Absorption In Organic Solar Cells Research Articles

“Superimposed” spectral characteristics of fluorophores arising from cross-conjugation hybridization

The demand for enhanced optical properties in advanced fluorescence technologies has driven research into the structure-property relationship of fluorophores. In this paper, we use naphthalene fluorophores NaDC-Aze and PhDO-Aze as a case study to emphasize the pivotal role of cross conjugation in tuning the optical structure-property relationship. NaDC-Aze and PhDO-Aze, formed by hybridizing two distinct conjugated systems in a single naphthalene molecule, exhibit spectral characteristics from both conjugated systems. Experimental data and theoretical calculations demonstrate the coexistence of two electron-delocalization systems in a cross-conjugation manner in both NaDC-Aze and PhDO-Aze. The cross-conjugation fluorophores exhibit high brightness, large Stokes shift, and a broad absorption wavelength range by combining distinct spectral properties from its parent fluorophores. These spectral properties will be advantageous for certain applications (i.e., panchromatic absorption in organic solar cells, and fluorophores compatible with a wide range of excitation wavelengths).

Unveiling the thermal stability of diketopyrrolopyrrole-based terpolymers: a key element for enhanced efficiency and stability of organic solar cells

Thermally stable DPP-based terpolymers with tunable broadband absorption for organic solar cells.

Dielectric light-trapping nanostructure for enhanced light absorption in organic solar cells

Dielectric scatterers where Mie resonances can be excited in both electric and magnetic modes have emerged as a promising candidate for efficient light trapping (LT) in thin-film solar cells. We present that light absorption in organic solar cells (OSCs) can be significantly enhanced by a front-sided incorporation of a core–shell nanostructure consisting of a high-refractive-index dielectric nanosphere array conformally coated with a low-refractive-index dielectric layer. Strong forward light scattering of the all-dielectric LT structure enables the absorption in an organic semiconductor to be remarkably boosted over a broad range of wavelengths, which is attributed to interference of a simultaneous excitation of the electric and magnetic dipole resonant modes. The OSC with the LT structure shows the short-circuit current density (Jsc) of 28.23 mA/cm2, which is 10% higher than that of a flat OSC. We also explore how the LT structure affects scattering cross-sections, spectral multipole resonances, and far-field radiation patterns. The approach described in this work could offer the possibility for the improvement of characteristic performances of various applications, such as other thin-film solar cells, photodiodes, light-emitting diodes, and absorbers.

Numerical Study of the Effect of Aluminum-Gold Nanorods on the Short-Circuit Current Density and the Absorption in Organic Solar Cells

Numerical Study of the Effect of Aluminum-Gold Nanorods on the Short-Circuit Current Density and the Absorption in Organic Solar Cells

Enhanced light absorption of organic solar cells based on stopped-trench metal grating.

Here, the influence of dimensional parameters of the trench metal grating on the absorption efficiency of organic solar cells (OSCs) was evaluated. The plasmonic modes were calculated. Due to the capacitance-like charge distribution in a plasmonic configuration, the platform width of grating has a significant influence on the intensity of wedge plasmon polaritons (WPPs) and Gap surface plasmon (GSPs). Stopped-trench gratings would lead to better absorption efficiency than thorough-trenched gratings. The stopped-trench gratings (STG) model with a coating layer showed 77.01% integrated absorption efficiency, which is 19.6% better than previously reported works with 19% less photoactive materials. This model offered 18% integrated absorption efficiency, better than an equivalent planar structure without a coating layer. Specifying the areas with maximum generation on the structure helps us to manage and reduce the thickness and volume of the active layer to control the recombination losses and the cost. We rounded the edges and corners with a curvature radius of 30 nm to investigate tolerance during fabrication. Results demonstrated that the integrated absorption efficiency profile of the blunt model is slightly different from the integrated absorption efficiency profile of the sharp model. Finally, we have studied the wave impedance (Zx) inside the structure. Between the spectrum of λ =∼700 nm to λ=900 nm, an extremely high wave impedance layer was formed. It creates an impedance mismatch between layers and helps us to better trap the incident light ray. STG with a coating layer (STGC) is a promising way to produce OCSs with extremely thin active layers.

Incorporation of a Boron-Nitrogen Covalent Bond Improves the Charge-Transport and Charge-Transfer Characteristics of Organoboron Small-Molecule Acceptors for Organic Solar Cells.

An organoboron small-molecular acceptor (OSMA) MB←N containing a boron-nitrogen coordination bond (B←N) exhibits good light absorption in organic solar cells (OSCs). In this work, based on MB←N, OSMA MB-N, with the incorporation of a boron-nitrogen covalent bond (B-N), was designed. We have systematically investigated the charge-transport properties and interfacial charge-transfer characteristics of MB-N, along with MB←N, using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT). Theoretical calculations show that MB-N can simultaneously boost the open-circuit voltage (from 0.78 V to 0.85 V) and the short-circuit current due to its high-lying lowest unoccupied molecular orbital and the reduced energy gap. Moreover, its large dipole shortens stacking and greatly enhances electron mobility by up to 5.91 × 10-3 cm2·V-1·s-1. Notably, the excellent interfacial properties of PTB7-Th/MB-N, owing to more charge transfer states generated through the direct excitation process and the intermolecular electric field mechanism, are expected to improve OSCs performance. Together with the excellent properties of MB-N, we demonstrate a new OSMA and develop a new organoboron building block with B-N units. The computations also shed light on the structure-property relationships and provide in-depth theoretical guidance for the application of organoboron photovoltaic materials.

Synergistic effects of dual-optical-nanocavity resonance and localized surface plasmons to enhance light absorption in organic solar cells

For organic solar cells, it is a vital issue to effectively confine more light in a thin photoactive layer to guarantee both efficient light absorption and effective collection for photogenerated carriers. Herein, a simple but high-performance composite structure consisting of a front indium tin oxide hemisphere (ITO-HS) array and a silver nanodisk (Ag-ND) array embedded in the photoactive layer is proposed. Benefiting from antireflection and light scattering induced by the ITO-HS array, dual-optical-nanocavity resonances formed by the ITO-HS array, Ag-ND array and Ag electrode, together with the secondary scattering and localized surface plasmons caused by the Ag-ND array, significantly enhanced light absorption can be achieved in a broad range from 300 to 800 nm. Simulation results indicate that the short-circuit current density and power conversion efficiency can be remarkably enhanced by 43.58% and 46.03%, respectively, through introducing the optimal composite light management structure into the normal inverted device architecture of ITO/ZnO/PTB7:PC 71 BM (100 nm)/MoO 3 /Ag, as compared to the control device. Furthermore, this composite light management structure exhibits excellent omnidirectional light confinement. Considering the notable device performance enhancement and the simple structure, this study may provide a guideline for designing and fabricating high-performance light management structures applicable in thin film-based optoelectronic devices. • A simple and efficient light management structure consisting of a front ITO-HS array and an Ag-ND array is proposed. • The enhancement of 43.58% and 46.03% for J sc and PCE is achieved for the device with the optimized light management structure. • Excellent omnidirectional light management is delivered for the proposed optical structure.

Synergetic effects of a front ITO nanocylinder array and a back square Al array to enhance light absorption for organic solar cells.

Efficient light management is critical to obtain high performance for organic solar cells (OSCs), which aims to solve the contradiction between limited carrier extraction and light absorption for the normally employed photoactive layers generally having both short exciton diffusion lengths and low extinction coefficients. In this study, we introduce a simple and efficient light management structure consisting of a front indium tin oxide nanocylinder (ITO-NC) array and a back square Al array. Thanks to the synergetic effects of antireflection and light scattering induced by the ITO-NC array, together with the secondary scattering and localized surface plasmon resonance because of the square Al array, remarkably enhanced light absorption in a broad spectral range can be achieved. Taking the most investigated photoactive layer of the P3HT:PC61BM blend as an example, simulation results reveal that, compared with the planar control device of the ITO/PEDOT:PSS/P3HT:PC61BM(80nm)/ZnO/Al, the short-circuit current density and power conversion efficiency can be enhanced by 36.58% and 38.38% after incorporating the light management structure with the optimal structural parameters. Furthermore, good omnidirectional light management can be achieved for the proposed device structure. Given the excellent performance and simple structure, we believe that this study would provide a meaningful exploration of developing light management structures applicable for thin film-based optoelectronic devices.

Synergistic Effects of Dual-Optical-Nanocavity Resonance and Localized Surface Plasmons to Enhance Light Absorption in Organic Solar Cells

Synergistic Effects of Dual-Optical-Nanocavity Resonance and Localized Surface Plasmons to Enhance Light Absorption in Organic Solar Cells

Enhancing light absorption for organic solar cells using front ITO nanograting and back ultrathin Al layer* *Project supported by the Natural Science Foundation of Gansu Province, China (Grant No. 20JR10RA611) and the Fundamental Research Funds for Central Universities, China (Grant Nos. lzujbky-2017-178 and lzujbky-2017-181).

To address the discrepancy between carrier collection and light absorption of organic solar cells caused by the limited carrier mobility and optical absorption coefficient for the normally employed organic photoactive layers, a light management structure composed of a front indium tin oxide (ITO) nanograting and ultrathin Al layer inserted in between the photoactive layer and the electron transport layer (ETL) is introduced. Owing to the antireflection and light scattering induced by the ITO nanograting and the suppression of light absorption in the ETL by the inserted Al layer, the light absorption of the photoactive layer is significantly enhanced in a spectral range from 400 nm to 650 nm that also covers the main energy region of solar irradiation for the normally employed active materials such as the P3HT:PC61BM blend. The simulation results indicate that comparing with the control device with a planar configuration of ITO/PEDOT:PSS/P3HT:PC61BM (80-nm thick)/ZnO/Al, the short-circuit current density and power conversion efficiency of the optimized light management structure can be improved by 32.86% and 34.46%. Moreover, good omnidirectional light management is observed for the proposed device structure. Owing to the fact that the light management structure possesses the simple structure and excellent performance, the exploration of such a structure can be believed to be significant in fabricating the thin film-based optoelectronic devices.

FDTD simulation studies on improvement of light absorption in organic solar cells by dielectric nanoparticles

In this paper, we present a systematic design and analysis of organic solar cell (OSC) by embedding dielectric nanoparticles layer at anode. Using numerical simulations, we show that there is improvement in the light absorption in the active layer of the device using nanoparticles. The nanoparticles will act as scattering medium for the incident light and hence trap the light inside the device. For the light scattered at different angles, the optical path length in the active medium is increased and this leads to more absorption of light in the active layer and thereby increase in the efficiency. The scattering efficiency and hence the absorption of solar radiation for generation of current depends on the particle size, interparticle separation and the refractive index contrast between the particles and the embedding medium. Mie theory has been used to calculate the scattering efficiency of nanoparticles. The effect on light absorption and current density of OSC due to nanoparticles has been carried out using finite difference time domain analysis. It is shown that the proposed OSC structure increases the light absorption in the active layer of the device by 40% and short circuit current density by 34%.

Broadband and wide-angle light absorption of organic solar cells based on multiple-depths metal grating.

A silver grating containing three grooves with different depths in one period was proposed as the back electrode for improving light absorption in organic solar cells. We found that the broadband absorption enhancement of the active layer covering the visible and near-infrared bands can be obtained due to the excitation of surface plasmon resonance and the multiple resonances of cavity mode. The integrated absorption efficiency of the proposed structure under TM polarization between 350 nm to 900 nm is 57.4%, with consideration of the weight of AM 1.5G solar spectrum, and is increased by 13.4% with respect to the equivalent planar device. Besides, the wide-angle absorption in proposed structure can be observed in the range from 0 to 50 degrees. These findings are of great importance for rationally designing composite nanostructures of metal gratings-based absorbers for sensing and photon-detecting applications.

Silver nanoparticles in organic photovoltaics: Finite size effects and optimal concentration

Using Transfer Matrix method combined with Effective Medium Approximation as well as Finite-Difference Time-Domain calculations we have determined the light absorption in organic solar cells with silver nanoparticles embedded in the active layer. In calculations, we introduced additional terms in permittivity of the nanoparticles due to their finite size. Our results show, that the optimal nanoparticle concentration in such a system is about 30% greater than when assuming that the permittivity of the Ag nanoparticles equates that of a continuous silver layer. We have found, that although the absorption curves for both models (with and without the finite size contribution) are similar, the dominant mechanism of absorption enhancement is different. When assuming that the permittivity of silver nanoparticles is that of a continuous Ag layer, the light scattering on the surface of the nanoparticles seems to dominate over localized surface plasmon (LSP) excitation, while when considering the finite sizes of the nanoparticles the opposite is true.

Performance improvement of ultrathin organic solar cells utilizing light-trapping aluminum-titanium nitride nanosquare arrays

Despite the crucial role of plasmonic nanostructures in enhancing the performance of organic solar cells, the actual absorption of the active layer in these cells is restricted by the parasitic metal absorption loss limiting their efficiencies. In this paper, a bilayer of Al-TiN nanosquare array has been proposed as a novel plasmonic scheme in order to improve the photocurrent and the solar absorption of organic solar cells. The hybrid design of Al and TiN materials reduces the parasitic loss issue offering extended application of plasmonic light-harvesting structures. Engineering the energy band diagram of the cell and concentrating the electrical field distribution inside the active layer result in boosting the absorption as well as the optical current densities for each layer. Utilization of TiN in the proposed design also benefits from the cost effectiveness, abundance, and CMOS compatibility of this material compared to noble metals as well as featuring appropriate light trapping properties. In addition, the symmetric scheme of the proposed structure makes it polarization insensitive under the normal incidence. The numerical simulations of this study based on the finite-difference time-domain (FDTD) method show an enhanced absorption of 70–88% over the spectral range of 300–625 nm and the maximum enhancement factor of 2.16 at the wavelength of about 665 nm. Moreover, an improved photocurrent density of 13.7082 (mA/cm2) has also been achieved despite the usage of an ultrathin active layer. The proposed design can also be developed to achieve performance enhancement of other thin film solar cells in the future.

Efficient Non-fullerene Organic Solar Cells Enabled by Sequential Fluorination of Small-Molecule Electron Acceptors.

Three small-molecule non-fullerene electron acceptors containing different numbers of fluorine atoms in their end groups were designed and synthesized. All three acceptors were found to exhibit relatively narrow band gaps with absorption profiles extending into the near-infrared region. The fluorinated analog exhibited enhanced light-harvesting capabilities, which led to improved short-circuit current densities. Moreover, fluorination improved the blend film morphology and led to desirable phase separation that facilitated exciton dissociation and charge transport. As a result of these advantages, organic solar cells based on the non-fullerene acceptors exhibited clearly improved short-circuit current densities and power conversion efficiencies compared with the device based on the non-fluorinated acceptor. These results suggest that fluorination can be an effective approach for the molecular design of non-fullerene acceptors with near-infrared absorption for organic solar cells.

The effect of hole transfer layers and anodes on indium-free TiO2/Ag/TiO2 electrode and ITO electrode based P3HT:PCBM organic solar cells

Transparent composite electrodes (TCE), TiO2/Ag/TiO2 (TAgT), have been demonstrated as a promising alternative of indium tin oxide (ITO) in organic solar cell application. TiO2/Ag/TiO2 have been incorporated into bulk heterojunction organic solar cells (OSCs) to replace ITO as the anode. Two different hole transfer layers (HTL), poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and MoO3, are compared in terms of their compatibility with the TiO2/Ag/TiO2 anode. The MoO3 layer has been shown to be a beneficial HTL for TiO2/Ag/TiO2 based OSC. However, the wettability of MoO3 for polymer blend is not as good as PEDOT:PSS, and results in a relatively thin active layer in OSCs. On the other hand, ITO based OSCs using PEDOT:PSS and MoO3 as the HTL are fabricated as control samples to compare the performance of TAgT and ITO anodes. Results of the investigation shows that the superior electrical property of TAgT anodes contributes to the effective collection of photo-carriers and its lower optical transmittance barely degrades the light absorption in OSCs. In a few words, the higher Haacke figure of merit of TAgT enables better performance of OSCs with MoO3 HTL, in comparison with ITO based OSC.

Investigating several ZrN plasmonic nanostructures and their effect on the absorption of organic solar cells

Broadband absorption enhancement is obtained in organic solar cells using zirconium nitride (ZrN) plasmonic nanostructures. Due to the thickness limitations imposed on organic solar cells, their absorption efficiency is limited. Plasmonic nanostructures have the ability to increase their optical efficiency by increasing the light path length inside the active material. Here, refractory plasmonics, a new type of plasmonic nanostructures, is used in organic solar cells instead of traditional metal plasmonics. ZrN, as an example of refractory plasmonics, has a high localized surface plasmon resonance quality factor in the visible range while being cheap, abundant, and C-MOS compatible. Here, several ZrN plasmonic nanostructures are studied including ZrN nanospheres, nanocubes, and nanoshells. Their Mie scattering and absorption efficiencies in a polymer environment are calculated. In addition, the effect of their incorporation on the absorbed power and short circuit current of organic solar cell is analyzed. ZrN nanodisks are also studied in different locations inside the solar cell with the best performance found for nanodisks placed at the interface between the active and the buffer layer. The highest absorption enhancement reported here is obtained when ZrN cubical nanoshells are added. A 34.7% increase in short circuit current is calculated for this structure.

Efficient Light Trapping in Organic Solar Cell Using a Short-Pitched Hexagonal Array of Metallic Nanocylinders

Plasmonic nanostructures have a great potential for enhancing light absorption of organic solar cells (OSCs). Our previous work has demonstrated that light absorption for OSCs with thin active layer can be significantly increased with a 1-D short-pitched metal grating, but the absorption enhancement is sensitive to the polarization of light and vanishes when the active layer is thicker than 60 nm. In this work, we extend the grating into 2-D space, specifically, with metallic nanocylinders packed in a hexagonal array embossed at the silver cathode. Numerical calculations indicate that the proposed OSC not only possesses polarization insensitivity but outperforms the corresponding equivalent planar device as well when the thickness of the active layer changes over a wide range from 40 to 120 nm. The absorption enhancement factor increases with the decrease of the active layer thickness, reaching 81.5% when the active layer is 40-nm thick. The evident enhancement in absorption is mainly due to the excitation of the strong dipole-like surface plasmon resonance, as well as their mutual coupling between neighboring nanocylinders. The present work could provide a promising route for the development of high-efficiency OSCs.

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon‐Optical and Plasmon‐Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

An insight on oxide interlayer in organic solar cells: From light absorption and charge collection perspectives

A comprehensive study of the effect of oxide interlayer on the performance of bulk-heterojunction organic solar cells (OSCs), based on poly[[4,8-bis[(2-ethylhexyl)oxy] benzo [1,2-b:4,5-b'] dithiophene-2,6- diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno [3,4-b] thiophenediyl]] (PTB7): [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) blend system, is carried out by optical simulation, interfacial exciton dissociation and charge collection analyses. It is found that a PTB7:PC70BM blend layer thickness optimized for maximum light absorption in OSCs does not generally give rise to the highest power conversion efficiency (PCE). OSCs, e.g., based on PTB7:PC70BM blend system, can benefit from the oxide interlayer in two ways, (1) to enhance the built-in potential for reducing recombination loss of the photo-generated charges, and (2) to improve charge collection by removal of unfavorable interfacial exciton dissociation. The combined effects result in ∼20% improvement in PCE over an optimized control cell, having an identical layer configuration without an oxide interlayer.