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.

Advanced characterization methods avoiding transient effects in combination with solar cell performance monitoring reveal details of reversible light-induced perovskite degradation under vacuum. A clear signature of related deep defects in at least the 1 ppm range is observed by low absorptance photocurrent spectroscopy. An efficiency drop, together with deep defects, appears after minutes-long blue illumination and disappears after 1 h or more in the dark. Systematic comparison of perovskite materials prepared by different methods indicates that this behavior is caused by the lead halide residual phase inherently present in material prepared by the two-step method. X-ray photoelectron spectroscopy confirms that lead halide when illuminated decomposes into metallic lead and mobile iodine, which diffuses into the perovskite phase, likely producing interstitial defects. Single-step preparation, as well as preventing lead halide illumination, eliminates this effect.

Metal-halide perovskites feature very low deep-defect densities, enabling thereby high operating voltages on solar cell level. Here, by precise extraction of their absorption spectra, we find that the low deep-defect density is unaffected when Cs + and Rb + are added during the perovskite synthesis. By comparing single-crystals and polycrystalline thin-films of methyl ammonium lead iodide/bromide, we find these defects to be predominantly localized at surfaces and grain boundaries. Furthermore, for the most important photovoltaic materials, we demonstrate a strong correlation between their Urbach energy and open-circuit voltage deficiency on the solar cell level. Via external quantum yield photoluminescence efficiency measurements, we explain these results as a consequence of non-radiative open-circuit voltage losses in the solar cell.Finally, we define practical power conversion efficiency limits of solar cells by taking into account the Urbach energy.

High-quality crystalline large grains with uniform morphologies of the perovskite films are particularly important for achieving stable, high-performance perovskite solar cells. Herein, an effective strategy to control the growth of large grains in the CH3NH3PbI3 perovskite films is demonstrated by modifying the perovskite film deposition process through forming an intermediate CH3NH3PbI3·methylammonium chloride (MACl)·xCH3NH2 liquid phase induced by CH3NH2 gas treatment in combination with a MACl additive without an antisolvent. By tuning the incorporation of the MACl additive to the perovskite precursor solution, this intermediate liquid phase enables the well-controlled growth of large grains up to 3 μm, highly uniform morphology, and higher crystallinity in the final CH3NH3PbI3 perovskite films. The high-quality CH3NH3PbI3 film derived from the CH3NH3PbI3·MACl·xCH3NH2 phase leads to enhanced carrier lifetime and reduced charge-trap density and nonradiative recombination of the perovskite films. In addition, the defect healing and reduced grain boundaries also greatly improve the environmental stability in ambient air. The perovskite solar cells made via the CH3NH3PbI3·MACl·xCH3NH2 phase exhibit high power conversion efficiency of 18.4%, much higher than that of the perovskite solar cells made without MACl (15.8%).

This article describes two simple classroom demonstrations that illustrate the principles of scanning probe microscopy (SPM) based on a macroscopic analogy. The analogy features the bumps in an egg carton to represent the atoms on a chemical surface and a probe that can be represented by a dwarf statue (illustrating an origin of the prefix “nano”), a plastic bottle, or a glass. The demonstrations allow students to visualize certain physical aspects of SPM, such as probing “hilly” atoms on a surface using changes in distance between probe and atoms. Both demonstrations are appropriate to introduce SPM into high school or undergraduate science education.