Multilayer Organic Solar Cells Research Articles

Determination of the Optimum Values of the Natural Frequency of the Nanocomposite Multilayer Organic Solar Cell by using Basic Differential Evolution Optimization Algorithms

By the analytical approach, the equation of natural frequency of nanocomposite multilayer organic solar cell (NMOSC) is analyzed in this work. The NMOSC is composed of 6 layers: Al, LiF, P3HT: PCBM, PEDOT: PSS, ITO and Glass. By using the classical plate theory, the Hooke’s Law, the geometrical compatibility equation and the nonlinear equilibrium equations are proposed. These equations combined with the Galerkin method in order to investigate the effect of elastic foundations, geometrical parameters and initial imperfection on the natural frequency. Besides, in order to determine the maximum natural frequency of NMOSC, basic differential evolution optimization algorithm (DE) was used with five variables: elastic foundations include Winkeler foudation and Pasternak foudation, temperature, width and length. In the numerical results, the influence of length to width ratio, imperfection, elastic foundations and temperature increment were evaluated in detail. The optimal results from DE algorithm are shown and compared.

Nonlinear static stability and optimal design of nanocomposite multilayer organic solar cells in thermal environment

Nonlinear static stability and optimal design of nanocomposite multilayer organic solar cells in thermal environment

Top‐Gate Field‐Effect Transistor as a Testbed for Evaluating the Photostability of Organic Photovoltaic Polymers

Light‐induced performance degradation in organic solar cells (OSCs) is a major impediment to their commercialization. As the photostability of OSCs strongly depends on the material's properties, the most effective solution for this concern is to develop a photostable material. However, the wide variety of causes of photo‐instability in a standard multilayered OSC structure complicates the evaluation of photostability of newly developed materials. To address this challenge, a top‐gate field‐effect transistor (FET) as a testbed for evaluating the photostability of OSC materials is proposed. This device test platform minimizes the internal and external origins of photo‐instability by employing a fluoropolymer gate dielectric. The photostability of an OSC material incorporated in this FET testbed can be evaluated by monitoring light‐induced mobility degradation. Two types of common donor polymers with similar chemical structures and crystallinity are employed as test materials, and their photostability is evaluated. The test results correspond to the photostability measurements conducted in the standard OSC structure, validating the proposed FET testbed. The proposed FET testbed enables rapid evaluation of the photostability of a newly developed OSC material, thereby providing timely feedback to material scientists. This boosts the development of photostable OSC materials.

Nonlinear vibration and geometric optimization of nanocomposite multilayer organic solar cell under wind loading

This paper presents an analytical approach on the nonlinear vibration of nanocomposite multilayer organic solar cell (NMOSC) subjected to the combination of wind load and uniform temperature change. The NMOSC comprises of five layers, which are made of Al, P3HT:PCBM, PEDOT:PSS, Graphene and Glass. Compared to common models of the NMOSC, ITO is replaced by graphene to control the output colors of NMOSC. The nonlinear motion equations and the compatibility equation are deriving by taking into account the effect of viscous damping, von Karman nonlinearity terms, elastic foundations and initial imperfection. The formula of the wind load is improved for consideration in the general case that the direction of the wind load is not just perpendicular to the structure. Four edges of the NMOSC are assumed to be simply supported and immovable in the transverse plane. The Galerkin and Runge – Kutta methods are applied to obtain the dynamic response and the natural frequency of the NMOSC. Four optimization algorithms (Bees Algorithm, Basic differential evolution algorithm, enhanced colliding bodies optimization algorithm, social group optimization algorithm) are used to determine the maximum value of natural frequency of the NMOCS, which depends on nine variables including the geometrical parameters, elastic foundations stiffness and temperature increment. In the numerical results, the effects of elastic foundations, initial imperfection, viscous damping ratio, temperature increment, wind load and the length to width ratio on the dynamic response and natural frequency are considered in details. The maximum values of the natural frequency of the NMOSC with four algorithms are obtained and compared. The results show that the optimal value obtained by four algorithms is close to each other. However, the calculation time with enhanced colliding bodies optimization algorithm is the fastest and with Bees Algorithm is the longest of all.

Nonlinear stability and optimization of thin nanocomposite multilayer organic solar cell using Bees Algorithm

Abstract This paper carries out the nonlinear stability of nanocomposite multilayer organic solar cell (NMOSC) subjected to axial compressive loads. The model of organic solar cell is assumed to consist five layers: Al, P3HT:PCBM, PEDOT:PSS, IOT and Glass. Based on the classical plate theory, the basic equations are established taking into account the effect of elastic foundations and initial imperfection. The approximation solutions are selected based on the boundary conditions of the four edges of NMOSC. The equation which indicates the relationship between axial compressive loads and deflection amplitude of NMOSC is obtained by using the Galerkin method. Bees Algorithm is applied to maximize the value of critical buckling load with nine variables including the thickness of five layers, the length and the width of NMOSC and two stiffness coefficients of elastic foundations. The numerical results show the effect of geometrical and material parameters, initial imperfection and elastic foundations on the nonlinear static stability and the critical buckling load of NMOSC. Optimal values of nine geometrical parameters of NMOSC are also determined.

Impact of concentration variation and thermal annealing on performance of multilayer OSC consisting of sandwiched P3HT layer between PEDOT:PSS and P3HT:PCBM

In this paper, comprehensive study on variation in composition ratio and thermal annealing of multilayer organic solar cells (OSC) has been demonstrated. Our previous work reported that in comparison to the conventional OSC structure, multilayer OSC consisting of the pure donor phase between the hole transport layer and active layer has shown a significant improvement in the power conversion efficiency (PCE). Since photovoltaic performance of the OSC strongly influenced by composition ratio, the impact of poly 3-hexylthiophene (P3HT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) weight ratio has been analyzed. It has been found that, the short-circuit current density (JSC) of the device is highest for the P3HT:PCBM composition ratio of 1.0:0.8 and decreases slightly for 1.0:1.0 weight ratio. This study has also been accompanied with the thermal annealing on spin coated P3HT:PCBM which further enhances the PCE of the device. The maximum JSC and the PCE of 9.47 mA/cm2 and 3.26% has been achieved for the multilayer OSC having P3HT:PCBM composition ratio of 1.0:0.8 and thermally annealed at 140 °C. This is attributed largely by the tuned phase morphology of the donor-acceptor. It has been found that, the P3HT:PCBM weight ratio of 1.0:0.8 results into efficient photon absorption and charge separation in the bulk heterojunction OSC. The reduced concentration of PCBM in 1.0:0.8 weight ratio leads to the strong interchain interaction of P3HT molecules which enhances the charge separation and collection ability of the bulk heterojunction OSC.

Performance Analysis and Thermal Reliability Study of Multilayer Organic Solar Cells

The progress in organic solar cell (OSC) technology mainly focuses on improving the power conversion efficiency (PCE), longer lifetime and the development of scalable cost-effective manufacturing. In this paper, the impact of P3HT layer inserted between PEDOT:PSS hole transport layer (HTL) and P3HT:PCBM active layer has been comprehensively studied. The comparative analysis of the proposed OSC structure with the conventional OSC illustrates that, the multilayer structure significantly improves the short circuit current density ( ${J} _{SC}$ ), fill factor (FF) and PCE of the device. This improvement in the device performance is mainly because of the increased photo absorption, reduction in the recombination losses due to improved vertical phase gradient and the efficient collection of photogenerated charge carriers at the electrodes. In addition to this, in order to determine the impact of thermal stress on the device performance, the thermal reliability study on the proposed multilayer OSC and the conventional OSC has been carried out. It has been found that, with annealing at 140 °C temperature for 10 minutes duration, the maximum PCE of 3.27% and 2.44% has been achieved for proposed multilayer OSC and the conventional OSC respectively. It has also been observed that, in comparison to conventional OSC the proposed multilayer OSC is less susceptible to the thermal degradation. Thus, the proposed multilayer OSC not only improves the device performance but also exhibits better thermal stability under different thermal conditions.

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoOx (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoOx. Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoOx molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoOx/BHJ. The high recombination resistance (~5 kΩ cm2) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

Nonlinear dynamic response and vibration of nanocomposite multilayer organic solar cell

In the recent years, organic solar cell (OSC) has attracted much interest of the research community due to its great promise as renewable sources. This paper presents the first analytical approach to investigate the nonlinear dynamic response and vibration of imperfect rectangular nanocompsite multilayer organic solar cell subjected to mechanical loads using the classical plate theory. Nanocompsite organic solar cell consists of five layers of Al, P3HT:PCBM, PEDOT:PSS, IOT and glass. Motion and compatibility equations are derived using the classical plate theory and taking into account the effects of initial geometrical imperfection and geometrical nonlinearity in Von Karman – Donnell sense. The Galerkin method and fourth – order Runge – Kutta method are used to give explicit expressions of natural frequencies, nonlinear frequency – amplitude relation and nonlinear dynamic responses of nanocompsite organic solar cell. The numerical results show the influences of geometrical parameters, the thickness of layers, imperfections, and mechanical loads on the nonlinear dynamic response and nonlinear vibration of nanocompsite organic solar cell.

Charge transport in quantum dot organic solar cells with Si quantum dots sandwiched between poly(3-hexylthiophene) (P3HT) absorber and bathocuproine (BCP) transport layers

We have modeled a multilayer quantum dot organic solar cell that explores the current-voltage characteristic of the solar cell whose characteristics can be tuned by varying the fabrication parameters of the quantum dots (QDs). The modeled device consists of a hole transport layer (HTL) which doubles up as photon absorbing layer, several quantum dot layers, and an electron transport layer (ETL). The conduction of charge carriers in HTL and ETL has been modeled by the drift-diffusion transport mechanism. The conduction and recombination in the quantum dot layers are described by a system of coupled rate equations incorporating tunneling and bimolecular recombination. Analysis of QD-solar cells shows improved device performance compared to the similar bilayer and trilayer device structures without QDs. Keeping other design parameters constant, solar cell characteristics can be controlled by the quantum dot layers. Bimolecular recombination coefficient of quantum dots is a prime factor which controls the open circuit voltage (VOC) without any significant reduction in short circuit current (JSC).

Crosslinked Semiconductor Polymers for Photovoltaic Applications

Organic solar cells (OSCs) have achieved much attention and meanwhile reach efficiencies above 10%. One problem yet to be solved is the lack of long term stability. Crosslinking is presented as a tool to increase the stability of OSCs. A number of materials used for the crosslinking of bulk heterojunction cells are presented. These include the crosslinking of low bandgap polymers used as donors in bulk heterojunction cells, as well as the crosslinking of fullerene acceptors and crosslinking between donor and acceptor. External crosslinkers often based on multifunctional azides are also discussed. In the second part, some work either leading to OSCs with high efficiencies or giving insight into the chemistry and physics of crosslinking are highlighted. The diffusion of low molar mass fullerenes in a crosslinked matrix of a conjugated polymer and the influence of crosslinking on the carrier mobility is discussed. Finally, the use of crosslinking to make stable interlayers and the solution processing of multilayer OSCs are discussed in addition to presentation of a novel approach to stabilize nanoimprinted patterns for OSCs by crosslinking.

Alkyl Thiophene Vinylene Electropolimerization in C<sub>8</sub>mimPF<sub>6</sub>, Potential Use in Solar Cells

We report the electrosynthesis of a novel semiconductor polymer based on alkyl vinylthiophene derivative in the presence of an ionic liquid (IL). The polymerization was performed under galvanostatic conditions and the polymer was studied as potential donor component of a multilayer heterojunction organic solar cell (OSC). The monomer used was (E)-1,2-di-(3-octyl-2-thienyl) vinylene (OTV) and the IL used for the electropolymerization was 1-octyl-3-methylimidazole hexafluorophosphate C8mimPF6. Optical properties, stability and morphology of the polymer were analyzed using FT-IR, UV-vis, Raman and XPS spectroscopy. Voltammetry analysis and scanning electron microscopy (SEM-EDX) were also performed on the polymer. The OSC assembled with the polymer of OTV was used as electro donor and C60 as acceptor. Molybdenum trioxide (MoO3) and bathocuproine (BCP) were used as buffer layer between anode and cathode respectively. I-V curves, in the dark and under AM 1.5 solar simulator were performed to measure its efficiency.

Synthesis of Benzoporphyrin-Diketopyrrolopyrrole Conjugates and Application for Organic Solar Cells

Organic solar cells (OSC) have great attractive attention as key technology for next energy harvesting source. They could be fabricated for low cost, large area and flexible devices. However, power-conversion efficiency (PCE) of OSC is limited compared with the inorganic materials-base solar cell. Therefore, OSC requires new organic functional dyes for OSC for the improvement of PCEs. Benzoporphyrin (BP) has a expanded structure with four benzannulation at pyrrolic β-position on porphyrin core. BP has a highly planer structure and excellent semiconducting properties. Ono and colleagues have developed the thermal-convertible precursors of BP, namely bicycle[2,2,2]octadiene-fused porphyrin (CP). CP can be converted to BP under heating conditions by retro-Diels-Alder reaction. This thermal conversion method allowed us to fabricate the multi-layered OSCs by solution process. However, the nature of BP has the problems and limitations for improvement of the PCE because BP has weak absorption in the NIR regions and the HOMO level of BP is higher than that of ideal donor materials for OSC. It is known that improvement of the PCE is necessary for the wide-range absorption in the visible to NIR regions and appropriated HOMO and LUMO levels. In this presentation, we will report the design, synthesis and properties of the benzoporphyrin-diketopyrrlopyrrole conjugates (BP–DPP) for the p-type material of the PCEs. The absorption spectrum of BP–DPP was measured as thin film on glass substrate. The absorption of BP­–DPP shows broader absorption with four peak tops from visible to NIR regions. We have fabricated the bulk heterojunction (BHJ) type OSC with BP–DPP as p-type material and PC61BM as n-type material. The PCEs of 4.0% for BP–DPP:PC61BM was achieved with short-circuit current (J SC) of 12.47 mA cm-2, open circuit voltage (V oc) of 0.68 V, and fill factor (FF) of 0.47.We will also report the evaluations of film structures of these devices by AFM and XRD measurements.

Structure and Thickness Optimization of Active Layer in Nanoscale Organic Solar Cells

This paper presents the development of a two-dimensional model of multilayer bulk heterojunction organic nanoscale solar cells, consisting of the thickness of active layer and morphology of the device. The proposed model is utilized to optimize the device parameters in order to achieve the best performance using particle swarm optimization algorithm. The organic solar cells under research are from poly (3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester type which are modelled to be investigated for performance enhancement. A three-dimensional fitness function is proposed involving domain size and active layer thickness as variables. The best results out of 20 runs of optimization show that the optimized value for domain size is 17 nm, while the short-circuit current vs. voltage characteristic shows a very good agreement with the experimental results obtained by previous researchers.

Impedance analysis of the multilayered organic solar cells with and without hole buffer layer

We studied the effects of a buffer layer [molybdenum oxide (MoO3) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)] and the thickness of a p-type tetraphenyldibenzoperiflanthene (DBP) layer on the performance of multilayered organic solar cells in indium–tin oxide (ITO)/buffer/DBP/fullerene C60/bathocuproine (BCP)/Al multilayered photovoltaic devices. The insertion of a hole buffer layer reduced the resistance across the cell in forward biasing under illumination and in the dark, and the negative capacitance behaviors were observed at a voltage higher than VOC, probably owing to the enhancement of the double injection. On the other hand, a hole-blocking ITO/DBP interface reduced the hole injection and extraction, and DBP became similar to a dielectric layer in the dark, as determined from the capacitance–frequency measurement. The photogenerated carriers caused an increase in the capacitance C and the conductance G in the devices with and without the MoO3 buffer layers. Both C and G reached their local minima at a voltage close to VOC, and increased with a decrease in external voltage probably owing to the enhancement of charge extraction efficiency. At a lower voltage, however, C and G reached their peak values and then decreased again with a decrease in external voltage, probably owing to the reduction (ejection) in the number of photogenerated space charges in active layers. The increase in the thickness of the dielectric DBP layer resulted in a reduction in charge extraction efficiency, and an S-shaped curve was observed for the thick DBP cell. The peak voltages in C and G moved toward negative values with an increment in thickness, and this was discussed using the simplified rate equation model.

MULTI-PHYSICAL PROPERTIES OF PLASMONIC ORGANIC SOLAR CELLS (Invited Paper)

(Invited Paper) Abstract|Organic solar cells (OSCs) have recently attracted considerable research interest. For typical OSCs, it is highly desirable to have optically thick and physically thin thickness for strong light absorption and e-cient carrier collection respectively. In the meantime, most organic semiconductors have short exciton difiusion length and low carrier mobility (1{3). As a consequence, the active layers of OSCs are generally thin with a thickness of a few hundred nanometers to ensure the e-cient extraction of carriers, hence limiting the total absorption of incident light. Optimizing both the optical and electrical (i.e., multi-physical) properties of OSCs is in demands for rationally designed device architectures. Plasmonic nanomaterials (e.g., metallic nanoparticles (4{6), nanorods (7,8), nanoprisms (9,10), etc.) have recently been introduced into difierent layers of multilayered solar cells to achieve highly e-cient light harvesting. The multilayered solar cells structures commonly have active layer, carrier (electron and hole) transport layer and electrode (anode and cathode). Through the localized plasmonic resonances (LPRs) (11{16) from metallic nanomaterials, very strong near-flelds will be generated, which can provide a large potential for enhancing optical absorption in the multilayered OSCs. Besides the optical efiects, it has been reported that metallic nanomaterials can modify the morphology, interface properties as well as the electrical properties of OSCs which will signiflcantly modify the performances of OSCs (17{23). In this article, the efiects of various optical resonance mechanisms and the theoretical studies of the multi- physical properties of OSCs will be reviewed. Meanwhile, the experimental optical and electrical efiects of metallic nanomaterials incorporated in difierent layers of OSCs will be studied. The morphology and interface efiects of metallic nanomaterials in the carrier transport layers on the performances of OSCs will also be described.

Spray coating fabrication of organic solar cells bypassing the limit of orthogonal solvents

The development of alternative deposition techniques is an important step towards the realization of low cost multilayered organic solar cells. While spin-coating needs orthogonal solvents to avoid an intermixing of stacked layers, thermal evaporation is expensive and not applicable to polymers. We show here how an innovative deposition technique called dry spray-coating may represent a promising way to manufacture bulk-hetero-junction (BHJ) and multilayered solar cells. Using standard materials such as poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl-C61-butyric acid methyl ester, we achieved efficiency of 2.6% for the BHJ device, while a value of 1.5% was obtained for a bilayer structure using the same solvent for both materials.

A Poly-(3-Hexylthiophene) (P3HT)/[6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM) Bilayer Organic Solar Cell Fabricated by Airbrush Spray Deposition

Airbrush spray deposition is applied to fabricate a bilayer heterojunction solar cell based on P3HT/PCBM. This solar cell device shows an open-circuit voltage of 0.36 V, a short circuit current density of 6.76 mA/cm2, a conversion efficiency of 0.74%, and a fill factor of 30.4%. The results demonstrate that airbrush spray deposition is an effective method to fabricate multilayer or other complex polymer-based organic solar cells. Although spin-coated bulk heterojunction devices have better performance than the airbrushed ones, the airbrush is indeed feasible as a low-cost yet simple process. It is noteworthy that such preliminary results of the airbrush spray solar cell is unoptimized and thus its performance can be further improved with the development of this technology. Furthermore, this method itself has huge potential as it can be used for other polymer-based organic thin film devices.

Realization of solution processed multi-layer bulk heterojunction organic solar cells by electro-spray deposition

Electro-spray deposition (ESD) was applied to fabricate solution processed donor–acceptor bulk heterojunction organic photovoltaic devices with multi-layer structure. Solvent effect was observed when using different organic solvents. Power conversion efficiency (PCE) of the devices prepared from dichlorobenzene increased dramatically comparing to the ones from chloroform, owing to improved homogeneity of the films. ESD enabled us to fabricate solution processed multi-layer (donor/donor:acceptor/acceptor) devices with simple successive deposition steps. Energy Dispersive X-ray Reflectometry analysis confirmed distinct three layered structure of the active layers. Solar cell device parameters of the trilayer devices were compared to single layer devices and those of spin coated devices with the same donor:acceptor ratio and film thickness. Post-thermal treatment results showed that after annealing at 125°C, trilayer devices exhibited best performance with the maximum PCE of 2.17%.

Relationship between Work Function of Hole Collection Electrode and Temperature Dependence of Open-Circuit Voltage in Multilayered Organic Solar Cells

We have investigated the photovoltaic properties of multilayered organic photovoltaic devices consisting of indium tin oxide (ITO)/(NiO)/donor/C60/bathocuproine (BCP)/Al structures. Open circuit voltage (VOC) increases with the decrease in temperature between 40 and 350 K. The VOC was, however, pinned at approximately 0.6 V for the device without NiO, probably owing to the insufficient work-function difference between ITO and Al electrodes. The hole injection was also markedly suppressed at the ITO/donor interface in the device with large IP donor materials without the buffer layer and abnormal S-shaped current density–voltage (J–V) characteristics were observed. On the other hand, the value of VOC increases with the increase in ionization potential (IP) of donor materials in the device with NiO buffer layers owing to the enhanced work-function difference of about 1 eV, and the S-shaped curves disappeared at the high temperatures above 200 K. The VOC is further improved to nearly 1.2 V by the UV–ozone treatment of the NiO surface. We have therefore concluded that the increment of work function of the anode caused by the insertion of an oxide buffer layer and the surface treatment of the electrode by UV–ozone treatment are essentially important for the improvement of VOC and charge transport/injection properties in the multilayered organic solar cell applications.