KEYWORDS: Perovskite, Solar cells, Zinc oxide, Lead, Nanoparticles, Electron transport, Energy conversion efficiency, Chemical elements, Iodine, Aluminum
Perovskite solar cells have attracted tremendous attention for their outstanding energy conversion efficiency in the past few years. Due to the development of active materials, device architectures and processing methods, power conversion efficiency (PCE) of perovskite solar cells is now growing up to 20%. Beyond the efficiency, to get rid of Lead, the widely-used toxic element in the perovskite layers, as well as to improve the device/module operation lifetime are the other two major challenges that need to be solved before their commercialization.
Here, we apply a layer of ZnO nanoparticles onto to a planar perovskite solar cell, which can not only improve the electron transport/extraction in the devices but highly improve the device operation lifetime. The devices were fabricated by spin-coating a poly(3,4-ethylenedioxythuiphene):polystyrene sulfonate (PEDOT:PSS) layer onto a glass/ITO substrate, followed by the deposition of a perovskite layer from a lead chloride (PbCl2) and methyl ammonium iodine (MAI) blend precursor solution. After that, a layer of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and a layer of ZnO nanoparticles were successively deposited as the electron transport layers, and the device was finished by thermally evaporation Al as the cathode. Such planar perovskite solar cell with ZnO NPs exhibits a maximum PCE of up to 14.1%, which is about 35% higher than that without the ZnO layer. Moreover, the device remains 80% of its initial PCE after 2500 hours under 1 sum illumination, majorly due to the protection of ZnO layer that prevent the diffusion of oxygen and moisture molecules into the perovskite layers as revealed by x-ray photoelectron spectroscopy studies.
Transparent conductive electrodes are one of the essential components for organic optoelectronic devices, including photovoltaic cells and light-emitting diodes. Indium-tin oxide (ITO) is the most common transparent electrode in these devices due to its excellent optical and electrical properties. However, the manufacturing of ITO film requires precious raw materials and expensive processes, which limits their compatibility with mass production of large-area, low-cost devices. The optical/electrical properties of ITO are strongly dependent on the deposition processes and treatment conditions, whereas its brittleness and the potential damage to underlying films during deposition also present challenges for its use in flexible devices. Recently, several other transparent conductive materials, which have various degrees of success relative to commercial applications have been developed to address these issues. Starting from the basic properties of ITO and the effect of various ITO surface modification methods, here we review four different groups of materials, doped metal oxides, thin metals, conducting polymers, and nanomaterials (including carbon nanotubes, graphene, and metal nanowires), that have been reported as transparent electrodes in organic optoelectronic materials. Particular emphasis is given to their optical/electrical and other material properties, deposition techniques, and applications in organic optoelectronic devices.
We demonstrated enhanced efficiency in small molecule organic photovoltaic devices using dual organic interfacial layers of PEDOT:PSS followed by tetracene between the ITO anode and the organic donor material. The use of a small molecular templating layer, such as tetracene, proved to increase the molecular stacking of the subsequent phthalocyanine (Pc) based donor materials. Upon application in planar heterojunction devices of ZnPc and C60, an enhancement of over 80 percent in the donor contribution to the external quantum efficiency was observed attributed to the combination of exciton blocking by the higher band gap tetracene layer and enhanced exciton diffusion and charge transport resulting from the increased crystallinity.
White light emitting devices were fabricated using blue emitting organic light emitting diodes (OLEDs) and down-conversion phosphor mixtures. Three different thicknesses of yellow and mixtures of yellow and red luminescent phosphor films were prepared on separate glass slides using a silicone matrix. The down-conversion films were optimized by varying the thickness and phosphor to silicone weight ratio. The phosphor films with different thickness were coupled to an optimized blue emitting OLED with a refractive index matching gel. Optimized down-conversion phosphor layers integrated with blue OLEDs exhibited 2× enhancement of efficiency (lumens per electrical Watt) for white to that of the blue OLED. The International Commission of Illumination color coordinates and average color rendering index for this device were (0.43, 0.46) and >80, respectively.
The most commonly used transparent electrode, indium-tin oxide (ITO), is costly and requires methods of deposition that are highly destructive to organic materials when it is deposited on top of the organic layers in top-emitting organic light-emitting devices (OLEDs). Here we have employed a trilayer electrode structure consisting of a thin layer of metal sandwiched between two MoO3 layers, which can be deposited through vacuum thermal evaporation without much damage to the organic active layers. Such MoO3/Au/MoO3 trilayer electrodes have a maximum transmittance of nearly 90% at 600 nm and a sheet resistance of <10 ohms per square (Ω/sq) with a 10-nm thick Au intermediate layer. Using these trilayers as the top transparent anode, we have fabricated top-emitting OLEDs based on either a fluorescent or phosphorescent emitter, and observed nearly identical emission spectra and similar external quantum efficiencies as compared to the more conventional bottom-emitting OLEDs based on the commercial ITO anode. The power efficiency of the top-emitting devices is 20% to 30% lower than the bottom-emitting devices due to the somewhat inferior charge injection in the top-emitting devices. The performance and emission characteristics of these devices indicate that this trilayer structure is a promising candidate as a transparent anode in top-emitting OLEDs.
We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.
Organic light emitting devices (OLEDs) have demonstrated the potential for solid state lighting as well as full color
display applications. Use of triplet harvesting phosphorescent materials has led to very high efficiency OLEDs especially
in green and red phosphorescent OLEDs. However in case of blue OLEDs the efficiency achieved is still room for
improvement. Charge balance is a very important factor for achieving high efficiency organic light emitting diodes. In
most OLED devices, hole mobility of hole transport layer is orders of magnitude higher than the electron mobility of
electron transport layer. We study how this affects the charge balance and hence the device performance in the blue
phosphorescent OLEDs with Iridium (III)bis
[(4,6-di-fluorophenyl)- pyridinato-N,C2´] picolinate (FIrpic) emitter.
Charge balance is studied in these devices and the devices are found to be hole dominant. Additionally, effect of charge
balance on device performance is demonstrated with different electron transport layers. Using this approach, a very high
efficiency of 60 Cd/A (50 lm/W) is achieved with
3,5´-N,N´-dicarbazole-benzene (mCP) host.
We report studies on blue and white organic light-emitting devices (OLEDs) based on the deep-blue electrophosphorescent
dye iridium(III)
bis(4',6'-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). Using high triplet
energy charge transport layers and a dual-emissive-layer structure as well as the p-i-n device structure, we have achieved
external quantum efficiencies of 20% and maximum power efficiency of 36 lm/W in these deep-blue OLEDs. White
OLEDs with a CRI of 79 and a maximum power efficiency of 40 lm/W were also demonstrated by incorporating red and
green phosphorescent dopants together with FIr6.
We have studied the effects of hole transporting layers and electron transporting layers on efficiencies of Iridium(III)bis
[(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) doped 3,5'-N,N'-dicarbazole-benzene (mCP) host blue
PHOLEDs. We found that the device efficiency is very sensitive to the hole transporting materials used and both the
triplet energy and carrier transport properties affect the device efficiency. On the other hand, there is no apparent
correlation between the device efficiency and the triplet energy of the electron transporting material used. Instead, the
device efficiency appears to be determined by the electron mobility of the electron transporting layer only.
One very important factor limiting the power conversion efficiency of the current state-of-the-art organic solar cells is the low energy conversion efficiency during the conversion process of an absorbed photon to an electron-hole pair collected at the electrodes. The absorption of a 2 to 3 eV photon typically leads to an open-circuit voltage of 0.5-0.6 V, representing approximately 80% energy loss. In this paper, we show that the open-circuit voltage of an organic donor-acceptor heterojunction cell is related to both the photocurrent and the dark current. Many factors, such as illumination intensity, organic heterojunction structure, electrode properties, operating temperature, can have significant impact on the open-circuit voltage. We also show that the conventional wisdom of using the "effective" gap of an organic donor-acceptor heterojunction to determine the maximum open-circuit voltage needs to be carefully re-examined. While the study shows that the open-circuit voltage in the copper phthalocyanine-C60 heterojunction cell still has some room for improvement, ultimately new materials will have to be used to boost the power conversion efficiency of organic solar cells to the 20% regime.
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