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 at the 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 study titled "Suppressed Recombination Loss in Organic Photovoltaics Adopting a Planar-Mixed Heterojunction Architecture" in *Nature Energy*.
This work leverages transient absorption spectroscopy (TAS) and molecular dynamics calculations to investigate the dynamic processes of photogenerated carriers and excited states within organic photovoltaic devices under various architectural configurations. For the first time, this research demonstrates that device engineering techniques can be used to regulate triplet excitons (T1) within the same material system. By doing so, it enhances the short-circuit current (JSC) without compromising the open-circuit voltage (VOC). This breakthrough surpasses the conventional voltage-current balance limitations in traditional organic photovoltaic devices, achieving a power conversion efficiency exceeding 19%. This study reveals a significant correlation between T1 and device performance in organic photovoltaics, offering a fresh perspective for understanding the structural-effect relationships and optimizing high-efficiency organic photovoltaic devices.
Organic solar cells represent a promising thin-film photovoltaic technology that can be fabricated at low temperatures via solution processing. These cells are compatible with flexible roll-to-roll manufacturing techniques and allow for tunable spectral transmission, making them ideal for applications in distributed photovoltaics, building-integrated photovoltaics, and agrivoltaics. Despite these advantages, the lower power conversion efficiency (PCE) remains a critical barrier to its broader adoption. Recent advancements in polymer donors and non-fullerene acceptors, particularly the discovery of the star molecules ITIC and Y6, have spurred rapid progress in PCE. Currently, boosting the PCE of single-junction organic photovoltaic devices beyond 20% is a major focus of research in the field.
In organic photovoltaic devices, T1 is thought to play a crucial role in device performance. Thus, understanding the regulatory mechanisms of T1 and its direct impact on device performance is essential for further enhancing organic photovoltaic performance. Typically, scientists manipulate acceptor materials through molecular engineering to adjust wavefunctions and excited-state energy levels in thin films, linking T1 to VOC. However, much of this research relies on different material systems, making it challenging to establish clear correlations between T1 signal strength and device VOC across diverse contexts. As VOC is influenced by multiple factors in individual material systems, the precise regulatory mechanisms of T1 and its direct link to device performance remain unclear.
Taking these considerations into account, the research team utilized TAS characterization techniques to demonstrate that altering the device architecture—from traditional bulk-phase heterojunctions (BHJ) to planar hybrid heterojunctions (PMHJ)—in the same material system significantly affects the formation of T1. Through molecular dynamics and TAS methods, the team found that upon excitation in D18 polymer and Y6-series small molecule films, localized excited-state excitons (LE) rapidly transform into delocalized singlet excitons (DSE) with lower binding energies. These DSEs can directly transition to a charge-separated state, generating photogenerated free electrons and holes, bypassing the need for the formation of charge transfer states (CT) at the interface. This mechanism ensures that the reduced donor-acceptor interfaces in PMHJ architecture devices do not hinder exciton dissociation.
Through TAS analysis, the study revealed that while the T1 signal strength in PMHJ architecture is significantly weaker compared to BHJ architecture, the excited state absorption signal (CS) of free carriers is notably stronger. This suggests that more free photogenerated carriers are produced in PMHJ architecture devices. Kinetic results from TAS tests indicate that T1 forms after the generation of CS, meaning that T1 in both PMHJ and BHJ devices arises from the bimolecular recombination behavior of free photogenerated carriers at the donor-acceptor interface. Consequently, the reduced donor-acceptor contact area in PMHJ architecture effectively minimizes the chances of photogenerated electrons in the acceptor meeting photogenerated holes in the donor at the donor-acceptor interface. In essence, this device engineering approach successfully blocks the carrier loss process from CS back to 3CT/1CT during the excited state evolution in organic photovoltaic devices.
The study posits that the lower probability of photogenerated carrier bimolecular recombination and higher number of photogenerated carriers, as evidenced by TAS experiments, result in PMHJ architecture devices exhibiting higher JSC than BHJ devices, while maintaining similar VOC levels. This highlights PMHJ devices’ superior voltage-current balance. Additionally, no correlation between the formation of T1 and VOC was observed in the D18 and Y6 series acceptor material systems. This work offers novel insights into exploring the mechanisms between T1 and device performance and further reducing voltage-current balance limitations to achieve even higher energy conversion efficiencies.
The research received support from 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.
**Figure 1:** Schematic diagram of the excited state evolution process in organic photovoltaic devices, TAS results, and theoretical calculations.
**Figure 2:** Dynamics of excited states like T1 in planar hybrid heterojunctions versus bulk phase heterojunctions.
**Figure 3:** Analysis of device performance and photovoltaic losses.
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