On November 14, Dr. Zhang Jie, an associate researcher at the Photonic Information and Energy Materials Research Center of the Institute of Advanced Materials Science and Engineering, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, collaborated with Professor Ren Guangyu’s team from City University of Hong Kong and Professor Zhang Chunfeng's team from Nanjing University to publish a groundbreaking research article titled "Suppressed Recombination Loss in Organic Photovoltaics Adopting a Planar-Mixed Heterojunction Architecture" in *Nature Energy*.
This research leverages transient absorption spectroscopy (TAS) and molecular dynamics simulations to explore the dynamic processes of photogenerated carriers and excited states within organic photovoltaic devices under various architectures. For the first time, the team demonstrated a device engineering method capable of regulating triplet excitons (T1) within the same material system. This advancement significantly enhances the short-circuit current (JSC) without compromising the open-circuit voltage (VOC), thus breaking through the conventional voltage-current balance limitations in traditional organic photovoltaic devices. Consequently, the study achieved a power conversion efficiency exceeding 19%. It also uncovered the relationship between T1 and device performance in organic photovoltaic devices, offering a novel perspective for analyzing structure-performance correlations and optimizing high-efficiency devices.
Organic solar cells represent a promising thin-film photovoltaic technology that can be fabricated via solution processing at low temperatures, making them compatible with flexible roll-to-roll production techniques. Additionally, their tunable spectral transmission holds significant potential for applications in distributed photovoltaics, building-integrated photovoltaics, and agrivoltaics. However, the relatively low power conversion efficiency remains a major challenge for further advancements in this field. Recent developments in polymer donors and non-fullerene acceptors, particularly the discovery of the star molecules ITIC and Y6, have spurred rapid progress in organic solar cell efficiencies. Currently, achieving a power conversion efficiency of over 20% in single-junction organic photovoltaic devices represents a critical focus for researchers in this domain.
Triplet excitons (T1) are believed to play a crucial role in determining the performance of organic photovoltaic devices. Therefore, understanding the regulatory mechanisms of T1 and its direct correlation with device performance is essential for improving device efficiency. Traditionally, scientists have relied on molecular engineering to adjust acceptor materials and modify the wavefunctions and excited-state energy levels in thin-films, often associating T1 with VOC. However, these studies were conducted across different material systems, making it challenging to establish a clear link between T1 and device performance due to varying factors influencing VOC in different material contexts.
In light of these challenges, the research team employed TAS characterization techniques to demonstrate that altering the device architecture can significantly impact T1 formation. By transitioning from a traditional bulk-phase heterojunction (BHJ) to a planar-mixed heterojunction (PMHJ) architecture within the same material system, the team confirmed that locally excited state excitons (LE) rapidly evolve into delocalized singlet excitons (DSE) with lower binding energies. These DSEs can directly transition to charge-separated states, generating photogenerated free electrons and holes. This mechanism ensures that the reduced donor-acceptor interfaces in PMHJ devices do not hinder exciton dissociation.
Through TAS analysis, the study revealed that while the T1 signal intensity in PMHJ devices is weaker than in BHJ devices, the excited state absorption signals of free carriers are significantly stronger. This suggests that PMHJ devices produce more free photogenerated carriers. Kinetic results indicate that T1 arises following the generation of charge-separated states, meaning that both PMHJ and BHJ devices derive T1 from the bimolecular recombinative behavior of free photogenerated carriers at donor-acceptor interfaces. Thus, the smaller donor-acceptor contact area in PMHJ devices reduces the likelihood of photogenerated electrons and holes recombining at donor-acceptor interfaces, effectively blocking the back-transition of carriers from charge-separated states to triplet charge transfer states (3CT/1CT). This approach significantly minimizes carrier losses during the excited state evolution process in organic photovoltaic devices.
The study found that PMHJ devices exhibit higher JSC compared to BHJ devices, with similar VOC values, demonstrating superior voltage-current balance. Furthermore, no correlation between T1 formation and VOC was observed in the D18 and Y6 acceptor material systems. These findings provide fresh insights into exploring the mechanisms linking T1 and device performance, as well as reducing voltage-current balance constraints to achieve even higher energy conversion efficiencies.
This research was supported by funding from organizations including the National Natural Science Foundation of China, the Ministry of Science and Technology, the Guangdong Provincial Department of Science and Technology, the Shenzhen Science and Technology Innovation Commission, the Hong Kong Innovation and Technology Agency, and the Hong Kong Research Grants Council.
[Image captions]
- Schematic diagram of the excited state evolution process in organic photovoltaic devices, TAS analysis of acceptor molecules, and theoretical calculation results.
- Dynamics of triplet excitons (T1) in planar hybrid heterojunction and bulk phase heterojunction architectures.
- Analysis of device performance and photovoltaic losses.
These figures provide visual representations of the experimental findings and theoretical models, reinforcing the study's conclusions.
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