Organic Electronics 2011 Volume 12 №02

Elsevier. — 177 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.227-233
Jongwoon Park, Hyokyun Ham, Cheolyoung Park. Heat transfer property of thin-film encapsulation for OLEDs
Abstract:
We investigate the heat transfer property of thin-film encapsulation (TFE) for organic lightemitting diodes (OLEDs). It is demonstrated that the TFE combined with a flexible heat sink shows better thermal performance, comparedwith the epoxy-filled glass encapsulation and the conventional glass encapsulation. By way of experiments and simulations, we verify that the multi-heterojunction configuration and the low thermal conductivity of the polymer layer in the TFE film have no impact on the thermal performance. Furthermore, we find through simulations that a significant temperature gradient appears inside the TFE layers only when the thermal conductivity of the polymer is lower than 1 x 10^-3 W/m K. This enables us to perform design optimization of the TFE configuration with relaxed heat dissipation.234-238
Yue-Hua Zhou, Li-Ping Zhu, Yu Qiu. Dynamics study of electron extraction from 1D conjugated polymers to metal electrodes
Abstract:
We simulate the processes of electron extraction from 1D conjugated polymers to metal electrodes in the presence of electric fields based on the Su–Schrieffer–Heeger (SSH) model extended to a polymer/electrode contact structure. From the simulation, we can obtain the lifetime of the process that a charged carrier known as polaron at the polymer chain end starts to dissociate and the released electron tunnels to the electrode immediately. An effective carrier mobility named interface mobility can be defined for this process and calculated from the lifetime. It is found that the interface mobility is independent on the external electric fields. However, this mobility increases with decreasing the work function of the electrode. By enhancing the coupling between the polymer and the electrode, the interface mobility is shown to increase. Kinetic energy of the electrons is also shown to favor the improvement of the mobility.239-243
Yasuhito Miyoshi, Takuya Fujimoto, Hirofumi Yoshikawa, Michio M. Matsushita, Kunio Awaga, Terufumi Yamada, Hiroshi Ito. Photoconductivity and FET performance of an n-type porphyrazine semiconductor, tetrakis(thiadiazole)porphyrazine
Abstract:
Photoconductivity and field effect transistor (FET) performance of thin films of a thiadiazole-annulated porphyrazine compound, tetrakis(thiadiazole)porphyrazine (abbreviated as H2TTDPz) were examined. The photocurrent-action spectra for ITO/H₂TTDPz/Au photocell exhibited symbatic and antibatic responses, depending on the bias polarity. This characteristic difference in photocurrent responses was interpreted as a filter effect in Schottky-type photocells; the dissociation of the excitons generated near the negative electrode can effectively produce photocurrent in n-type semiconductors. We also found n-type semiconductor behavior of H₂TTDPz in the thin-film FETs operated by the gates of an ionic liquid and SiO₂, and obtained a field effect mobility of 7,2 x 10^-4 cm² V^-1 s^-1 and an on/off ratio of 10⁴.244-248
R.A. Street, K.W. Song, S. Cowan. Influence of series resistance on the photocurrent analysis of organic solar cells
Abstract:
The series resistance of a bulk heterojunction solar cell diode affects the measurement of the voltage dependence of the diode photocurrent, P(V). An empirical model for the effect is described and shows that P(V) can be significantly modified, particularly at high bias voltages. Experimental measurements on PCDTBT:PCBM solar cells with added series resistance demonstrate the effect and confirm the model. The model provides a method to correct the photoconductivity data using measured quantities.249-256
Huai-Yuan Tseng, Vivek Subramanian. All inkjet-printed, fully self-aligned transistors for low-cost circuit applications
Abstract:
Layer-to-layer registration is a significant problem in printed electronics since parasitic overlap capacitances resulting from the poor registration of conventional printers substantially degrade device operating speeds. To overcome this, a novel inkjet printing process was utilized to demonstrate an all inkjet-printed and fully self-aligned transistor, as well as fully self-aligned inverters for the first time. Self-alignment of transistor source/drain electrodes to gates was achieved by a printed wetting-based roll-off process, resulting in an improved overlap capacitance as small as 0,14–0,23 pF mm^-1. Circuit building blocks including transistor arrays and inverters were demonstrated using a novel self-split source/drain and a self-aligned interconnect printing technique. Due to the substantial reduction in overlap capacitance, demonstrated inverters showed a propagation delay as low as 0,488 ms at a fan-out of 1, despite using materials with mobility of 0,01 cm² V^-1 s^-1, thus showing a performance that is suitable for use in a range of low-cost electronics applications, realized using a simple, scalable process.257-262
T. Xiao, W. Cui, J. Anderegg, J. Shinar, R. Shinar. Simple routes for improving polythiophene:fullerene-based organic solar cells
Abstract:
Improved power conversion efficiency (PCE), by up to ~27%, of organic solar cells based on poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):[6,6]-phenyl-C₆₀-butyric acid methyl ester (P3HT:PCBM) were obtained via simple modifications, widely applicable, in the fabrication of the spin-coated PEDOT:PSS layer. These included (i) further diluting the original PEDOT:PSS solution with deionized water, (ii) mixing the original PEDOT:PSS solution with ethylene glycol (EG), and (iii) spin coating EG over a PEDOT:PSS layer fabricated using the original solution. The optimal dilutions, spin coating rates, and durations were determined. Approach (iii) resulted in the best cell with a PCE of 4,7% as compared to 3,7% for the untreated PEDOT:PSS. To evaluate the origin of the improvements we monitored the PEDOT:PSS conductivity, external quantum efficiency of the devices, and their I–V curves that indicated an increase of ~16% in the short-circuit current ISC. Other characteristics included the PEDOT:PSS layer thickness, its transmittance, P3HT:PCBM absorption spectra, its morphology, and surface chemical composition. The results indicate that in addition to the enhanced PEDOT:PSS conductivity (following some of the treatments) that improves charge extraction, enhanced PEDOT:PSS transmission and especially, enhanced P3HT:PCBM absorption contribute to improved solar cell performance, the latter by increasing ISC. While the various treatments in the optimized devices had a minor effect on the PEDOT:PSS thickness, its morphology, and consequently that of the active layer, were affected. The surface roughness of the active layer increased significantly and, importantly, in devices with PEDOT:PSS/EG/P3HT:PCBM, PCBM aggregates were observed near the cathode. Such aggregates may also result in increased absorption and improved charge extraction.263-268
Arsen Babajanyan, Harutyun Melikyan, Jongchel Kim, Kiejin Lee, Mitsumasa Iwamoto, Barry Friedman. Direct imaging of conductivity in pentacene field-effect transistors by a near-field scanning microwave microprobe
Abstract:
A near-field scanning microwave microprobe (NSMM) is employed to visualize the carrier distribution in the active layer of an organic field-effect transistor (OFET). Experiments using this technique with a pentacene OFET reveal changes of the conductivity profiles in the channel which arise from the development and exhaustion of an accumulated charge region. Electric field profiles that are visualized by using the electric field induced optical second harmonic generation (EFISHG) method, verify the results. NSMM is a powerful tool, and in combination with EFISHG will provide a way to directly probe carrier transport.269-278
U Ra Lee, Tae Wan Lee, Mai Ha Hoang, Nam Su Kang, Jae Woong Yu, Kyung Hwan Kim, Kyung-Geun Lim, Tae-Woo Lee, Jung-Il Jin, Dong Hoon Choi. Photoreactive low-bandgap 4H-cyclopenta[2,1-b:3,4-b']dithiophene and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-based alternating copolymer for polymer solar cell
Abstract:
Polymer solar cells (PSCs) are often fabricated using a well-known 4H-cyclopenta[2,1-b:3,4-b0]dithiophene and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole-based polymer as a low-bandgap polymer. Further, PSCs are also fabricated by mixing methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM) with the polymer. We prepared poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl-alt-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole-5',5''-diyl] after anchoring the penta-1,4-diene in the side chain of the cyclopentadithiophene and investigated the long-term performance stability of a photocrosslinked bulk heterojunction (BHJ) photovoltaic (PV) cell based on the polymer. The photocrosslinking reaction was monitored via infrared spectroscopy without the use of a photoinitiator, by carrying out a spontaneous radical coupling reaction. The polymer film has a broad absorption band extending from 300 to 850 nm with an optical bandgap as low as 1,52 eV. The polymer was employed to fabricate a PSC with PC₆₁BM. The resultant PSC device, which was treated by UV irradiation (λ = 254 nm, I = 40 mW/cm², 5 min), had good preliminary results with an open-circuit voltage of 0,62 V, a shortcircuit current density of -5,37 mA/cm², a fill factor of 0,41, and an overall power conversion efficiency of 1,37%. All parameters of the UV-cured PSC device were more stable over the course of 300 h than those of P3HT-PC₆₁BM devices, indicating the long-term stability of the polymer.279-284
Takahiro Nagata, Seungjun Oha, Toyohiro Chikyow, Yutaka Wakayama. Effect of UV–ozone treatment on electrical properties of PEDOT:PSS film
Abstract:
We reveal the effect of UV–ozone treatment on the electrical properties of poly (styrenesulfonate) doped poly (3,4-ethylenedioxythiophene) (PEDOT:PSS) film by clarifying the respective roles of UV light irradiation and exposure to ozone gas. The UV–ozone treatment induced increases in both work function and resistivity. Furthermore, the film thickness was reduced at a rate of 0,13 nm/min. The ozone-exposed films also exhibited a marked increase in the work function. However, such variations were not observed in the resistivity and film etching. Angle resolved X-ray photoelectron spectroscopy revealed that the main role of the UV light was to decompose the chemical bonds in the PEDOT:PSS film, resulting in a resistivity increase and film etching. In contrast, the ozone and atomic oxygen were absorbed and oxidized the surface, which was responsible for the increase in the work function. Due to these different functions, UV–ozone treatment is capable of controlling the work function and resistivity of PEDOT:PSS film thus allowing them be adjusted to the device application.285-290
Joo Hyung Kim, You Jong Lee, Yoon Sung Jang, Jin Nyoung Jang, Doo Hyun Kim, Byung Chul Song,
Dong Hyeok Lee, Soon Nam Kwon, MunPyo Hong. The effect of Ar plasma bombardment upon physical property of tungsten oxide thin film in inverted top-emitting organic light-emitting diodes
Abstract:
This study examined the mechanism of performance degradation induced by the plasma sputtering process for the formation of the ITO top anode in inverted top-emitting organic light-emitting diodes (ITOLEDs) with WO₃ thin film as the hole injection layer (HIL) and the buffer layer. The degradation is mainly caused by the changes in the physical properties of the WO₃ thin film as well as the damages of organic layers, which are caused by the bombardment of energetic particles induced by the plasma sputtering process. The number of oxygen vacancies in the WO₃ thin film is increased with the increase of the bombardment energy of energetic particles such as Ar ions, resulting in the decrease in the work function and optical band gap and the dramatic increase in conductivity. Above all, the increase in the energy potential difference between WO₃ HIL and NPB HTL, which was attributed to the reduced work function of the WO₃ HIL, reduces the efficiency of carrier injection and dominantly affects the device performance. These results show that the plasma damage effect, induced by the sputtering process for ITO deposition, could not be effectively removed, even when the metal oxide thin film is used as the buffer layer in ITOLEDs.291-294
Kyoung Soo Yook, Jun Yeob Lee. Solution processed white phosphorescent organic light-emitting diodes with a double layer emitting structure
Abstract:
Solution processed multilayer white phosphorescent organic light-emitting diodes with a double layer emitting structure were developed using an alcohol soluble phosphorescent host and dopant materials. An alcohol soluble blue phosphorescent emitting layer was spin coated on a toluene soluble red/green emitting layer. A high current efficiency of 17,8 cd/A with a color coordinate of (0,31, 0,46) was obtained from the solution processed double emitting layer white phosphorescent organic light-emitting diodes. In addition, all solution processed multilayer white phosphorescent organic light-emitting diodes were also developed by using the two layer emitting structure.295-299
Kenji Toyoda, Ikutaro Hamada, Susumu Yanagisawa, Yoshitada Morikawa. Density-functional theoretical study of fluorination effect on organic/metal interfaces
Abstract:
Density-functional theory with a semi-empirical dispersion correction was used to systematically examine how the number of fluorine (F) atoms affects atomic and electronic structures of fluorinated pentacene (C₂₂F_nH_14-n) adsorbed on Cu(111) surfaces. The fluorination effect on the carrier injection efficiency at organic/metal interfaces was investigated. We found that as the number of F atoms decreases, the electron affinity of isolated molecules decreases, suggesting that the molecule becomes less reactive. However, for adsorbed systems, as the number of F atoms decreases, molecular orbitals of C₂₂F_nH_14-n strongly hybridize with the substrate states while retaining the n-type energy level alignment, resulting in lowering the barrier height of the carrier injection. Based on the calculation results, we propose using C₂₂F_nH_14-n (n ≤ 8) with Cu electrodes for efficient electron injection.300-305
L. Mattias Andersson. Charge transport and energetic disorder in polymer:fullerene blends
Abstract:
Mobility versus temperature measurements on two different polymer:fullerene blends intended for solar cell applications have been evaluated in terms of the energetic disorder and how this varies with stoichiometry. The charge transport is shown to be confined to the intended phases in both cases, but with fundamentally different interaction properties. In one case the energetic disorder is a function of stoichiometry for the hole transport, while it is constant and equal to that of the pure fullerene for electrons, and in the other case it is the hole transport that has a stoichiometry independent energetic disorder. Transport site dilution is argued to be present in the fixed energetic disorder system and this is offered as a partial explanation to molecular weight dependent currents in solar cells. Determination of the glass transition temperature through the use of field effect transistors is also demonstrated.306-311
X.A. Cao, Z.Y. Jiang, Y.Q. Zhang. Organic thin film structures for high-sensitivity imaging of contact stress distributions
Abstract:
We studied the responses of the electrical and optical characteristics of organic lightemitting diodes (OLEDs) with green fluorescent and phosphorescent dyes doped in a polymer matrix to compressive stresses. The phosphorescent OLED converted stresses into measurable and reversible changes in both current density and electroluminescence (EL) intensity. The current showed a nearly linear characteristic response with sensitivity up to 205 μA kPa^-1, whereas EL decreased by over three orders of magnitude at 107 kPa. In contrast, the fluorescent OLED exhibited much smaller stress-induced modulations in current and light intensity, and the responses were saturated at stresses above 26 kPa. The discrepancy has been attributed to stress-enhanced back exciton energy transfer between guest and host molecules, which quenches the EL of the phosphorescent OLED, but has a much smaller impact on the performance of the fluorescent OLED.312-321
Senol Mutlu, Iskender Haydaroglu, Ali Osman Sevim. Realization of polymer charge pump circuits using polymer semiconductors
Abstract:
Vertical diodes made of chromium/gold/poly(3-hexylthiophene) (P3HT)/aluminum are used to implement AC–DC charge pump circuits. A single stage polymer charge pump with 470 nF capacitors is able to supply 7,2 V DC to a load of 1 MΩ || 20 pF when an AC signal with a peak value of 10 V is applied. The circuit can be used up to 10 kHz. High turn-on voltages and internal resistances and capacitances of the diodes limit the DC output voltages. The ramp-up time of the circuit built with 10 nF capacitors is 9,4 ms for a 1 kHz, 10 V peak AC signal. Diodes are modeled and simulated in SPICE using space charge limited conduction model. The simulated charge pump circuit produces consistent results with measurements. Performances of air exposed vertical diodes deteriorate under vacuum, prolonged operation at atmospheric conditions and electrical bias stress resulting in blistering, emphasizing the importance of packaging the devices under inert atmosphere. The DC output voltage and the input impedance of the circuit can be adjusted by using multiple stages.322-328
Jun Xie, Chunyan Chen, Shufen Chen, Yang Yang, Ming Shao, Xu Guo, Quli Fan, Wei Huang. Blue top-emitting organic light-emitting devices based on wide-angle interference enhancement and suppression of multiple-beam interference
Abstract:
Blue top-emitting organic light-emitting devices (TEOLEDs) based on blue phosphor iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C^2'] picolinate (FIrpic) are demonstrated. Instead of using microcavity effects to obtain the blue top emission in conventional reports, the blue emission with a good chromaticity is achieved in this paper through the suppression of the multiple-beam interference by introducing a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline light outcoupling layer onto the semitransparent Sm/Ag cathode. The electroluminescent performances of the blue emission is further improved based on the effective carrier transport and the consideration of the wide-angle interference at the emission peak of FIrpic with a maximum luminance of 8029 cd/m² and a luminous efficiency of 4,02 cd/A achieved at 14 V and 10 V, respectively. With a polarizer on the top cathode, a high pixel contrast ratio of 113:1 is realized under an ambient illumination of 140 lx and a pixel brightness of 1000 cd/m².329-335
Antonio Sánchez-Díaz, Roberto Pacios, Udane Muñecas, Tomás Torres, Emilio Palomares. Charge transfer reactions in near IR absorbing small molecule solution processed organic bulk-heterojunction solar
Abstract:
We have measured the charge transfer processes on near IR small molecule solution processes solar cells under working conditions. The results show that there are two different behavior, for the device under working conditions, controlled by the intrinsic properties of the organic bulk-heterojunction phase to store charges at the different organic materials. Furthermore, our recombination kinetic studies show the origin of the photovoltage on these thin organic devices (80 nm thick) and the reason for the difference between the observed open circuit cell potential and the theoretical expected value based on the LUMOPCBM–HOMO phthalocyanine difference.336-340
Franziska D. Fleischli, Katrin Sidler, Michel Schaer, Veronica Savu, Jürgen Brugger, Libero Zuppiroli. The effects of channel length and film microstructure on the performance of pentacene transistors
Abstract:
With the use of the stencil lithography, we fabricated pentacene thin-film transistors prepared directly on thermally oxidized silicon gates with channel lengths ranging from 2 to 600 μm. By performing 4-probe measurements or by using the transfer line method on these field-effect transistors, we were able to separate the respective contributions of the channel and the contacts to the transistor performance. The contact resistance depends strongly on the gate voltage and on the grain size and morphology; this behavior has been attributed to screening effects at the contact barriers. The low-field mobility in the channel is 0,5–0,6 cm²/Vs in long-channel transistors and reaches mobility in the order of 10 cm²/Vs in short-channel transistors consisting essentially of a single grain.341-347
Ines Rabelo de Moraes, Sebastian Scholz, Björn Lüssem, Karl Leo. Analysis of chemical degradation mechanism within sky blue phosphorescent organic light emitting diodes by laser-desorption/ionization time-of-flight mass spectrometry
Abstract:
We present an investigation of the intrinsic chemical degradation mechanisms of phosphorescent OLEDs based on the common sky blue emitter bis(2-(4,6-difluorophenyl)-pyridyl-N,C^2')iridium(III)picolinate (FIrpic). The OLEDs are investigated using the laserdesorption/ionization time-of-flight mass spectrometry. The comparison between the collected spectra for electrically aged and unaged OLEDs allows the identification of different reaction products, like fragments which are mainly related to the chemical dissociation of FIrpic molecules during the OLED operation. We present different reaction pathways of the blue emitter FIrpic. One proposed reaction indicates that the short lifetimes of the OLEDs may be related to the irreversible dissociation of the FIrpic molecule by the loss of carbon dioxide (CO₂) from the picolinate ligand. Additionally, the formation of chemical complexes between different fragments of the FIrpic molecules with their neighbour materials is visible. The cesium adducts of FIrpic formation indicate a possible contribution of the dopant to the OLED degradation process. Finally, we show rearrangement effects in the Cs doped electron transporting layer. This rearrangement is indicated by the presence of m/z signals of the BPhen dimer and adducts of the dimer with Cs.348-352
Yerok Park, Kyu S. Han, Byoung H. Lee, Sangho Cho, Kwang H. Lee, Seongil Im, Myung M. Sung. High performance n-type organic–inorganic nanohybrid semiconductors for flexible electronic devices
Abstract:
We report a high-performance and air-stable flexible and invisible semiconductor which can be substitute for the n-type organic semiconductors. N-type organic–inorganic nanohybrid superlattices were developed for active semiconducting channel layers of thin film transistors at low temperature of 150 °C by using molecular layer deposition with atomic layer deposition. In these nanohybrid superlattices, self-assembled organic layers (SAOLs) offer structural flexibility, whereas ZnO inorganic layers provide the potential for semiconducting properties, and thermal and mechanical stability. The prepared SAOLs–ZnO nanohybrid thin films exhibited good thermal and mechanical stability, good flexibility, transparent in the visible range, and excellent field effect mobility ( 7cm²/V s) under low voltage operation (from -1 to 3 V). The nanohybrid semiconductor is also compatible with pentacene in p–n junction diodes.353-357
Yeon-Il Lee, Jun-Ho Youn, Mi-Sun Ryu, Jungho Kim, Hie-Tae Moon, Jin Jang. Highly efficient inverted poly(3-hexylthiophene): Methano-fullerene [6,6]-phenyl C₇₁-butyric acid methyl ester bulk heterojunction solar cell with Cs₂CO₃ and MoO₃
Abstract:
We have studied the fabrication of an inverted P3HT:PC₇₁BM bulk hetero-junction (IBHJ) solar cell using ultrathin cesium carbonate (Cs₂CO₃) and molybdenum trioxide (MoO3) as the cathodic and anodic buffer layers, respectively. The influence of MoO₃ thickness on the performance of the IBHJ solar cells was investigated for thicknesses ranging from 0,75 to 3 nm and the optimum thickness was found to be 2 nm, which showed the highest power conversion efficiency of 4,01%. We also found that the power conversion efficiency of the IBHJ solar cells is relatively retained even in low temperature while that of BHJ solar cell rapidly decreased with decreasing temperature and the stability in air is also much better.358-363
Silu Tao, Yilan Jiang, Shiu-Lun Lai, Man-Keung Fung, Yechun Zhou, Xiaohong Zhang, Weiming Zhao, Chun-Sing Lee. Efficient blue organic light-emitting devices with a new bipolar emitter
Abstract:
A new multifunctional compound, 4,4'-di-(1-pyrenyl)-4''-[2-(9,9'-dimethylfluorene)]-triphenylamine (DPFA) has been designed, synthesized and applied respectively as hostemitter, electron- and hole-transporters in organic light-emitting devices (OLEDs). The compound exhibits good thermal stability with a glass transition temperature (Tg) of 161 °C and shows blue emissions in both solution and films. OLEDs with DPFA playing various roles have been fabricated and characterized. The results show that DPFA has efficient bipolar charge transport properties for both hole and electron. By using DPFA as a blue emitter in a typical three layer device with a configuration of ITO/NPB(50 nm)/DPFA(20 nm)/TPBI(30 nm)/LiF(0,5 nm)/MgAg, highly efficient blue emission with a maximum luminescence efficiency of 5,1 cd/A (5,3 μm/W) is obtained.364-371
Thue T. Larsen-Olsen, Eva Bundgaard, Kristian O. Sylvester-Hvid, Frederik C. Krebs. A solution process for inverted tandem solar cells
Abstract:
Tandem solar cells with normal and inverted device geometries were prepared by a solution process. Both device types were based on the use of zinc(II)oxide as the electron transporting layer (ETL). The hole transporting layer (HTL) was either PEDOT:PSS for normal geometry tandem solar cells or vanadium(V)oxide in the case of inverted tandem cells. It was found that the inverted tandem solar cells performed comparable or better than the normal geometry devices, showing that the connection structure of vanadium(V)oxide, Ag nanoparticles and zinc(II)oxide functions both as a good recombination layer, ensuring serial connection, and as a solvent barrier, protecting the first photoactive layer from processing of the second layer. This successfully demonstrates a tandem solar cell fabrication process fully compatible with state-of-the-art solution based automated production procedures.372-375
V.Yu. Aristov, O.V. Molodtsova, M. Knupfer. Potassium doped Co phthalocyanine films: Charge transfer to the metal center and the ligand ring
Abstract:
The electronic properties of potassium doped cobalt phthalocyanine (CoPc) films have been investigated using near-edge X-ray absorption fine structure measurements. Our results demonstrate that charge transfer to the CoPc molecules changes both the valence of the phthalocyanine ring as well as that of the central Co ion. The first electron transferred to the molecule occupies a Co 3d state, which is connected to a change of the magnetic moment of the molecule.376-382
Chi-Ping Liu, Wei-Ben Wang, Cheng-Wei Lin, Wei-Chun Lin, Chia-Yi Liu, Che-Hung Kuo, Szu-Hsian Lee, Wei-Lun Kao, Guo-Ji Yen, Yun-Wen You, Hsun-Yun Chang, Jwo-Huei Jou, Jing-Jong Shyue. Molecular migration behaviors in organic light-emitting diodes with different host structures
Abstract:
Over the past years, organic light-emitting diodes (OLEDs) have attracted increasing interest because of their great potential for use in high-quality flat-panel displays and solidstate lighting. One of the basic requirements in any emissive device is to provide adequate stability to ensure a sufficiently long lifetime. Recently, it was observed that small molecules migrate toward the ITO anode under a direct driving voltage while retaining their original structures. To prevent this bias-driven migration of small molecules, a chemical structure with a higher steric hindrance could be introduced as a blockade, thus molecular migration could be suppressed and the device half-life increased. In this work, OLED devices with different hosts, including CBP, mCP, SimCP2, and SimCP, with increasing steric hindrances are fabricated. The spatial distribution of the tracking molecules after operation for different lengths of time is examined by using X-ray photoelectron spectroscopy (XPS) with in situ high-energy C₆₀⁺ and low-energy Ar⁺ co-sputtering for depth profiling. It is found that the bias-driven molecular migration is suppressed and the device half-life prolonged as the steric hindrance of the host increases.383-393
Weining Wang, Diogenes Placencia, Neal R. Armstrong. Planar and textured heterojunction organic photovoltaics based on chloroindium phthalocyanine (ClInPc) versus titanyl phthalocyanine (TiOPc) donor layers
Abstract:
Planar and textured heterojunction solar cells (OPVs) are reported for vacuum deposited chloroindium phthalocyanine (ClInPc)/C₆₀ heterojunctions, and their response compared to previously explored OPVs based on titanyl phthalocyanine (TiOPc)/C₆₀ heterojunctions. As for TiOPc/C₆₀ OPVs, the photoelectrical activity of ClInPc/C₆₀ OPVs extends well into the near infrared, with good activity out to ca. 900 nm. As-deposited ClInPc films (Phase I) produce open-circuit photopotentials, V_OC as high as ca. 0,8 V (Phase I form of ClInPc), ca. 0,15 V larger than previously observed for Phase I TiOPc/C₆₀ OPVs. The offsets in frontier orbital energies (E_HOMO^Pc–E_LUMO^C60) revealed by UV-photoemission studies (UPS) are slightly smaller for ClInPc/C60 versus TiOPc/C₆₀ heterojunctions, and the interface dipole contribution (shift in local vacuum level) to these offsets is in the opposite direction for ClInPc/C₆₀ versus TiOPc/C₆₀ heterojunctions, or missing altogether, suggesting differences in molecular interaction at the Pc/C₆₀ interface. Higher VOC values are correlated with lower reverse saturation currents, Jo, for ClInPc/C₆₀, versus other Pc or pentacene/C₆₀ heterojunctions, suggesting weak intermolecular interactions at the ClInPc/C₆₀ interface and large barriers to dark charge injection. Solvent annealing of the ClInPc films enhances the near-IR response, and textures the Pc film, enhancing the Pc/C₆₀ interfacial contact area and the short-circuit photocurrent, J_SC. J_SC under AM 1,5 illumination conditions was estimated by integration of the incident photon current efficiency (IPCE) response, to compare relative power conversion efficiencies for the two different device types. The estimated efficiency of Phase I ClInPc/C₆₀ OPVs is ca. 2,6%. The estimated AM 1,5 efficiency of ClInPc/C₆₀ OPVs with solvent annealed Pc layers is estimated to be ca. 3,3%, arising from the extensive texturing achieved of the Pc layer, which nearly doubles JSC for the Phase II versus Phase I Pc films.394-403
P.E. Schwenn, P.L. Burn, B.J. Powell. Calculation of solid state molecular ionisation energies and electron affinities for organic semiconductors
Abstract:
Energy level engineering has become one of the central ideas in organic optoelectronics; particularly in the field of organic photovoltaics. However, this paradigm has led to significant ambiguities and errors in the literature. To investigate and address these issues we calculate solid state ionisation energies and electron affinities of a range of organic molecular semiconductors from density functional theory and the polarizable continuum model. We show that the differences between these results and measurements from (inverse) photoemission are on the same scale as the differences between the measured values reported by different groups. We compare our results with in vacuo calculations and estimates of the ionisation energies and electron affinities from the eigenvalues of the Kohn–Sham equations. In vacuo calculations overestimate the ionisation energies measured in the solid state, but underestimate solid state electron affinities. However, the Kohn–Sham eigenvalues predict the measured ionisation energies nearly as well as the full calculation. However, we show that the apparent accuracy of the Kohn–Sham eigenvalues is fortuitous and arises from the cancellation of the errors due to the use of Kohn–Sham molecular orbital energies as predictions of ionisation energies and electron affinities and the neglect of the polarizable solid state environment. These results stress the importance of descriptions based on molecular states rather than molecular orbitals when designing and characterising materials for organic electronic and optoelectronic devices.