Perovskite: Light Harvesting Materials for Future Alternative Solar Cells
Assoc. Prof. Dr. Jatuphorn Wootthikanokkhan
Nanotec-KMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy
School of Energy, Environment and Materials
King Mongkut’s University of Technology Thonburi
Keywords: Solar cell, Dye sensitized solar cell, Hybrid materials, light harvesting materials, Perovskite
Introduction to the new generation solar cells
It has been known that solar cells can be generally classified into 3 main categories, in accordance with their revolution (Figure 1). The first generation solar cells are those of which based on crystalline silicon materials whereas the second generation solar cells utilized amorphous silicon and alternative inorganic materials such as CIGS as photoactive semiconductor. Last but not least are the third generation solar cells which included dye sensitized solar cell (DSSC), organic photovoltaic cell (OPV), quantum dot solar cell, and the recently developed perovskite solar cell.
The DSSC and OPV systems have been developed for more than decade. Advantages of these solar cells included their relatively low cost of materials and a simple fabrication process. The solar cell products can also be flexible, using TCO coated plastic substrate. However, power conversion efficiency values of these cells are remaining low as compared to those of the first and second generation solar cells. Besides, durability, stability and life time of these cells have yet to be further improved before their real applications can be launched.
Figure 1 Classification of solar cells based on their technology and revolution
Figure 2 OPV (left) and DSSC (right) laboratory scale prototypes
From solid state DSSC to hybrid material solar cell
Among the various types of new generation solar cells, dye sensitized solar cell is considered interesting. It was pioneered by O’Regan and Gratzel in 1991 . In principle, DSSC utilizes dye as a light absorber. The photo-excited electron from the excited dye was then transported though the TiO2 layer toward the electrode, whereas the excited dye accepted electrons from NaI/I2 electrolyte before returning to ground state. (Fig.3).
At the presence time, the highest claimed PCE of DSSC is 14% . However, stability and lifetime of the DSSC have yet to be further improved. This is attributed to the fact that liquid electrolyte and solvents used in the DSSC are usually evaporated and leak out of the cell. Alternatively, a kind of room temperature ionic liquid (RTIL) might be used as a replacement of the liquid electrolyte. However, the leakage problem still exists if an adhesion between a sealing film and glass substrate was insufficiently strong.
Figure 3 Schematic diagram representing components and working mechanism of the DSSC
(picture drawn by SWG)
Consequently, attempts were made to develop and use solid and quasi-solid (gel) electrolytes. These include various types of hole transport materials such as SpiroOMeTAD, P3HT, and CsSnI3. It was also of noteworthy that stability of the DSSC containing solid electrolyte is usually improved at the expense of its power conversion efficiency. Besides, iodine used as a redox couple in the cell is known to be corrosive and cost of the ruthenium dye is considerably high. Until in 2009, the development of a DSSC containing perovskite as a light harvesting material with a power conversion efficiency of 3.8 % was reported . In that case, perovskite layer was deposited on top of the meso-porous TiO2 layer and the photoactive material was sandwiched between Au and FTO.
Structure of Perovskie
Perovskite is a common name of crystal structure with a general chemical formula of ABX3 (Figure 4). This structure is resembled to CaTiO3 crystal which was discovered by Gustav Rose and so it was named after Russian mineralogist, Lev Perovski. In the ABX3 compound, B represents halide atoms whereas A represents cations (which can be either metal or hydrocarbon). In general, there are several types (more than 100 types) of perovskite compounds, both inorganic based and organic based, with a variety of properties such as antiferromagnetic, piezoelectric, thermoelectric, semiconducting, conducting, and superconducting.
Figure 4 Crystal structure of perovskite
The use of perovskite in solar cell applications has gained more and more interest over the last few years. This was attributed to the fact that perovskite exhibit good light absorption property. Besides, perovskite can be simply prepared from raw chemical which are naturally abundant. For example, it can be obtained from a chemical reaction between methylammonium iodide (CH3NH3I) and PbI2 at 60 °C . In addition, electronic properties of perovskite, such as band gap energy, can be adjusted by tailoring made chemical structure of the material. Noteworthy, experimental results from photoluminescence quenching technique  revealed that diffusion length of the photo generated exciton is a long as 1 µm. This is rather exciting and interesting as compared to that of the exciton obtained from organic photovoltaic cells (OPV) usually of less than 100 nm. This means that the exciton from perovskite has less chance to experience recombination. Furthermore, this enables the use of an active layer, made from perovskite, with a greater thickness (higher than 1 µm). This was in turn promoted more photon absorption capacity of the active layer. These factors contributed to the considered high power conversion efficiency of perovskite solar cell.
R&D trends for perovskite solar cell
Solar cell based on perovskite has been developed from DSSC system, using perovskite as a kind of hole-transport material (HTM) (Figure 5). However, the use of solid HTM still has some limitations in terms of pore-filling or contact between HTM and the electrode. Consequently, attempts have been made to use 1 D TiO2 as a replacement of mesoporous TiO2 (Figure 5, middle)
Notably, work by Snaith et al., [8,9] demonstrated that by using a simple configuration of the cell (Figure 5, right), PCE of the perovskite solar cell as high as 10% can be obtained, provided that morphology of the active layer must be uniform and dense.
In addition, work by Lee et al.,  revealed that, apart from functioning as a light absorber, pervoskite can also serve as an electron acceptor. This suggests that the perovskite solar cell can be a kind of mesoporous TiO2-free cell. In this regard, it was possible that the annealing step at high temperature can be eliminated and so the use of plastic substrate made from poly(etnylene terephtharate), PET or poly(ethylene naphthanate), PEN, can be more realized.
Figure 5 Configuratuion of various types of perovskite solar cells
(Reproduced from J. Fan, B. Jia, M. Gu, Photon Research, 2(2014)111-120)
Consideration of the literature review in this field during 2009 to 2013 reveals that power conversion efficiency of perovskite solar cells have been improved rapidly as compared to those of other new generation solar cell analogues. Until now, the PCE as high as 17.9 % was claimed  and it seems that the raising trend is ongoing.
Figure 6 Progress in power conversion efficiency of various types of solar cell
(Figure from J. Fan, B. Jia, M. Gu, Photon Research, 2(2014) 111-120)
However, even though the PCE of pervoskite can be comparable to those of the traditional inorganic solar cells, there are some limitations and disadvantages for the perovskite solar cell which have to be corrected before applications. These include the fact that traditional perovskite compound such as CH3NH3PbI3 contains lead atoms. Therefore, attempts have been made to develop a kind of lead free perovskite compound, for example, CH3NH3SnI3 . Moisture sensitivity of the perovskite is another problem .To ensure stability and long term performance of the device, the pervoskite solar cell should be encapsulated by some suitable materials. These include the use of glass frit technique  and Surlyn film . Last but not least, since perovskite is can undergo phase change at 55 °C, thermal stability and durability of the cell should be determined and evaluated before uses.
 Anonymous, 2014, Types of Solar Cells [Online], Available: http://www.solartech-usa.com/types-of-solar-cells [2014, January 13]
 Anonymous, 2010, Classification of Solar Cell Technologies [Online], Available: http://sovoxglobal.com/cell_classification.html [2014, January 13]
 O’Regan, B., Grätzel M., 1991, “A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films”. Nature 353 (6346): 737–740. doi:10.1038/353737a0
 Burschka, J., Pellet, N., Moon, S.-J., Humphry-Baker, R., Gao, P., Nazeeruddin M. K. and Grätzel, M., 2013, “Sequential Deposition as a Route to High-Performance perovskite-Sensitized Solar Cells”, Nature, Vol. 499, pp. 316–319.
 Kojima, A., Teshima, K., Shirai, Y. and Miyasaka, T., 2009, “Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells”, Journal of the American Chemical Society, Vol. 131, pp. 6050–6051.
 Chen, C., Li, C., Li, F., Wu, F., Tan, F., Zhai, Y. and Zhang, W., 2014, “Efficient Perovskite Solar Cells based on Low-Temperature Solution-Processed (CH3NH3)PbI3 Perovskite/CuInS2 Planar Heterojunctions”, Nanoscale Research Letters, Vol. 9, pp. 457-464.
 Xing, G., Mathews., Sun, S., Lim, S. S., Lam, Y. M., Grätzel, M., Mhaisalkar, S. and Sum, T. C., 2013, “Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3”, Science, Vol. 342, pp. 344-347.
 Liu, M., Johnston, M. B. and Snaith, H. J., 2013, “Efficient Planar Heterojunction Perovskite Solar cells by Vapour Deposition”, Nature, Vol. 501, pp. 395-398.
 Eperon, G. E., Burlakov, V. M., Docampo, P., Goriely, A. and Snaith, H. J., 2013, “Morphological Control for High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells”, Advanced Functional Materials, Vol. 24, pp. 151–157.
 Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. and Snaith, H. J., 2013, “Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites”, Science, Vol. 338, pp. 643-647.
 NREL, 2014, Research Cell Efficiency Records [Online], Available: http://www.nrel.gov /ncpv/images/efficiency_chart.jpg [2014, January 13]
 Hao, F., Stoumpos, C. C., Cao, D. H., Chang, R. P. H. and Kanatzidis, M. G., 2014, “Lead-Free Solid-State Organic–Inorganic Halide Perovskite Solar Cells”, Nature Photonics, Vol. 8, pp. 489-494.
 Smith, I. C., Hoke, E. T., Solis-Ibarra, D., McGehee, M. D. and Karunadasa, H. I., 2014, “A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability”, Angewandte Chemie, Vol. 53, pp. 1-5.
 Hinsch, A., Mastroianni, S., Brandt, H., Heinz, F., Schubert, M.C. and Veurman, W., 2014, “Introduction to in situ Produced Perovskite Solar Cells: A New Concept Towards Lowest Module Manufacturing Costs”, Presented at the 29th European PV Solar Energy Conference and Exhibition, September 22-26, Amsterdam, Netherlands, pp. 1-5.