Candidate
Graeme Williams
Title
The role of device structure in charge collection for organic solar cells
Supervisor
Hany Aziz
Abstract
Organic photovoltaics employ small molecules or polymers as their primary light absorbing materials and thus differ strongly from traditional silicon-based photovoltaics. Their primary technological benefit is a significant reduction in materials and module fabrication costs. While research on organic solar cells (OSCs) has increased dramatically in the past decade, both OSC efficiencies and device lifetimes must be improved before they can compete with existing second generation photovoltaic technologies. Many of the gains in OSC efficiency to date can be attributed to the vast and concurrent trial-and-error experiments on new donor materials and processing techniques to form traditional bulk heterojunction structures. The field is consequently lacking in predictive power, and many stipulations regarding ideal device architectures and optimal interfacial layers remain ambiguous. It is thus beneficial to systematically study charge transport and charge extraction in modern OSCs.
This seminar comprehensively examines charge collection in OSCs as a function of OSC device architecture. To maintain a coherent test platform, vacuum-deposited OSCs are fabricated with various metal phthalocyanine donor materials and a fullerene acceptor. This is in contrast to the solution-processed OSCs that have been the focus of most OSC research since 2005. By removing complications in solution coating (especially film formation and phase separation considerations), it is significantly more straightforward to study photo-physics and charge collection behaviour. In this regard, the role of interfacial layers in charge extraction is investigated, the optimal combination/proportion of neat or mixed donor and acceptor layers in terms of the photo-active materials’ properties is studied, and the impact of adding a third component to the mixed layer (i.e. ternary OSCs) is elucidated. The culmination of this work illuminates limitations in charge collection, especially in terms of the distribution of donor and acceptor material in the OSC (both in the bulk mixed layers and with regard to vertical distribution), as well as with variations made at the organic/electrode interface. The results provide guidelines to overcome device performance limitations that are pertinent for future research in both vacuum-deposited and solution-coated OSCs.