**6. The key challenges in photosystem-based solar cell development**

#### **6.1. Methods of immobilization and orientation of biocatalysts**

It is necessary to immobilize photoactive molecules on a conductive substrate for the optimum functioning of a solar cell. Most of these cells require peptides for immobilization of pigments on the electrode surface. Another important question is the correct orientation of pigment molecules. The studies conducted at Stanford [62] were focused on the orientation of photo‐ synthetic RCs towards the electrode surface. According to this construction, a poly-histidine tag was attached to the C-terminus of the M-subunit of the *Rhodobacter sphaeroides* RC. With the help of the tag, the construction was immobilized on a gold electrode containing selfassembling layer of alkanethiols with Ni2+-NTA as a head part. It has been experimentally shown that the proximity of RCs to the electrode is important for the cell effective operation [63].

Many techniques were used for immobilization of photosynthetic complexes, including bioelectrocatalystic self-assembling monolayers (bio-SAMs) [45, 47, 63, 64]; Ni2+-NTA attached to poly-histidine tagged PSI complexes (Fig. 6a) [24, 54]; the redox-active hydrogels [38] (Fig. 8a) and fixation on CNTs by means of molecular binding reagents [32]. Each of these techniques provides various beneficial properties, including the increase of the electrode surface area, the rise of the electron transfer rate between the electrode and photobiocatalyst and/or orientation of specific enzymes on the electrode. Unfortunately, enzyme immobiliza‐ tion reduces their activity in comparison with their native state. Therefore, the enzyme activity should be retained for a long time with the help of correct immobilization methods. In a study conducted by Meunier et al. [65], thylakoids were adsorbed on a silicon matrix, thereby the stability of the native thylakoid suspension increased and it remained active for 30 days. Immobilization should provide an optimum rate of electron transfer from the protein to the electrode, with minimal resistance. This can be achieved by correct orientation of proteins on the electrode surface or by the usage of intermediate carriers. Many investigations have shown that the correct orientation of a photosystem on the electrode results in an improved electron transport [45]. In many studies, the correct orientation of the photosystems provides specific binding of a histidine-tagged protein complex with Ni2+-NTA molecule anchored to a gold electrode [24, 50]. Badura et al. [38] used the osmium-containing polymer of polyvinylimidazole as modified electrode acting as both an immobilizing agent and an electron acceptor for the PSII complex (Fig. 8a). Binding of sensitizer with redox polymer is rather interesting way of sensitizer immobilization. Due to that, an electron transfer between neighbouring redox centres covalently bound to the polymer backbone is possible by means of a hopping mecha‐ nism. Thus, there could be the shuttling of electrons from a reactive site within a redox protein towards an electrode surface. Several parameters determine the rate of electron transfer: the polymer backbone composition (flexibility, swelling behaviour, and amount of cross-linking), the distance between the polymer-bound redox centres and potential of the mediator. Hence, the properties of the redox polymer can be adapted to find an appropriate redox polymer for a specific application. In these modified systems, immobilized PSII were capable of generating a current density of 45 mA/cm2 at light intensity of 2.65 mW/cm2 (maximum wavelength at 675 nm) [38].

#### **6.2. The stability of the isolated proteins**

The main problem of the usage of isolated proteins as photosensitizers in photovoltaic cells is their extremely low stability. Photoinhibition of photosystems is the main reason for protein destruction, especially in case of the PSII. Caused by an excessive amount of radiation, photoinhibition may damage the photosynthetic apparatus and hence destroy the chloroplast proteins. Photosystems are provided with some protective mechanisms in vivo [2]. However, once the proteins are isolated, natural self-healing mechanisms do not work. Thus, isolated proteins are more susceptible to damage and have a short lifetime. There is one of the methods to stabilize the photosynthetic complexes through the simulation of the natural states of proteins. Surfactant peptides can be used to imitate the lipid membrane naturally stabilizing photosynthetic complexes. Such surfactants are designed as molecular nanomaterial to study the membrane protein's stability [47]. It consists of hydrophilic amino acids (aspartate or lysine) as the head of the chain and hydrophobic amino acids (alanine) as the tail. For the stabilization of the photosynthetic complex during the construction of a solid electrical device, Das et al. [24] used the peptides A6K and V6D, a sequence of six valines and one aspartic acid (VVVVVVD), as cationic and anionic surfactant peptides, respectively. They showed a shortcircuit current density of 0.12 mA/cm2 at the excitation light intensity of 10 W/cm2 with a wavelength of 808 nm. Presumably, this direction is promising.

#### **6.3. Increase of surface area**

The increase of the electrode surface area is a method conventionally used to improve the efficiency of functioning solar cells. In many cases, the electrode itself is originally flat, and changes of its geometry can destroy its structure. However, the electrode can be modified via nanomaterials, which could increase the real surface area due to the formation of nanostruc‐ tures with non-planar topology on the electrode surface. In this case, the working electrode area is larger than the area of the initial flat surface, and it can absorb more pigment molecules. In their study, Mershin et al. [47] compared two different forms of electrode modification using nanocrystals of titanium dioxide and zinc oxide nanowires (Fig. 7). In contrast with the flat electrodes of the same size, the electrodes using TiO2 and ZnO had 200 and 30 times larger active areas, respectively. The results of this study demonstrated that the samples based on ZnO were less effective due to the smaller coefficient of roughness. On the other hand, ZnO was found to be more conductive and less expensive in comparison with the TiO2 [47].

electrode, with minimal resistance. This can be achieved by correct orientation of proteins on the electrode surface or by the usage of intermediate carriers. Many investigations have shown that the correct orientation of a photosystem on the electrode results in an improved electron transport [45]. In many studies, the correct orientation of the photosystems provides specific binding of a histidine-tagged protein complex with Ni2+-NTA molecule anchored to a gold electrode [24, 50]. Badura et al. [38] used the osmium-containing polymer of polyvinylimidazole as modified electrode acting as both an immobilizing agent and an electron acceptor for the PSII complex (Fig. 8a). Binding of sensitizer with redox polymer is rather interesting way of sensitizer immobilization. Due to that, an electron transfer between neighbouring redox centres covalently bound to the polymer backbone is possible by means of a hopping mecha‐ nism. Thus, there could be the shuttling of electrons from a reactive site within a redox protein towards an electrode surface. Several parameters determine the rate of electron transfer: the polymer backbone composition (flexibility, swelling behaviour, and amount of cross-linking), the distance between the polymer-bound redox centres and potential of the mediator. Hence, the properties of the redox polymer can be adapted to find an appropriate redox polymer for a specific application. In these modified systems, immobilized PSII were capable of generating

at light intensity of 2.65 mW/cm2

The main problem of the usage of isolated proteins as photosensitizers in photovoltaic cells is their extremely low stability. Photoinhibition of photosystems is the main reason for protein destruction, especially in case of the PSII. Caused by an excessive amount of radiation, photoinhibition may damage the photosynthetic apparatus and hence destroy the chloroplast proteins. Photosystems are provided with some protective mechanisms in vivo [2]. However, once the proteins are isolated, natural self-healing mechanisms do not work. Thus, isolated proteins are more susceptible to damage and have a short lifetime. There is one of the methods to stabilize the photosynthetic complexes through the simulation of the natural states of proteins. Surfactant peptides can be used to imitate the lipid membrane naturally stabilizing photosynthetic complexes. Such surfactants are designed as molecular nanomaterial to study the membrane protein's stability [47]. It consists of hydrophilic amino acids (aspartate or lysine) as the head of the chain and hydrophobic amino acids (alanine) as the tail. For the stabilization of the photosynthetic complex during the construction of a solid electrical device, Das et al. [24] used the peptides A6K and V6D, a sequence of six valines and one aspartic acid (VVVVVVD), as cationic and anionic surfactant peptides, respectively. They showed a short-

The increase of the electrode surface area is a method conventionally used to improve the efficiency of functioning solar cells. In many cases, the electrode itself is originally flat, and changes of its geometry can destroy its structure. However, the electrode can be modified via

at the excitation light intensity of 10 W/cm2

(maximum wavelength at

with a

a current density of 45 mA/cm2

178 Applied Photosynthesis - New Progress

**6.2. The stability of the isolated proteins**

circuit current density of 0.12 mA/cm2

**6.3. Increase of surface area**

wavelength of 808 nm. Presumably, this direction is promising.

675 nm) [38].

The cells of that construction used in Mershin's experiments are worth discussing in more detail. It is a stable and acknowledged design of solar cells. These cells are called dye-sensitized solar cells (DSSC) or Grätzel cell named after the one of its inventors, Michael Grätzel [21]. The advantage of such cells over the others is exactly in the usage of mesoscopic material as a substrate for photoactivator. The mesoscopic material is the material with a complicated inner structure represented by interpenetrating network of inorganic or organic semiconductor particles of mesoscopic size (2–50 nm) forming connections of very high contact area [4]. The structure of DSSC can be described as follows (Fig. 10). The main components of DSSC are two flat glass electrodes. Each of them has one conductive side. The conductive side is provided by application of thin layer of indium tin oxide (ITO) or fluorine tin oxide (FTO). The layer of mesoscopic semiconductive material is deposited on the one of the electrodes. Also, there is a monolayer of the dye attached to the surface of the nanocrystalline film. Photoexcitation of the dye results in the injection of an electron into the conduction band of the semiconductor. For the original state of the dye to be subsequently restored, there is the electrolyte, usually an organic solution containing redox system, such as the iodide/triiodide couple. It donates an electron to the dye. Timely regeneration of the sensitizer by iodide retards the recapture of the conduction band electron by the oxidized dye. The reduction of triiodide at the counter electrode regenerates the iodide, and the circuit is completed via electron migration through the external load. The mesoscopic oxide films are made of networks of thin crystals of a few nanometers. The components mostly preferred are the oxides such as TiO2, ZnO, SnO2, Nb2O5 or chalcogenides such as CdSe. They are bound inside and this allows the electron conduction to occur. Generally, the size of TiO2 particles is about 20 nm [4]. Before Grätzel cells, many cells were designed by the application of photosensitizer on flat electrode. In case of flat topology solar cells, a low density of pigments is one of low efficiency reasons. In DSSC, there are several advantages of nanocrystalline structure of semiconducting oxide (usually TiO2) used for sensitizer support [4]:

**1.** It enables the effective capture of light by the surface with sensitizer absorbed. On the flat surface, a monolayer absorbs less than a few percent of light as it covers an area approx‐ imately two order of magnitude larger than its optical cross-section [4]. The use of multilayered sensitizer would not solve this problem as molecules only in contact with semiconductor could excite it; the others act like filter. Significant increase of interface enhances absorption and leads to thousand-fold increase of photocurrent in comparison with flat surface DSSC.


Ruthenium dyes are used as sensitizers in most studies connected with Grätzel cells [4, 21]. But isolated components of photosynthetic apparatus as dyes are also quite attractive since ruthenium dyes are rather expensive. Some studies including Mershin's investigations offer significant possibilities in this area.

As it has already been shown, the mesoscopic materials cannot be the only way to increase an active surface area of solar cell. In fuel cells of another kind, nanotextured surfaces are also used to increase the amount of absorbed dye molecules. As the technique for creating nano‐ wires, nanotubes and other structures for carbon materials is quite well developed, carbon is rather attractive as a material for the creation of such surfaces. These approaches include the usage of GNPs [50], nanoporous gold electrodes [46] and redox hydrogels [38].

#### **6.4. Direct or mediated transfer of electrons**

Another way to achieve the maximum current density in the cells based on photosynthetic sensitizers is to create a system that carries out direct electron transfer from photosystem to electrode without using a mediator. As was mentioned earlier, the mediators have lower redox potential required for the efficient electron transfer compared to the native electron source. If the electrons are transferred to the mediator, they lose some part of their energy in contrast to transfer from the real source. The distance between the redox site and the electrode should also be minimized in order to ensure efficient transfer of electrons. The difficulty in ensuring continuous contact between the electron source and the electrode is the main disadvantage of direct electron transfer.

Furukawa et al. used polyaniline as an electronic catalyst instead of mediators to develop a photosynthetic biofuel cell [66]. Polyaniline has a good electrical conductivity; it is compatible with the photosystem. Due to its nanostructure, polyaniline also increases the surface area. During their experiment, they managed to achieve a good efficiency of the developed cell: peak current density was about 150 mA/cm2 and power density was measured at 5.3 mkW/cm2 . According to the study conducted by Sekar et al. [54], MWCNTs were successfully used for direct electron transfer, both in isolated spinach thylakoids and cyanobacteria *Nostoc sp*.

**Figure 10** Scheme of operation of the dye-sensitized solar cell. The photoanode, made of a mesoporous dye-sensitized semiconductor, receives electrons from the photo-excited dye which is thereby oxidized, and which in turn oxidizes the mediator, a redox species dissolved in the electrolyte. The mediator is regenerated by reduction at the cathode by the electrons circulated through the external circuit. Energy levels are measured in relation to normal hydrogen elec‐ trode (NHE). S –ground state of dye molecule. S+ – its oxidized state, S\* – its exited state (adapted from [21]).

#### **6.5. Extension of the spectral range of the light absorption by photosystems**

Previous four problems were closely linked: stability of isolated complexes directly depends on the way of its immobilization on electrode. For creating electrodes with complex surface, it is necessary to consider the ability of the surface to adsorb sensitizer molecules. In their experiments, Badura et al. [38] have been solving all these four problems at once. Ni2+-NTA and 6-His tag are both mediators of electron transport and a means of photosystem attachment on electrode surface. Immobilization, dye stability, working surface area and mediation of electron transport are connected with 'dye/electrode' contact. The increase of the spectral absorption region is the matter that is connected only with sensitizer. Extension of the spectral range of the light absorption is possible using Chl *d* or *f* [15,21,67-70]. Though the creation of artificial solar cells based on these chlorophylls is still at the early stages of its development. Overall, designing of solar cells using these chlorophylls seems to be quite promising.

#### **7. Conclusion**

**2.** Nanocrystals of TiO2 should not be somehow doped to have the conductivity. Injection of one electron from sensitizer to TiO2 particle is enough for titanium dioxide to get its conducting state. This photo-induced conductivity allows gathering the electron without any significant ohmic losses. In contrast, it is necessary for compact semiconductive films to be n-doped so that semiconductor could conduct the current. In this case, the energy transport from excited sensitizer to conducting band of semiconductor will inevitably

**3.** Size of TiO2 particles allows effectively screening electrons from the electrolyte or hole conductor present in pores. As a result, photocurrent is not declined by a repulse between

Ruthenium dyes are used as sensitizers in most studies connected with Grätzel cells [4, 21]. But isolated components of photosynthetic apparatus as dyes are also quite attractive since ruthenium dyes are rather expensive. Some studies including Mershin's investigations offer

As it has already been shown, the mesoscopic materials cannot be the only way to increase an active surface area of solar cell. In fuel cells of another kind, nanotextured surfaces are also used to increase the amount of absorbed dye molecules. As the technique for creating nano‐ wires, nanotubes and other structures for carbon materials is quite well developed, carbon is rather attractive as a material for the creation of such surfaces. These approaches include the

Another way to achieve the maximum current density in the cells based on photosynthetic sensitizers is to create a system that carries out direct electron transfer from photosystem to electrode without using a mediator. As was mentioned earlier, the mediators have lower redox potential required for the efficient electron transfer compared to the native electron source. If the electrons are transferred to the mediator, they lose some part of their energy in contrast to transfer from the real source. The distance between the redox site and the electrode should also be minimized in order to ensure efficient transfer of electrons. The difficulty in ensuring continuous contact between the electron source and the electrode is the main disadvantage of

Furukawa et al. used polyaniline as an electronic catalyst instead of mediators to develop a photosynthetic biofuel cell [66]. Polyaniline has a good electrical conductivity; it is compatible with the photosystem. Due to its nanostructure, polyaniline also increases the surface area. During their experiment, they managed to achieve a good efficiency of the developed cell: peak

According to the study conducted by Sekar et al. [54], MWCNTs were successfully used for direct electron transfer, both in isolated spinach thylakoids and cyanobacteria *Nostoc sp*.

and power density was measured at 5.3 mkW/cm2

.

usage of GNPs [50], nanoporous gold electrodes [46] and redox hydrogels [38].

decrease the coefficient of efficiency.

180 Applied Photosynthesis - New Progress

significant possibilities in this area.

**6.4. Direct or mediated transfer of electrons**

direct electron transfer.

current density was about 150 mA/cm2

electrons diffusing through the particle network.

Researchers in the area of artificial photosynthesis have focused on the development of total inorganic and hybrid semi-natural systems [71, 72] that could effectively produce a sustainable energy from sunlight without requiring external fuels. These systems should have a high quantum yield and generate energy fluxes of high density to satisfy the energy requirements. The more we learn about the nature, the closer we come to the creation of the efficient energy solar cells using the components of photosynthetic apparatus. The usage of systems imitating the photosynthetic apparatus and the elements of photosynthetic systems in current energy generators and fuel cells is a quite promising direction [72, 73]. However, the biophotovoltaics requires a lot of changes and improvements to be widely used.
