A Conversation with Michael Grätzel

A Conversation with Michael Gra ẗzel D uring the recent HOPV17 conference in Lausanne, Switzerland, and SISF17 conference in Seoul, Korea, we had the opportunity to meet with Prof. Michael Graẗzel and discuss scientific advances in solar cell research (see Figure 1).Prof. Graẗzel has been a visionary in advancing the field of dye-sensitized solar cells and perovskite solar cells.The conversation we had with Prof. Graẗzel is summarized here.EL (ACS Energy Letters).For many years you were involved in investigating micellar and colloidal systems.How did this research motivate you to become involved in solar energy conversion?Graẗzel.The reason why we started studying the micellar systems is because we were interested in natural photosynthesis and micellar systems are the simplest mimics of membrane-type organizational architectures.So, we were in fact studying the primary event of photosynthesis, that is, photoinduced charge separation, in these micellar systems.From there on, we turned to colloidal semiconductors.We were actually the first to make colloidal semiconductors.And the reason why we changed from micelles to inorganic colloids was mainly stability and ease of preparation of these systems.So, that is how we turned to the colloidal semiconductors.EL.What led you to the design of dye-sensitized solar cells (DSSCs) using mesoscopic TiO 2 f ilms?Graẗzel.Well, we were working on sensitizing colloidal TiO 2 because the material itself has only a UV response to sunlight, and so, a lot of people were trying to move the response into the visible, but that had been without success.The dye-sensitization offered a way to shift the light response of the TiO 2 into the visible.While carrying out these curiosity-driven fundamental studies, we found that the injection of an electron in the colloidal particles was much faster, 9 orders of magnitude faster, than the recombination process.So, it looked like a very good system for light-induced charge separation.In parallel, we were working on mesoscopic TiO 2 films; so the two concepts were united when I