# Alkylammonium Spacer-Directed Charge-Transfer in 2D|3D Perovskite Solar Cells

## TL;DR

This study explores how different alkylammonium spacers affect the stability and efficiency of perovskite solar cells.

## Contribution

The paper reveals how spacer chemistry and concentration influence the balance between charge transport and durability in 2D|3D perovskite solar cells.

## Key findings

- Low concentrations of BAI and BDAI2 act as surface passivators without reducing efficiency.
- High BAI concentrations improve stability and moisture resistance but BDAI2 causes rapid degradation.
- Excess spacers create insulating barriers that hinder charge transport.

## Abstract

Long-term stability
is a key challenge for Perovskite Solar Cells
(PSCs). A promising strategy is to construct two-dimensional|three-dimensional
(2D|3D) lead-halide perovskite heterostructures, which integrate the
high efficiency of 3D phases with the high durability of 2D layers.
These layers are constructed on 3D perovskites by spreading spacer
solutions onto their surfaces, forming 2D|3D structures. Here, we
systematically investigate the role of two alkylammonium spacers by
using butylammonium iodide (BAI, a Ruddlesden–Popper spacer)
and butyl-1,4-diammonium diiodide (BDAI2, a Dion-Jacobson
spacer) in two different concentration regimes. At low spacer concentrations
(5 mmol L–1 BAI and 0.5 mmol L–1 BDAI2), both spacers primarily acted as surface passivators,
yielding efficiencies comparable to those of pristine methylammonium
lead iodide perovskite. High spacer concentrations (50 mmol L–1 BAI and 5 mmol L–1 BDAI2) induced layered 2D|3D phases with contrasting effects. BAI promoted
structural flexibility (by varying n from 1 to 2)
and improved moisture resistance, thereby enhancing device stability.
BDAI2 unexpectedly leads to rapid degradation under ambient
processing conditions. Photoluminescence, conductive atomic force
microscopy, and electrochemical impedance spectroscopy confirmed that
excessive spacer incorporation introduces insulating barriers that
hinder charge transport. Optimized concentrations suppress nonradiative
recombination without compromising conductivity. Durability tests
demonstrated that BAI consistently prolonged device lifetime, while
BDAI2 devices degraded rapidly. These results reveal that
spacer chemistry and concentration critically determine the trade-off
between charge transport and environmental resilience in 2D|3D PSCs.
By clarifying these mechanisms, this work establishes insights into
the rational design of spacer-assisted perovskites, offering a pathway
toward more durable and commercially viable perovskite photovoltaics.