In the latest of our author commentaries shortly to appear in the special 10th Anniversary Edition of Advanced Functional Materials, Dr. Tsukasa Yoshida of Gifu University, Japan, provides a personal perspective for his Feature Article on hybrid thin films.
Electrodeposition of Inorganic / Organic Hybrid Thin Films
By Tsukasa Yoshida, Jingbo Zhang, Daisuke Komatsu, Seiichi Sawatani, Hideki Minoura, Thierry Pauporté, Daniel Lincot, Torsten Oekermann, Derck Schlettwein, Hirokazu Tada, Dieter Wöhrle, Kazumasa Funabiki, Masaki Matsui, Hidetoshi Miura and Hisao Yanagi
I am extremely pleased to have this article been selected for the reproduction in the 10 years anniversary issue of AFM, since this article is undoubtedly the most important one from my past research work.
The discovery of the phenomenon that we call “electrochemical self-assembly (ESA)” today was a product of serendipity, or better say, of accident. When I started my university carrier in 1995, I wished to combine studies on chemical/electrochemical deposition of compound semiconductor thin films that I started in the group of Prof. Hideki Minoura and functional molecules that I learned during my PhD thesis.
Dye-sensitized solar cell (DSSC) was exactly such a system in which combination of inorganic semiconductor and organic photosensitizer was crucial. Together with my student, Mr. Miyamoto, I was struggling to adsorb sulfonated phthalocyanines that I received from Prof. Dieter Wöhrle to ZnO films electrodeposited from aqueous Zn(NO3)2 solutions. As they were so nicely crystallized, they had too small surface area to adsorb the dyes. When we were almost giving up, I told to Mr. Miyamoto, “Just put some dyes to the deposition bath!” as the last trial. Then, he came back with a deep blue colored film that obviously contained a lot of phthalocyanines but the film was still made of crystalline ZnO as checked by XRD. Together with Prof. Derck Schlettwein who is a long lasting collaborator and a good friend of mine, photoelectrochemical properties were studied. A few μA of photocurrent was measured on illumination to confirm photosensitization by the loaded phthalocyanines. That was the beginning of the study. We were very curious about the mechanism of hybridization and how adsorptive organic molecule modifies the crystal growth of ZnO. But we could never think at that time, that we would continue the work for such a long time and it would become something really useful for solar cells.
Important event for progress of the study was the encounter with Prof. Daniel Lincot in 1998 for the occasion of ECS/ISE conference in Paris. I was learning a lot from his publications on solution phase deposition of compound thin films. Later in 1999, I visited his institute to present the work on hybrid electrodeposition. He and his colleagues found great interest in the work and we agreed to work together. In 2000, I spent a few weeks in ENSCP that resulted in many important discoveries. Together with Dr. Thierry Pauporté, we added eosinY to ZnCl2 bath with dissolved O2 that still is the standard composition use today. We found deeply red colored and highly transparent ZnO/eosinY hybrid thin films. Photocurrent under illumination jumped up to a few mA, making us to think “This may become something really useful!”
Another important event was to be selected for industrial R&D program from NEDO with the title “Development of Solar Cell Painting for Electric Vehicles” in 2001 that boosted the research. The idea was to develop colorful, flexible and light weight solar cell that is perfect for body coating for EVs. Having discovered that eosinY loaded during electrodeposition was aggregated but could be removed to obtain porous electrode to which any sensitizers could be re-adsorbed, we succeeded in making solar cells with efficiencies close to 3% under AM 1.5. Unfortunately, we still could not make a solar powered EV, but demonstrated a toy car driven by our solar cell that appealed to NEDO for extension of the program! During this 5 years program, I also encountered many people from industries. Through working with these people, I further developed a mind to contribute to industrial research with this technology. “Universal Solar Cell Consortium” was founded in 2004 together with Sekisui Jushi Corporation and GUNZE Ltd. to promote application studies.
Following this experience, we are now forming a group to take part in Japan’s national project on PV systems. Having had strong partners for development of photosensitizer dyes for ZnO, Prof. Masaki Matsui, Prof. Kazumasa Funabiki of Gifu University, Dr. Hidetoshi Miura of Chemicrea Inc., the highest efficiency has been boosted up to 6.5%. The group of Sekisui Jushi guided by Mr. Yoshiya Fujishita achieved 5.8% efficiency for 10 × 10 cm plastic solar cell sub-module and confirmed 3 years life in outdoor use. Although these numberes are still to be improved, the joint efforts are continuing to realize light weight and low cost plastic solar cells for versatile installations.
Use of the technology for solar cells has always been the main driving force of the research. However, efforts have also been put to extend the idea of ESA. Cathodic formation of p-CuSCN/dye hybrid and anodic titanate/quinone hybrid described in the article can be seen as counter parts of n-ZnO/dye system. EL devices using rare earth metal complexes instead of dye is a new direction of applications and that is further developed together with Prof. Hisao Yanagi and Prof. Aishi Yamamoto today. As I tried to emphasize in the article, self-assembly of inorganic/organic hybrid materials offers literally “unlimited” possibilities in seeking of new materials, most importantly, without consuming high energy in the process. It is also a very exciting area of science as it finds a close relevance to biomineralization that is essential in development of living bodies. I hope this article could be something instructive not only to the people looking for new materials in solar cells but also for those working on broader kind of materials combination and applications.
On this occasion, I looked back the story behind this article and tried to indicate names of important collaborators and how they got involved. I also wished their names to be indicated as co-authors of this article. Naturally, I could count many more names of contributors from the last 15 years research, who unfortunately could not be indicated here. Without their input, none of these things could have been realized. Working with them made my life so fruitful, exciting and happy! I wish to thank them all. I also wish to thank Dr. Karen Grieve, then deputy editor of AFM, who originally suggested me to write a feature article and was so patient to wait for the finish of the article as it took nearly 3 years! Last but not least, I thank my wife Dr. Ruth Wahlster-Yoshida from the bottom of my heart for her encouragement and support.