Small Molecule Passivation Leading to Efficient Hole Transport Layer‐Free Sn–Pb Mixed Perovskite Solar Cells with High Open‐Circuit Voltage

Abstract

There have been a number of remarkable signs of progress achieved in tin–lead mixed narrow‐bandgap perovskite solar cells (PSCs) due to the high theoretical power conversion efficiency (PCE) and their promising application in tandem devices. Indeed, Sn–Pb mixed PSCs without a hole transport layer (HTL) also have been more attractive owing to lower cost and simplification of the device structure. However, the defects in perovskite film introduced by Sn2+ oxidation severely restrict device efficiency and stability.Herein, a small organic molecule, 4,4'‐sulfonyldiphenol, is employed to passivate perovskite (Eg = 1.26 eV) surface to decrease the interfacial defects and suppress the nonradiative carrier recombination. Furthermore, by regulating energy‐level alignment, charge carrier extraction is greatly facilitated. The device performance is significantly enhanced in that the champion PCE is enlarged to 21.43% with an open‐circuit voltage (Voc) of 0.876 V from only 18.02% with a Voc of 0.770 V. The stability of unencapsulated devices is improved substantially as well while retaining 80% PCE of its initial value after being stored in the glovebox for around 600 h. This facile but highly effective strategy successfully proposes the promising development of HTL‐free Sn–Pb mixed PSCs.

Ligand Engineering in Tin-Based Perovskite Solar Cells

“…One of the main challenges is still the Sn 2+ oxidation and relevant defects. Ligand engineering in Sn-Pb mixed perovskites have also been widely studied, including SnX 2 additive [169], coordination with I anions [170][171][172] or Sn cations [173,174], formation of low-dimensional structure [175], improvement of stability by A-& X-site mixing [176,177] or posttreatment [178], etc. The research on Sn-based perovskite and Sn-Pb mixed perovskite can be mutually referenced to solve the problem of commonality.…”

Eco‐friendly perovskite solar cells: From materials design to device processing and recycling

“…Moreover, Lewis base molecules with electron withdrawing groups (such as sulfonate group, pyridine unit, thiophene unit, and cyano groups) could also efficiently reduce the trap density of Sn‐based perovskite 76–80 . Li et al employed the SCN functional group to strongly coordinate with charged defects and reduce the deep‐level trap state density of CsSnI 3 perovskite (Figure 2E).…”

Enhancement and Broadening of the Internal Electric Field of Hole-Transport-Layer-Free Perovskite Solar Cells by Quantum Dot Interface Modification

“…However, the efficiency of HTL-free PSCs is still much lower than that of fully structured perovskite solar cells, which is not only related to the film formation quality of each functional layer but also a key challenge is the severe energy-level mismatch between the direct contact of the perovskite layer and the counter electrode, which causes a large number of photogenerated carriers to compound, thus limiting the device efficiency. − Therefore, a large amount of work has been devoted to exploring and designing methods that favor charge separation, including the quality of the perovskite layer film, − the better charge transport of the electron transport layer, , suppression of the active defects, − surface modification, and other modulation strategies, which have made significant contributions to high-performance HTL-free PSCs with efficient charge separation. At present, inorganic perovskite quantum dots (QDs) covered with hydrophobic ligands provide an effective strategy for simultaneously passivating surface and internal defects of perovskite materials. , The organic ligands attached to QDs can self-assemble at interfaces and grain boundaries to achieve defect passivation (such as undercoordinated Pb 2+ ions and halide vacancies), , while hydrophobic quantum dots benefit from their soft ionic lattice, which can be well bonded into halide perovskite films by ion exchange and diffusion to passivate the vacancy defects .…”