This entry was written by Juan Bisquert and Emilio J. Juárez Pérez
It has been 4 years since Tom Miyasaka and collaborators first reported the use of halide perovskites as a light harvester material in hybrid solar cells (A. Kojima et al., J. Am. Chem. Soc., 2009, 131, 6050). This solar cell was originally intended as an alternative to binary lead chalcogenide based sensitized cells. In fact, this early design of the cell was interfaced with a liquid electrolyte heterojunction like in the classical dye sensitized Grätzel cell. The power conversion efficiency reached a 3.8 % value, which would be quite good for this kind of solar cell in those times, in which quantum dot sensitized solar cell remained at low efficiencies.
Three years after this finding, Nam-Gyu Park and collaborators made two consecutive crucial modifications in the original design (H.S. Kim et al., Sci. Rep. 2012, 2 , 591). They prepared a halide perovskite solution five-fold more concentrated than the original recipe and replaced the liquid hole conductor electrolyte by a solid phase one, the spiro-MeOTAD. The easy manufacture of the active material in this design, that is solution processable and can be synthesized using solutions of simple organic (CH3NH3)I and inorganic PbI2 salts, together with the great power conversion efficiency of 9.7% achieved by Park brought these cells into the scientific community spotlight. Recently Park summarized these developments (Nam-Gyu Park J. Phys. Chem. Lett. 2013 4, 2423-2429).
Currently, there are about 20 published papers dealing with the study of these devices using halide perovskite as light harvester. Over this short period of research time, the scientific experts in solar cells have applied different variations to build up the light active material. The most important developments have been to change the iodide perovskite by a mixed- or bromide based perovskite which develops higher open circuit voltages, to change the organic methylammonium by ethylammonium cation, to substitute spiro-OMeTAD by polymeric hole conductors, to use inert scaffolds for the perovskite like Al2O3 and even do not use them at all, as further explained below.
In the summer of 2013, two major reports have been published showing efficiencies higher than 15%, already raising the attention of photovoltaic industry and investors, but using quite different methods to deposit the halide perovskite.
First Michael Grätzel’s research group discovered (J. Burska et al. Nature, 2013, 499, 316–319) a sequential method that consists of first a stage of spin-coating a concentrated PbI2 solution in a mesoporous layer of TiO2, followed by a second stage of dip-coating the substrate in the organic salt solution of CH3NH3I. They observed that the conversion of PbI2 nanocrystals to CH3NH3PbI3 perovskite was completed in a time of seconds.
Henry Snaith had already shown that one can get rid of the mesostructured electron transporter, using alumnina (an insulator) instead of TiO2 as the nanostructure framework (Lee MM, et al., Science, 2012, 338, 643-647). Last week, he shows in Nature (M. Liu et al. Nature, 2013) how to synthesize the mixed halide perovskite in situ by vapor deposition in a vacuum chamber and overcome the use of a mesoporous layer to scaffold the perovskite. The dual-source evaporation method uses CH3NH3I and PbCl2 to form the mixed halide perovskite CH3NH3PbI3-xClx.
In spite of the different active layer deposition techniques, both methods are based on dual precursors, and both designs share the original strategy of capturing electron and holes in the corresponding electrodes by using electron (TiO2 compact layer) and hole (spiro-OMeTAD) selective layers. From the point of view of the manufacturing, the two-step Grätzel approach offers a solution processed, low temperature route that provides efficient use of the reagents and economically feasible techniques to produce cells at industrial scale. The advantage of the Snaith’s method is that the mesoporous layer is unnecessary, showing that the perovskite absorber can stand on its own as any other thin film solar cell. Furthermore he shows in the Nature paper that it is rather difficult to prepare high quality photovoltaic film of single perovskite phase using solution processed methods, due to defficient morphology.
Now there are many scientific teams looking for the highest efficiencies and novel cell architectures. Several groups are also elucidating the mechanisms of transport and accumulation of charges in this challenging two carrier system, and certainly new discoveries and solution to the dilemmas presented now, about the best way to go towards viable large scale production, will appear when the electronic functions are better understood. For the moment it has been shown that the perovskite displays an unprecedented amount of charge accumulation (H. S. Kim et al., Nature Communications 4, 2013, 2242, doi:10.1038/ncomms3242 ).
Thanks to the basic research of a japanese group about five years ago, and several discoveries of people that combined mastery of materials and photovoltaics, effort, wit, and sometimes good luck , an old known halide perovskite compound is waiting that countries and industry environmentally responsible and concerned with the use of renewable energy, willbring economic effort to explore its recently discovered role in the immense future of photovoltaic energy conversion.