An all-weather solar cell that can harvest energy from sunlight and rain Qunwei Tang a,n , Hongna Zhang a , Benlin He a , Peizhi Yang b,n a Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR China b Key Laboratory of Advanced Technique & Preparation for Renewable Energy Materials, Ministry of Education, Yunnan Normal University, Kunming 650500, PR China article info Article history: Received 20 June 2016 Received in revised form 28 July 2016 Accepted 8 September 2016 Keywords: All-weather solar cell Conducting composite Charge transfer Energy conversion abstract Future solar cells may produce electricity in all weathers. We present here a cost-effective and high- efficiency graphene based conducting composite tailored all-weather solar cell that can be actuated with raindrops and sunlight. Due to the formation of π-electron|cation double-layer pseudocapacitance at conducting composite/raindrop interface, the delocalized π-electrons on graphene can migrate along percolating pathways forward at spreading process and backward at shrinking periods, resulting in charging and discharging of the pseudocapacitor. The optimized solar cell yields a photoelectric con- version efficiency of 9.8% under simulated sunlight irradiation (air mass 1.5, 100 mW cm 2 ) as well as current over several microamps and voltage of hundreds of microvolts under simulated raindrops. This work could extend our knowledge of future all-weather solar cells. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction The energy-harvesting systems with unprecedented perfor- mances call for rational design of advanced devices in transporting charges [1–6]. One of the solutions for this is to realize energy conversion from nature to electricity. Till now, many prestigious devices have been successfully developed to utilize solar energy [7–11], chemical energy [12], wind energy, tidal energy, etc. All the systems for energy harvesting have limitations in either operation time or applicable region or fabrication cost. Consequently, the electricity generation devices regardless of these limitations seem to be crucial to meet the growing energy demand. All-weather solar cell featured with cost-effectiveness and high-efficiency is promising to break this impasse. To address this issue, we present here a preliminary study on all-weather solar cells that can harvest energy under either sunlight irradiation or raindrop dripping by integrating graphene based composite film with a solar cell. Recent work has carefully demonstrated that the cations from ionic liquids can be adsorbed onto graphene surface to form π- electron|cation electrical double-layer (EDL) at ionic liquid/gra- phene interface [13], arising from Lewis acid-base interaction [14,15]. When dragging a droplet of ionic liquid over monolayer graphene, an EDL pseudocapacitor is created at forward droplet/ graphene interface and is subsequently decomposed at backward droplet/graphene interface. Raindrops are not pure water, they contain gigantic salts that split up into positive (Na þ , NH 4 þ , Ca 2 þ , Mg 2 þ , etc) and negative (Cl  , NO 3  , SO 4 2  , etc) ions. The rain- drops suffer expanding and subsequent shrinking processes when dropping from cloud layer to solar panels, charging and dischar- ging the π-electron|cation EDL pseudocapacitance for current and potential outputs [13,16]. Similarly, the raindrops spread quickly to periphery, forming EDL pseudocapacitance at raindrop/graphene interface and dragging electron migration and charging at the front of the raindrops. The raindrops can subsequently shrink, releasing electrons to graphene and discharging the pseudocapa- citance. The repeated charging/discharging processes yield per- sistent electric signals including current and voltage. Using the peculiar performance of graphene, we have recently fabricated an all-weather solar cell by integrating reduce graphene oxide film with a solar cell on flexible substrate [17], producing a photo- electric conversion efficiency of 6.53% under simulated sunlight irradiation (AM1.5) as well as maximal current of  0.50 μA and voltage of  150 μV for each simulated raindrop. However, this strategy remains a problem that it is not applicable for high-effi- ciency all-weather solar cells with conductive glass. Moreover, the film-forming technique of graphene is still a challenge for its large-scale applications. To reduce graphene dosage and to in- crease film-forming ability, we have combined electron-conduct- ing graphene and carbon black with insulating polytetra- fluorethylene (PTFE) for a G-CB/PTFE conducting composite. Car- bon black is beneficial to increase compatibility of graphene and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nanoen Nano Energy http://dx.doi.org/10.1016/j.nanoen.2016.09.014 2211-2855/& 2016 Elsevier Ltd. All rights reserved. n Corresponding authors. E-mail addresses: tangqunwei@ouc.edu.cn (Q. Tang), pzhyang@hotmail.com (P. Yang). Please cite this article as: Q. Tang, et al., An all-weather solar cell that can harvest energy from sunlight and rain, Nano Energy (2016), http://dx.doi.org/10.1016/j.nanoen.2016.09.014i Nano Energy ∎ (∎∎∎∎) ∎∎∎–∎∎∎