Co-Adsorbent Effect on the Sensitization of TiO₂ and ZnO Surfaces: A Theoretical Study

Abstract

Co-adsorbents are often employed in DSSC to avoid electron/hole recombination at the hybrid organic/inorganic interface. Here we investigate, by means of density functional theory simulations, the changes induced in the structural and electronic properties of a hybrid heterostructure by the presence of a co-adsorbent. In particular, we focus on a prototype interface obtained by co-grafting a dye molecule (the hemi-squaraine dye) and imidazole at the oxide surfaces mostly employed in DSSC (anatase-TiO2(101) and ZnO­(1100)). Our results show that beside avoiding dye aggregation, the co-adsorbent plays an active role in stabilizing the binding between the chromophore and oxide substrate and in tuning the optoelectronic properties of the system. We found that imidazole shifts downward the dye energy levels (HOMO and LUMO) while raising the oxide valence and conduction bands, strongly influencing the energy alignment of the interface. As such, a co-adsorbent can either modify the type of heterostructure (straddling, staggered, or broken gap) or change the open circuit voltage (V oc), with potentially both beneficial or detrimental effect of the final device performance.

Electronic and Optical Excitations at the Pyridine/ZnO(101¯0) Hybrid Interface

“…[1][2][3] These properties together with a type-I level alignment at the hybrid interface would be ideally suited for light-emitting applications. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The adsorption of a molecular layer on inorganic surfaces has also been exploited to tune the work function of the inorganic component. [6][7][8][9][10] Prerequisite for that is the lightinduced creation of hybrid or charge-transfer excitons, which exhibit the hole to a large extent on one side of the interface while the excited electron would reside on the other side.…”

“…Prerequisite for that is the light‐induced creation of hybrid or charge‐transfer excitons, which exhibit the hole to a large extent on one side of the interface while the excited electron would reside on the other side. Investigations along these lines on hybrid inorganic/organic materials composed of wide‐gap semiconductors like ZnO and organic chromophores have created an active field of research …”

Visible Light Driven Photocatalytic Cross‐Coupling Reactions on Nano Pd/ZnO Photocatalyst at Room‐Temperature

“…Figure a ‐b displays the UV‐vis absorption spectra of nano ZnO and nano Pd/ZnO. ZnO has an absorption peak at 360 nm which is in the UV light region, this peak is due to the charge‐transfer absorption from the oxide anions 2p orbital valence band to the conduction band of 3d orbital of Zn 2+ cations . Using ZnO as a photocatalyst by visible light irradiation due to its wide band gap ( ∼ 3.2 eV at room temperature) and large excitation binding energy (60 meV) is difficult .…”

Role of co-sensitization in dye-sensitized and quantum dot-sensitized solar cells

“…Here, BDT effectively played a vital role in reducing the charge recombination process. Theoretical studies show that the improper selection of co-adsorbed dye with squaraine dye result poor performance [210] and hence careful selection of secondary dye for co-sensitization is an imperative process. Co-sensitization of squaraine dye SQ2 with two D-π-A type compounds called 'H' type of sensitizers has also reached with moderate efficiency [54] and proper tuning of core structure of these kind of compounds would be a good approach to enhance the efficiency further.…”