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.     

Electrochemically grown ZnO nanorods for hybrid solar cell applications

A hybrid solar cell is designed and proposed as a feasible and reasonable alternative, according to acquired efficiency with the employment of zinc oxide (ZnO) nanorods and ZnO thin films at the same time. Both of these ZnO structures were grown electrochemically and poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester; (P3HT:PCBM) was used as an active polymer blend, which was found to be compatible to prepared indium-tin-oxide (ITO) substrate base. This ITO base was introduced with mentioned ZnO structure in such a way that, the most efficient configuration was optimized to be ITO/ZnO film/ZnO nanorod/P3HT: PCBM/Ag. Efficiency of this optimized device is found to be 2.44%. All ZnO works were carried out electrochemically, that is indeed for the first time and at relatively lower temperatures.     

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.     

Characteristics of indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrode grown by dual target DC sputtering at room temperature for low-cost organic photovoltaics

We compared the electrical, optical, structural and surface properties of indium-free Ga-doped ZnO (GZO)/Ag/GZO and Al-doped ZnO (AZO)/Ag/AZO multilayer electrodes deposited by dual target direct current sputtering at room temperature for low-cost organic photovoltaics. It was shown that the electrical and optical properties of the GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes could be improved by the insertion of an Ag layer with optimized thickness between oxide layers, due to its very low resistivity and surface plasmon effect. In addition, the Auger electron spectroscopy depth profile results for the GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes showed no interfacial reaction between the Ag layer and GZO or AZO layer, due to the low preparation temperature and the stability of the Ag layer. Moreover, the bulk heterojunction organic solar cell fabricated on the multilayer electrodes exhibited higher power conversion efficiency than the organic solar cells fabricated on the single GZO or AZO layer, due to much lower sheet resistance of the multilayer electrode. This indicates that indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes are a promising low-cost and low-temperature processing electrode scheme for low-cost organic photovoltaics.     

Synthesis and characterization of ZnO nanowires grown on different seed layers: The application for dye-sensitized solar cells

Zinc oxide (ZnO) nanowire electrodes which were grown on different seed layers and examination of their significant effects on the performance of dye sensitized solar cells were studied. Through chemical bath deposition process, the ZnO nanowires were grown on an indium tin oxide (ITO) coated glass using sputter-deposited aluminum doped zinc oxide (AZO) and ZnO seed layers. Afterward, main parameters such as solution concentration, growth temperature, and time were systematically investigated based on morphology of nanowires. The X-ray diffraction (XRD), field emission scanning microscopy (FESEM), and photoluminescence (PL) were applied to investigate the characteristics of the samples. The results showed ZnO nanowires, which were grown by AZO seed layer, had a high density array with hexagonal wurtzite structure distributed vertically and uniformly on ITO coated glass. The mentioned zinc-oxide nanowires grown under an optimum condition on different seed layer were used to fabricate dye solar cells afterward. The seed layer was effective on morphologic, optical, and structural features. The overall light-conversion efficiency of dye sensitized solar cell with ZnO nanowires grown on AZO seed layer was almost 2 times higher than that of those grown on ZnO seed layer. Electrochemical impedance spectroscopy analysis was measured under standard light to investigate the electron transport properties in the both ZnO-NW DSSCs. As the results showed, photoanode electron recombination rate with electrolyte was 6.02 Hz for dye solar cells of zinc oxide (ZnO-NWDSSC) produced by ZnO seed layer, which is 2.5 times faster than cells with AZO seed layer.     

Flexible Ag electrode for use in organic photovoltaics

High efficiency organic photovoltaic cells discussed in literature are normally restricted to devices fabricated on glass substrates. This is a consequence of the extreme brittleness and inflexibility of the commonly used transparent conductive oxide electrode, indium tin oxide (ITO). This shortcoming of ITO along with other concerns such as increasing scarcity of indium, migration of indium to organic layer, etc. makes it imperative to move away from ITO. Here we demonstrate a highly flexible Ag electrode that possesses low sheet resistances even in ultra-thin layers. It retains its conductivity under severe bending stresses where ITO fails completely. A P3HT:PCBM blend organic solar cell fabricated on this highly flexible electrode gives an efficiency of 2.3%.     

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.     

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%).     

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.     

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.     

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 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%.     

Light harvesting in organic solar cells

We present a methodology which allows designing photonic crystals slabs (PCs) able to couple incident light into “slow Bloch modes” (SBMs) and dealing with their incorporation in an organic solar cell (OSC). We theoretically study different structures based on the same couple of organic materials (poly-3-hexylthiophène (P3HT) as donor and [6,6]-phenyl-C61-butiryc acid methyl ester (PCBM) as acceptor): a 2D photonic crystal based on a perfectly ordered P3HT/PCBM blend (placed in the air), a 1D photonic crystal based on a nanostructured PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)) layer embedded in a P3HT:PCBM host matrix (first placed in the air and then inserted in an organic solar cell) and finally a 1D photonic crystal based on a nanostructured P3HT:PCBM layer covered by a metallic electrode and inserted in an OSC ( with and without nanostructuration of the PEDOT:PSS layer). We show that the light coupling into SBMs in an OSC depends on vertical interferences and that optical spacers are needed. We then demonstrate that the P3HT:PCBM active layer nanostructuring covered by a thick metallic electrode exhibits the highest gain (4% in the 400–700 nm spectral range) thanks to a simultaneous optimisation of the optical properties of the photonic crystal (coupling of SBM) and of the stack of the organic solar cell (vertical interferences).     

Effects of ITO precursor thickness on transparent conductive Al doped ZnO film for solar cell applications

Al doped ZnO (AZO) film was continuously deposited on ITO precursor on glass substrate by d.c. magnetron sputtering. The thickness of ITO was varied from 30 to 120 nm in order to investigate the effect of ITO thickness on crystallinity of AZO film. X-ray diffraction measurement shows that AZO film grown on ITO has an enhanced (0 0 2) preferred orientation as the ITO thickness was increased. The crystalline structure improvement of AZO film with an increase of ITO precursor thickness is due to the near-epitaxial growth of AZO on ITO precursor. As the ITO thickness was increased, mobility of AZO film by the Hall measurement was significantly increased from 5.4 cm²/V s (no ITO) to 23.6 cm²/V s (ITO 120 nm), and resistivity was about 81.7% improved from 1.99×10⁻³ to 3.63×10⁻⁴ Ω cm. The AZO films with ITO revealed excellent average transmission of visible (90.0%) and NIR (89.6%) regions, whereas those of AZO film without ITO were 82.1% and 88.1%, respectively. The haze values of AZO film with ITO of 90 and 120 nm are similar or higher than those of AZO film without ITO. The surface textured AZO film with ITO precursor is promising for optoelectronic applications such as the front TCO of thin film solar cells.     

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%.     

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.     

Effect of solution processed graphene oxide/nickel oxide bi-layer on cell performance of bulk-heterojunction organic photovoltaic

We report the solution processed graphene oxide (GO), NiO_x and GO/NiO_x bi-layer used as an anode interfacial layer in organic bulk-heterojunction solar cells. The bulk-heterojunction solar cells using GO, NiO_x and GO/NiO_x bi-layer exhibited the conversion efficiency of 2.33%, 3.10% and 3.48%, respectively. The cell efficiency is correlated with the matching of energy levels between ITO, hole transport layer and P3HT and thus a well-matched stack layer of ITO/GO/NiO_x/P3HT:PCBM/LiF/Al shows the best cell efficiency of 3.48% with the J_SC of 8.71 mA/cm², V_OC of 0.602 V and FF of 66.44%.     

Transparent electrode with ZnO nanoparticles in tandem organic solar cells

The transparent inter-electrodes with the p/n heterojunction consisting of the solution-processible ZnO nanoparticles as the n-type and the conventional hole injection layers (MoO₃ or PEDOT:PSS) as the p-type materials are studied for developing tandem organic solar cells employing different band gap active materials (i.e., P3HT:PCBM blend layer for larger band gap material in the bottom cell and ZnPc/C₆₀ bilayer for smaller band gap material in the top cell). For the ZnO/PEDOT:PSS inter-electrode, the V_OC corresponding to the sum of V_OC’s of the top and bottom unit cells is obtained, denoting that the two unit cells are successfully connected in series. For the ZnO/MoO₃ inter-electrode, the open-circuit voltage (V_OC) of the tandem cell is smaller than the sum of V_OC’s of the top and bottom unit cells, but it can be increased by inserting a very thin Al layer (∼3 nm) between ZnO and MoO₃ (ZnO/Al/MoO₃) as the recombination center for carriers.     

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.     

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.