Colloidal quantum dots (QDs) are attractive for optoelectronic applications due to their tunable properties, efficient multiple exciton effects, and relatively good stability. Lead sulfide (PbS) QDs have achieved 13.8% power conversion efficiency (PCE) in solar cells. However, all-inorganic CsPbI3 QDs have emerged as a promising alternative because their surface strain helps maintain the perovskite phase, unlike metastable thin films. CsPbI3 QDs exhibit high photoluminescence (PL) quantum yields due to defect tolerance, leading to high open-circuit voltages. Efficiencies exceeding 15% have been reported for CsPbI3 QD solar cells. The colloidal synthesis of perovskite QDs using nonpolar organic solvents offers advantages over the toxic polar solvents used in thin-film perovskite processing. QD materials also offer inherent mechanical flexibility, but experimental evidence of their mechanical endurance compared to thin films, particularly in high-PCE flexible photovoltaics, is scarce. Flexible QD devices offer two key benefits: room-temperature deposition on flexible substrates and high power output-per-weight. While previous studies showed excellent flexural endurance, high PCEs were not achieved due to poor charge transfer and carrier extraction at interfaces. This work addresses these limitations by demonstrating efficient flexible QD photovoltaics using CsPbI3 perovskite QDs and a novel hybrid interfacial architecture.

The literature review extensively covers the existing research on colloidal quantum dots (QDs), particularly focusing on PbS and CsPbI3 QDs for photovoltaic applications. It highlights the advancements in PCE achieved using various strategies like surface passivation and improved device structures for PbS QDs and the unique advantages of CsPbI3 QDs in terms of phase stability and high PL quantum yields. The review also discusses the challenges faced in achieving high-efficiency flexible QD photovoltaics and the limited success in this area compared to flexible organic and thin-film perovskite solar cells. The authors emphasize the need for improved charge transfer and carrier extraction at QD heterointerfaces to overcome these limitations.

CsPbI3 QDs were synthesized and purified using previously published procedures. Phenyl-C61-butyric acid methyl ester (PCBM) was incorporated into the CsPbI3 QD layer in chlorobenzene (CB) to create a hybrid solution. Transmission electron microscopy (TEM) confirmed that the QDs, with or without PCBM, maintained a similar cubic shape and size (∼10 nm). The QD solution was spin-coated onto substrates, and the native long oleate ligands were removed by soaking in anhydrous methyl acetate (MeOAc). Fourier transform infrared (FTIR) spectroscopy, femtosecond transient absorption (fs-TA) spectroscopy, and photoluminescence (PL) spectroscopy were used to characterize the CsPbI3 QD films with and without PCBM. Fs-TA measurements revealed improved charge separation in the hybrid films, and time-resolved PL analysis confirmed faster carrier dynamics in the PCBM/CsPbI3 QD hybrid film. Space-charge-limited current (SCLC) measurements showed that PCBM reduced surface trap density. Control and target (PCBM/CsPbI3 QD) solar cells were fabricated, with the latter showing significantly improved short-circuit current density (Jsc). External quantum efficiency (EQE) measurements confirmed enhanced EQE across the entire response region for the target device. Electrochemical impedance spectroscopy (EIS) provided insights into improved charge transport and suppressed recombination in the target device. Transient photocurrent and photovoltage measurements showed faster carrier transport in the target device. Ultraviolet photoelectron spectroscopy (UPS) and bandgap calculations determined energy level alignments, confirming an energy cascade enabling efficient electron injection into PCBM and then SnO2. Density functional theory (DFT) simulations and X-ray photoelectron spectroscopy (XPS) further investigated the molecular interaction between CsPbI3 QDs and PCBM, showing coordination bonds between PCBM's carboxyl groups and under-coordinated Pb2+ ions on the QD surfaces, leading to surface passivation. Flexible solar cells were fabricated using PET/ITO substrates and tested for mechanical stability under bending cycles. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) examined the morphology and mechanical properties of the films.

The key findings include: 1. A novel hybrid interfacial architecture (HIA) incorporating PCBM into the CsPbI3 QD layer significantly improved the performance of both rigid and flexible solar cells. 2. The HIA facilitated efficient charge transfer and suppressed interfacial recombination, as confirmed by fs-TA, PL, EIS, and transient photocurrent/photovoltage measurements. 3. The champion rigid CsPbI3 QD solar cell achieved a PCE of 15.1% (stabilized power output of 14.61%), representing a substantial improvement (∼25%) compared to the control device. 4. Flexible CsPbI3 QD solar cells with a PCE of 12.3% were demonstrated, exceeding previous reports and showcasing superior mechanical stability compared to thin films. 5. DFT simulations and XPS analysis revealed that PCBM interacts strongly with under-coordinated Pb2+ ions on the QD surface, providing surface passivation and enhancing charge transfer. 6. The mechanical durability of QD films was significantly higher than that of thin films, as indicated by SEM analysis after bending cycles and AFM measurements of Young's modulus. The nanoscale boundaries and surface ligands in QD films effectively release internal stress during bending.

The findings demonstrate the efficacy of the HIA strategy in enhancing both the efficiency and flexibility of CsPbI3 QD solar cells. The significant improvements in PCE and mechanical stability compared to the control device and previous reports highlight the importance of optimizing QD/ETL interfaces. The HIA's ability to promote charge transfer and suppress recombination is a crucial factor in achieving high efficiencies. The superior mechanical stability of QD films over thin films is attributed to the nanoscale boundaries and soft ligands that can accommodate stress during bending. These results have important implications for the development of flexible and high-performance optoelectronic devices. The work suggests that strategies focusing on interface engineering could lead to advancements in other QD-based optoelectronic applications.

This study successfully demonstrated highly efficient (15.1% PCE) and flexible (12.3% PCE) CsPbI3 QD solar cells through a simple yet effective hybrid interfacial architecture (HIA) strategy. The introduction of PCBM significantly enhanced charge transfer, suppressed recombination, and improved mechanical stability. This work establishes a new pathway for improving QD photovoltaic devices and potentially other QD-based optoelectronics. Future research could explore different organic molecules or polymers for HIA modification, optimize the thickness and composition of the hybrid layer, and investigate the long-term stability of the devices under various environmental conditions.

The study primarily focused on the efficiency and flexibility of the CsPbI3 QD solar cells. While long-term stability tests were conducted, further investigation is needed to assess long-term performance under different environmental stresses (e.g., humidity, temperature). The study used a specific type of PCBM; further research could explore other similar molecules or alternative interfacial engineering approaches to potentially achieve even better results. The investigation of the mechanical properties was focused on bending tests; other mechanical stress conditions should be evaluated to provide a more thorough analysis of the mechanical durability of the QD films.