Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm(-1). These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.

To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH 3 NH 3 PbI 3 ), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH 3 NH 3 PbI 3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.

Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH3NH3PbI3), tribromide (CH3NH3PbBr3), and mixed iodide-bromide (CH3NH3PbI3-xBrx) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH3NH3PbI3 layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI2. Moreover, we describe CH3NH3PbI3-xBrx Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio.

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition. We show a dramatic improvement in passivation when H 2 plasma treatments are used during film deposition. Although the bulk of the a-Si:H layers is slightly more disordered after H 2 treatment, the hydrogenation of the wafer/film interface is nevertheless improved with as-deposited layers. Employing

Thin films of colloidal quantum dots (QDs) are promising solar photovoltaic (PV) absorbers in spite of their disordered nature. Disordered PV materials face a power conversion efficiency limit lower than the ideal Shockley–Queisser bound because of increased radiative recombination through band-tail states. However, investigations of band tailing in QD solar cells have been largely restricted to indirect measurements, leaving their ultimate efficiency in question. Here we use photothermal deflection spectroscopy (PDS) to robustly characterize the absorption edge of lead sulfide (PbS) QD films for different bandgaps, ligands, and processing conditions used in leading devices. We also present a comprehensive overview of band tailing in many commercial and emerging PV technologiesincluding c-Si, GaAs, a-Si:H, CdTe, CIGS, and perovskitesthen calculate detailed-balance efficiency limits incorporating Urbach band tailing for each technology. Our PDS measurements on PbS QDs show sharp exponential band tails, with Urbach energies of 22 ± 1 meV for iodide-treated films and 24 ± 1 meV for ethanedithiol-treated films, comparable to those of polycrystalline CdTe and CIGS films. From these results, we calculate a maximum efficiency of 31%, close to the ideal limit without band tailing. This finding suggests that disorder does not constrain the long-term potential of QD solar cells.

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The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley−Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.