Organic solar cells (OSCs) have attracted a lot of attention due to their potential as flexible, lightweight, and low-cost renewable energy sources. Significant improvements have been made in increasing the devices’ power conversion efficiency (PCE) and extensive efforts to understand degradation mechanisms and increase OSCs’ lifetimes are ongoing. OSCs with higher than 10% efficiency have been reported. Enhanced stability and efficiency of inverted poly(3-hexylthiophene) (P3HT) solar cells with Cesium (Cs) halides were achieved by spin-coating Bphen (4,7-di(phenyl)-1,10-phenanthroline) on the halide layer and adding an ~100nm polystyrene beads layer on the blank side of the OSC.

To investigate photodegradation in a low-bandgap polymer, PBDTTT-EFT (benzo[1,2-b:4,5-b’]dithiophene (BDT) and thieno[3,4-b]-thiophene), PBDTTT-EFT:PCBM bulk heterojunction (BHJ) solar cells were irradiated under 1X sun intensity and their electronic measurements were monitored over time. The electronic measurements revealed an increase in deep defect density in the polymer and at the donor-acceptor (D/A) interface of BHJ SCs and an increase in charge recombination as well as a decrease in external quantum efficiency, charge collection, short circuit current, open circuit voltage and hole mobility. Filtering blue and UV light resulted in drastically reduced photodegradation.

Electron paramagnetic resonance measurements were performed on the photodegraded polymer and BHJ films, and revealed for the first time experimental evidence for metastable carbon dangling bonds (g=2.0029à ±0.0004) formed by blue/UV irradiation of the films. Dark EPR showed an increase in densities of other spin-active sites in the polymer, fullerene and polymer:fullerene blends in agreement with electronic measurements. The EPR results revealed that the carbon dangling bonds were at the D/A interface. These studies helped to better understand degradation mechanisms in a low-bandgap polymer, PBDTTT-EFT, ways to enhance its stability as well as the stability of P3HT (high bandgap) polymer SCs.