New Type Organic–Inorganic Solar Cells based on All Chl Derivative
Developing and utilizing renewable and clean energy have become a top priority for society today due to the energy and environmental crisis. Solar cells, which could convert solar energy efficiently into heat and electrical energy, have been developed greatly.46-48 However, the high costs and uneasy degradation have limited the further development of traditional inorganic- or organic materials-based photovolta- ics. Photosynthesis is the most basic channel for converting renewable solar energy into chemical energy and biological energy on the earth.49 As an essential natural pigment in photosynthesis, Chi plays an important role in light capturing, energy/ charge transferring in natural photosynthetic complex.50
Chi derivatives are widely used in DSSCs, perovskite solar cells, and organic solar cells due to their main merits of easy synthesis, non-toxics, and low costs.51-53 Furthermore, their outstanding photoelectron performance and strong absorption ability around near-infrared region provide new research ideas and methods for the usage of clean and renewable energy.54 In addition, previous studies about DSSCs have proved that charge dissociation could take place at the Chi sensitizer and TiO, interface.55-56 Considering above-mentioned advantages, a series of Chi derivatives are semi-synthesized and applied into OISCs to achieve the goal of using “green” photovoltaics with high efficiency.
Three kinds of carboxyl-functioned Chi derivatives methyl trans-32-carboxypy- ropheophorbide a (Chi-1) or its zinc complex (ZnChl-1) as sensitizers and zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide a (named as ZnChl-2) as hole transporter materials (HTM) were employed to fabricate the mesoporous OISC with an architecture of FTO/compact TiOVmesoporous Ti02/Chl-1 or ZnChl-l/ZnChl-2/Ag (see their molecular structure in Figure 8.20 and device architecture in Figure 8.2 la).57 The photoexcited excitons are dissociated at the Ti02 and Chi sensitizer interface due to the strong driving force between their Ti02 and Chi heterojunction. The electrons
FIGURE 8.20. Molecular structures of the Chi and porphyrin derivatives used in the organic-inorganic heterojunction based BSCs. Reprinted with permission from Ref. 19, Copyright 2019. American Chemical Society.
FIGURE 8.21. (a) Device structure and (b) energy alignments of the whole Chi derivative as photoactive material-based OISCs, (c) IPCE spectra of the Chl-l/ZnChl-3-based OISCs via solvents engineering, and (d) possible charge transfer pathway for the Chl-l/ZnChl-3-based OISCs with CF:CB=2:1 as solvents for ZnChl-3. Reprinted with permission from Ref. 20. Copyright 2015, Elsevier B.V.
are extracted by the Ti02 and are collected at FTO side. In the meanwhile, the holes are transferred to Ag side through ZnChl-2 (see their working principle in Figure 8.21b).
It is worthy to mention that the hole transferring ability among ZnChl-2 is greatly increased due to their formed aggregates. As an initial attempt of fabricating the OISCs with Chi derivatives as sensitizer and hole transporter, such an interesting and unique device, could work normally but only gave an efficiency of 0.11%. Here, the ZnChl-2 had no contribution to the photocurrent since there was no incident photon to converted electron (IPCE) single from ZnChl-2, which means that ZnChl-2 only functioned as a hole transporter instead of photoactive layer. The photocurrents were generated only at the Chi-1 and Ti02 such an organic-inorganic interface. Although the current efficiency was relatively low. this study offered new possibility of using bio-resources as the whole photoactive materials for next generation of photovoltaics and also deepen our recognition of Chi derivatives.
Considering the relative low efficiency for the previous OISC devices, molecular engineering is employed to improve the hole extraction ability of hole transporter. Four kinds of hole transport materials were synthesized with either Chi or porphyrin skeleton together with different substitutes at C3 position named as ZnChl-3/4 and ZnPor-1/2, respectively.58 All the HTMs showed suitable energy levels and well- aggregated state for a favorable hole transfer from the sensitizer to Ag. In the meanwhile, it proved that the larger (highest occupied molecular orbital) HOMO energy gap between sensitizer and HTM contributes to a larger photovoltage. As a result, with an improved carrier mobility as maximum as 4.1 lxl 0-3 cm2-V_l-s-1 and highest HOMO energy level for ZnChl-3, a better PCE of 0.86% was achieved for this ZnChl-3-based OISC devices with the same working principle as previous one. And it is noted that the fabrication process of this device is without employing of any additives, which is favorable to realize a true “green energy” utilization.
Although the PCE for this whole Chl-based OISC device has improved through optimizing the molecular structure of the Chi HTM, their overall efficiency still needs to be further improved. Previous study found that the final photovoltaic performance is greatly influenced by the intermolecular arrangement.59-60 And solvents engineering could influence the molecular arrangement. Thus, the aggregation of ZnChl-3 HTM was controlled by solvents engineering. Different solvents (chloroform (CF), chlorobenzene (CB), CF:CB=2:1, CF:CB=1:1, and CF:CB=1:2) were employed as the chosen solvents forZnChl-3 HTM.61 The spin-coated ZnChl-3 films prepared by different solvents differ from their absorption ability, film morphology, and carrier mobility. When controlling the CF and CB as the ratio as 2:1, the ZnChl-3 film showed the highest carrier mobility and the most uniform film morphology, which means the ZnChl-3 showed the best aggregation state in such a situation, leading to the most favorable charge transfer from Chi-1 to Ag. Here, the device architecture of this device is FTO/compact TiO,/mesoporous Ti02/Chl-l/ZnChl-3/Ag. And the PCE of this system was further enhanced to 2.13% through solvent engineering.
The most surprising thing was that, for the first time, the ZnChl-3 HTM contributes to the photocurrent generation after solvents optimization, which means both Chi sensitizer and HTC can be excited to generate photocurrent in this solar cell, as proved by the IPCE spectra in Figure 8.21c. However, the photocurrent of the device based on CB as solvents showed a major contribution from sensitizer, which is similar to the previous study. And for the device with CB as the solvents for ZnChl-3, the ZnChl-3 only could act as HTL. Furthermore, the heterojunction between the Chl-1 sensitized TiO, and HTC may form, thus leading to an enhanced photocurrent generation through analyzing the spectral analysis of the devices and films (Figure
8.2 Id). The excitons of ZnChl-3 are dissociated at the ZnChl-3 and Chl-1-sensitized Ti02 interface. And then electrons from ZnChl-3 are transferred to TiO, and finally collected by FTO. In the meanwhile, the holes from both Chl-1 and ZnChl-3 are gathered at Ag side.
In order to clear out the excited state dynamics at the Chl-1 sensitizer and ZnChl-3 HTL interface, sub-picosecond time-resolved absorption spectroscopy (TAS) was employed to investigate above-mentioned system.62 After pumping the Chl-1- sensitized TiO, and ZnChl-3 bilayer at both 680 nm and 720 nm, there are two bleaching signals at sorel and Qv bands together with an positive absorption signal at 675 nm (Figure 8.22a and b). A charge transfer state between Chl-1 sensitizer and ZnChl-3 was observed at 640 nm after exciting at the 680 nm and 720 nm. A fast electron injection process from Chl-1 to TiO, was observed followed by the process of radical cation transferring from Chi-1 to ZnChl-3. Such a new charge transfer state was observed when exciting both at 680 nm and 720 nm, indicating that charge dissociation and transfer could take place at the Chl-1 to ZnChl-3 interface.
FIGURE 8.22. The TAS of TiO,-Chl-l/ZnChl-3 films pumped at (a) 680 nm and (b) 720 nm.
- (c) device architecture of the Chi derivative-based OISC devices with P3HT as HTL, and
- (d) IPCE spectra of the OISC devices with or without P3HT as HTL. Reprinted with permission from Ref. 21. Copyright 2019, Royal Society of Chemistry.
Considering the formation of heterojunction between Chi-1 sensitized Ti02 and ZnChl-3, thus there may be no electron-blocking layer or hole-extracting layer between ZnChl-3 and Ag interface. A traditional polymer P3HT was employed as hole transporter to improve the charge collection efficiency of the whole Chi derivative-based organic-inorganic heterojunction solar cells63 (see device structure in Figure 8.22c). After adding the P3HT layer, the charge transfer resistance of the device is reduced while the charge recombination resistance is increased, leading to a smoother and more efficient charge transfer and collection from ZnChl-3 to Ag. Therefore, the photocurrents and fill factor of this OISCs device (FTO/compact Ti02/ mesoporous Ti02/Chl-l/ZnChl-3/P3HT/Ag) were enhanced greatly. In addition, the photon-to-electron conversion in both 300-540 nm and 660-725 nm wavelength regions have enhanced significantly (Figure 8.22d). Therefore, the P3HT enhanced the charge transport ability and suppressed the charge recombination simultaneously, thus achieving a higher efficiency for the OISC device.
Recently, our group has successfully fabricated a kind of bilayer, Chl-based BSCs, to simulate the electron transfer process of the natural Z-scheme oxygenic photosynthesis.64 However, charge extraction between the planar ZnO and Chi derivative interface might be insufficient, w'hich origins from the relatively small contact area. In order to improve the charge extraction ability and also further mimic photosynthesis systems, we proposed a trilayer Chl-based OISC device by employing TiO,-sensitized Chl-l as primary electron acceptor, ZnChl-5 as PSI simulator as Chl-6 as PSII simulator65 (see molecular structure in Figure 8.20). The device structure we fabricated here is FTO/compact Ti02/mesoporous Ti02/Chl-l/Chl-5/Chl-6/Ag (Figure 8.23a). Compared the bilayer Chi based devices with the trilayer Chi based ones, the planar ETL ZnO is replaced by Chl-l-sensitized Ti02, which owns larger interface contact areas and could capture more complimentary light than previous ZnO. Such an enhanced light absorption ability and charge extraction ability will lead to a higher photocurrent and lower charge recombination.
The possible charge transfer pathway is shown in Figure 8.23b. The Ti02- sensitized Chl-l, ZnChl-5, and Chl-6 could be photoexcited simultaneously. The photoexcited electrons in ZnChl-5 (PSI simulator) is extracted by Chl-l-sensitized Ti02 (primary electron acceptor simulator) and collected at FTO side while the holes of Chl-6 (PSII simulator) are gathered at Ag side. The rest holes of ZnChl-5 (PSI simulator) are recombined with the electrons of Chl-6 (PSII simulator), this charge transfer step is similar to the charge transfer of natural Z-scheme photosynthesis. The PCE for such a unique device could reach to 3.21%. Considering that the electron injection efficiency could be further improved by со-sensitizing different sensitizer. In order to get a best electron injection efficiency at Chl-l-sensitized TiO, and ZnChl-5 interface, Chl-7 is introduced as co-sensitizer together with Chl-l. After optimizing the ratio of Chl-l and Chl-7 as 100:1 wt%, the highest PCE of 4.14% was reached by this со-sensitized trilayer Chl-based OISC system, which is due to the decreased charge transfer resistance. And this study shows endless possibility to get a high efficiency for the Chl-based OISC devices by mimicking the natural photosynthesis process.
The above-mentioned series of exploration by using natural Chi derivatives as photoactive materials for OISCs prove that Chi derivatives, as an abundant natural pigment with low costs and non-toxic property together with biodegradable ability, are promising for the next generation photovoltaics. Improving the efficiencies of Chi derivatives-based OISCs with organic-inorganic heterojunction also makes them
FIGURE 8.23. (a) Device architecture of the trilayer Chi derivative-based OISCs and (b) energy alignments together with possible electron transfer pathway for this natural Z-scheme photosynthesis-simulated OISC devices. Reprinted with permission from Ref. 19, Copyright 2019, American Chemical Society.
charming and unique, thus gathering more and more attentions. We believe in deep that Chi derivative-based OISCs will be further developed and ultimately applied commercially.