Over the years, the global energy demand has increased due to the increased human population and activities to sustain their life. Sunlight is a renewable energy source with huge potential to meet energy demands. However, the high solar energy conversion and storage cost is a major challenge. Therefore, converting solar energy into electricity requires an effective and affordable harnessing technique.

Dye-sensitized solar cells (DSSCs) are potential candidates to replace traditional silicon-based solar cells owing to their low manufacturing cost, easy fabrication process, better performance under diffuse light conditions, flexibility and ecofriendliness [1,2]. The DSSC mainly comprises a Nanoporous metal oxide-coated photoanode, dye, redox electrolyte, and a counter electrode. The low light absorption, poor electron injection and poor dye regeneration are obstacles to high-performance DSSCs. Therefore, several materials and techniques have been employed to improve counter electrodes, electrolytes, dyes and photo anodes. The high bandgap semiconductor (TiO2) plays a role in DSSCs by acting as a scaffold for the adsorption of the dye molecules and as an electron acceptor and charge carrier [3]. Several metal oxide Semiconductors have been researched as photoanodes, including TiO2, ZnO, SnO2, and Nb2O5 [4]. Among these, ZnO and TiO2 satisfy to be used as photoanodes [5,6]. Anatase TiO2 is commonly used compared to ZnO due to its wide bandgap, corrosion resistance, ease of availability, low cost, mechano-chemical stability, and non-toxic composition [7,8].

In DSSCs, the dye (sensitizer) plays a significant role in determining light-harvesting, electron injection and regeneration efficiency [9]. It helps absorb the light, transfer an electron into the conduction band of the semiconductor layer, accept an electron from the redox couple, and then repeat the cycle [10]. Dyes are classified into three major groups: metal complex dyes, metal-free organic dyes, and natural dyes. Metal Complex dyes as a sensitizer have demonstrated wide light absorption and high power conversion efficient but face the challenges of toxicity and high cost imparted by complex synthesis [11,12]. Metal-free organic dyes have been developed as an alternative to complex metal dyes; however, their cost is still high. Compared to metal complex and metal-free organic synthetic dyes, natural dyes are abundantly available in plants and easily extracted by facile methods at low cost. Also, they are non-toxic, environmentally friendly and biodegradable; hence, they are being explored as alternative photosensitizers. Natural pigments such as betalains [13], chlorophylls [14,15], anthocyanin [16,17] and carotene [18] have been investigated as photo-sensitizers for solar cell applications. However, the DSSCs sensitized with these natural dyes demonstrated low efficiency. The reasons for low efficiency are degrading tendency, poor light absorption capability, low absorption coefficient, the molecular structure of the natural dye, and the weak bonding of natural dyes with the semiconductors [19]. Molecular structural modification of natural dye and combining two or more natural dyes are considered the best approaches to improve the light absorbance efficiency and the photovoltaic properties of DSSCs [6,20,21]. Moreover, modulates the HOMO-LUMO molecular energy level possible to obtain the smallest energy gap and the largest absorption range.

Brazilin, brazilein, and betanidin zwitterionic dyes are selected in this study owing to their abundant, easily extracted, environmentally friendly, and high absorbance for visible light. The Betanidin zwitterionic has high molar extinction coefficients in the visible spectrum (green portion of the spectrum) region with absorption maxima of about 550 nm [22] and possess the necessary (COOH) functional groups for better binding to the TiO2 nanoparticle [23,24]. The brazilin dye and brazilein dyes absorb the light in the ultraviolet region and visible region with a maximum absorbance of 292 nm [25] and 445 nm [26], respectively. However, the unstable binding modes of brazilin and brazilein to the semiconductor through the OH functional group limit its efficiency in application in DSSCs. The DSSCs sensitized with brazilin dyes demonstrated a maximum power conversion efficiency of 1.65% [27], while those sensitized with betanidin were 1.7% [28]. To our knowledge, neither computational nor experimental studies have been reported about combining brazilin and brazilien with betanidin zwitterionic. For the first time, we report the complexes designed from a combination of brazilin dye with betanidin dye and a combination of brazilein dye with the betanidin dye to enhance the performance of these dyes for the DSSCs.

In this work, quantum chemical calculations were carried out to get insight into the optoelectronic and photovoltaic properties of the newly designed complexes derived from a combination of two natural dyes, i.e. brazilin and betanidin, brazilein and betanidin via etherification and bi-etherification reactions. The reorganization energies, molecular electrostatic potentials surface and chemical reactivity parameters were obtained. Moreover, the light-harvesting efficiencies, free-energy change injection driving forces, electron regeneration driving forces, injection rate constant, electron excited lifetime, electron injection time, broadening lifetime and open-circuit voltage of the dyes were calculated to evaluate the photovoltaic characteristics of dyes. The results indicate that Braz-Bd-Oxane and Braze-Bd-ether show prominent optical and electrical properties among the investigated dyes. Braz-Bd-Oxane and Braze-Bd-ether are good potential candidates to be used as photosensitizers in DSSCs.