Development of nanofabrication processes for light management strategies in ultrathin CIGS solar cells

• Autor(a) principal: Ribeiro, Enzo de Jesus
• Data de Publicação: 2023
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/39498
• topic: Nanoimprint lithography; Light management strategies; CIGS; Moth eye; Photonic crystals

The ever-increasing global energy demand requires an urgent need for sustainable energy sources, for which photovoltaics technology is a strong contender. Within this framework, Cu(In,Ga)Se₂ (CIGS) solar cells are amongst the best performing thin-film technologies, allowing for building-integrated photovoltaics and space applications. To reduce production costs, an ultrathin absorber thickness is required due to In and Ga scarcity. However, the incomplete light absorption inherent to ultrathin absorbers demands light management strategies to improve light absorption, which, in turn, requires high resolution lithography techniques to develop subwavelength schemes. Hence, the present work aims to address and overcome challenges regarding the development of a cost-efficient process-flow for the nanofabrication of front and rear light management architectures through nanoimprint lithography (NIL). Front light management was addressed through a broadband anti-reflective (AR) Moth Eye (ME) architecture, for which an ME master stamp was implemented followed by the optimisation of the pattern transfer to different surfaces. Finite-difference time-domain (FDTD) simulations predict a broadband AR effect with the integration of the ME pattern into an ultrathin CIGS’s top layer, resulting in an improved short circuit current density of 0.45 mA cmˉ² over a reference cell. Rear light management was approached through photonic crystals (PC) light scattering schemes, silicon oxide pillars (PL) and hemispheres (HS), to be implemented on the solar cell’s rear contact. FDTD guided the PC’ optimal dimensions that provided the highest optical gain, up to 4.06 mA cmˉ², for each PC solar cell configuration, relative to a reference cell, which were considered for the development of PL and HS structures. To develop such architectures, the challenges in obtaining the PL and HS NIL master stamps were first addressed, followed by the pattern transfer on to proof of concept substrates. During this Thesis, NIL master stamps for both front and rear light management architectures were successfully established, from which four architectures were developed: AR ME, and light scattering PL, conical HS, and ellipsoid HS. All the considered architectures presented optical gains, relative to reference samples without light management, through a decrease in relative total reflectance and increase in relative total transmittance for the ME architecture, and increase in relative diffuse reflectance/transmittance for the PC architectures, showing an efficient AR effect and light scattering, crucial for ultrathin CIGS to fulfil its potential, with PL as the most promising architecture to provide the best optical gain.