**2. Photosynthesis**

Photosynthesis is the process of conversion of the sunlight energy into chemical energy of various organic compounds, which is carried out by photosynthesizing organisms. Photosyn‐ thesis serves as the primary source of energy for all kinds of life on Earth. Photosynthetic organisms are sources of energy and essential metabolites for heterotrophic organisms [8, 9]. This process proceeds in two stages: the light stage of the light absorption by photosynthetic pigments and the formation of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) and the dark stage when the biosynthesis of carbohydrates occurs. During the dark stage, carbon dioxide (CO2) acts as a carbon substrate, NADPH molecule is a proton source, and ATP molecule is a source of energy. The electron transport chain (ETC) is an essential element of the light stage of photosynthesis. An electron is trans‐ ferred to the NADP+ molecule through ETC, which leads to the reduction of the NADP+ to NADPH. An external source of electrons is required to reduce the oxidized pigment molecule. The type of photosynthesis using water as an electron source is called oxygenic one, since molecular oxygen is a by-product of the water decomposition [10].

Nowadays, the available sources of renewable energy, including solar, wind, rain energy, energy of waves and geothermal heat, could generate only approximately 16% of the energy used [2]. Global energy consumption is about 17 TW according to the information of the year 2014 [3]. The flux density of sunlight emission near the ground surface is about 100 PW, which exceeds 5000 times our current needs [4]. Even though the solar period and the presence of clouds are taken into account, the sun is the extremely attractive source of energy, given we know how to extract it. Thus, sunlight is the most accessible and reliable source among the

Photosynthesis is one of the main pathways of solar energy conversion, performed by higher plants, microalgae and some bacteria. Over 2.5 billion years, plant photosynthesis has evolved to convert solar energy into the chemical energy using only water as electron donor and proton source. This photosynthesis realises oxygen and is called oxygenic. Water, carbon dioxide and light are necessary for oxygenic phototrophic organisms to produce carbohydrates. The lightdependent reaction of photosynthesis takes place in the thylakoid membrane of photosynthetic organisms. The thylakoid membrane involves two photosystems (PSI and PSII), cytochrome b6f complex and other protein complexes embedded in a lipid bilayer. PSI and PSII can capture sunlight and create an electron-hole pair [5, 6]. The latter process operates with a quantum yield closer to 100%. Water, one of the most abundant substances on Earth, is the donor of

For several decades, photovoltaic semiconductor devices have also been developed to generate electric power by converting sunlight directly into the electricity. The coefficient of efficiency of the light energy conversion into the electric current produced by commercial silicon photovoltaic cells is typically less than 20% [7]. Unfortunately, exhaustible materials and components used in photovoltaic systems cannot be fully recycled. Considering that the efficiency of energy conversion in the primary processes of photosynthesis is close to 100%, it

Recently, after critical analysis of the photosynthetic and photovoltaic energy conversion mechanisms, experts in the area of artificial photosynthesis concluded that it is difficult to compare the conversion efficiency of the current photovoltaic cells with that of the living photosynthesizing cells, as they are completely different systems [7]. The efficiency of photovoltaic cells can be calculated by dividing the cell's output power by the total solar radiation spectrum. However, the storage and energy transfer are not considered by this approach. Photovoltaic batteries, in which energy is stored, have high production cost and the expenses required for the maintenance of such systems. Photosynthesis stores solar energy in the form of chemical energy, which can further be converted into electrical energy [2].

Photosynthesis is the process of conversion of the sunlight energy into chemical energy of various organic compounds, which is carried out by photosynthesizing organisms. Photosyn‐ thesis serves as the primary source of energy for all kinds of life on Earth. Photosynthetic

is reasonable to use this natural process for energy conversion applications.

other renewable energy sources.

162 Applied Photosynthesis - New Progress

electron for PSII [2].

**2. Photosynthesis**

The oxygenic photosynthesis is explicit in all plants, microalgae and cyanobacteria. It is the key source of oxygen in the atmosphere.

The oxygenic photosynthesis could be summarized through the following general equation:

$$\text{C}\text{O}\_{2} + \text{H}\_{2}\text{O} \xrightarrow[\text{H}]{\text{h}^{\text{h}}} \text{[C}\text{H}\_{2}\text{O}] + \text{O}\_{2}$$

Light stage processes of the oxygenic photosynthesis occur in membrane structures called thylakoids. In eukaryotic cells of green plants, thylakoids are localized in specific photosyn‐ thetic organelles - chloroplasts. The space limited by chloroplast membrane is determined as stroma, and the space inside the thylakoid is determined as lumen. Thus, one side of the thylakoid membrane faces the stroma, and the other side faces the lumen. In cyanobacteria cells, thylakoids are located directly in the cytoplasm [11].

Light energy is not immediately converted into ATP energy. In fact, it is initially stored in the form of a transmembrane electrochemical potential formed due to the proton transfer by lipophilic transporters through the thylakoid membrane from the stroma to the lumen. As a result, the lumen becomes acidic and the stroma is alkalized. Due to the energy of the created potential difference (ΔμH<sup>+</sup> ), the enzyme ATP-synthase embedded in the thylakoid membrane starts to function [12].

The light stage of photosynthesis is a sequence of enzymatic reactions. There are four trans‐ membrane protein enzymes that catalyse these reactions in higher plants: PSI, PSII, cytochrome b6f complex (Cyt b6f) and ATP-synthase [11]. PSII catalyses the electron transfer reaction from the water molecule to plastoquinone (PQ). The Cyt b6f is involved in the oxidation of plasto‐ quinole and reduction of plastocyanin (Pc). It mediates the transfer of electrons from PSII to PSI as well as of protons from stroma to lumen [12]. PSI catalyses the oxidation of lipophilic electron carrier plastocyanine, and the ferredoxin (Fd) reduction. The enzyme ferredoxin: NADP+ oxidoreductase (FNR) catalyses the NADP+ reduction due to the electrons from the reduced Fd (Fig. 1).

**Figure 1** The scheme of the non-cyclic electron transport pathway in thylakoids of higher plants and the redox poten‐ tials of the components of electron transport chain. P680 – primary electron donor in photosystem II; P680\* – singlet exit‐ ed state of P680; P700 – primary electron donor in photosystem I; P700\* – singlet exited state of P700; *QA* and *QB* are primary and secondary quinone electron acceptors, respectively. Red crosses represent reactions that can be inhibited by a) 3- (3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); b) dibromothymoquinone (DBMIB); c) potassium cyanide (KCN) (adapted from [2]).

The primary charge separation involving photosynthetic pigments occurs in the special part of photosystem complex called photosynthetic reaction centre (RC). In the RC, the primary electron donor is at the inner lumenal side of thylakoid membrane, whereas the primary electron acceptor is closer to the outer stromal side. Thus, an electron from the molecules of the primary electron donor moves onto the opposite side of the thylakoid membrane [12].

The ETC is activated by light. First, photons are absorbed by the pigments of special antenna complex. Then, the energy of the light quanta is transferred to RC by hopping mechanism [13]. In the RC, a special pair of chlorophyll is excited by the photon energy. Chlorophyll is the pigment molecule that can be excited by light of a certain wavelength (Fig. 2). The basis of the chlorophyll structure is a heterocyclic ring consisting of four pyrrole rings connected by methine bridges [14]. Four nitrogen atoms within the chlorine ring are associated with magnesium ion (Mg2+). A long hydrophobic phytol tail is attached to the fourth pyrrole ring, whereas a pigment molecule is correctly oriented in the membrane. In nature, there are two widespread forms of chlorophyll: Chl *a* and Chl *b*.

Chl *a* serves as the primary electron donor in the RC, and Chl *b* is the accessory pigment of the antenna complexes. A free Chl *a* molecule absorbs light preferably in the wavelength ranges of 400-500 nm and 600-700 nm. Due to the usage of other pigments, for example, carotenoids, the absorption spectrum of the photosystems is much broader [16]. In addition to Chl *a* and Chl *b*, other forms of chlorophyll, Chl *d* and Chl *f*, could also be found in antenna complexes

**Figure 2** The structural formula of chlorophylls: Chl *a*, Chl *b*, Chl *d*, and Chl *f* (adapted from [15]).

of phototrophic organisms, such as cyanobacteria. Also Chl *d* can be found in the photosyn‐ thetic RC [17, 18]. The chemical difference among the Chl *b*, Chl *d*, Chl *f* and the Chl *a* is that methyl or vinyl group is substituted by formyl one. The chlorophylls also differ from each other in their absorption spectra. More specifically, the long-wavelength maximum in the absorption spectrum of Chl *d* and Chl *f* markedly shifts towards longer wavelengths compared to that of the Chl *a* (shift up to 40 nm). The energy region (i.e., 380-710 nm) consists of photo‐ synthetically active radiations that constitute about 40% of the total solar radiation reaching the Earth's surface [19]. However, further expansion in the region ranging from 700 to 750 nm leads to the increase in the overall energy conversion intensity by about 19% [20].

#### **3. Solar cells**

H2O

*–1.2*

*–0.8*

*–0.4*

*+0.4*

E° (V vs. SHE)

164 Applied Photosynthesis - New Progress

(adapted from [2]).

*+0.8*

*0*

O2+4H<sup>+</sup>

widespread forms of chlorophyll: Chl *a* and Chl *b*.

*hn*

*+1.2 hn*

1

*QA QB PQ*

2

**Figure 1** The scheme of the non-cyclic electron transport pathway in thylakoids of higher plants and the redox poten‐ tials of the components of electron transport chain. P680 – primary electron donor in photosystem II; P680\* – singlet exit‐ ed state of P680; P700 – primary electron donor in photosystem I; P700\* – singlet exited state of P700; *QA* and *QB* are primary and secondary quinone electron acceptors, respectively. Red crosses represent reactions that can be inhibited by a) 3- (3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); b) dibromothymoquinone (DBMIB); c) potassium cyanide (KCN)

The primary charge separation involving photosynthetic pigments occurs in the special part of photosystem complex called photosynthetic reaction centre (RC). In the RC, the primary electron donor is at the inner lumenal side of thylakoid membrane, whereas the primary electron acceptor is closer to the outer stromal side. Thus, an electron from the molecules of the primary electron donor moves onto the opposite side of the thylakoid membrane [12].

The ETC is activated by light. First, photons are absorbed by the pigments of special antenna complex. Then, the energy of the light quanta is transferred to RC by hopping mechanism [13]. In the RC, a special pair of chlorophyll is excited by the photon energy. Chlorophyll is the pigment molecule that can be excited by light of a certain wavelength (Fig. 2). The basis of the chlorophyll structure is a heterocyclic ring consisting of four pyrrole rings connected by methine bridges [14]. Four nitrogen atoms within the chlorine ring are associated with magnesium ion (Mg2+). A long hydrophobic phytol tail is attached to the fourth pyrrole ring, whereas a pigment molecule is correctly oriented in the membrane. In nature, there are two

Chl *a* serves as the primary electron donor in the RC, and Chl *b* is the accessory pigment of the antenna complexes. A free Chl *a* molecule absorbs light preferably in the wavelength ranges of 400-500 nm and 600-700 nm. Due to the usage of other pigments, for example, carotenoids, the absorption spectrum of the photosystems is much broader [16]. In addition to Chl *a* and Chl *b*, other forms of chlorophyll, Chl *d* and Chl *f*, could also be found in antenna complexes

*P680*

*P680*\*

*P700*\*

*Fd*

*FNR*

*NADP+*

*P700*

3

*PC*

*Cyt b6f*

Solar cells are used to convert solar energy into electrical energy. The development of effective and inexpensive solar cells is of particular interest because of the importance of alternate energy sources. Currently, there are many different types of solar energy converters. The solar cells, or photoelements, are devices that can convert solar energy into usable electrical energy. They are divided into two types: regenerative cells and photosynthetic cells [21]. In the regenerative cells, the sunlight energy is converted into electricity. This process is unaccom‐ panied by any subsequent chemical reactions. Sometimes, such cells are called photobioelec‐ trochemical cells. In photosynthetic cells, the sunlight energy is converted to the molecular fuel energy, for example, that of hydrogen [20-23]. Photosynthetic cells based on biological objects such as isolated photosystems [22] or the whole bacterial cells [23] are called photo‐ biochemical fuel cells. This article focuses generally on the regenerative solar cells.

#### **3.1. Operating of solar cells**

The main steps can be identified for all types of solar cells [24]:

**1.** *Absorption of light by photoactive component*.

Photoactive component is the substance that absorbs photons inside the solar cell. A semiconductor acts as a photoactive component in conventional photovoltaic solar cells; while an organic pigment (photosensitizer molecule) serves as a photoactive component in dye-sensitized solar cells. Absorption of a photon leads to certain changes in the energy of the photosensitizer molecule, which is necessary for the further generation of current or the synthesis of molecular hydrogen [20].

**2.** *The charge separation.*

In photoelements using plant or bacterial photosystems, charge separation occurs due to a series of redox reactions. After the absorption of incident photon energy by special pigment molecule, a primary electron donor, a charge separation between primary electron donor and primary electron acceptor occurs. Then, the molecule of the primary donor is reduced by electrons from the secondary one and electron from the primary acceptor is transferred into ETC components. This stage is termed as charge stabilization. Some voltage is generated in the photoelement as a result of these processes.

**3.** *The transfer of electrons to an external circuit for biofuel generation.*

For the elements acting as a photoelectric converter (regenerative cells), this step implies an electron transfer to the electrode, and further to an external circuit. For photosynthetic cells, charge separation leads to the activation of the sequence of redox reactions, resulting in the formation of molecular hydrogen [20].

#### **3.2. The coefficient of efficiency of regenerative solar cells**

One of the basic estimation parameters of regenerative solar cells is the coefficient of efficiency. The efficiency of solar cell energy conversion is determined as a ratio of power electrical output to the intensity of incident light.

$$
\eta = \frac{P\_{\text{ell}}}{J\_{\text{light}}} \tag{1}
$$

Some conditions of accepted standard tests of solar cells are: air mass of AM 1.5, light intensity of 1 kW/m2 with a temperature of 298 K. Air mass is a ratio of the way, where sunlight passes in the atmosphere, to the thickness of the atmosphere; the value of AM 1.5 means that the sun is set at an angle of 48° to its position in zenith point [4], which is explained in Figure 3.

**Figure 3** Explanation of air mass (AM) notion.

fuel energy, for example, that of hydrogen [20-23]. Photosynthetic cells based on biological objects such as isolated photosystems [22] or the whole bacterial cells [23] are called photo‐

Photoactive component is the substance that absorbs photons inside the solar cell. A semiconductor acts as a photoactive component in conventional photovoltaic solar cells; while an organic pigment (photosensitizer molecule) serves as a photoactive component in dye-sensitized solar cells. Absorption of a photon leads to certain changes in the energy of the photosensitizer molecule, which is necessary for the further generation of current

In photoelements using plant or bacterial photosystems, charge separation occurs due to a series of redox reactions. After the absorption of incident photon energy by special pigment molecule, a primary electron donor, a charge separation between primary electron donor and primary electron acceptor occurs. Then, the molecule of the primary donor is reduced by electrons from the secondary one and electron from the primary acceptor is transferred into ETC components. This stage is termed as charge stabilization.

For the elements acting as a photoelectric converter (regenerative cells), this step implies an electron transfer to the electrode, and further to an external circuit. For photosynthetic cells, charge separation leads to the activation of the sequence of redox reactions, resulting

One of the basic estimation parameters of regenerative solar cells is the coefficient of efficiency. The efficiency of solar cell energy conversion is determined as a ratio of power electrical output

> ell light *P J* h

Some conditions of accepted standard tests of solar cells are: air mass of AM 1.5, light intensity of 1 kW/m2 with a temperature of 298 K. Air mass is a ratio of the way, where sunlight passes in the atmosphere, to the thickness of the atmosphere; the value of AM 1.5 means that the sun is set at an angle of 48° to its position in zenith point [4], which is explained in Figure 3.

= (1)

Some voltage is generated in the photoelement as a result of these processes.

**3.** *The transfer of electrons to an external circuit for biofuel generation.*

in the formation of molecular hydrogen [20].

to the intensity of incident light.

**3.2. The coefficient of efficiency of regenerative solar cells**

biochemical fuel cells. This article focuses generally on the regenerative solar cells.

The main steps can be identified for all types of solar cells [24]:

**1.** *Absorption of light by photoactive component*.

or the synthesis of molecular hydrogen [20].

**3.1. Operating of solar cells**

166 Applied Photosynthesis - New Progress

**2.** *The charge separation.*

Traditionally, the efficiency coefficient of photoelement is defined by means of voltammetric methods [25]. The controlled voltage source is attached to the solar cell. Then, the values of the current passing through the photoelement under different values of voltage are obtained using galvanometer. Current dependence on voltage received is called current–voltage characteristic (I–V). A typical I–V curve of photoelement in the darkness and in the light is presented in Figure 4. In the darkness, the photoelement acts as a diode in reverse bias, almost no current flows through the cell in conditions of increasing voltage – there is no free charge carrier. Once the external voltage becomes higher than the potentials, which are holding the electrons in atoms, a sharp increase of the current occurs. This phenomenon is called break‐ down. In the light of sufficient intensity, there will be the current in the circuit even under the voltage equal to zero: at the light, solar cell generates photovoltage. The direction of this current will be the opposite to that in case of breakdown. While the external voltage acting in the reverse direction increases, the photocurrent will decrease until it reaches value of zero. Then, there will be the situation similar to the breakdown: the reverse current significantly increases.

**Figure 4** Typical I–V curve of photoelement.

On the I–V curve, it is possible to determine four parameters of the cell: short-circuit current, open-circuit voltage, values of current and voltage defining maximum power generated by the cell [25].

Short-circuit current *I*sc (the current at an external voltage equal to zero) is the point of I–V intersection with the vertical axis. Open-circuit voltage *V*oc is the voltage equal in absolute value to photovoltage and opposite to it in sign: if it is applied to the cell, no current flows. Opencircuit voltage is determined by the I–V curve intersection with the axis of abscissa. Current power generated on the cell is determined by the voltage *V* and current *I*.

$$P\_{all} = I \cdot V \tag{2}$$

There is a point on the I–V curve, where the value *P* reaches its maximum, *P*max. The product of *I*sc and *V*oc presents the value proportionate to an area of rectangle AB'C'D' (Fig. 4). The ratio of maximum power *P*max, corresponded to the area of rectangle ABCD, to the product of *I*sc by *V*oc is called fill factor.

$$FF = \frac{S\_{ABCD}}{S\_{\, \, \_{AB'C'D'}}} = \frac{P\_{\text{max}}}{I\_{sc} \cdot V\_{oc}} \tag{3}$$

Thereby, the maximum coefficient of efficiency could be expressed by the following equation:

$$
\eta\_{\text{max}} = \frac{P\_{\text{max}}}{J\_{\text{light}}} = \frac{I\_{\text{sc}} \cdot V\_{\text{oc}} \cdot FF}{J\_{\text{light}}} \tag{4}
$$
