Optical properties of nanoparticulate organic photovoltaics and pathways to implementation

Research Doctorate - Doctor of Philopsophy (PhD)This thesis begins by examining the optical characteristics of organic photovoltaics (OPV) as the thesis continues the focus shifts steadily to practical considerations around the implementation of the OPV technology. The first chapter demonstrates an implementation of transfer matrix methods to model absorption in the active layer. The calculation is separated into two distinct material components and used to deduce the exciton generation rates for the two species in the blend. The calculation of the individual component absorption is performed for bulk hetrojunctions (BHJ) that all use P3HT as the donor polymer and use three different types of fullerenes, PCBM, ICBA and PC₇₀BM. The second chapter examines the P3HT:PCBM case again but in this case the active layer has now be formed by the deposition of the blend particles in discrete nanoparticles (NP). The NP films are examined optically and dielectric models that accurately describe their behaviours are developed for a range of weight ratios of polymer to fullerene. The calculation in the first chapter that attributed absorption to each of the species in the blend is now performed for the NP case over the weight ratios examined. The optimisation curve that examines the relationship between thickness and absorbed photons is calculated for the nanoparticles and compared to an equivalent curve using a BHJ dielectric function. In the third chapter the angular dependent current and performance characteristics is examined for both BHJ and NP₋OPV devices. The fourth experimental chapter examines a solution processable material that improves device performance in NP-OPVs in a similar manner to the evaporated calcium layer. The goal of this work was to find a material that was solution processable, that would enable the scale up of the nanoparticle system to roll-to-roll processes, and allow for the electrical inversion of devices. Different formulations of the thoroughly researched ZnO interface are examined to find a interface material, with the best candidate to be found being zinc acetylacetonate Zn(ac)₂. While the material found did not function as well in this role as calcium it did provide a significant improvement over its absence from the interface. In the final experimental chapter efforts to coat organic layers are outlined. Initially the coating efforts began as a group project in organic solvents with the lessons learned from that process being transferred to the NP coating. The coating regimes for achieving uniformity from chloroform and water were found to be significantly different. Additional challenges such as dewetting of the substrate, low evaporative rate and viscosity made the process of finding optimal conditions to print uniform films harder. Suitable conditions were discovered and functional devices were manufactured using a drawbar coating technique that had been developed earlier for the coating of materials from solvent

Fabrication of large-area organic photovoltaics using a draw-bar coating technique

Organic photovoltaic (OPV) devices were fabricated using a novel draw bar premetered coating technique, whereby a meniscus of fluid is dragged across a substrate to leave a trailing wet film. The results showed that coating thickness could be controlled by varying the coating speed, rod diameter, gap height, amount of solution injected, rod diameter, rod composition material and number of layers. Devices on PET with active areas of 10 cm² and active layer thicknesses ranging from 35 to 475 nm were produced using the technique. Active layers of 160 nm were the optimum of thicknesses trialled, achieving typical best efficiencies around 0.4 %. Devices with films thinner than 90 nm did not function due to short-circuiting. The draw-bar coating method has the advantage of allowing controlled deposition of a wide range of film thicknesses with no solution wastage

Inspiration for tomorrow technicians: online physics remote

Internet communication technology has already blended into the modern education, changed it and provides new opportunities for students to study at a university of their choice. There are increasing numbers of open universities, global university collaborations and networks that provide online courses but subjects with compulsory experimental laboratories such as Physics are not well presented. Software simulation cannot substitute the behaviour of real equipment and remote laboratory that allows students to control real experimental apparatus via the Internet provides the best strategic approach. This poster demonstrates the Gamma Ray Spectroscopy remote lab developed for Atomic and Nuclear Physics course at the University of Newcastle. LabVIEW software was used for the control and communication support. Integrated USB cameras provide real-time video of the apparatus in operation

Comparative degradation and regeneration of polymer solar cells with different cathodes

A comparative degradation study of solar cells based on a bulk-heterojunction (BHJ) blend of poly(3-hexylethiophene) (P3HT) and phenyl [6,6] C₆₁ butyric acid methyl ester (PCBM) with two different cathodes is reported. Poly(ethylene-dioxythiphene):poly(styrene sulfonate) (PEDOT:PSS) coated ITO electrodes were used as the anode, whereas Ca/Al and Ca/Ag electrodes were used as cathodes. Fully degraded devices were subjected to thermal annealing under inert atmosphere. The performance of degraded solar cells with a Ca/Al cathode exhibited no improvement after treatment. However the solar cells with a Ca/Ag cathode exhibited a considerable recovery in their performance following annealing under a nitrogen atmosphere. Indeed, these solar cells could be subjected to many degradation and regeneration cycles. Current density–voltage (J-V) characteristics and X-ray photoelectron spectroscopy (XPS) studies show that this behavior arises from the complex chemical thermodynamics of the reactions that can occur at the cathode/active layer interface. In particular, the recovery of device performance for solar cells with a Ca/Ag cathode is due to the reversible oxidation of Ag upon thermal annealing

Nano-pathways: Bridging the divide between water-processable nanoparticulate and bulk heterojunction organic photovoltaics

Here we report the application of a conjugated copolymer based on thiophene and quinoxaline units, namely poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), to nanoparticle organic photovoltaics (NP-OPVs). TQ1 exhibits more desirable material properties for NP-OPV fabrication and operation, particularly a high glass transition temperature (T-g) and amorphous nature, compared to the commonly applied semicrystalline polymer poly(3-hexylthiophene) (P3HT). This study reports the optimisation of TQ1:PC71BM (phenyl C-71 butyric acid methyl ester) NP-OPV device performance by the application of mild thermal annealing treatments in the range of the T-g (sub-T-g and post-T-g), both in the active layer drying stage and post-cathode deposition annealing stage of device fabrication, and an in-depth study of the effect of these treatments on nanoparticle film morphology. In addition, we report a type of morphological evolution in nanoparticle films for OPV active layers that has not previously-been observed, that of PC71BM nano-pathway formation between dispersed PC71BM-rich nanoparticle cores, which have the benefit of making the bulk film more conducive to charge percolation and extraction

Nano-pathways: bridging the divide between water-processable nanoparticulate and bulk heterojunction organic photovoltaics

Here we report the application of a conjugated copolymer based on thiophene and quinoxaline units, namely poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), to nanoparticle organic photovoltaics (NP-OPVs). TQ1 exhibits more desirable material properties for NP-OPV fabrication and operation, particularly a high glass transition temperature (T) and amorphous nature, compared to the commonly applied semicrystalline polymer poly(3-hexylthiophene) (P3HT). This study reports the optimisation of TQ1:PCBM (phenyl C butyric acid methyl ester) NP-OPV device performance by the application of mild thermal annealing treatments in the range of the T (sub-T and post-T), both in the active layer drying stage and post-cathode deposition annealing stage of device fabrication, and an in-depth study of the effect of these treatments on nanoparticle film morphology. In addition, we report a type of morphological evolution in nanoparticle films for OPV active layers that has not previously been observed, that of PCBM nano-pathway formation between dispersed PCBM-rich nanoparticle cores, which have the benefit of making the bulk film more conducive to charge percolation and extraction

Solution processable interface materials for nanoparticulate organic photovoltaic devices

Nanoparticulate zinc oxide can be prepared at low temperatures from solution processable zinc acetylacetonate. The use of this material as a cathode interfacial layer in nanoparticulate organic photovoltaic devices results in comparable performances to those based on reactive calcium layers. Importantly, the enhanced degradation stability and full solution processability make zinc oxide a more desirable material for the fabrication of large area printed devices

Polymer Photodetectors for Printable, Flexible, and Fully Tissue Equivalent X-Ray Detection with Zero-Bias Operation and Ultrafast Temporal Responses

A new printable organic semiconducting material combination as a tissue equivalent photodetector for indirect X-ray detection is demonstrated in this work. The device exhibits a higher optical-to-electrical conversion efficiency than any other reported printable organic systems for X-ray photodetection while also operating efficiently with zero applied bias. Complete X-ray detectors fabricated by coupling the photodiode with a plastic scintillator are among the first flexible and fully tissue equivalent X-ray detectors capable of operating without external bias. The response to X-rays is energy independent between 50 keV and 1.2 MeV, with a detection sensitivity equivalent to inorganic direct X-ray detectors and one of the fastest temporal responses ever reported for organic X-ray detectors. The materials can be printed into arrays with a pixel pitch of 120 μm, providing 2D spatial detection. The devices are found to be highly stable with respect to time, mechanical flexing, and large (5 kGy) radiation doses. The new materials and fully tissue equivalent X-ray detectors reported here provide stable, printable, flexible, and tissue equivalent detectors with a high radiolucency that are ideally suited for wearable applications, where simultaneous monitoring and high transmission of the X-ray absorbed dose into the human body is required