The Unusual Properties of the Methylammonium Lead Iodide Perovskite Solar Cell

Abstract

Solar cells based on organic-inorganic perovskites, such as CH3NH3PbX3 (X = I, Br, Cl), as light absorber materials, have recently emerged as one of the most promising class of photovoltaic devices. Given their high performances, along with their low fabrication costs, such devices are appealing for building integrated photovoltaics products. However, in the most organic-inorganic perovskites based solar cell, the device architectures required other materials to make the hole transport media (HTM) layer and electron transport media (ETM) layer. As a result, the production cost is increased and the fabrication process becomes relatively complicated. Understanding the unusual defect structure of CH3NH3PbI3 (MAPbI3) perovskite is important to achieve high efficient single-junction solar cells with a MAPbI3 as absorbers. In addition, MAPbI3 is readily available hence cheap and is easy to manufacture. Moreover, it has a high absorption coefficient making it a promising material for commercialization. Aiming to understand the unusual defect of MAPbI3, the structural, electronic, and optical properties of the low temperature tetragonal phase of the MAPbI3 was first examined using Density Functional Theory (DFT). The numerically predicted structure was in agreement with existing experimental data. DFT electronic structure calculations showed that relativistic effects are important for the heavy lead atom and spin-orbit coupling has to be included for accurate results. The experimental band gap of 1.63 to 1.66 eV was similar in magnitude to the DFT direct gap of 1.72 eV, which suggests that many-body and relativistic effects cancel in this compound. The calculated fundamental gap, at the G0W0 level of approximation, is 2.48 eV. Optical anisotropy of tetragonal MAPbI3 was investigated by including many-body effects at the time dependent Hartree Fock and the Bethe-Salpeter equation level of approximation, with input data from a range separated Heyd-Scuseria-Ernzerh of DFT functional calculation. The optical edge for radiation polarized parallel to the a- and b-axes differ by about 0.15 eV and for polarization parallel to the b- and c-axes the difference is about 0.05 eV. Secondly, the formation energy of all simple point defects of MAPbI3 has been calculated in an attempt to identify the dominant defects. Three types of defects were created including: the three vacancies created by removing a Pb, or I atom or methylammonium molecule (VPb, VI and VMA), three interstitials, (Pbi, Ii and MAi) and six substitutions (PbMA, MAPb, PbI, MAI, IPb and IMA). The formation energy of each defect as a function of Fermi level position at chemical potential was calculated. A chemical potential A, under (I-rich/Pb-poor), (VPb) acceptor defects have the lowest formation energy while the donor defect (VI) has high formation energy, leading to the Fermi level pinned close to the valance band maximum (VBM) and making p-type semiconductor. For moderate conditions, a chemical potential B, the donor (VI) decreased its formation energies, while the acceptor defect increase the formation energy, but the Fermi level is not pinned on the mid-gap. Under (I-poor/Pb-rich) condition, chemical potential C, (VI) donor defect have the lowest formation energy while the acceptor defect (VPb) have the highest formation energy, leading to the Fermi level pinned close to CBM and making n-type semiconductor. The representative points A (μMA=-3.8eV, μPb=-3.5 eV, μI= 0 eV), B (μMA=-2.41 eV, μPb=-2 eV, μI=-1eV), and C (μMA=-2.05 eV, μPb= 0 eV,μI= -1.75 eV) were used to assign the respective stoichiometric molar concentration of perovskite precursors. vi Acknowledging the importance of controlling the PbI2/MAI ratio in MAPbI3 perovskite solutions, the above calculated chemical potentials were used to control the compositions of the films. The films thus obtained, including n-type, intrinsic and p-type MAPbI3 perovskite were systemically investigated in terms of morphology, elemental composition, and optical properties to elucidate their successful formation. Thereafter, three planar MAPbI3 pervoskite based solar cell devices with configuration of glass/fluorine doped tinoxide (FTO)/intrinsic MAPbI3/p-type MAPbI3/Al, glass/FTO/n-type MAPbI3/intrinsic MAPbI3/Au, and glass/FTO/ZnO/intrinsic MAPbI3/p-type MAPbI3/Au were fabricated. The resultant device fabricated with glass/FTO/intrinsic MAPbI3/p-type MAPbI3/Al exhibited improved device efficiency (power conversion efficiency (PCE) of 1.01%) with high short circuit current (JSC) of 5.23 mA cm-2 than that of the reference device using conventional P3HT based polymer (JSC=3.23 mA cm-2, PCE of 0.91%). Also, device with configuration glass/FTO/n-type MAPbI3/intrinsic MAPbI3/Au showed comparable efficiency (PCE of 1.22%) with that of ZnO based device (PCE of 1.99%). Device fabricated with glass/FTO/ZnO/intrinsic MAPbI3/p-type MAPbI3/Au showed a PCE of 0.30% compared to the 0.19% efficiency value obtained for solar cell fabricated using commercially available P3HT