In the present review, the main degradation mechanisms occurring in the different layer stacking (i.e. photoactive layer, electrode, encapsulation film, interconnection) of polymeric organic solar cells and modules are discussed. Bulk and interfacial, as well as chemical and physical degradation mechanisms are reviewed, as well as their implications and external or internal triggers. Decay in I-V curves in function of time is usually due to the combined action of sequential and interrelated mechanisms taking place at different locations of the device, at specific kinetics. This often makes the identification of specific root causes of degradation challenging in non-model systems. Additionally, constant development and refinement in terms of type and combination of materials and processes render the ranking of degradation mechanisms as a function of their probability of occurrence and their detection challenging. However, it clearly appears that for the overall stability of organic photovoltaic devices, the actual photoactive layer, as well as the properties of the barrier and substrate (e. g. cut of moisture and oxygen ingress, mechanical integrity), remain critical. Interfacial stability is also crucial, as a modest degradation at the level of an interface can quickly and significantly influence the overall device properties. (C) 2011 Elsevier B. V. All rights reserved.
Solution-based NiOx outperforms PEDOT:PSS in device performance and stability when used as a hole-collection layer in bulk-heterojunction (BHJ) solar cells formed with poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) and PC70BM. The origin of the enhancement is clarified by studying the interfacial energy level alignment between PCDTBT or the 1: 4 blended heterojunctions and PEDOT: PSS or NiOx using ultraviolet and inverse photoemission spectroscopies. The 1.6 eV electronic gap of PEDOT: PSS and energy level alignment with the BHJ result in poor hole selectivity of PEDOT:PSS and allows electron recombination at the PEDOT: PSS/BHJ interface. Conversely, the large band gap (3.7 eV) of NiOx and interfacial dipole (>= 0.6 eV) with the organic active layer leads to a hole-selective interface. This interfacial dipole yields enhanced electron blocking properties by increasing the barrier to electron injection. The presence of such a strong dipole is predicted to further promote hole collection from the organic layer into the oxide, resulting in increased fill factor and short circuit current. An overall decrease in recombination is manifested in an increase in open circuit voltage and power conversion efficiency of the device on NiOx versus PEDOT: PSS interlayers. (C) 2012 Elsevier B.V. All rights reserved.
A hybrid white organic light-emitting diode (WOLED) with an emission layer (EML) structure composed of red phosphorescent EML/green phosphorescent EML/spacer/blue fluorescent EML was demonstrated. This hybrid WOLED shows high efficiency, stable spectral emission and low efficiency roll-off at high luminance. We have attributed the significant improvement to the wide distribution of excitons and the effective control of charge carriers in EMLs by using mixed 4,4',4 ''-tri(9-carbazoyl) triphenylamine (TCTA) and bis[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp2) as the host of phosphorescent EMLs as well as the spacer. The bipolar mixed TCTA: Bepp(2), which was proved to be a charge carrier switch by regulating the distribution of charge carriers and then the exciton recombination zone, plays an important role in improving the efficiency, stabilizing the spectrum and reducing the efficiency roll-off at high luminous. The hybrid WOLED exhibits a current efficiency of 30.2 cd/A, a power efficiency of 32.0 lm/W and an external quantum efficiency of 13.4% at a luminance of 100 cd/m(2), and keeps a current efficiency of 30.8 cd/A, a power efficiency of 27.1 lm/W and an external quantum efficiency of 13.7% at a 1000 cd/m(2). The Commission Internationale de l'Eclairage (CIE) coordinates of (0.43, 0.43) and the color rendering index (CRI) of 89 remain nearly unchanged in the whole range of luminance. (C) 2012 Elsevier B.V. All rights reserved.
Tungsten oxide layer is formed uniformly by a sol-gel technique on top of indium tin oxide as a neutral and photo-stable hole extraction layer (HEL). The solution processed tungsten oxide layer (sWO(3)) is fully characterized by UV-Vis, XPS, UPS, XRD, AFM, and TEM. Optical transmission of ITO/sWO(3) substrates is nearly identical to ITOs. In addition, the sWO(3) layer induces nearly ohmic contact to P3HT as PEDOT: PSS layer does, which is determined by UPS measurement. In case that an optimized thickness (similar to 10 nm) of the sWO(3) layer is incorporated in the organic photovoltaic devices (OPVs) with a structure of ITO/sWO(3)/P3HT: PCBM/Al, the power conversion efficiency (PCE) is 3.4%, comparable to that of devices utilizing PEDOT: PSS as HEL. Furthermore, the stability of OPV utilizing sWO3 is significantly enhanced due to the air-and photo-stability of the sWO(3) layer itself. PCEs are decreased to 40% and 0% of initial values, when PEDOT: PSS layers are exposed to air and light for 192 h, respectively. In contrast, PCEs are maintained to 90% and 87% of initial PCEs respectively, when sWO(3) layers are exposed to the same conditions. Conclusively, we find that solution processed tungsten oxide layers can be prepared easily, act as an efficient hole extraction layer, and afford a much higher stability than PEDOT: PSS layers. (C) 2012 Elsevier B.V. All rights reserved.
In this work, two high boiling-point solvents (1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO)) were utilized as co-additives in ortho-dichlorobenzene (ODCB) and chlorobenzene (CB) solutions to fine tune the donor and acceptor domains in the bulk heterojunction (BHJ) of poly(benzo[1,2-b:4,5-b'] dithiophene-alt-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) and fullerene derivatives. A power conversion efficiency of 7.1% and a fill factor up to 70% were obtained for solar cells with active area of 1 cm(2) when using [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) as acceptor, suggesting that an optimized morphology was achieved. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.
Eliminating processing with halogenated solvents is desirable to achieve sustainable large-scale fabrication of organic solar cells. This work demonstrates a device processing approach completely free of halogenated solvents to yield high-performance (power conversion efficiency, eta(P) > 6%) polymer: fullerene bulk-heterojunction solar cells comprising a conjugated polymer PIDT-phanQ and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Introducing 2% 1-methylnaphthalene (Me-naph) as a processing additive to toluene alleviates PC71BM solubility problems, reduces phase domain size by two orders of magnitude, and boosts efficiency from eta(P) = 0.02% to 6.10%. Both AFM and TEM imaging show that the Me-naph additive promotes a more finely phase-separated morphology in spin-coated films, while photoluminescence quenching and photoinduced absorption spectroscopy confirm that this finer morphology results in both better exciton quenching and more efficient charge separation. (C) 2012 Elsevier B.V. All rights reserved.
Organic photovoltaic devices based on the donor:acceptor blend of poly[N-9 ''-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) have received considerable attention in recent years due to their high power conversion efficiencies and the ability to achieve close to 100% internal quantum efficiency. However, the highest efficiencies were all attained using active layers of less than 100 nm, which is not ideal for either maximised potential performance or commercial viability. Furthermore, more recent reports have documented significant charge carrier trapping in these devices. In this paper two charge extraction techniques (photo-CELIV and time-of-flight) have been used to investigate the mobility and recombination behaviour in a series of PCDTBT:PCBM devices. The results not only confirm significant charge carrier trapping in this system, but also reveal close to Langevin-type bimolecular recombination. The Langevin recombination causes a short charge carrier lifetime that results in a short drift length. The combination of these two characteristics (trapping and fast bimolecular recombination) has a detrimental effect on the charge extraction efficiency when active layers greater than similar to 100 nm are used. This accounts for the pronounced decrease in fill factor with increasing active layer thickness that is typically observed in PCDTBT:PCBM devices. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.
We report thin-film moisture barriers based on Al2O3/ZrO2 nanolaminates grown by ALD for an encapsulation of OLEDs. In order to optimize the moisture-barrier performance of the nanolaminates, the most important factors affecting the performance were sought by measuring WVTR of the nanolaminates via an electrical Ca test. We found out that both the number of interfaces in the nanolaminates and the thickness of ZrO2 in a unit layer were responsible for the performance. By optimizing the nanolaminate structure, the moisture-barrier performance was enhanced up to 350% from a single layer of the same thickness. The WVTR of 30-nm-thick optimized nanolaminate barrier was 2 x 10(-4) g/(m(2) day) or less at ambient condition. A storage-lifetime measurement of an OLED with a 100-nm-thick encapsulation layer showed that it could exceed 70,000 h if stored at ambient condition. (C) 2012 Elsevier B.V. All rights reserved.
Highly efficient deep blue phosphorescent light emitting diodes were developed using a newly synthesized series of blue emitting tridentate platinum emitters. Devices employing a cohost of hole and electron transport materials yielded high external quantum efficiencies with low turn on voltage and low efficiency roll off. A maximum EQE of 15.7% and CIE coordinates of (0.16, 0.13) was achieved in a device based on platinum(II) bis(N-methyl-imidazolyl)benzene chloride (Pt-16). (C) 2012 Elsevier B.V. All rights reserved.
We present a simple hybrid white organic light-emitting diodes (WOLED) consisting of only two layers, i.e., a hole-transporting layer and an emitting layer. The emitting layer is formed by simply co-doping a green phosphor and a red phosphor in bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp(2)), which acts as the blue emitter, electron-transport material, and high triplet energy host for the phosphors, i.e., a multifunctional chromophore. This simple device exhibits a maximum power and quantum efficiency of 46.8 lm W-1 and 16.5%, respectively, with a good CRI up to 90. The versatile experimental techniques are performed to gain a deep understanding of the emission mechanism. We believe that this simple design concept can provide a new avenue for achieving ultrahigh performance WOLEDs for lighting application. (C) 2012 Elsevier B.V. All rights reserved.
We present a QQVGA top emitting monochrome AMOLED display with 85dpi resolution using an organic TFT backplane on low temperature PEN-foil. The backplane process flow is based on a 7 layer photolithography process that yields a final mobility of the OTFT of similar to 0.4 cm(2)/Vs. The aperture ratio of the top-emitting OLEDs is over 75%. For operation at 10 V supply voltage (V-DD), the brightness of the display using red and green OLEDs exceeds 200 cd/m(2). (C) 2012 Elsevier B.V. All rights reserved.
A DNA sensor based on a water-gated organic field-effect transistor is described. The semiconductor is poly [3-(5-carboxypentyl)thiophene-2,5-diyl] onto which DNA probes are covalently grafted via NHS/EDC chemistry. Clear changes in the output characteristic of the device are observed upon DNA immobilization and after DNA hybridization. Experimental data point out the importance of the electrolyte Debye length that can screen negative DNA charges and impede transduction. For this reason, deionized water was used in order to increase the Debye length up to several hundreds of nanometers. In this case, a decrease in the off current was observed upon hybridization, whereas no significant change occurred when using saline solutions. (C) 2011 Elsevier B. V. All rights reserved.
We compare standard and inverted bulk heterojunction solar cells composed of PCPDTBT:PC70BM blends. Inverted devices comprising 100 nm thick active layers exhibited short circuit currents of 15 mA/cm(2), 10% larger than in corresponding standard devices. Modeling of the optical field distribution in the different device stacks proved that this enhancement originates from an increased absorption of incident light in the active layer. Internal quantum efficiencies (IQEs) were obtained from the direct comparison of experimentally derived and modeled currents for different layer thicknesses, yielding IQEs of similar to 70% for a layer thickness of 100 nm. Simulations predict a significant increase of the light harvesting efficiency upon increasing the layer thickness to 270 nm. However, a continuous deterioration of the photovoltaic properties with layer thickness was measured for both device architectures, attributed to incomplete charge extraction. On the other hand, our optical modeling suggests that inverted devices based on PCPDTBT should be able to deliver high power conversion efficiencies (PCEs) of more than 7% provided that recombination losses can be reduced. (c) 2012 Elsevier B.V. All rights reserved.
A series of simple structured small molecules based on diketopyrrolopyrrole (DPP) are synthesized and their photovoltaic properties are investigated in terms of the type of electron donating unit. By introducing a donor unit with different electron-donating power such as thiophene (T) and phenylene (Ph), into A-D-A type small molecule, the frontier orbital energy levels of small molecules can effectively be tuned. The small molecule with a weak donor unit of Ph, Ph(TDPP)(2) exhibits a power conversion efficiency of 4.01% with a remarkably high open circuit voltage of 0.93 V when it is blended with [6,6]-phenyl-C-71-butyric acid methyl ester as an active layer material in bulk heterojunction solar cells. (C) 2012 Elsevier B V. All rights reserved.
In this paper, we successfully improved the spectral stability in blue/orange complementary white organic light-emitting diodes (OLEDs) by utilizing hole-type single host double emissive layer structure. The demonstrated double emissive layer structure effectively suppresses the direct recombination of electron-hole pairs on the hole-trapping orange phosphor and thus reduces the deteriorated effect of charge trapping on electroluminescence spectrum stability by controlling exciton recombination zone. It is shown that the white light emission is a cascade energy transfer process from host to blue phosphor and then to orange phosphor, which seems to be less affected by the driving conditions. Thus, the change in Commission Internationale de L'Eclairage coordinates (CIE) in the white OLEDs is less than (+/- 0.010, +/- 0.007) as the voltage increases from 4 V to 9 V, which correspond to the luminance increasing from 200 cd m(-2) to about 20,000 cd m(-2). This is superior to that of co-doped single emissive layer devices, which show much larger CIEs variation of (+/- 0.05, +/- 0.02) in the same driving voltage range. We gave detailed analysis on the exciton recombination processes and well elucidated the working mechanism of the fabricated double emissive layer structure white OLEDs. (C) 2012 Elsevier B.V. All rights reserved.
Organic solar cells (OSCs) have attracted much attention as a clean and renewable energy convention system, owning to the low-cost and easy-processing nature of organic semiconductors. While indium tin oxide (ITO) is commonly used in OSCs as the transparent conductive electrode, the rising cost of indium, the high temperature process and the poor flexibility of ITO, make it incompatible with large-scale roll-to-roll manufacture of OSCs. In this paper, the MoO3/thin metal/MoO3 trilayer structure was used to replace the ITO electrode in OSCs. The optical and electrical properties of the trilayer were shown to depend on the material and thickness of the intermediate metal layer. The maximum power conversion efficiency of up to 2.5% under simulated 1 sun AM 1.5 solar illumination was achieved for OSCs based on poly(3-hexylthiophene) (P3HT) and [6,6[-phenyl-C-61-butyric acid methyl ester (PCBM), compared to a maximum efficiency of 3.1% for the ITO-based devices. Moreover, due to the flexible nature of the trilayer structure, the OSCs with the trilayer electrode exhibited good mechanical flexibility. The efficiency of the flexible device was only reduced by similar to 6% from its original performance after 500 bending cycles with a bending radius of 1.3 cm. Therefore, the performance of the ITO-free devices on rigid/flexible substrates suggests that this oxide/metal/oxide trilayer electrode is a promising ITO replacement in OSCs. (C) 2012 Published by Elsevier B.V.
The molecular design strategies for the host materials suitable for highly efficient, blue fluorescent organic light-emitting diodes (OLEDs) are demonstrated. The device characteristics of blue fluorescent OLEDs are compared with different host materials. Some devices exhibit a highly efficient blue electroluminescence with a high external quantum efficiency of more than 7%. The correlation between OLED efficiency and triplet-triplet annihilation is characterized by measuring the up-conversion of triplet excited states into singlet ones. The host materials require an anthracene unit and a bulky molecular structure to prevent the overlap of anthracene units between adjacent molecules in the film. (C) 2012 Elsevier B. V. All rights reserved.
Nonvolatile memory devices, based on electrical conductance tuning in thin films of poly(N-vinylcarbazole) (PVK)-graphene composites, are fabricated. The current density-voltage characteristics of the fabricated device show different electrical conductance behaviors, such as insulator behavior, write-once read-many-times (WORM) memory effect, rewritable memory effect and conductor behavior, which depend on the content of graphene in the PVK-graphene composites. The OFF and ON states of the WORM and rewritable memory devices are stable under a constant voltage stress or a continuous pulse voltage stress at a read voltage of -1.0 V. The memory mechanism is deduced from the modeling of the nature of currents in both states in the devices. (C) 2012 Elsevier B.V. All rights reserved.
Organic light emitting diodes (OLEDs) with surface plasmon (SP) enhanced emission have been fabricated. Gold nanoclusters (GNCs) deposited using thermal evaporation technique has been used for localization of surface plasmons. Size of GNCs and distance of GNCs from the emissive layer have been optimized using steady state and time resolved photoluminescence (PL) results. 3.2 Times enhancement in PL intensity and 2.8 times enhancement in electroluminescence intensity of OLED have been obtained when GNCs of size 9.3 nm has been introduced at a distance of 5 nm from emissive layer. Distance dependence of energy transfer efficiency between exciton and SPs was found to be of 1/R-4 type, which is typically the dependence for dipole-surface energy transfer. (C) 2011 Elsevier B. V. All rights reserved.