Bulk and contact resistance in P3HT:PCBM heterojunction solar cells

In this paper, the series resistance of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been studied. The series resistance of thermal annealed and un-annealed devices with different active layer thicknesses was measured. The series resistance of the organic solar cells consists of the bulk resistance of the active layer itself and the specific contact resistance between the active layer and the electrode. The bulk resistance and contact resistance were extracted from the measured series resistance using the vertical transmission line model (TLM) method. By fabricating solar cell devices with different active layer thicknesses, a relationship of the series resistance with thickness was established from which bulk and contact resistances were derived. We have also found that thermal annealing helps reduce both contact resistance and bulk resistance significantly; the contact resistance dropped by a factor of 2, while the bulk resistance decreased by a factor of 8. Results have shown that for an annealed P3HT:PCBM device that has an active layer thickness of 85 nm (optimum thickness for high efficiency), 17% of the total series resistance was due to the contact resistance, and bulk resistance contributed the rest 83%. The bulk resistance value for thermal annealed organic solar cell device with an active area of 0.1 cm² was found to be 150 Ω, and the measured specific contact resistance was 3.1 Ω cm². The measured bulk and contact resistance values are much higher as compared to the high efficiency silicon solar cells. Bulk resistance and contact resistance need to be further decreased in order to achieve higher organic solar cell efficiency.     

Effect of P3HT:PCBM concentration in solvent on performances of organic solar cells

Focused on phase separation and morphologies of poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PCBM) active layers, we studied the effect of preparation conditions of the active layer on photovoltaic performance by changing concentration of P3HT:PCBM in the solvent. The performances of the cells varied depending on concentration of P3HT:PCBM (1:1 ratio by weight) in solvent even with the same thickness. The P3HT:PCBM active layer is prepared in cell structure of ITO/PEDOT/P3HT:PCBM/Al by changing spin-coating speed with different concentrations (1, 2 and 3 wt%) in chlorobenzene. Here, it was found that both the P3HT:PCBM concentrations and spin-coating conditions affected the crystalline structure formation, interchain interaction, morphology and phase separation during drying process of solvent and subsequent annealing.     

The effect of porphyrin inclusion on the spectral response of ternary P3HT:porphyrin:PCBM bulk heterojunction solar cells

Cyanoporphyrins have been included into the active layer of bulk heterojunction poly (3-hexylthiophene) (P3HT): [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) solar cells. The amount of porphyrin, P3HT and PCBM were systematically varied and the characteristics of the devices from the corresponding active layers were recorded. The spectral responses of the devices showed that the addition of the porphyrin to the active layer broadened the absorption efficiency of the device and led to a porphyrin contribution to the photocurrent of the solar cell. The porphyrin molecules did not contribute to the photocurrent unless both P3HT and PCBM were present in the active layer. In most cases, the porphyrin was unable to contribute to the photocurrent after the devices had been annealed, suggesting changes to the morphology of the active layer.     

Modeling the short-circuit current density of polymer solar cells based on P3HT:PCBM blend

We have investigated the short-circuit current density of organic solar cells based on poly (3-hexylthiophene)(P3HT)/6,6-phenyl C61-butyric acid methyl ester (PCBM) blend. In order to model charge collection efficiencies with respect to short circuit density in such blends, a full optical modeling of the cell is performed. From the distribution of the electromagnetic field, we compute the rate of exciton generation. This exciton generation rate is used as input in the transport equations of holes and electrons. Charge densities at steady state are obtained as solutions are used for computing short-circuit current densities generated in the cell. The dependence of short-circuit current densities versus the thickness of the blend is analyzed and compared with our experimental data and with data extracted from the literature.     

Widening of harvesting layer and area of P3HT/PCBM bulk-heterojunction photovoltaic cells

We have fabricated P3HT/PCBM based bulk-heterojunction photovoltaic cells with P3HT layer as the hole transport layer and PCBM layer as the electron transport layer between electrode and blended P3HT/PCBM layer in order to widen the photon harvesting layer. Current density has increased by about 1 mA/cm² by the insertion of P3HT layer and the resulting conversion efficiency has been improved by about 20%. We have also fabricated a centimeter-scale active area with an efficiency of ∼1%.     

P3HT/PCBM bulk heterojunction solar cells: Relation between morphology and electro-optical characteristics

The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by blend composition and thermal annealing conditions. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) diffraction measurements show that in the considered blends, ordering of P3HT plays a key role in understanding the PV-performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. Annealing however improves the crystallinity, explaining the observed spectral broadening and is also resulting in a higher mobility of the holes in P3HT.     

Polymeric solar cells based on P3HT:PCBM: Role of the casting solvent

Polymeric photovoltaic (PV) solar cells have been fabricated using six solvents: chloroform (CHCl₃), toluene (T), chlorobenzene (CB), orthodichlorobenzene (ODCB), 1,2,3,4-tetrahydronaphthalene (THN) and 1,2,4-trichlorobenzene (TCB). The active layers were composed of poly(3-hexyl)thiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Special care has been taken to keep all experimental parameters constant (thickness of the active layers, donor/acceptor weight ratio, area of active surface and electrodes) in order to avoid artefacts and truly study the effect of solvents. Studies using atomic force microscopy (AFM) and optical absorption (UV-vis) showed the relationship between the photovoltaic performance and the evaporation rate of solvents. The use of solvents with high boiling point results in a higher degree of organization in the structure of P3HT. A direct comparison with devices processed with thermal treatment has also been performed. As often reported thermal annealing increases photo-conversion efficiency of devices created from common solvents, due to better separation of phase between the two materials of the blend. In the case of solvents with high boiling point such as THN and TCB, neither phase separation nor modification of P3HT crystallization induced by thermal annealing has been observed. However thermal treatment appears to enhance performance, ensuing the evaporation of remaining solvent in the active layers. An overview of the effect of solvent on the electrical properties of films containing pure P3HT and P3HT:PCBM blend reported in the literature has been completed for the discussion.     

Flexible organic P3HT:PCBM bulk-heterojunction modules with more than 1 year outdoor lifetime

Flexible organic solar cells and modules based on P3ht:PCBM bulk-heterojunctions were fabricated and their lifetime was investigated under laboratory and outdoor conditions. In the laboratory cells were exposed to 1 sun illumination at 65 °C in order to accelerate the degradation. The outdoor behavior of modules was investigated at the Konarka rooftop testing setup in Lowell, MA (USA). We show that these flexible polymer solar cells have a good light stability, passing 1000 h under accelerated light soaking conditions in the laboratory, and that flexible modules survived over 1 year of outdoor exposure without performance losses.     

Morphological and electrical effect of an ultrathin iridium oxide hole extraction layer on P3HT:PCBM bulk-heterojunction solar cells

An ultrathin iridium layer was treated with O₂-plasma to form an iridium oxide (IrO_x), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrO_x layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the Ir layer on ITO changed to IrO_x by the O₂-plasma treatment. The OPV cell with IrO_x (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm⁻² illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS.     

Abnormally high photocurrent of a degraded organic solar cell under chopped light

Photocurrent of an organic solar cell under chopped monochromatic light was studied at different stages, i.e. just after fabrication (without degradation) and after serious degradation. A large photocurrent under chopped light was observed for a seriously degraded device, unexpected from intuitionistic reckoning. This work demonstrates that both light bias and chopper frequency affect the photocurrent measurement of an organic solar cell, especially for a degraded device. The influence of light bias and chopper frequency is explained as the result of traps produced during the degradation of an organic solar cell. Thus measuring photocurrent under one sun light bias at low chopper frequency is suggested to more accurately determine the external quantum efficiency.     

P3HT/ZnO bulk-heterojunction solar cell sensitized by a perylene derivative

In this work, a soluble perylene-derivative dye, N, N′-didodecyl-3,4,9,10-perylene tetracarboxylic diimide (PDI), was used to improve the photovoltaic performance of poly(3-hexylthiophene) (P3HT)/ZnO bulk heterojunction cells through blending with the composite. Results show that by incorporation of PDI in the P3HT/ZnO composite, the light absorption and exciton separation can be significantly improved. The photocurrent under white-light irradiation can be increased from 6.35 to 9.55 mA/cm². Solar decay experiment shows that V_OC of the ITO/PEDOT:PSS/P3HT:ZnO:PDI/Al device decreases rapidly to almost zero in 1 h under persistent white-light illumination. After placing a 420 nm cutoff filter between the cell and the xenon lamp, the stability of the cell can be significantly improved. The device performance can maintain about 80% of the original value within 30 h and I_SC degraded to zero after 142 h. The addition of PDI into the P3HT/ZnO device up to 5 wt% does not show observable effect on the solar cell decay behavior.     

Sputtered NiO as electron blocking layer in P3HT:PCBM solar cells fabricated in ambient air

We fabricated P3HT:PCBM bulk hetero-junction solar cells, where a 5 nm layer of NiO sputtered on top of ITO is used as an effective electron blocking layer. All the steps involving the processing of the organic materials were performed in ambient air. Under 1 sun of AM1.5G illumination such NiO cells exhibited a power conversion efficiency of 3.3% compared to 3.1% exhibited by a PEDOT:PSS cell fabricated under similar conditions. We observed that for the NiO cell processing in air was not detrimental to the ulterior performance of the cell, which in ambient air degraded with a time constant of 303 h. On the contrary, the PEDOT:PSS cell degraded very rapidly and the loss in efficiency was shown to be 29 times faster when compared to that of the NiO cell.     

A study of stabilization of P3HT/PCBM organic solar cells by photochemical active TiOx layer

We describe a study of the stabilization behavior of P3HT/PCBM organic solar cells under air and UV irradiation using a 20 nm thin TiO_x protection layer made by partial hydrolysis of a Ti-alkoxide and spin coating in air. Data on the degradation of solar cell performance under air and under UV exposure are presented indicating that significant improvements are observed with TiO_x layer protection. The protection mechanism has been investigated by transmission IR and UV spectroscopy and by ESR spectroscopy. The results of this study suggest how sol-gel derived TiO_x films containing organic functionalities serve as effective passivation films for protection from oxygen when excited by photons, where the photooxidation of the bound organic moieties causes oxygen gas scavenging.     

Dye-sensitized solar cells based on natural and artificial phycobiliproteins to capture low light underwater

We developed dye-sensitized solar cells (DSSCs) based on natural and artificial phycobiliproteins (PBPs) to capture low light underwater. We assembled DSSCs with seven types of PBPs as sensitizers and studied their photoelectric properties. The results showed that the PBPs could markedly improve the photoelectrical properties of the DSSCs. The sensitization achieved by B-phycoerythrin (B-PE) from Porphyridium cruentum was superior to that of the other PBPs. The short-circuit current density, open circuit voltage, fill factor, and photoelectric conversion efficiency of the DSSC containing B-PE with a mesoporous titanium dioxide film as a photoelectrode were 3.236 mA/cm², 0.545 V, 0.569, and 1%, respectively. The DSSC based on B-PE displayed its highest photoelectric conversion efficiency between 525 and 570 nm. However, the maximum photoelectric conversion efficiencies of the DSSCs based on C-phycocyanin from Spirulina platensis and artificial PBPs were observed around 690 nm. DSSCs containing B-PE show great potential for use in underwater photovoltaic applications.     

Longterm stability of efficient inverted P3HT:PCBM solar cells

We report on the longterm stability of 1.5-3% efficient inverted P3HT:PCBM solar cells sealed with glass plates, which have been glued onto the solar cells. We employed two different electron contact materials, titanium and chromium, and two different P3HT:PCBM layer thicknesses. By means of Auger electron spectroscopy we could show that the electron contact is partially oxidised during the processing of the inverted solar cells. It turns out that both, the electrode material and the thickness of the photoactive layer, have an influence on the longterm stability of such devices. After 1500 h of continuous illumination under a sulphur plasma lamp with a light intensity of ca. at the most stable devices maintained 90% of the initial efficiency and the most efficient cells still had more than 2.5% power conversion efficiency. These best results are obtained with chromium as electron contact material. The light dose corresponds to approximately 1.5 years exposure to sunlight and is therefore a promising result. To our knowledge, this is the first systematic study on the longterm stability of organic solar cells using the inverted layer sequence. Compared to state of the art devices with usual layer sequence our results suggest that inverted organic solar cells can be at least as stable.     

Pathways for the degradation of organic photovoltaic P3HT:PCBM based devices

We report on studies of device degradation in organic photovoltaic devices based on blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Since delamination, oxidation, and chemical interactions at the metal electrode/organic interface have long been posited as degradation pathways in organic electronic devices, we first investigated the stability of a variety of electrodes for devices stored in an inert, dark environment. Second, a set of experiments was designed to separate the effects at the metal/organic interface from the degradation of the active layer or the hole extraction interface. To do this, Ca/Al electrodes were deposited to complete half of a substrate's devices, and samples were left both under constant illumination and 10% illumination (10% duty cycle of 1 sun illumination) in a glovebox environment. After more than 200 h of measurement, additional electrodes were deposited and device performance of each set was compared. Third, to assess the degree of degradation caused by photo-induced processes, device stability in an inert atmosphere under constant illumination, and 10% illumination conditions was also investigated. Last, various degradation mitigation strategies in air under constant illumination were explored. The results showed that the active layer itself is not inherently unstable on the timescales studied here. Choosing the appropriate electrode (Ca/Al) reduced interfacial degradation, storing the active layer in an inert, dark environment did not cause significant degradation, and storing the active layer under constant illumination caused only a limited reduction in performance. Our results indicate that the metal/organic interface can be a significant source of degradation in the devices, and we discuss approaches that could reduce this instability.     

Fe3O4 nanoparticles induced magnetic field effect on efficiency enhancement of P3HT:PCBM bulk heterojunction polymer solar cells

Fe₃O₄ magnetic nanoparticles (mean size of about 10 nm) capped by surfactant oleic acid (OA) were incorporated into P3HT:PCBM BHJ-PSCs by doping in the P3HT:PCBM photoactive layer for the first time. The PCE of the OA-Fe₃O₄:P3HT:PCBM BHJ-PSC device is enhanced by ∼18% at the optimum OA-Fe₃O₄ NPs doping ratio of 1%. The role of the magnetic property of Fe₃O₄ NPs on the PCE of OA-Fe₃O₄:P3HT:PCBM devices was studied, confirming the exclusive contribution of the Fe₃O₄ NPs to the observed enhancement of PCE. The enhancement of the PCE of the OA-Fe₃O₄:P3HT:PCBM BHJ-PSC device is found to be primarily due to the increase of short-circuit current (J_sc) by ∼14%, which is attributed to the magnetic field effect originated from the superparamagnetism of Fe₃O₄ NPs, resulting in the increase of the population of triplet excitons. Finally, the effect of Fe₃O₄ NPs on the enhancement of PCE of OA-Fe₃O₄:P3HT:PCBM device is further investigated by comparing different means of doping in P3HT:PCBM or PEDOT:PSS layer, confirming that such an effect can be achieved only when Fe₃O₄ NPs are doped in the P3HT:PCBM photoactive layer.     

P3HT/TiO2 bulk-heterojunction solar cell sensitized by a perylene derivative

In this work, a new type of dye-sensitized bulk-heterojunction hybrid solar cells has been developed. The heterojunction films were prepared to contain poly(3-hexylthiophene) (P3HT), N,N′-diphenyl glyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide (PDI) and TiO₂. In the architecture, TiO₂ and P3HT were designed to act as the electron acceptor and donor. PDI was used as sensitizer to enhance the photon absorption. Results showed that by incorporation of PDI in the P3HT/TiO₂ composite, the light absorption, exciton separation and photocurrent under white light were dramatically enhanced. Solar decay analyses showed that devices contained TiO₂ required 12 h to obtain maximum current density and the addition of PDI did not affect the solar decay behavior and stability of device composed of P3HT/TiO₂. The devices of P3HT, P3HT/TiO₂, P3HT/TiO₂/PDI could work for 5, 42, 45 h under continuous white light illumination (100 mW/m²) under the ambient condition.     

Exploiting optical anisotropy to increase the external quantum efficiency of flexible P3HT:PCBM blend solar cells at large incident angles

The external quantum efficiencies of P3HT:PCBM blend solar cells decrease significantly when they are bent or illuminated at large incident angles because of (i) optical anisotropy of the P3HT:PCBM films—primarily because a mismatch between the direction of the electric field of the incoming light and the orientation of the P3HT:PCBM blend nanocrystallites results in a significant reduction in the amount of TM-polarized light absorbed and (ii) interfacial reflection of multilayer structures - primarily because the outermost air-flexible substrate interface exhibits a distinct refractive index difference - at large incident angles. Textured moth-eye structures fabricated by nanoimprint lithography on the flexible substrates of organic solar cells reduce the degree of interfacial reflection at high incident angles; they should allow more TE-polarized light to absorb in the P3HT:PCBM films (active layers) of the organic solar cells.     

Study of field effect mobility in PCBM films and P3HT:PCBM blends

We report on field effect mobility measurements in methanofullerene [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) films and in blends of poly(3-hexylthiophene) (P3HT) and PCBM, identical to those used in polymer solar cells. Electron mobilities in the order of were found in the pure PCBM films, and electron mobilities of were found in P3HT:PCBM blends at room temperature. Mobility measurements on the blends yielded electron mobilities of a factor 10 lower than those in the pure films. Temperature dependent measurements were performed to investigate the temperature dependence of the mobility in both films. In order to understand the discrepancy in the electron mobility between pure film and blend, we investigated the parasitic effects of the contacts on the measured value of mobility, but found that losses in the bulk dominate the current.