**5. Improving the efficiency of solar cells**

#### **5.1. The redox-active components: changing the direction of the electron flow**

Various redox-active components accept electrons from specific sites of ETC in accordance with their redox potential. Redox active sites of metalloproteins are usually hidden inside the PSII complex [38]. Therefore, the electron transfer from immobilized photosystems onto the electrode may be limited. This limitation could be overcome by redirecting the electrons from the inner part of the protein to the surface [2]. For instance, Larom et al. [55] successfully used an artificial mediator to redirect electrons from QA-site to an artificial acceptor at a distance of about 1.3 nm from the stromal side of the membrane. This change in the direction of electron flow together with additional blocking of QB-site has reduced oxidative damages at the expense of reducing the time of the intermediate electron transfer at the QA/QB stage. In another study, Sekar et al. [54] used whole cells of cyanobacteria as photobiocatalysts in a solar cell. They achieved a power density of about 10 μW/cm2 by adding 1,4-benzoquinone (BQ) as a mediator. It was three times higher in comparison with power density of photoelements without a mediator using the systems *Nostoc*/MWCNT and laccase/MWCNT. Since both BQ and PQ have the similar structure, this addition facilitated electron transfer from the PSII to the MWCNT [54]. Previously, mediators such as BQ, 2,6-dimethyl-1-benzoquinone (DMBIB) and 2-hydroxy-1,4-naphthoquinone (HNQ) have been also used for accepting the electrons from the cyanobacterial photosynthetic ETC [51, 52].

#### **5.2. Bioengineering of photosynthetic RCs**

Primary processes of photosynthesis have a high quantum yield reaching almost 100%, but the energy storage efficiency can reach about 27% under ideal conditions, and much less under non-ideal ones [56]. This value is comparable to the efficiency of the modern silicon-based solar panels operating with an efficiency of approximately 20%. It is notable that some laboratory models have demonstrated an efficiency of 40% [2, 7, 21]. Moreover, the photosynthetic pigments usually absorb light only from the visible region of the spectrum (from 400 to 700 nm) [7] unlike photovoltaic cells that are capable of absorbing the light from ultraviolet and near infra-red regions as well. Thus, photosynthetic organisms utilize only about a half of the incident solar energy. Nevertheless, photosynthetic efficiency can be improved by expanding the region of photosynthetic absorption using bioengineering techniques. Since two photo‐ systems used in photosynthesis absorb light under the same conditions, the variation of absorbance could increase the efficiency [20]. This approach may be performed using photo‐ elements based on RC containing far-red and infra-red absorbing pigments similar to bacter‐ iochlorophyll that absorbs light in the region up to 900 nm [57] or Chl *d* or Chl *f* capable of absorbing light in the region of 400-750 nm [21]. As a result, the absorption region may be significantly increased.

#### **5.3. Biomimetics**

**5. Improving the efficiency of solar cells**

176 Applied Photosynthesis - New Progress

from the cyanobacterial photosynthetic ETC [51, 52].

**5.2. Bioengineering of photosynthetic RCs**

significantly increased.

**5.1. The redox-active components: changing the direction of the electron flow**

Various redox-active components accept electrons from specific sites of ETC in accordance with their redox potential. Redox active sites of metalloproteins are usually hidden inside the PSII complex [38]. Therefore, the electron transfer from immobilized photosystems onto the electrode may be limited. This limitation could be overcome by redirecting the electrons from the inner part of the protein to the surface [2]. For instance, Larom et al. [55] successfully used an artificial mediator to redirect electrons from QA-site to an artificial acceptor at a distance of about 1.3 nm from the stromal side of the membrane. This change in the direction of electron flow together with additional blocking of QB-site has reduced oxidative damages at the expense of reducing the time of the intermediate electron transfer at the QA/QB stage. In another study, Sekar et al. [54] used whole cells of cyanobacteria as photobiocatalysts in a solar cell. They achieved a power density of about 10 μW/cm2 by adding 1,4-benzoquinone (BQ) as a mediator. It was three times higher in comparison with power density of photoelements without a mediator using the systems *Nostoc*/MWCNT and laccase/MWCNT. Since both BQ and PQ have the similar structure, this addition facilitated electron transfer from the PSII to the MWCNT [54]. Previously, mediators such as BQ, 2,6-dimethyl-1-benzoquinone (DMBIB) and 2-hydroxy-1,4-naphthoquinone (HNQ) have been also used for accepting the electrons

Primary processes of photosynthesis have a high quantum yield reaching almost 100%, but the energy storage efficiency can reach about 27% under ideal conditions, and much less under non-ideal ones [56]. This value is comparable to the efficiency of the modern silicon-based solar panels operating with an efficiency of approximately 20%. It is notable that some laboratory models have demonstrated an efficiency of 40% [2, 7, 21]. Moreover, the photosynthetic pigments usually absorb light only from the visible region of the spectrum (from 400 to 700 nm) [7] unlike photovoltaic cells that are capable of absorbing the light from ultraviolet and near infra-red regions as well. Thus, photosynthetic organisms utilize only about a half of the incident solar energy. Nevertheless, photosynthetic efficiency can be improved by expanding the region of photosynthetic absorption using bioengineering techniques. Since two photo‐ systems used in photosynthesis absorb light under the same conditions, the variation of absorbance could increase the efficiency [20]. This approach may be performed using photo‐ elements based on RC containing far-red and infra-red absorbing pigments similar to bacter‐ iochlorophyll that absorbs light in the region up to 900 nm [57] or Chl *d* or Chl *f* capable of absorbing light in the region of 400-750 nm [21]. As a result, the absorption region may be

Biomimetic approach is aimed to construct an artificial systems mimicing the natural photo‐ synthesis for the production of electricity or hydrogen. Synthetic sensitizers and catalysts are considered as a suitable alternative to unstable native systems. As a first-line strategy, porphyrins, phthalocyanines and their metal complexes that absorb light in the same optical region as the native chlorophyll molecules are regarded as such synthetic RCs. Covalently linked cyclic porphyrins are more durable, but they are difficult to synthesize [58]. Noncovalently associated porphyrins easily degrade due to their sensitivity to changing environ‐ mental conditions [59]. The advantages of utilizing the porphyrin structures include stability of the RCs and accessibility compared to synthetic products, while their disadvantage is the short lifetime in their excited state. Polypyridines containing transition metals have a longer lifetime in their high energy excited states [60]. Nevertheless, they generally require more expensive metals [53]. It should be noted that the biomimetic-based semiconductor materials mimicking the oxygen-evolved complex were designed to create energy devices during the early 1970s [61].
