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

Low stability of organic-inorganic perovskite (CH3NH3PbI3) solar cells in humid air environments is a serious drawback which could limit practical application of this material severely. In this study, from real-time spectroscopic ellipsometry characterization, the degradation mechanism of ultra-smooth CH3NH3PbI3 layers prepared by a laser evaporation technique is studied. We present evidence that the CH3NH3PbI3 degradation in humid air proceeds by two competing reactions of (i) the PbI2 formation by the desorption of CH3NH3I species and (ii) the generation of a CH3NH3PbI3 hydrate phase by H2O incorporation. In particular, rapid phase change occurs in the near-surface region and the CH3NH3PbI3 layer thickness reduces rapidly in the initial 1 h air exposure even at a low relative humidity of 40%. After the prolonged air exposure, the CH3NH3PbI3 layer is converted completely to hexagonal platelet PbI2/hydrate crystals that have a distinct atomic-scale multilayer structure with a period of 0.65 ± 0.05 nm. We find that conventional x-ray diffraction and optical characterization in the visible region, used commonly in earlier works, are quite insensitive to the surface phase change. Based on results obtained in this work, we discuss the degradation mechanism of CH3NH3PbI3 in humid air.

A variety of organic-inorganic hybrid perovskites (APbX 3 ) consisting of mixed center cations [A = CH 3 NH 3 + , HC(NH 2 ) 2 + , Cs + ] with different PbX 3 − cages (X = I, Br, Cl) have been developed to realize high-efficiency solar cells. Nevertheless, clear understanding for the effects of A and X on the optical transition has been lacking. Here, we present MAPbI 3 is replaced with a formamidinium (FA) cation [HC(NH 2 ) 2 + ] having a larger molecular weight. For FAPbI 3 , no major structural change occurs upon thermal treatment up to 150 o C. 46 However, the FAPbI 3 perovskite has limited long-term stability and a cubic FAPbI 3 crystal (α-FAPbI 3 ) shows a gradual phase transformation into a transparent δ-FAPbI 3 phase having a one-dimensional crystal structure. 49,50 Such instability is caused by the larger size of FA + and, quite fortunately, FAPbI 3 -based perovskites can be stabilized by including a small amount of MA + and Cs + having smaller ionic radii. 51-57 The Cs addition to α-FAPbI 3 is also beneficial for suppressing degradation induced by humid air and light illumination. 53 A recent study further demonstrates the improved overall stability of α-FAPbI 3 by the incorporation of Cs and Br atoms. 57 Accordingly, by the optimum combination of different A-site cations and X-site halogen atoms, an ideal hybrid perovskite compound with high stability could be realized. To date, very high conversion efficiencies exceeding 20% have been demonstrated in (FA, MA)Pb(I, Br) 3 and (FA, MA, Cs)Pb(I, Br) 3 solar cells. 52,57-59 Nevertheless, despite the rapid progress for the solar cell fabrication, the optical process in the complex hybrid perovskite remains unclear. For the light absorption in APbX 3 , there is a common belief that the A-site cation plays a minor role in the optical transition 12-19 and only the band gap (E g ) changes slightly according to the size of the A-site cation. 17,20 However, although many studies have been devoted to determine the dielectric functions (ε = ε 1 -iε 2 ) of MAPbI 3 (Refs. 5, 23-27), MAPbBr 3 (Refs. 27, 63) and MAPbCl 3 (Ref. 27), only limited experimental results are available for the quantitative effect of the A-site cation on the light absorption. 53,60,61 So far, the optical properties of FAPbI 3 (Refs. 20, 65) and CsPbI 3 (Refs. 65-67) perovskites have beeninvestigated by applying density functional theory (DFT), but the influences of the A-site cation and X-site halogen atom on the absorption properties remain ambiguous.

We demonstrate a method to reduce the thermal conductivity of fully dense (above the rigidity percolation threshold) amorphous thin films below the minimum limit by systematically changing the coordination number through hydrogenation. Studying a-SiO:H, a-SiC:H, and a-Si:H thin films, we measure the thermal properties using time-domain thermoreflectance to show that thermal conductivity can be reduced below the amorphous limit by a factor of up to two. By experimentally investigating the thermophysical parameters that determine thermal conductivity, we show that sound speed, atomic density, and heat capacity cannot explain the measured reduction in thermal conductivity, revealing that the coordination number can significantly alter the scattering length scale of heat carriers. Reformulating the minimum limit to consider the propensity for energy to transfer through the non-hydrogen network of atoms, we observe greatly improved agreement with experimental data.

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