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Cadmium selenide (CdSe) nanocrystals are one of the most studied colloidal nanocrystal systems in terms of their size and shape control. CdSe nanorods (NRs) with narrow size distribution is more interesting, because of the long axis of the nanorods serves a continuous pathway for electron transport precluding the need for electron hopping in nanoparticle-based photovoltaic. Although CdSe/conjugated polymer solar cells have been studied in several studies, the photo conversion efficiency (PCE) of photovoltaic devices is majorly limited by the low compatibility between inorganic CdSe nanoparticles and the conjugated polymers. Consequently, good dispersion of the CdSe nanocrystals in the polymer donor phase is required for the CdSe/conjugated polymer nanocomposite in solar cells to create a larger interfacial surface area, enhancing the charge transfer between polymer matrix and nanocrystals. Based on this study, we fabricate bulk heterojunction solar cells based on a binary system consisting of blending Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with CdSe NRs as active layer.
Hydrothermal technique is used to prepare CdSe NRs with the aim of using them in bulk heterojunction solar cell. The CdSe NRs are found to have crystallite size of about 8 nm. The CdSe NRs are characterized by energy dispersive X-ray, X-ray diffraction and high-resolution transmission electron microscope.
CdSe NRs are mixed with (different concentrations 0.04, 0.06, 0.08, 0.1 and 0.12 g denoted as S1, S2, S3, S4 and S5, respectively) with MEH-PPV to form CdSe NRs/MEH–PPV nanocomposites. The structural properties of the nanocomposites in the form of thin films prepared by DROP coating technique are studied in terms of X-ray diffraction, field emission scanning microscope and atomic force microscope. In addition, the UV–Vis spectra of the active layers together with the pure polymer and CdSe are investigated.
The transmittance and the reflectance of the nanocomposite films are measured at the normal incidence of the light over wavelength range 200–2500 nm. The optical functions such as absorption coefficient and refractive index of the nanocomposites are calculated and analyzed. The optical band gap of MEH–PPV is found to decrease with the addition of CdSe NRs. In addition, the single oscillator model is used to evaluate the optical oscillator parameters. The ratio of the free carriers to their effective mass is found to increase by increasing CdSe content.
Finally, the CdSe NRs/MEH–PPV nanocomposites are used as active layer in the bulk heterojunction solar cells in the structure of ITO/MoO3/MEH-PPV:CdSe NRs/LiF/Al, where ITO is indium tin oxide. The effect of CdSe NRs concentration on the photovoltaic properties of the devices has been studied. By varying the concentration of CdSe NRs, the efficiency of the devices is optimized in which a maximum PCE of about 2% is obtained.