Optical Transitions in Hybrid Perovskite Solar Cells: Ellipsometry, Density Functional Theory, and Quantum Efficiency Analyses forCH3NH3PbI3

Abstract

Light-induced photocarrier generation is an essential process in all solar cells, including organic-inorganic hybrid (CH 3 NH 3 PbI 3 ) solar cells, which exhibit a high short-circuit current density (J sc ) of approximately 20 mA/cm 2 . Although the high J sc observed in the hybrid solar cells relies on strong electron-photon interaction, the optical transitions in the perovskite material remain unclear. Here, we report artifact-free CH 3 NH 3 PbI 3 optical constants extracted from ultra-smooth perovskite layers without air exposure and assign all the optical transitions in the visible/ultraviolet region unambiguously based on density functional theory (DFT) analysis that assumes a simple pseudo-cubic crystal structure. From the self-consistent spectroscopic ellipsometry analysis of the ultra-smooth CH 3 NH 3 PbI 3 layers, we find that the absorption coefficients of CH 3 NH 3 PbI 3 (α = 3.8 × 10 4 cm −1 at 2.0 eV) are comparable to those of CuInGaSe 2 and CdTe, and high α values reported in earlier studies are overestimated seriously by extensive surface roughness of CH 3 NH 3 PbI 3 layers. The polarization-dependent DFT calculations show that CH 3 NH 3 + interacts strongly with the

Intriguing Optoelectronic Properties of Metal Halide Perovskites

“…139, 192−197 Shirayama et al suggest these widely varying and often overestimated α values stem from two surface-related phenomena: (i) formation of a hydrate layer and (ii) roughness. 193 The hydrated surface layer is purported to be the typical 0D compound (CH 3 NH 3 ) 4 [PbI 6 ]·2H 2 O. However, monohydrate formation is perhaps more likely.…”

“…A simulation of the apparent α derived from the effective-medium approximation with a surface roughness layer thickness ranging from 0−25 nm is shown is Figure 11C. 193 Clearly, roughness has a marked impact, artificially enhancing apparent absorption across the entire visible spectrum, especially near the band edge. A characteristic feature that arises with increasing roughness is nonzero absorption extending well below E g , which also hinders accurate determination of the band gap when using the Tauc method.…”

“…202 Regardless of the precise phase, formation of this layer modifies α most significantly at photon energies ≥3 eV due to the hydrate's large band gap (vide supra) and the high surface sensitivity at these frequencies resulting from shallow photon penetration depths (∼20 nm at 3.4 eV). 193 While proper precautions can preclude deleterious effects of hydrate formation, typical solution-based thin film deposition methods inherently yield rough surfaces. A simulation of the apparent α derived from the effective-medium approximation with a surface roughness layer thickness ranging from 0−25 nm is shown is Figure 11C.…”

“…Such effects can be difficult to address empirically or theoretically, indicating that extremely high quality thin films are necessary to accurately determine the optical properties of MHPs. The absorption coefficient of a CH 3 NH 3 PbI 3 (MAPbI 3 ) thin film prepared by laser evaporation is provided in Figure 11D, 193 along with CH 3 NH 3 PbBr 3 (MAPbBr 3 ) and a number of other PV-relevant semiconductors. The broadband absorption of CH 3 NH 3 PbI 3 , with E g = 1.61 eV as determined by critical point analysis, 193 is on par with GaAs and CdTe, and even slightly greater at moderate energies ca.…”

GeP3: A Small Indirect Band Gap 2D Crystal with High Carrier Mobility and Strong Interlayer Quantum Confinement

“…As shown in Figure 5, the absorption coefficients of 1L and 2L GeP3 reach to the order of 10 6 cm -1 , which are comparable to those of organic perovskite solar cells. 46,47 In-plane absorption is always larger (due to the larger cross section), suggesting that flakes should be aligned normal to the surface of the photovoltaic cell for most efficient application. The bilayer shows excellent absorption, in particular in the important region between 1 and 4 eV, which marks the infrared, visible and near UV range of the solar spectrum.…”