High-efficiency photovoltaics through mechanically stacked integration of solar cells based on the InP lattice constant

Solar Energy is a clean and abundant energy source that can help reduce reliance on fossil fuels around which questions still persist about their contribution to climate and long-term availability. Monolithic triple-junction solar cells are currently the state of the art photovoltaic devices with champion cell efficiencies exceeding 40%, but their ultimate efficiency is restricted by the current-matching constraint of series-connected cells. The objective of this thesis was to investigate the use of solar cells with lattice constants equal to InP in order to reduce the constraint of current matching in multi-junction solar cells. This was addressed by two approaches: Firstly, the formation of mechanically stacked solar cells (MSSC) was investigated through the addition of separate connections to individual cells that make up a multi-junction device. An electrical and optical modelling approach identified separately connected InGaAs bottom cells stacked under dual-junction GaAs based top cells as a route to high efficiency. An InGaAs solar cell was fabricated on an InP substrate with a measured 1-Sun conversion efficiency of 9.3%. A comparative study of adhesives found benzocyclobutene to be the most suitable for bonding component cells in a mechanically stacked configuration owing to its higher thermal conductivity and refractive index when compared to other candidate adhesives. A flip-chip process was developed to bond single-junction GaAs and InGaAs cells with a measured 4-terminal MSSC efficiency of 25.2% under 1-Sun conditions. Additionally, a novel InAlAs solar cell was identified, which can be used to provide an alternative to the well established GaAs solar cell. As wide bandgap InAlAs solar cells have not been extensively investigated for use in photovoltaics, single-junction cells were fabricated and their properties relevant to PV operation analysed. Minority carrier diffusion lengths in the micrometre range were extracted, confirming InAlAs as a suitable material for use in III-V solar cells, and a 1-Sun conversion efficiency of 6.6% measured for cells with 800 nm thick absorber layers. Given the cost and small diameter of commercially available InP wafers, InGaAs and InAlAs solar cells were fabricated on alternative substrates, namely GaAs. As a first demonstration the lattice constant of a GaAs substrate was graded to InP using an InxGa1-xAs metamorphic buffer layer onto which cells were grown. This was the first demonstration of an InAlAs solar cell on an alternative substrate and an initial step towards fabricating these cells on Si. The results presented offer a route to developing multi-junction solar cell devices based on the InP lattice parameter, thus extending the range of available bandgaps for high efficiency cells.

Public policies and work integration social enterprises: the challenges of institutionalism in a neoliberal era.

One particular field of Social Enterprise – WISEs or Work Integration Social Enterprises – has become increasingly recognised as being emblematic of the dynamics of social enterprises and now constitutes a major sphere of their activity globally. From their early roots, focusing on providing productive activity for the blind and those with other physical and/or intellectual disabilities, WISEs are pioneers in promoting the integration of excluded persons through a productive activity. In recent decades, WISEs have incrementally evolved as a tool for implementing national and regional labour market policies. The papers in this special edition explore how populations of WISEs in different country contexts have emerged, and in some instances, shifted in their identities over time in relation to changing national or regional public policies. This special issue is part of the ICSEM project, a worldwide research project aiming to identify, analyze and compare social enterprise models across countries, regions and fields. The special issue features five country specific analyses from the first stage of the ICSEM project where researchers focusing on WISEs examined the policy environment surrounding WISEs and the heterogeneity of the organizational WISE models that have emerged in different contexts: Ireland, the United States, Japan, Austria and Switzerland.