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%.     

Gravure printed flexible organic photovoltaic modules

In this letter, organic solar cell modules based on poly-3-hexylthiophene (P3HT) and [6.6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films with a module active area of 15.45 cm² prepared by roll-to-roll (R2R) compatible gravure printing method are demonstrated. The gravure printed organic photovoltaic modules consist of eight serially connected solar cells in same substrate. Indium-tin-oxide (ITO) is patterned by screen printable etching paste. Hole injection layer and active layer are prepared by gravure printing method. All processing steps excluding cathode evaporation are performed in air. Electrical measurements are done to modules consisting of 5-8 serially connected solar cells. The photovoltaic modules comprising 5, 7 and 8 serially connected cells exhibit an active area power conversion efficiency of 1.92%, 1.79% and 1.68%, respectively (Oriel Sol3A Class AAA, AM1.5G, 100 mW cm⁻²).     

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.     

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.     

Organic photovoltaic devices with a crosslinkable polymer interlayer

Organic photovoltaic devices with a photo-crosslinkable interlayer were fabricated. This photo-crosslinkable interlayer acted as a leakage current reducing buffer layer. The performance of the small area OPV cell (0.04 cm²) was enhanced by the increase in the short circuit current and the fill factor. When a larger area cell (1 cm²) was used, the performance of OPV cell increased when the appropriate interlayer thickness was used. In the case of a 10 cm×10 cm module, the power conversion efficiency was about double than that without the interlayer. The insertion of the interlayer increased the current extraction by lowering the barrier height and attenuated the fill factor reduction by enhancing the rectification with a better leakage current sealing. From this study, it is clearly proved that the insertion of the appropriate photo-crosslinkable layer improves the performance of OPV devices, the effect was especially evident for large area cells.     

High-performance polymer photovoltaic devices with inverted structure prepared by thermal lamination

Use of a lamination process for the introduction of Au electrode, instead of conventional metal evaporation, improved the power conversion efficiency (PCE) of inverted-structure photovoltaic devices from 1.6% to 2.6% based on a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C₆₁ butyric acid methyl ester (PCBM). X-ray photoelectron spectroscopy indicated that a thin layer of P3HT is spontaneously formed at the air/polymer blend layer interface during the spin-coating process. It is suggested that the vacuum-evaporated gold could destroy the surface-segregated thin layer of P3HT, while the lamination process preserves the surface structure working as an electron-blocking layer. The insertion of a PEDOT:PSS layer, between the metal electrode and polymer layer, in the lamination process further improved the PCE to 3.3% with a short-circuit current density of 9.94 mA cm⁻², an open-circuit voltage of 0.60 V, and a fill factor of 55% under AM1.5 100 mW cm⁻² irradiation.     

ITO-free flexible inverted organic solar cell modules with high fill factor prepared by slot die coating

We report on the fabrication of inverted ITO-free P3HT:PCBM solar cell modules on glass and PET foil as substrate where the organic functional layers are deposited with slot die coating, a reel to reel compatible coating technique. The active layers have been processed in ambient atmosphere, which will be of advantage in a future production and is especially remarkable as the metallic cathode is already deposited on the substrate at this stage of fabrication. The modules comprise two busses of 11 cell elements connected in series each. The series connection leads to an open circuit voltage of up to 6.88 V on glass substrate, which translates to 625 mV per cell element, a very competitive value for P3HT:PCBM based solar cells on glass. Although the designated area is as large as 41 cm² and the active area 26.4 cm², we obtain fill factors of up to 65% for these modules, which again is a typical value for small area laboratory cells. Remarkably the values for PET foil as substrate with an open circuit voltage of 6.5 V and a fill factor of 64% are very close to the results on glass and to our knowledge the highest fill factors for flexible organic solar cells, even if compared to small area devices. The short circuit current densities and therefore efficiencies are also comparable to small area devices, if only the photoactive area is accounted for. Therefore we have demonstrated that the scale up of organic solar cells can be achieved with a suited circuitry scheme.     

Optimization of inverted tandem organic solar cells

Inverted tandem organic solar cells, consisting of two bulk heterojunction sub-cells with identical poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ (PCBM) active layer and a MoO₃/Ag/Al/Ca intermediate layer, have been presented and optimized. Indium tin oxide (ITO) modified by Ca acts as a cathode for electron collection and Ag is used as the anode for hole collection for the tandem device. A proper thickness of Ca (3 nm) forms a continuous layer, working as a cathode for the top sub-cell. MoO₃ as the anode buffer layer prevents exciton quenching and charge loss at the anode side, which could result in increase in interfacial resistance. The variance of sub-cell thickness adjusts the optical field distribution in the entire device, facilitating light absorption and good current matching in both sub-cells. The optimal inverted tandem device achieves a maximum power conversion efficiency of 2.89% with a short-circuit current density of 4.19 mA/cm², an open-circuit voltage of 1.17 V, and a fill factor of 59.0% under simulated 100 mW/cm² (AM 1.5G) solar irradiation.     

Polymer photovoltaic devices using highly conductive poly(3,4-ethylenedioxythiophene-methanol) electrode

In this work, modified poly(3,4-ethylenedioxythiophene) (PEDOT) was used as an anode in polymer photovoltaic devices (PVDs) based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PCBM). We synthesized poly(3,4-ethylenedioxythiophene methanol) (PEDTM) with a transmittance of 87% (at 510 nm) and a conductivity of 700 S/cm. PEDTM was applied in photovoltaic devices as a hole transporting layer on indium-tin oxide (ITO) electrode as well as a direct anode layer. PVDs with PEDTM as hole transporting layers on ITO showed a very high short-circuit density of 14.87 mA/cm² and power conversion efficiency of 2.67% under an illumination of AM 1.5 G (100 mW/cm²). In addition, we also fabricated ITO-free PVDs using PEDTM as an anode, which exhibited a performance of 0.61% with a result of J_sc of 4.48 mA/cm², V_oc of 0.51 V, and FF of 27%.     

Flexible large area polymer solar cells based on poly(3-hexylthiophene)/fullerene

Photovoltaic devices based on regioregular poly(3-hexylthiophene) (P3HT) and ([6,6]-phenyl-C61-butyric acid methyl ester) (PCBM) were fabricated and characterized using 5×5 cm ITO polyester foils with an active cell area of 0.5×0.5 cm². The HOMO/LUMO of P3HT and PCBM were estimated from cyclic voltammetry data. The complete quenching of photoluminescence of P3HT after mixing with PCBM indicates an effective charge transfer from P3HT to PCBM. The absorption spectrum of a blend (1:3 wt%) of both components shows that there is no ground state doping. Following device parameters without any special postproduction treatment were determined: V_OC=600 mV, I_SC=6.61 mA/cm², FF=0.39 and η_AM1.5 (P_IN:100 mW/cm²)=1.54%.     

Efficient hybrid carboxylated polythiophene/nanocrystalline TiO2 heterojunction solar cells

Efficient hybrid solar cells fabricated from TiO₂, novel carboxylated polythiophene poly (3-thiophenemalonic acid) P3TMA as sensitizer as well as hole conductor and poly (3-hexylthiophene) (P3HT) as hole transporter was described. UV-Vis absorption and morphology of the active layer were investigated. Device J/V characterizations with different P3HT layer thickness were measured and discussed. Efficiency improvements were observed in thinner P3HT layer thickness and with poly[3,4-(ethylenedioxy)-thiophene]:poly(styrene sulfonate) (PEDOT:PSS) as charge collection layer, and such device showed a short-circuit current density of 1.32 mA/cm², an open-circuit voltage of 0.44 V, a fill factor of 0.43, and a energy conversion efficiency of 0.25% at A.M. 1.5 solar illumination (100 mW/cm²).     

Effect of organic salt doping on the performance of single layer bulk heterojunction organic solar cell

The effect of organic salt, tetrabutylammonium hexafluorophosphate (TBAPF₆) doping on the performance of single layer bulk heterojunction organic solar cell with ITO/MEHPPV:PCBM/Al structure was investigated where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. In contrast to the undoped device, the electric field-treated device doped with TBAPF₆ exhibited better solar cell performance under illumination with a halogen projector lamp at 100 mW/cm². The short circuit current density and the open circuit voltage of the doped device increased from 0.54 μA/cm² to 6.41 μA/cm² and from 0.24 V to 0.50 V, respectively as compared to those of the undoped device. The significant improvement was attributed to the increase of built-in electric field caused by accumulation of ionic species at the active layer/electrode interfaces.     

Patternable polymer bulk heterojunction photovoltaic cells on plastic by rotogravure printing

We study the fabrication of poly(3-hexylthiophene)—P3HT and [6,6]-phenyl-C61 butyric acid methyl ester—PCBM based polymer bulk heterojunction photovoltaic cells using rotogravure printing. By studying the dependencies of device performance on material and process parameters including contact angles, ink concentrations, ink viscosities, solvent characteristics, and gravure printing parameters, optimized hole transport layers [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)—PEDOT:PSS] and active layers (P3HT:PCBM) are printed, resulting in devices with power conversion efficiencies as high as 1.68% under AM 1.5 G and a spectrally matched intensity of 100 mW/cm².     

Spray deposition of electrohydrodynamically atomized polymer mixture for active layer fabrication in organic photovoltaics

Printing and spray technologies are the most recent and novel approaches to form organic photovoltaics (OPV) with inexpensive, high speed, and environmentally friendly process. With an electrohydrodynamic atomization (EHDA) approach, the active layer composed of polymer mixture (P3HT:PCBM) was successively fabricated. Operating conditions for obtaining the stable cone jet mode were determined with various applied voltages and liquid feed flow rates. The size distribution of EHDA droplets was characterized by aerodynamic particle sizer (APS) measurement. The mode diameters of the droplets were 580 and 670 nm, respectively, when the liquid flow rates were 1 and 20 μl/min. The maximum power conversion efficiency of 0.48% was obtained under AM 1.5 solar simulation for an OPV device fabricated in air.     

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.     

Enhancing the short-circuit current and efficiency of organic solar cells using MoO3 and CuPc as buffer layers

Efficient bulk-heterojunction (BHJ) (regioregular poly (3-hexylthiophene) (P3HT): (6, 6)-phenyl C₆₁ butyric acid methyl ester (PCBM)) solar cells were fabricated with molybdenum trioxide (MoO₃) and copper phthalocyanine (CuPc) as buffer layers. The insertion of MoO₃ layer was found to be critical to the device performance, effectively extracting holes to prevent the exciton quenching and reducing the interfacial resistance because of alignment of energy levels. The introduction of CuPc buffer layer was observed to be ameliorative for device performance, further enlarging the visible absorption spectra range of the devices. The effect of the MoO₃ and CuPc layer thickness on device performance was studied. The optimized thickness was achieved when MoO₃ layer was 12 nm and CuPc layer was 6 nm, resulting in optimized power conversion efficiency (PCE) of 3.76% under AM1.5G 100 mW/cm² illumination.     

Well-aligned single-crystalline silicon nanowire hybrid solar cells on glass

Hybrid solar cells are fabricated on the glass substrate using well-aligned single-crystalline Si nanowires (SiNWs) and poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). Their key benefits are discussed. The well-aligned SiNWs are fabricated from Si wafer and transferred onto the glass substrate with the P3HT:PCBM. Such SiNWs provide uninterrupted conduction paths for electron transport, enhance the optical absorption to serve as an interesting candidate of the absorber, and increase the surface area for exciton dissociation. Our investigations show that SiNWs are promising for hybrid organic photovoltaic cells with improved performance by increasing the short-circuit current density from 7.17 to 11.61 mA/cm².     

Highly durable inverted-type organic solar cell using amorphous titanium oxide as electron collection electrode inserted between ITO and organic layer

An indium tin oxide/titanium oxide/[6,6]-phenyl C₆₁ butyric acid methyl ester:regioregular poly(3-hexylthiophene)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid)/Au type organic solar cell (ITO/TiO_x/PCBM:P3HT/PEDOT:PSS/Au) with 1 cm² active area, which is called “inverted-type solar cell”, was developed using an ITO/amorphous titanium oxide (TiO_x) electrode prepared by a sol-gel technique instead of a low functional electrode such as Al. The power conversion efficiency (η) of 2.47% was obtained by irradiating AM 1.5G-100 mW cm⁻² simulated sunlight. We found that a photoconduction of TiO_x by irradiating UV light containing slightly in the simulated sunlight was required to drive this solar cell. The device durability in an ambient atmosphere was maintained for more than 20 h under continuous light irradiation. Further, when the air-stable device was covered by a glass plate with a water getter sheet which was coated by an epoxy-UV resin as sealing material, the durability was still higher and over 96% of relative efficiency was observed even after continuous light irradiation for 120 h.     

Synthesis and photovoltaic properties of biindene-C70 monoadduct as acceptor in polymer solar cells

A new fullerene derivative, biindene-C₇₀ monoadduct (BC₇₀MA), was synthesized by [4+2] cycloaddition reaction between 1,1′-biindene and C₇₀, for the application as acceptor in polymer solar cells (PSCs). BC₇₀MA is soluble in common organic solvents such as tetrahydrofuran, chloroform, toluene, o-dichlorobenzene, etc., and shows stronger absorption in the visible region and a slightly up-shifted lowest unoccupied molecular orbital (LUMO) energy level than that of PCBM. PSCs were fabricated with BC₇₀MA as acceptor and poly(3-hexylthiophene) (P3HT) as donor for investigating the photovoltaic properties of BC₇₀MA. The power conversion efficiency of the PSC based on P3HT/BC₇₀MA (1:1, w/w) with the additive of 3% octane-1,8-dithiol and thermal annealing at 110 °C for 10 min reached 3.44% with open circuit voltage of 0.64 V, short circuit current of 8.02 mA/cm² and fill-factor of 0.67, under the illumination of AM1.5, 100 mW/cm².     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².