Exciton Generation Rate Research Articles

Theoretical studies on the influence of graphene layers as an anode on the performance of P3HT:PC61BM and PTB7:PC71BM bulk heterojunction organic solar cells

In the present work, photovoltaic properties of P3HT:PC61BM bulk heterojunction solar cells based on graphene anode are investigated using the transfer matrix model and drift-diffusion model for optical and electrical performances, respectively. First, we study the effect of the number of graphene layers on optical and electrical performance. Light absorption and total exciton generation rate in active layer as well as photovoltaic parameters including the short circuit current density, open circuit voltage, fill factor, and power conversion efficiency are determined. The optoelectronic model suggests that short circuit current density decreases upon increasing the number of graphene layers, which is attributed to the reduction of light absorption in the active layer. Power conversion efficiency is found to be between 3.49 and 4.31 % for various numbers of graphene layers. Based on the calculation results, five-layer graphene anode is found to have the same sheet resistance as 150 nm-thick ITO anode. Second, the performance of five-layer graphene-based organic solar cell is compared with that of ITO-based anode device. The results indicate that the former can reach 84 % power conversion efficiency of the latter. Finally, P3HT:PC61BM replaces with PTB7:PC71BM as the active layer and its influence on device performance is studied.

Optical absorption and electrical properties of enhanced efficiency in organic solar cells as interfacial layer with Au NPs

The effects of the gold nanoparticles (Au NPs) on performance of organic solar cells (OSCs) are systematically investigated based on blend of the low band gap polymer and fullerene. The localized surface plasmon resonance (LSPR) induced by the Au NPs enhance the light absorption in the active layer and the photoluminescence spectra showed a significant enhancement in their intensity which mainly contribute to increased light absorption of active layer induced by LSPR. The impedance spectroscopy study revealed that the introduction of Au NPs as interfacial layer decreases the sheet and charge-transport resistance between ITO/PEDOT:PSS or PEDOT:PSS/active layer. From the results, the introduction of Au NPs increased the rate of exciton generation and the probability of exciton dissociation, thus enhancing the short-circuit current density and the fill factor The optimized OSCs incorporated with Au NPs were found to have power conversion efficiency of 5.40% compared to control device (4.65%), measured by using an AM 1.5G solar simulator at 100mW/cm2 light illumination intensity.

Inverted Ultrathin Organic Solar Cells with a Quasi-Grating Structure for Efficient Carrier Collection and Dip-less Visible Optical Absorption

We propose a metallic-particle-based two-dimensional quasi-grating structure for application to an organic solar cell. With the use of oblate spheroidal nanoparticles in contact with an anode of inverted, ultrathin organic solar cells (OSCs), the quasi-grating structure offers strong hybridization between localized surface plasmons and plasmonic gap modes leading to broadband (300~800 nm) and uniform (average ~90%) optical absorption spectra. Both strong optical enhancement in extreme confinement within the active layer (90 nm) and improved hole collection are thus realized. A coupled optical-electrical multi-physics optimization shows a large (~33%) enhancement in the optical absorption (corresponding to an absorption efficiency of ~47%, AM1.5G weighted, visible) when compared to a control OSC without the quasi-grating structure. That translates into a significant electrical performance gain of ~22% in short circuit current and ~15% in the power conversion efficiency (PCE), leading to an energy conversion efficiency (~6%) which is comparable to that of optically-thick inverted OSCs (3–7%). Detailed analysis on the influences of mode hybridization to optical field distributions, exciton generation rate, charge carrier collection efficiency and electrical conversion efficiency is provided, to offer an integrated understanding on the coupled optical-electrical optimization of ultrathin OSCs.

High-performance perovskite solar cells by incorporating a ZnGa2O4:Eu3+ nanophosphor in the mesoporous TiO2 layer

ZnGa2O4:Eu3+ nanophosphor was synthesized through a hydrothermal method and used as an effective light down-shifting/converting material in mesoporous TiO2 layer of perovskite solar cells (PSCs). The nanophosphor can convert the high energy incident photon into the photon(s) with lower energy, which can excite CH3NH3PbI3 to generate more photo generated electron–hole pairs, and thus promote the incident light use ratio and increase the power conversion efficiency (PCE) of PSCs. After the incorporation of suitable amount of ZnGa2O4:Eu3+ into PSCs, the cell PCE is increased to 13.80% with a photocurrent density of 23.68mAcm−2 and a highest PCE of 14.34% with a photocurrent density of 25.02mAcm−2 is achieved, much higher than those of the cell without ZnGa2O4:Eu3+ (PCE of 10.67% and photocurrent density of 20.2mAcm−2). The enhanced photovoltaic performance by incorporating the nanophosphor into PSCs should be ascribed to the increased incident light use ratio and improved exciton generation rate.

Electro‐Optics of Colloidal Quantum Dot Solids for Thin‐Film Solar Cells

The electro‐optics of thin‐film stacks within photovoltaic devices plays a critical role for the exciton and charge generation and therefore the photovoltaic performance. The complex refractive indexes of each layer in heterojunction colloidal quantum dot (CQD) solar cells are measured and the optical electric field is simulated using the transfer matrix formalism. The exciton generation rate and the photocurrent density as a function of the quantum dot solid thickness are calculated and the results from the simulations are found to agree well with the experimentally determined results. It can therefore be concluded that a quantum dot solid may be modeled with this approach, which is of general interest for this type of materials. Optimization of the CQD solar cell is performed by using the optical simulations and a maximum solar energy conversion efficiency of 6.5% is reached for a CQD solid thickness of 300 nm.

Effects of the active region thickness on the exciton generation mechanisms in polymer light emitting diodes

In this study, we investigate the influence of the active region thickness on the device characteristics parameters in single layer polymeric light emitting diode (PLED). Calculation of the key characteristic parameters versus layer thickness has been done at four different thicknesses that consist of 50, 80, 100 and 120nm. We perform the analysis of the PLED with the drift-diffusion model, and the current and voltage thickness dependency. Finite element method has been taken into account to investigate active region thickness effects on the exciton generation rate versus distance from anode and also short time after injection of carriers in the polymer light emitting diode. In fact by decreasing the layer thickness, charge carrier distributions are changed in the organic material and concentrations of them are increased and whereby the probability for formation of excitons is increased, as well. Simulation results shown that thickness of active region has impressive effects on the electrical properties of the devices that resulted from the location of exciton generation along the active organic region.

Influence of the charge carrier mobility on the dynamic behavior and performance of the single-layer OLED

In this paper, by solving the drift-diffusion and Poisson equations, a numerical simulation for single layer organic light emitting diode (OLED) has been presented. Finite element method has been taken into account in calculations and the effects of electron and hole mobilities on the electrical properties of OLEDs have been investigated. The influences of the hole and electron mobilities on the charge carrier and exciton generation rates in the terms of distance from anode and carrier injection time have been calculated and it was shown that by increasing the carrier mobilities, the carrier concentrations as well as exciton generation rate were increased. Charge distribution densities versus distance from anode for various electron and hole mobilities, have been calculated and it was found that due to high mobility of holes, the electron–hole recombination happens near the cathode. From the simulation results it was found that the mobility of electrons is the most important parameter, which decides directly about the transport of free carriers and the recombination in the organic material.

Dithiol treatments enhancing the efficiency of hybrid solar cells based on PTB7 and CdSe nanorods

We report the fabrication of polymer/inorganic hybrid solar cells (HSCs) based on CdSe nanorods (NRs) and the semiconducting polymer PTB7. The power conversion efficiency of HSCs can be significantly enhanced by engineering the polymer/nanocrystal interface with ethanedithiol (EDT) and 1,4-benzenedithiol (1,4-BDT) treatments and reached 2.58% and 2.79%, respectively. These results were preferable to that of a pyridine-coated NR-based device (1.75%). This improvement was attributed to the thiol groups of EDT and 1,4-BDT, which can tightly coordinate the Cd ions to form Cd-thialate on CdSe NR surfaces, thereby effectively passivating the NR surface and reducing the active layer defects. Therefore, the rate of exciton generation and dissociation was enhanced and led to the improvement of the device performance.

Study of Nano-Filtration and Solvent Effects for Improving Efficacy of Organic Photovoltaic

Conductive polymer such as poly (3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) plays an important role for improving performance of organic photovoltaics (OPVs). In this study, we investigated the effect of filtrating the buffer layer between the indium tin oxide (ITO) and bulk heterojunction (BHJ); poly(3-hexylthiophene) and [6,6]-phenylC71 butyric acid methyl ester (P3HT:PC71BM) used in our organic photovoltaics (OPVs) as shown in Figure 1. This buffer layer consists of poly (3,4-ethylenedioxythiophene) and polystyrene sulfonate acid (PEDOT:PSS) doping with different solvents [isopropyl alcohol (IPA), ethanol, and methanol]. For comparison, we also studied the OPV devices without filtering the buffer layers. The OPV devices fabricated with filtered buffer layer show better performance than OPV devices without buffer layer filtration. The OPV devices with buffer layers; PEDOT:PSS/IPA, PEDOT:PSS/Ethanol, and PEDOT:PSS/Methanol after filtration showed 24 %, 16 %, and 11 % improvement in short circuit current density (Jsc) , respectively compared to the same OPV devices without filtration. The maximum exciton generation rates (Gmax), and the exciton dissociation probabilities [P (E, T)] were calculated for all fabricated OPV devices to explain the performance enhancement. The data showed that the performance enhancement of OPVs devices after filtering the buffer layers originate from the large increase of Jsc. Filtering the buffer layers enhanced light absorption of BHJs, increased the surface roughness, and decreased the sheet resistance of buffer layer after filtration. Figure 1. A schematic diagram of ITO/PEDOT:PSS:different organic solvents/P3HT: PC71BM/Al Figure 1

Achieving EQE of 16,700% in P3HT:PC71BM based photodetectors by trap-assisted photomultiplication.

We report a trap-assisted photomultiplication (PM) phenomenon in solution-processed polymer photodetectors (PPDs) based on P3HT:PC71BM as the active layer, the maximum EQE of 16,700% is obtained for the PPDs with PC71BM doping weight ratio of 1%. The PM phenomenon is attributed to the enhanced hole tunneling injection assisted by trapped electrons in PC71BM near Al cathode, which can be demonstrated by the transient photocurrent curves and EQE spectra of PPDs with different PC71BM doping ratios. The positive effect of trapped electrons in PC71BM near Al cathode on the hole tunneling injection is further confirmed by the simulated optical field and exciton generation rate distributions in the active layer and the EQE spectra of PPDs with Al(1)/P3HT:PC71BM(100:1)/Al(2) device structure under forward and reverse biases. This discovery may open a new road for organic materials to be used in highly sensitive photodetectors while preserving the advantages of organic materials.

Optically-Enhanced Polymer Bulk Heterojunction Solar Cells by Addition of Gold Nanostructures

We investigate the effects of Au nanoparticles (Au NPs) on poly [N-9′′-hepta-decanyl-2,14-carbazole-alt-5,5-(4′,14′-di-2-thienyl-2′,1,3′-benzothiadiazole)]:ph-enyl-C61-butyric acid methyl ester (PCDTBT:PCBM) based organic photovoltaic (OPV) devices by thermal evaporating Au NPs onto polytetrafluoroethylene (PTFE) layer which is based on indium-tin-oxide (ITO) glass substrate.Significant improvement in terms of short-circuit current density (Jsc) by 33.6%, fill factor (FF) by 0.9%, and thereby commensurate power conversion efficiency (PCE) by 40.7% were achieved compared to devices without Au NPs. The OPVs performance enhancement is attributed to the formation of Au NPs-induced surface plasmons that increases the rate of exciton generation, and the probability of exciton dissociation.

Optical engineering of uniformly decorated graphene oxide nanoflakes via in situ growth of silver nanoparticles with enhanced plasmonic resonance.

A nanocomposite of silver-nanoparticle-decorated graphene oxide (GO-Ag NPs), enhanced by the surface plasmon resonance (SPR) effect, improved the performance of polymer solar cells (PSCs). The GO-Ag NPs were fabricated in situ via ultraviolet (UV) irradiation (254 nm) of GO and an aqueous solution of AgNO3. The photoexcited GO accelerated reduction of Ag(+) ions into silver nanoparticles (Ag NPs) upon UV irradiation, and the Ag NPs spontaneously deposited on the GO nanoflakes because the numerous functional groups on GO enable efficient adsorption of Ag(+) ions and Ag NPs via electrostatic interactions. The strong coupling between the SPR effect of GO-Ag NPs and incident light offers the probability of improved light absorption and corresponding exciton generation rate with enhanced charge collection, resulting in significant enhancement in short-circuit current density and power conversion efficiency (PCE). Therefore, the PCE of PSCs based on poly[4,8-bis(2-ethylhexylthiophene-5-yl)-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethylhexanoyl)thieno[3,4-b]thiophen-4,6-diyl] and [6,6]-phenyl C71-butyric acid methyl ester has been substantially elevated to 7.54% from 6.58% by introducing GO-Ag NPs at the indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid interface. In addition, the excellent properties of GO-Ag NPs, including its simple preparation, processability in aqueous solution, cost-effectiveness, and sustainability, make it suitable for the roll-to-roll manufacturing of PSCs.

Solvents induced ZnO nanoparticles aggregation associated with their interfacial effect on organic solar cells.

ZnO nanofilm as a cathode buffer layer has surface defects due to the aggregations of ZnO nanoparticles, leading to poor device performance of organic solar cells. In this paper, we report the ZnO nanoparticles aggregations in solution can be controlled by adjusting the solvents ratios (chloroform vs methanol). These aggregations could influence the morphology of ZnO film. Therefore, compact and homogeneous ZnO film can be obtained to help achieve a preferable power conversion efficiency of 8.54% in inverted organic solar cells. This improvement is attributed to the decreased leakage current and the increased electron-collecting efficiency as well as the improved interface contact with the active layer. In addition, we find the enhanced maximum exciton generation rate and exciton dissociation probability lead to the improvement of device performance due to the preferable ZnO dispersion. Compared to other methods of ZnO nanofilm fabrication, it is the more convenient, moderate, and effective to get a preferable ZnO buffer layer for high-efficiency organic solar cells.

Universal and Versatile MoO3-Based Hole Transport Layers for Efficient and Stable Polymer Solar Cells

Two solution-processed and highly dispersed MoO3 called d-(MoO3)120 and d-(MoO3)15 with sizes of 120 nm and extremely smaller 15 nm, respectively, are applied into polymer solar cells, and the evaporated MoO3 as hole transport layers (HTLs) in devices is also compared. It is the first time it has been found that the different size of MoO3 can induce the quite different morphologies of the HTLs and their upper active layers due to the unexpectedly caused difference in the surface energy levels. It is worthy to note that the performance of the device with solution-processed d-(MoO3)15 is higher than that of the device with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTLs and even comparable to that of the device with optimized evaporated-MoO3. Simulated by the transfer matrix method, the light intensity and the exciton generation rate in the active layer are found to be greatly enhanced by incorporation of an ultrathin MoO3 combined with PEDOT:PSS. As a result, by inserting a layer of evaporated MoO3 (e-MoO3) between the ITO and PEDOT:PSS, power conversion efficiency (PCE) can be dramatically improved to 7.10% for PBDTTT-C-T:PC71BM. Moreover, the e-MoO3/PEDOT:PSS bilayer also ensures good stability for the devices, due to the MoO3 preventing moisture and oxygen attack and protecting ITO from corrosion caused by the acid PEDOT:PSS.

Interface Modification of ZnO-Based Inverted PTB7:PC71BM Organic Solar Cells by Cesium Stearate and Simultaneous Enhancement of Device Parameters

We used cesium stearate (CsSt) to modify the interface of the electron-extracting contact in inverted organic solar cells. Surface microstructure, optical properties, and electrical characterization as well as exciton generation rate and dissociation probability were investigated to understand the impact of CsSt on the interface contact. The results indicated that by incorporation of CsSt, the surface morphology and energy level as well as conductivity of a zinc oxide (ZnO) film were improved. On the basis of the above properties, highly efficient inverted organic solar cells have been demonstrated by using a ZnO nanoparticle film and CsSt stacked bilayer structure as the cathode interfacial layer. The insertion of a CsSt layer between the ZnO film and active layer improved the electron extraction efficiency, and a high power conversion efficiency (PCE) of 8.69% was achieved. The PCE was improved by 20% as compared to the reference device using a ZnO-only electron extraction layer.

Optical optimization of organic solar cell with bulk heterojunction

AbstractThe work is devoted to the optimization of layer thickness in an organic photovoltaic cell. It presents the applied calculation method which is based on the optical transfer matrix 2×2 formalism. We present the influence of thickness of a PEDOT:PSS layer and of an active layer on the normalized modulus squared of optical electric fields distribution inside devices and on the distributions of exciton generation rate. We present the relationship between optimal thicknesses of the PEDOT:PSS layer and the active layer. We also present the influence of antireflection coating on distributions of optical electrical fields, as well as the distributions of exciton generation rate. Perpendicular and oblique illumination of the photovoltaic structure is discussed.

Solution-Processed Nanocomposites Containing Molybdenum Oxide and Gold Nanoparticles as Anode Buffer Layers in Plasmonic-Enhanced Organic Photovoltaic Devices

Solution-processed nanocomposites containing molybdenum oxide (MoO3) and gold nanoparticles (Au NPs) have been used as anode buffer layers in organic photovoltaic devices (OPVs). The resulting devices exhibit a remarkable enhancement in power conversion efficiency after Au NPs were incorporated into the device. Such enhancements can be attributed to the localized surface plasmon resonance induced by the metallic nanostructures. We have also found that the rate of exciton generation and the probability of exciton dissociation were increased. Furthermore, the devices made of the MoO3 buffer layer containing Au NPs exhibited superior stability. This work opens up the possibility of fabricating OPVs with both high efficiency and a prolonged lifetime.

Effects of ITO film annealing temperature on hybrid solar cell performance

The effects of indium tin oxide (ITO) film annealing temperature on the performance of organic solar cells are investigated. The roughness of the ITO film surface morphology increased with increasing annealing temperature. The optical penetration and rate of exciton generation both increased with increasing ITO film annealing temperature, enhancing the short-circuit current density. The maximum efficiency (2.62 %) was obtained with an annealing temperature of about 500 °C. The incident-photon-to-current efficiency value for a hybrid photovoltaic device with an ITO film annealed temperature at 500 °C was 45 % at 475 nm.

A quantitative study of the formation of PCBM clusters upon thermal annealing of P3HT/PCBM bulk heterojunction solar cell

Several thermal annealing strategies are employed to account for the formation of PCBM aggregates in a blend of P3HT/PCBM thin films. Optical absorption spectroscopy reveals dependence of PCBM cluster formation upon different annealing strategies as indicated by optical microscopic images. Using Raman image mapping of the selected locations we were able to identify PCBM rich regions. Exciton generation rate as a function of thermal annealing is in good correlation with short circuit current density. The probability of exciton dissociation as a function of thermal annealing reveals an almost static behaviour in response to the temperature rise up to 75°C. The enhanced intensity of photoluminescence signal upon thermal annealing supports the increase in exciton generation rates resulting from formation of PCBM clusters and recrystallisation of P3HT domains, removing nonradiative recombination centres. Current density voltage characterisation reveals dependence of device parameters on thermal annealing strategies.

Optical optimization of organic solar cells

The work is devoted to the optimization of layer thickness in an organic photovoltaic cell. It presents the applied calculation method which is based on the optical transfer matrix 2×2 formalism. We present the influence of the thickness of the PEDOT:PSS layer and of the active layer on the normalized modulus squared of the optical electric fields distribution inside devices and on the distributions of exciton generation rate. We present the relationship between the optimal thicknesses of the PEDOT:PSS layer and the active layer.