Development of Superstrate CuInGaSe₂ Thin Film Solar Cells with Low-Cost Electrochemical Route from Nonaqueous Bath

Electrodeposition of Cu­(In,Ga)­Se₂ (CIGS) thin film is an attractive approach for the development of highly efficient low-cost solar cells. This work focuses on the effects of various electrodeposition parameters on the growth and properties of CIGS layers. The films deposited at −0.9 V tend to drive the growth of CIGS favoring (112) crystal orientation, whereas the films deposited at −1.6 V show the orientation along (220)/(204). Interplanar distances corresponding to (112) and (204/220) planes could be observed in the high resolution transmission electron microscopy (HRTEM) images of the respective films, confirming the dependence of the texture on the deposition potential. Films with larger grains could be grown by maintaining higher temperature (130 °C) during the deposition of layers. X-ray photoelectron spectroscopy (XPS) confirmed the presence of Cu⁺, In³⁺, Ga³⁺, and Se^2– valence states in the CIGS layers prepared at −0.9 and −1.6 V. The film deposited at −1.6 V with (220/204) orientation showed high efficiency as compared to the film deposited at −0.9 V with (112) orientation. The observed solar cell parameters, measured under illuminated condition of input power intensity 100 mW/cm², were <i>V</i>_OC = 0.357 V; <i>J</i>_SC = 27 mA/cm², FF = 44, and η = 4.90; and <i>V</i>_OC = 0.460 V, <i>J</i>_SC = 34 mA/cm², FF = 58, and η = 9.07 for the deposition potentials of −0.9 and −1.6 V, respectively<sub>.</sub

Experimental and Theoretical Investigations on the Activity and Stability of Substitutional and Interstitial Boron in TiO₂ Photocatalyst

Effects of boron doped in TiO₂ at (a) interstitial site (B_int), (b) substitutional site (B_sub), and (c) combination of both the sites (B_int+sub) have been investigated experimentally and theoretically to understand the origin of enhanced photocatalytic activity and stability. B-doped TiO₂ powders were synthesized by sol–gel method with different concentrations of boron. XPS results indicate that boron first prefers B_int site when doped with low concentration (up to 1 at. % B), but as the concentration increases (2 at. % and above) B also occupies substitutional O position in addition to B_int to form TiO₂ containing B_int+sub (TiO₂–B_int+sub). Higher absorption of visible light is achieved for TiO₂–B_int+sub due to the presence of two absorption edges (2.4 and 2.2 eV) as observed in the absorption spectra, while insignificant narrowing of band gap is observed for TiO₂–B_int. Electronic structure calculated by DFT for TiO₂ with B_int, B_sub, and B_int+sub revealed that the two localized deep levels are formed in the mid gap region which are responsible for these optical transitions for TiO₂–B_int+sub. Photoluminescence (PL) emission spectra showed that the shallow level (as inferred from the DFT calculations) created below the conduction band is able to decrease the radiative recombination process in TiO₂–B_int by trapping electrons and prolonging the lifetime of charge carriers as observed in the time-resolved PL decay curve. Furthermore, lower effective mass ratio of charge carriers calculated using DFT for TiO₂–B_int also suggests better charge mobility and low recombination rate. Photocatalytic degradation rate of organic pollutants in water was significantly higher after B-doping with higher performance obtained with TiO₂ containing B_int as compared to B_int+sub. By imposing the destabilizing circumstances it was established that TiO₂–B_sub is metastable and collapses under mild conditions, whereas TiO₂–B_int is highly stable and retains all its properties. All these unprecedented findings disclose that higher activity of TiO₂–B_int as compared to that of TiO₂–B_int+sub is mainly because of the delayed recombination processes even though the optical band gap is not significantly varied