黄佳明

The ordered arrangement of methylamine dipoles is induced by the polar molecule to form a top-down macroscopic polarization of the film. Reorientation of dipoles contributes to forming a gradient energy band and changes the surrounding dielectric environment, facilitating efficient separation and directional transport of carriers. The photoelectric conversion efficiency and stability of the perovskite solar cells are significantly improved.

Abstract

Despite great progress in perovskite photovoltaics, it should be noted that the intrinsic disorder dipolar cations in organic–inorganic hybrid perovskites exert negative effects on the energy band structure as well as the carrier separation and transfer dynamics. However, oriented polarization achieved by applying an external electric field may cause irreversible damage to perovskites. Herein, a unique and efficient strategy is developed to modulate the intrinsic dipole arrangement in perovskite films for high-performance and stable perovskite solar cells (PSCs). The spontaneous reorientation of dipolar cation methylamine is triggered by a polar molecule, constructing a vertical polarization during crystallization regulation. The oriented dipole determines a gradient energy-level arrangement in PSCs and more favorable energetics at interfaces, effectively enhancing the built-in electric field and suppressing the nonradiative recombination. Besides, the dipole reorientation induces a local dielectric environment to remarkably reduce exciton binding energy, leading to an ultralong carrier diffusion length of up to 1708 nm. Accordingly, the n–i–p PSCs achieve a significant increase in power conversion efficiency, reaching 24.63% with negligible hysteresis and exhibiting outstanding stabilities. This strategy also provides a facile route to eliminate the mismatched energetics and enhance carrier dynamics for other novel photovoltaic devices.

A solution-processed SnOCl ternary tin (II) alloy as hole-transport material (HTM) for Sn–Pb perovskite solar cells is reported, which results in efficiencies of 23.2% and 26.3% for Sn–Pb single-junction and all-perovskite tandem cells, respectively. Greatly enhanced light stabilities with T87 of >1200 h and 85 °C thermal stability with T85 of >1500 h are achieved by using the SnOCl HTM.

Abstract

Tin–lead (Sn–Pb) narrow-bandgap (NBG) perovskites show great potential in both single-junction and all-perovskite tandem solar cells. Sn–Pb perovskite solar cells (PSCs) are still limited by low charge collection efficiency and poor stability. Here, a ternary Sn (II) alloy of SnOCl is reported as the hole-transport material (HTM) with a work function of 4.95 eV for Sn–Pb PSCs. The solution-processed SnOCl layer has a texture structure that not only reduces the optical loss of the devices, but also changes grain growth of Sn–Pb perovskites and boosts the carrier diffusion length to 3.63 µm. The formation of small perovskite grains at the HTM/perovskite interface is suppressed. These result in an almost constant internal quantum efficiency (IQE) of 96 ± 2% across the absorption spectrum of Sn–Pb perovskites. The SnOCl HTM significantly enhances the stability of Sn–Pb PSCs with 87% of its initial efficiency retained after 1-sun illumination for 1200 h, and keeps 85% efficiency under 85 °C thermal stress for 1500 h. The hybrid HTM further improves the stabilized efficiencies of single-junction Sn–Pb PSCs and all-perovskite tandem solar cells to 23.2% and 25.9%, respectively. This discovery opens an avenue to the multicomponent metal alloys as HTM in PSCs.

Novel electron-accepting material, PDI–Cb, is synthesized by introducing diphenyl-o-carboranyl groups at the bay positions of the PDI unit and its photophysical and electrochemical properties are systematically investigated. The inverted perovskite solar cells with PDI–Cb interlayer demonstrate enhanced 22.31% power conversion efficiency from efficient electron extraction capability and exhibit excellent photo- and thermal stability due to suppressed ion migration.

To investigate the synergistic effect of perylene diimide (PDI) unit and diphenyl-o-carboranyl (Cb) group, PDI–Cb with Cb groups at the bay positions of the PDI unit is synthesized. By introducing 3D carboranyl group into the bay position, PDI–Cb shows distorted geometry between PDI core and the adjacent phenyl ring of Cb due to the ring torsions, which can suppress the aggregation tendency. Furthermore, the proper lowest unoccupied molecular orbital (LUMO) energy level of −4.12 eV may be beneficial to extract electrons from the perovskite efficiently. By introducing PDI–Cb interlayer at perovskite/C60 interface, the inverted perovskite solar cells (PSCs) have significantly enhanced efficiency from 19.98% to 22.31% at 1 sun condition (AM1.5G 100 mW cm⁻²) because the PDI–Cb interlayer promotes charge extraction thanks to the electron-accepting ability of Cb group; thereby, it reduces carrier recombination significantly. In addition, the unencapsulated inverted PSC with the PDI–Cb interlayer has good photo- and thermal stability because it shows 9.3% efficiency degradation after continuous 1 sun light soaking for 1000 h at 85 °C in N₂ atmosphere.

Herein, the effect of silver alloying, absorber stoichiometry, and sodium supply on the performance of wide-gap Cu(In,Ga)Se₂ solar cells with transparent back contacts (TBCs) is studied. Tin- and hydrogen-doped In₂O₃ films are used as TBC materials. Efficiencies up to 12% (without antireflection coating) and very high infrared transparency are reached for an absorber bandgap of 1.44 eV.

Herein, the performance of wide-gap Cu(In,Ga)Se₂ (CIGS) and (Ag,Cu)(In,Ga)Se₂ (ACIGS) solar cells with In₂O₃:Sn (ITO) and In₂O₃:H (IOH) as transparent back contact (TBC) materials is evaluated. Since both TBCs restrict sodium in-diffusion from the glass substrate, fine-tuning of a NaF precursor layer is crucial. It is found that the optimum Na supply is lower for ACIGS than for CIGS samples. An excessive sodium amount deteriorates the solar cell performance, presumably by facilitating GaO _x growth at the TBC/absorber interface. The efficiency (η) further depends on the absorber stoichiometry, with highest fill factors (and η) reached for close-stoichiometric compositions. An ACIGS solar cell with η = 12% at a bandgap of 1.44 eV is processed, using IOH as a TBC. The best CIGS device reaches η = 11.2% on ITO. Due to its very high infrared transparency, IOH is judged superior to ITO for implementation in a top cell of a tandem device. However, while ITO layers maintain their conductivity, IOH films show an increased sheet resistance after absorber deposition. Chemical investigations indicate that incorporation of Se during the initial stage of absorber processing may be responsible for the deteriorated conductivity of the IOH back contact in the final device.

The effect of A-site doping with guanidinium cation (GA⁺) on ion migration is explored based on the temperature-dependent ion conductivity. The increase of activation energy proves the lift of ion migration barrier. It elaborates that doping of GA⁺ can effectively suppress the ion migration and enhance the operational stability of the device.

Perovskite solar cells (PSCs) develop great potential to make photovoltaic power generation systems more cost-effective due to the high power conversion efficiency (PCE), low material cost, and easy fabrication. Alloyed A-site cations surrounded by PbI₆ octahedra play decisive roles in the crystal structure, bandgap, and phase stability, as well as ion migration. Herein, based on the temperature-dependent ion conductivity measurement, the activation energy for iodide ion migration is systematically studied with different proportions of guanidinium cation (GA⁺) substitution. It is found that partial GA⁺ doping could effectively suppress iodide ion migration. The triple-cation perovskite (MA_0.8FA_0.1GA_0.1PbI₃) PSCs achieve a PCE of 22.17% with superior operational stability maintaining 90% of their initial efficiency after 1200 h under continuous light soaking. Furthermore, it is extended to mini perovskite solar modules, 14 cm² active area, and achieves a PCE of 19.18%.

Nickel oxide serves as an inexpensive and stable charge-transporting layer for perovskite solar cells (PSCs). However, its high-temperature processing limits its applicability to low-temperature-processed devices. Herein, ligand-modified NiO nanoparticles are shown to permit low-temperature (140 °C) processing into high-quality thin films using a Tesla-valve microfluidic mixer, which are suitable for developing stable and efficient PSCs.

Nickel oxide (NiO) is used as a hole-transporting layer (HTL) in perovskite solar cells (PSCs) because of its high optical transmittance, intrinsic p-type doping, and suitable valence band energy level. However, fabricating high-quality NiO films typically requires high-temperature annealing, which limits their applicability for low-temperature, printable PSCs. Herein, the need for such postprocessing steps is circumvented by coupling 4-hydroxybenzoic acid (HBA) or trimethyloxonium tetrafluoroborate (Me₃OBF₄) ligand-modified NiO nanoparticles (NPs) with a Tesla-valve microfluidic mixer to deposit high-quality NiO films at a temperature <150 °C. The NP dispersions and the resulting thin films are thoroughly characterized using a combination of optical, structural, thermal, chemical, and electrical methods. While the optical and structural properties of the ligand-exchanged NiO NPs remain comparable with those possessing the native long-chained aliphatic ligands, the ligand-modified NiO thin films exhibit dramatic reductions in surface energy and an increase in hole mobilities. These are correlated with concomitant and significant enhancements in performance and stability factors of PSCs when the ligand-modified NiO NPs are used as HTL layers within p−i−n device architectures.