Organic Thin-film Solar Cells Research Articles

Investigations on the charge transfer mechanism at donor/acceptor interfaces in the quest for descriptors of organic solar cell performance.

Herein, we theoretically and experimentally investigated the mechanisms of charge separation processes of organic thin-film solar cells. PTB7, PTB1, and PTBF2 have been chosen as donors and PC71BM has been chosen as an acceptor considering that effective charge generation depends on the difference between the material combinations. Experimental results of transient absorption spectroscopy show that the hot process is a key step for determining external quantum efficiency (EQE) in these systems. From the quantum chemistry calculations, it has been found that EQE tends to increase as the transferred charge, charge transfer distance, and variation of dipole moments between the ground and excited states of the donor/acceptor complexes increase; this indicates that these physical quantities are a good descriptor to assess the donor-acceptor charge transfer quality contributing to the solar cell performance. We propose that designing donor/acceptor interfaces with large values of charge transfer distance and variation of dipole moments of the donor/acceptor complexes is a prerequisite for developing high-efficiency polymer/PCBM solar cells.

Extremely Efficient Design of Organic Thin Film Solar Cells via Learning-Based Optimization

Design of efficient thin film photovoltaic (PV) cells require optical power absorption to be computed inside a nano-scale structure of photovoltaics, dielectric and plasmonic materials. Calculating power absorption requires Maxwell’s electromagnetic equations which are solved using numerical methods, such as finite difference time domain (FDTD). The computational cost of thin film PV cell design and optimization is therefore cumbersome, due to successive FDTD simulations. This cost can be reduced using a surrogate-based optimization procedure. In this study, we deploy neural networks (NNs) to model optical absorption in organic PV structures. We use the corresponding surrogate-based optimization procedure to maximize light trapping inside thin film organic cells infused with metallic particles. Metallic particles are known to induce plasmonic effects at the metal–semiconductor interface, thus increasing absorption. However, a rigorous design procedure is required to achieve the best performance within known design guidelines. As a result of using NNs to model thin film solar absorption, the required time to complete optimization is decreased by more than five times. The obtained NN model is found to be very reliable. The optimization procedure results in absorption enhancement greater than 200%. Furthermore, we demonstrate that once a reliable surrogate model such as the developed NN is available, it can be used for alternative analyses on the proposed design, such as uncertainty analysis (e.g., fabrication error).

Surrogate based modeling and optimization of plasmonic thin film organic solar cells

In this study, we demonstrate that surrogate models can be trained and used to accurately predict the optical properties of thin film solar cells and to optimize their structures. We consider organic an active thin film poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric-acid-methyl ester (P3HT:PCBM) layer coated by indium-free highly conductivepolymer poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) on top with aluminum as cathode, and study optical absorptivity enhancement of such a structure when embedded with periodic spherical silver nano-particles. Metallic nano-structures, including textures, gratings and particles are known to induce plasmonic effects on the surface and inside absorbing semiconductors, thereby increasing light trapping in thin film cells. However, design of such structures requires precise characterization of the dependencies of electromagnetic field distribution to geometry parameters and material choices. Optical properties of structures at sub-wavelength scales are measured by numerically solving first principle electromagnetic equations, e.g., by means of finite difference time domain (FDTD) method. These methods are time-consuming, and therefore limit the possibility of exhaustive optimization. Surrogate modeling can be used to overcome this challenge. In the present work, we design a two-layer neural network (NN) surrogate model to estimate the optical absorptivity of the cell for any given geometry vector as well as any radiation wavelength and incident angle. Training of the network is done using the Levenberg–Marquardt (LM) method with generalization techniques such as early stopping and Bayesian regularization using a pool of training and validation data. The resulting surrogate model is first demonstrated to yield accurate out-of-sample estimation of absorptivity. Then, the model is used to investigate the individual contributions of each input by means of sensitivity analysis. In addition, optimization of the parameters is efficiently done using the surrogate model for different source light irradiance spectra and incident angles. Resulting optimizations are very efficient. At the end, solutions found by surrogate-based optimization demonstrate enhancement factors greater than 270% for optical absorptivity.

Bimetallic nanocomposite as hole transport co-buffer layer in organic solar cell

Silver–zinc bimetallic nanocomposite (Ag:Zn BiM-NPs) was used as an inter-facial buffer layer in the preparation of thin film organic solar cell (TFOSC). The current investigation focuses on the effect of bimetallic nanoparticles on the performance of TFOSC. A number experiments were conducted by employing Ag:Zn nanocomposite buffer layer of thickness 1 nm at various positions of the device structure. In all cases, we found significant improvement on the power conversion efficiency of the solar cells. It is also noted that the open circuit voltage of the devices are decreasing when Ag:Zn form direct contact with the ITO electrode and without the inclusion of PEDOT:PSS. However, all results show that the introduction of Ag:Zn nanocomposite layer close to PEDOT:PSS could be beneficial to improve the charge transport processes in the preparation of thin film organic solar cell. The Ag:Zn BiM-NPs and the device properties were presented and discussed in terms of optical, electrical and film morphologies of the devices.

Investigation of Gold Quantum Dot Enhanced Organic Thin Film Solar Cells

Gold quantum dots (AuQDs) are employed as photosensitizers in organic thin-film solar cells (OSCs) to improve their photoelectric conversion properties. Three types of AuQDs with different fluorescence emission wavelengths are used: blue (B-AuQDs), green (G-AuQDs), and red (R-AuQDs). AuQDs are loaded into the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) thin-film layer of OSCs. UV–vis spectra, atomic force microscope images, current density–voltage characteristics, and impedance spectra of the fabricated devices are measured for the three aforementioned types of AuQDs. All types of AuQDs improve the photoelectric conversion properties, and the G-AuQD-loaded OSCs exhibit the best improvement, exhibiting an efficiency increase of 10% compared with OSCs without the AuQDs. The fluorescence/photosensitization of the AuQDs plays an important role in the enhancement of the OSCs. Finite-difference time-domain simulations indicate increased electric field intensity due to a small degree of AuQDs aggregation.

Energy band alignment in operando inverted structure P3HT:PCBM organic solar cells

Inverted structure thin-film organic solar cells (OSCs) are becoming increasingly important as they deliver higher power conversion efficiency and demonstrate better long-term stability than conventional devices. However, the energy band alignment and the built-in field across the device, which are crucial in understanding the device operation, is yet to be directly characterized. Here we present a direct visualization of the energy level alignment in operando inverted structure poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) OSCs using cross-sectional scanning Kelvin probe microscopy. The raw data of measured energy level alignment appear to be inconsistent with each other, and sometimes can even be contradictory to the device polarity observed in current density-voltage measurements. It is identified to be caused by the tip/cantilever induced convolution effect, which may severely mask abrupt energy level offsets at the thin electrode interlayers. A numerical deconvolution method is devised to quantitatively recover the energy level alignment across the device, and reveals the non-uniform electric field distribution in photoactive layer.

Effect of gold nanoparticles in titanium oxide layer on the photovoltaic performance of inverted-type organic thin-film solar cells

ABSTRACTGold nanoparticle/titanium oxide composite materials were prepared at low temperature and were used as electron-transport layers for inverted-type organic thin-film solar cells. The application of gold nanoparticle/titanium oxide composite materials led to an improvement in the photoelectric conversion efficiency of the solar cells under simulated sunlight irradiation.

Organic and Polymeric Semiconductors Enhanced by Noncovalent Conformational Locks.

Constructing highly planar, extended π-electron systems is an important strategy for achieving high-mobility organic semiconductors. In general, there are two synthetic strategies for achieving π-conjugated systems with high planarity. The conventional strategy connects neighboring aromatic rings through covalent bonds to restrict the rotation about single bonds. However, this usually requires a complex sequence of synthetic steps to achieve this target, which can be costly and labor-intensive. More recently, noncovalent through-space intramolecular interactions, which are defined here as noncovalent conformational locks, have been employed with great success to increase the planarity and rigidity of extended π-electron systems; this has become a well-known and important strategy to design and synthesize highly planar π-conjugated systems for organic electronics. This review offers a comprehensive and general summary of conjugated systems with such noncovalent conformational locks, including O···S, N···S, X···S (where X = Cl, Br, F), and H···S through-space interactions, together with analysis by density functional theory computation, X-ray diffraction, and microstructural characterization, as well as by evaluation of charge transport in organic thin-film transistors and solar cells.

The effect of uni- and binary solvent additives in PTB7:PC61BM based solar cells

The effect of uni- and binary solvent additives on the performance of organic solar cells based on poly4,6-(2-ethylhexyl-3-fluorothieno [3,4-b]thiophene-2-carboxylate)alt-2,6(4,8-bis(2-ethylhexyloxy) benzo [1,2-b:4,5-b]dithiophene) (PTB7) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was investigated using chloroform host solvent. The use of binary solvents additive was found to be more effective in improving the collection of photo-generated current in thin film organic solar cells (TFOSC). The difference in the boiling points of the solvents and their ability to dissolve fullerene molecules from the blend are some of the critical factors that determines the effect of binary solvents additive. The inclusion of solvent additives in device processing improved the power conversion efficiency of TFOSC which can be attributed to the polymer crystallinity and the formation of favourable phase separated domains for the percolation of free charge carriers. Moreover, the investigations on the charge transport properties suggest that a little reduction in the non-germinate recombination was realized leading to the improved charge extraction in the devices with binary solvents additive compared to those processed without additive solvent.

Theoretical Calculation of the Optical Properties of Dielectric Material @ Noble Metal Core-Shell Composite Nanoparticles

Objectives: Surface plasmon exhibited in nano-sized particles such as gold, silver, copper has recently attracted a great deal of attention due to the enhancement and tunable optical properties in the visible to near-infrared region of the electromagnetic spectrum. In this work, the optical properties of dielectric material at noble metal core-shell nanoparticles were investigated. Methods: Mie theory is the analytical method is used to study via parameters of extinction, scattering and absorption efficiencies. The samples for studying were the composites of a dielectric material core coated by a thin noble metal (Ag, Au, and Cu). The core size was fixed at 50 nm, while the metal shell thicknesses were varied from 3 nm to 40 nm. Findings: The calculated optical properties were found that the surface plasmon resonance can be enhanced and tuned the wavelength from the near ultraviolettonear infrared regions by varying the shell thickness. Application: The dielectric core coated with a thin noble metal to promote the properties of the core can be applied in the field of plasmonic nanolaser, nonlinear plasmonic antenna, light trapping in thin-film organic solar cells, and biomedical research.

Zinc oxide doped single wall carbon nanotubes in hole transport buffer layer

Abstract Zinc oxide doped single wall carbon nanotubes (ZnO:CNT) are incorporated in PEDOT:PSS aqueous solution to serve as a hole transport buffer layer in the preparation of thin film organic solar cells (TFOSC). The solar cells were fabricated according to bulkheterojunction design whose photoactive layer is composed of poly (3 hexythiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend. Significant improvement in harvesting photo-generated currents and low series resistances were observed which lead to higher power conversion efficiencies compared to the devices without ZnO:CNT. The optical properties and surface morphologies of ZnO:CNT/PEDOT:PSS hole transport layers are investigated and compared with the changes in the measured parameters of the solar cells. The power conversion efficiency of the devices increased by nearly 116%, 63% and 42% for ZnO:CNT loading at 2.5%, 5% and 10% by weight, respectively, from the devices that uses only PEDOT:PSS as a hole transport layer. Furthermore, a complete recovery of device performance was found by storing the device in warm nitrogen atmosphere.

Optimization of photocurrent in bulk heterojunction organic solar cells using optical admittance analysis method

The optimized thicknesses of the active individual layers in organic thin film solar cells are obtained using optical admittance analysis method (OAAM). We have used OAAM to simulate the optical properties of two bulk-heterojunction (BHJ) organic solar cells (OSCs) of structures: (1) ITO/PEDOT:PSS/P3HT:PCBM/Lif/Al and (2) ITO/PTB7:PCBM/Lif/Ag. The optimal thicknesses of 75 nm and 115 nm of P3HT:PCBM and PTB7:PCBM blend layers, respectively, are obtained by maximising the absorbance in these layers through this simulation, which agree very well with the experimental results. The simulated short-circuit current density J SC is plotted as a function of the active layer thickness for a few selected thicknesses of the Al cathode in these two OSCs and it is found that J SC becomes maximum when the thickness of Al cathode is 40 nm. Using these optimised thicknesses of the active layers in these two cells the short-circuit current density is found to increase in ITO/PEDOT:PSS/P3HT:PCBM/Lif/Al BHJ OSC by 4.8% and in ITO/PTB7:PCBM/Lif/Ag by 13.3%.

Regenerated cellulose fiber solar cell

Wearable electronics and smart textiles are growing fields in the cause to integrate modern communication and computing tools into clothing instead of carrying around smart phones and tablets. Naturally, this also requires power sources to be integrated in textiles. In this paper, a proof-of-concept is presented in the form of a photovoltaic cell based on a commercially available viscose fiber. This was realized using a silver nanowire network around the viscose fiber to establish electrical contact and a photoactive coating using the standard workhorse among organic thin film solar cells, a blend of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester. The structure and performance of single fiber devices demonstrate their feasibility and functionality. The applied materials and methods are compatible with solution processing, and therefore qualify for potential roll-to-roll large-scale production.

Detailed optical modelling and light-management of thin-film organic solar cells with consideration of small-area effects.

We present detailed numerical and experimental investigation of thin-film organic solar cells with a micro-textured light management foil applied on top of the front glass substrate. We first demonstrate that measurements of small-area laboratory solar cells are susceptible to a significant amount of optical losses that could lead to false interpretation of the measurement results. Using the combined optical model CROWM calibrated with realistic optical properties of organic films and other layers, we identify the origins of these losses and quantify the extent of their influence. Further on, we identify the most important light management mechanisms of the micro-textured foil, among which the prevention of light escaping at the front side of the cell is revealed as the dominant one. Detailed three-dimensional simulations show that the light-management foil applied on top of a large-area organic solar cell can reduce the total reflection losses by nearly 60% and improve the short-circuit current density by almost 20%. Finally, by assuming realistic open-circuit voltage and especially the realistic fill factor that deteriorates as the absorber layer thickness is increased, we determine the optimal absorber layer thickness that would result in the highest power conversion efficiency of the investigated organic solar cells.

Organic photovoltaic annealing process analysis using impedance spectroscopy

Organic solar cells based on P3HT:PCBM without annealing process, with annealing process before and after cathode deposition have been investigated in dark and under light by using high magnitude and phase resolution impedance spectrometer. The impedance data in dark can be interpreted as the properties and positions of layers and interfaces of the sample to nanoscale range. Therefore, the fabrication process and device nanoscale layer and interface quality can be monitored. Under light conditions, the impedance data obtained were fitted into equivalent circuit and electronic parameters such as diffusion time of electrons, τd and effective carrier lifetime, τn were calculated. Correlation of the nanoscale layers and interface parameters to the dynamic carrier performance parameters were established. Applying the high magnitude and phase resolution impedance method to understand the properties of organic thin-film solar cells in both static and dynamic status will give useful insights not only on the device performance improvement in terms of process, layer morphology and interfaces quality; but also deeper understanding of how the device kinetic characteristics at the interface states induced correlate with the improvement of current-potential characteristics under device operation conditions.

Co-solvent additives influence on the performance of PTB7:PCBM based Thin film organic solar cell

The effect of co-solvents additives on the performance of thin film organic solar cells (TFOSC) composed of Poly {4,6-(2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate)alt-2,6(4,8-bis(2-ethylhexyloxy) benzo[1,2-b:4,5-b]dithiophene)} (PTB7), a donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as an acceptor were investigated. We found significant improvement on device performance when 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) mixture was added to the chloroform based solution of PTB7:PCBM blend as compared to devices without additives (pristine) as well as with sole DIO or CN. The role of co-solvents on device performance has been investigated and discussed in terms of changes in optical, electrical and micro-structural properties of the photoactive medium.

Grating-coupled surface plasmon resonance enhanced organic photovoltaic devices induced by Blu-ray disc recordable and Blu-ray disc grating structures.

In this work, we studied the performance enhancement of organic thin-film solar cells (OSCs) originating from the presence of diffraction gratings on the surface of the active layer. Two types of diffraction gratings, periodic gratings (Blu-ray disc recordable: BD-R) and quasi-random gratings (Blu-ray disc: BD), were employed as master templates for grating structures. The grating structures were introduced to the surfaces of poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) films, which were the active layers of the solar cells. The addition of the grating structures led to an increase of light absorption in the absorption region of P3HT:PCBM induced by light scattering. Furthermore, the grating-coupled surface plasmon resonance generated additional light absorption peaks. With illumination of non-polarized light at a normal incident angle, the short-circuit current densities of the BD-R and BD solar cells improved by 11.05% and 10.6%, respectively. Efficiency improvements of 19.28% and 3.21% were also observed for the BD-R and BD devices, respectively. Finally, the finite-difference time-domain simulation results revealed an enhanced electric field in the P3HT:PCBM layer, especially in the BD-R OSC devices.

Effects of PBr<sub>3</sub> Addition to Polysilane Thin Films on Structures and Photovoltaic Properties

Organic thin film solar cells are potential next generation solar cells. Many p-type semiconductors have been used in organic solar cells, but there have been far fewer reports involving n-type organic semiconductors. Developing new n-type organic semiconductors is therefore desirable. Polysilane thin films were spin-coated from solutions containing phosphorus (P), and the effects of P addition on film microstructures and electronic properties as n-type semiconductors were investigated. Microstructures of poly-methyl-phenyl-silane (PMPS), dimethyl-polysilane (DMPS) and deca-phenyl-penta-silane (DPPS) thin films were investigated by X-ray diffraction and transmission electron microscopy. PMPS thin films doped with P (PMPS(P)) were amorphous upon annealing at 300&#176C, while DMPS(P) and DPPS(P) thin films exhibited crystalline structures. PMPS(P) and DMPS(P) thin films exhibited decreased electrical resistances upon P doping. The energy gaps of PMPS(P), DMPS(P) and DPPS(P) were 3.5, 3.9 and 3.8 eV, respectively. Decreased photoluminescence intensities of PMPS, DMPS and DPPS were observed upon P doping. The desorption of phenyl and methyl groups from PMPS(P) thin films was observed from Raman scattering measurements. Solar cells containing polysilane(P)/poly[3-hexylthiophene] heterojunction structures were fabricated and exhibited photovoltaic behavior.

Fabrication and Characterization of PCBM: P3HT-based Thin-film Organic Solar Cells with Zinc Phthalocyanine and 1,8-Diiodooctane

[6,6]-phenyl C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT) bulk heterojunction solar cells added with zinc-tetra-tertiary-butyl-phthalocyanine (ZnPc) and 1,8-diiodooctane (DIO) were fabricated and characterized. Incident photon-to-current conversion efficiencies in the range of 400-650 nm were increased by the ZnPc addition, and the absorption range of ZnPc overlapped with the photoluminescence range of P3HT. Photovoltaic properties of the solar cells with an inverted structure were improved by the ZnPc and DIO addition. Microstructures of the thin films were analyzed by X-ray diffraction. The improvement would be due to the Forster energy transfer mechanism the direct charge transfer from ZnPc to PCBM, and the phase separation of PCBM and P3HT by the DIO addition would also contribute the improvement.

Research into Fabrication and Popularization of Organic Thin Film Solar Cells

An analysis was conducted herein on the research status of several popular solar cells at the present stage, including silicon solar cell, thin film photovoltaic cell, and dye-sensitized solar cell (DSSC). In doing so, we concluded that the current situations provide a favorable objective environment for the popularization of organic thin film solar cells. Finally, we reviewed the merits and demerits of the organic thin film solar cell together with the major research focus on and progress of it, and summarized obstacles to and development trails of the popularization of organic thin film solar cells.