Stability, Scale-up, and Performance of Quantum Dot Solar Cells with Carbonate-Treated Titanium Oxide Films

A novel yet simple approach of carbonate (CBN) treatment of TiO₂ films is performed, and quantum dot solar cells (QDSCs) with high power conversion efficiencies (PCEs), reasonably good stabilities, and good fill factors (FFs) are fabricated with TiO₂-CBN films. The ability of carbonate groups to passivate defects or oxygen vacancies of TiO₂ is confirmed from a nominally enhanced band gap, a lowered defect induced fluorescence intensity, an additional Ti-OH signal obtained after carbonate decomposition, and a more capacitive low frequency electrochemical impedance behavior achieved for TiO₂-CBN compared to untreated TiO₂. A large area QDSC of 1 cm² with a TiO₂-CBN/CdS/Au@PAA (poly(acrylic acid)) photoanode delivers an enhanced PCE of 4.32% as opposed to 3.03% achieved for its analogous cell with untreated TiO₂. Impedance analysis illustrates the role of carbonate treatment in increasing the recombination resistance at the photoanode/electrolyte interfaces and in suppressing back-electron transfer to the electrolyte, thus validating the superior PCE achieved for the cell with carbonate-treated TiO₂. QDSCs with the configuration TiO₂-CBN/CdS/Au@PAA-polysulfide/SiO₂ gel-carbon-fabric/WO_3-x and active areas of 0.2-0.3 cm² yield efficiencies in the range of 5.16 to 6.3%, and the average efficiency of the cells is 5.9%. The champion cell is characterized by the following photovoltaic parameters: J_SC (short circuit current density), 11.04 mA cm^-2; V_OC (open circuit voltage), 0.9 V; FF, 0.63; and PCE, 6.3%. Stability tests performed on this cell show that dark storage has a less deleterious effect on cell performance compared to extended illumination. In dark, the PCE of the cell dropped from 5.69 to 5.52%, and under prolonged continuous irradiance of 5 h, it decreased from 5.91 to 4.83%. A scaled-up QDSC with the same architecture of 4 cm² size showed a PCE of 1.06%, and the demonstration of the lighting of a LED accomplished using this cell exemplifies that this cell can be used for powering electronic devices that require low power.