Ligand-Induced In Situ Epitaxial Growth of PbI2 Nanosheets/MAPbI3 Heterojunction Realizes High-Performance HTM-Free Carbon-Based MAPbI3 Solar Cells.

Hole-transporting layer-free carbon-based perovskite solar cells (HTL-free C-PSCs) hold great promise for photovoltaic applications due to their low cost and outstanding stability. However, the low power conversion efficiency (PCE) of HTL-free C-PSCs mainly results from grain boundaries (GBs). Here, epitaxial growth is proposed to rationally design a hybrid nanostructure of PbI2 nanosheets/perovskite with the desired photovoltaic properties. A post-treatment technique using tri(2,2,2-trifluoromethyl) phosphate (TFEP) to induce in situ epitaxial growth of PbI2 nanosheets at the GBs of perovskite films realizes high-performance HTL-free C-PSCs. The structure model and high-resolution transmission electron microscope unravel the epitaxial growth mechanism. The epitaxial growth of oriented PbI2 nanosheets generates the PbI2 /perovskite heterojunction, which not only passivates defects but forms type-I band alignment, avoiding carrier loss. Additionally, Fourier-transform infrared spectroscopy, 31 P NMR, and 1 H NMR spectra reveal the passivation effect and hydrogen bonding interaction between TFEP and perovskite. As a result, the VOC is remarkably boosted from 1.04 to 1.10 V, leading to a substantial gain in PCE from 14.97% to 17.78%. In addition, the unencapsulated PSC maintains the initial PCE of 80.1% for 1440 h under air ambient of 40% RH. The work offers a fresh perspective on the rational design of high-performance HTL-free C-PSCs.

Eu-Based Porphyrin MOF Enables High-Performance Carbon-Based Perovskite Solar Cells.

The low power conversion efficiency (PCE) of hole transport materials (HTM) - free carbon-based perovskite solar cells (C-PSCs) poses a challenge. Here, a novel 2D Eu-TCPP MOF (TCPP; [tetrakis (4-carboxyphenyl) porphyrin]) sandwiched between the perovskite layer and the carbon electrode is used to realize an effective and stable HTM-free C-PSCs. Relying on the synergistic effect of both the metal-free TCPP ligand with a unique absorption spectrum and hydrophobicity and the EuO4(OH)2 chain in the Eu-TCPP MOF, defects are remarkably suppressed and light-harvesting capability is significantly boosted. Energy band alignment is achieved after Eu-TCPP MOF treatment, promoting hole collection. Förster resonance energy transfer results in improved light utilization and protects the perovskite from decomposition. As a result, the HTM-free C-PSCs with Eu-TCPP MOF reach a champion PCE of 18.13%. In addition, the unencapsulated device demonstrates outstanding thermal stability and UV resistance and keeps 80.6% of its initial PCE after 5500 h in a high-humidity environment (65%-85% RH).