Biomolecular photosensitizers for dye-sensitized solar cells: Recent developments and critical insights
Graphical abstract
Introduction
Non-renewable energy resources such as fossil fuels pose environmental concerns and will be diminished due to the limited oil/gas reserves as well as the increasing demand for energy. Econometric models estimated that fossil fuel reserves will be totally consumed by the year 2042 [1]. In contrast, solar energy is the most abundant, unlimited, free, and environmental-friendly energy with power approximately 1.8 × 1011 MW from sun intercepted by the earth [2]. Photovoltaics (PV) is an efficient tool designed to harness the solar power by converting incident photons to excitons for electricity generation [2]. However, commercial silicon-based solar cells (first-generation) are expensive to manufacture and are restricted to the terrestrial PV market only as compared to the second-generation PV systems (multi-crystalline Si) [[3], [4], [5], [6], [7]] and the emerging third-generation PV systems such as organic/inorganic perovskite solar cells [[8], [9], [10], [11], [12], [13], [14], [15], [16]], inorganic solar cells (Si, III-V compounds, alloys, CdTe, CIGS) [[17], [18], [19], [20], [21], [22], [23], [24]], organic tandem solar cells [[25], [26], [27], [28], [29], [30], [31]], quantum dot solar cells [[32], [33], [34], [35], [36], [37]], and dye-sensitized solar cells (DSSCs) [[38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]], which are still in the development phase for being commercialized [49]. In 1991, O'Regan and Grätzel [38] initiated the idea of DSSCs inspired by natural photosynthesis and photography processes [50]. Similar to natural photosynthesis, DSSCs utilize plants and fruits pigment molecules to transfer energy and electrons converting light energy into electricity (instead of the chemical energy conversion stored as sugars and carbohydrates in natural photosynthesis). Thus, DSSC (Grätzel cell) is defined as a thin-film photovoltaic (solar) cell that efficiently converts any visible-light into electrical energy.
An intense research work [38] has been devoted to DSSCs from 1991 to 2014 which resulted in improving DSSCs efficiency from 7.1% to 13% for commercialization viability [51]. In 2015, Ye et al. reported a maximum DSSC efficiency of 15% achieved with a solid-state mesoscopic TiO2 DSSC sensitized with lead iodide perovskite (CH3NH3PbI3) under AM1.5 illumination; which is expected to reach a 20% future cell performance. The maximum recorded efficiency of commercial crystalline silicon solar cells is approximately 25% ($2.7–3.57/W) [52]. However, DSSCs are cost-effective solar cells (<$0.5/W) owing to their inexpensive materials/components and explicit fabrication design with little maintenance requirements [2,40,49]. A typical DSSC system includes four major components: photoanode, photosensitizer, electrolyte, and counter electrode (cathode) [49]. The cell converts visible-light energy into electrical energy by sensitizing a wide bandgap semiconductor (e.g. TiO2, ZnO, and SnO2) to inject a photo-excited electron at the interface between the semiconductor material and the monolayer sensitizer [50]. Nanoporous large-area semiconductors provide anchoring sites for dye molecules acceptor segments for easy electron injection from the generated electron-hole (e-h) pairs. Charge separation occurs in femtoseconds due to the electron injection from the dye molecules into the conduction band (CB) of the semiconductor [41,53]. To date, commercial photosensitizers used in DSSCs are limited to plant-based organic dyes [43], ruthenium dyes [54], and platinum dyes [55]. However, inorganic dyes are scarce in nature, synthetically produced, very expensive, and pose a high risk of toxicity to humans (ecosystem pollution) [56]. Thus, research efforts have been shifted to focus on natural photosensitizers, and specifically on biomolecular sensitizers from bacterial sources such as naturally optimized light harvesting (LH) ligands, photochemical reaction center (RC) protein, and carotenoid complexes for photons-to-electrons conversion [57].
The chemical structure of a photosensitizer material involves a donor-acceptor-substituted -conjugated bridge (D––A), Fig. 1(A).The dye's anchoring group exists in the acceptor part allows dye molecules to chemically attach themselves to the semiconductor surface [58]. Anchoring groups in dye molecules bind to oxide layers via a surface hydroxylation chemical reaction [59]. Covalently-bonded dye particles reduce interfacial resistance for the electron flow. Perfect bonding and dye attachment occur from surface interaction between functional groups as carboxyl and/or other peripheral acidic anchoring groups with the semiconductor surface. Earlier works [60] suggest that acidic dye solutions are preferred since de-attachment of dye molecules usually occur around pH = 9. A photosensitizer is considered efficient for DSSCs when it fulfills these requirements [50]: (i) intense visible-light absorption, (ii) strong chemisorption onto the semiconductor surface, (iii) fast electron injection into the semiconductor CB, and (iv) involve several =O or –OH groups to anchor dye molecules onto the semiconductor surface. The pigment's molecular structure, properties (i.e. hydrophilicity/hydrophobicity, solubility, surface chemistry, and stability of dye molecules), surface morphology, self-assembly, aggregation tendency, anchoring groups, and photosensitizer-electrolyte interactions are some of the basic parameters need to be well understood in order to optimize DSSCs performance through using commercial and/or natural photosensitizers [52]. Uniformly dispersed dyes in an optimal solvent prevent dye agglomeration and enhance dye/semiconductor surface interactions required for the attachment of dye acceptor segments, reducing series resistance and improving electron injection at the interfacial contacts.
Natural dyes extracted from different biological sources (e.g. anthocyanin, carotenoid, flavonoid, aurone, chlorophyll, tannin, and betalain obtained from fruits, flowers, leaves, seeds, barks, and various parts of plants or other biological sources) [[61], [62], [63]] have been proposed to be used as sensitizers in DSSCs due to their low cost and environmental friendliness [50,52,53,64]. Previous studies have mostly investigated on the use of anthocyanins [64,65], flavonoids [66,67], and carotenoids [42] as plant-source photosensitizers. However, Hug et al. (2014) [52] showed that bixin, crocetin, crocin, betaxanthin, betalains, mangostin, rutin, neoxanthin, violaxanthin, and lutein were among the investigated natural sensitizers extracted from plant-based sources. More importantly, anthocyanins (e.g. cyanin and nasunin), anthocyanidin (e.g. delphinidin, cyanidin, and peonidin), chlorophyll (e.g. methyl-3-carboxy-3-devinylpyropheophorbide), and carotenoids (e.g. -carotene) have been identified as the most promising biomolecular dyes. Carotenoids are highly light-sensitive pigments [39,52] due to their conjugated double -bonds structure with optimal chain length of seven [68] giving an approximated light-absorption range of 400–500 nm [56,69]. The highest observed performance with single carotenoids in DSSCs was 2.6% [70], where the integration of chlorophyll derivatives with carotenoids can increase the DSSC efficiency up to 4.2% [71]. A major challenge in optimizing DSSC efficiency is the expansion of the absorption range of the photoanode photoactive and/or semiconductor layers [72]. For example, proteins pigment complexes (PPCs) are good alternatives to carotenoids since they have higher absorption coefficient, wider absorbance range (300–1100 nm), and higher conversion efficiency [53,73].
Bacterial pigments have many advantages over the commercial metal-synthetic dyes for DSSCs. Natural pigments from biological sources are promising candidates to be integrated in DSSCs which can be simply installed as rolls in many daily used items such as handbags and clothing as well as building walls, windows and integrated bio-photovoltaics [74,75]. Advantages of using natural pigments from biomolecular sources include [53,56,74,75]:
- 1)
Bacteria and their protein complexes and carotenoids are abundant and cost-effective.
- 2)
Extraction of bio-dyes is easy, feasible, and can be also utilized in large scales (scalable).
- 3)
Biological pigments are biodegradable, renewable, and sustainable which makes them very convenient.
- 4)
Pigments from bacterial sources are usually noncarcinogenic and pose no health concerns to humans which makes them environmental-friendly alternatives.
- 5)
Biosensitizers can absorb most of the light energy due to their wide absorption spectrum (multi colors and wavelengths).
Very few works and minimal progress have been devoted towards the use of bacterial protein complexes in bio-sensitized DSSCs which may become a potential alternative as a natural sensitizer. In this context, this review focuses on understanding, analyzing, and exploring available biological pigments extracted from bacteria as potential sensitizers. The aim is to critically evaluate the novelty of utilizing biomolecular bacterial-source pigments as photo-electron sensitizers in DSSCs for solar-to-electricity applications. Several previously designed bio-sensitized DSSC systems have been reviewed, studied, and discussed thoroughly with current knowledge and advancements on the selected biomolecular photosensitizers. Biomolecular pigments discussed in this work include reaction center (RC) proteins [76], chlorophyll a (BChl) [77], chromatophores [45], PPCs including LH2, LH4, and RC; mimicking the principles of natural photosynthesis [78], light-harvesting complex II (LHCII) [79], bacteriorhodopsin (BR) proteins [53,80], xanthophylls carotenoids [56,81], lycopene carotenoids [82], and RC photosystem I trimer (PSI) [83]; where we have studied pigments biological sources, chemical structure, biological information, and their abilities to convert photons-to-electrons in bio-sensitized DSSCs as an attempt to improve the photoelectrochemical performance.
Section snippets
Architecture
A regular DSSC consists of four important components [49,50,52,[84], [85], [86]] to initiate the conversion of visible-light photons to electrons. These components and their roles in electron transportation and current generation are briefly: (i) photoanode for charge separation/conduction, (ii) counter electrode for electron collection, (iii) photosensitizer for electron injection, and (iv) redox electrolyte for dye regeneration; as shown in Fig. 1(B). Transparent conductive oxide (TCO) glass
Sources of biomolecular pigments
Bioactive and/or bio-passive bacterial-based pigments are extracted from complex molecules and small particles found in the cytoplasm of various genetics and living bacterial cells. For example, ribosomes in the bacteria cytoplasm (outside the cell nucleus) is capable of creating different PPCs such that LH2, LH4, and RC as well as producing unique enzymes as hydrogenase for solar-to-energy applications [[98], [99], [100]]. Organic carotenoid pigments (yellow to orange-red) are synthesized by
Current advancements in bio-sensitized DSSCs
Photosynthetic RC proteins consist of a transmembrane pigment-protein complex across the bacteria which act as a charge separator of the photo-formed e-h pairs [76]. Semiconductor/protein films and their functionalization have been recently extensively studied due to their promising role in the development of bioelectronics and bio-photoelectric devices [76,121,122]. Yet, researchers are still unable to overcome two major problems: (i) loss of energy due to the formation of the final charge
Bacteria isolation and extraction processes of bimolecular pigments
Biomolecular pigments can be extracted from various biomaterials including: (i) plant extracts: fruits, flowers, leaves, seeds, peels, and vegetables, (ii) different amino acids and proteins, and (iii) nucleic acids DNA, fungi, and bacteria (bacteria proteins/carotenoids our focus in this work!). The extracted biomolecules (natural products) may also be used as raw materials for cost-effective and green synthesis of carbon quantum dots (C-QDs) or graphene quantum-dots (Q-QDs) utilized for
Bandgap energies of biomolecular dyes
In biomolecular organic dyes, two typical energy levels exist for electrons called LUMO and HOMO levels. Under illumination, dye molecules get excited allowing electron injection into the CB of the semiconductor as long as LUMO energy level is closer to the vacuum level and higher than semiconductor CB. Effective electron-diffusion and high dye reduction rates occur with the existence of large energy difference between dye-HOMO and electrolyte-redox-potential [139]. Dye light-absorption
Photoanode degradation
Excessive ultraviolet (UV) radiations and oxygen environments cause quick dye degradation of the pigmented-photoanode in DSSCs. Hence, it is important to apply a protective layer such as a UV protection foil and/or UV absorbing luminescent chromophores on top of the photoanode for the protection of the pigmented-layer from high UV radiations. Moreover, the addition of antioxidants to the applied pigments might be useful in protecting the pigmented-photoanode from oxidation and further
Conclusions and outlook
We demonstrated the potential of using bacterial-based biomolecular pigments extracted from protein and carotenoid complexes. Bacterial-pigments are found to be promising photosensitizer candidates since they are cost-effective, scalable, renewable, sustainable, environmental-friendly, biodegradable, and noncarcinogenic. Biomolecular pigments can be extracted from plant extracts, amino acids, and bacteria. The authors have elucidated the structural components of DSSCs (anode, photosensitizer,
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
HAM would like to acknowledge the Saudi Arabian Cultural Mission (SACM) and King Abdulaziz University (KAU) for their support and funding to pursue the graduate studies. SKB and VB thank Dimerond Technologies, LLC for the support to conduct renewable energy research at the University of Illinois at Chicago. All the authors thank University of Illinois at Chicago for the support. VB thanks funding support from National Science Foundation (grant: 1054877) and Office of Naval Research (grants:
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