Photosensitizers

Photosensitizers play a crucial role in DSCs, as they harvest light energy and convert this light energy to electric energy. The photovoltaic processes in DSCs are initiated by the photosensitizers.

Therefore, photosensitizers have been developed over the three decades and continue to be studied for enhancing the photovoltaic performances of DSCs.^13-15 In the 1990s and 2000s, researchers synthesized and investigated ruthenium (Ru) photosensitizers intensively.^74-79 Figure 1.14 shows representative Ru photosensitizers. DSCs with a Ru photosensitizer (N3) provided a high PCE of 10% in the late 1990s for the first time.⁷⁴ In 2009, Chen et al. achieved the highest PCE of 11.5% with a Ru photosensitizer (CYC-B11).⁷⁷ The high PCEs of Ru photosensitizers are attributed to their broad light-harvesting range from the visible to the near-infrared (NIR). In addition, the carboxyl groups on the bipyridyl units provide effective charge injection process from photosensitizers to TiO2 conduction band because of a metal-to-ligand charge transfer. However, Ru photosensitizers have somedisadvantages for practical application in DSCs because of their expensive elemental ruthenium, environmental hazards, potential toxicity, low molar extinction coefficients, and difficult purification.

Figure 1.14. Representative Ru photosensitizers.

In contrast to Ru photosensitizers, organic photosensitizers have much attracted interest due to their high molar extinction coefficients, versatile molecular modification, tunable electronic properties, low toxicity, environmental-friendly materials, easy purification, and facile synthesis.^13,14 Since the early 2000s, organic photosensitizers have been synthesized and investigated for enhancing the PCE of DSCs (Figure 1.15).66,67,72,80-91 Up to now, DSCs with organic photosensitizers have achieved PCEs approaching 12%> for aesthetic blue-colored organic dyes,⁶⁷ >13% for porphyrin dyes,^66,88 and 14%>

for metal-free organic dyes⁹² under standard AM 1.5G illumination. Furthermore, co-sensitized DSCs have reached >30% of PCEs under indoor light condition.^70-73

Generally, organic photosensitizers have been designed with donor–π-bridge–acceptor configuration for maximizing ICT properties (Figure 1.16a). Inducing ICT is an effective strategy for steering the photophysical properties of organic photosensitizers, such as light-harvesting ability, energy levels, and charge transfer. Furthermore, the molecular units and structures of organic photosensitizers also significantly affect the interfacial charge transfer between the photosensitizer and TiO2, between the photosensitizer and electrolytes, and between photosensitizers, which determines the photovoltaic performances of DSCs. The role of each molecular unit in organic photosensitizers is summarized in Figure 1.16b. Molecular engineering of organic photosensitizers is essential for achieving high- performance DSCs.

Figure 1.16. Molecular design strategy of organic photosensitizers for DSCs. (a) Design principle of organic photosensitizers. (b) A role of each molecular unit in organic photosensitizers.

Although the development of photosensitizers has advanced significantly, there is a lack of molecular design strategy for absorbing the NIR region capable of generating large photocurrent values (Figure 1.17a).⁹³ For absorbing the NIR region, a band gap of photosensitizers should be lower than a NIR wavelength (eV = 1240/λ, λ is a wavelength in nm). The band gap and energy levels of photosensitizers depend strongly on their molecular units and electronic interactions. An increase in electronic coupling facilitates the ICT of photosensitizers, lowering the HOMO–LUMO energy gap.

Therefore, photosensitizers have been generally comprised of donor and acceptor groups and arranged

in a planar structure to induce strong electronic coupling. To increase electronic coupling further, strong acceptor units, such as benzothiadiazole,^66,94 diketopyrrolopyrrole,⁹⁵ and isoindigo,⁹⁶ have been introduced into photosensitizers, as shown in Figure 1.17b. The strong acceptor units reduce the LUMO energy level and increase the ICT property of photosensitizers, inducing strong electronic coupling effectively. As a result, the photosensitizers with strong electronic coupling exhibit an extended light- harvesting range up to 800 nm with high light-harvesting ability (>30,000 M⁻¹ cm⁻¹).⁹⁶ However, the strong electronic coupling in photosensitizers also increases electronic communication between TiO2

and photosensitizers, causing severe charge recombination. This charge recombination called BET occurs between the injected electrons in TiO2 and the holes of oxidized photosensitizers. The BET reduces both current and voltage of DSCs, decreasing PCE values, as shown in Figure 1.18.

Figure 1.17. The current limitation of DSCs by BET problem. (a) Solar spectrum and current density (b) Schematic BET problem and previous method.

Figure 1.18 shows the reported photovoltaic parameters of DSCs with organic photosensitizers and a comparison to the Shockley–Queisser (SQ) limit. Although photosensitizers have been developed for two decades, the PCE of DSCs has not exceeded 15% because of severe BET and energy level mismatch. To achieve a highly efficient DSCs, photosensitizers should have an optimal band gap for generating large current densities while reducing energy loss of DSCs, such as charge recombination and overpotential. Thus, researchers have been struggling for the synthesis of photosensitizers with a low band gap of 1.2–1.6 eV. The photosensitizers with a low band gap can produce large current densities of approximately 25–30 mA/cm² under the quantum yield of 90% (Figure 1.18a). Furthermore, the low band gap of the photosensitizers can reduce the overpotentials between the excited oxidation potential of the photosensitizers and TiO2 conduction band, and between the ground-state oxidation potential of the photosensitizers and the redox potential of electrolytes, achieving the loss-in-potential of 0.2–0.3 eV (Figure 1.18b). According to the SQ limit, DSCs can achieve 23.6% PCE at a band gap of 1.38 eV under the loss-in-potential of 0.2 eV and the quantum yield of 90% (Figure 1.18c), and the feasible efficiency of DSCs in the near future is 18.7% PCE at a band gap of 1.39 eV under the loss-in- potential of 0.3 eV and the quantum yield of 80% (Figure 1.18d).

However, as shown in Figure 1.18, the photosensitizers with a low band gap of 1.3–1.6 eV exhibit frustrated photovoltaic parameters. DSCs with the photosensitizers show very low current densities of 0.5–12 mV/cm², far from the quantum yield of 90%, and low voltages of 0.3–0.7 V (Figure 1.18a and 1.18b). These are because the severe BET decreases the quantum yield of DSCs and reduces the electron densities of TiO2 conduction band, resulting in low current densities and voltage of the DSCs. As a result, the photosensitizers exhibit a disappointing PCE value of 0.1–5% despite having optimal band gaps capable of achieving high PCE values (Figure 1.18c and 1.18d). If the BET can be suppressed in the photosensitizers possessing a low band gap, the DSCs can achieve beyond the current PCE limitation of 15%. Unfortunately, the BET has been overlooked in DSCs, and there is a lack of molecular design strategy for suppressing the BET. Therefore, to address this problem, I controlled the electronic coupling of photosensitizers by conformational effect. The strategies of electronic coupling engineering for suppressing the BET and their results will be discussed in chapters 1 and 2.

Figure 1.18. Photovoltaic parameters of reported DSCs in comparison to the SQ limit. (a) Short- circuit current density (JSC), (b) open-circuit voltage (VOC), PCE under the loss-in-potential of (c) 0.2 eV and (d) 0.3 eV. IPCE: Incident-photon-to-electron conversion efficiency. The denoted percentages in panels are the quantum yields of DSCs. The band gaps were evaluated by the onset of reported IPCE spectra of DSCs.