A new approach to building quantum dots has shown considerable gains in generating photocurrent. This could potentially create a new generation of solar cells. In a new study, a team from the Center for Advanced Solar Photophysics (CASP) at Los Alamos National Laboratory has demonstrated that nano-engineered quantum dots can significantly increase charge carrier multiplication — in which a single photon excites multiple electrons and holes — compared to conventional quantum dots. While conventional quantum dots are made of lead selenide (PbSe), the CASP team’s nano-engineered quantum dots are made of a PbSe core and a thick cadmium selenide (CdSe) shell. This combination has allowed the researchers to better understand the effect of slowed carrier cooling. This illustration shows core/shell PbSe/CdSe quantum dots (a) and a carrier multiplication pathway (b) in nanostructures. Courtesy of CASP/Los Alamos National Laboratory. “Typical solar cells absorb a wide portion of the solar spectrum, but because of the rapid cooling of energetic, or hot, charge carriers, the extra energy of blue and ultraviolet solar photons is wasted in producing heat,” said CASP director Dr. Victor Klimov. “This lost energy can be recovered by converting it into additional photocurrent via carrier multiplication.” With conventional quantum dots, carrier multiplication is not efficient enough to boost the power output of practical devices. “A striking feature of the thick-shell PbSe/CdSe quantum dots is fairly bright visible emission, from the shell, observed simultaneously with the infrared emission from the core,” said Qianglu Lin, a CASP student working on the study. “This shows that intraband cooling is slowed down dramatically, so that holes reside in the shell long enough to produce emission.” The researchers also plan to study the concept of carrier multiplication engineering through control of intraband cooling. “Further enhancement in carrier multiplication should be possible by combining this new approach with other demonstrated means for increasing multicarrier yields, such as by using shape control (as in nanorods) and/or materials in which cooling is already naturally slower,” said Dr. Jeff Pietryga, lead chemist at CASP. “Applied together, these strategies might provide a practical route to nanostructures exhibiting carrier multiplication performance approaching the limits imposed by energy conservation.” The research was published in Nature Communications (doi: 10.1038/ncomms5148). For more information, visit casp.lanl.gov.