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

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.

In developing photovoltaic devices with high efficiencies, quantitative determination of the carrier loss is crucial. In conventional solar-cell characterization techniques, however, photocurrent reduction originating from parasitic light absorption and carrier recombination within the light absorber cannot be assessed easily. Here, we develop a

In this article, the optical properties of various APbX 3 -type hybrid perovskites (A: center cation; X: halogen atom) and operational principles of hybrid perovskite solar cells are reviewed and discussed. In particular, controversy observed for the absorption coefficient (α) of CH 3 NH 3 PbI 3 is found to originate from large surface roughness of the solution-processed perovskite layers. The comparison of the accurate experimental α with the theoretical results indicates clearly that the hybrid perovskite has a direct band gap with negligible indirect nature. Moreover, the A-site cation plays a critical role in determining the absorption strength of hybrid perovskites through hydrogen bonding interaction. In contrast to general understanding that hybrid perovskite solar cells operate in p-i-n type solar cells, a simple p-n junction formation explains the observed solar cell characteristics well. The high short-circuit current observed in the solar cells is further interpreted by the suppressed parasitic light absorption and efficient optical confinement in the solar cells, rather than the high α of hybrid perovskite materials. This article provides renewed principles for the optical processes and operation of hybrid perovskite solar cells.

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