High-performed flexible organic photovoltaics towards wearable applications

Abstract

Organic photovoltaic (OPV) is an emerging technology for converting sunlight into electricity that shows the advantages of low cost, light weight, non-toxic, large-area, and flexible compatibility. Such features enable the OPV to exhibit huge potential in wearable applications. However, there are several major issues with the advanced flexible OPV. First, the efficiency of flexible OPV still lags the rigid devices due to the lack of high-quality flexible transparent electrodes (FTEs). Second, emerging FTEs usually suffer from stability issues. Third, the fabrications of widely used FTE (e. g. conductive metal oxides) need high vacuum processes which impede large-area application. Limited by their mechanical brittleness, these FTEs are not ideal for wearable applications. This thesis introduces the fundamentals of semiconductors, photovoltaics, and covers an overview of the development of FTE and their applications in organic photovoltaics. Moreover, several works have been conducted to tackle the aforementioned issues. The first work is designing a novel high-performance FTE via displacement-diffusion-etch process to obtain ultrathin gold electrodes (UTAuEs) by solution-process on a wide variety of plastic substrates. UTAuEs fabricated on polyethylene terephthalate show superior chemical stability and bendability. Moreover, flexible OPVs made with UTAuEs show similar efficiency but much-enhanced flexibility, in comparison to that of ITO-based devices. The second work is constructing stretchable anti-reflecting substrate and their outdoor/indoor applications in OPV. By duplicating the surface texture from abrasive papers, the surface-textured polydimethylsiloxane (PDMS) was obtained with higher diffuse transmittance compared to the non-textured substrate. Benefitting from the high light utilization, the ITO-free device based on the surface-textured PDMS yields a high PCE of 15.3% under 1-sun illumination. More excitingly, the device based on textured PDMS/PEDOT:PSS maintained a comparable PCE to the rigid device under dim-light illumination. The third work is aiming to construct intrinsically stretchable organic photovoltaic (is-OPV). A ferroconcrete-liked AZO@silver nanowires (AgNWs)@AZO (AAA) was designed for replacing the ultra-thin metal back electrode. The AAA not only offers a 3D long-range pathway for efficient charge transport and collection but also reinforces interfacial stability. The OPV based on AAA exhibits an efficiency of 12.8% with an average visible transmittance of 26.7%. Furtherly, the semi-transparent is-OPV based on the full-solution-process achieved a record PCE of 10.9% and excellent mechanical robustness. The fourth work is an attempt at the application of flexible OPV in wearable electronics. We proposed a tandem self-powered flexible energy supplier (SPFES) that ‘harvests and stores energy from sunlight, dim-light, and human body motion. Two FTEs are shared by three functional components: OPV, triboelectric nanogenerator, and electrochromic supercapacitor. Interestingly, the SPFES shows distinctive in-built features including energy indication, self-modulation, self-protection, and a higher power-to-weight ratio than independent devices. In summary, the main works in my Ph.D. study include the following aspects in a systematic self-sustain manner. First, developing new types of flexible/stretchable transparent electrodes. Second, exploring the application of these flexible/stretchable transparent electrodes in OPV. Third, attempt the multifaceted potential applications of OPV, and beyond OPV in wearable applications. We hope these works would provide new insight into the development of flexible/stretchable OPV.