Flexibility, lightness and printability make organic solar cells (OSC) strong candidates to power low consumption devices such as envisioned for the Internet of Things. Such devices may be placed indoors, where light levels are well below typical outdoors level. Here, we demonstrate that maximizing the efficiency of OSC for indoor operation requires specific device optimization. In particular, minimizing the dark current of the solar cells is critical to enhance their efficiency under indoor light. Cells optimized for sunlight reach 6.2 % power conversion efficiency (PCE). However when measured under simulated indoor light conditions, the PCE is to 5.2 %. Cells optimized for indoor operation yield 7.6 % of PCE under indoor conditions. As a proof-of-concept, the solar cells are combined with fully printed super-capacitors to form a photo-rechargeable system. Such a system with a 0.475 cm 2 indoor-optimized solar cell achieved a total energy conversion and storage efficiency (ECSE) of 1.57 % under 1-sun, providing 26 mJ of energy and 4.1 mW of maximum power. Under

No abstract

As industrial and commercial demand for thin, small-footprint energy storage devices increases (e.g., for wearable electronics, wireless sensor networks, etc.), it is essential to develop theoretical descriptions for those devices in order to make effective design choices. Of particular interest are porous interdigitated electrodes, which can function as supercapacitors but which are nontrivial to analyze due to their unique geometry. In this paper, we develop a purely mathematical model to determine the dependence of a cell's capacitance and/or resistance on electrode height, width, and spacing, and electrolyte layer height. We then extend this model to incorporate effects from the porous nature of the electrodes, and show that in many cases of interest, the system can be described using a simple equivalent circuit.

No abstract

A NEW PDSA PetAid hospital will open its doors later this year in Kirkdale, Liverpool. In designing the building, the PDSA worked closely with architects to ensure that the new hospital was as environmentally friendly as possible. Tony Keith and Martin Cowell outline the considerations when planning the hospital, and suggest ways in which any practice can be made more energy efficient and sustainable.