# Deepening the LUMO: Brominated Naphthalene Diimide Electron Transport Layers for Low-Hysteresis Perovskite Solar Cells

## TL;DR

This paper introduces a new organic material for solar cells that improves efficiency and reduces performance issues by adjusting its electronic structure.

## Contribution

The novel brominated naphthalene diimide derivative with a deeper LUMO is introduced for perovskite solar cells.

## Key findings

- Br2-NDI-(BnPA)2 achieved a 13.67% power conversion efficiency in PSCs.
- The brominated derivative reduced hysteresis and improved electron extraction.
- Bromination deepened the LUMO by 0.29 eV, enhancing energy alignment with perovskite.

## Abstract

Precise energy level
alignment at the interfaces between the charge
transport layers, active layer, and electrodes plays a key role in
maximizing photovoltaic performance and operational stability in perovskite
solar cells (PSCs). Organic electron transport layers (ETLs) have
received little attention compared to their inorganic counterparts
but offer the unique advantage of facile functionalization for fine-tuning
of electronic properties. Here, we report the design, synthesis, and
characterization of two benzyl-phosphonic acid (BnPA)-functionalized
naphthalene diimide (NDI) derivatives, NDI-(BnPA)2 and
Br2-NDI-(BnPA)2 as organic ETLs for PSCs in
an n-i-p device configuration. Bromination of the NDI core at the
4,9-positions deepens the lowest unoccupied molecular orbital (LUMO)
in Br2-NDI-(BnPA)2 by 0.29 eV, enabling improved
energy alignment with the conduction band minimum of the perovskite.
Devices incorporating Br2-NDI-(BnPA)2 demonstrated
enhanced short-circuit current density (J
SC), reduced hysteresis, and a maximum power conversion efficiency
of 13.67%, compared to 13.20% for the unsubstituted NDI-(BnPA)2 analog. The deeper LUMO of Br2-NDI-(BnPA)2 is hypothesized to facilitate more efficient electron extraction,
suppress interfacial charge accumulation, and reduce field-driven
ion migration, collectively contributing to the observed reduction
in hysteresis. These results highlight the effectiveness of combining
molecular-level LUMO tuning with robust interfacial anchoring to advance
the performance and durability of organic ETLs in PSCs.