《厦门大学学报（自然科学版）》

结合稳态和时间分辨光谱技术,研究了CdSe量子点与富勒烯C60构成的模型太阳能电池体系中,荧光发射猝灭以及基态漂白恢复的动力学过程,发现二者间的荧光猝灭是由CdSe量子点与C60间的光致电子转移所引起的,并且电子转移的速率随着量子点粒径的增大而减小:4.3×109 s-1(3.0 nm),1.1×109s-1(3.5 nm),0.60×109 s-1(4.0 nm).该结果为设计、构筑以CdSe量子点为中心的光电转化体系,以及提高其光电转化效率提供了实验依据.

Objective s: The carrier migration process in quantum dots (QDs) plays significant roles in the photoelectric conversion of solar cells. Understanding the mechanism of carrier migration will be helpful to improve photoelectric conversion efficiency in QD-based solar cells. Physically mixed CdS QDs and C60 is an interesting candidate for building solar cells, and the investigation of the photo-induced electron transfer between this donner-acceptor pairs will provide information about the pathways anddynamics of the ultrafast electron transfer process. Methods : Laser-induced steady state and time-resolved fluorescence spectroscopy can provide information of the excited state emission and its quenching processes. In time-resolved transient (TA) spectroscopy, a pump laser beam excited the sample to electronically excited states, and a following broadband laser beam monitored the changes in the sample with controlled time delays relative to the pump pulse. High time resolution TA was used to monitor the dynamics of carrier migrations in QDs through ground state bleaching (GSB), excited state absorption (EA), and stimulated emission (SE). Results : Absorption and fluorescence spectra of CdSe QDs (with diameters of 3.0 nm, 3.5 nm and 4.0 nm) and CdSe-C60 mixture with different C60 concentrations clearly showed that there was fluorescence quenching. There was no overlap between the C60 absorption and QDs fluorescence peaks, which implies that there was no fluorescence resonance energy transfer between QDs and C60.The fluorescence quenching was through direct energy transfer between CdSe QDs and C60. From transient fluorescence of QDs and theCdSe-C60 mixture, C60 concentration dependent lifetimes were observed. Thus, the major quenching mechanismwasphoto-induced electron transfer between the excited state of QDs and C60. And, the relative energy levels of the CdSe QDs and the highest occupied molecular orbit (HOMO)/lowest unoccupied molecular orbit (LUMO) levels of C60 also suggest thatthe electron transfer from the excited state of QDs to C60was favored. From TA spectroscopy of CdSe QDs and CdSe-C60 mixture pump with 390 nm laser pulses, the GBS dynamics was investigated. From exponential fitting the GBS curves of CdSe QDs, the shallow and deep surface states trapping of photo-induced electrons in the excited state of QDs occurred with time constants of a few tens and hundreds of picosecond, respectively. The recombination of electrons and holes within QDs was on the order of nanosecond. With the C60 acceptors, the charge separation (electron-hole) and electron transfer from QDs to C60were with time constants of a few picosecond, the surface state trapping were at tens of picosecond, and the recombination of carriers in QDs were on the order of nanosecond. All the decay processes became slower and electron transfer components got smaller with bigger QDs. The average electron transfer rate decreased with the QD’s diameter, 4.3×109s-1 for 3.0 nm QD, 1.1×109s-1 for 3.5 nm QD, and0.60×109 s-1 for 4.0 nm QD, respectively. Conclusion s: From the steady state and transient absorption and fluorescence spectroscopy of CdSe QDs and CdSe-C60 mixtures, fluorescence quenching mechanism was discussed, and efficient electron transfer between the QDs excited state and C60was investigated. The electron transfer, surface state trapping and carrier recombinationdynamics were observed. With the presence of C60, all dynamic processes became faster. With the increase of CeSe QDs diameter, the energy level of the QDs excited state was getting closer to the LUMO of C60, which led to a smaller driving force for the electron transfer between QDs and C60.The bigger QDs, the less efficient the photo-induced electron transfer. It is worth mentioning that even physically mixed CdSe QDs and C60 can still produce enough photo-induced electron transfer to build a solar cell. These results provide useful experiment data for development of high efficient QD-based solar cells. Further investigations with core/shell QDs to reduce surface state trapping of photo-induced carriers will be performed to get deeper insights about the mechanism and dynamics of electron transfer and construction of QD-based solar cells.

引言
1 实验部分
2 结果与讨论
3 结论

图1 TA光谱的原理示意图<br/>Fig.1 Schematic diagram of TA spectroscopy

图1 TA光谱的原理示意图
Fig.1 Schematic diagram of TA spectroscopy

图2 TA光谱实验装置示意图<br/>Fig.2 Schematic diagram of TA spectrum experimental setup

图2 TA光谱实验装置示意图
Fig.2 Schematic diagram of TA spectrum experimental setup

图3 CdSe 量子点的TEM图<br/>Fig.3 TEM images of CdSe quantum dots

图3 CdSe 量子点的TEM图
Fig.3 TEM images of CdSe quantum dots

图4 CdSe量子点和C60甲苯溶液的稳态吸收和荧光光谱<br/>Fig.4 Steady state absorption and fluorescence spectra of CdSe quantum dots and C60 in toluene solution

图4 CdSe量子点和C60甲苯溶液的稳态吸收和荧光光谱
Fig.4 Steady state absorption and fluorescence spectra of CdSe quantum dots and C60 in toluene solution

图5 在不同C60浓度下3.0 nm CdSe量子点(3 μmol/L)的瞬态荧光衰减曲线<br/>Fig.5 Transient time-resolved fluorescence of 3.0 nm CdSe quantum dots(3 μmol/L)under different C60 concentration

图5 在不同C60浓度下3.0 nm CdSe量子点(3 μmol/L)的瞬态荧光衰减曲线
Fig.5 Transient time-resolved fluorescence of 3.0 nm CdSe quantum dots(3 μmol/L)under different C60 concentration

表1 CdSe量子点的荧光寿命随C60质量浓度的变化<br/>Tab.1 C60 concentration-dependent fluorescence lifetime of CdSe quantum dots

表1 CdSe量子点的荧光寿命随C60质量浓度的变化
Tab.1 C60 concentration-dependent fluorescence lifetime of CdSe quantum dots

图6 CdSe量子点和C60的能级位置图<br/>Fig.6 Energy level position diagram of CdSe quantum dots and C60

图6 CdSe量子点和C60的能级位置图
Fig.6 Energy level position diagram of CdSe quantum dots and C60

图7 3.0 nm CdSe量子点(a)和3.0 nm CdSe-C60(b)甲苯溶液的TA光谱(激发波长390 nm)<br/>Fig.7 TA spectra of 3.0 nm CdSe quantum dots(a)and 3.0 nm CdSe-C60(b)in toluene solution(with 390 nm excitation)

图7 3.0 nm CdSe量子点(a)和3.0 nm CdSe-C60(b)甲苯溶液的TA光谱(激发波长390 nm)
Fig.7 TA spectra of 3.0 nm CdSe quantum dots(a)and 3.0 nm CdSe-C60(b)in toluene solution(with 390 nm excitation)

图8 CdSe量子点与CdSe-C60的漂白峰恢复动力学曲线<br/>Fig.8 Bleach peak recovery kinetic curves of CdSe quantum dots and CdSe-C60

图8 CdSe量子点与CdSe-C60的漂白峰恢复动力学曲线
Fig.8 Bleach peak recovery kinetic curves of CdSe quantum dots and CdSe-C60

表2 CdSe 量子点和CdSe-C60漂白恢复的动力学参数<br/>Tab.2 Kinetic parameters of the bleach recovery of CdSe quantum dots and CdSe-C60

表2 CdSe 量子点和CdSe-C60漂白恢复的动力学参数
Tab.2 Kinetic parameters of the bleach recovery of CdSe quantum dots and CdSe-C60

表3 CdSe量子点与C60间的电子转移速率常数<br/>Tab.3 Electron transfer rate constant between CdSe quantum dots and C60

表3 CdSe量子点与C60间的电子转移速率常数
Tab.3 Electron transfer rate constant between CdSe quantum dots and C60

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备注

引言

1 实验部分

2 结果与讨论

3 结论

学报简介

备注

引言

1 实验部分

2 结果与讨论

3 结 论

学报简介

3 结论