Polymer assisted deposition of high-quality CsPbI2Br film with enhanced film thickness and stability

Inorganic halide perovskites such as cesium lead iodide(CsPbI3)have drawn tremendous attention,as their tunable band gaps are desirable for solar cells as well as light emitting diodes.However,due to their low Goldschmidt tolerance factor,the cubic phase of bulk CsPbX3-the variant with desirable band gap-is not stable in ambient,especially in humid air.Besides,the low solubility of CsX in precursor makes it difficult to control the film thickness and morphology of CsPbX3,which becomes another obstacle for the practical application of inorganic perovskite.Here,we report a polymer assisted deposition of high-quality CsPbI2Br film by spin-coating a polymer-blended CsPbI2Br precursor.The long-chained polymer increases the viscosity of the solution,which enables us to achieve a ca.700-nm thick film with a low solution concentration of CsPbI2Br.Moreover,the polymer network helps to regulate the crystallization process and provides more crystallization sites for perovskite film,reducing grain size and thus improving the film coverage.Perovskite solar cells with the polymer network exhibit improved efficiency and reproducibility(0.72%standard deviation).Moreover,the device demonstrates excellent robustness against moisture and oxygen,and maintains 90%of its initial power conversion efficiency(PCEs)after aging 4 months in ambient conditions.The conception of polymer incorporation into inorganic perovskite films paves a way to further increase the performance,stability and reproducibility of inorganic perovskite devices.

Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect

2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum.Nevertheless,the effective oscillator strengths of these transitions have bee n scarcely reported,nor is there a con sistent interpretati on of the obtained values.Here,we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect(OSE).Intriguingly,we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient.We propose that the pronounced exciton absorption line should be seen as the sum of multiple,low oscillator strength transitions,rather than a single high oscillator strength one,a feat we assign to strong exciton center-of-mass localization.Within the quantum mechanical description of excitons,this 50-fold difference between both oscillator strengths corresponds to the ratio between the cohere nee area of the exciton's center of mass and the total area,which yields a coherence area of a mere 6.1 nm2.Since we find that the coherence area in creases with reducing temperature,we conclude that thermal effects,related to lattice vibrations,contribute to exciton localization.In further support of this localization model,we show that FStark is in dependent of the n anoplatelet area,correctly predicts the radiative lifetime,and lines up for strongly confined quantum dot systems.

Rapid and robust control of single quantum dots

The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules,quantum dots and plasmonic nanoparticles.Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive,yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics.In ensemble measurements,such control is often achieved by resorting to a closed-loop optimization strategy,where the spectral phase of a broadband laser field is iteratively optimized.This scheme needs long measurement times and strong signals to converge to the optimal solution.This requirement is in conflict with the nature of single emitters whose signals are weak and unstable.Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature,using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser.Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps,even when operating at the few-photon level.Full optimization of the single dot luminescence is obtained within~100 trials,with a typical integration time of 100 ms per trial.These times are faster than the typical photobleaching times in single molecules at room temperature.Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules,thus extending the available experimental toolbox to the active control of nanoscale coherences.