The Sun, a near perfect spherical ball of hot plasma, formed some 4.5 billion years ago and from its birth the Earth and our Solar System were created. Without nuclear fusion occurring in the belly of our star life would cease to exist. Everything we do is controlled by the sun and depends on it. For millions of years gazing upon the sun was as close as it got to staring into the face of God. Deep in its core the fusion of hydrogen atomic nuclei forms helium which powers the star’s furnace. Through this reaction millions of tons of matter are converted into energy every second of every day. We feel this energy in the form of heat and see it as light. This turbulent sea of incandescent gas is over 150 million kilometers from the earth and its energy takes 8 minutes to reach us. It accounts for over 99% of the total matter of our entire solar system and is by far the most important source of energy for life on Earth.

Mankind, through science, has learned to harness the energy of the sun by converting solar radiation into electricity to power whole cities, heat houses and drive vehicles. Korean scientists from the Institute for Basic Science (IBS) and the Korea Institute of Science and Technology (KIST) have demonstrated an efficient method of converting this energy into electricity. The Center for Nanoparticle Research within IBS, headed by Director Taeghwan Hyeon, and Min Jae Ko from KIST have reported copper indium selenide (CISe) quantum dot (QD)-sensitized solar cell {QDSC} which was greatly enhanced by optimizing the thickness of zinc sulfide (ZnS) on the quantum dots sensitized titanium dioxide electrodes. ZnS is a typical wide band gap material and reduces carrier recombination that occurs in QDSC. Consequently the team’s best results yielded a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal free QDs solar cells.

Previous to the team’s finding, most of the highly efficient QD solar cells have employed a toxic heavy metal compound of cadmium and lead chalcogenide as a photosensitizer. By switching to the less toxic metal free compound of CISe, it could be possible to fabricate the QDSC that satisfies both the high conversion efficiency and environmental friendliness. The conversion efficiency of the produced QDSC is enhanced by about 40% when the thickness of the ZnS overlayer is increased to twice that of a conventional one.

According to the team’s paper they state “the results exhibit a great advancement in technology and facilitate important insight into the development of highly efficient heavy-metal-free QD solar cells.” The paper, entitled, Highly Efficient Copper Indium Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers was published on the 2nd of October on the online issue of ACS Nano.

Figure 1:(L-R) Quantum dot solar cells using CISe compound nano particles. Zn and S EPMA images of the CISe-sensitized TiO2 electrodes with respect to the number of ZnS SILAR cycles. Recombination resistance of the CISe-sensitized TiO2 electrodes with respect to the number of ZnS SILAR cycles, evaluated from impedance spectra taken in the dark state.

Figure 2: (a,b) TEM images and (c) absorption spectrum of 4 nmCISe QDs dispersed in dichloromethane. The inset of (c) shows the extrapolated plot of (Rhν)2 vs hν obtained from the absorption spectrum and used to obtain the CISe QD band gap.

Figure 3: (a) JV characteristics under illumination (one sun light intensity with AM 1.5G filter, measured under reverse scan condition) and (b) EQE values (ηQ) for CISe QDSCs with respect to the number of ZnS SILAR cycles. (c) Extrapolated plot of [hν ln(1 ηQ)]2 vs hν from the EQE spectrum, used to obtain the optical band gap of the CISe QDs.

By Neil Mannix

Notes for editors

- References
-Jae-Yup Kim, Jiwoong Yang Jung Ho Yu, Woonhyuk Baek, Chul-Ho Lee, Hae Jung Son, Taeghwan Hyeon, and Min Jae Ko

- Title of Paper
-Highly Efficient Copper Indium Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers
-DOI: acsnano.5b04917

- Media Contact
-For further information or to request media assistance, please contact:
Mr. Shi Bo Shim, Head of Department of Communications, Institute for Basic Science (+82-42-878-8189; sibo@ibs.re.kr) or
Ms. Sunny Kim, Department of Communications, Institute for Basic Science (+82-42-878-8135; sunnykim@ibs.re.kr)

- About the Institute for Basic Science (IBS)
IBS was founded in 2011 by the government of the Republic of Korea with the sole purpose of driving forward the development of basic science in Korea It comprises a total of 50 research centers in all fields of basic science, including mathematics, physics, chemistry, life science, earth science and interdisciplinary science. IBS has launched 25 research centers as of October 2015.There are eight physics, one mathematics, six chemistry, eight life science, and two interdisciplinary research centers.