Organic solar cells and fully printed super-capacitors optimized for indoor light energy harvesting

Abstract

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

Dye-sensitized solar cells for efficient power generation under ambient lighting

“…Given that the spectral region in which the DSCs absorb light is limited to the visible part of the solar spectrum, it is expected that their efficiency in the visible region (400-700 nm) is twice the fullspectrum value 20 . Taking into consideration that the indoor-light sources emit mostly visible light, a DSC with an 11.3% PCE under full solar illumination is expected to have an efficiency over 20% in its particular spectrum of operation indoors.…”

“…They show an outstanding performance under indoor, conditions with an artificial light source in comparison with other solar cell technologies 14 . The unique properties of DSCs make them the best alternative to wired and battery energy sources, as DSCs are capable of maintaining a high photovoltage even in diffuse light conditions [15][16][17][18][19][20] .…”

Theoretical analysis and characterization of the energy conversion and storage efficiency of photo-supercapacitors

“…[9]. ITO-covered glass (from Thin Film Devices) were sequentially cleaned in acetone, isopropanol and water, then passed under UV-ozone plasma treatment for 30 min.…”

“…PSC can be fabricated as a single stack where the generation and storage components share a common electrode, which considerably reduces resistive losses and minimizes footprint [7]. They can also be fabricated in specific formats, like a fiber [8], or using low-cost solution processing techniques compatible with flexible substrates [1,2,9].…”

“…The duration of the charging phase and the incident light power influence the measured ECSE value of a PSC: as the supercapacitor is charging, its voltage increases, which modifies the output power of the solar cell over time [9,12]. For a given light power, each system thus displays a maximum ECSE value.…”