A study of the effects of a thermally evaporated nanoscale CsBr layer on the optoelectronic properties and stability of formamidinium-rich perovskite solar cells

Richard K Koech,Dahiru M Sanni,Winston O Soboyejo,Sharafadeen A Adeniji,Deborah O Oyewole,Reisya Ichwani,Omolara V Oyelade,Esidor Ntsoenzok,Julia L Martin,Abdulhakeem Bello,Ronald L Grimm,Yusuf A Olanrewaju,Oluwaseun K Oyewole

doi:10.1063/5.0064398

Abstract

Incorporation of cesium (Cs) into the perovskite layer has become a good strategy to boost the stability and power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, a suitable and scalable method of Cs incorporation in a perovskite film that does not cause a significant increase in the optical bandgap is needed. In this paper, we introduce a thin layer of CsBr into a formamidinium (FA)-rich mixed halide perovskite film using the thermal evaporation technique. The effects of the thickness of the CsBr layer on the microstructural, structural, and optoelectronic properties and surface chemical states of the perovskite film are then studied. The results indicate that the CsBr layer thickness is able to tune the microstructural and optoelectronic properties of the perovskite film. Planar PSCs fabricated with different thicknesses of CsBr layers in the perovskite absorber exhibited different photovoltaic performance characteristics. The CsBr-modified PSC device with a 50 nm layer of CsBr in the perovskite layer showed a better PCE of 16.19% ± 0.17%, which was about 15% higher than that of the control device, and was able to retain nearly 70% of its initial PCE value after 120 days of storage in an unencapsulated state.

Highlights

We observe a change in the morphology of perovskite films with the incorporation of cesium bromide scitation.org/journal/adv (CsBr), which implies that the thickness of the CsBr layer affected their crystallization dynamics
The CsBr30 and CsBr50 show surfaces with no distinct boundaries between adjacent grains, while the CsBr100 shows some grain boundaries, but they are not as pronounced as those of CsBr0. This shows that the CsBr-modified perovskite films had more compact films with well-passivated grain boundaries. This grain boundary passivation effect of CsBr is important in addressing the problems in perovskite solar cells (PSCs) that are caused by the presence of defects
A thermally evaporated CsBr layer of appropriate thickness in a FA-rich perovskite film passivates the grain boundaries and modifies the interfacial energetics with adjacent charge transport layers (CTLs). This improves the charge transport and PV performance characteristics of PSC while simultaneously suppressing the degradation of the perovskite layer when exposed to ambient conditions

Summary

Introduction

Perovskite solar cells (PSCs) are emerging solar cells with unique optoelectronic properties and simple processing routes that make them stand out from other photovoltaic (PV) technologies in the pursuit of high performance and low-cost solar energy harnessing systems. Within a short period of time, PSCs have achieved a remarkable progress in their power conversion efficiencies (PCEs) and are currently almost at par with the conventional PV technologies based on crystalline silicon. This progress was a culmination of many concerted efforts from researchers drawn from different disciplines that yielded a better understanding of their structure, optoelectronic properties, and their working principles. the PSC technology is still faced with myriads of challenges, which, among others, include non-radiative recombination power losses and performance degradation under the real outdoor operating conditions.7,8Various studies that have been carried out to probe the origin of the abovementioned challenges have pointed out that they are majorly caused by perovskite phase instability and the presence of defects in the bulk of the active layer and at the interfaces with the charge transport layers (CTLs). The morphology of the perovskite film plays a big role in controlling the defect states in the bulk and at the interfaces. Within a short period of time, PSCs have achieved a remarkable progress in their power conversion efficiencies (PCEs) and are currently almost at par with the conventional PV technologies based on crystalline silicon.4 This progress was a culmination of many concerted efforts from researchers drawn from different disciplines that yielded a better understanding of their structure, optoelectronic properties, and their working principles.. Research efforts have been expended to improve the morphology of the perovskite active layer through compositional engineering, solvent engineering, optimization of perovskite pre-cursors, additive engineering, improving deposition conditions, and optimization of post-treatment techniques.17 These strategies are aimed at controlling the nucleation and crystallization dynamics of the perovskite film with a view of passivating the defects at grain boundaries, which are the genesis of most problems in PSCs.. Research efforts have been expended to improve the morphology of the perovskite active layer through compositional engineering, solvent engineering, optimization of perovskite pre-cursors, additive engineering, improving deposition conditions, and optimization of post-treatment techniques. These strategies are aimed at controlling the nucleation and crystallization dynamics of the perovskite film with a view of passivating the defects at grain boundaries, which are the genesis of most problems in PSCs. The tunability of the perovskite (ABX3) structure through elemental substitution and mixing has given rise to perovskite films with mixed cations and mixed halides, which have better morphologies and superior photophysical properties. The elemental substitution/mixing tunes the tolerance factor that stabilizes the perovskite phase and improves its stability.

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Conclusion