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Achieving Fast Charge Separation and Low Nonradiative Recombination Loss by Rational Fluorination for High‐Efficiency Polymer Solar Cells
摘要： Four low-cost copolymer donors of poly(thiophene-quinoxaline) (PTQ) derivatives are demonstrated with different fluorine substitution forms to investigate the effect of fluorination forms on charge separation and voltage loss (Vloss) of the polymer solar cells (PSCs) with the PTQ derivatives as donor and a A–DA’D–A-structured molecule Y6 as acceptor. The four PTQ derivatives are PTQ7 without fluorination, PTQ8 with bifluorine substituents on its thiophene D-unit, PTQ9, and PTQ10 with monofluorine and bifluorine substituents on their quinoxaline A-unit respectively. The PTQ8- based PSC demonstrates a low power conversion efficiency (PCE) of 0.90% due to the mismatch in the highest occupied molecular orbital (HOMO) energy levels alignment between the donor and acceptor. In contrast, the devices based on PTQ9 and PTQ10 show enhanced charge-separation behavior and gradually reduced Vloss, due to the gradually reduced nonradiative recombination loss in comparison with the PTQ7-based device. As a result, the PTQ10-based PSC demonstrates an impressive PCE of 16.21% with high open-circuit voltage and large short-circuit current density simultaneously, and its Vloss is reduced to 0.549 V. The results indicate that rational fluorination of the polymer donors is a feasible method to achieve fast charge separation and low Vloss simultaneously in the PSCs.
关键词： voltage loss,nonradiative recombination,low-cost copolymer donors,fluorination,charge separation
更新于2025-09-12 10:27:22
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Al <i> <sub/>x</sub></i> Ga <sub/>1?</sub><i> <sub/>x</sub></i> N-Based Quantum Wells Fabricated on Macrosteps Effectively Suppressing Nonradiative Recombination
摘要： AlxGa1?xN-based quantum wells (QWs) are fabricated on AlN with macrosteps, which are formed by using vicinal sapphire and AlN (0001) substrates. The QWs on macrosteps (MS-QWs) show photoluminescence lifetimes of nearly 2 ns at an emission wavelength of 242 nm at room temperature (RT). This is the longest lifetime so far reported for AlxGa1?xN-based QWs emitting below 270 nm, indicating the suppression of nonradiative recombination, the dominant recombination process at RT. Compared with planar QWs without macrosteps, the emission internal quantum efficiency estimated by photoluminescence spectroscopy is improved approximately by two orders of magnitude under a weak excitation condition. Consequently, AlxGa1?xN-based MS-QWs are promising structures for highly efficient ultraviolet emitters.
关键词： AlGaN,macrosteps,nonradiative recombination,quantum wells
更新于2025-09-10 09:29:36
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-related Materials
摘要： Dominant recombination paths in AlN and AlxGa1?xN-related structures are investigated using cathodoluminescence (CL) mapping measurements and photoluminescence (PL) spectroscopy. The dark spot contrasts originating from nonradiative recombination at threading dislocations (TDs), which are observed in CL intensity maps, drastically decrease upon elevating the temperature. This is because carriers can reach TDs at low temperatures (9–60 K), but are captured by point defects (PDs) even in the vicinity of TDs near RT. Calculations based on the experimental results indicate that in the current AlN and Al-rich AlxGa1?xN crystals, TDs scarcely affect the internal quantum efficiency (IQE) at RT as long as the TD density is less than 2.6 × 1010 cm?2. Because a TD density less than 2.6 × 1010 cm?2 has already been achieved even for heteroepitaxially grown AlN films on sapphire substrates, it is evident that the most effective method to further improve the IQE of AlxGa1?xN-related materials is to reduce PDs not TDs. Moreover, we clarify the existence of two types of PD states, which mainly degrade the emission efficiency, using temperature-dependent PL measurements. Combining the CL and PL results allows the activation energies of these PDs and TDs to be evaluated. Furthermore, we highlight the probability that PDs, which predominantly act as nonradiative recombination centers at room temperature, are complexes formed by Al vacancies and oxygen impurities that enhance the deep-level emissions at 3.2 and 3.5–3.7 eV near room temperature. Such a large impact of PDs on the efficiency degradation may be attributed to the high density of Al-vacancy–related PDs in AlN and Al-rich AlxGa1?xN compared with that of Ga-vacancy–related PDs in GaN due to the small formation energy.
关键词： AlxGa1?xN,cathodoluminescence,internal quantum efficiency,point defects,photoluminescence,nonradiative recombination,AlN,threading dislocations
更新于2025-09-04 15:30:14