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.     

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.     

退火对活性层P3HT:PCBM性能的影响

采用匀胶机旋涂成膜的方法在手套箱中制备了P3HT:PCBM薄膜,详细研究了退火处理对薄膜的吸收、晶体结构以及表面形貌的影响,并全面阐述了它们之间的联系。结果发现退火后薄膜的吸收增强,吸收峰的峰位有所红移,通过XRD测试研究表明是因为薄膜出现了不同程度的晶化,主要是P3HT发生了部分晶化。对表面形貌进一步研究发现退火后薄膜的表面粗糙度有所增加,并形成了一定清晰可见的互穿网络,薄膜发生品化,使薄膜性能得到大大改善。将优化得到的材料用于太阳电池中,电池结构为glass/ITO/PEDOT:PSS/P3HT:PCBM/Al,在AM1.5,100mW/cm~2条件下测试获得电池效率1.41%。     

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%.     

Light-induced degradation of the P3HT-based solar cells active layer

We report on the effects of long-term artificial accelerated ageing on the active layer of organic solar cells in the absence of oxygen. The samples were composed of a bulk heterojunction formed by poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) deposited on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). First, a set of experiments was performed to study the modifications resulting from prolonged exposure to UV-vis light. A gradual decrease in absorbance was recorded, and TEM results clearly indicated that the initial morphology was unstable upon long irradiation times. Second, we revealed that the annealing temperature of PEDOT:PSS strongly influenced the degradation of the active layer. Indeed, an increase in the PEDOT:PSS annealing temperature resulted in an important improvement in stability. Third, a comparison was made between different active layers obtained by changing the P3HT type, polymer:fullerene weight ratio and solvent nature. As expected, the polymer:fullerene weight ratio was shown to significantly impact the degradation kinetics. The ageing effects on the photovoltaic properties were then explored, and extrapolation of the data to outdoor exposure is also discussed.     

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.     

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.     

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.     

Long-term operational lifetime and degradation analysis of P3HT:PCBM photovoltaic cells

We study the degradation of photovoltaic cells with poly(3-hexyl thiophene) (P3HT) and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) blends under long-term continuous illumination as well as in shelf-life conditions, both in inert N₂ atmosphere. Degradation of the illuminated solar cells mainly occurs by a rapid decrease of the fill factor (FF) after 300 h, while short-circuit current and open-circuit voltage follow a linear decay after the initial burn-in. The sudden drop of the FF is correlated with an increase of the series resistance and proves irreversible upon annealing. Electrical measurements indicate that it stems from reduced charge extraction due to the photodegradation of the organic-electrode interfaces. Furthermore, as the external quantum efficiency (EQE) spectrum is evenly lowered over the entire wavelength range, we could exclude major changes in the blend morphology or significant changes to optical properties of the active layer. Introducing a thin C₆₀ layer leads to complete suppression of the FF decay over 1000 h, further proving that interface degradation dominates. Interestingly, similar improved lifetime over 1000 h was achieved by separate substitution of MoO₃ for PEDOT:PSS.     

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.     

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.     

Temperature dependence of polymer/fullerene organic solar cells

The temperature dependence of bulk heterojunction organic solar cells fabricated from poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) was studied in detail. Individual materials as well as blends and solar cell devices were examined. Light absorption, photoluminescence, quantum efficiency, total efficiency, and current-voltage characteristics were studied from temperatures −10 to 140 °C. A method and apparatus for testing these parameters at various temperatures is described. Parameters were measured for both unannealed and annealed samples to give insight into the annealing process. It was found that absorption and photoluminescence of devices shift both position and intensity with varying temperatures. Quantum efficiency and total efficiency were monitored as they increased with annealing. Once annealed, device efficiency peaked at temperatures from 10 to 60 °C because of competing temperature dependent effects of the materials. The temperature dependence study provides valuable information on device properties and thermal annealing.     

Effect of solvent on fabrication of active layers in organic solar cells based on poly(3-hexylthiophene) and fullerene derivatives

Investigating the effect of solvent on the fabrication of photo-active layers in bulk heterojunction organic solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl C₆₁-butyric acid methyl ester, we found the short-circuit current density (J_SC) and fill factor (FF) of a cell with a photo-active layer made using materials dissolved in a mixture of o-dichlorobenzene with chloroform to be slightly higher than those of a cell made using the same materials dissolved only in chlorobenzene. Evaluating the absorption spectra, surface morphology, charge mobility, and solubility of photo-active films made using different solvents, we concluded that the higher J_SC and FF were due to a larger interfacial area between the photo-active materials in the layer.     

Improvement in light harvesting and performance of P3HT:PCBM solar cell by using 9,10-diphenylanthracene

We have studied the effect of 9,10-diphenylanthracene (DPA) as a conjugated dye with different concentrations on light harvesting and performance of solar cell composed from poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁butyric acid methyl ester (PCBM) blend films. The dye concentration effect was investigated with optical absorption spectroscopy, photocurrent spectroscopy, and current density-voltage characteristic measurements on devices under AM1.5 white light illumination with intensity of 100 mW/cm². The incorporation of the conjugated DPA inside P3HT:PCBM blend improved the light harvesting, slightly, and conjugation length indicated from the optical absorption and external quantum efficiency spectra. By adding specific amounts of the DPA into P3HT:PCBM blend, the external quantum efficiency and solar cell performance parameters, i.e., short circuit current density, fill factor, and power conversion efficiency improved as a result of improvement in the light harvesting and charge carrier transfer taking place between P3HT and PCBM through the conjugated DPA molecules.     

On the use of Ga–In eutectic and halogen light source for testing P3HT–PCBM organic solar cells

In this paper, we report a simple and inexpensive method for testing the photovoltaic characteristics of organic devices. We report measurement data on P3HT–PCBM organic solar cells spin-coated from chloroform solutions, obtained using a Gallium–Indium eutectic top contact, and a halogen light source. We propose a method to correct the spectral mismatch of this light source with the solar spectrum. The effect of annealing and changing the P3HT:PCBM ratio are studied, and similar results as with evaporated aluminum are obtained. We conclude that Ga–In eutectic and halogen light source can conveniently be used instead of evaporated aluminum and a solar simulator for the study of polymer–fullerene bulk-heterojunction devices.     

Thermally stable organic bulk heterojunction photovoltaic cells incorporating an amorphous fullerene derivative as an electron acceptor

A highly soluble amorphous fullerene derivative substituted with dihexylfluorene (DHFCBM) was synthesized and used as an electron acceptor material for P3HT-based bulk heterojunction solar cells. By fitting the experimental J-V curves with space charge limited current equation, the electron mobility of DHFCBM was determined to be 4×10⁻⁴ cm²/Vs, possibly leading to balanced charge transport with P3HT. From structural and morphological analysis using X-ray diffraction, UV-vis absorption, and atomic force microscopy, we found that the amorphous nature of DHFCBM stabilized the nanomorphology of P3HT:DHFCBM blend films under high temperature annealing. By optimizing blend ratios and annealing conditions, P3HT:DHFCBM-based solar cells yielded power conversion efficiencies in excess of 3%. In addition, the fabricated cells maintained their initial performances even after high temperature annealing for long times, as predicted from the stable nanomorphology. We believe that the use of thermally stable amorphous fullerene as an electron acceptor can be a promising strategy for commercialization of organic solar cells.     

Bulk heterojunction organic solar cells utilizing 1,4,8,11,15,18,22,25-octahexylphthalocyanine

Bulk heterojunction solar cells utilizing soluble phthalocyanine derivative, 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH₂) have been investigated. The active layer was fabricated by spin-coating the mixed solution of C6PcH₂ and 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 (PCBM). The photovoltaic properties of the solar cell with bulk heterojunction of C6PcH₂ and PCBM demonstrated the strong dependence of active layer thickness, and the optimized active layer thickness was clarified to be 120 nm. By inserting MoO₃ hole transport buffer layer between the positive electrode and active layer, the FF and energy conversion efficiency were improved to be 0.50 and 3.2%, respectively. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH₂ and poly(3-hexylthiophene) and the interlayer of LiF/Al/MoO₃ structure, and a high V_oc of 1.27 V has been achieved.     

Multiparametric optimization of polymer solar cells: A route to reproducible high efficiency

We carried out a detailed optimization of P3HT:PCBM polymer solar cells by variation of blending ratio, film thickness and annealing conditions. From our studies it became evident that the film thickness and the fullerene concentration are mutually dependent parameters, what the overall performance concerns. In detail, we revealed a clear relationship between film thickness, PCBM concentration and the blend film morphology. We varied the PCBM concentration in our polymer solar cells between 25% and 50%, and found the best results for 33.3% of PCBM. In this case, the optimum between competing processes like effective charge carrier separation and percolation path establishment was realized. In thicker films, the growth of PCBM aggregates via phase separation leads to formation of percolation paths and therefore improves the photocurrent. In contrast, for thinner films a high PCBM concentration is favourable to achieve optimal efficiencies.