Molecular engineering of simple phenothiazine-based dyes to modulate dye aggregation, charge recombination, and dye regeneration in highly efficient dye-sensitized solar cells

A series of simple phenothiazine-based dyes, namely, TP, EP, TTP, ETP, and EEP have been developed, in which the thiophene (T), ethylenedioxythiophene (E), their dimers, and mixtures are present to modulate dye aggregation, charge recombination, and dye regeneration for highly efficient dye-sensitized solar cell (DSSC) applications. Devices sensitized by the dyes TP and TTP display high power conversion efficiencies (PCEs) of 8.07 (J_sc=15.2mA cm ^-2, V_oc=0.783V, fill factor (FF)=0.679) and 7.87 % (J _sc=16.1mA cm^-2, V_oc=0.717V, FF=0.681), respectively; these were measured under simulated AM 1.5 sunlight in conjunction with the I^-/I₃^- redox couple. By replacing the T group with the E unit, EP-based DSSCs had a slightly lower PCE of 7.98 % with a higher short-circuit photocurrent (J_sc) of 16.7mA cm^-2. The dye ETP, with a mixture of E and T, had an even lower PCE of 5.62 %. Specifically, the cell based on the dye EEP, with a dimer of E, had inferior J_sc and V_oc values and corresponded to the lowest PCE of 2.24 %. The results indicate that the photovoltaic performance can be finely modulated through structural engineering of the dyes. The selection of T analogues as donors can not only modulate light absorption and energy levels, but also have an impact on dye aggregation and interfacial charge recombination of electrons at the interface of titania, electrolytes, and/or oxidized dye molecules; this was demonstrated through DFT calculations, electrochemical impedance analysis, and transient photovoltage studies. Shape sensitivity: A solar cell sensitized with a phenothiazine-based dye containing a bithiophene unit (TTP) as the electron donor has a high power conversion efficiency (PCE) of 7.87 %, whereas a related dye containing an ethylenedioxythiophene and thiophene unit (ETP) and that containing an ethylenedioxythiophene dimer (EEP) have much lower PCE values. The photovoltaic performance is sensitive to structural modifications (see picture).