Indoor Light Harvesting: A New Frontier for Organic Photovoltaics

There is rapidly growing interest in indoor photovoltaics because of their potential in the huge technology field of the Internet of Things (IoT) [1]. IoT refers to a smart network of internet connected electronic and electrical devices which can communicate with each other and respond promptly. Wireless sensors, requiring only µW-mW range of electrical power for their efficient functioning, are the most fundamental components in these smart devices. By 2025, there will be more than 75 billion IoT connected devices with half of the components to be installed inside the buildings (web reference). Sustainably powering these sensors is a huge challenge. Light energy from artificial light sources such as white LEDs and fluorescent lamps are ubiquitous inside the building and can be converted to electricity using the photovoltaic effect, to autonomously power IoT sensors. Among the various photovoltaic technologies available today such as silicon, dye sensitised, hybrid perovskites and organic photovoltaics (OPVs), the latter one is very promising as they are scalable, flexible, conformable and possess excellent optoelectronic properties suitable for efficient light-harvesting [2, 3]. Furthermore their wider band gap than most other photovoltaic materials, makes them better matched to the spectrum of indoor lighting, which is very different from the sun.

The first generation of organic photovoltaics (OPVs) mainly used fullerene-based acceptors and the power conversion efficiency (PCE) of the corresponding bulk-heterojunction (BHJ) devices levelled off around 10% under 1 Sun irradiance (100 mW/cm²). However, with the emergence of non-fullerene acceptors, the PCE of the analogous organic solar cells are soaring towards 20% [4]. These non-fullerene acceptor based BHJs are particularly interesting for indoor OPVs, because of their tunable bandgap, low open-circuit voltage loss, high absorption coefficient in the visible range and demonstrated maximum PCE of ~ 30% under indoor illumination (< 1 mW/cm²). However, this demonstrated efficiency is only half of the theoretically predicted maximum PCE value.

In this PhD project, you will explore the bandgap tuning of NFA based organic solar cells to maximise the spectral overlap of their absorption with the emission spectra of indoor light sources. You will also explore bulk and buried interface engineering to reduce the recombination losses, maximise the carrier extraction and push the PCE of indoor OPVs closer to its theoretically predicted value. The kinetics of photogenerated carriers will be studied using time-resolved photoluminescence (TR-PL) with measurements of transient photovoltage (TPV), transient current (TPC) and impedance spectroscopy measurements to further understand device operation and guide the improvement of low-intensity indoor light harvesting. You will have the opportunity to conduct the proposed research in a very supportive research environment with access to a breadth of advanced device fabrication and characterisation facilities.

Energy Harvesting Research Group