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

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 simulated indoor light the system yielded an ECSE of 2.9%, while delivering 13.3 mJ and 2.8 mW. Those energy and power levels would be sufficient to power low-consumption electronic devices with low duty cycles. (C) 2016 Elsevier Ltd. All rights reserved.