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Review

Interfacial Charge-Transfer Transitions for Direct Charge-Separation Photovoltaics

by
Jun-ichi Fujisawa
Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan
Energies 2020, 13(10), 2521; https://doi.org/10.3390/en13102521
Submission received: 30 March 2020 / Revised: 2 May 2020 / Accepted: 8 May 2020 / Published: 15 May 2020
(This article belongs to the Special Issue Advanced Dye-Sensitized Solar Cells)
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Abstract

:
Photoinduced charge separation (PCS) plays an essential role in various solar energy conversions such as photovoltaic conversion in solar cells. Usually, PCS in solar cells occurs stepwise via solar energy absorption by light absorbers (dyes, inorganic semiconductors, etc.) and the subsequent charge transfer at heterogeneous interfaces. Unfortunately, this two-step PCS occurs with a relatively large amount of the energy loss (at least ca. 0.3 eV). Hence, the exploration of a new PCS mechanism to minimize the energy loss is a high-priority subject to realize efficient solar energy conversion. Interfacial charge-transfer transitions (ICTTs) enable direct PCS at heterogeneous interfaces without energy loss, in principle. Recently, several progresses have been reported for ICTT at organic-inorganic semiconductor interfaces by our group. First of all, new organic-metal oxide complexes have been developed with various organic and metal-oxide semiconductors for ICTT. Through the vigorous material development and fundamental research of ICTT, we successfully demonstrated efficient photovoltaic conversion due to ICTT for the first time. In addition, we revealed that the efficient photoelectric conversion results from the suppression of charge recombination, providing a theoretical guiding principle to control the charge recombination rate in the ICTT system. These results open up a way to the development of ICTT-based photovoltaic cells. Moreover, we showed the important role of ICTT in the reported efficient dye-sensitized solar cells (DSSCs) with carboxy-anchor dyes, particularly, in the solar energy absorption in the near IR region. This result indicates that the combination of dye sensitization and ICTT would lead to the further enhancement of the power conversion efficiency of DSSC. In this feature article, we review the recent progresses of ICTT and its application in solar cells.

1. Introduction

Photoinduced charge separation (PCS) at heterogeneous interfaces between electron-donating (D) and -accepting (A) substances (organic compounds, inorganic semiconductors, metals, etc.) play an important role in various solar energy conversions ranging from photovoltaic conversion in solar cells to photocatalytic reactions such as solar-to-fuel energy conversion. Usually, PCS at heterogeneous interfaces takes place by the following two steps; the absorption of solar energy by light absorbers and the subsequent charge transfer at D–A heterogeneous interfaces, as shown in Figure 1a. In the two-step PCS mechanism, the interfacial charge transfer process requires the energy level offset of at least ca. 0.3 eV between heterogeneous D and A substances. Accordingly, the two-step PCS occurs with a relatively large energy loss of at least ca. 0.3 eV. In order to realize efficient solar energy conversion, the exploration of a new PCS mechanism to minimize the energy loss is of great importance. Interfacial charge-transfer transitions (ICTTs) enable direct PCS at D–A heterogeneous interfaces without energy loss in principle, as shown in Figure 1b. However, the research field of ICTT has not been cultivated yet. Recently, several progresses have been reported in the research of ICTT between organic compounds and inorganic semiconductors by our group, ranging from the material development to the application of ICTT in solar cells. At first, we developed several new organic-metal oxide hybrids for ICTT. By applying these hybrid materials in photovoltaic cells, we successfully demonstrated efficient photoelectric conversion based on the direct PCS via ICTT for the first time [1]. We revealed that the efficient photoelectric conversion originates from the effective suppression of charge recombination in the hybrid materials, providing a theoretical insight into the control of charge recombination after ICTT [2]. Moreover, we showed that ICTT can be successfully applied to dye-sensitized solar cells (DSSCs), particularly, in the photovoltaic conversion in the near IR region [3]. This result indicates that the appropriate combination of dye sensitization and ICTT would be useful for the improvement of the energy conversion efficiency of DSSC. We introduce the current situation of DSSC concisely below.
Dye-sensitized solar cells (DSSCs) have attracted much attention as a next-generation solar cell [4,5,6] and recently gained increasing interest in the potential indoor application, for example, as a power supply for the Internet of Things (IoT) because of the efficient power generation in low light conditions [7,8,9,10]. DSSC works by the two-step PCS mechanism via light absorption by dyes and the subsequent electron transfer from excited dyes into the conduction band of wide band-gap semiconductors such as TiO2, as shown in Figure 2a. So far, considerable effort has been devoted to the research and development of DSSC using various dyes (organic dyes and metal-complex dyes) and hole-transporting redox electrolytes [11,12,13,14,15,16,17,18,19]. Recently, the power conversion efficiency (PCE) over 14% under the standard solar irradiation (AM1.5G, 1 sun) was achieved by two groups [15,19]. In addition, solid-state DSSC with PCE over 11% was developed [20,21,22]. However, the highest PCE of DSSC is still lower than those (>20%) of other solar cells such as crystalline Si solar cells and perovskite solar cells [23]. To realize more efficient solar energy conversion is a high-priority subject for DSSC. As mentioned above, the two-step PCS occurs with a large amount of energy loss. In DSSC, the electron injection from excited dyes to TiO2 occurs with an energy loss of ca. 0.3 eV that corresponds to the energy level offset between the LUMO of dyes and the conduction band minimum (CBM) of TiO2 [5,6,24,25,26,27], as shown in Figure 2a. ICTT between organic compounds including dyes and inorganic semiconductors is anticipated to overcome the energy loss issue in the electron injection process due to the direct PCS nature. There are two schemes for the application of ICTT to photovoltaic conversion, in which solar energy is absorbed only by ICTT and by both dye sensitization and ICTT, as shown in Figure 2b,c, respectively. In the former scheme, ICTT opens up a new potentiality of organic compounds for photovoltaic conversion. Photovoltaic materials with two functions of solar energy absorption and direct PCS can be prepared by using low-cost organic compounds with a wide HOMO–LUMO gap. Because of the different PCS mechanisms and light absorbing materials and methods to realize efficient photovoltaic conversion, ICTT-based photovoltaic cells should be distinguished from DSSC. In the latter scheme, on the other hand, ICTT between dyes and TiO2 can expand the spectral sensitivity of DSSC on the longer wavelength side. This scheme is included in the framework of DSSC. We have examined the two approaches of ICTT to the development of solar cells. Here, we review the background and the recent progresses in the research of ICTT and the application in solar cells.

2. Interfacial Charge-transfer Transitions and Their Photovoltaic Conversion Properties

First, we briefly introduce the history of the fundamental research of ICTT. To our best knowledge, ICTT between organic compounds and inorganic semiconductors was first reported in 1983 by Houlding et al. [28]. They observed that TiO2 nanoparticles were colored bright yellow orange upon the chemisorption of 8-hydroxyquinoline (HOQ) via the hydroxy group and showed a broad absorption band in the visible region. They predicted that the visible light absorption is due to ICTT from the chemisorbed OQ molecule to TiO2. Interestingly, they also demonstrated photocatalytic H2 generation via the ICTT excitation, showing the high potentiality of ICTT in the solar energy conversion. Then, Moser et al. and Rajh et al. reported in 1991 and 1999 that catechol (CA) [29] and ascorbic acid (AA) [30] show ICTT bands in the visible region upon the chemisorption on TiO2 surfaces via their two hydroxy groups, respectively, as shown in Table 1. Persson et al. computationally verified ICTT between TiO2 and CA with quantum chemical calculations [31]. Stimulated by their pioneering studies, ICTTs in the surface complexes of TiO2 nanoparticles with aromatic hydroxy compounds (mono-hydroxy compounds [32,33,34,35,36], enediol compounds [37,38,39,40,41], etc.) have been examined in detail.
Against the background, photovoltaic conversion properties of ICTT have been examined from around 1996 employing the surface complexes of TiO2 nanoparticles with aromatic hydroxy compounds. In 1996, Tennakone et al. first reported the photocurrent generation by ICTT using TiO2-CA and TiO2-gallic acid surface complexes [42]. Unfortunately, they did not estimate incident photon-to-current conversion efficiencies (IPCEs) in their experiments. Then, Xagas el al. [43] and Sirimanne et al. [44] reported in 2000 and 2003 photovoltaic conversion due to ICTT in the TiO2-AA surface complex and estimated IPCE to be ca. 5% and ca. 12%, respectively, as shown in Table 2. In 2005, Tae et al. reported low IPCE (ca. 9%) for ICTT in the TiO2-CA surface complex [45]. Contrary to the anticipation, it was seen that ICTTs in the TiO2-enediol surface complexes give rise to quite inefficient photovoltaic conversion with the low IPCE values. In order to examine the charge recombination process after ICTT, Wang et al. performed femtosecond transient absorption measurements of the TiO2-CA complex [46]. They found that the charge recombination occurs very rapidly after the ICTT excitation, in which ca. 80% of electrons injected into the conduction band of TiO2 by ICTT recombine with holes on the adsorbed CA molecules within ca. 10 psec [46]. This result is consistent with the above IPCE data. Accordingly, the charge recombination should be suppressed for efficient photovoltaic conversion. These results show that enediol compounds such as CA and AA are not suitable for direct PCS photovoltaic conversion. Since organic-inorganic semiconductor hybrids for ICTT were quite limited at that time, there had been no further progress on the photovoltaic conversion based on direct PCS until around 2010.
New kinds of organic-inorganic semiconductor complexes for ICTT have been developed with various organic compounds and inorganic semiconductors since around 2010, as shown in Table 1 [47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. In 2011, our group reported unique surface complexes of bis(dicyanomethylene) compounds (TCNXs) such as tetracyanoquinodimethane (TCNQ), which are well-known to be strong electron acceptors, with TiO2 nanoparticles [51,52]. As mentioned above, organic compounds typically chemisorb on metal-oxide surfaces (TiO2, ZnO, etc.) via their chemical anchoring groups (-OH, -SH, -COOH, -NH2, etc.) through the dehydration condensation reaction with surface hydroxy groups on TiO2, as shown in Figure 3a. In contrast, TCNQ has no chemical anchoring groups and chemisorbs on TiO2 surfaces via the nucleophilic addition reaction of a surface hydroxy group, producing the negatively-charged TCNQ adsorbate on TiO2, as shown in Figure 2b. Upon the surface complex formation, the color of TiO2 nanoparticles is drastically changed to violet, as shown in Figure 4. The TiO2-TCNQ complex shows a broad ICTT band in the visible region between 400 and ca. 700 nm, in which ICTT takes place from the HOMO of the adsorbed TCNQ to the conduction band of TiO2. The coloration and the wavelength range of ICTT band are drastically changed by introducing electron-donating or -withdrawing substituent groups or modification of the benzene ring, as shown in Figure 4 [52,55]. These spectral changes are directly associated to the variation in the HOMO energy of the adsorbed TCNX due to the chemical modifications [56]. With increasing and decreasing the electron accepting property of TCNX, the ICTT band blue- and red-shifts, respectively. Notably, our research showed that the adsorption mechanism and structure of organic compounds can be clarified by means of FT-IR measurements and density functional theory (DFT) calculations and the electronic structure and ICTT excitation can be clarified by ionization potential measurements based on photoelectron yield spectroscopy (PYS) and DFT and time-dependent DFT (TD-DFT) calculations, which provide a firm basis for the research of ICTT [51,52,53,54,55,56].
We examined photovoltaic properties of ICTT in the TiO2-TCNQ surface complex [50]. The TiO2-TCNQ photovoltaic cell was fabricated with an anatase TiO2-TCNQ nanoporous photoelectrode and iodide electrolyte (I-/I3- in acetonitrile). The fundamental structure is shown in Figure 5a. We observed efficient photovoltaic conversion with IPCE of ca. 80%, as shown in Figure 5b. This IPCE value is remarkably higher than those (IPCE < ca. 10%) reported for the direct PCS photovoltaics with the TiO2-enediol complexes. Figure 5c shows the current density-voltage (J-V) curve of the TiO2-TCNQ photovoltaic cell under simulated solar irradiation (AM1.5G, 100 mW/cm2). The short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), and PCE were estimated to be 9.9 mA/cm2, 0.36 V, 0.62, and 2.2%, respectively. Although VOC is rather low, this is the first time that such high JSC was obtained by ICTT.
More efficient photoelectric conversion due to ICTT was achieved by employing the surface complex of TiO2 with 2-anthracene carboxylic acid (ATCA) [1]. Upon the immersion into the ATCA solution, anatase TiO2 nanoparticles were colored yellow, as shown in Figure 6a. The TiO2-ATCA complex is formed via a carboxy group similarly to the TiO2-dye system in DSSC and shows an ICTT band in the visible region between 400 and ca. 650 nm. Figure 6b,c shows IPCE spectrum and J-V curve under simulated solar irradiation (AM1.5G, 100 mW/cm2) of the TiO2-ATCA photovoltaic cell with iodide electrolyte (LiI: 1 M, I2: 0.025 M, solvent: acetonitrile), respectively, which was fabricated in a similar way to the TiO2-TCNQ photovoltaic cell. The TiO2-ATCA photovoltaic cell showed higher IPCE (86% at 440 nm) than that obtained for the TiO2-TCNQ cell, as shown in Figure 6b. Taking into account the reflection (ca. 90%) of incident light on the FTO surface, almost all incident photons were absorbed by ICTT and converted to photocurrent. JSC, VOC, FF, and PCE were estimated to be 6.6 mA/cm2, 0.50 V, 0.66, and 2.2%, respectively [1]. The JSC value is lower than that of the TiO2-TCNQ photovoltaic cell because of the narrower spectral range despite the higher IPCE maximum value. Table 2 summarizes the IPEC values reported for the direct PCS photovoltaic conversion. Inefficient photovoltaic conversion with IPCE values lower than ca. 10% was reported with the enediol compounds. In contrast, efficient photoelectric conversion with high IPCE values was realized with TCNQ and ATCA. Since the low IPCE is attributed to the rapid charge recombination after ICTT, it is likely that charge recombination is suppressed significantly in the TiO2-TCNQ and TiO2-ATCA systems. We theoretically analyzed the charge recombination process based on the Marcus theory with DFT and TD-DFT calculations [2].

3. Suppression of Charge Recombination for Direct PCS Photovoltaics

Figure 7a shows the harmonic potential curves of the ground singlet state (S0) and lowest excited charge-separated state (S1) with the same curvature. The aforementioned ICTT systems correspond to the so-called inverted region in the Marcus theory, in which the energy gap (ΔE) between the S0 and S1 potential minima is larger than the reorganization energy (λ) in the S1 state. Charge recombination occurs via thermal activation in the S1 state and the subsequent jumping to the S0 potential curve at the cross point, as shown by the dotted arrows in Figure 7a. The activation energy (Ea) for charge recombination is a key factor to reduce the charge recombination rate (krecom).
According to the Marcus theory, krecom after ICTT is given by the following equation.
k recom = 2 π 2 h β 2 π λ k B T exp ( − E a k B T )
h, kB, T, and β are the Plank constant, Boltzmann constant, absolute temperature, and the transfer integral between the S0 and S1 states at the cross point, respectively. From this equation, it is seen that krecom is predominantly governed by Ea, which is given by the following equation.
E a = ( λ − Δ E ) 2 4 λ
this equation indicates that Ea increases with decreasing λ, slowing down the charge recombination and vice versa. By DFT and TD-DFT calculations using very simple model complexes shown in Figure 7b, λ values for the four ICTT surface complexes in Table 2 were calculated to be 0.79 eV for AA, 0.71 eV for CA, 0.34 eV for TCNQ, and 0.25 eV for ATCA [2]. Note that the geometrical optimization of each model complex in the S1 state for the estimation of λ was carried out by fixing the Cartesian coordinates of OH and H2O ligands to those in the S0 optimized structure and relaxing other atoms. Accordingly, the reorganization energies originate from structural changes of the Ti atom and each adsorbed molecule in the S1 state. Figure 8a shows the relationship between the reported IPCE values and the calculated reorganization energies. The TiO2-ATCA and -TCNQ model complexes featuring high IPCE show relatively small reorganization energies, while the TiO2-CA and -AA model complexes showing low IPCE exhibit much larger reorganization energies. This correlation is consistent with the tendency predicted from Equations (1) and (2). Ea for each the organic molecule was estimated from Equation (2) using the calculated λ and ΔE experimentally estimated from the absorption onset of the ICTT band. Figure 8b shows the relationship between the reported IPCE values and the calculated activation energies. We confirm a reasonable correlation between IPCE and Ea, which indicates that the high IPCE is attributed to the higher activation energy (slow charge recombination) and the low IPCE is due to the low activation energy (fast charge recombination). In addition, it is seen that IPCE abruptly decreases with Ea around 1.5 eV. In order to understand this behavior, we formulated IPCE based on the two kinetic processes including charge recombination and escape from the TiO2 surface generating free electron-hole pairs that are detected as photocurrent, as shown in Figure 9a. Based on the kinetic scheme in Figure 9b, IPCE is given by the following equation [2]:
IPCE   ( % ) = 100 × LHE 1 + c × exp ( − E a k B T )
c = 2 π 2 k escape h β 2 π λ k B T
LHE is the light-absorption quantum efficiency. Since the dependence of c on λ is much weaker than the dependence of IPCE on Ea, we tentatively treat c as a constant. Figure 8b shows the dependence of IPCE on Ea with LHE of 0.86 and c of 1 × 1016 and 1 × 1026. The calculated Ea dependence of IPCE well reproduces the IPCE-Ea correlation. This result clearly reveals that the reorganization energy should be small to suppress charge recombination for obtaining high IPCE. Our DFT analysis indicates that the reorganization energy strongly depends on the kind of chemical anchoring group and a carboxy group is the most useful anchor to suppress the charge recombination [2].

4. Photovoltaic Conversion Based on ICTT in DSSC

Generally, it has been reported that carboxy-anchor dyes adsorbed on TiO2 exhibit light absorption at longer wavelengths than their intra-molecular electronic transitions. Figure 10a shows the energy level diagram of TiO2 and the LEG4 dye, which was employed in the DSSC achieving the highest PCE (14.3%) [15]. The absorption onset of the LEG4 dye red-shifts from ca. 620 to ca. 780 nm upon the adsorption on TiO2, enabling photovoltaic conversion in the near IR region, as shown in Figure 10b. However, the origin of the near IR light absorption was not unknown. The above-mentioned result of the efficient photoelectric conversion due to ICTT in the TiO2-ATCA complex suggests that the red-shift of the absorption band is attributable to ICTT from LEG4 to TiO2. In fact, the absorption onset energy (1.6 eV) of the TiO2-LEG4 complex well corresponds to the energy difference between the CBM of TiO2 and the LUMO of the dye, as shown in Figure 10a. In order to get an insight into the near IR absorption, we examined the absorption properties of the TiO2-LEG4 surface complex with TD-DFT calculations [3]. Figure 11a shows HOMO and LUMO of the bridge- and chelate-type model complexes. For the both models, the HOMO is delocalized over the LEG4 molecule and the LUMO is predominantly distributed in the TiO2 cluster, but slightly delocalized on the carboxylate group and cyclopentadithiophene moiety of the dye. Figure 11b shows the TD-DFT calculated electronic excitation spectra of the model complexes. The TD-DFT calculations indicate that dye-to-TiO2 ICTTs appear at longer wavelengths than the intra-dye electronic transition consistent with the experimental result. Since the lowest electronic excitation is assigned to the HOMO⇒LUMO transition, the ICTT undergoes direct electron injection into the conduction band. We confirmed that other DSSC dyes bearing a carboxy anchoring group also show ICTT bands on longer wavelength side than their intra-dye absorption. From the DFT analysis, it is seen that the energy levels in the conduction band close in energy to the LUMO of the dyes are strongly coupled with the LUMO, which results in the delocalization on the dyes, rendering ICTT dipole-allowed. The magnitude of the electronic coupling with LUMO for each conduction-band level tends to decrease with increasing the energy difference between them. In addition, the density of states (DOS) of the conduction band of anatase TiO2 gradually increases with the energy of the conduction band [70]. Taking into account these factors, the absorption intensity of ICTT decreases as the excitation energy approaches to ECBM-EHOMO, as observed experimentally in the absorption spectra of the TiO2-dye complexes and IPCE spectra of the DSSCs. Based on these results, it is concluded that ICTT plays an important role in the photovoltaic conversion in the near IR region for enhancing PCE of DSSC effectively.

5. Summary

In summary, we reviewed the background and recent progresses on ICTT between organic compounds and metal-oxide semiconductors and its application in solar cells. As mentioned above, there are two approaches of ICTT to solar cells, in which solar energy is absorbed only by ICTT and by both dye sensitization and ICTT. The former approach widely opens up the novel functionality of organic compounds for photovoltaic conversion. ICTT enables to prepare photovoltaic materials with two functions of light absorption and direct PCS using low-cost organic compounds with a large HOMO-LUMO gap. Recently, new organic-metal oxide semiconductor hybrids for ICTT have been developed using various organic compounds and metal-oxide semiconductors. Based on the vigorous material development and fundamental research of ICTT, efficient ICTT-based photovoltaic conversion was demonstrated with the TiO2-TCNQ and TiO2-ATCA complexes. The obtained IPCE values (>ca. 80%) are much higher than those (<ca. 10%) reported for the TiO2-enediol complexes. Our DFT analysis indicates that the efficient photoelectric conversion results from the effective suppression of charge recombination in the surface complexes and reveals that the charge recombination rate strongly depends on the kind of chemical anchoring group. These studies provide an important basis for the development of direct PCS photovoltaic devices. In the latter approach, we found that the carboxy-anchor dyes typically used in DSSC show ICTT in the visible to near IR region. Particularly, ICTT plays a crucial role in the absorption of near IR sunlight in DSSC. This result indicates the possibility that appropriate combination of dye sensitization and ICTT could improve the PCE of DSSC exceeding 14%. For both the approaches, further material development and fundamental research of ICTT is necessary to realize efficient direct PCS photovoltaic conversion.

Funding

Our research on ICTT was partially supported by the Precursory Research for Embryonic Science and Technology (PRESTO) program of the Japan Science and Technology Agency (JST).

Acknowledgments

The author (J.F.) is grateful to M. Hanaya (Gunma Univ.) and H. Segawa (Univ. Tokyo) for fruitful collaboration.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Schematic pictures of photoinduced charge separation (PCS) between heterogeneous electron-donating and -accepting substances. (a) Conventional two-step PCS mechanism and (b) direct PCS mechanism based on interfacial charge-transfer transitions (ICTT).
Figure 1. Schematic pictures of photoinduced charge separation (PCS) between heterogeneous electron-donating and -accepting substances. (a) Conventional two-step PCS mechanism and (b) direct PCS mechanism based on interfacial charge-transfer transitions (ICTT).
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Figure 2. Schematic energy level diagrams of (a) dye sensitization of TiO2 (two-step PCS), (b) ICTT between an organic compound with a wide HOMO–LUMO gap and TiO2 (direct PCS), and (c) dye sensitization and ICTT between a dye and TiO2. CB and VB denote the conduction band and valence band, respectively. HOMO and LUMO stand for the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively.
Figure 2. Schematic energy level diagrams of (a) dye sensitization of TiO2 (two-step PCS), (b) ICTT between an organic compound with a wide HOMO–LUMO gap and TiO2 (direct PCS), and (c) dye sensitization and ICTT between a dye and TiO2. CB and VB denote the conduction band and valence band, respectively. HOMO and LUMO stand for the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively.
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Figure 3. Chemisorption reactions of (a) organic compounds bearing proton-donating chemical anchoring groups via dehydration condensation reaction and (b) TCNQ via nucleophilic addition reaction.
Figure 3. Chemisorption reactions of (a) organic compounds bearing proton-donating chemical anchoring groups via dehydration condensation reaction and (b) TCNQ via nucleophilic addition reaction.
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Figure 4. Photographs of anatase TiO2 nanoparticles immersed in the solution of various TCNXs.
Figure 4. Photographs of anatase TiO2 nanoparticles immersed in the solution of various TCNXs.
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Figure 5. (a) Structure of TiO2-TCNQ photovoltaic cells fabricated in our study and (b) incident photon-to-current conversion efficiency (IPCE) excitation spectrum and (c) J-V curve under simulated solar irradiation (AM1.5G, 100 mW/cm2) of TiO2-TCNQ photovoltaic cell with iodide electrolyte (LiI: ca. 2 M, I2: 0.025 M, solvent: acetonitrile).
Figure 5. (a) Structure of TiO2-TCNQ photovoltaic cells fabricated in our study and (b) incident photon-to-current conversion efficiency (IPCE) excitation spectrum and (c) J-V curve under simulated solar irradiation (AM1.5G, 100 mW/cm2) of TiO2-TCNQ photovoltaic cell with iodide electrolyte (LiI: ca. 2 M, I2: 0.025 M, solvent: acetonitrile).
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Figure 6. (a) Coloration of anatase TiO2 nanoparticles immersion into the ATCA solution and (b) IPCE spectrum and (c) J-V curve under simulated solar irradiation (AM1.5G, 100 mW/cm2) of TiO2-ATCA photovoltaic cell with iodide electrolyte (LiI: 1 M, I2: 0.025 M, solvent: acetonitrile). Adapted with permission from Reference 1. Copyright 2015 Elsevier.
Figure 6. (a) Coloration of anatase TiO2 nanoparticles immersion into the ATCA solution and (b) IPCE spectrum and (c) J-V curve under simulated solar irradiation (AM1.5G, 100 mW/cm2) of TiO2-ATCA photovoltaic cell with iodide electrolyte (LiI: 1 M, I2: 0.025 M, solvent: acetonitrile). Adapted with permission from Reference 1. Copyright 2015 Elsevier.
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Figure 7. (a) Potential energy curves of the ground state (S0) and the lowest excited charge-separated state (S1) and (b) DFT optimized structures of the model complexes in the S0 state. Gray: carbon, white: hydrogen, blue: nitrogen, red: oxygen, large white: titanium atom. Adapted with permission from Reference 2. Copyright 2015 the PCCP Owner Societies.
Figure 7. (a) Potential energy curves of the ground state (S0) and the lowest excited charge-separated state (S1) and (b) DFT optimized structures of the model complexes in the S0 state. Gray: carbon, white: hydrogen, blue: nitrogen, red: oxygen, large white: titanium atom. Adapted with permission from Reference 2. Copyright 2015 the PCCP Owner Societies.
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Figure 8. (a) Correlation between the reported IPCE and calculated reorganization energy (λ) and (b) correlation between the reported IPCE and activation energy (Ea) calculated with the calculated reorganization energy and experimentally-estimated ΔE value. See text for detail. Adapted with permission from Reference 2. Copyright 2015 the PCCP Owner Societies.
Figure 8. (a) Correlation between the reported IPCE and calculated reorganization energy (λ) and (b) correlation between the reported IPCE and activation energy (Ea) calculated with the calculated reorganization energy and experimentally-estimated ΔE value. See text for detail. Adapted with permission from Reference 2. Copyright 2015 the PCCP Owner Societies.
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Figure 9. (a) Schematic picture of two kinetic processes of electrons injected into TiO2 by ICTT and (b) kinetic scheme of the charge-separated state. Adapted with permission from Reference 2. Copyright 2015 the PCCP Owner Societies.
Figure 9. (a) Schematic picture of two kinetic processes of electrons injected into TiO2 by ICTT and (b) kinetic scheme of the charge-separated state. Adapted with permission from Reference 2. Copyright 2015 the PCCP Owner Societies.
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Figure 10. (a) Energy level diagram of TiO2 and LEG4 along with the molecular structure and (b) absorption spectrum of LEG4 in ethanol and IPCE spectrum of TiO2-LEG4 DSSC. Adapted with permission from Reference 3. Copyright 2018 American Chemical Society.
Figure 10. (a) Energy level diagram of TiO2 and LEG4 along with the molecular structure and (b) absorption spectrum of LEG4 in ethanol and IPCE spectrum of TiO2-LEG4 DSSC. Adapted with permission from Reference 3. Copyright 2018 American Chemical Society.
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Figure 11. (a) Isosurface plots of HOMO and LUMO and (b) TD-DFT calculated electronic excitation spectra of bridge- and chelate-type TiO2-LEG4 model complexes. Adapted with permission from Reference 3. Copyright 2018 American Chemical Society.
Figure 11. (a) Isosurface plots of HOMO and LUMO and (b) TD-DFT calculated electronic excitation spectra of bridge- and chelate-type TiO2-LEG4 model complexes. Adapted with permission from Reference 3. Copyright 2018 American Chemical Society.
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Table
Table 1. Representative organic-inorganic semiconductor complexes for ICTT, bridging atoms, and the year of the first report for each organic-inorganic complex.
Table 1. Representative organic-inorganic semiconductor complexes for ICTT, bridging atoms, and the year of the first report for each organic-inorganic complex.
Inorganic SemiconductorOrganic CompoundBridging AtomYear
TiO2Aromatic
hydroxy
compound		Mono-hydroxy compoundO1983
TiO2CatecholO1991
TiO2Ascorbic acidO1999
TiO2Aromatic carboxylic acidO2008
TiO2Bis(dicyanomethylene) compoundO2009
TiO2Aromatic amineN2011
TiO2BenzenedithiolS2015
BaTiO3CatecholO2016
SrTiO3CatecholO2018
ZnOBenzenethiolS2020
Table
Table 2. Incident photon-to-current conversion efficiency (IPCE) maximum values reported for ICTT based photovoltaic conversion in organic-metal oxide complexes.
Table 2. Incident photon-to-current conversion efficiency (IPCE) maximum values reported for ICTT based photovoltaic conversion in organic-metal oxide complexes.
Organic-Metal Oxide ComplexIPCE Maximum Value
TiO2-AA5%
TiO2-AA12%
TiO2-CA9%
TiO2-TCNQ81%
TiO2-ATCA86%

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