The Organic Solar Cell

A solar cell converts solar energy into electricity by the photovoltaic effect. Most of the cost of a solar panel comes from the photoactive materials. Recently, it has been shown that the inorganic components can be replaced by organic semiconductors, which are inexpensive and can be processed in a roll-to-roll fashion with high throughput. There are two approaches for organic photovolatic

Organic Solar Cell

Organic Solar Cells (OSC) are solar cells where the active layer of the device is made of organic material

On photon absorption, an electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), forming an exciton.

After exciton dissociation, each charge carrier must be transported through the device to the appropriate contact while avoiding traps and recombination.

The simplest device that promotes exciton separation is a planar heterojunction sandwiched between a transparent conductor (e.g., indium-tin-oxide, ITO) coated with one or more inorganic layer and a reflecting metal (usually Al or Ag) on top of the whole structure. Bulk heterojunction, where a blend of two polymer are used instead of a planar junction is a more effective way to promote the exciton dissociation via a charge transfer complex

Fig. 1

Dye-sensitized Solar Cells

DSCs (dye-sensitized solar cells) are promising alternative to conventional silicon solar cells in many application fields, due to various advantages, such as low cost and the suitability for architectonic integration.

The structure of the DSC consists of: a transparent conductive oxide (ITO), which acts as photo-anode; a transparent semiconductor oxide (TiO2); a dye-sensitizer; an electrolyte solution; and a second electrode, which acts as cathode

The sensitizer material absorbs part of the incident sunlight into photocurrent. Charge separation occurs at TiO2/dye interface: electrons are transfered toward the titanium dioxide; the oxidized dye is reduced by iodide ions, which are dissolved in the electrolyte; The reduction process forms tri-ioidide ions, which diffuse toward the Pt counter electrode, where they are reduced back to iodide by electrons that have passed through the external circuit.

Fig. 2

Research activity

Our research in OSC involve: the characterization of the efficiency of cells with different structures and layer composition; the analysis of the dependence of the main devices parameters (efficiency, fill factor, short-circuit current and open circuit voltage) on temperature, illumination conditions, UV exposure, electrical stresses, etc; the study of the reliability of the devices and its dependence on stress temperature and illumination level.

Measurement techniques include not only standard DC characterization by means of current-voltage and capacitive measurements, but also advanced characterisation, such as the Impedance Spectroscopy tecnique to investigate the degradation of the interfaces, the Open Circuit Voltage Decay, the Applied Bias Voltage Decay, Deep-level transient spectroscopy.

Fig. 3 shows a comaprison between the impedance spectroscopy taken in a DSC after thermal stress (a) and UV exposure (b), highlighting different degradation of the cell depending on the stress type.

Fig. 3

MOST - Molecular and Organic Semiconductor Technology
Microelectronic Group - University of Padova