Large-area organic photovoltaic module—Fabrication and performance

Here we describe the fabrication of the largest (233 cm² total area) organic photovoltaic (OPV) module (polymer:fullerene) to be certified by the National Renewable Energy Laboratory (NREL). OPV solar cells were fabricated at Plextronics by spin coating a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C₆₁ butyric acid methyl ester (PCBM) on top of our hole transport layer (HTL), Plexcore^® OC. In laboratory-scale devices (0.09 cm²), this system routinely exhibits power conversion efficiencies exceeding 3.7%. This P3HT:PCBM active layer and HTL ink system was used to scale up to the larger area module (15.2 cm×15.2 cm module size, i.e. 233 cm² total area; 108 cm² active area), which was certified by NREL as having 1.1% total area efficiency (3.4% active area efficiency).     

Triphenylamine-substituted methanofullerene derivatives for enhanced open-circuit voltages and efficiencies in polymer solar cells

A new class of triphenylamine substituted methanofullerene derivatives, bis(4'-(diphenylamino)biphenyl-4-yl)methanofullerene (1) and the bisadduct (2), were synthesized. The incident photon to current efficiency (IPCE) studies revealed that the diphenylamino components have contribution to the photocurrent that expands the light harvesting window around 400 nm. When being blended with poly (3-hexylthiophene) (P3HT) to fabricate the solar cell, the device of P3HT:1 (1:0.7) shows high open circuit voltage (V_oc) of 0.69 V under the illumination of AM 1.5, 100 mW/cm² with high power conversion efficiency (PCE) of 3.16%, which is about 0.1 V higher than that of the corresponding [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) devices. This indicates that the arylamine substituents on 1 have played some special roles on the high V_oc performance. Similar effects are also observed for 2. The device of P3HT:2 (1:1) shows even higher V_oc of 0.87 V with the PCE of 1.83%. These results indicate that 1 and 2 are alternative high performance acceptors.     

Effect of proton irradiation on electrical properties of CuInSe2 thin films

High-energy proton irradiation (380 keV and 1 MeV) on the electrical properties of CuInSe₂ (CIS) thin films has been investigated. The samples were epitaxially grown on GaAs (0 0 1) substrates by Radio Frequency sputtering. As the proton fluence exceeded 1×10¹³ cm⁻², the carrier concentration and mobility of the CIS thin films were decreased. The carrier removal rate with proton fluence was estimated to be about 1000 cm⁻¹. The electrical properties of CIS thin films before and after irradiation were studied between 80 and 300 K. From the temperature dependence of the carrier concentration in CIS thin films, we found N_D=9.5×10¹⁶ cm⁻³, N_A=3.7×10¹⁶ cm⁻³ and E_D=21 meV from the fitting to the experimental data on the basis of the charge balance equation. After irradiation, a defect level was created, and N_T=1×10¹⁷ cm⁻³ for a fluence of 3×10¹³ cm⁻², N_T=5.7×10¹⁷ cm⁻³ for a fluence of 1×10¹⁴ cm⁻² and E_T=95 meV were also obtained from the same fitting. The new defect, which acted as an electron trap, was due to proton irradiation, and the defect density was increased with proton fluence.     

Enhanced efficiency of organic photovoltaic cells using solution-processed metal oxide as an anode buffer layer

We report performance improvement of organic (P3HT:PCBM) photovoltaic cell by introducing a solution-based WO_x anode buffer layer, between the PH500 conducting polymer and the active layer. By introducing a solution-based WO_x on PH500, the cell efficiency increases by 13% compared to that of the organic photovoltaic cell with only PH500. The organic cell exhibits J_SC of 12.31 mA/cm², V_OC of 0.61 V, FF of 58.22%, and the power conversion efficiency of 4.35%. The improved cell performance is due to effective hole collection injected from electron donors (P3HT) in P3HT:PCBM active layer by forming WO_x nanoparticle on the conducting anode.     

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%.     

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.     

Comparison of normal and inverse poly(3-hexylthiophene)/fullerene solar cell architectures

Polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and ([6,6]-phenyl-C61-butyric acid methyl ester) (PCBM) were fabricated with two different architectures (normal and inverse). Normal cells using indium tin oxide (ITO) as anode and Al as cathode were fabricated on polyester foils and illuminated from substrate side. Inverse cells using Ti as cathode and ultrathin Au layer as anode were illuminated from the top side covered by a transparent Au contact. Both Au layer and PET/ITO show comparable transmission in the spectral range where P3HT absorbs. Inverse cells showed comparable device parameters to normal cell (open circuit voltage 550 mV, short circuit current 6.25 mA/cm², fill factor 0.33 and white light power conversion efficiency 1.12%).     

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.     

Electrodeposition zinc-oxide inverse opal and its application in hybrid photovoltaics

This article reports the preparation of three-dimensional (3D) mesoporous zinc oxide (ZnO) films and their application in solar cells. The films were obtained through electrochemical deposition in DMSO solutions by using PS colloidal crystal as templates. The ZnO films with inverse opal (IO) structure were obtained after removing the templates by thermolysis. The ordered porous ZnO films were used to prepare hybrid solar cells by infiltrating the films with poly(3-hexylthiophene) (P3HT) or P3HT:ZnO nanocomposite. Results showed that the interpenetrating network of both ZnO(IO) and P3HT can form continuous pathways for electron and hole transport. By infiltrating a P3HT:ZnO nanocomposite into the porous ZnO films, the photocurrent of the solar cell can be dramatically improved. The cell shows the V_oc and I_sc of 462 mV and 444.3 μA/cm², respectively. By using a 420 nm cutoff filter, the cell retains about 80% and 50% of its original V_oc and I_sc after continuous white-light illumination (100 mW/cm²) for 10 h. Stability of the device under above conditions was estimated to be 51 h.     

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.     

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T_fil≈1600°C, pressure, p=8 mTorr and silane flow, F(SiH₄)=100 sccm, keeping the substrate temperature at T_S=200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d_fil=4 cm were used. The optimum filament-to-substrate distance was found to be d_fil–S=8.4 cm. While studying the influence of different d_fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r_d≈4 Å/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ_Ph/σ_D=(2.46±0.7)×10⁵, microstructure factor, R=0.17±0.05, defect densities, N_d(PDS)=(2.06±0.6)×10¹⁷ cm⁻³ and N_d(CPM)=(2.05±0.5)×10¹⁶ cm⁻³ (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η=(6.1±0.2)% on the 20 cm×20 cm area.     

Improving the current density Jsc of organic solar cells P3HT:PCBM by structuring the photoactive layer with functionalized SWCNTs

Several works concerning the incorporation of carbon nanotubes (CNTs) in bulk polymer RR-P3HT (regio-regular poly(3-hexylthiophene-2,5-diyl)):PCBM (methanofullerene phenyl–C₆₁–butyric-acid–methyl-ester) heterojunction have been already reported by a number of research groups. The optical and electrical properties of organic cells have been extensively studied. We investigated the incorporation of functionalized single wall carbon nanotubes (SWCNTs) into the matrix of P3HT:PCBM photovoltaic (PV) cells. The photovoltaic characteristics of the cells depend on the concentration of SWCNT. The incorporation of low concentrations of SWCNT in the photoactive layer increases the current density J_sc before annealing and it can reach above 9 mA/cm². We attribute the improved PV performances to partial crystallization of the RR-P3HT. As revealed by XRD studies and confirmed by the absorbance spectra, which exhibit the typical shoulder at 600 nm and absorbance in the near infrared region. Interestingly, we observe also that doping the P3HT:PCBM active layer by the functionalized SWCNTs increases the open circuit voltage V_oc.     

High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a novel In(OH)3:Zn buffer layer

We propose the inclusion of a novel In(OH)₃:Zn²⁺ buffer layer for fabricating high-efficiency CIGS solar cells. This buffer layer was deposited using a solution consisting of ZnCl₂, InCl₃·4H₂O, and thiourea. The In(OH)₃:Zn²⁺ films showed high resistivities of 2.1×10⁸ Ω cm and transmittance of above 95% in the visible range. We expected two effects due to this new buffer layer: first is the formation of a passivation layer on the CIGS surface and the second is Zn-doping into CIGS layer, resulting in the formation of a buried junction. A cell efficiency of 14.0% (V_oc: 0.575 V, J_sc: 32.1 mA/cm², FF: 0.758) was achieved by using an In(OH)₃:Zn²⁺ buffer layer, without the light soaking effect.     

Novel hybrid polymer photovoltaics made by generating silver nanoparticles in polymer:fullerene bulk-heterojunction structures

Here, we developed the high efficient poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁-butyric acid butyl ester (PCBB):Ag nanoparticle hybrid photovoltaic cells by generating the silver nanoparticles in the bulk-heterojunction structures of P3HT:PCBB prior to the spin-coating process. Hybrid bulk-heterojunction solar cells with an ITO/(G)-PEDOT/P3HT:PCBB:Ag/Al was fabricated, which shows open-circuit voltage, short-circuit current density, and power conversion efficiency of 0.639 V, 12.29 mA/cm², and 4.30% under AM1.5 irradiation (100 mW/cm²), respectively.     

Reduction of defects of polycrystalline silicon thin films by heat treatment with high-pressure H2O vapor

An improvement of electrical properties of pulsed laser crystalllized silicon films was achieved by simple heat treatment with high-pressure H₂O vapor. The electrical conductivity of 7.4×10¹⁷ cm⁻³ phosphorus-doped 50-nm-thick pulsed laser crystallized silicon films was markedly increased from 1.6×10⁻⁵ S/cm (as crystallized) to 2 S/cm by heat treatment at 270°C for 3 h with 1.25×10⁶ Pa H₂O vapor because of reduction of density of defect states localized at grain boundaries. Spin density was reduced from 1.7×10¹⁸ cm⁻³ (as crystallized) to 1.2×10¹⁷ cm⁻³ by heat treatment at 310°C for 3 h with 1.25×10⁶ Pa H₂O vapor.     

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.     

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%.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Photoelectrochemical response and differential capacitance of poly(3-methylthiophene)

Following the theory used to study the semiconductor/electrolyte interface the differential capacitance of poly(3-methylthiophene) films has been determined from measurements with a lock-in amplifier and by electrochemical impedance spectroscopy (EIS). According to our findings, the best results were obtained by EIS because the space charge capacitance can be separated from the other capacitances. Using Mott–Schottky plots (C⁻² vs. E) we obtained the flat band potential E_fb=80 mV and the carrier density N=6×10¹⁷ cm⁻³ for the PMeT film in contact with the electrolyte, where dissolved O₂ played the role of the electron acceptor. The determined width of the depletion layer is 0.04 μm. We also investigated the photoelectrochemical response of the PMeT film. The plot of the square of the photocurrent vs. potential yields E_fb=90 mV, in good agreement with the EIS measurement. The dependence of the photocurrent with the frequency of the incident light shows that PMeT has a long response time (order of ms), compared to an inorganic semiconductor. The band gap was also determined from the photocurrent spectra. The value obtained, for a direct transition is 1.9 eV and is coincident with the value obtained from the absorption spectra.     

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.