Different interface engineering approaches have been adopted for development of high efficiency and stable perovskite solar cells (PSCs). In this work, we report our efforts for realizing high efficiency PSCs using a poly(methyl methacrylate) (PMMA):2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)-modified poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA) hole-transporting layer (HTL). The results reveal that PMMA molecules on PTAA induce an interface dipole at the PTAA/perovskite interface, caused by the electrostatic interaction between the ester groups in the PMMA molecules and the electron-deficient N⁺ free radicals in the PTAA molecules. The existence of the PMMA-induced interface dipole at the PTAA/perovskite interface increases the built-in potential in the PSCs, resulting in a 60 mV increase in the open circuit voltage. The increase in the built-in potential aids in an efficient charge collection process. The incorporation of F4-TCNQ molecules into PMMA further promotes the hole extraction by creating an intermediate state. In addition, PMMA and F4-TCNQ, especially PMMA, also passivate interface defects and promote perovskite crystallization. As a result, the MAPbI₃-based PSCs with a PMMA:F4-TCNQ-modified PTAA HTL achieve a power conversion efficiency of >20%, which is evidently higher than that of a control PSC with a pristine PTAA HTL (∼17%). Moreover, the stability of PSCs is also improved efficiently. Our findings unveil the importance of PMMA:F4-TCNQ interfacial modification for mitigating charge recombination loss through controlled growth of the perovskite semiconductor layer, reduced interface defects and high built-in potential for efficient operation of the PSCs.

Constructing a van der Waals (vdW) heterostructure is a promising approach to tackle the bandgap hurdle of graphene meanwhile preserving its excellent electronic properties. The unique symmetry of atomic arrangement in C₃B and C₃N monolayers could induce interesting properties in the graphene/C₃B (C₃N) heterostructure. Based on the density functional theory calculation, we demonstrate that the bandgap near the Dirac cone depends sensitively on the stacking order in the graphene/C₃B (C₃N) vdW heterostructure. The bandgap is opened in the AB stacking order, but is completely closed in the AA stacking order. The bandgap closing is protected by the AA stacking order even under vertical strain, while the bandgap increases in the AB stacking order with a positive vertical strain. Based on the tight-binding model, the origin of the bandgap opening and closing is explained by the stacking-order dependent charge transfer. Moreover, the transport properties of the devices assembled by the graphene/C₃B vdW heterostructures are investigated based on the non-equilibrium Green's function method plus the density functional theory. The conductance can be turned on or off depending on the stacking order. This finding is useful for the future design of on/off devices based on graphene/C₃B (C₃N) heterostructures.

Electronic and optical responses of zigzag- and armchair-edge quasi-one-dimensional phosphorene nanoribbons (Q1D-PNRs) to strain and external field are comparatively studied based on the tight-binding calculations. The results show that: (i) Zigzag-edge Q1D-PNR has the metallic ground state; applying global strains can not open the gap at the Fermi level but applying the electric field can achieve it; the direct/indirect character of the field-induced gap is determined by the electron-hole symmetry; an electric-field-enhanced optical absorption of low-energy photons is also predicted. (ii) Armchair-edge Q1D-PNR turns out an insulator with the large direct band gap; the inter-plane strain modulates this gap non monotonically while the in-plane one modulates it monotonically; in addition, the gap responses to electric fields also show strong direction dependence, i. e., increasing the inter-plane electric field will monotonically enlarge the gap but the electric field along the width direction modulates the gap non monotonically with three characteristic response regions.

In donor–acceptor (D–A) heterojunction organic solar cells, hot and cold charge transfer (CT) states are formed at the interface as the precursor for subsequent charge separations. Hot CT states dissociate easily because they are loosely bound, while for cold CT states, the origin of their high-efficiency charge separations still remains heavily debated. Here, we propose a simple but effective methodology that can be used to simulate the cold CT dissociation process and, thereby, the multiple factors which may essentially affect the charge separation efficiency and can be conveniently investigated. The energy barriers on the path from cold CT to the separated charges are analyzed by calculating the adiabatic potential energy surfaces of the lowest-energy excitonic state. The calculation results indicate that the D–A molecular coupling strength and coupling area, D–A energetic offset, charge carrier delocalizations, interfacial Coulomb screening strength, and interfacial disorders can essentially affect the charge separation efficiency of a cold CT state.

The synergistic effect of the carrier velocity and density is studied to understand chirality-induced spin selectivity (CISS) in helical spin-filtering devices. Unlike most previous theories that emphasize the asymmetry between the transmitted spin-related electron densities, we reveal two types of dynamics for the spin polarization: (i) spin conversion dynamics between spin-up and spin-down electrons and (ii) spin-dependent transport dynamics leading to different propagation velocities of spin-up and spin-down electrons. The former dominate in the high-bias-voltage regime by electron tunneling, while the latter dominate in the low-bias-voltage regime by polaron transport. Our theory reproduces well the recently observed extreme spin polarization under a certain bias voltage in a helical spin-filtering device, and it provides a general microscopic perspective for reconsidering CISS.