Organic photovoltaics (OPVs) are a type of solar cell that has recently been receiving much research. Though such polymer solar cells have low conversion efficiencies when compared with expensive nonorganic multijunction solar cells, OPVs are also much cheaper to produce and can be inexpensively manufactured in large quantities. Low conversion efficiencies is not the only drawback of OPVs; organic materials are much more vulnerable to degradation from environmental factors, and the lifetimes of OPVs must be seriously considered as part of the cost benefit analysis. Peters et al. (2011) present conclusions from their study of the efficiency decay of two types of OPVs: the well-studied P3HT and relatively new PCDTBT devices. The study concludes that the less researched PCDTBT devices decayed at a slower rate than P3HT though the P3HT had, in general, a higher absolute conversion efficiency figure. Researchers expect that further development of PCDTBT solar cells will raise their conversion efficiency while maintaining their long lifetime.—Alan Hu
Peters, C., Sachs-Quintana, I., Kastrop, J., Beaupre, S., Leclerc, M., McGehee, M., 2011. High Efficiency Polymer Solar Cells with Long Operating Lifetimes. Advanced Energy Materials 1, 491–494.
Peters et al. at Stanford University and University of Laval test the lifetimes of P3HT and PCDTBT devices by exposing an experimental group of the two types of solar cells to a standardized environment. Initial device efficiencies of P3HT and PCDTBT respectively were 4 ± 0.05%. and 5.5 ± 0.15%. Since UV radiation is known to cause defects in polymer solar cells and that commercial OPVs are likely to carry UV blockers, an LG sulfur plasma lamp, which emits very little UV radiation, was used as a source of light. The solar cells were kept in a dark room for a week after fabrication before being placed under the lamp. The cells were then aged at maximum power point for 4400 hours under one-sun intensity at 37ºC. Light intensity was calibrated with a National Renewable Energy Lab-certified silicon photodiode. Both light intensity and temperature were measured every 5 seconds; current-voltage curve data was collected every hour. The researchers defined burn-in of solar devices as the short period of exponential loss in efficiency after initial use. Lifetime ends by convention when the efficiency of the device has dropped below 80% of its initial value.
Results from the experiment show that PCDTBT solar cells had lower efficiency ratios immediately after burn-in but suffer from less degradation than P3HT. PCDTBT had a burn-in period of about 400 hours after which its efficiency figures remained relatively stable. The VOC and fill factor of PCDTBT in particularly remained practically flat for 4000 hours after the initial burn-in. P3HT devices on the other hand experienced a roughly 10% drop in efficiency per 1000 hours. This was caused by a simultaneous decrease in both VOC and JSC.
A burn-in demarcation was set at 1300 hours and the lifetime of each type device tested was found with a linear regression. Assuming 5.5 hours of one-sun intensity per day and 365 days per year, the average lifetimes of PCDTBT and P3HT solar cells were found to be 6.2 and 3.2 years respectively. As such, PCDTBT solar cells demonstrated a clear advantage over P3HT solar cells in terms of durability; one particular PCDTBT device was so remarkably stable that it was projected to have a lifetime of 11 years.
A laser beam-induced current map, a test used to determine if a portion of the solar cell has lost effectiveness, shows that both P3HT and PCDTBT showed no loss of device area after 200 hours of aging.
PCDTBT solar cells experience a more dramatic loss of efficiency over the burn-in period but are far more stable than P3HT solar cells after this initial period. P3HT cells currently hold a conversion efficiency advantage over PCDTBT cells though considering the nascent stage of research into the PCDTBT polymer, PCDTBT cells have the potential for large advances in efficiency. The researchers predict that as PCDTBT cells become more optimized, both longevity and greater efficiency can be achieved in the inexpensive OPVs.