Abstract
Interfacial charge-transfer (ICT) transitions involved in charge-separation mechanisms are expected to enable efficient photovoltaic conversions through one-step charge-separation processes. With this in mind, the charge-transfer complex fabricated from TiO2 nanoparticles and 7,7,8,8-tetracyanoquinodimethane (TCNQ) has been applied to dye-sensitized solar cells. However, rapid carrier recombination from the conduction band of TiO2 to the highest occupied molecular orbital (HOMO) of TCNQ remains a major issue for this complex. In this study, to inhibit surface-complex recombinations, we prepared Nb-doped TiO2 nanoparticles with different atomic ratios for enhanced electron transport. To investigate the effects of doping on electron injection through ICT transitions, these materials were examined as photoelectrodes. When TiO2 was doped with 1.5 mol % Nb, the Fermi level of the TiO2 electrode shifted toward the conduction band minimum, which improved electron back-contact toward the HOMO of TCNQ. The enhancement in electron transport led to increases in both short circuit current and open circuit voltage, resulting in a slight (1.1% to 1.3%) improvement in photovoltaic conversion efficiency compared to undoped TiO2. Such control of electron transport within the photoelectrode is attributed to improvements in electron injection through ICT transitions.
Highlights
Interfacial charge-transfer (ICT) transitions between inorganic semiconductors and π-conjugated organic compounds are characteristic electronic transitions that enable direct photoinduced charge separation
Photovoltaic conversions and electron injections between surface complexes, such as bis(dicyanomethylene) compounds (TCNX) [TCNE, TCNQ, and 11,11,12,1 2-tetracyanonaphtho-2, 6-quinodimethane (TCNAQ)] and TiO2, have been studied theoretically by Fujisawa et al using density functional theory (DFT) on the basis of Marcus theory, which revealed that the structure and formation mechanism of the surface complex need to be considered to control interfacial electronic transitions and carrier recombinations by adjusting the electron affinity of TCNX [6,7,8]
To inhibit rapid electron recombinations in surface complexes, we examined the effects of Nb-doped TiO2 electrodes with TCNQ on photovoltaic performance
Summary
Interfacial charge-transfer (ICT) transitions between inorganic semiconductors and π-conjugated organic compounds are characteristic electronic transitions that enable direct photoinduced charge separation. Photovoltaic conversions and electron injections between surface complexes, such as bis(dicyanomethylene) compounds (TCNX) [TCNE, TCNQ, and 11,11,12,1 2-tetracyanonaphtho-2, 6-quinodimethane (TCNAQ)] and TiO2 , have been studied theoretically by Fujisawa et al using density functional theory (DFT) on the basis of Marcus theory, which revealed that the structure and formation mechanism of the surface complex need to be considered to control interfacial electronic transitions and carrier recombinations by adjusting the electron affinity of TCNX [6,7,8].
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