Context. Technical and social innovations are central to the transition to renewable energy systems. Despite the availability of innovative technologies for renewable energy production, management, and e-mobility, their widespread societal adoption remains limited. Proximity between actors is key for energy technologies' diffusion. Yet, it is challenging to bring into a cohesive interdisciplinary conceptualisation.

Goal. This thesis examines proximity in the diffusion of energy innovations in Switzerland. I conceptualise proximity related to innovation-diffusion as relational and circumstantial. Relational proximity is studied by drawing on information networks to examine connectivity and diversity within five proximity dimensions: geographical, social, cognitive, organisational and institutional. I analyse circumstantial proximity by researching which socio-spatial contexts bring actors into situations where they exchange information and what characterises these situations.

Methods. Using data from 36 interviews with Swiss energy actors and surveys with 157 professionals and 4,000 adopters of photovoltaic panels, electric vehicles, and energy management systems, I analysed information exchanges that led to the diffusion of these technologies. I included exchanges among professionals, between professionals and adopters, and between adopters and their personal contacts. Results. Proximity relates to the diffusion of energy innovations through linking infrastructures that create the potential for interactions and circumstances that generate spaces of interface where information can be exchanged. The relational proximity results show linking infrastructures are geographically and socially connected and introduce a balance of actors with similar and diverse cognitive, institutional and organisational characteristics. Circumstantial proximity reveals face-to-face interactions to be key for the diffusion of innovations, allowing the flow of tacit knowledge and the development of trust. Specific contexts foster spaces of interface: a) diverse urban environments that increase interactions with people outside of the closest circle, b) socio-economic conditions that allow actors to have these interactions and, c) professionals and convinced adopters that support less urban and innovative ones.

Discussion. The interrelations between relational and circumstantial proximity highlight three avenues to nurture spaces of interface for innovation-diffusion. First, an appropriate configuration of the space, through densification, urban planning, design or event organisation, can be key to increasing connecting dimensions and opportunities for interaction. Second, establishing trustful relationships is important to transform these interactions into diffusion. Third, the integration of people from different backgrounds in social circles supports opportunities to discuss with "others" bringing novel information, and encourages diffusion from early adopters to the people from different sociodemographic backgrounds.

Conclusion. By complementing relational with circumstantial proximity, this research enriches the theory of the diffusion of innovations and offers practical recommendations for advancing the diffusion of energy technologies in Switzerland. It highlights the importance of socio-spatially mixed contexts that also foster trust and openness for innovation to flow in society.