March 2, 2020.
Thailand has moved into the fully industrial country, thus, the need of the electrical power is much higher than the past years. The development of the renewable energy resources is a very important path for reducing the future energy requirement. The renewable energy resources should fulfill these criterial: 1) the power resource should be from our country, 2) it has a low cost, 3) it is a clean energy, 4) it can provide energy enough for the whole country and 5) it uses our own technology in fabricating and assembling. Through all these requirements, the solar cell technology is a suitable candidate to fulfil these requirements. Solar cell is the device that can convert the solar energy directly into the electrical energy. However, the use of solar cell modules is not yet widely. This is because the price of the solar cell module is still expensive. Therefore, there are enormous ongoing researches all around the world on developing the high solar cell efficiency and a low cost solar cell device.
Perovskite solar cell is received a highly attention in this present time because of its outstanding efficiency. The fast advanced efficiency from 3.81% in 2009  to 23.7% in 2019  is subjected to the discovery of CH3NH3PbI3 (MAPbI3) perovskite film. The CH3NH3PbI3 perovskite solar cell delivers the superior efficiency because of its high light absorption coefficient, high carrier mobility rate and long carrier diffusion length in CH3NH3PbI3 film [3-8]. The perovskite solar cell composes of three main important parts: 1) the anode electrode, which absorb the sun light, is consisted of TiO2 film and perovskite layer, 2) hole-transporting-layer (HTL), extracting holes from the perovskite film, is normally used Spiro-OMeTAD and 3) the cathode electrode, which transfers holes to the external load, is mostly used the Au film. The structure of the perovskite solar cell is shown in Figure 1.
Fig. 1 The structure of the perovskite solar cells.
The main important parts of the perovskite solar cells are the perovskite film and the hole-transporting-layer (HTL). The inorganic-organic CH3NH3PbI3 film is the mostly used in the perovskite solar cell device, which has the perovskite structure in ABX3 form as shown in Figure 2, where, A is CH3NH3+, B is Pb+, and X is I-. CH3NH3PbI3 can be obtained from the chemical reaction of CH3NH3I + PbI2 ® CH3NH3PbI3. The bandgap of CH3NH3PbI3 calculated from the density functional theory (DFT) is about 1.5-1.8 eV , which is the appropriated value for the superior solar cell performance. Hole-transporting-material (HTM) is normally used 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD). Spiro-OMeTAD is the organic material that transports holes to the hole-collector (Au film), and it has the bandgap of ~3 eV.
Fig. 2 The structure of CH3NH3PbI3. (https://en.wikipedia.org/wiki/Methylammonium_lead_halide#/media/File:CH3NH3PbI3_structure.png)
Perovskite solar cell will generate the electrical current when the device is kept under the sunlight. Perovskite film absorbs sunlight and generates the electron-hole pairs. Electrons are injected into the conduction band of TiO2 film and then collected at the conductive glass (FTO). Holes are extracted by the hole-transporting-layer (HTL), and they will accumulate at the Au film as illustrated in Figure 3. The accumulation of the opposite charges on FTO and Au films creates the potential difference called “open-circuit voltage, Voc”. When connecting the external load to this solar cell device, electrons from the FTO surface and holes from the Au layer flow through the external load. The current density flowing out from the device at the short-circuit condition is called “short-current density, Jsc”.
Fig. 3 Schematic illustrates the movement of electrons and holes.
However, it was found that CH3NH3PbI3 film is degraded in the ambient environment via the interaction with water molecules. To enhance the perovskite film stability, researchers has doped the stable moieties such as Cs+, SCN-, NH2CH=NH2+ (FA+) or C(NH2)3+ (GA+) into CH3NH3PbI3 film. They observed that the doped perovskite film has higher film stability than that of the pristine CH3NH3PbI3 film. Moreover, they are several attempts to fabricate other types of perovskite solar cell, which is known as the carbon-based HTL-free perovskite solar cells as shown in Figure 4(a). Y. Zhang et al.  reported that the price of hole-transporting-layer (HTL) Spiro-OMeTAD and Au films was ~50% of the total perovskite solar cell cost as seen in Figure 4(b). Therefore, the achievement of the high efficiency and stability from the carbon-based HTL-free perovskite solar cells attracts the great interested from people. In the present time, the highest reported efficiency of the carbon-based HTL-free perovskite solar cell is about 12.3% .
Fig. 4 (a) Structure of the carbon-based HTL-free perovskite solar cell, and (b) the percent of the cost of each layer in the normal perovskite solar cells .
In the research group of Associate Professor Dr. Vittaya Amornkitbamrung, we fabricated the CH3NH3Pb(SCN)xI3-x (x=0, 0.25, 0.5, 1 and 2) films. CH3NH3Pb(SCN)xI3-x film properties were investigated by means of experiments and theoretical calculations. Interestingly, the optical color of CH3NH3Pb(SCN)xI3-x films is changed from the black to yellow color with increasing SCN- dopant levels as seen in Figure 5. The calculated result of the tetragonal CH3NH3Pb(SCN)xI3-x structures reveals the increasing bandgap with the increasing SCN- dopant levels in similarly trend to the UV-vis spectra.
Fig. 5 Optical image of the CH3NH3PbI3-x(SCN)x coated on the conductive glass (FTO) .
CH3NH3Pb(SCN)xI3-x films were used as the light absorber in the carbon-based HTL-free perovskite solar cells. It is found that the increasing of SCN- dopant ratios in the CH3NH3PbI3 films reduces the solar cell efficiency as seen in Figure 6(a). This is because the bandgap increases with the increasing SCN- ratio. However, the stability of the CH3NH3Pb(SCN)0.25I2.75 and CH3NH3Pb(SCN)0.5I2.5 based solar cells is much better than that of the pure CH3NH3PbI3 based solar cells as displayed in Figure 6(a). This is attributed to the hydrogen bond formation between SCN- and MA- in the CH3NH3Pb(SCN)xI3-x structures as illustrated in Figure 6(b). This work was submitted to Materials Chemistry and Physics and it was under reviewed . The highest performance of the carbon-based HTL-free solar cells in our work is about 4.5%, which is lower than that of the highest reported literature of 12.3%. Therefore, our carbon-based HTL-free perovskite solar cell devices are still needed the further investigation for improving its efficiency and stability.
Fig. 6 (a) The stability of the carbon-based HTL-free CH3NH3PbI3-x(SCN)x (x=0, 0.25, 0.5, 1 and 2) solar cells, and (b) the presence of the hydrogen bonding in CH3NH3Pb(SCN)0.25I2.75 .
 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, “Organometal halide perovskites as visible-light sensitizers for photovoltaic cells”, J. Am. Chem. Soc. 131 (2009) 6050.
 “National Renewable Energy Laboratory. Best research-cell efficiencies”. www.nrel.gov/pv/assets/pdfs/pv-efciency-chart.20190103.pdf (2019).
 M.A. Green, A. Ho-Baillie, H.J. Snaith, “The emergence of perovskite solar cells”, Nat.
Photonics 8 (2014) 506–514.
 C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, and L. M. Herz, “High charge carrier mobilities and lifetimes in organolead trihalide perovskites”, Adv. Mater. 26 (2014) 1584–1589.
 N. G. Park, “Perovskite solar cells: an emerging photovoltaic technology”, Materials Today 18 (2015) 65-72.
 J. Fan, B. Jia, M. Gu, “Perovskite-based low-cost and high-efficiency hybrid halide solar cells”, Photonics Research 2 (2014) 111-120.
 D. Shi, V. Adinolfi, R. Comin, M. J. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland,
A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed,
E. H. Sargent, O. M. Bakr, “Low trap-state density and Long carrier diffusion in organolead trihalide perovskite single crystals”, Science 347 (2015) 519–522.
 S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens,
L. M. Herz, A. Petrozza, H. J. Snaith, “Electron-hole diffusion lengths exceeding 1
micrometer in an organometal trihalide perovskite absorber”, Science 342 (2013)
 P. Umari, E. Mosconi and F. de Angelis, “Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 perovskites for solar cell applications”, Scientific Reports, 4 (2014) 4467.
 Y. Zhang, H. Zhang, X. Zhang, L. Wei, B. Zhang, Y. Sun, G. Hai, Y. Li, “Major Impediment to Highly Efficient, Stable and Low-Cost Perovskite Solar Cells”, Metals, 8 (2018) 964.
 L. Fagiolari, F. Bella, “Carbon-based materials for stable, cheaper and large-scale processable perovskite solar cells”, Energy & Environmental Science, 12 (2019) 3437-3472.
 P. Kumlangwan, P. Suksangrat, M. Towannang, N. Faibut, V. Harnchana, P. Srepusharawoot, A. Chompusor, P. Kumnorkaew, W. Jarernboon, S. Pimanpang and V. Amornkitbamrung, Submitted to Materials Chemistry and Physics, MATCHEMPHYS-D-19-04286.
Assoc. Prof. Dr. Samuk Pimanpang 1), Assoc. Prof. Dr. Pornjuk Srepusharawoot 2) and Assoc. Prof. Dr. Vittaya Amornkitbumrung 2)
1)Department of Physics, Faculty of Science, Srinakharinwirot University, Bangkok 10110, Thailand
2)Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand