Negative capacitance at the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, where the origin is still under discussion. Here we use Impedance Spectroscopy to analyse these responses in methylammonium lead bromide cells treated with lithium cation at the electron selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the Surface Polarization Model, we obtain a time constant associated to the kinetics of the interaction of ions/vacancies with the surface, τkin, in the range of 10 0 -10 2 s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding to a flattening of the cyclic voltammetry. As consequence of these results we find that that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.

We have synthesized 1-(2-hydroxyacetyl)piperidine-2-one (2) and 1-(2-hydroxyacetyl)azepan-2-one (3). Equilibrium (K(f)) between the free alcohol (open form) and the tetrahedral intermediate (cyclol) is readily established, and both forms are observed in the D(2)O (1)H NMR spectra of 2 and 3. Therefore, their interconversion can be considered as an almost thermoneutral non-identical one. Pseudo-first-order rate constants (k(obs)) were obtained by simulating the AB (1)H NMR system observed for the cyclol. By best fitting the experimental points of a k(obs) versus pD profile to the equation k(obs) = 0.5k(0r) + 0.5k(r) K(ac)/(K(ac) + [D(+)]) + 0.5k(f)K(ao)/(K(ao)+ [D(+)]), the parameters involved were obtained: rate constants of rupture and formation (k(0r) and k(0f) = K(f)k(0r)) catalyzed by water, rate constants of rupture (k(r)) and formation (k(f)) from the conjugated bases of the cyclol form and the open form, and their acidity equilibrium constants K(ac) and K(ao). The system studied mimics the serine alcohol attack on the peptide bond and its reverse reaction in serine protease enzymes. In fact, the reaction rates are similar or perhaps even faster than the ones obtained for enzymatic reactions. The results also show the participation of water molecules forming catalytic proton bridges in stable potentials with the two interconverted forms. The position change of the bridged proton is sensitive to lactam ring size, and it is manifested by considerable change in the pKa values of both cyclol and open forms. Other evidence such as kinetics, DeltaS degrees , DeltaS, and proton inventory experiments and semiempirical molecular calculations support this proposal.

Using two‐dimensional energy diagrams, results on the formation and cleavage of detectable tetrahedral intermediates are used to predict the position of transition states involved in the cleavages of unstable and stable tetrahedral intermediates (STI). A late transition state (anti‐Hammond) for the former and synchronous cleavage for the latter are predicted and supported with experimental results. For instance, in the case of STI, intrinsic leaving abilities in the order CO > CNHCO ≫ CNH were experimentally observed, confirming the departure of these groups without full heteroatom protonation. The STI designed herein mimics serine proteases enzymes due to the thermoneutral relationship between the enzyme substrate (ES) and enzyme‐acyl tetrahedral intermediate (EATI) analogs. In the STI models, a dramatic change in pKa is observed when changing the lactam ring size due to the strain in the 3‐fused‐ring system of the STI‐H2O complex formed and exo‐anomeric effect. A plot of ΔSo (STI formation) versus pKa provides evidence for the STI–water interaction. Since only one water molecule is required to produce the effect it is also detected when using ‘pure’ CDCl3 as the solvent and therefore may also play a role in the active sites of serine proteases. Copyright © 2010 John Wiley & Sons, Ltd.