2.2 Cadmium Selenide
Cadmium Selenide (CdSe) is a good candidate material for hybrid solar cells, as its
shape can be relatively easily controlled, itcontributes useful absorption in the visible
region, and forms a strong type II heterojunction with most polymers. Due to these
factors, CdSe currently represents the state of the art in hybrid solar cell efficiency, with
a certified PCE of 3.13% (Dayal et al., 2009).
The well-established synthesis methods of this material allow for the fabrication of
nanoparticles with complicated shapes. Dayal et al. report of CdSe tetrapods dispersed
in a polymer matrix. These tetrapods are reported as having average dimensions of 5 nm
arm diameter and 30 nm –50nm arm lengths. This structure allows for an improved
nanomorphological structure which facilitates excitonic dissociation and allows
improved conductive pathways. As a result, the ordered structure leads to improved
electrical characteristics. The Current-voltage characteristics for devices using this
structure are displayed in Fig. 2 (a). In their communication detailing the current state of
the art system, Dayal et al. report 34% of the absorption of the system is attributed to
CdSe. This contribution has a positive impact on the External Quantum Efficiency
profile for the device, as shown in Fig. 2 (b). This shows that the absorption
contribution of the inorganic acceptor material leads to the production of photocurrent.
The band gap of bulk CdSe is approximately 1.74 eV; however, this is altered by
changing the physical dimensions of the nanoparticle. Zhou et al. report a CdSe band
gap of ~2 eV for nanoparticles with a mean diameter of 4.7 nm (Zhou et al., 2011). This
value is slightly larger than desirable but still allows for absorption up to ~650 nm.
One major drawback of this material is the toxicity; however, it may be a good model
system to study to gain a deeper understanding of hybrid solar cells.