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... order to optimize the silver grid geometry a simplified model of a solar module was developed (Fig. 1). It is based on values of a well working reference solar cell with ITO and evaporated silver electrodes. The model considers the physical parameters of the used materials such as the conductivities of the PEDOT:PSS and silver, the contact resistances at the interconnect region and between PEDOT:PSS and silver grid as well as the ...
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... Owing to its unique physical characteristics and excellent electrical stability, Silver Nanowire (AgNW) has emerged as a promising candidate for the charge transport conducting layer in organic devices and also in flexible devices to replace the well-known ITO [7][8][9][10]. AgNWs are chemically synthesised by the polyol reaction of silver nitride, and the layer can be produced by non-vacuum deposition machines such as inkjet printing [11,12], spray printing [13][14][15], Meyer rod [16], and roll-to-roll techniques [17], which makes them a cost-effective, scalable, and simple approach as device electrodes [18]. ...
Despite a great potential for low-voltage display applications, vertical organic light-emitting transistors (VOLETs) suffer serious issues of high-cost and complex fabrication techniques, notably for the intermediate electrode. To address this problem, this study demonstrates a cost-effective and simple approach to fabricate a VOLET device by utilising spin-coated silver nanowires (AgNWs) as an intermediate electrode. AgNWs exhibit high electrical conductivity, high porosity and high optical transparency, which qualify them as a perfect candidate for the intermediate electrode in VOLETs. To show the potential of AgNWs in VOLET devices using a facile, cost-effective spin-coated method, two types of VOLETs, namely, the Schottky barrier (SB) VOLET and static induction transistor (SIT) VOLET, were fabricated and analysed. Interestingly, both the devices show transistor behaviour when the Vg is varied, implying a fully functional VOLET device. We believe that this is one of the simplest methods to fabricate VOLETs without compromising the device characteristics demonstrated to date.
... Thus, it has been shown that transport and emitting layers can be deposited using this technique and integrated in a full device [29][30][31][32][33][34]. Other functional layers, such as electrodes, metallic grids or secondary optics can also be fabricated [35][36][37][38][39][40]. By printing insulating materials, a lateral structuring and therefore a pixellation of the device can be achieved [41]. ...
We have developed inkjet-printed scattering layers for enhanced light extraction in organic light emitting diodes (OLEDs). These layers are based on scattering polymer/nanoparticle composites, which are prepared from a solvent-free process and used for the outcoupling of waveguide modes to free propagating modes, thus improving the device efficiencies. Two different monomers are examined and an inkjet-printing process is developed for each of them. We first analyze the inks' rheological properties, followed by the optical and morphological properties of the resulting layers. Upon integration of these printed layers into an OLED stack, the device efficiencies are increased by up to 40% with respect to an unpatterned reference device. Furthermore, we show that these internal light extraction layers improve the angular and spectral stability of the devices.
Organic solar cells (OSCs) have attracted much attention due to their advantages in fabricating flexible and semi‐transparent devices. Especially, the light weight, flexibility and spectral adjustability make OSCs superior to silicon, perovskite and other thin film based solar cells in applications of integrated photovoltaic devices and wearable electronics. In flexible and semi‐transparent OSCs, transparent conducting electrodes (TCEs) play a key role in obtaining high performances. Among various TCEs, silver nanowire (AgNW) has become a promising candidate due to its low sheet resistance, high optical transparency, excellent mechanical flexibility and solution processability. In this article, we review the recent advances in AgNW‐based TCEs and their applications in the field of OSCs. Firstly, we introduce the general properties of AgNW, including optoelectronic and mechanical characteristics. Secondly, the preparation methods of AgNW are discussed, along with some approaches on the optimization of AgNW to overcome the shortcomings of TCEs. Thirdly, we discuss the applications of AgNW as TCEs in fabricating flexible and semi‐transparent OSCs, including the use of AgNW as bottom and top electrodes. Finally, we point out the challenges in AgNW‐based TCEs and suggest several guidelines for preparing AgNW so as to meet the demands for the practical use of OSCs.
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This chapter provides a review on solution processed barriers for flexible organic electronics. After elaborating on the necessity for encapsulation of organic devices by describing the degradation mechanisms caused by oxygen and water, the state of the art of vacuum-deposited barriers is briefly addressed as it presently defines the benchmark of barrier technology. Subsequently, the fundamentals of permeation of gases through barriers and the characterization methods for barrier performance are described. The main part of the chapter gives an overview of the material systems used for manufacturing solution-processed barriers. We start with the methods relying on reducing diffusion and solubility coefficients of oxygen and water in barrier films. Subsequently, barriers based on tortuosity and barriers based on covering substrates with layers of zero or very low permeability are described. Finally, the role of getters is elucidated. The chapter is concluded with highlighting some of the present challenges in the field of solution processed flexible barriers.
A simple spin-coating process for fabricating vertical organic light-emitting transistors (VOLETs) is realized by utilizing silver nanowire (AgNW) as a source electrode. The optical, electrical and morphological properties of the AgNW formation was initially optimized, prior VOFET fabrication. A high molecular weight of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] MEH-PPV was used as an organic semiconductor layer in the VOFET in forming a multilayer structure by solution process. It was found that current density and luminance intensity of the VOLET can be modulated by a small magnitude of gate voltage. The modulation process was induced by changing an injection barrier via gate voltage bias. A space-charge-limited current (SCLC) approach in determining transistor mobility has been introduced. This preliminary and fundamental work is beneficial towards all-solution processing display devices.
Thin-film photovoltaics (PV) is an emerging technology. However, further improvement and analysis of this technology are needed in order to become well-established in the PV market. This requires extensive experimental investigations which are often very cost- and time-intensive. Here, simulations can complete and verify experimental work. In cases where the experiments are not possible, they may even go beyond experimentally accessible parameters. Furthermore, simulations yield profound understanding of physical processes in the PV devices that cannot be measured by characterization methods. Most of the time, simulations are used for the optimization of products. In this work simulations were used to investigate relevant topics in thin-film PV with different objectives. One common problem in thin-film PV modules are shunts that can be generated already during the production process and which can dramatically reduce the output power of the devices. Therefore, it is important to understand the shunt’s influence on the module performance. However, shunts also cause hot spots in PV devices. Depending on their temperatures, the hot spots can cause severe damage of the entire module. Hence, the information on the temperatures and shunt parameters for exceeding critical temperatures is of interest for estimating the relevance and probability of detrimental shunts. As a well-controlled generation of shunts of specified properties can hardly be realized and verified experimentally, simulations were used for the investigations. One of the main tasks of simulations of PV devices is the optimization. Thereby, the simulations deal with a large set of parameters and improve for instance the design of grid electrodes or multi-junction cells. Grid electrodes are a very suitable electrode technology for PV devices. TF PV modules can be produced with a large variety in grid design, especially for fully inkjet printed ones. Multi-junction cells can exceed the efficiencies of single-junction cells. Hence, the prediction of their efficiencies for different material combinations in the cell stack is of special interest. Due to recently developed transparent and electrical conductive interface materials the preparation of parallel connected subcells became possible. Therefore, series, parallel and mixed electrical connection types were simulated for double- and triple-junction cells and compared to each other. This was done for organic PV cells with absorber materials, which are by part not yet developed, and for different absorption windows. In order to examine the electrical influence of a shunt in a copper indium gallium diselenide (CIGS) module and to optimize the grid electrodes in organic PV modules, dedicated 2D spatially resolved electrical finite element models were developed. For temperature simulation of shunted organic PV cells, the model was extended to a 3D electrothermal one for taking the thermal effects in perpendicular direction into account. The efficiencies of multi-junction organic PV cells were simulated by using a semi-empirical model based on a simplified calculation approach of the electrical connection types. Using these methods the following results were obtained. The relative maximum power output and the relative open-circuit voltage of a shunted monolithically integrated CIGS module revealed that the irradiance has the highest influence besides the shunt resistance. For increasing irradiance the relative performance values increased due to shunt screening in the resistive (front) electrode. Hence, the shunt has most impact under low light conditions. The shunt size and its position inside the module were investigated as well as the distance between two shunts in one cell. The latter shows maximum performance reduction for equal distribution of the two shunts. Visualization of the lateral current densities in the electrode layers reveals current loops in the shunted cell which even progress into the neighboring cells. Thereby, the shunt current flows mainly in the electrode with the lower sheet resistance (except for short distances to the shunt). Due to this, the shunt position determines the geometry of the lateral currents in the module and the shunt’s impact on module performance. Hereby, the shunt screening due to the electrodes plays an important role. The temperature development and distribution was investigated in organic PV cells with diketopyrrolopyrrole-quinquethiophene alternating copolymer (pDPP5T-2):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction as active absorbing layer and a metallic ohmic shunt under open-circuit conditions and 1000 W/m² irradiance by varying the shunt radius, its electrical conductivity, the encapsulation material and the sheet resistance of the transparent front electrode. Critical temperatures of more than 400 K (that can damage the cell) are easily exceeded for shunt radii < 2.44 µm for plastic foil encapsulated cells. The value of electrical shunt conductivity only plays a minor role. For most radii the highest temperatures do not appear in the shunt, but in the lower electrical conductive electrode around the shunt caused by higher resistive losses. For glass encapsulated cells the critical shunt radius decreases to 0.71 µm due to the higher thermal conductivity compared to plastic. Organic PV modules with different grid electrode structures (lines, triangles, squares, hexagons) were optimized with respect to active cell length, unit cell width, silver grid width and thickness of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-layer using poly(3-hexylthiophene-2,5-diyl) (P3HT):PCBM bulk heterojunction as active absorbing layer. Thereby, the features of inkjet printers (e.g. line width ≥ 50 µm) were taken into account. The optimum parameters are: active cell length = 5 - 6 mm, grid width = 50 µm and PEDOT:PSS-thickness = 59 nm. All grid structures show almost the same maximum power output (≈ 27 W/m²) – with a slightly higher value (< 1.6 %) for the line structure. However, for grid interruptions in or close to the patterning region the other three grid structures have lower losses due to their “network” structures and, thus, yield marginally higher maximum efficiencies. Theoretical efficiency studies were performed for organic multi-junction PV cells by varying the band gap energies of the absorber materials, the electrical connection types between the subcells and the absorption window types. The results reveal a maximum efficiency of 15.0 % for the series and 13.6 % for the parallel connected double-junction cells. (Best single-junction cell has 11 % efficiency.) For the triple-junction cells the parallel connection of the front subcell with the series connected middle and back subcells (maximum efficiency = 17.3 %) exceeds the purely series connected cell in maximum efficiency and in variety of absorber material combinations for efficiencies higher than 14.5 %. A realistic limitation of the band gap energies (Eg ≥ 1.4 eV) available for organic absorber materials decreases the maximum efficiencies to 14.5 % and 16.5 % for double- and triple-junction cells, respectively. Many of the findings have general characters and are applicable to other thin-film PV technologies. The spatially resolved analysis of electric current, electric potential and temperature distribution give a profound understanding of the physics in cells and modules. The specific simulation results for shunted module performance, hot spot temperatures and grid electrode optimizations set a good starting point for further experimental work and investigations. The efficiency calculations of multi-junction cells with generic connection types provide a roadmap to future highly efficient organic PV cells.
