Inverted Organic Photovoltaics Research Articles

Inverted Organic Photovoltaic Cells Using Three-Dimensionally Interconnected TiO<SUB>2</SUB> Nanotube Arrays

Here we describe a simple sol-gel method to fabricate inverted organic photovoltaics (OPV) using interconnected TiO2 nanotubes (inter-TiO2 NT) as an efficient electron transport layer. Three-dimensionally inter-TiO2 NT arrays were prepared by spin-coating a TiO2 precursor solution on the ZnO nanorod (NR) template grown via the liquid phase deposition method. Upon etching of ZnO NRs, inter-TiO2 NT arrays were generated, as confirmed by X-ray diffraction (XRD), energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (FE-SEM). A blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) deeply infiltrated into the pores of inter-TiO2 NT, as revealed by FE-SEM and atomic force microscopy (AFM) images. The power conversion efficiency (PCE) of inter-TiO2 NT-based inverted OPV reached 3.0% at an air mass of 1.5 (100 mW/cm2), which is a 25% performance improvement compared to flat TiO2 films derived from the sol-gel process or commercial paste. The efficiency improvement arises from facilitated charge separation and collection due to the increased TiO2 interface arera and efficient transport pathway.

Phase separation-driven stratification in conventional and inverted P3HT:PCBM organic solar cells

We have used neutron reflectivity to investigate the stratification of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend films. Films were spun-cast on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and titanium oxide (TiOx) layers to mimic the procedures followed for the fabrication of conventional and inverted organic photovoltaics respectively. The resultant scattering length density profiles reveal a PCBM-rich layer is formed in the vicinity of PEDOT:PSS or TiOx, while PCBM is depleted at the free surface of the film. PCBM segregation close to the substrate is further enhanced by annealing. This stratification is considered to be favorable only for inverted devices.

Fluorinated Copper Phthalocyanine Nanowires for Enhancing Interfacial Electron Transport in Organic Solar Cells

Zinc oxide is a promising candidate as an interfacial layer (IFL) in inverted organic photovoltaic (OPV) cells due to the n-type semiconducting properties as well as chemical and environmental stability. Such ZnO layers collect electrons at the transparent electrode, typically indium tin oxide (ITO). However, the significant resistivity of ZnO IFLs and an energetic mismatch between the ZnO and the ITO layers hinder optimum charge collection. Here we report that inserting nanoscopic copper hexadecafluorophthalocyanine (F(16)CuPc) layers, as thin films or nanowires, between the ITO anode and the ZnO IFL increases OPV performance by enhancing interfacial electron transport. In inverted P3HT:PC(61)BM cells, insertion of F(16)CuPc nanowires increases the short circuit current density (J(sc)) versus cells with only ZnO layers, yielding an enhanced power conversion efficiency (PCE) of ∼3.6% vs ∼3.0% for a control without the nanowire layer. Similar effects are observed for inverted PTB7:PC(71)BM cells where the PCE is increased from 8.1% to 8.6%. X-ray scattering, optical, and electrical measurements indicate that the performance enhancement is ascribable to both favorable alignment of the nanowire π-π stacking axes parallel to the photocurrent flow and to the increased interfacial layer-active layer contact area. These findings identify a promising strategy to enhance inverted OPV performance by inserting anisotropic nanostructures with π-π stacking aligned in the photocurrent flow direction.

Ag Interlayered Transparent Conducting Electrode for Photovoltaic Cells

We fabricated the organic photovoltaic (OPV) cells on the transparent and low resistive multilayer electrode of a few nm Ag layer embedded Al doped zinc oxide film (AZO/Ag/AZO) on a glass. AZO and Ag consisting of the continuous multilayer were fabricated using rf magnetron at room temperature successively. AZO (45 nm)/Ag (11 nm)/AZO (45 nm) electrode shows an electrical resistivity of 6.56×10-5 Ω cm, optical transmittance of 89.3% at 550 nm and figure of merit (FOM) value of 4.94×10-2 Ω-1. An inverted structure OPV was fabricated on AZO/Ag/AZO transparent electrode in which TiOx and PEDOT:PSS were used as an electron transport and hole transport layer respectively. The OPVs showed power conversion efficiency and fill factor as high as 2.07%, 0.51 respectively.

Towards fabrication of high-performing organic photovoltaics: new donor-polymer, atomic layer deposited thin buffer layer and plasmonic effects

Using a novel polymer (polythienothiophene-co-benzodithiophenes 7 F-20) as a donor and phenyl-C71-butyric acid methyl ester as an acceptor of bulk heterojunction, inverted organic photovoltaics (OPVs) were fabricated. Wet-chemically prepared ZnO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) were used as buffer layers. Particularly, for PEDOT:PSS deposition, no annealing step was employed. This inverted OPV showed a power conversion efficiency (PCE) of ∼7.0%, which is comparable to the hitherto reported highest efficiency of the inverted OPV with vacuum-deposited MoO3 as hole-collecting buffer layers without plasmonic enhancements. Incorporation of Au nanoparticles into PEDOT:PSS was performed for plasmonic enhancement of the electromagnetic field, whereas ZnO thin layers were deposited on ZnO using atomic layer deposition for quenching electron–hole recombination at surface defects of ZnO ripples. These additional treatments could be used for improving the performance of OPV, which ultimately resulted in a PCE of 7.9%.

Inverted Organic Photovoltaic Cells with Solution-Processed Zinc Oxide as Electron Collecting Layer

In this work, inverted polymer:fullerene organic photovoltaic (OPV) cells with solution-processed zinc oxide (ZnO) as the electron collecting layer are investigated. ZnO films are prepared simply by the spin-casting of a zinc acetate dehydrate precursor solution, followed by sintering under ambient conditions. The performance of the fabricated inverted OPV cells shows a clear dependence on precursor concentration and sintering conditions. With the ZnO film derived from a sol–gel concentration of 0.1 M and sintered at 350 °C for 10 min, the inverted OPV cell shows optimum performance.

Inverted Organic Photovoltaic Cells with Solution-Processed Zinc Oxide as Electron Collecting Layer

In this work, inverted polymer:fullerene organic photovoltaic (OPV) cells with solution-processed zinc oxide (ZnO) as the electron collecting layer are investigated. ZnO films are prepared simply by the spin-casting of a zinc acetate dehydrate precursor solution, followed by sintering under ambient conditions. The performance of the fabricated inverted OPV cells shows a clear dependence on precursor concentration and sintering conditions. With the ZnO film derived from a sol–gel concentration of 0.1 M and sintered at 350 °C for 10 min, the inverted OPV cell shows optimum performance.

Photonic Crystal Geometry for Organic Polymer:Fullerene Standard and Inverted Solar Cells

We describe a 2-D photonic crystal (PC) geometry that enhances the absorption of light for organic photovoltaic (OPV) cells relative to conventional planar cells. The PC geometry is developed by patterning an organic photoactive bulk heterojunction blend of P3HT (poly(3-hexylthiophene)) and PCBM (phenyl-C61-butyric acid methyl ester) via PRINT (pattern replication in nonwetting templates), a method that lends itself to large area fabrication of nanoscale features. Replication flexibility for various patterns and ranges of materials emphasizes the utility of PRINT for both inverted and standard OPV architectures. The optical properties of PC cells for different device architectures are investigated theoretically and experimentally in order to determine critical physical dimensions that influence device performance.

Air-processed organic photovoltaic devices fabricated with hot press lamination

A detailed method for air-processed, inverted Organic Photovoltaic (OPV) devices using lamination of the metal electrode with a doped conducting polymer adhesive is presented. A poly(3-hexylthiophene) (P3HT) - [6,6] phenyl C 61 butyric methyl ester (PCBM) bulk heterojunction is used, with a solution-processed ZnO electron collection layer. The top contact consists of modified polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) that is spin-coated onto a Ag electrode, and the entire structure is laminated onto the active layer. Lamination produces devices with 3.19% power conversion efficiency at 1 sun illumination, which is higher than the value for control devices produced with the standard sequential process with an evaporated top metal electrode. The laminated interface yields an ohmic contact, as shown by the high fill factor and low series resistance of the laminated devices compared to devices with a thermally evaporated top metal electrode. We examine the lamination parameters and their effect on device performance, and we identify the treatment of the adhesion layer to be one of the more important parameters to control for efficient devices. The optimized protocol described here allows one to fabricate efficient inverted OPV devices in air, without the need for an expensive, high quality evaporation chamber for top contact deposition.