p-CuSCN/n-Fe 2 O 3 heterojunctions were electrochemically prepared by sequentially depositing α-Fe 2 O 3 and CuSCN films on FTO (SnO 2 :F) substrates. The α-Fe 2 O 3 and CuSCN films and the α-Fe 2 O 3 /CuSCN heterojunctions were characterized by Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray spectroscopy (EDX), and X-Ray Diffraction (XRD). Pure crystalline CuSCN films were electrochemically deposited on α-Fe 2 O 3 films by fixing the SCN/Cu molar ratio in the electrolytic bath to 1:1.5 at 60 °C and at a potential of-0.4 V. The photocurrent measurements showed an increase of the intrinsic surface states or defects at the α-Fe 2 O 3 /CuSCN interface. The photoelectrochemical performance of the α-Fe 2 O 3 /CuSCN heterojunction was examined by chronoamperometry and linear sweep voltammetry techniques. It was found that the α-Fe 2 O 3 /CuSCN structure exhibits a higher photoelectrochemical activity when compared to α-Fe 2 O 3 thin films. The highest photocurrent density was obtained for α-Fe 2 O 3 /CuSCN films in 1 M NaOH electrolyte. This high photoactivity was attributed to the high active surface area and to the external applied bias;, which favors the transfer and the separation of the photogenerated charge carriers in α-Fe 2 O 3 /CuSCN heterojunction devices. The flat band potential and the donor density were found to be maximal for the heterojunction. These results suggest a substantial potential to achieve heterojunction thin films in photoelectrochemical water splitting applications.
In this study, uniform and dense iron oxide α-Fe 2 O 3 thin films are used as an electron-transport layer (ETL) in CH 3 NH 3 PbI 3-based perovskite solar cells (PSCs), in substiting the Titanium dioxide (TiO 2) ETL conventionally usedin planar heterojunction perovskite solar cells. The α-Fe 2 O 3 films were synthesized using an electrodeposition method for the blocking layer and a hydrothermal method for the overlaying layer, while 2,2',7,7'-tetrakis (N, N'-di-pmethoxyphenylamine)-9,9' spirobifluorene (spiro-OMeTAD) was employed as a hole conductor in solar cells. Based on the above synthesized α-Fe 2 O 3 films the photovoltaic performances of the PSCs were studied. The α-Fe 2 O 3 layers were found to have a significant impact on the photovoltaic conversion efficiency (PCE) of the PSCs. This was attributed to an efficient charge separation and transport due to abetter coverage of the perovskite on the α-Fe 2 O 3 films.As a result, the PCE measured under standard solar conditions (AM 1.5G, 100mW cm-2) reaches5.7%.
Using the easily applicable hydrothermal method Cr-doped hematite thin films have been deposited polycrystalline on conductive glass substrates. The hydrothermal bath consisted of an aqueous solution containing a mixture of FeCl3.6H2O and NaNO3 at pH = 1.5. The samples were introduced in an autoclave and heated for a fixed time at a fixed temperature and then annealed in air at 550ºC. The concentration of the incorporated Cr atoms (Cr 4+ ions) was controlled by varying the concentration of the Cr(ClO4)3 precursor solution, varied from 0 % to 20 %. All samples followed morphological and structural studies using field-emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. Chronoamperometry measurements showed that Cr-doped hematite films exhibited higher photoelectrochemical activity than the undoped films. The maximum photocurrent density and incident photon conversion efficiencies (IPCE) were obtained for 16 at.% Cr-doped films. This high photoactivity can be attributed to both the large active surface area and increased donor density caused by Cr-doping in the α-Fe2O3 films. All samples reached their best IPCE at 400 nm. IPCE values for 16 at.% Cr-doped hematite films were thirty times higher than that of undoped samples. This high photoelectrochemical performance of Cr-doped hematite films is mainly attributed to an improvement in charge carrier properties.
CuGaS2 thin films were obtained by sulfurization of CuGaSe2. CuGaSe2 thin films were first electrodeposited from aqueous solutions containing CuCl2, GaCl3, and H2SeO3 and subsequently annealed at 400 °C for 10 min in forming gas atmosphere and in the presence of molecular sulfur. This sulfurization process resulted in the complete conversion of CuGaSe2 into CuGaS2. The formation of CuGaS2 was proven by X-Ray diffraction and optical spectroscopy. Diffraction peaks of CuGaS2 shifted to higher angles than those observed for CuGaSe2 films, and the optical band gap shifted to blue rising from 1.66 eV for CuGaSe2 to 2.2 eV for CuGaS2. When Cr ions were added to the initial electrolyte, the final CuGaS2 films exhibited a broad in-gap absorption band centred at 1.63 eV that can be ascribed to Cr atoms in Ga sites. The performance of solar cells based on CuGaS2:Cr absorbers containing an in-gap absorption band was then estimated by numerical simulation using Solar Cell Capacitance Simulator Software. Both quantum efficiency and short circuit current of simulated Mo/CuGaS2:Cr/CdS/ZnO solar cells rose up proportionally to the amount of Cr present in CuGaS2:Cr absorbers. As a result, the photo conversion efficiency of the simulated devices changed from 14.7% for CuGaS2 to 34% for CuGaS2:Cr absorbers. Nevertheless, when neutral defects related to Cr-doping were introduced in the absorber layer, the positive effect of the enhancement of photon harvesting due to IGB was compensated by a decline in the carrier collection and the overall efficiency of the device fell considerably.
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