Effect of a 2D-Modification of Cs2AgBiBr6 on Nucleation and Contact Formation of Subsequently Deposited Hole Transport Layers as Revealed by In Situ Growth Studies

TL;DR

This study shows how adding a 2D perovskite layer improves the growth of hole transport materials in solar cells, leading to better performance.

Contribution

The novel contribution is revealing how 2D perovskite modification enhances nucleation and contact formation of hole transport layers through in situ growth studies.

Findings

Modified Cs2AgBiBr6 improves energy alignment and homogenous growth of HTMs like CuPc and Pn.

CuPc forms different crystal phases on modified surfaces, while Pn forms dendritic islands.

2D perovskite interlayers enhance contact characteristics in perovskite solar cells.

Abstract

A two-dimensional (2D) perovskite interlayer prepared
by modification
of a three-dimensional (3D) perovskite absorber with organic ammonium
ions such as butylammonium (BA+) or phenethylammonium (PEA+) between the 3D perovskite and contact layers is widely known
to significantly improve the performance of perovskite solar cells.
This has also been confirmed previously for the lead-free double perovskite
absorber Cs2AgBiBr6. In this work, film growth
of copper phthalocyanine (CuPc) or pentacene (Pn), used as model hole
transport materials (HTM), was investigated. Mimicking solar cell
geometry, the HTMs were evaporated onto thin films of 2D perovskites
BA4AgBiBr8 or PEA4AgBiBr8, as well as on 3D Cs2AgBiBr6, either in its
pristine form or after modification by BA+ or PEA+. The morphology and work function were inspected intermittently
with respect to the evaporation of the HTMs by Kelvin probe force
microscopy at different average film thicknesses. By these means,
the origin of device improvements following a 2D-modification in contact
with HTMs, as established earlier, was revealed by analyzing in detail
the interface of the HTM with the respective perovskite starting at
monolayer coverage and proceeding toward bulk thickness. On modified
Cs2AgBiBr6, the energy alignment between the
perovskite and the HTM was found to be well confined, and the growth
of both HTMs was improved compared to pristine Cs2AgBiBr6. HTM growth occurred more homogeneously and led to layer
formation, even at early stages of deposition. For CuPc as HTM, these
changes were accompanied by preferential formation of needles in a
crystal phase different from that formed on pristine Cs2AgBiBr6, as also detected on 2D PEA4AgBiBr8. Pn formed large dendritic islands on the 2D perovskites
as well as on layered terraces formed upon ammonium modification of
Cs2AgBiBr6, in contrast to the growth of small
grains on pristine Cs2AgBiBr6. Implications
of these observed changes in film growth and energy level alignment
on the observed contact characteristics with the HTMs in model solar
cells are discussed. Insight into the mechanism of improving perovskite-based
devices by use of 2D/3D perovskite heterostructures is, thereby, provided
by these measurements using CuPc or Pn as model HTMs.

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