Photoelectronic Properties of CdSe and CdTe Nanowires

Photoelectronic properties of CdSe and CdTe nanowires (NWs) were investigated for the application of photodetector and photovoltaic devices. Component NWs were typically synthesized by solution-liquid-solid (SLS) method. TEM images reveal that NWs are highly crystalline with diameters between 7-20 nm and lengths exceeding 1 Ì_å_m.

A modified chemical vapor deposition process was also developed to synthesize long (>10 Ì_å_m), 20-60 nm diameter CdSe and CdTe NWs at low temperatures on a plastic substrate. The approach applies synthetic strategies developed during the growth of solution-based semiconductor nanowires (NWs). Namely, Bi film was deposited on the substrate and acted as a catalyst to induce the growth of NWs. The length, width, and overall density of the wires can be modified by varying the growth temperature, Bi film thickness, as well as the introduced precursor metal:chalcogen stoichiometry. Additional studies were conducted to grow wires on other substrates such as silicon, glass, indium-tin-oxide coated glass coverslips and TeflonÌ¢'_å¢.

While the polarization sensitivity of single or aligned NW ensembles is a well known phenomenon, we surprisingly found the existence of a residual photocurrent polarization sensitivity even in random NW networks. When illuminated with visible (linearly polarized) light, such random NW networks exhibit significant photocurrent anisotropies ̏=0.25 (̏®Õ=0.04) [̏=0.22 (̏®Õ=0.04)] for CdSe (CdTe) NWs. Additional studies have investigated the effects of varying the electrode potential, gap width, and spatial excitation profile. These experiments suggest electrode orientation as the determining factor behind the polarization sensitivity of NW devices. A simple geometric model has been developed to qualitatively explain the phenomenon. The main conclusion from these studies, however, is that polarization sensitive devices can be made from random NW networks without the need to align component wires.

CdSe NWs are also investigated to build solar cell devices. Incorporating colloidal CdSe quantum dots (QDs) into CdSe nanowire (NW)-based photoelectrochemical solar cells increases their incident-photon-to-carrier conversion efficiencies (IPCE) from 13% to 25% at 500 nm. This beneficial effect originates from an interplay between NWs and QDs where the latter fill voids between interconnected NWs, providing electrically accessible conduits, in turn, enabling better carrier transport to electrodes. The presence of QDs furthermore reduces the residual polarization anisotropy of random NW networks. The effect appears to be general and may aid the future design and implementation of other NW-based photovoltaics.

To improve charge separation of NW based solar cells, single-walled carbon nanotube (SWNT) was added. Both emission quench measurement and transient absorption measurement confirms the ultrafast electron transfer to SWNTs with estimated rate constant about 2.54ÌÄ' 10⁸ s^-1. The improved charge separation and transport doubled the photocurrent generation of CdSe NW based solar cells.