# Trap states impact photon upconversion in rubrene sensitized by lead halide perovskite thin films

## TL;DR

This study investigates how trap states in lead halide perovskite thin films influence photon upconversion efficiency when sensitized by rubrene, revealing that film thickness and trap passivation affect recombination mechanisms and triplet sensitization.

## Contribution

It demonstrates the impact of trap states on photon upconversion in rubrene-perovskite systems and suggests charge transfer pathways as an alternative to excitonic singlet-to-triplet conversion.

## Key findings

- Thicker MAFA films show a shift from monomolecular to bimolecular recombination.
- Rubrene passivates the perovskite surface but introduces charge transfer quenching.
- Efficient triplet-triplet annihilation occurs at lower powers with increased MAFA thickness.

## Abstract

The same optical and electronic properties that make perovskite thin films ideal absorber materials in photovoltaic applications are also beneficial in photon upconversion devices. In this contribution, we investigate the rubrene-triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA). To elucidate the role of trap states which affect the free carrier lifetimes, we fabricate MAFA perovskite thin films with three different thicknesses. By measuring the change in the photoluminescence properties under different excitation fluences, we find that the prevalent recombination mechanism shifts from monomolecular for thinner films to bimolecular recombination for thicker MAFA films, indicating a reduction in shallow trap-assisted recombination. The addition of rubrene shows a passivating effect on the MAFA surface, but adds an additional quenching pathway due to charge transfer to the triplet state of rubrene. We observe that the threshold for efficient triplet-triplet annihilation shifts to lower incident powers with increasing MAFA thickness, which suggests that the charge transfer to the triplet state competes with non-radiative trap filling. Hence, injection of free electrons and holes into the upconverting organic semiconductor can provide a new avenue for sensitization of rubrene, and may allow us to move away from the necessity of efficient excitonic singlet-to-triplet converters.

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Source: https://tomesphere.com/paper/1902.01371