Why you need to have a different design for solar cells that are used indoors?

Paul berger

Author: Professor Paul Berger

As the Internet of Things (IoT), focused toward indoor applications, evolves and expands, the importance of energy harvesting to power its nodes has to be emphasized. The energy from heat, vibration, and light, extracted from thermoelectric generators, piezoelectric generators, and photovoltaic cells respectively, can all be used. But light is one of the most efficient and ubiquitous among these energy harvesting sources [1-2].

Our vision of these IoT nodes will need to be printable and resemble a postage stamp, costing a microcent, and including computing, memory, communications and sensors. Each organic node will contain no toxic batteries, but instead be self-powered by energy scavenging from the environment (light, heat, motion) and storage of energy locally into a non-toxic supercapacitor.

Organic photovoltaics are considered the most plausible candidate as a light harvesting source for IoT nodes owing to the abundance of visible light, the optoelectronic characteristics of organic semiconductors and their scalability to affordable printing. Under indoor lighting, light intensities for photovoltaic energy conversion are typically in the range of 0.1-10 Wm-2, much lower than those of AM0 or AM 1.5 [3]. According to the simulation result by Freuneck et al. [4], the optimum bandgap of single-junction photovoltaic cells for energy-efficient indoor light sources is in the range of 1.5-2.0 eV. Organic semiconductors used in organic photovoltaics have bandgaps in this range, and these are more air-stable for OPV longevity. Furthermore, mass-production capability by roll-to-roll (R2R) process and flexibility suggests OPV as a promising niche opportunity for IoT.

The two Achilles heels hurled at OPV are their inability to harvest the longer wavelengths of the sun without moving to less stable conjugated systems, and lifetime for outdoor installations pales in comparison to conventional silicon PV. However, OPV is poised to be well-suited for indoor IoT applications, especially on disposable household goods, where shelf-lives are limited. Also the possibility to pattern the devices into decorative functional elements by printing is a significant advantage for indoor applications.

paulbergerProf. Berger has a 30+ year history of inorganic materials and novel devices, for which he was recognized as an IEEE Fellow. Prof. Berger, additionally entered the world of organic semiconductor devices back in 1999, during his first sabbatical leave to the Max Planck Institute for Polymer Research in Mainz, Germany, followed by a longer stay with Cambridge Display Technology (CDT) under their CTO, Jeremy Burroughes. In collaboration with Finland, Berger is working closely with Prof. Don Lupo’s group at Tampere University of Technology (TUT), where Berger now is a Distinguished Visiting Professor on a year-long sabbatical leave (2015-2016), to build out this vision together using TUT’s extensive organic printing infrastructure.


  1. “You can’t avoid the ‘Internet of things’ hype, so you might as well understand it”: http://www.digitaltrends.com/home/heck-internet-things-dont-yet/#!SWd96
  2. “Internet of Things IS Poised to Change Everything, Says IDC”, http://www.idc.com/getdoc.jsp?containerId=prUS24366813, 3. Oct. 2013.
  3. R. Steim, T. Ameri, P. Schilinsky, C. Waldauf, G. Dennler, M. Scharber, and C. J. Brabec, Sol. Ener. Mater. & Sol. Cells, 95 (2011) p. 3256-3261.
  4. M. Freunek, M. Freunek, and L. M. Reindl, IEEE J. of Photovol. 3 (2013) p. 59-64.