MAI Termination Favors Efficient Hole Extraction and Slow Charge Recombination at the MAPbI₃/CuSCN Heterojunction

Abstract

Photoinduced charge separation is the key step determining the efficiency of photon-to-electron conversion in solar cells, while charge carrier lifetimes govern the overall solar cell performance. Experiments report that copper­(I) thiocyanate (CuSCN) is a very promising hole extraction layer for perovskite solar cells. Using nonadiabatic molecular dynamics combined with ab initio time-domain density functional theory, we show that termination of CH3NH3PbI3 (MAPbI3) at MAPbI3/CuSCN heterojunctions has a strong influence on both charge separation and recombination. Both processes are favored by MAI termination, compared to PbI2 termination. Because the MAPbI3 valence band originates from iodine orbitals while the conduction band arises from Pb orbitals, MAI termination places holes close to CuSCN, favoring extraction, and creates an MAI barrier for recombination of electrons in MAPbI3 and holes in CuSCN. The opposite is true for PbI2 termination. The origin of these effects is attributed solely to the properties of the MAPbI3 surfaces, and therefore, the conclusions should apply to other hole-transporting materials and can be generalized to other perovskites. Importantly, the simulations show that the injected hole remains hot for several hundreds of femtoseconds, allowing it to escape the interfacial region and prevent formation of bound excitons. This study suggests that metal halide perovskites should be treated with an organic precursor, such as MAI, prior to the formation of their interfaces with hole-transporting materials. The reported results advance the fundamental understanding of the highly unusual properties of metal halide perovskites and provide specific guidelines for optimizing the performance of perovskite solar cells and other devices.

Edge Influence on Charge Carrier Localization and Lifetime in CH3NH3PbBr3 Perovskite: Ab Initio Quantum Dynamics Simulation

“…The lighter and faster electrons are treated quantum mechanically, whereas the heavier and slower nuclei are handled semiclassically. , Decoherence, driven by coupling of the electronic system to atomic motions, destroys superposition formed between pairs of electronic states due to NA coupling and leads to branching of nuclear trajectories. , The decoherence time is estimated as the pure-dephasing time in the optical-response theory . Decoherence effects are needed because the decoherence time is significantly shorter than the nonradiative electron–hole recombination time. , The method has been applied successfully to investigate photoexcitation dynamics in a broad range of condensed-phase materials, including perovskites containing grain boundaries, chemical dopants, defects, and water molecules; low-dimensional Ruddlesden–Popper and Dion–Jacobson perovskites; , semiconducting − and metallic nanoparticles; , black phosphorus; , and so on. − …”

Dual-functional inorganic CuSCN for efficient hole extraction and moisture sealing of MAPbI3 perovskite solar cells

“…Here, we explore the CuSCN layer's dual function as a HTM and moisture sealant and correlate it with the device performance and stability in MAPbI 3 based devices. [18][19][20][21][22] Also, the aging characteristics of efficient working devices for more than 1500 h under ambient conditions (25 AE 3 1C and 50 AE 10% RH ) without interfacial and perovskite compositional modifications are not reported. In this study, we have investigated (i) the MAPbI 3 film formation by a one-step method, without any modifications of the MAPbI 3 perovskite or charge transport layers, and the structural and morphological evolution of the devices when stored under ambient conditions for 1500 h; (ii) the device fabrication in an n-i-p configuration using an FTO/compact TiO 2 /mesoporous TiO 2 /MAPbI 3 /HTM(spiro/CuSCN/HTM free)/Au architecture; (iii) the aging of the devices at 25 AE 3 1C and 50 AE 10% RH under ambient conditions for 1500 h. CuSCN HTM based PSC devices are found to have superior stability than spiro devices without compromising on the performance.…”

Core Fusion Engineering of Hole-Transporting Materials for Efficient Perovskite Solar Cells

“…As a solution-processable photovoltaic technology, perovskite solar cells (PSCs) have realized ultrahigh efficiency up to 25.2% during the past years due to the extraordinary optoelectronic properties of perovskite materials and great attention worldwide. − Generally, the structure of PSCs consists of a thick layer of perovskite light-harvesting material sandwiched between electron- and hole-transporting materials and two contact electrodes . Among them, the hole-transporting materials (HTMs) play a considerable role in hole extraction and transport in PSCs, especially for high-performance devices. , Compared to inorganic HTMs, − which usually show more surface defects, organic HTMs possess good solution-processability and tuneable film morphological properties, enabling superior interfacial contact, effective passivation effect, and reduced surface defects. − Among organic HTMs, small molecules exhibit well-defined structures, high purity, and good repeatability, which have been regarded as promising candidates for HTMs. , However, the widely used small molecular HTM spiro-OMeTAD suffers from relatively low thermal stability with the T g of 121 °C and unsatisfactory film morphology, which usually acquire a high concentration of HTM solution (60–80 mg mL –1 ), and the pinhole defects cannot be avoided in some cases . In addition, intrinsic low hole mobility is observed for spiro-OMeTAD due to its symmetric globular structure and the resultant weak π–π interactions. − Hence, it is imperative to explore new HTMs with good hole mobility and excellent film morphology.…”