Quantum dot sensitized solar cells use small inorganic quantum dot absorbers, or ultrathin absorber layers, on a nanostructured TiO2 electron collector framework, and the hole extraction from the absorber is realized by a liquid electrolyte or solid hole conducting material. Some years ago, efficiencies and stabilities of this type of solar cell were really negligible. But recently, a steady raise of the results shows that these solar cells should be taken very seriously. The QDSSC have the credit that the perovskite solar cells started in this version, in the work of Miyasaka and coworkers in 2009. But apart from the great consequences of that discovery, QDSSC show other developments that are very interesting on their own.
In 2013 Xinhua Zhong and coworkers presented in JACS a CdTe/CdSe QD-based solar cell exhibiting a record PCE of 6.76%. The key advancement of this result was the core-shell structure of the QD provoking a charge transfer or excitonic absorption that extends light harvesting up to 900 nm. A new result published this week by the same group uses CIS (CuInS2), a famous photovoltaic material, to make QDs for sensitization. Using a core-shell CIS-ZnS structure, adequate passivation reduces recombination and leads to the development of high efficiency “green” solar cell of 7.04% efficiency (with certified efficiency of 6.66%), which greatly surpasses the previous best “green” QDSSC of 2.5% efficiency. This achievement shows that of Cd and Pb free QDs can provide similar or better results than their more toxic and widely used counterparts.
Another material showing great progress is the QDs of Sb2S3. Pioneered by Gerardo Larramona in IMRA, France, Sb2S3 Qds have been used for some years to build all solid QDSSC. Typical results of Pablo Boix, Larramona and coworkers in 2012 was 3.20% efficiency. This was increased to 5.7% by Seigo Ito in 2013, and now a series of result by the group of Sang Il Seok shows impressive power conversion efficiencies exceeding 7%. Again the key to these results is the use of graded or core shell QD structures, enhancing the light absoroption range of the QDs, as well as effective passivation treatments that reduce recombination.
The next great opportunity to monitor the developments of this field is the nanoGe conference on Solution Processed Semiconductor Solar Cells, from 10 to 12 September 2014, in Oxford, organized by Henry Snaith.
Jin Wang, Iván Mora-Seró, Zhenxiao Pan, Ke Zhao, Hua Zhang, Yaoyu Feng, Guang Yang, Xinhua Zhong, and Juan Bisquert, Core/Shell Colloidal Quantum Dot Exciplex States for the Development of Highly Efficient Quantum-Dot-Sensitized Solar Cells, J. Am. Chem. Soc., 2013, 135, pp 15913–15922
Zhenxiao Pan, Ivan Mora-Sero, Qing Shen, Hua Zhang, Yan Li, Ke Zhao, Jin Wang, Xinhua Zhong, and Juan Bisquert, High Efficiency “Green” Quantum Dot Solar Cells, Journal of the American Chemical Society, 2014, DOI: 10.1021/ja504310w
P.P. Boix, G. Larramona, A. Jacob, B. Delatouche, I. Mora-Seró, J. Bisquert: Hole transport and recombination in all-solid Sb2S3-sensitized TiO2 solar cells using CuSCN as hole transporter J Phys Chem C, 116 (2012), pp. 1579–1587
Seigo Ito, Kazuki Tsujimoto, Duy-Cuong Nguyen, Kyohei Manabe, Hitoshi Nishino, Doping effects in Sb2S3 absorber for full-inorganic printed solar cells with 5.7% conversion efficiency, International Journal of Hydrogen Energy, Volume 38, Issue 36, 2013, Pages 16749-16754,
Efficient Inorganic-Organic Heterojunction Solar Cells Employing Sb2(Sx/Se1-x)3 Graded-Composition Sensitizers, Yong Chan Choi , Yong Hui Lee , Sang Hyuk Im , Jun Hong Noh , Tarak Nath Mandal , Woon Seok Yang , and Sang Il Seok, Advanced Energy Materials, 2014, DOI: 10.1002/aenm.201301680
Highly Improved Sb2S3 Sensitized-Inorganic–Organic Heterojunction Solar Cells and Quantification of Traps by Deep-Level Transient Spectroscopy. Yong Chan Choi , Dong Uk Lee , Jun Hong Noh , Eun Kyu Kim , and Sang Il Seok, Advanced Functional Materials, 2014, DOI: 10.1002/adfm.201304238