One of the key issues affecting the performance of solar cells is the behavior of carrier transfer. In this work, the time-resolved photoluminescence (TRPL) technique was utilized to investigate the electron transfer at the CdS/CdSe, TiO2/CdS, and TiO2/CdSe heterointerfaces. By varying the excitation wavelengths, photoluminescence lifetimes of CdSe and CdS in TiO2/CdSe, TiO2/CdS, TiO2/CdS/CdSe, and TiO2/CdSe/CdS photoelectrodes were measured. The results show that, for the single sensitizer electrodes (TiO2/CdS, TiO2/CdSe), the average PL lifetime of CdS (0.69 ns) is shorter than CdSe (0.99 ns), suggesting that CdS has higher electron transfer rate toward TiO2 compared with CdSe. For the TiO2/CdSe/CdS electrode, the PL lifetime of CdSe exhibits an excitation-wavelength-dependent behavior. A shorter excitation wavelength leads to a longer PL lifetime of CdSe. This additional long lifetime is ascribed to the rapid carrier transfer from the photoexcited carriers in CdS layer into the CdSe layer. On the contrary, the PL lifetime of CdSe is independent of the excitation wavelength in the TiO2/CdS/CdSe electrode, indicating that the excited electrons in the CdS layer did not inject into the CdSe layer. This observation confirms that the charge transfer from the cosensitizers toward the TiO2 is much more efficient in the TiO2/CdS/CdSe electrode rather than in the TiO2/CdSe/CdS electrode.
Highly efficient
quasi-solid-state dye-sensitized solar cells (QS-DSSCs)
are fabricated using nanocomposite gel electrolytes and applied under
room light conditions (200 lx). To obtain high energy conversion efficiency
in QS-DSSCs, the important components of the DSSC are systematically
optimized based on their performance in liquid-state DSSCs. It shows
that the liquid cell using the 3-methoxypropionitrile-based cobalt
electrolyte has higher efficiency (18.91%) than the cell using the
acetonitrile-based electrolyte (17.82%) under 200 lx illumination
due to the higher charge recombination resistance at the photoelectrode/electrolyte
interface for the 3-methoxypropionitrile system. Poly(vinylidene
fluoride-co-hexafluoropropylene) is utilized as the
gelator of the liquid electrolytes to prepare polymer gel electrolytes.
Furthermore, to improve the performance of the QS-DSSCs, different
metal oxide nanoparticles are introduced as nanofillers of the polymer
gel electrolytes. It shows that the zinc oxide nanofillers have a
superior performance in increasing the cell efficiency and the energy
conversion efficiencies of the QS-DSSCs are higher than those of the
corresponding liquid cells. By regulating the concentration of the
zinc oxide nanofillers, the efficiency of the 3-methoxypropionitrile
based QS-DSSC can achieve a value of 20.11% under 200 lx illumination.
This QS-DSSC has a long-term stability at 35 °C.
Power
generation in indoor environments is the next step in dye-sensitized
solar cell (DSSC) evolution. To achieve this goal, a critical recombination
route which is usually inhibited by the TiCl4-derived blocking
layers (BLs), that is, charge transfer at the fluorine-doped tin oxide
substrate/electrolyte interface, is of concern. In this study, we
demonstrate that because of low surface coverage, the conventional
TiCl4 BLs are unable to suppress such electron leakage,
thus limiting the photovoltaic performance of Co(bpy)3
2+/3+-mediated DSSCs (bpy = 2,2′-bipyridine) under ambient
lighting. On the other hand, by introducing compact BLs prepared by
spray pyrolysis, the DSSCs show lower dark current and operate efficiently
not only under high-intensity sunlight but also under ambient light
conditions. The better blocking function of the compact BL is verified
by the cyclic voltammetry; other thin-film preparation methods, except
for the common TiCl4 treatment, are anticipated to realize
a similar blocking effect. This study illustrates that dense thin
film with a predominant blocking function is highly required as the
BL for DSSCs under low-light conditions, and this concept will pave
the way for more development of indoor DSSCs.
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