When a solar cell absorbs light, the photons “excite” an electron, which becomes separated from the “positive hole” left behind in the atom. This charge separation is the crucial first step that captures the energy derived from the incident photon. The negative electron migrates to an electrode, and the hole attracts another electron from a neighbouring atom, effectively moving the hole which ultimately migrates to the other electrode. Thus the electrons and the holes are collected at different electrodes giving an electrical potential (voltage). This drives a flow of the electrons (current) round the outer circuit where they can do some work (e.g. light a bulb or turn a motor) before combining with the holes at the other electrode and returning the system to its starting point..
Crystalline silicon cells are solid-state devices which use a continuous lattice of silicon. When the electron is excited, it is easy to separate it from its positive hole because they're both spread out across the “infinite” lattice, so the electrostatic attraction they feel for each other is low.
In a dye-sensitised solar cell (DSSC), the electron and the hole are created in the same way, but they're very close together because they are confined to one small molecule, rather than spread out over a very large lattice. This means there is a strong negative-positive electrostatic attraction between them and they tend to stick together. The electron-hole pair is called an "exciton,” and to get the electrons to flow and produce an electrical current, they are pulled apart using an interface between dissimilar materials – a dye with TiO2 on one side and a redox electrolyte on the other side.
The major difference between DSSC and crystalline silicon cells is therefore that DSSC use molecules rather than an infinite lattice, and also have to use a different mechanism to separate electrons from their positive holes – a process described as "excitonic" because it deals with excitons or electron-hole pairs.
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