Browsing by Author "Xu, Liang"
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Item Charge Collection in Bulk Heterojunction Organic Photovoltaic Devices: An Impedance Spectroscopy Study(Amer Inst Physics) Xu, Liang; Lee, Yun-Ju; Hsu, Julia W. P.; 0000 0003 8600 0978 (Hsu, JWP); 243648305 (Hsu, JWP)Through thickness and applied bias variation, charge collection in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction organic photovoltaic (OPV) devices was investigated with impedance spectroscopy. An equivalent circuit model incorporating chemical capacitance (Cμ), recombination resistance (R₂), and transport resistance (R₁) was used to analyze the results. Insufficient carrier extraction, exhibiting diffusion transport characteristics at high frequencies, was found in devices with a thick active layer. These devices also display a higher chemical capacitance, indicating greater carrier accumulation, and a lower recombination resistance, signaling increased bimolecular recombination. Increasing internal field with negative applied bias enhances carrier collection by reducing carrier accumulation and recombination. Moreover, we showed explicitly that charge collection can be quantified by (R₂/R₁)½, which is proportional to device fill factor. These results demonstrate that impedance spectroscopy is an effective tool for investigating charge collection in OPV devices.Item Charge Dynamics and Device Physics in Bulk-Heterojunction Organic Photovoltaics(2017-05) Xu, Liang; Hsu, Julia W. P.Organic photovoltaic (OPV) technology is still advancing towards commercialization with its potential for low-cost and light-weight flexible electricity generating applications. However, the lack of thorough understanding on the charge dynamics including charge generation, recombination, and transport processes still hinders the further boost of OPV device performance. The aim of my research is applying advanced characterization techniques to gain comprehensive understandings on charge dynamics in various OPV systems. Particularly the impedance spectroscopy was extensively applied as a powerful and non-destructive tool to study charge carrier collection and recombination. Meanwhile, time-resolved charge extraction measurement was performed as a complementary method for recombination study. External quantum efficiency spectroscopy was also carefully adapted to probe the device energetics. On the other hand, to better understand the experimental results, numerical drift-diffusion simulation and transfer matrix method modeling were carried out to provide theoretical guidance. With the combination of these techniques morphology effects, including the donor-acceptor interface and structural order, on recombination in OPV devices were thoroughly investigated. The photocurrent loss mechanisms in a novel fullerene-based OPV system were systematically and quantitatively analyzed. Moreover, the defect characterization using capacitance-frequency analysis was discovered to have severe artifacts when applied to OPV devices. In general, this research not only carefully addressed the charge dynamics in several OPV systems, but the multifaceted approaches it demonstrated also provide insights for future photovoltaic research.Item Minimizing Performance Degradation Induced by Interfacial Recombination in Perovskite Solar Cells through Tailoring of the Transport Layer Electronic Properties(Amer Inst Physics) Xu, Liang; Imenabadi, Rouzbeh Molaei; Vandenberghe, William G.; Hsu, Julia W. P.; 0000-0003-2710-5227 (Xu, L); 0000-0002-7821-3001 (Hsu, JWP); Xu, Liang; Imenabadi, Rouzbeh Molaei; Vandenberghe, William G.; Hsu, Julia W. P.The performance of hybrid organic-inorganic metal halide perovskite solar cells is investigated using one-dimensional drift-diffusion device simulations. We study the effects of interfacial defect density, doping concentration, and electronic level positions of the charge transport layer (CTL). Choosing CTLs with a favorable band alignment, rather than passivating CTL-perovskite interfacial defects, is shown to be beneficial for maintaining high power-conversion efficiency, due to reduced minority carrier density arising from a favorable local electric field profile. Insights from this study provide theoretical guidance on practical selection of CTL materials for achieving high-performance perovskite solar cells.Item Probing Defect States in Organic Polymers and Bulk Heterojunctions Using Surface Photovoltage Spectroscopy(American Chemical Society, 2019-04-10) Murthy, Lakshmi N. S.; Barrera, Diego; Xu, Liang; Gadh, Aakash; Cao, F. -Y; Tseng, C. -C; Cheng, Y. -J; Hsu, Julia W. P.; 0000-0002-7821-3001 (Hsu, JWP); Murthy, Lakshmi N. S.; Barrera, Diego; Xu, Liang; Gadh, Aakash; Hsu, Julia W. P.We performed frequency-modulated (AC) and steady-state (DC) surface photovoltage spectroscopy (SPS) measurements on a bilayer structure consisting of an organic semiconductor (P3HT, P3HT:PC₆₁ BM, or PFBT₂Se₂Th:PC₇₁ BM) on top of a ZnO electron-transport layer. The AC spectra overlap with the absorption spectra of the organic layer, providing evidence that AC SPS corresponds to band-to-band transitions. The DC spectra are generally broader than the AC spectra, with responses extended below the absorption edge. Thus, DC SPS also probes transitions between band states and trap states within the band gap in addition to band-to-band transitions. When a hole-transport layer (HTL) is deposited on top of the organic layer, the DC spectra of P3HT and P3HT:PC₆₁ BM are narrower than those without the HTL, suggesting that the sub-band gap states exist at the surface of these organic semiconductors. In contrast, PFBT₂Se₂Th:PC₇₁ BM does not show signature of surface states or optically active trap states in the band gap. External quantum efficiency and capacitance measurements are employed to explain the nature of sub-band gap states that contribute to surface photovoltage signals and the differences between the two bulk heterojunction systems. ©2019 American Chemical Society.