Dynamic Nuclear Polarization Enables NMR of Surface Passivating Agents on Hybrid Perovskite Thin Films

Abstract

Surface and bulk molecular modulators are the key to improving the efficiency and stability of hybrid perovskite solar cells. However, due to their low concentration, heterogeneous environments, and low sample mass, it remains challenging to characterize their structure and dynamics at the atomic level, as required to establish structure–activity relationships. Nuclear magnetic resonance (NMR) spectroscopy has revealed a wealth of information on the atomic-level structure of hybrid perovskites, but the inherent insensitivity of NMR severely limits its utility to characterize thin-film samples. Dynamic nuclear polarization (DNP) can enhance NMR sensitivity by orders of magnitude, but DNP methods for perovskite materials have so far been limited. Here, we determined the factors that limit the efficiency of DNP NMR for perovskite samples by systematically studying layered hybrid perovskite analogues. We find that the fast-relaxing dynamic cation is the major impediment to higher DNP efficiency, while microwave absorption and particle morphology play a secondary role. We then show that the former can be mitigated by deuteration, enabling 1H DNP enhancement factors of up to 100, which can be harnessed to enhance signals from dopants or additives present in very low concentrations. Specifically, using this new DNP methodology at a high magnetic field and with small sample volumes, we have recorded the NMR spectrum of the 20 nm (6 μg) passivating layer on a single perovskite thin film, revealing a two-dimensional (2D) layered perovskite structure at the surface that resembles the n = 1 homologue but which has greater disorder than in bulk layered perovskites.

A Complete Picture of Cation Dynamics in Hybrid Perovskite Materials from Solid-State NMR Spectroscopy

“…Having demonstrated this relaxometry methodology for MA + , FA + , and GUA + , we note that it can be readily extended to understand the dynamics of other relevant cations used in hybrid perovskites, such as dimethylammonium (DMA + ) − and methylenediammonium (MDA + ). , We further highlight that by understanding the FA + cation dynamics, we were recently able to mitigate the detrimental 1 H relaxation by deuteration and enable dynamic nuclear polarization experiments that show the structure of a surface coating on a single thin film . Overall, the complete picture of the cation dynamics presented here will help reveal the underlying causes of the beneficial optoelectronic properties seen in some hybrid perovskite formulations and thereby aid the design of perovskite solar cells with higher efficiencies in the future.…”

Redox Reactions at Colloidal Semiconductor Nanocrystal Surfaces

“…In recent years, dynamic nuclear polarization (DNP) enhanced MAS NMR spectroscopy has been used to overcome the sensitivity issues of MAS NMR methods. , Transfer of spin polarization from added stable organic radicals to the nuclear spin enhances the measurement up to 100-fold, resulting in shorter measurement times by 2–4 orders of magnitude and experiment times of tens of minutes. , Importantly, DNP enhanced NMR increases the intensity of resonances corresponding to nuclei at the NC surface. Studies by Kovalenko, Rossini, and others of CdSe NCs have been able to reveal the coordination environment of core and surface Cd 2+ and Se 2– centers. ,, These methods have also been applied toward other materials such as lead halide perovskite films and alkaline earth chalcogenide NCs …”

The race between complicated multiple cation/anion compositions and stabilization of FAPbI3 for halide perovskite solar cells

“…70,72 Very recently, MAS DNP has been demonstrated for the first time on an organic passivation layer in a single perovskite thin film with the total amount of passivation agents within the sample estimated to be 6 μg. 73 One of the areas where MAS DNP could be particularly insightful is the hypothesis that potassium ions passivate surface defects in hybrid halide perovskites, which is suggested to be their main mechanism of action. 23 MAS DNP has a unique capacity to enhance the sensitivity of surface NMR signals and, consequently, could be used to detect the hypothetical potassium species bound to perovskite surfaces.…”

“…71 Some work has been done exploring these concepts but effective application of MAS DNP to a wide range of halide perovskite materials has a tantalizing potential and is yet to be realized. 70,72 Very recently, MAS DNP has been demonstrated for the first time on an organic passivation layer in a single perovskite thin film with the total amount of passivation agents within the sample estimated to be 6 mg. 73 One of the areas where MAS DNP could be particularly insightful is the hypothesis that potassium ions passivate surface defects in hybrid halide perovskites, which is suggested to be their main mechanism of action. 23 MAS DNP has a unique capacity to enhance the sensitivity of surface NMR signals and, consequently, could be used to detect the hypothetical potassium species bound to perovskite surfaces.…”