The proliferation of technology on a global scale has led to a surge in the utilization of electronic devices, consequently escalating the energy requirements in the present century. To address this rising global energy demand, and replace fossil fuels to mitigate their detrimental effect on climate change there is a growing focus on the consumption of renewable energy sources that are environment friendly. Among the various alternatives of renewable energies, solar cells possess the potential to satisfy these energy needs effectively. The challenge for the scientific community is either to develop new effective materials for creating effective devices or to explore various possibilities to enhance the efficiency of the existing devices with lower cost and sustainability [1]. The framework of the dye sensitized solar cells allows the sensitizer to anchor on the major surface area at the semiconductor/electrolyte interface forming a bulk junction [2]. The bulk junction forming between the sensitizer and semiconductor facilitates the sensitizer to attain effective light harvesting and better energy conversion [2]. Nonetheless, the charge recombination of photoinjected electrons taking place at the sensitizer and semiconductor interface hinders the expected efficiency that would result from the bulk junction between the sensitizer and the semiconductor surface [2]. In the presence of sunlight the sensitizer gets irradiated and injects an electron into the conduction band of the semiconductor surface (1) creating an electron hole in the dye ground state (Scheme 1) [3]. The dye ground state is regenerated through an electron transfer from the electrolyte, mostly iodide (5 Scheme 1) [3]. However, electrons introduced into the conduction band (CB) of a semiconductor in a Dye-Sensitized Solar Cell (DSSC) tend to either combine with the hole generated in the oxidized dye ground (7) or the oxidized form of the redox couple (8) (Scheme 1) [3]. These processes are known as recombination leading to “dark current” and consequently a reduction in the overall power conversion efficiency of the cells (Scheme 1) [3]. Additionally, when the dye molecules are densely aggregated on the semiconductor surface, that leads to significant overlap of the sensitizers. This substantial overlap often results in the excited states dye molecules being quenched by other non-excited dyes before electron transfer can take place (6, Scheme 1) [2]. Further, the stability of the device is compromised due to the protonation of the surface of the sensitized semiconductor film [2]. The long-term stability of the device is compromised due to this protonation of the semiconductor surface, which leads to alteration in the dye adsorption mode or even detachment of the adsorbed dye from the semiconductor surface [2]. The dye aggregation, semiconductor surface protonation, and charge recombination are major unwanted processes hindered the DSSC operations [3].

Therefore, several attempts have been made to comprehend and importantly to prevent their occurrence. So far, the most effective technique has involved the use of molecules that can be adsorbed onto the photoanode of DSSCs with the dye molecules, effectively blocking the unoccupied regions on the semiconductor surface that are not covered by the dye molecules [2,[4], [5], [6], [7]]. These substances are known as co-adsorbents, as they are competitively chemisorbed with the dye molecules [8]. Co-adsorbents, when combined with dye molecules, form a denser monolayer compared to the dye layer alone, thereby preventing the formation of vacant sites on the semiconducting surface and reducing the possibility of electron recombination [8]. The decreasing recombination rate also improves the open circuit voltage leading to overall enhancement of the DSSC performance [8]. The presence of co-adsorbent with sensitizer effectively improves the performance of DSSCs by increasing their open circuit voltage (Voc) while maintaining the short circuit current (Jsc) at optimal levels [5]. Generally, co-adsorbents are small organic amphiphilic molecules that possess carboxylic or phosphonic groups at one end and long hydrophobic alkyl groups at the other end [8]. The presence of the carboxylic or phosphonic moiety facilitates the effective grafting (anchoring) of the co-adsorbent onto the surface of mesoporous semiconducting nanoparticles [8]. In the case of co-adsorbents having carboxylic acid group chenodeoxycholic acid (CDCA) [9], deoxycholic acid (DCA) [10] are widely known for reducing dye aggregation and improving the overall efficiency of the DSSCs either by increased FF, JSC or by improved VOC. Further, DCA with an organic dye, coumarin, showed a significant enhancement of JSC value of 33 % [11]. In addition to DCA and CDCA, hexadecylmalonic acid (HDMA) [12], 3, 3′-dithiopropionic acid (DTA), 3-phenyl propionic acid (PPA) [5] and, 4-guanidinobutyric acid (GBA) [2], benzoic acid derivatives, octanoic acid [13], and stearic acid [14] were employed as co-adsorbents and it was found that these co-adsorbents assists to reduce the charge recombination resulting in higher power conversion efficiency [15]. The phosphonate group possesses enhanced strength and stability to the TiO2 surface compared to the carboxylate linkage in organic solvents [15]. Therefore, researchers have synthesized and examined co-adsorbents containing phosphonate groups to explore their impact on photovoltaic characteristics. The usage of 1-dodecylphosphonic acid (DPA) with Z907 dye resulted in a 7 % increase in VOC and overall efficiency [16]. Further, it was reported that a systematic study was carried out modifying the phosphonic acid group with a longer alkyl chain using octadecylphosphonic acid (OPA) [4]. The comparative study suggests OPA co-adsorbent is more competent than DPA in reducing the recombination and a maximum of 8.5 % photocurrent efficiency was observed [4]. The co-adsorbents having phosphinates as anchoring groups such as dialkylated phosphinates [5], bis-(4-methoxyphenyl) phosphinic acid (BMPPA) [17] were reported in the literature. However, the literature predominantly focused on the phosphonic acid anchoring group as it strongly binds with TiO2 semiconductor surfaces. There were a few reports on the non-proton-releasing co-adsorbents phosphate diester group. A comparative study established that proton releasing –OH group has a detrimental effect on the DSSC performance whereas the less proton-releasing group gives better performance in DSSCs using diethyl 4-methylphenylphosphonate(DEMPP), ethyl 4-methylphenylphosphonate (EMPP) and 4-methoxyphenylphosphonic acid (MOPPA) as co-adsorbents [18]. In these co-adsorbents the –OH group was replaced with ester groups in a stepwise manner, and DEMPP with no –OH group shows the highest efficiency [18]. All studies with different co-adsorbents are experimentally reported with only metal containing organic dyes. However, there are a few reports that express how these co-adsorbents will affect DSSCs performance with organic dyes [11]. Although it was well established that the co-adsorbents improve different parameters which contributes significantly in the overall performance of DSSCs however, it is not clear how the co-adsorbents tune the parameters. The understanding on the role of co-adsorbents to improve the efficiency of DSSCs will help to assist in designing new DSSCs with superior performance. Computational studies can provide understanding on such parameters to augment the performances of DSSCs. In this work, we have studied Ru-dye with different co-adsorbents and studied how different co-adsorbents affect the DSSC efficiency. The findings were further employed with organic dyes to explore the effect of co-adsorbents in DSSC.