Key Takeaways & Executive Findings
- •• By anchoring the perovskite sites with the functional groups of CzBP (P=O···Pb, N–H···I and P=O···N–H), the bulk nonradiative recombination is suppressed and ion migration is inhibited. Doping perovskite films with CzBP led to enhanced intercrystallite interactions in the bulk and improved photoluminescence quantum yield. • Using a typical electron-rich moiety as the π-linker to replace the classic alkyl spacer in CzBP facilitated the charge-carrier transport processes and the passivation effect of carbazole further contributed to high VOC. The optimized 2,7-CzBP-treated device achieves the highest power conversion efficiency (PCE) of 25.88%, with VOC of 1.189 V for 0.090 cm2 and the perovskite solar cell module with a PCE of 21.04% for 14 cm2. • For 2,7-CzBP, the more extended conjugation and the more linear molecular geometry result in a more effective improvement in the performance. • The study demonstrates that tuning the substitution positions and molecular shapes of organic additives is crucial for enhancing both efficiency and stability of perovskite solar cells, providing a universal guideline for designing next-generation multifunctional aromatic additives.
Abstract
Organic additives with multiple functional groups have shown great promise in improving the performance and stability of perovskite solar cells. The functional groups can passivate undercoordinated ions to reduce nonradiative recombination losses. However, how these groups synergistically affect the enhancement beyond passivation is still unclear. Specifically, isomeric molecules with different substitution patterns or molecular shapes remain elusive in designing new organic additives. Here, we report two isomeric carbazolyl bisphosphonate additives, 2,7-CzBP and 3,6-CzBP. The isomerism effect on passivation and charge transport process was studied. The two molecules have similar passivation effects through multiple interactions, e.g., P=O···Pb, P=O···H–N and N–H···I. 2,7-CzBP can further bridge the perovskite crystallites to facilitates charge transport. Power conversion efficiencies (PCEs) of 25.88% and 21.04% were achieved for 0.09 cm2 devices and 14 cm2 modules after 2,7-CzBP treatment, respectively. The devices exhibited enhanced operational stability maintaining 95% of initial PCE after 1000 h of continuous maximum power point tracking. This study of isomerism effect hints at the importance of tuning substitution positions and molecular shapes for organic additives, which paves the way for innovation of next-generation multifunctional aromatic additives.
1. Introduction
The power conversion efficiency (PCE) of single-junction perovskite solar cells (PSCs) has skyrocketed to >26% in recent years [1]. However, the efficiency is still far below the Shockley–Queisser limit and the durability remains a major barrier to commercialization. Organic additives are widely used in improving efficiency and stability of PSCs by passivating defects [2–4]. These additives contain polar functional groups such as amino, hydroxy and carbonyl. Related works researched the chemical space of organic additives by permutation of these groups, yet the synergy remains unclear. Typically, most organic additives are insulating materials that may hinder charge transport, and knowledge about other effects beyond passivation is limited [5, 6]. Custom-tailored organic additives that simultaneously work as superior morphology modulators, phase stabilizers, energy-level adjusters, defect passivators and stability enhancers are rare. There is much room to unveil the mechanism and develop universal guidelines for innovation of next-generation organic additives.
Beginning in 1970s, conjugated organic materials have found extensive use in electronics and photonics. Specifically, in single-molecule electronics, conjugated molecules installed by symmetric functional groups at the terminals present a myriad of interesting electrical properties. We expect that such structural motif shows great potential in designing bulk additives for PSCs, because the conjugated units can undoubtedly facilitate charge transport. A conjugated bulk additive may function as a bridge to connect cracked crystallites, with two binding sites to passivate undercoordinated ions at PVK surface. Different substitution positions of the functional groups further allow us to study the isomerism effect on device performance. Such structure–property relationship is valuable to unveil the underlying mechanism and expand the reach of new bulk additives.
With these concepts in mind, we designed and synthesized two isomeric molecules (Fig. 1a), 9H-carbazole-2,7-diyl tetraethyl bisphosphonate (2,7-CzBP) and 9H-carbazole-3,6-diyl tetraethyl bisphosphonate (3,6-CzBP).
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Qi Zhang, Qiangqiang Zhao, Han Wang, Yiguo Yao, Lei Li, Yulin Wei, Ruida Xu, Chenyang Zhang, Erik O. Shalenov, Yongguang Tu, Kai Wang, Mingjia Xiao (2025). Tuning Isomerism Effect in Organic Bulk Additives Enables Efficient and Stable Perovskite Solar Cells. Nano-Micro Letters. https://doi.org/10.1007/s40820-024-01613-z
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Frequently Asked Questions
What is the main finding of this study?
The study demonstrates that the isomerism effect of organic bulk additives significantly influences the performance and stability of perovskite solar cells. Specifically, the 2,7-CzBP additive, with its extended conjugation and linear geometry, achieves a high PCE of 25.88% for small-area devices and 21.04% for modules, while maintaining 95% of initial efficiency after 1000 hours of operation.
How do the two isomeric additives differ in their passivation effects?
Both 2,7-CzBP and 3,6-CzBP exhibit similar passivation effects through multiple interactions such as P=O···Pb, P=O···H–N, and N–H···I. However, 2,7-CzBP can further bridge perovskite crystallites, facilitating charge transport, which leads to superior device performance.
What is the significance of the isomerism effect in organic additives?
The isomerism effect highlights the importance of tuning substitution positions and molecular shapes in organic additives. This structural variation can enhance charge transport and passivation, providing a universal guideline for designing next-generation multifunctional aromatic additives for perovskite solar cells.
What are the key performance metrics achieved in this study?
The optimized 2,7-CzBP-treated devices achieved a power conversion efficiency (PCE) of 25.88% with an open-circuit voltage (VOC) of 1.189 V for 0.090 cm2 area, and a module PCE of 21.04% for 14 cm2 area. The devices also showed enhanced operational stability, retaining 95% of initial PCE after 1000 hours of continuous maximum power point tracking.
What is the potential application of this research?
This research provides a new strategy for designing organic bulk additives that can simultaneously passivate defects and facilitate charge transport, leading to more efficient and stable perovskite solar cells. This could accelerate the commercialization of perovskite photovoltaics.
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