Solar cells' stability is measured as function of time. When we plot time against the stability of solar cells, it is called Decay Curve. The curve varies depending upon the situations or environment in which the stability has been measured. Decay curve is very useful but it does not give much detail about underlying degradation mechanisms. Decay Curve is a graph obtained when we plot stability of a solar cell in electrical measurements at regular intervals, hence it gives us information about how fast or slow degradation process is under specific circumstances. It also gives clues about what role factors like oxygen, water, temperature, etc. play in degradation of a solar cell. Terms Used in a Decay Curve :- E0 : Initial efficiency of a solar cell. It decreases over time. T80 : The time it takes to reduce the initial efficiency to 80%. It is sometimes taken as a measure of the useful lifetime of a device. Burn-In Phase : It has been observed that there is a rapid initial decrease in the power-conversion efficiency, which is followed by a more stable phase. This initial stage is called the burn-in phase. It is not always considered part of the proper decay curve. Ts : The time at the end of the burn-in phase is T-s. Ts80 : The time it takes to reduce the efficiency to 80% from the T-s time.

Oxygen, Water and Light are the main degradation causing elements. Oxygen and Light: Active material react with oxygen in the presence of light and a photo oxidation reaction occurs. This destroys active material's ability to absorb light and create electrical carriers. Oxygen also reacts with metal electrodes. Aluminium is most popular as electrode but it is more susceptible to degradation. Silver is more stable as compared to aluminium and hence used in stable solar cells. Water : Water may react with both the hole transport layer and the electrodes and cause degradation of solar cell. There are also a number of important physical mechanisms that degrade the solar cell functionality. The active layer is made up of certain components. The size of domains of each component is optimized for exciton transport and charge separation. This junction structure may get destroyed by slow rearrangement of these constituents, and the destruction can be accelerated using heat. The multi-layer structure of polymer solar cell can also delaminate at weak interfaces between the layers, for example, between the hole conductor, PEDOT:PSS and the active layer. Moreover, if the top and bottom electrodes get connected, it may lead to an electrical short. Similar situation can arise during the printing process (we'll come back to this subject in a separate lesson). So the most important factors are oxygen and humidity, sometimes together with light and higher temperatures.

Polymerization is the process of combining many small molecules (the monomers) into a covalently bonded chain or network. Stille cross coupling - The Stille cross coupling reaction can generate new carbon-carbon bonds, through transmetallation of an organotin compound with a halogenide as coupling partner, with the use of a palladium catalyst system in an a solvent. Suzuki cross coupling - This comprises an organoboron functionalized compound together with an organic halide compound and also apply a palladium catalytic system. Direct arylation - This is a Heck-type coupling and compared to Stille and Suzuki reactions the direct arylation polymerization does not require preparation of stannylated or boronated reagents which eliminates one of the normally more complicated steps in the monomer preparation. Source : http://www.plasticphotovoltaics.org/lc/lc-materials/lc-synthesis.html