Properties Of Organic Solar Cells Research Articles

Structural and electrical properties of mixture based on P3HT:PCBM and low band gap naphthalene diimide-imines

Three small organic molecules, namely naphthalene diimide-imines, were investigated by electrical techniques and theoretically calculated by density functional theory. Electrical properties of the devices were tested in dark and under illumination. Nyquist plots were presented for devices measured by impedance spectroscopy. Additionally, compounds were investigated by ultraviolet–visible spectroscopy and atomic force microscopy. The obtained results showed that chemical structure of the organic compound and device architecture influence on the electrical properties of organic solar cells. The device comprising 5% w/w of naphthalene diimide-imine end-capped with bithiophene units in layer consist of poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester showed lower value of power conversion efficiency in comparison with the device comprising 33% w/w of this compound in active layer. Up to now to our best knowledge, there is no any study concerns the properties of the solar cell constructed with such type of molecules as an additive.

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

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

The Effect of Solution Processing on the Power Conversion Efficiency of P3HT-based Organic Solar Cells

Chloroform, Chlorobenzene, 1, 2-Dichlorobenze and co-solvent Chlorobenzene with Chloroform (1:1) were used to investigate the influence of the solvent on the morphology of P3HT:PCBM (with the ratio 1:1) as well as the photovoltaic properties of organic solar cells based on this polymer blend. Using co-solvents leads to higher absorption intensity as compared to other studied solvents. XRD measurements reveal improved crystallinity in P3HT films prepared by chloroform and chlorobenzene while large aggregates appeared in films prepared using dichlorobenzene. Films produced using co-solvents show well-ordered structure and improved crystallinity; these results were further confirmed by SEM images. The J(V) measurements show that a maximum PCE of 1.47% is obtained in films produced using co-solvents. This was associated with significant improvement in current density (11.77mA/cm2) and a fill factor of 0.4 which was determined from the series and shunt resistances. Open circuit voltage has decreased in film prepared by co-solvents (0.31 Volt), and improved significantly (0.54 Volt) in film prepared in chloroform.

Impact of alkyl side chains on the photovoltaic and charge mobility properties of naphthodithiophene–benzothiadiazole copolymers

A series of 5,6-bis(2-ethylhexyloxy)naphtho[2,1-b:3,4-b′]dithiophene and 4,7-bis(3-alkylthiophen-2-yl)-2,1,3-benzo[c][1,2,5]thiadiazole alternating copolymers, PNB-Cn, with different alkyl chains, varying from 8-carbon (C8, 2-ethylhexyl) to 16-carbon (C16, hexadecyl) chains attached onto thienyl moieties were designed and synthesized. The effect of the conjugated copolymer side chain on the photovoltaic and charge mobility properties of organic solar cells (OSCs) and polymer field-effect transistors (PFETs) was investigated. It is found that the short alkyl side chains on PNB-Cn copolymers are favourable for the efficient operation of bulk heterojunction (BHJ) OSCs, while the long alkyl side chains enhance the charge mobility of PFETs. As a result, the BHJ OSCs fabricated from the blend of PNB-C2,6 and PC71BM, using diiodooctane as a solvent additive, yielded a power conversion efficiency of 4.8% and the solution-processed bottom gate bottom contact type PFETs fabricated from PNB-C16 exhibited a hole mobility of 3.4 × 10−2 cm2 V−1 s−1 with an on/off current ratio of 106. Our results suggest that copolymers constructed from 5,6-bis(2-ethylhexyloxy)naphtho[2,1-b:3,4-b′]dithiophene and 4,7-bis(3-alkylthiophen-2-yl)-2,1,3-benzo[c][1,2,5]-thiadiazole are very stable, and solution-processable organic semiconductors for application in organic electronics.

Photovoltaic properties of organic solar cell with octafluorophthalocyanine as electron acceptors

Organic photovoltaic (OPV) cells were fabricated using copper-2,3,9,10,16,17,23,24-octafluorophthalocyanine (F8CuPc) as an electron acceptor. Normal and inverted OPV cells using F8CuPc were showed rectification in dark and photovoltaic characteristics under illumination. The inverted-type cell had good durability. The power conversion efficiency (η) of the inverted-type cell was 0.11%, approximately 6.5 times higher than that of the normal-type cell. The cell using F8CuPc was more stable than that using fullerene in air. These results indicate the possibility of using F8CuPc as an electron acceptor for air-stable OPV cells.

Photovoltaic Characteristics of Hybrid MEH-PPV and TiO<sub>2</sub> Nanoparticle Based Organic Solar Cells

The photovoltaic properties of organic solar cells based on hybrid poly [2-methoxy-5-(2-ethylhexyloxy-p-phenylenevinylen) (MEH-PPV) and anatase titanium dioxide (TiO2) nanoparticles as a function of TiO2 concentration were investigated. Synthesis of TiO2 nanoparticles was performed by sol-gel immerses heated method and been used as a filler in MEH-PPV polymer matrix. The hybrid MEH-PPV: TiO2 solar cells exhibited increased in light absorption and power conversion efficiency than the pristine organic solar cell. By further optimizing the concentration of TiO2 nanoparticles, the short-circuit current of the hybrid MEH-PPV: TiO2 was reached up to 0.004823 (mA/cm2) and the corresponding power conversion efficiency was 0.000378% was obtained under Air Mass 1.5 illumination which was more than 80% higher compared to the device without TiO2 nanoparticles. This indicates by embedded TiO2 nanoparticles in MEH-PPV matrix encouraging the charge transportation in the active layer of organic solar cells device.

Enhancing Performance and Nanomechanical Properties of Carbon Nanotube Doped P3HT:PCBM Solar Cells

The effects of single-walled carbon nanotubes (SWCNTs) on the performance and nanomechanical properties of organic solar cells are explored. The maximum surface root mean square roughness of the hybrid film was obtained with 0.5 wt% SWCNT doping. The addition of SWCNTs increased the rate of exciton generation, thereby enhancing the short-circuit current density. The optical absorption increased and the absorption peak position slightly blue-shifted with increasing SWCNT content. The SWCNT doping of hybrid organic solar cells improved efficiency and increased hardness and Young's modulus.

Carrier transport and charge transfer properties in coumarin‐doped bulk‐heterojunction materials

AbstractWe have investigated photovoltaic properties of organic solar cells using polymer‐fullerene bulk‐heterojunction films doped with coumarin dyes. Whereas the coumarin molecules used in this study had similar absorption bands, evident difference was observed in the open‐circuit voltage as well as in the short‐circuit current. In particular, the doping of coumarin 307 was found to cause a distinct enhancement in the open‐circuit voltage. On the other hand, the doping of coumarin 30 gave a serious degradation in the device performance. These results were strongly associated with calculated molecular energies of the doped dyes, especially with the highest occupied molecular orbital energy. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of Microscopic Properties of Organic Solar Cells by Light-Induced Electron Spin Resonance

Evaluation of Microscopic Properties of Organic Solar Cells by Light-Induced Electron Spin Resonance

Evaluation of Microscopic Properties of Organic Solar Cells by Light-Induced Electron Spin Resonance

We have investigated organic thin-film solar cells from the microscopic viewpoint with light-induced electron spin resonance (ESR). The utilized cell structures are indium–tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methylester (PCBM)/Pd/LiF/Al. We performed simultaneous measurements of light-induced ESR and device characteristics using the same device. Under simulated solar irradiation, the light-induced ESR intensity of a solar cell increases, while the short-circuit current and open-circuit voltage of the cell concomitantly decrease with increasing duration of irradiation. We have successfully observed a clear correlation between the light-induced ESR intensity and the device characteristics. Trapping sites of photogenerated hole carriers (positive polarons) are ascribed to P3HT by the analysis of the ESR signals.

Toward mechanically robust and intrinsically stretchable organic solar cells: Evolution of photovoltaic properties with tensile strain

This paper describes the effect of tensile strains of up to 20% on the photovoltaic properties of organic solar cells based on two different conjugated polymers and their blends with (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The two conjugated polymers are poly(3-hexylthiophene) (P3HT) and a donor–acceptor polymer whose repeat unit comprises a diketo pyrrolo-pyrrole moiety, thiophene, thienothiophene, and thiophene (DPPT-TT). The stretchable substrate, transparent electrode, and top electrode are, respectively, poly(dimethylsiloxane) (PDMS), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and eutectic gallium–indium (EGaIn). Analysis of the results of mechanical testing using a buckling based metrology, atomic force and optical microscopy, and linear and cyclic applications of strain yielded six major conclusions. (i) The identity of the conjugated polymer significantly affects the response of the solar cells to strain. Blending of PCBM in bulk heterojunction films stiffens P3HT more than it stiffens DPPT-TT, as determined by buckling-based measurements of elastic moduli. (ii) The greater elastic modulus of P3HT:PCBM than that of DPPT-TT:PCBM is correlated with a greater tendency of P3HT:PCBM to fracture under strain. (iii) PEDOT:PSS has the effect of an adhesion layer of the bulk heterojunction films to PDMS; its presence increases the crack-onset strain from <2% to 8% for P3HT:PCBM and from 6% to 12% for DPPT-TT:PCBM. (iv) Cracks dominate the characteristics of the devices under strain. The differences in the response to strain between devices comprising P3HT:PCBM and DPPT-TT:PCBM are correlated to the unequal brittleness of the polymer:fullerene films. (v) Devices based on both P3HT and DPPT-TT exhibit increased open-circuit voltage with strain, which is greatest for devices containing P3HT. These effects are attributed to disruption of the gallium oxide “skin” that covers the top electrode, EGaIn. (vi) Wave-like buckles appear in the films upon stretching ≤10% for the first time. These buckles permit reversible stretchability.

Photo-induced changes in fundamental properties of organic solar cells

We report on the measurement of fundamental properties such as deep defects and hole mobility in poly-3-hexyl-thiophene (P3HT)/[6,6]-phenyl-C60-butyric acid methyl ester(PCBM) solar cells when the cells are exposed to solar radiation without any atmospheric exposure. It is found that the midgap defect density in P3HT and the interface density between P3HT and PCBM increase significantly upon light soaking along with a reduction in hole mobility in P3HT. The increase in defect density leads to a corresponding increase in reverse saturation current of the diode, and the corresponding decrease in open circuit voltage of the cell upon light soaking.

Metallic nanomesh electrodes with controllable optical properties for organic solar cells

We have fabricated a metallic nanomesh using nanosphere lithography and metal evaporation. The metallic nanomesh has a precisely controlled nanostructure showing excellent uniformity with hexagonally arrayed periodic circular holes. A P3HT:PCBM organic solar cell, with the gold nanomesh electrode, demonstrates a high fill factor of 61% and a considerable power conversion efficiency of 3.12%. Electromagnetic simulation indicates that the optical properties of the metallic nanomesh can be optimized for organic photovoltaic devices by tuning the film thickness, hole diameter, and periodicity. These results show the promising potential of using a metallic nanomesh as the transparent electrode in organic solar cells.

Investigation of Hole Transport Layer in Relation to the Properties of Organic Solar Cells

Organic solar cells based on a blend of copper phthalocyanine and bulk fullerene are fabricated with a double hole transport layer system. The double hole transport layer was composed of poly3,4-ethylenedioxythiophene:polystyrenesulfonate, and copper phthalocyanine and inserted between the anode and active layer. The double hole transport layer system utilizes advantages of both layer. The poly3,4-ethylenedioxythiophene:polystyrenesulfonate layer modifies the surface morphology of the ITO anode and the copper phthalocyanine layer enhances hole transport. In order to enhance the conductivity of the modification layer, the optimal amount of glycerol is doped into poly3,4-ethylenedioxythiophene:polystyrenesulfonate. Furthermore, the photovoltaic characteristics are further improved. Insertion of the double hole transport layer with a 4 nm-thick copper phthalocyanine layer resulted in open circuit voltage, short current, and power conversion efficiency as high as 0.46 V, 8.8 mA/cm2 and 1.37%, respectively.

Effects of anode buffer layers on the properties of organic solar cells

We fabricated organic solar cells (OSCs) using a poly(3-hexythiophene) (P3HT) and methanofullerene(6,6)-phenyl C61 butyric acid methyl ester (PCBM) active layer for the electron donor and acceptor materials, respectively. The active layer was spin-coated onto various substrates with different anode buffer layer positions, such as lithium fluoride (LiF)/indium tin oxide (ITO)/glass (device A), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/ITO/glass (device B) and LiF/PEDOT:PSS/ITO/glass (device C). The effects of the anode buffer layer position on the electrical and morphological properties of the devices were investigated. The performance of the OSC coated on the device C substrate showed the best electrical property among the other samples, indicating that the maximum short circuit current density, open circuit voltage, fill factor and power conversion efficiency values were about 9.1mA/cm2, 0.65V, 54.1% and 3.2%, respectively. The LiF buffer layer deposited on the PEDOT:PSS/ITO substrate (device C) showed a smooth film roughness and a lower contact angle compared to the other samples, which led to an improvement of the hole injection efficiency into the anode electrode.

Effect of molar mass and regioregularity on the photovoltaic properties of a reduced bandgap phenyl-substituted polythiophene

Among the numerous reduced bandgap polymers currently being developed, poly[3-(4-octylphenyl)thiophene)]s (POPT) may present attractive properties for organic solar cells due to its facile preparation and improved absorption with respect to poly(3-hexylthiophene). This article appraises methods of preparation, including the use of diphenyl ether as a reaction medium, and discusses the effects of variations in molar masses, from about 3200 to 65,000 g mol -1 and regioregularity on its optoelectronic properties. The photovoltaic properties of POPT with [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) in bulk heterojunction devices are also discussed in the light of morphological variations, as indicated by atomic force microscopy characterizations. With an initial screening of conditions, namely POPT:PCBM ratios and deposition solvent, a power conversion efficiency of 1.58% was obtained using a relatively high molar mass POPT sample.

Effects of various solvent addition on crystal and electrical properties of organic solar cells with P3HT:PCBM active layer

We investigated the effect of the preparation conditions used for the active layer on the performance of polymer/fullerence bulk-hetero junction solar cells by changing the solvent (toluene, monochlorobenzene, dichlorobenzene) combinations. The organic solar cells (OSCs) with an ITO/PEDOT:PSS/P3HT:PCBM/Al structure were fabricated by using regioregular poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C61-butyric acid methyl ester fullerene (PCBM) for the active layer. The P3HT and PCBM used for the electron donor and acceptor materials were spin-casted on the indium tin oxide (ITO) coated polyethylene naphthalate (PEN) flexible substrates. The optimum mixing concentration ratio for the active layer was found to be P3HT:PCBM=50:50 in wt%. In order to investigate the effects of the precursor solvents under the same P3HT:PCBM concentration in the mixing ratio, the performance of the OSCs was studied by introducing various pure and blended co-solvents. The best property was obtained in the active layer by using monochlorobenzene (MCB) for the P3HT and dichlorobenzene (DCB) for the PCBM. The results show that the maximum short circuit current density (Jsc), the open circuit voltage (Voc), the fill factor (FF) and the power conversion efficiency (PCE) were about 10.4mA/cm2, 606mV, 45.5%, and 2.9%, respectively.

Coarse-grained kinetic modelling of bilayer heterojunction organic solar cells

The on-lattice kinetic Monte Carlo (KMC) method provides a powerful tool to simulate the J–V properties of organic solar cells. However, the computational cost associated with charge injection may limits its applicability. In the attempt to overcome this limitation, we describe in this paper a coarse-grained numerical approach to photocurrent generation in bilayer heterojunction solar cells. Starting from the KMC algorithm, a self-consistent numerical procedure is proposed to find the steady-state solutions of the kinetic equations describing particle dynamics in one dimension across the layer thickness. Our model incorporates the generation, transport and recombinations of charge carriers, excitons, and electron/hole pairs, whose introduction is required to correctly describe interfacial phenomena at the coarse-grained level. A continuum model of the electrostatic interactions among charge carriers is proposed and used to compute their hopping rates during the simulation. The model is used to investigate the J–V properties of Cathode/PCBM/P3HT/PEDOT:PSS/ITO bilayer devices, showing that Fermi level pinning at the Cathode/PCBM interface must be invoked to accurately model the experimental behavior. From the fitting to the experimental J–V data, we conclude the short-circuit current density to be mainly associated with a high exciton diffusion length. The analogies and differences between our model and existing KMC and drift–diffusion approaches are also discussed.

Optical and electrical study of organic solar cells with a 2D grating anode

We investigate both optical and electrical properties of organic solar cells (OSCs) incorporating 2D periodic metallic back grating as an anode. Using a unified finite-difference approach, the multiphysics modeling framework for plasmonic OSCs is established to seamlessly connect the photon absorption with carrier transport and collection by solving the Maxwell's equations and semiconductor equations (Poisson, continuity, and drift-diffusion equations). Due to the excited surface plasmon resonance, the significantly nonuniform and extremely high exciton generation rate near the metallic grating are strongly confirmed by our theoretical model. Remarkably, the nonuniform exciton generation indeed does not induce more recombination loss or smaller open-circuit voltage compared to 1D multilayer standard OSC device. The increased open-circuit voltage and reduced recombination loss by the plasmonic OSC are attributed to direct hole collections at the metallic grating anode with a short transport path. The work provides an important multiphysics understanding for plasmonic organic photovoltaics.

In situ monitoring of photovoltaic properties in organic solar cells during thermal annealing

This paper reports a simple and useful technique that monitors the changes of poly(3-hexylthiophene) (P3HT):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61-based organic solar cells (OSCs) during thermal annealing in situ. Thermal annealing was divided into five stages in which the variations of the cell parameters were obtained in detail. Annealing temperature that was higher than the glass transition temperature of P3HT (127°C) was found critical to the improvement of open-circuit voltage. The initial rise of the short-circuit current was explained by in situ monitoring of the transport behaviors of electrons and holes. Finally, in situ monitoring was adopted to compare OSCs that were or were not solvent-annealed, indicating the effectiveness in optimizing, modeling and understanding in depth the effects of the thermal annealing of OSC with a blended active layer.