Transport and Spectroscopic Studies of the Effects of Fullerene Structure on the Efficiency and Lifetime of Polythiophene-based Solar Cells

Time-dependent measurements of both power conversion efficiency and ultraviolet-visible absorption spectroscopy have been observed for solar cell blends containing the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) with two different functionalized C60 electron acceptor molecules: commercially available [6,6]-phenyl C61 butyric acid methyl ester (PCBM) or [6,6]-phenyl C61 butyric acid octadecyl ester (PCBOD) produced in this laboratory. Efficiency was found to decay with an exponential time dependence, while spectroscopic features show saturating exponential behavior. Time constants extracted from both types of measurements showed reasonable agreement for samples produced from the same blend. In comparison to the PCBM samples, the stability of the PCBOD blends was significantly enhanced, while both absorption and power conversion efficiency were decreased.Comment: manuscript submitted to Solar Energy Materials and Solar Cell

Inverted polymer fullerene solar cells exceeding 10% efficiency with poly(2-ethyl-2-oxazoline) nanodots on electron-collecting buffer layers

Polymer solar cells have been spotlighted due to their potential for low-cost manufacturing but their efficiency is still less than required for commercial application as lightweight/flexible modules. Forming a dipole layer at the electron-collecting interface has been suggested as one of the more attractive approaches for efficiency enhancement. However, only a few dipole layer material types have been reported so far, including only one non-ionic (charge neutral) polymer. Here we show that a further neutral polymer, namely poly(2-ethyl-2-oxazoline) (PEOz) can be successfully used as a dipole layer. Inclusion of a PEOz layer, in particular with a nanodot morphology, increases the effective work function at the electron-collecting interface within inverted solar cells and thermal annealing of PEOz layer leads to a state-of-the-art 10.74% efficiency for single-stack bulk heterojunction blend structures comprising poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-3-fluorothieno[3,4-b]thiophene-2-carboxylate] as donor and [6,6]-phenyl-C71-butyric acid methyl ester as acceptor

High-efficiency robust perovskite solar cells on ultrathin flexible substrates.

Wide applications of personal consumer electronics have triggered tremendous need for portable power sources featuring light-weight and mechanical flexibility. Perovskite solar cells offer a compelling combination of low-cost and high device performance. Here we demonstrate high-performance planar heterojunction perovskite solar cells constructed on highly flexible and ultrathin silver-mesh/conducting polymer substrates. The device performance is comparable to that of their counterparts on rigid glass/indium tin oxide substrates, reaching a power conversion efficiency of 14.0%, while the specific power (the ratio of power to device weight) reaches 1.96 kW kg(-1), given the fact that the device is constructed on a 57-μm-thick polyethylene terephthalate based substrate. The flexible device also demonstrates excellent robustness against mechanical deformation, retaining >95% of its original efficiency after 5,000 times fully bending. Our results confirmed that perovskite thin films are fully compatible with our flexible substrates, and are thus promising for future applications in flexible and bendable solar cells

Oxygen doping-induced photogeneration loss in P3HT:PCBM solar cells

This work investigates the loss in performance induced by molecular oxygen in bulk heterojunction solar cells. We observe that upon exposure to molecular oxygen both formation of P3HT+:O2− complex and metal oxidation at the interface between the active layer and metallic contact occur. These two different effects were separately investigated using NOBF4 as an oxidant. Our procedure has allowed studying p-doping of the active layer independently from contact degradation. A loss in photocurrent is associated with formation of P3HT+:O2− complex, which reduces the concentration of neutral P3HT present in the film in accordance with absorption and external quantum efficiency spectra. This complex is regarded as a source of a pathway of reversible degradation. Capacitance–voltage measurements allow for an accurate extraction of p-doping levels of the active layer produced by the presence of charged O2− species. In addition, one of the irreversible degradation pathways is identified to be oxidation of the metallic contact to form CaO. This oxide forms a thin dipole layer producing a voltage drop across the active layer/Ca interface, which has a direct impact on the open circuit voltage and fill factor

A Study of the Relationship Between Microstructure and Photophysics in Organic Semiconductor Blends for Solar Cell Applications

This thesis is a study of material blends involving organic semiconductors and their application to opto-electronic devices, particularly photovoltaic diodes. Its principal aim is to examine the microstructure of the blend, where microstructure is defined as molecular ordering and spatial arrangement on the nanometer to micrometer scale. In general, each chapter of the thesis presents a novel means by which to influence the microstructure of organic semiconductor blends. These techniques are used as a means to understand how the photophysics of optoelectronic devices is influenced by microstructure. We establish some general principles of how microstructure relates to device performance and also find high performance in some entirely novel device structures and architectures. It is hoped that understanding developed here will lead to improvements in the performance of organic photovoltaic devices

Organic solar cells: electrodes, performance enhancement and degradation mechanisms

In this thesis I focus on a number of aspects associated with the fabrication and characterization of organic photovoltaics. Specifically, my work focuses on evaluating solution processed graphene electrodes for use in organic photovoltaics, improving the performance of indium tin oxide transparent contacts by coating them with Au nanoparticles, and understanding the degradation pathways of Poly(3-hexylthiophene-2,5-diyl): Phenyl-C61-butyric acid methyl ester (P3HT:PCBM) organic solar cells. In my work on graphene electrodes for organic solar cells I worked out a relationship between the sheet resistance and the film transmittance that is useful to optimize such electrodes. Investigation of organic solar cell degradation in a controlled 70-70-70 test (i.e. keeping a device for 70 hours at 70oC under 70% humidity conditions) showed several possible pathways in which the active layer of these photovoltaics degrade. In addition to the typical morphological degradation, a strong increase in paramagnetic defect density in the active layer contributes to their degradation. Formation of paramagnetic defects in P3HT:PCBM layers was attributed by us to the creation of charge transfer complexes between P3HT and oxygen. Our attempts on improving indium tin oxide electrodes for their use in organic photovoltaics included a study of the effects of nucleation of Au-containing molecular nanoclusters. It was discovered that different types of Au nanoparticles with specific properties can be formed by annealing such clusters at different temperatures and under different conditions. This discovery was utilized to fabricate Au nanoparticle layers on indium tin oxide which were then utilized as plasmonic enhancement layers for organic solar cells

Fullerene based organic solar cells

The direct conversion of the sunlight into electricity is the most elegant process to generate environmentally-friendly renewable energy. Plastic solar cells offer the prospect of flexible, lightweight, lower cost of manufacturing, and hopefully an efficient way to produce electricity from sunlight. Since the discovery of photo induced charge transfer from a conjugated polymer to C60, followed by introduction of the bulk heterojunction concept, this material combination has been extensively studied in organic solar cells leading to a power conversion efficiency approaching 6% nowadays. A typical bulk heterojunction solar cell consists of a photoactive layer sandwiched between two different electrodes. The photoactive layer is based on a blend of an electron donating material (p-type semiconductor) and an electron accepting material (n-type semiconductor) forming nanostructured bicontinuous interpenetrating networks. Although significant progress has been made for bulk heterojunction photovoltaic devices, the current efficiency of these solar cells does not guarantee (large scale) commercialization. The most commonly used n-type semiconductor in the bulk heterojunction solar cells is [60]PCBM. Up till now [60]PCBM remains the best performing soluble fullerene derivative in combination with rr-P3HT. Improving the performance and lifetime of bulk heterojunction solar cells requires careful design and material engineering, and more insight in the operation of these devices. This thesis addresses the possibility of using new fullerene derivatives in organic bulk heterojunction photovoltaic devices, and discusses the preparation, and the morphological and electrical characterization of devices made from rr-P3HT and a library of new methanofullerenes

Improved Efficiency Organic Photovoltaic Cells through Morphology Control and Process Modification

Organic photovoltaic (OPV) cells have drawn great attention due to the potential to produce flexible, light weight, affordable solar cells using polymer organic photovoltaic materials; however, the current power conversion efficiency achieved for these systems is too low for widespread implementation of the technology. Morphology and phase separation are key factors determining the performance of organic photovoltaic cells. Precise control of the size and distribution of the phase-separated photoactive domains is necessary for optimum photon-electron conversion. Polyhedral oligomeric silsesquioxane (POSS) nanostructered chemicals have the potential to provide enhanced control of morphology, crystallinity, and phase dispersion in polymeric blend systems. In this work, POSS molecules with different organic functionalities were utilized to control OPV film morphology. The light absorption, crystallinity, and phase separated domain size were evaluated to determine the relationship between POSS structures and film characteristics. The selected POSS molecules were utilized for further device fabrication and performance measurements, with which the POSS enhanced performance was revealed. Furthermore, processing conditions are also important in determining the performance and phase separated morphology of the OPV devices. The effects of solvent vapor annealing and thermal annealing were evaluated in terms of light absorption, crystallinity, long-term stability, and device performance