Stabilizer-free CdSe/CdS core/shell particles from one-step solution precipitation and their application in hybrid solar cells

CdSe/CdS core/shell nanoparticles were synthesized in aqueous solution by one-step process. CdSe precipitates were first obtained at 80 °C in a solution of cadmium chloride, sodium citrate, ammonia and sodium selenosulfate. After 4 h of reaction, the solution with CdSe products was cooled to 60 °C and thiourea (TU), a sulfur precursor, was added while stirring. The amount of the TU solution added was 2, 5 and 10 mL, and the obtained products were named as CdSe/CdS-2, CdSe/CdS-5 and CdSe/CdS-10, respectively. It is observed that CdSe core particles had predominantly a crystalline cubic structure, and the addition of TU in the CdSe solution led to the formation of a very low concentration of hexagonal crystalline phase of CdS whose intensity increased with the amount of TU. The energy-dispersive X-ray spectroscopy in a scanning electron microscope indicated an increase of sulfur concentration in CdSe/CdS products as a function of the amount of TU solution. The presence of CdS-like morphology on the surface of CdSe/CdS particles was also evident. Photoluminescence emission spectra of the CdSe/CdS particles exhibited an increasing CdS emission as the volume of TU solution increased. The photovoltaic performance of solar cells based on CdSe/CdS particles and poly(3-hexylthiophene) (P3HT) suggested that the CdSe particles should be surrounded by CdS because of the similarity between the current–voltage curves of CdSe/CdS/P3HT heterojunctions and that of CdS/P3HT solar cells.