Technology Review – Polymer solar cells are inefficient compared to silicon solar cells, but they are much cheaper to make. Organic materials—whether made of polymers or so-called “small molecules,” which are organic compounds with a low molecular weight—can be made into inks and printed over large areas. They’re also lightweight and flexible, which makes them promising for applications like rooftop installations or solar-cell patches for charging portable electronics.
Using a new small molecule designed by Bazan, Heeger built a solar cell that converts 6.7 percent of the light energy that strikes it into electricity. Bazan expects to reach 9 percent efficiency within a year. Although efficiencies in lab tests tend to be much greater than those in a manufactured cell, this would put these materials on par with the best polymer solar cells on the market.
Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6–8% range for conjugated polymer/fullerene blends. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh2)2. A record PCE of 6.7% under AM 1.5 G irradiation (100 mW cm−2) is achieved for small-molecule BHJ devices from DTS(PTTh2)2:PC70BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25% v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.
It may be hard for organic solar cells to become real contenders in the energy market, especially when silicon cells are getting cheaper. “The performance and lifetimes are not there yet,” says Yang Yang, professor of materials science and engineering at the University of California, Los Angeles. Yang is working on polymer solar materials at his company, Solarmer, as well as small-molecule solar cells in his academic lab; his goal is 15 percent efficiency in a lab-made cell. But Yang says the Santa Barbara work is an important demonstration of the potential of small-molecule solar.