The researchers detailed their approach in a paper titled “Silicon Nanowire/Polymer Hybrid Solar Cell-Supercapacitor: A Self-Charging Power Unit with a Total Efficiency of 10.5%” published in Nano Letters.
First, they built a hybrid silicon nanowire/polymer heterojunction solar cell consisting of highly-ordered vertically aligned Si nanowires (about 250nm in diameter at an approximate pitch of 360nm in a hexagonal array) spin-coated with highly conductive PEDOT:PSS and later covered with a 200nm-thick silver finger grid as the front electrode while a 200nm-thick layer of aluminium served as a rear contact.
One thing the researchers documented was the use of a low temperature, chemical solution process to reduce the nanowire surface roughness, suppressing the charge recombination at the SiNW/polymer interface and enhancing the solar cell’s power conversion efficiency to 13.39%.
They then built a polypyrrole-based supercapacitor affixed to the back of the solar cell. To do so, a titanium film was thermally deposited on the back side of the Si substrate, acting both as the rear electrode of the hybrid solar cell and one of the electrodes of the supercapacitor. Then polypyrrole films were electrochemically deposited on the titanium film to form a symmetric supercapacitor (using a porous polyethylene membrane as a separator and H3PO4/PVA (poly(vinyl alcohol)) gel as the electrolyte.
Thanks to the efficiency enhancement of the hybrid nanowire solar cells and the dual-functional titanium film serving as conjunct electrode of the solar cell and supercapacitor, the integrated system was able to yield a total photoelectric conversion to storage efficiency of 10.5%, a record according to the researchers.
In such a configuration, the energy-harvesting system could serve as a buffer diminishing the solar power fluctuations from light intensity while being self-charging. The researchers also implemented a flexible self-charging power unit based on ultrathin flexible Si substrates (etched down to 25μm).
The flexible integrated device was able to charge a 680mF supercapacitor to 0.52V in 78 seconds. The ultrathin solar cell (with a lower performance than that based on a bulk silicon wafer) and the large capacity of the supercapacitor explained the long charging time, yielding a total conversion and storage efficiency of 5.68%. But the flexible device remained at a stable voltage of about 0.40V when discharged at 5mA/cm2 with the solar cell under sustainably light illumination, proving that such a flexible integrated system could constitute a continuous stable output power source.