Characteristics Of Organic Solar Cells Research Articles

Approach toward Low Energy Loss in Symmetrical Nonfullerene Acceptor Molecules Inspired by Insertion of Different π-Spacers for Developing Efficient Organic Solar Cells.

In this quantum approach, by adding bridge/π-spacer fragments between the donor and acceptor parts of a newly constructed DF-PCIC (A-D-A type) molecule, it is the aim to improve the photovoltaic characteristics of organic solar cells (OSCs). After π-spacer insertion into the reference molecule (DF-R), six new molecules (DF-M1 to DF-M6) were designed. The optoelectronic attributes of newly inspected molecules were theoretically calculated using MPW1PW91/6-31G(d,p) level of theory. All newly proposed molecules possessed a lower band gap (Eg), a higher value of absorption, lower reorganization energy, greater dipole moment, and lower energies of excitations than the DF-R molecule. The frontier molecular orbital study proclaimed that the DF-M1 molecule has the lowest band gap of 1.62 eV in comparison to the 2.41 eV value of DF-R. Absorption properties represented that DF-M1 and DF-M2 molecules show the highest absorption values of up to 1006 and 1004 nm, respectively, in the near-infrared region. Regarding the reorganization energy, DF-M2 has the lowest value of λe (0.0683896 eV) and the lowest value of λh (0.1566471 eV). DF-M2 and DF-M5 manifested greater dipole moments with the values of 5.514665 and 7.143434 D, respectively. The open circuit voltage (VOC) of all the acceptors was calculated with J61, a donor complex. DF-M4 and DF-M6 molecules showed higher values of VOC and fill factor than the DF-R molecule. Based on the given results, it was supposed that all the newly presented molecules might prove themselves to be better than the reference and thus might be of great interest to experimentalists. Thus, they are suggested to be used to develop proficient OSC devices with improved photovoltaic prospects in the near future.

Dependence of the characteristics of organic solar cells on cathode polymer interface

Increasing energy assumption and rising energy prices in the world forces to look for energy alternatives, one of the most promising is the photovoltaic energy. Solar cells made from organic materials are particularly attractive because they are relatively easy to produce, structurally flexible, and can be applied to large areas at low cost. Fill Factor is a more sensitive parameter compared to open-circuit voltage (Voc) and shortcircuits current density (Jsc). This FF depends also on the thickness of the active-polymer layer, on the morphology of the cathode-polymer interface and on temperature. In this article, we study the dependence of fill factor (FF) on different parameters in organic bulk heterojunction solar cells.

Promising SnOx electron transport layer for polymer solar cells

In this study, SnOx films were synthesized via thermal oxidation of Sn films at various temperatures ranging from 300 °C to 500 °C. The impact of oxidation temperature on the structural, optical, and electronic properties of the SnOx films was investigated using SEM, EDX, optical absorption, and impedance spectroscopy techniques. It was found that the oxidation temperature has a significant impact on the SnOx resistance, which decreases as the temperature increases due to improved SnOx crystallinity and enhanced electron mobility. Furthermore, an increase in recombination resistance suggests an improvement in interface quality and reduced charge recombination processes. Finally, the impact of SnOx electron transport layer properties on the photovoltaic characteristics of organic solar cells was investigated. It was observed that OSCs based on SnOx ETL obtained at higher oxidation temperature reveal enhanced photovoltaic characteristics.

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Plasmonic enhancement of photovoltaic characteristics of organic solar cells by employing parabola nanostructures at the back of the solar cell.

In this paper, we demonstrate the enhanced performance of organic solar cells (OSCs) comprising low band gap photoactive layers (PMDPP3T:PC70BM) and 2-dimensional (2D) arrays of either Ag nano-spheres, nano-hemispheres, or nano-parabolas embedded at the back of the OSCs. Finite-difference time-domain (FDTD) simulations were performed to compare the performance of the OSCs containing the different plasmonic nanostructures, in terms of optical absorption, short circuit current density (JSC) and power conversion efficiency (PCE). The results demonstrate that single junction OSCs consisting of this new active layer polymer (PMDPP3T), blended with PC70BM, and plasmonic nanostructures at the back of the OSC can enhance the optical absorption in the visible and the NIR region. We demonstrate that the aspect ratio of the nanoparticles embedded at the back of OSCs is a vital parameter for light absorption enhancement. It is observed that the performance in terms of JSC and PCE enhancement of OSC having 2D arrays of Ag nano-parabola at the back of the solar cell improved by 26.41% and 26.37%, respectively, compared to a planar OSC. The enhancement in photon absorption can be attributed due to the enhancement of light scattering from metallic nanostructures near their localized plasmon resonance.

Effects of Oxygen Vacancies in a Zinc Oxide Electron Transport Layer on Long-Term Degradation and Short-Term Photo-Induced Changes in the Operation Characteristics of Organic Solar Cells

Effects of Oxygen Vacancies in a Zinc Oxide Electron Transport Layer on Long-Term Degradation and Short-Term Photo-Induced Changes in the Operation Characteristics of Organic Solar Cells

An effective method of reconnoitering current–voltage (IV) characteristics of organic solar cells

Current–voltage (IV) characterization is the most fundamental measurement performed on solar cells. This measurement is commonly used to extract basic solar cell parameters, such as open circuit voltage, short circuit current density, fill factor, and power conversion efficiency. We were able to obtain a fast tool to find defective behavior using Simulation Program with Integrated Circuit Emphasis simulations and generate an understanding of which device property can create such defective behaviors by analyzing the second derivative of IV curves.

The role of surface defects in the charge transport in organic solar cells based on oxidized indium thin films

In this work, indium oxide (In2O3) thin films were obtained by the thermal oxidation method of metallic indium thin films. The effect of thermal annealing on the morphology, structure, optical properties of In2O3 and also on the photovoltaic characteristics of organic solar cells (OSCs) was studied. Annealing of the films was carried out in the temperature range of 200–500 °C. The rise of annealing temperature leads to the increase in the grain size and to the widening of the In2O3 bandgap. Based on the In2O3 films with various annealing temperatures, inverted organic solar cells (OSCs) with FTO/In2O3/P3HT:ICMA/MoOx/Ag structure were assembled. OSCs with In2O3 oxidized at 200 °C generated low power conversion efficiency (PCE) of 0.45%. The increase in the oxidation temperature up to 300 °C results in a significant improvement in PCE, which has reached 1.38%. However, a further rise of the annealing temperature deteriorates the photovoltaic performance of OSCs. The dynamics of charge carrier transfer in OSCs based on In2O3 were investigated by the impedance spectroscopy (IS) technique. It follows from the IS data that the growth of In2O3 oxidation temperature leads to the improvement in the In2O3 conductivity, however, it also results in enhanced recombination rate of charge carriers at the In2O3/P3HT:ICMA interface, which indicates increased surface defects in In2O3 with the rise in the oxidation annealing temperature.

A pH‐Neutral Polyelectrolyte Hole Transport Layer for Improved Energy Band Structure at the Anode/PTB7 Junction and Improved Solar Cell Performance

In organic solar cells (OSCs), interfacial properties between the donor phase and hole transport layers (HTLs) are critical factors which govern charge extraction efficiency. Many ionic and polar materials are known to function as effective interfacial layers; however, an understanding of how ionic moieties affect the electronic band structure and characteristics of OSCs is lacking. Herein, a new, pH‐neutral polyelectrolyte is introduced that resolves several problems which are encountered with the commonly used HTL, poly(3,4‐ethylenedioxythiopene):polystyrenesulfonate (PEDOT:PSS). An effective p‐type polyelectrolyte dopant is designed, comprising an anionically charged PSS backbone with easily reduced Cu2+ counterions (Cu:PSS), and interfacial properties for HTL/donor interfaces by photoelectron spectroscopy are analyzed. The effects of the polyelectrolyte on interfacial energy levels and charge extraction efficiency between the active layer and HTL are quantified. Using optimized processing conditions, the efficiency can be improved from 8.31% to 9.28% in conventional OSCs compared with a standard PEDOT:PSS HTL. The energy‐level alignment at the HTLs/donor interface determined by UV photoelectron spectroscopy measurements reveals the origin of distinct differences in device performances. The reduced ionization potential (IP) and hole injections barrier (Φh) at the HTL/donor interface play a crucial role in efficient charge extraction in conventional OSCs.

Effects on Photovoltaic Characteristics by Organic Bilayer- and Bulk-Heterojunctions: Energy Losses, Carrier Recombination and Generation

We investigate the photovoltaic characteristics of organic solar cells (OSCs) for two distinctly different nanostructures, by comparing the charge carrier dynamics for bilayer- and bulk-heterojunction OSCs. Most interestingly, both architectures exhibit fairly similar power conversion efficiencies (PCEs), reflecting a comparable critical domain size for charge generation and charge recombination. Although this is, at first hand, surprising, a detailed analysis points out the similarity between these two concepts. A bulk-heterojunction architecture arranges the charge generating domains in a 3D ensemble across the whole bulk, while bilayer architectures arrange the specific domains on top of each other, rather than sharp bilayers. Specifically, for the polymer PBDB-T-2F, we find that the enhanced charge generation in a bulk composite is partially compensated by reduced recombination in the bilayer architecture, when nonfullerene acceptors (NFAs) are used instead of a fullerene acceptor. Overall, we demonstrate that bilayer-heterojunction OSCs with NFAs can reach competitive PCEs compared to the corresponding bulk-heterojunction OSCs because of reduced nonradiative open-circuit voltage losses, and suppressed trap-assisted recombination, as a result of a vertically separated donor-to-acceptor nanostructure. In contrast, the bilayer-heterojunction OSCs with the fullerene acceptor exhibited poor photovoltaic characteristics compared to the corresponding bulk devices because of highly aggregated acceptor molecules on top of the polymer donor. Although free carrier generation is reduced in a in a bilayer-heterojunction, because of reduced donor/acceptor interfaces and a limited exciton diffusion length, more favorable transport pathways for unipolar charge collection can partially compensate the aforementioned disadvantages. We propose that the unique properties of NFAs may open a technical venue for the bilayer-heterojunction as a great and easy alternative to the bulk heterojunction.

The Impact of Surface Processes on the J–V Characteristics of Organic Solar Cells

In this article, the impact of surface recombination on the J–V characteristics of organic solar cells (OSC) was analyzed. The investigation was carried out using the drift-diffusion model in which the surface recombination of holes and electrons on both electrodes, anode and cathode, were taken into account through boundary conditions. In order to perceive the individual influence of the surface recombination, the equilibrium thermionic concentrations for holes and electrons on contact interfaces were neglected. As the reference point of our analysis, the measured In2O5Sn/ poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/ poly(3-hexylthiophene-2,5-diyl): indene-C60 bisadduct /Al solar cell J–V curve was reproduced by our model. The surface recombination velocities (SRVs) for electrons and holes on both contacts were further varied and J–V curves were generated. For simplicity, it was assumed that diffusion current dominates at contacts. For the sake of systematization, three categories of SRV values were introduced, S —small for blocking contact, L —large for conductive contact, and M —medium for contact which is neither blocking nor conductive. The analysis was conducted for different ratios of electron and hole mobilities. It was established that in order to obtain regular J-shape J–V characteristics in OSCs, both contacts should be conductive, for both holes and electrons. This implies that the concept of contact selectivity in OSCs should be reexamined in terms of its physical description and relation to the SRVs values.

Two different types of S-shaped J-V characteristics in organic solar cells

In this paper, S-shaped characteristics of organic solar cells (OSCs) are analyzed by a drift–diffusion model which includes charge carrier surface recombination and thermal injection on the anode and cathode through boundary conditions. By varying surface recombination velocities (SRVs) for electrons and holes on both contacts and the injection barrier heights for majority carriers, two different S-shaped deviations in OSCs J-V characteristics were observed. The first type of S-shaped J-V characteristic manifests the S-shape bending in the vicinity of the voltage axis, after which it rises almost exponentially. This kind of S-shape deformation was found to arise from the finite SRVs. The second type of the S-shaped J-V characteristic also makes a kink near the voltage axis but proceeds to grow monotonically, having only one saddle point. The S-shaped J-V curve of this kind turned out to be the consequence of the electron injection barrier height larger than 0.2 eV. The validity of our model is confirmed by comparing the simulated J-V curves with the experimentally obtained data. The model has been applied to the ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/PEDOT:PSS/P3HT:ICBA/Al solar cells. For the P3HT:PCBM based solar cells, the regular J-shaped J-V curves were measured, while for the P3HT:ICBA solar cells, the J-V curves with anomalous S-shape behavior were recorded. All experimentally obtained J-V curves were reproduced very well with our model. It seems that the ITO/PEDOT:PSS/P3HT:ICBA/Al solar cell S-shaped J-V curve originates from the electron barrier height on the cathode contact, rather than from the low surface recombination on the electrodes.

Fabrication and Characteristics of Organic Solar Cell Based on Conductive Polymer

Polyaniline (PANi) is one of imported polymers for synthesis solar cells. The quality of thin film depended on the method of polymerization. In this paper PANi have been prepared by the chemical oxidative polymerization of aniline by presence oxidative agent. The electrical conductivity of these films was measured by two probe methods. The electrical conductivity is influenced by preparation conduction such as concentration of the protonic acid HCl and temperature. The best electrical conductivity about 1.2x10-4 S/cm was found at 383K.

Enhancing the photovoltaic characteristics of organic solar cells by introducing highly conductive graphene as a conductive platform for a PEDOT:PSS anode interfacial layer

We propose a remarkably conductive polymer composite that results from highly conductive pristine graphene (PG) being doped with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). With the addition of PG to a PEDOT:PSS (B-PE) dispersion acting as a conductive platform, the sheet resistance can be lowered from 368.54 Ω/sq to 114.67 Ω/sq for the graphene-PEDOT:PSS (G-PE) over a bare glass substrate. Importantly, this only causes a minor decrease in optical transmittance of approximately 4.16% at 550 nm, and generates a noticeable decrease in the surface roughness profile of ~ 11.42 nm. XRD, Raman spectroscopy, and XPS analyses were used to verify that various types of chemical bonds are formed between the PG sheets and B-PE molecules. Due to these chemical interactions, a huge number of electrons are transferred from the PG sheets to the PEDOT, creating a net positive charge among the carbon atoms in the PG sheets. This results in an increase in the conductivity of the G-PE composite and, consequently, improves the power-conversion efficiency (PCE; 4.52%) of organic solar cells utilizing G-PE composite hole transport layers (HTLs). This enhancement in the PCE of G-PE HTL-based devices is compared with devices fabricated from either B-PE or PG HTLs alone, these devices achieved a PCE of only 4.18% and 3.87%, respectively.

Improvement of Solar Cell Characteristics Using PbS Quantum Dot Superlattice Prepared by Sedimentation

We report the characteristics of organic solar cells on Si substrates using quantum dot superlattices (QDSLs) prepared by sedimentation. When colloidal quantum dots (QDs) are deposited on a substrate in a solvent, they are closely packed and form multiple grains in a film. We found that the PbS QD grains grew into a large superlattice when the deposition proceeded for a long period of time. A solar cell was fabricated using the QD superlattice film as an absorbing layer. When the deposition was slow, the short-circuit current density of the solar cell doubled compared with that of the rapid deposition case. Simulations based on the superposition model suggest that the superlattice formation process is responsible for the observed change in the cell characteristics.

An analysis for S-shaped I-V characteristics of organic solar cells using lumped-parameter equivalent circuit model

A lumped-parameter equivalent circuit model of organic solar cells (OSCs) is proposed to analyze the S-shaped current-voltage (I-V) characteristics and the explicit solution to I-V equation is derived so that this model is suitable to be compactly implemented for the simulations of photovoltaic applications. Comparing our explicit solutions with the method of least squares and the Newton-Raphson root-finding scheme, the simulations quantificationally reproduce the S-shaped I-V characteristics of OSCs especially for both linear- and exponential-like S-shaped kink in the third and first quadrants, respectively. Simultaneously, the simulations facilitate us to qualitatively analyze the S-shaped I-V characteristics of OSCs influenced by the different model parameters. Furthermore, we verify the simulation results of our proposed model by using the reconstructed experimental data. Good agreements indicate that such a model can be used to accurately predict the S-shaped I-V characteristics of OSCs and regarded as a serviceable tool implemented compactly into simulations of the wide photovoltaic applications.

An Improved Organic Solar Cell Lumped-Parameter Equivalent Circuit Model

An improved lumped-parameter equivalent circuit model is proposed to describe S-shaped I–V characteristics of organic solar cells (OSCs). This model originates but differs from Mazhari’s model. As a minor but important modification, a shunt resistance is added to Mazhari’s model to increase the accuracy of simulating the S-shaped kink in the third quadrant. Subsequently, we present a terminal current-voltage equation set and derive an analytical solution to the improved model. Furthermore, we verify the analytical solution to our model by using the least square method and validate our model by using the experimental I–V curves examined from OSCs. Compared with Mazhari’s model, our model has greater accuracy in interpreting the S-shaped kink with linear-like rise in the third quadrant. As a result, our improved model is suitable to explain the S-shaped I–V characteristics of organic solar cells in the whole operational region, especially for the S-shaped kink in the third quadrant.

Increasing the Efficiency of Organic Solar Cells by Antireflection Coatings Based on Fluoride Composites

Results are presented obtained in measurements of current–voltage characteristics of organic solar cells with and without antiref lection coatings produced by melting in a solar furnace of 95 : 5 and 55 : 45 wt % MgF2 and CaF2 fluoride mixtures. The relative increase in the efficiency of the organic solar cells was 3.2% in the first case and 3.3% in the second.

Irradiance intensity dependence of the lumped parameters of the three-diodes model for organic solar cells

In this paper the current-voltage characteristics of organic solar cells (OSC) is analyzed in terms of equivalent lumped parameter circuit at different level of insolation. In particular, starting from a circuital model based on a three-diode configuration, a set of formulas is proposed to describe the dependence of lumped circuital parameters on irradiance intensity and consequently the behaviour of organic solar cells from dark to high irradiance conditions. These expressions are achieved by using a trail and error approach applied to the fitting procedure of measured I-V curves at different irradiance levels. The achieved model with parameters dependent on irradiance level allows us to use a single circuit for the analysis of OSC under different operating conditions: this is the first step for the integration of OSC circuital model in photovoltaic system simulator, but also can furnish a novel point of view for the advance of the OSC applied research. Experimental measurements are used to setup the model and for its successive validation on two different kind of organic solar cells.

Увеличение эффективности органических солнечных элементов с помощью просветляющих покрытий на основе фторидных композиций

AbstractResults are presented obtained in measurements of current–voltage characteristics of organic solar cells with and without antiref lection coatings produced by melting in a solar furnace of 95 : 5 and 55 : 45 wt % MgF_2 and CaF_2 fluoride mixtures. The relative increase in the efficiency of the organic solar cells was 3.2% in the first case and 3.3% in the second.