Lewis Base Plays a Double-Edged-Sword Role in Trap State Engineering of Perovskite Polycrystals

Abstract

We report the double-edged-sword effect of the thiourea (a typical Lewis base) additive for tailoring the trap state distribution of perovskite polycrystalline films. Through the thiourea treatment, the polycrystal grain size is greatly increased because of the reduced crystallization activation energy, which, together with the surface defect passivation, alters the density of the energetically “deep” and “shallow” trap states in a trade-off manner. Based on this finding and further photoelectric and spectral studies, the nonmonotonic dependence of the photoluminescence intensity on the thiourea concentration and the complicated time-resolved photoluminescence behavior are excellently clarified. As a proof of concept, the photophysical performance of perovskite polycrystals is optimized via a modified Lewis base treatment by taking the proposed double-edged-sword effect into account.

Low‐Temperature Preparation of High‐Quality Perovskite Polycrystalline Films via Crystallization Kinetics Engineering

“…The deviation between the experimental data and the fitting model in the early‐time region may be attributed to delayed heat conduction [30,31] and/or to the nucleation‐induced absorption redshift in the context of the quantum confinement [32] . From the fitting results, it is unambiguously seen that the TU treatment shortens the crystallization time constant from 87.5 s to 75.7 s, in great agreement with our previous study demonstrating that the faster crystallization of the TU‐treated film links with the reduced activation energy ( E a ), [27] which is suggested to result from the strong interaction between the Lewis base and the precursor compounds [33] …”

“…The former stems from the conversion of the intermediates into the perovskite film resulting in more homogeneous size distribution, and the latter is related to the weakened oscillator strength in large‐sized perovskite crystals, in excellent agreement with the quantum confinement theory as discussed above. Notably, after annealed for 5 min (Figure 2j), the TRPL kinetics of the TU‐treated film becomes highly monoexponential, in stark contrast to the biexponential TRPL profile of the pristine film, corroborating the efficacy of TU on surface defect passivation [27] that suppresses the nonradiative charge recombination loss and leads to stronger PL emission of the TU‐treated sample (Figure 2g). It has been established that TU can effectively remove the lead‐based dangling bonds by closely coordinating with Pb 2+ , [38] and a more detailed discussion regarding the defect passivation mechanism and the exponential behavior of the TRPL kinetics can be found in Ref.…”

“…As mentioned in the previous section, the high annealing temperature is widely employed to facilitate the crystallization of perovskite polycrystals, which, however, brings more structural defects in the meantime (see Figure S9 and S10) [12] . In the recently reported work, we propose that the aforementioned tradeoff issue can be judiciously overcome via a modified Lewis base treatment protocol [27] . Herein, based on the concept of crystallization kinetics engineering, perovskite polycrystals with large grain sizes and high crystallinity are achieved in a low‐temperature manner through the viable TU treatment, while the structural defects are only slightly increased (Figure S9 and S10).…”

“…In recent works, we report that the value of E a can be remarkably decreased through a Lewis base (e. g., thiourea, abbreviated as TU) treatment, whereby the perovskite crystallization kinetics can be effectively promoted without elevating the annealing temperature [27] . Based on this finding, here we thoroughly investigate the efficacy of the TU additives on the low‐temperature preparation of perovskite polycrystalline films.…”

Zwitterions Narrow Distribution of Perovskite Quantum Wells for Blue Light‐Emitting Diodes with Efficiency Exceeding 15%

“…This is mainly due to that the interaction between the sulfhydryl group and Pb 2+ is too strong to disturb nucleation and the following crystal growth. [ 40,41 ]…”