Optimizing the Cosensitization Effect of SQ02 Dye on BP‑2 Dye-Sensitized Solar Cells: A Computational Quantum Chemical Study

Cosensitization of the semiconducting
electrode in dye-sensitized
solar cells (DSCs), with two or more light-harvesting dyes, is a chemical
fabrication method that aims to achieve a panchromatic absorption
spectrum emulating that of the solar emission spectrum. In this paper,
SQ02 and BP-2 cosensitizers have been investigated, as isolated monomers/dimer
and adsorbed monomers/dimer on the TiO2 (101) anatase surface,
by employing density functional theory (DFT) and time-dependent DFT
calculations. Computed results showed that the dominant electron injection
pathway is direct injection from each dye into the conduction band
of TiO2. The almost complete spectral overlap between the
simulated absorption spectrum of BP-2 and fluorescence emissions of
SQ02 implies that excitation energy transfer occurs between cosensitizers
via the trivial reabsorption mechanism. However, the results showed
very limited unidirectional intermolecular charge transfer (CT) from
SQ02 dye to BP-2 dye (0.04 |e–|). Therefore, this
study also presents a stepwise molecular engineering of BP-2 dye,
aiming at optimizing the cosensitization functionality. First, 14
redesigned dye candidates are reported to identify dyes with photophysical
properties matching the requirements for efficient DSCs. Second, the
four most promising dyes are shortlisted for testing as cosensitizers
with the SQ02 dye. The molecular design factors of cosensitization
that need validation are chemical compatibility, availability of CT
between cosensitizers, and complementarity of the absorption spectra.
This screening suggests the judicious choice of the modeled difluorenyl
amine donor-based dye (BP-D4) as a very promising cosensitizer. In
particular, the SQ02/BP-D4 dimer showed 10 times larger (0.53 |e–|) unidirectional CT than that of SQ02/BP-2 dimer,
in addition to the maximum increased electron population of acceptor
moieties upon photoexcitation.

染料敏化太阳能电池（dye-sensitized solar cells, DSCs）中采用两种及以上捕光染料对半导体电极实施共敏化，是一种旨在构建与太阳发射光谱相匹配的全光谱吸收特性的化学制备工艺。本文基于密度泛函理论（density functional theory, DFT）与含时密度泛函理论计算，对SQ02与BP-2两种共敏染料进行了研究，分别考察了其孤立单体与二聚体形态，以及吸附于锐钛矿相TiO2(101)晶面的单体与二聚体形态。计算结果显示，主导电子注入路径为各染料分子直接向TiO2导带注入电子。BP-2的模拟吸收光谱与SQ02的荧光发射光谱几乎完全重叠，这表明共敏染料之间可通过常规再吸收机制实现激发态能量转移。但研究同时发现，SQ02向BP-2的单向分子间电荷转移（intermolecular charge transfer, CT）量极低，仅为0.04 |e–|。
鉴于此，本研究针对BP-2染料开展分步分子工程改造，以优化其共敏化性能。首先，本文报道了14种重新设计的染料候选物，用于筛选具备高效DSCs所需光物理特性的染料；其次，筛选出其中性能最优的4种染料，与SQ02搭配作为共敏剂开展测试。共敏化设计需验证的核心要素包括：化学相容性、共敏染料间电荷转移的可行性，以及吸收光谱的互补性。通过上述筛选，本文遴选出基于二芴基胺给体基团的建模染料BP-D4，其具备极佳的共敏化应用前景。具体而言，SQ02/BP-D4二聚体的单向分子间电荷转移量较SQ02/BP-2二聚体提升了10倍（达0.53 |e–|），同时光激发过程中受体基团的电子占据数也实现了最大幅度提升。