ITO-free inverted polymer solar cells using a GZO cathode modified by ZnO

A gallium-doped ZnO (GZO) layer was investigated and compared with a conventional indium-tin-oxide (ITO) layer for use as a cathode in an inverted polymer solar cell based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ). By modifying the GZO cathode with a ZnO thin layer, a high power conversion efficiency (3.4%) comparable to that of an inverted solar cell employing the same P3HT:PCBM BHJ photoactive layer with a conventional ITO/ZnO cathode was achieved. This result indicates that GZO is a transparent electrode material that can potentially be used to replace high-cost ITO.     

Effects of intrinsic ZnO buffer layer based on P3HT/PCBM organic solar cells with Al-doped ZnO electrode

Organic solar cell devices were fabricated using poly(3-hexylthiophene) (P3HT) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), which play the role of an electron donor and acceptor, respectively. The transparent electrode of organic solar cells, indium tin oxide (ITO), was replaced by Al-doped ZnO (AZO). ZnO has been studied extensively in recent years on account of its high optical transmittance, electrical conduction and low material cost. This paper reports organic solar cells based on Al-doped ZnO as an alternative to ITO. Organic solar cells with intrinsic ZnO inserted between the P3HT/PCBM layer and AZO were also fabricated. The intrinsic ZnO layer prevented the shunt path in the device. The performance of the cells with a layer of intrinsic ZnO was superior to that without the intrinsic ZnO layer.     

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.     

Influence of ZnO interlayer on the performance of inverted organic photovoltaic device

Inverted organic photovoltaic devices with a structure of fluorine tin oxide (FTO)/ZnO/poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM)/Ag were fabricated, in which ZnO interlayer serves as an electron selective layer. The ZnO interlayer includes three different nanostructures: polycrystalline seed layer, polycrystalline seed layer/loose nanopillars and polycrystalline seed layer/dense nanopillars. The influences of the different ZnO interlayers on the device performance were investigated. It is concluded that the polycrystalline seed layer/loose nanopillars offer more interfacial area with the P3HT:PCBM blends and acts as a continuous conducting path to the cathode. Our results demonstrate that effective infiltration of the blends into the ZnO nanopillars is critical for optimizing the device performance.     

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%.     

Optimization of process parameters for high-efficiency polymer photovoltaic devices based on P3HT:PCBM system

Here, we report the fabrication of high-efficiency poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend photovoltaic device. Process parameters like solvent, solvent drying conditions, electron donor to acceptor ratio and cathodes structures are optimized in making the devices. For the first time, we used cosolvent systems to make active layer of P3HT:PCBM composite and G-PEDOT:PSS, made by mixing 6 wt% glycerol to PEDOT:PSS, is used as a buffer layer. Highest efficiency of 4.64% was obtained for the device made with 1:0.7 ratio of P3HT to PCBM, o-dichlorobenzene:chloroform cosolvent, newly developed slow process and G-PEDOT:PSS. Film morphology is evaluated by atomic force microscopy (AFM). Time-of-flight (TOF) and incident photon-to-current conversion efficiency (IPCE) measurements are also performed for the best device.     

Enhancing the durability of polymer solar cells using gold nano-dots

The durability of organic photovoltaic devices is improved by (a) replacing thermally labile poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) with gold nano-dots and (b) stabilizing the morphology of photoactive layers through thermally induced reaction. Gold nano-dots (Au-ND) (3–6 nm in diameter and 0.8 nm in height) were thermally deposited on ITO substrates prior to depositing a hole transporting layer (40 nm) of an azide-functionalized poly(3-hexylthiophene), P1, which was insolubilized by heating to 150 °C. A blend of P1 and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) was deposited and heated to 150 °C prior to the deposition of a Ca/Al cathode. The reaction of P1 with PCBM stabilized the bulk heterojunction film as evidenced by the suppression of crystallization of PCBM. Replacement of PEDOT:PSS with Au-ND, in combination with morphological stabilization, greatly improves the durability of PV devices under accelerated lifetime testing at 150 °C. Power conversion efficiencies (PCE) for the P1:PCBM devices stabilized at 1.25% after 28 h of accelerated testing at 150 °C, whereas conventional P3HT:PCBM devices on PEDOT/ITO dropped to 0.58% after only 7 h of accelerated testing. Prospects for similarly enhancing the durability of highly efficient PV devices are discussed.     

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.     

Numerical study of electrical behavior of P3HT/PCBM bulk heterojunction solar cell

In this work the program AMPS-1D was used to optimize the performance of the organic solar cells. The cells considered consist of poly(3-HexylThiophène) [P3HT] as electron donors, and (6,6)-phenyl- C61-butyric acid methyl ester [PCBM] as electron acceptor, (P3HT/PCBM) is used as photo-active material, sandwiched between a transparent indium tin oxide (ITO) and layer of poly(3,4 ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT/PSS) on top of the ITO electrode and an AL backside contact. The results showed that the optimum thickness of the solar cell is about 400 nm, V_oc = 0.61 at T = 300 K. This is in the good agreement with the corresponding computer simulation value of 0.63 V. The maximum limit for the organic solar cell efficiency is about 8%, provided that the band-gap of the cell is about 1.5 eV.     

A life cycle analysis of polymer solar cell modules prepared using roll-to-roll methods under ambient conditions

A life cycle analysis was performed on a full roll-to-roll coating procedure used for the manufacture of flexible polymer solar cell modules. The process known as ProcessOne employs a polyester substrate with a sputtered layer of the transparent conductor indium-tin-oxide (ITO). The ITO film was processed into the required pattern using a full roll-to-roll process, employing screen printing of an etch resist and then applying etching, stripping, washing and drying procedures. The three subsequent layers; ZnO, P3HT:PCBM and PEDOT:PSS were slot-die coated and the silver back electrode was screen printed. Finally the polymer solar modules were encapsulated, using a polyester barrier material. All operations except the application of ITO were carried out under ambient conditions. The life cycle analysis delivered a material inventory of the full process for a module production, and an accountability of the energy embedded both in the input materials and in the production processes. Finally, upon assumption of power conversion efficiencies and lifetime for the modules, a calculation of energy pay-back time allowed us to compare this roll-to-roll manufacturing with other organic and hybrid photovoltaic technologies. The results showed that an Energy Pay-Back Time (EPBT) of 2.02 years can be achieved for an organic solar module of 2% efficiency, which could be reduced to 1.35 years, if the efficiency was 3%.     

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.     

Investigations of efficiency improvements in poly(3-hexylthiophene) based organic solar cells using calcium cathodes

In this study, we investigate the mechanisms leading to the power conversion efficiency improvement in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) based organic solar cells using calcium (Ca) in cathode structures. Ultraviolet and x-ray photoemission spectroscopy (UPS and XPS) indicate that chemical reactions occur at the P3HT/Ca interface. Upon Ca deposition, UPS results illustrate a 0.8 eV-downward shift in energy levels of P3HT, but not in those of PCBM. In addition to forming an ohmic contact at the cathode the presence of Ca widens the energy difference between the HOMO of P3HT and the LUMO of PCBM at the cathode interfaces, which results in the increase of open circuit voltage and the enhancement of device efficiency.     

Nano-scale mechanical properties of polymer/fullerene bulk hetero-junction films and their influence on photovoltaic cells

Mechanical properties of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend films, prepared under different processing conditions, were evaluated by nanoindentation. Photovoltaic devices fabricated using above active layers presented the highest power conversion efficiencies for blend films having lowest Young’s modulus (20.73 GPa) and hardness (649 MPa), as measured by a nanoidentator under optimized conditions of blend proportion (1:1), film drying rate (slow) and annealing temperature and time (110 °C and 10 min). It implies that the degree of nano-scale phase separation for the P3HT:PCBM blend is strongly correlated with the mechanical properties in the nanodimension. The nanoindentation is a method to estimate nano-scale mechanical properties of blend films and the performance of photovoltaic cells.     

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.     

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%).     

In-plane anisotropy of photovoltaic effects in aligned polymer solar cells

We herein describe our investigation of in-plane anisotropic polymer photovoltaic effects in aligned bulk-heterojunction layers that consisted of an oriented composite of regioregular poly(3-hexylthiophene) and methanofullerene (P3HT:PCBM). By means of simple rubbing, a fairly uniform in-plane alignment of the P3HT:PCBM layer was achieved. The macroscopic orientation of the main chain of the P3HT polymer in the aligned layer was observed to be significantly greater in the direction of rubbing, while the nanoscopic crystalline packing along the side chains of the P3HT decreased significantly. The polymer solar cells that contained aligned P3HT:PCBM photoactive films exhibited a greater degree of anisotropy of the photovoltaic effects under polarized illumination along the two principal axes. These findings form a promising foundation for new types of polarization-dependent opto-electrical applications.     

ITO-free low-cost organic solar cells with highly conductive poly(3,4 ethylenedioxythiophene): p-toluene sulfonate anodes

Indium tin oxide (ITO)-free organic solar cells were fabricated with highly conductive and transparent tosylate-doped poly(3,4-ethylenedioxythiophene: p-toluene sulfonate) (PEDOT:PTS) anodes of various thicknesses that were prepared by the vapor-phase oxidative polymerization of EDOT using Fe(PTS)₃ as an oxidant. Both solution-processable layers - PEDOT:PSS and photoactive P3HT:PCBM - were spin coated. The anodes transmittance and conductivity varied with thickness. Power conversion efficiency was maximized at 1.4%. The ITO-free organic solar cells photovoltaic characteristics are qualitatively compared with those of ITO-based organic solar cells to explore the possibility of replacing costly, vacuum-deposited ITO with highly conductive, patterned polymer films fabricated by inexpensive vapor-phase polymerization.     

Solution-processed bulk heterojunction organic solar cells with high polarity small molecule sensitizer

A new sensitizer molecule, HMBI (9,18-(di-2-hexyldecyl)-2,11-dimethoxy-9,18-dihydrobenzo[5,6]-s-indaceno[1,2-b]indeno[2,1-h]carbazol-6,15-dione), containing electron-donating carbazole and electron-accepting diketone units, has been synthesized for solution-processed bulk heterojunction organic solar cells. The HMBI material has good solubility in common organic solvents. Its HOMO and LUMO energy levels were found to be at 5.6 and 3.0 eV, respectively. It has absorption bands ranging from 300 to 500 nm. Dispersion of HMBI molecules in the P3HT/PCBM blend broadens the absorption bands over the spectral range of 350-500 nm. Uniform thin film devices doped with varying concentration of HMBI, incorporated within the P3HT/PCBM blend, were fabricated. The 3 wt% of HMBI doping produces an improvement in power-conversion efficiency (PCE) up to 11.5% compared with the reference P3HT/PCBM device. Efficient light harvesting caused by HMBI sensitizer molecules primarily yields increased carrier generation and short-circuit current. In addition, some morphological improvements in the P3HT/PCBM system may contribute to the generation of enhanced photocurrent.     

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.