Jason D. Myers

A universal optical approach to enhancing efficiency of organic-based photovoltaic devices

We report a new optical approach that can be used to enhance light harvesting in many different organic-based photovoltaic cells. A transparent polymer microlens array moulded on the light incident surface increases the light path in the active layer and reduces surface reflection, resulting in a 15–60% relative increase in overall cell efficiency.

Organic Semiconductors and their Applications in Photovoltaic Devices

In recent years, organic semiconductors have emerged as a promising and, in some situations, viable commercial alternative to traditional inorganic materials such as silicon. Organic-based light emitting diodes, photovoltaic devices, photodetectors, and transistors have attracted intense interest in the scientific community. In this review, we first present a discussion of the fundamental electronic nature of organic semiconductors, processing techniques, and their application to two main classes of optoelectronic devices, light emitting diodes, and photovoltaics. The second part of the review introduces organic photovoltaics in depth, including their operation principles, development history, current state of the art, and routes for further improvement.

Enhancing light harvesting in organic solar cells with pyramidal rear reflectors

We report enhanced light absorption in semi-transparent organic solar cells by using pyramidal rear reflectors to induce light trapping in the photoactive layer. Pyramidal rear reflectors with a base angle of 30° were molded from a transparent polymer on planar substrates. Compared with a planar rear reflector, the pyramidal structure leads to a more than 2.5 times longer path length in the active layer for the incident light. Experimental demonstration showed an 11%–75% enhancement in the photocurrent and overall efficiency of the solar cells, depending on the device size and active layer thickness.

Computational and experimental studies of phase separation in pentacene:C60 mixtures

Phase separation in molecular donor-acceptor mixtures composed of pentacene and C60 is examined using a combination of computational and experimental methods. Classical molecular dynamics simulations of the relaxation process of pentacene:C60 mixtures predict the formation of pentacene stacks and C60 clusters in the equilibrated structures. These findings are consistent with experimental observations, where x-ray diffraction and atomic force microscopy characterization of the mixed films confirm the existence of polycrystalline pentacene domains. The scanning electron and atomic force micrographs of mixed films deposited at different rates as well as with various mixing ratios show that the aggregation of pentacene, and therefore the degree of phase separation in the mixtures, can be manipulated by the processing conditions. Finally, bulk heterojunction photovoltaic devices using different pentacene:C60 mixtures as an active layer are fabricated and their photovoltaic performance characteristics are compa...

Quaternary Sputtered Cu(In,Ga)Se 2 Absorbers for Photovoltaics: A Review

Quaternary sputtering is a promising alternative to more established deposition methods for the fabrication of Cu(In,Ga)Se2 (CIGS) thin films for photovoltaics (PV). In this technique, a single sputtering target containing all four constituents is employed to deposit the CIGS film. Quaternary sputtering offers several advantages over other deposition methods, including excellent uniformity over large areas, high material usage, and less reliance on toxic Se precursors such as H2Se. Despite these advantages, several drawbacks remain. To date, devices fabricated by quaternary sputtering without additional selenization have been limited in efficiency to about 11%, and realizing bandgap grading in order to match the performance of the best evaporated devices presents a challenge. We discuss the prospects for quaternary sputtering as a fabrication technique for CIGS and highlight areas of research that may result in improved performance. Target fabrication and usage is reviewed. We also present results for films and devices including data for the optical constants of sputtered CIGS. Some recent previously unpublished results, including a study of impurities in CIGS sputtering targets and the first demonstration of a CIGS device on a flexible glass substrate, are discussed

ORMOCHALCs: organically modified chalcogenide polymers for infrared optics

A novel method combining elemental sulfur and selenium was developed, yielding crystalline sulfur-selenium compounds. The compounds were melted, and an organic comonomer added. Once the organic comonomer was consumed, the viscous compound was vitrified and allowed to cool yielding organic-inorganic hybrid polymers that are termed Organically Modified Chalcogenide (ORMOCHALC) polymers.

Structural and electronic characteristics of Cu(In,Ga)Se 2 thin films sputtered from quaternary targets

Although the advantages of sputter deposition for large area, uniform deposition are well known, it has long been believed that sputtering Cu(In,Ga)Se2 (CIGS) from a quaternary sputtering target yields films with morphological and electronic properties that make them unsuitable for use in high-efficiency photovoltaic devices. Recent work, however, has demonstrated that this deposition method can produce dense, polycrystalline, highly oriented films with the desired stoichiometry. Devices built with these films exhibit efficiencies >;10%. While effective parameters for target composition and deposition conditions have been achieved, variation from these conditions can result in a wide array of morphologies, even while composition remains near that of stoichiometric CIGS. In this paper, we review the broad range of structural and electronic properties that result from various sets of target compositions and deposition conditions. Films deposited under some conditions are similar in important respects - their composition, a dense structure composed of ~1 μm sized grains, and the presence of a MoSe2 layer - to those of evaporated CIGS. We discuss how these results point towards the possibility of higher-efficiency sputtered CIGS.

Transparent conducting oxide-based, passivated contacts for high efficiency crystalline Si solar cells

In this work, we investigate a transparent conducting oxide (TCO)-based, passivated contact for the potential use as a passivated tunnel contact to p-type Si. As a surface passivation layer, the Al₂O₃ films with varying the thickness are deposited using plasma-enhanced atomic layer deposition (PEALD) at 200 °C, followed by post-deposition annealing. For a ~15 nm thick Al₂O₃ layer, a high level of surface passivation is achieved, characterized by the effective surface recombination velocity (S_eff,max) of <;30 cm/s. The samples with ultrathin Al₂O₃ layer <;3 nm, however, shows degradation in passivation quality, reaching the S_eff,max<;500 cm/s. When Al-doped zinc oxide (ZnO:Al) as TCO contact is directly deposited onto a ~10.6 nm thick Al₂O₃ coated p-Si via RF magnetron sputtering, the final passivation quality (p-Si/Al₂O₃/ZnO:Al) is characterized by the saturation current density at contact (J_0,contact) of 92.1 fA/cm² with the implied open-circuit voltage (iVoc) of 653 mV, showing the passivation quality is not severely degraded after sputtering without thermal treatment. Further process optimization of PEALD is in progress to produce an improved quality of surface passivation with the S_eff,max<;10 cm/s for ultrathin passivation layers less than 2 nm, enabling a passivated tunneling contact.

Preparation and layer-by-layer solution deposition of Cu(In,Ga)O2 nanoparticles with conversion to Cu(In,Ga)S2 films.

We present a method of Cu(In,Ga)S2 (CIGS) thin film formation via conversion of layer-by-layer (LbL) assembled Cu-In-Ga oxide (CIGO) nanoparticles and polyelectrolytes. CIGO nanoparticles were created via a novel flame-spray pyrolysis method using metal nitrate precursors, subsequently coated with polyallylamine (PAH), and dispersed in aqueous solution. Multilayer films were assembled by alternately dipping quartz, Si, and/or Mo substrates into a solution of either polydopamine (PDA) or polystyrenesulfonate (PSS) and then in the CIGO-PAH dispersion to fabricate films as thick as 1–2 microns. PSS/CIGO-PAH films were found to be inadequate due to weak adhesion to the Si and Mo substrates, excessive particle diffusion during sulfurization, and mechanical softness ill-suited to further processing. PDA/CIGO-PAH films, in contrast, were more mechanically robust and more tolerant of high temperature processing. After LbL deposition, films were oxidized to remove polymer and sulfurized at high temperature under flowing hydrogen sulfide to convert CIGO to CIGS. Complete film conversion from the oxide to the sulfide is confirmed by X-ray diffraction characterization.

Mid-IR Emission in Erbium-Doped Gallium Lanthanum Sulfide Glass Integrated Optic Waveguides

We demonstrate mid-IR emission at a wavelength of 2.7 μm in chalcogenide glass integrated optic waveguides. Slab and rib waveguides are fabricated in RF magnetron sputtered films of erbium-doped gallium lanthanum sulfide glass. Elevated substrate temperature during deposition is found to enhance fluorescence lifetime, resulting in 2.7 μm emission from slab waveguides with a lifetime of 0.7 ms.