescent emitters. The host for EML1 (adjacent to the anode side) was N,N-dicarbazolyl-3,5-benzene (mCP), with a hole mobility higher than the electron mobility, whereas the host for EML2 (adjacent to the cathode side) was 2,2'-bis[5-phenyl-2-(1,3,4)oxadazolyl]biphenyl (OXD), with a higher electron mobility than the hole mobility. Phosphorescent blue emitters, 9% iridium(III)bis[4,6-di-fluorophenyl-pyridinato-N,C2]picolinate (FIrpic), and phosphorescent green emitters, 9% fac-tris(phenylpyridine) iridium [Ir(ppy)₃], were doped into EML1 and EML2, respectively for dichromatic emission. In the dichromatic blue/green OLEDs, both the blue and
green dopants helped to reduce the driving voltage. The dopants essentially played the role of carrier traps and recombination centers, from which the current density–voltage characteristics and electroluminescence (EL) spectra were derived. In this structure, EML1 and EML2 exhibited hole- and electron-rich bipolar transporting characteristics, respectively. Hence, the recombination was extended to the two EMLs, which reduced the efficiency roll-off and resulted in high efficiency levels even at high luminance levels. Through further incorporation of 0,5% of tris(2-phenylquinoline)iridium(III) [Ir(2-phq)₃] into EML1, a white OLED with maximum current efficiency of 35,8 cd/A and quantum efficiency of 10,73% at 0,643 mA/cm² was obtained. In this white OLED, the current efficiency decreased slightly by 2,51% from 35,8 cd/A at 230 cd/m² to 34,9 cd/A at 1000 cd/m². Carrier confinement was achieved at the D-EML interface, which not only contributed to the high current efficiency, but also resulted in stable color coordinates that varied a mere (-0,014, 0,004) from 100 to 10,000 cd/m², without additional blocking layers between the two EMLs.766-773
Bing Chen, Yanhu Li, Wei Yang, Wen Luo, Hongbin Wu. Efficient sky-blue and blue-green light emitting electrochemical cells based on cationic iridium complexes using 1,2,4-triazole-pyridine as the ancillary ligand with cyanogen group in alkyl chain
Abstract:
Sky-blue and blue-green light-emitting cationic iridium (III) complexes with triazolepyridine as the ancillary ligand, with formula [Ir(dfppy)₂(trzpy-cn)](PF₆) (C1), [Ir(mppy)₂-(trzpy-cn)](PF₆) (C2), [Ir(dfppy)₂(trzpy-hx)](PF₆) (C3) and [Ir(mppy)₂(trzpy-hx)](PF₆)(C4) were synthesized, where mppy, trzpy-cn, trzpy-hx are 2-methylphenylpyridine, 2,2-dimethyl-6-(3-methyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)hexanenitrile, and 2-(5-methyl-4-octyl-4H-1,2,4-triazol-3-yl)pyridine, respectively. C1/C3 and C2/C4 emit sky-blue and blue-green light peaked at 458 and 484 nm, respectively. Single layer light-emitting electrochemical cells (LECs) based on C1 and C2 show luminous efficiencies (LE) of 2,6 and 6,8 cd/A (corresponding to an external quantum efficiency (EQE) of 2,7 and 2,8%), with CIE coordinates of (0,21, 0,33) and (0,31, 0,53), respectively. As compared with the devices from C3 and C4, the device efficiencies based on C1 and C2 are found to be significantly enhanced by 10 times and 4 times, respectively, owing to the incorporation of cyanogen group in the alkyl chain. The results demonstrate that using the triazole-pyridine as ancillary ligand with a cyanogen group in the side alkyl chain are effective strategies to tune the emission into blue region and to improve the efficiency of the cationic iridium complex.774-784
J.A. Mikroyannidis, A.N. Kabanakis, S.S. Sharma, G.D. Sharma. Low band-gap phenylenevinylene and fluorenevinylene small molecules containing triphenylamine segments: Synthesis and application in bulk heterojunction solar cells
Abstract:
Starting from triphenylamine, two low band gap small molecules, PH and FL, based on phenylenevinylene and fluorenevinylene, respectively were synthesized. They were soluble in common organic solvents such as tetrahydrofuran, chloroform and dichloromethane. Their long-wavelength absorption maximum was at 605–643 nm with optical band gap of 1,64–1,66 eV. These small molecules showed a band gap lower than that of P3HT and also have deeper HOMO level, which is beneficial for improvement of the open circuit voltage. The photovoltaic properties have been investigated using the bulk heterojunction (BHJ) active layer of PH or FL with PCBM. The device based on FL:PCBM displayed higher power conversion efficiency (PCE) (1,42%) than the device based on PH:PCBM (1,02%), when the blend films were cast from chloroform solvent. Moreover, various casting solvents were used for the BHJ solar cells based on FL:PCBM blend and their effect on the photovoltaic performance was investigated. The results indicate that high boiling point solvents lead to an enhanced self-organization of FL in the active layer, which causes an increased charge transport. Finally, we have used a modified PCBM, i.e. F as electron acceptor along with FL as electron donor, to increase the light harvesting in the wavelength region below 500 nm and the PCE is about 4,38% when the BHJ (FL:F blend) device was spin casted from mixed 1-chloronaphthalene/o-dichlorobenzene solvents. The improved PCE has been attributed to the increased light absorption and higher hole mobility in the active layer, which resulted in more balanced charge transport.785-793
Min-Gi Shin, K. Thangaraju, Seul-ong Kim, Jong-Won Park, Yun-Hi Kim, Soon-Ki Kwon. A new N-fluorenyl carbazole host material: Synthesis, physical properties and applications for highly efficient phosphorescent organic light emitting diodes
Abstract:
A new N-fluorenyl carbazole material, 9,90-bis-(9,9-dimethyl-9H-fluoren-2-yl)-9H,9'H-[3,3']bicarbazolyl (BDFC), was synthesized by bromination, Ullmann and Yamamoto coupling reactions and confirmed using various spectroscopic studies. Thermogravimetric analysis and differential scanning calorimetry studies show the thermal stability (DT5%) of 494,7 °C with high glass transition temperature (T_g) of 177,8 °C. The photophysical and electrochemical studies of BDFC show the photoluminescence at 408 nm and a band gap of 3,01 eV with higher triplet energy of 2,72 eV. The phosphorescent organic light emitting diode using BDFC as host, ITO/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC)/host: fac-tris(2-phenylpyridine)-iridium [Ir(ppy)₃] (5%)/1,3,5-tris(m-pyrid-3-ylphenyl) benzene (TmPyPB)/LiF/Al, shows the effective confinement of triplet excitons and efficient energy transfer to the guest emitter in the emissive layer, resulted in the higher device efficiencies of 56,3 cd/A, 18,1% and 21,3 μm/W compared with that (48,1 cd/A, 15,3% and 16,3 μm/W) of device based on (4,4'-N,N'-dicarbazole)biphenyl (CBP) as host. The results show that the new host material BDFC could be useful for the efficient organic light emitting diodes.794-801
Kung-Shih Chen, Yong Zhang, Hin-Lap Yip, Ying Sun, Joshua A. Davies, Chin Ting, Chih-Ping Chen, Alex K.-Y. Jen. Highly efficient indacenodithiophene-based polymeric solar cells in conventional and inverted device configurations
Abstract:
Highly efficient bulk-heterojunction polymer solar cells based on two indacenodithiophene- containing low bandgap, high mobility polymers in both of their conventional and inverted device configurations are demonstrated. The highest power conversion efficiency obtained from the conventional structure device is 6,3%, while the PCE of the inverted device reaches 4,9%. These results are very encouraging for further material and device optimization.802-808
Jong-Kwan Bin, Jong-In Hong. Efficient blue organic light-emitting diode using anthracene-derived emitters based on polycyclic aromatic hydrocarbons
Abstract:
We have synthesized two light-emitting materials based on polycyclic aromatic hydrocarbons (PAHs) for use in blue organic light-emitting diodes (OLEDs). 9-(Phenanthryl)-10-(3-(9-phenylcarbazole-9-yl)anthracene (PPCA) acts as the host and 9,10-bis-biphenyl-4-yl-2,6-diphenylanthracene (BDA) acts as the dopant. PPCA contains an electron-transporting phenanthrene moiety and a hole-transporting 9-phenyl-9H-carbazole moiety in the anthracene core. BDA contains PAHs without amino substituents in the anthracene core. The optimized device structure, ITO/DNTPD (600 Å)/α-NPB (300 Å)/PPCA:BDA 5 wt.% (250 Å)/N1PP (300 Å)/LiF (5 Å)/Al (1000 Å), is characterized by blue electroluminescence (EL), with a current efficiency of 6,9 cd/A, power efficiency of 3,23 μm/W, and external quantum efficiency of 5,1% at 10 mA/cm² and CIE coordinates of (0,15, 0,18).809-817
Jörg Frischeisen, Daisuke Yokoyama, Ayataka Endo, Chihaya Adachi, Wolfgang Brütting. Increased light outcoupling efficiency in dye-doped small molecule organic light-emitting diodes with horizontally oriented emitters
Abstract:
Small molecule organic light-emitting diodes (SM-OLEDs) are efficient large area light sources facing their market entry. However, a low light outcoupling efficiency of typically 20% still strongly limits device performance. Here, we highlight the potential of employing dye-doped emission layers with emitting molecules having horizontally oriented transition dipole moments. The effect of molecular orientation is explained by studying optical simulations that distinguish between horizontal and vertical dipole orientation. In addition, an experimental method that enables straightforward determination of dipole orientation in guest–host systems is presented and used for the analysis of two materials that are very similar except for their orientation. By measuring the external electroluminescence quantum efficiency of SM-OLEDs based on these materials, evidence is found that a mainly horizontal dipole orientation enhances light outcoupling by around 45%. Furthermore, the effect of orientation in SM-OLEDs offers many additional benefits concerning stack design and has fundamental implications for material choice.818-822
S. Cheylan, D.S. Ghosh, D. Krautz, T.L. Chen, V. Pruneri. Organic light-emitting diode with indium-free metallic bilayer as transparent anode
Abstract:
Bilayer Cu–Ni transparent electrodes were room-temperature grown on glass by sputtering technique and used as anodes for polymer light-emitting diodes (PLEDs). The bilayer electrode structure allows combining the low sheet resistance and high transparency of Cu with the excellent stability and high work function of Ni. We demonstrate that Cu–Ni bilayer based PLED devices exhibit comparable efficiency and lifetime decay behavior to indium tin oxide (ITO) based device, with potentially significant advantages, such as easy processing, low cost and mechanical flexibility. In addition, the ductile nature of the metal electrode and its low thickness ( 10 nm) make the proposed structure particularly suitable for flexible organic electronic and optoelectronic devices.823-826
Y. Yan, X.J. She, H. Zhu, S.D. Wang. Origin of bias stress induced instability of contact resistance in organic thin film transistors
Abstract:
We report a study on the contact resistance instability induced by the bias stress in staggered pentacene thin film transistors, combining the bias stress measurements with the transfer line method. The contact resistance is increasing with the stress time, and two device parameters are found to contribute to this contact resistance instability: one is the threshold voltage increase due to the charge trapping in the charge accumulation layer; the other is the effective contact length increase due to the charge trapping in the pentacene bulk in the contact region. The gold contact shows lower contact resistance stability compared with the copper contact, which is ascribed to higher density of the deep trap states at the gold contact. This work suggests that the time-dependent charge trapping is responsible for the bias stress effect in organic thin film transistors.827-831
Yinhua Zhou, Hyeunseok Cheun, Seungkeun Choi, Canek Fuentes-Hernandez, Bernard Kippelen. Optimization of a polymer top electrode for inverted semitransparent organic solar cells
Abstract:
We report on semitransparent organic solar cells using a single-layer blend based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the top electrode. The PEDOT:PSS blend was prepared by mixing a high-conductivity formulation of PEDOT:PSS (H.C. Starck CLEVIOS PH-1000) and another formulation of PEDOT:PSS (H.C. Starck CLEVIOS CPP 105D). The PEDOT:PSS blend yields good wetting properties on the hydrophobic surface of a blend of poly(3-hexylthiophene) (P3HT) with phenyl-C₆₁-butyric acid methyl ester (PC₆₀BM), and shows a conductivity over 400 S cm^-1. Semitransparent organic solar cells using the PEDOT:PSS blend as the top electrode with a structure of glass/ITO/ZnO/P3HT:PC₆₀BM/PEDOT:PSS-blend exhibited an average power conversion efficiency of 2,4% estimated for 100mW cm^-2 AM 1,5G illumination.832-842
J.A. Jiménez Tejada, K.M. Awawdeh, J.A. López Villanueva, J.E. Carceller, M.J. Deen, N.B. Chaure, Tamara Basova, A.K. Ray. Contact effects in compact models of organic thin film transistors: Application to zinc phthalocyanine-based transistors
Abstract:
A new compact model is developed for organic thin film transistors (OTFTs) by incorporating the effects of the contacts on the transistor’s output characteristics. The model is based on physically realistic expressions that describe charge flow in the vicinity of the contacts and a previously developed drift model. The resulting new model maintains the compactness of our original drift-based model and is very suitable for circuit simulations. For proper modeling, accurate model parameters are important. Therefore, we also propose a modified method to extract the transistor’s parameters, including ones related to the contact region. The validity of the model is examined by applying it to experimental data obtained for OTFTs which used solution processed films of substituted zinc phthalocyanine derivatives as an active layer between the gold contacts as the source and the drain terminals. Both linear and non-linear responses in the low drain voltage regime of the output characteristics of the transistors are analyzed. The new parameter extraction scheme provides a way to study the evolution with the gate voltage of the ratio of free to total charge density in the low conductivity region close to the contact.843-850
Chao Cai, Shi-Jian Su, Takayuki Chiba, Hisahiro Sasabe, Yong-Jin Pu, Kenichi Nakayama, Junji Kido. High-efficiency red, green and blue phosphorescent homojunction organic light-emitting diodes based on bipolar host materials
Abstract:
In this paper, we demonstrate red, green and blue (RGB) phosphorescent p–i–n homojunction devices by using a series of bipolar host materials, including 2,6-bis(3-(carbazol-9-yl)phenyl) pyridine (26DCzPPy), 3,5-bis(3-(carbazol-9-yl)phenyl) pyridine (35DCzPPy) and 4,6-bis(3-(carbazol-9-yl)phenyl) pyrimidine (46DCzPPm). Homojunction devices comprise a MoO₃-doped host as a hole transport layer, and a Cs₂CO₃-doped host as an electron transport layer. Emission layer consists of a host doped with iridium(III) bis(4,6-(di-fluorophenyl) pyridinato-N,C^2') picolinate (FIrpic) for blue, fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) for green, and tris(1-phenylisoquinolinolato-C2,N)iridium(III) (Ir(piq)3) for red devices. External quantum efficiencies of 12,9% for the 35DCzPPy-based blue, 9,5% for the 46DCzPPm-based green, and 8,5% for the 46DCzPPm-based red devices were achieved at 100 cd m^-2. By systematically investigating carrier balance in these homojunction devices, well-balanced charge carrier injection and transport in emission layers are found to be critical factors for constructing efficient homojunction devices.851-856
Annalisa Calò, Pablo Stoliar, Massimiliano Cavallini, Yves H. Geerts, Fabio Biscarini. Doping and photo-induced current in discotic liquid crystals thin films: Long-time and temperature effects
Abstract:
Here we address the problem of time and temperature instability of the electrical current related to parasitic doping effects in discotic liquid crystals (DLCs). We investigated the electrical transport in thin films of a phthalocyanine-based DLC in-dark and upon laser irradiation during long thermal cycles. Measuring the electrical transport between two electrodes we observed that in-dark the current is mostly due to doping levels while, upon laser irradiation, the photocurrent reflects the diffusion length of the photogenerated charge carriers and it is limited by the coherence length of a column. We prove here that the two contributions are completely independent. Our result experimentally supports the theoretical studies performed at the molecular level on discotic liquid crystals.857-864
Ilias Katsouras, Victor Geskin, Auke J. Kronemeijer, Paul W.M. Blom, Dago M. de Leeuw. Binary self-assembled monolayers: Apparent exponential dependence of resistance on average molecular length
Abstract:
We investigate the electrical transport through mixed self-assembled monolayers of alkanemonothiols and alkanedithiols in large-area molecular junctions. To disentangle the role of the molecular length and the interfacial composition, monothiol–monothiol, dithiol–dithiol, and monothiol–dithiol binary combinations are studied. In all cases, we find that the resistance of these mixed SAMs appears to depend exponentially on the average number of carbon atoms, thus resembling monocomponent SAMs, whose resistance is known to depend exponentially on molecular length. However, in monocomponent SAMs this behavior has a single-molecule tunneling origin, which is not directly relevant for mixtures. Furthermore, in certain mixed SAMs the resistance decreases with increasing average layer thickness (the case of monothiol–dithiol systems). We suggest an explanation for the observed dependence of the resistance in the mixed SAMs on their composition within an equivalent circuit model based on a simple assumption concerning their microdomain structure. The simulated dependence is non-exponential but leads to a good agreement between calculated and measured resistances with only two fit parameters.865-868
Jwo-Huei Jou, Shih-Ming Shen, Chuen-Ren Lin, Yi-Shan Wang, Yi-Chieh Chou, Sun-Zen Chen,
Yung-Cheng Jou. Efficient very-high color rendering index organic light-emitting diode
Abstract:
Very-high color rendering index (CRI 90) is extremely crucial for lighting in surgery, photography and exhibition of museums etc. We demonstrate herein an efficient very-high CRI organic light-emitting diode with CRI of 98 with an efficacy of 8,3 μm/W at 100 cd/m², or CRI of 96 with 5,2 μm/W at 1000 cd/m². The very high CRI may be attributed to the successful deposition and emission of the two full-spectrum complementary white emissive layers, especially as a thin interlayer is inserted in between to regulate the injection of carriers. Without the interlayer, the resultant CRI drops to 73 and efficacy to 3,6 μm/W at 1000 cd/m². The employment of the carrier regulating layer also helps disperse the injected carriers, leading recombination to occur in a wider area and hence a higher efficiency.869-873
Jeramy D. Zimmerman, Eric K. Yu, Vyacheslav V. Diev, Kenneth Hanson, Mark E. Thompson, Stephen R. Forrest. Use of additives in porphyrin-tape/C₆₀ near-infrared photodetectors
Abstract:
We demonstrate organic heterojunction photodetectors sensitive to wavelengths λ 1500 nm based on a solution processed Zn–porphyrin-dimer donor layer that includes phenyl-C₆₁-butyric acid methyl ester (PCBM) and/or 4,4'-bipyridyl (Bipy) additives, and an evaporated C₆₀ acceptor layer. The inclusion of PCBM results in up to a 20-fold increase in differential resistance, while the inclusion of Bipy results in up to a 100% increase in external quantum efficiency (EQE) compared to devices lacking additives. The devices have peak specific detectivities of D* = 8,8 ± 0,2 x 10¹¹ Jones (EQE = 3.6 ± 0.1%) at λ = 1130 nm, and D* = 8,2 ± 0,2 x 10¹⁰ Jones (EQE = 10,5 ± 0,2%) at λ = 1400 nm. The maximum external quantum efficiency at λ = 1400 nm is 13,5 ± 0,3%, approximately double that of previously reported organic near infrared detectors.