Organic photovoltaic diodes appear to be a promising technology because of their potential use in synthesizing low-cost, flexible, large area and lightweight electronics. Unfortunately, the limited lifetime of organic photovoltaic diodes hinders their commercialization. For example, a thousand hours lifetime reported for poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester solar cells is inferior to that of silicon photovoltaics whose lifetime extends up to 25 years.
The degradation of these solar cells remains a limitation and many researchers are still looking for ways to improve their performance. Organic photovoltaic diodes degrade owing to chemical and physical processes. Chemical degradation results from oxygen, light, water and temperature. However, water and oxygen are considered the principle factors; these oxidize the organic photovoltaic materials as well as electrodes leading to poor performance and electronic traps. Degradation may also result from the bulk heterojunction morphology since the constituent materials may aggregate with time, leading to reduced exciton dissociation and poor performance.
Blending the active semiconductor with an inert polymer appears to improve the lifetime of the organic photovoltaic diodes. A team of researchers under the guidance of Professors Chris Groves and Michael Petty at Durham University in United Kingdom investigated the use of an insulating polymer, poly(methyl methacrylate), as a ternary component in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester solar cells as a way of enhancing their lifetime and reducing degradation. Their research work is now published in Solar Energy Materials & Solar Cells.
The authors separately prepared poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester solutions before mixing in a 1:1 weight ratio. In a bid to make the ternary blend, they first prepared poly(methyl methacrylate) solution by dissolving in anhydrous 1,2-dichlorobenzene to obtain a clear solution.
The authors stirred both the ternary and binary organic photovoltaic blends before spin coating them onto the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). They annealed all devices before lifetime tests.
The research team observed that the addition of poly(methyl methacrylate) improved both the initial performance as well as the lifetime of the solar cells. Measurement of the different relative humidity values suggested that poly(methyl methacrylate) absorbed water, thereby reducing the rate of chemical degradation in the solar cell. The lifetime improvement with poly(methyl methacrylate) reduced with decreasing humidity. This suggested that the poly(methyl methacrylate) becomes saturated. A number of studies revealed that the addition of poly(methyl methacrylate) led to a morphology containing poly(methyl methacrylate) pillars dissimilar to the morphology seen in the binary film.
Electrical conductivity did not degrade at different rates across the ternary film. This suggested that water diffused microns through the film before reacting with the active material. The rate of conductivity degradation was similar for binary and ternary devices. This indicated that various degradation mechanism were present, and that poly(methyl methacrylate) only assisted in extending the lifetime associated with a selected degradation pathways.
Power conversion efficiency of the poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester solar cells is severely limited by reaction with water. The incorporation poly(methyl methacrylate) slows down this degradation process through water absorption. The results of their study suggest that electrically inert and hydroscopic polymers can be blended with an organic photovoltaic active layer to extend the device lifespan.
Zakiya AL-Busaidi, Christopher Pearson, Christopher Groves, Michael C. Petty. Enhanced lifetime of organic photovoltaic diodes utilizing a ternary blend including an insulating polymer. Solar Energy Materials & Solar Cells, volume 160 (2017), pages 101–106.Go To Solar Energy Materials & Solar Cells