In the Aseanian Conference of dye-sensitized solar cells and organic solar cells, celebrated this week in Taipei, a number of important recent developments have been reported. The dye-sensitized solar cell, also known as Grätzel cell, in the most widely used and efficient configuration, is formed by nanostructured TiO2 as electron conductor, and organic dye as light absorber, and a redox electrolyte. It is believed that the liquid part is a weakness of the device, towards the realization of long lasting configurations for efficient conversion of solar light to electricity in the rooftop.
For many years there has been a search for a full solid version of the Grätzel cell, in which a solid absorber replaces the dye, and an organic or inorganic solid conductor makes the role of the liquid electrolyte. The first suggestions in this direction came in late 1990s from Rolf Könenkamp, then at Hahn-Meitner Institut Berlin, and it was knwon as the Extremelly Thin Absorber solar cell. A few years later Gerardo Larramona, working at IMRA Europe, a japanese-owned company at Nice, France, first tried the Sb2S3 solid absorber, with promising results, in combination with inorganic p-type semiconductor CuSCN. Althouh this team reported 4% in a meeting in Jerusalem, they did not succeed to obtain high efficiencies. Nonetheless, it has been recognized that such class of solid absorbers have very large absorption (extinction) coefficient and are likely candidates for complete solid cells of sensitized TiO2. Using a variety of hole transporters, such as OMETAD and P3HT, the antimonium cell was improved to consistent efficiencies above 5% by work of the teams of Gary Hodes, Michael Grätzel and Sang Il Seok . This type of work has been, furthermore, complementary and strongly connected to the application of quantum dots as absorbers for nanostructured TiO2 solar cells, pioneered by Nozik, Zaban, Kamat, and many others. These concepts have been summarized in review articles by Prashant Kamat, Thomas Dittrich et al, Arie Zaban et al, and Ivan Mora-Sero et al.
In 2009, a paper by Tom Miyasaka and coworkers showed 3.8% efficiency with a (CH3NH3)PbI3 perovskite compound. This result was very promising, and at this time, Nam-Gyu Park, a well known DSC scientist from South Korea that happened to have had perovskites as the subject of his pHD thesis, decided to develop this system. Recently in cooperation with M. Grätzel, Park and their coworkers presented in Nature a 9.7% efficiency based on the (CH3NH3)PbI3 absorber. Meanwhile Miyasaka joined forces with Henry Snaith, a young but tough-pushing DSC scientist at Oxford, and they reported a 10.7 % efficiency based on the perovskite, but without TiO2! It seems that the absorber perovskite layer on an insulating scaffold is sufficient to provide this power conversion efficiency without need for additional electron transport material. In fact, recently Lioz Etgar, Michael Grätzel, and their coworkers, showed that the (CH3NH3)PbI3 perovskite works well simply sandwiched between TiO2 and gold contacts, that is, without a specific hole conductor. Another major development is the report in Nature of a cell of CsSnI 2.95F0.05 exceeding 10% efficiency by a group of Northwestern University led by Mercouri Kanatzidis.
These developments were presented at the Aseanian Conference in Taiwan, where Seigo Ito reported for the first time a 5% efficiency in the Sb2S3 cell with the hole conductor CuSCN. It seems that solid cells, especially those based on perovskites, offer a great potential for further development. However some participants in the Conference expressed concerns about reproducibility and stability of some of the reported devices. As in the past, we have to wait some time to see that other groups are able to reproduce these conversion efficiencies with the same materials, so that the results are robust and do not demand excepcional conditions of preparation, which is an important requisite for photovoltaic devices, that ultimately should be produced on a large scale.
Nevertheless it is clear that all solid Semiconductor Sensitized solar cells have experienced an impresive increase of efficiencies that follows the pace of all organic solid cells (see the status in may 2012 here). While the liquid cell has increased recently to 12.3% at 1 sun with the porphyrin dye and cobalt redox couple in the work of Eric Diau, Chen-Yu Ye, Michael Grätzel, and their coworkers, the solid cells with inorganic absorber seem to have entered an unstopable upward trend, as seen in the graph presented below.
The next opportunity to learn more of this topic is the 3rd International Conference on Semiconductor Sensitized and Quantum Dot Solar Cells, a nanoGe Conference which will take place from the 9th to the 11th of June 2013, in Granada, Spain .
“CdTe and CdS extremely thin absorber materials in An solar cell (ETA)”Siebentritt, S.; Ernst, K.; Fischer, C.-H.; Könenkamp, R.; Lux-Steiner, M. C.; 14Th European Photovoltaic Solar Energy Conference, 1997, Barcelona.,
Solid Nanostructured Solar cells made with nanocrystalline TiO2 films and absorber semiconductors, G. Larramona, A. Jacob, C. Choné, Nanoenergy Symposia : Semiconductor Sensitized Solar Cells Conference in Jerusalem, Israel, 2010, edited by A. Zaban (SEFIN, Castelló).
Hole Transport and Recombination in All-Solid Sb2S3-Sensitized TiO2 Solar Cells Using CuSCN As Hole Transporter, Pablo P. Boix, Gerardo Larramona, Alain Jacob, Bruno Delatouche, Ivan Mora-Sero, and Juan Bisquert, J. Phys. Chem. C (2011) 10.1021/jp210002c
High-Performance Nanostructured Inorganic−Organic Heterojunction Solar Cells. Chang, J. A.; Rhee, J. H.; Im, S. H.; Lee, Y. H.; Kim, H.-J.; Seok, S. I.; Nazeeruddin, M. K.; Grätzel, M., Nano Letters 2010, 10, 2609–2612.
From Flat to Nanostructured Photovoltaics: Balance between Thickness of the Absorber and Charge Screening in Sensitized Solar Cells, Pablo P. Boix, Yong Hui Lee, Francisco Fabregat-Santiago, Sang Hyuk Im, Ivan Mora-Sero, Juan Bisquert, and Sang Il Seok, acs Nano (2012) 10.1021/nn204382k
Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. Kamat, P. V. The Journal of Physical Chemistry C 2008, 112, 18737–18753.
“Concepts of inorganic solid-state nanostructured solar cells” Dittrich, T.; Belaidi, A.; Ennaoui, A. Solar Energy Materials and Solar Cells (2011), 10.1016/j.solmat.2010.1012.1034.
“Quantum-Dot-Sensitized Solar Cells “Rühle, S.; Shalom, M.; Zaban, A. . ChemPhysChem 2010, 11, 2290 – 2304.
Breakthroughs in the Development of Semiconductor-Sensitized Solar Cells Iván Mora-Seró and Juan Bisquert, J. Phys. Chem. Lett., 2010, 1, 3046–3052
Semiconductor Nanocrystal Quantum Dots as Solar Cell Components and Photosensitizers: Material, Charge Transfer, and Separation Aspects of Some Device Topologies. Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L., The Journal of Physical Chemistry Letters 2011, 2, 1879-1887.
Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells, Akihiro Kojima, Kenjiro Teshima, Yasuo Shirai, and Tsutomu Miyasaka J. Am. Chem. Soc.,(2009) 10.1021/ja809598r
Towards Printable Sensitized Mesoscopic Solar Cells: Light Harvesting Management with Thin TiO2 Films, T. Miyasaka, J. Phys. Chem. Lett. 2010, 2, 262–269.
Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%, Hui-Seon Kim, Chang-Ryul Lee, Jeong-Hyeok Im, Ki-Beom Lee, Thomas Moehl, Arianna Marchioro, Soo-Jin Moon, Robin Humphry-Baker,Jun-Ho Yum, Jacques E. Moser, Michael Grätzel and Nam-Gyu Park, Nature Scientific Reports, 2, 591 (2012) 10.1038/srep00591
Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites, Lee MM, Teuscher J, Miyasaka T, Murakami TN, Snaith HJ, Science (2012) Oct 4
All-solid-state dye-sensitized solar cells with high efficiency, In Chung, Byunghong Lee, Jiaqing He, Robert P. H. Chang and Mercouri G. Kanatzidis, Nature 485, 486–489 (2012)
“Porphyrin-Sensitized Solar Cells with Cobalt (II/III)Based Redox Electrolyte Exceed 12 Percent Efficiency” Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. . Science 2011, 334, 629-634.
“Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells” Lioz Etgar, Peng Gao, Zhaosheng Xue, Qin Peng, Aravind Kumar Chandiran, Bin Liu, Md. K. Nazeeruddin and Michael Grätzel, J. Am. Chem. Soc. 2012, 134, 17396−17399