Metal-halide perovskite photovoltaic devices (PPVs) are showing great promise as a potential replacement or complement to silicon-based solar cells. While state-of-the-art single-junction PPVs already boast impressive electrical properties and high power conversion efficiencies (PCEs), further improvement toward the detailed balance (DB) limit requires them to approach the behavior of ideal light emitters. Electroluminescence (EL) quantum efficiencies (ELQEs) exceeding 10% have been reported in high-efficiency PPVs, but these results often stem from optimizing light incoupling and charge collection, resulting in device architectures distinct from those of high-performance perovskite light-emitting diodes (PeLEDs) designed for maximum light outcoupling. This paper challenges the conventional wisdom by demonstrating that PPVs with thicker perovskite layers can be brighter than PeLEDs with thin layers at the radiative limit due to the enhanced benefit of photon recycling. The key to achieving this lies in suppressing interfacial quenching by employing perovskite multiple quantum wells (MQWs) with thick energy barriers. This study explores the use of long (~3 nm) organic spacers (oleylammonium, OLA) to create these thick barriers, overcoming previous limitations associated with the reduced conductivity of such thick structures. The successful implementation of this strategy results in high ELQE and PCE, along with substantial improvements in device stability.

Extensive research has focused on optimizing perovskite crystals and reducing trap densities to improve PPV performance. Previous studies on organic treatment of 3D perovskites mostly aimed at passivating trap sites or forming 3D/2D junctions to facilitate charge transfer, employing short (~1 nm) organic spacers like octylammonium (OA) and phenethylammonium (PEA). These short spacers allow for efficient charge conduction but limit the suppression of interfacial quenching. The authors cite several relevant papers that demonstrate high-efficiency PPVs using various strategies such as optimizing the perovskite layers and managing interfaces. The literature review emphasizes the need for a new approach to enhance both efficiency and stability, focusing on achieving highly radiative PPVs.

The researchers employed a sequential coating process involving octylammonium iodide (OAI) and oleylamine on the 3D perovskite layer. OAI treatment forms OA (C8)-based 2D perovskites, while subsequent oleylamine treatment exchanges the L-site cation with OLA (C18), resulting in MQWs with thicker (~3 nm) barriers. X-ray diffraction (XRD) analysis confirmed the formation of the desired Ruddlesden-Popper-phase 2D perovskites with n = 2. The charge carrier probability density in the MQWs was calculated using a transfer-matrix method. Photoluminescence (PL) measurements on 3D perovskite films with and without MQWs were conducted, both with and without a hole transporting layer (HTL), revealing the significant role of MQWs in suppressing interfacial quenching. PL decay measurements were used to investigate charge transfer dynamics. The stability of C8 and C18 MQWs on 3D perovskites was also studied using XRD. To characterize the devices, the researchers performed current density-voltage (*J*-*V*) measurements under AM 1.5G illumination, electroluminescence (EL) measurements, external quantum efficiency (EQE) measurements, and stability tests under various conditions (continuous illumination, elevated temperatures, and air exposure). Optical modeling using a recently proposed model was used to simulate photon recycling effects, and compare ELQE in thin and thick perovskite devices. Detailed material characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), ultraviolet photoelectron spectroscopy (UPS), and inverse photoelectron spectroscopy (IPES) were employed. Transient PL measurements utilizing time-correlated single-photon counting (TCSPC) were used to probe charge transfer processes.

The key findings include: 1. **Optical Modeling:** Optical analysis, using a model accounting for photon recycling, revealed that PPVs with thick perovskite layers can exhibit significantly higher ELQE than thin-layer PeLEDs at high internal radiative efficiency, due to the much larger photon recycling contribution in thick layers. 2. **L-site Exchange:** The L-site exchange process using oleylamine resulted in the formation of stable interfacial structures with reasonable charge carrier conductivity despite the thick (~3nm) barriers of OLA molecules, overcoming electrical limitations previously associated with thick barriers. 3. **Enhanced ELQE and PCE:** Devices fabricated using the C18 (OLA) MQWs exhibited a peak ELQE of 19.7% and 17.8% under 1-sun equivalent conditions, surpassing those of devices with the conventional C8 (OA) MQWs (16.8% peak). This resulted in a significantly higher PCE of 26.0% (certified to 25.2%), compared to 25.1% for the C8-based devices. 4. **Improved Stability:** The devices with C18 MQWs showed greatly enhanced stability compared to those with C8 MQWs. They retained 92% of their initial efficiency after 500 h of operation under 1-sun illumination, along with significantly improved air stability at room temperature and 60 °C. The C18 devices also demonstrated superior long-term stability, maintaining a considerable PCE even after 2 years of storage in air. 5. **Mechanism of Improvement:** The enhanced performance was attributed to the effective suppression of interfacial quenching, achieved through the charge selectivity of the thick C18 barriers. The XRD analysis revealed that the C18 MQWs are significantly more stable than C8 MQWs, preventing spontaneous deformation of the MQW structure. 6. **Approaching DB Limits:** The high ELQEs achieved demonstrate that the devices are approaching the detailed balance limit for V_OC (96.4% of the DB limit), outperforming other photovoltaic devices and nearing the performance of GaAs.

The results demonstrate that the strategy of using thick quantum barriers in PPVs, although counterintuitive due to the potential for reduced charge conductance, can yield significant improvements in efficiency and stability. The enhanced ELQE and PCE are primarily attributed to the suppression of interfacial quenching, which is more pronounced at the interface between the perovskite and the HTL. The study shows that the optical benefits of using thick barriers, particularly those related to photon recycling, outweigh the increase in electrical resistance. The greatly improved stability is correlated to the stable crystalline structure of the OLA-based MQWs. This work highlights the importance of considering the balance between charge transport and radiative efficiency in perovskite device design, emphasizing the potential to improve PV performance by optimizing photon recycling. The approach successfully demonstrates that even minor improvements in internal radiative efficiency can substantially boost ELQE through photon recycling, particularly in devices with thick perovskite absorbers, low parasitic absorption, and high radiative lifetimes.

This research presents a novel approach to enhancing the efficiency and stability of perovskite photovoltaic devices by employing thick quantum barriers via an L-site exchange process with OLA molecules. The method successfully mitigates interfacial quenching, resulting in high ELQE and PCE, exceeding previous state-of-the-art results and approaching the detailed balance limits. The remarkable stability improvements highlight the potential of this strategy for the development of highly efficient and durable perovskite solar cells. Future research could focus on further optimizing the device architecture to minimize electrical losses while maximizing the benefits of photon recycling to achieve even higher efficiencies. Exploring different organic spacers and investigating the influence of other device parameters could also lead to further advancements.

While the study demonstrates significant improvements in efficiency and stability, some limitations exist. The increase in series resistance due to thicker barriers slightly reduces the fill factor, though this is outweighed by the gains in V_OC. The optical model used assumed uniform distribution of dipoles and Lambertian emission, which might not perfectly represent the actual light emission characteristics. Further investigation into the long-term stability under various operating conditions and environmental factors is needed to fully assess the durability of the devices. The study also focuses on a specific perovskite composition, and exploring the generalizability of the method to other perovskite materials is warranted.