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.