For semiconducting polymers, such as regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the molecular weight has been correlated to charge carrier field-effect mobilities, surface morphology, and gelation rates in solution and therefore has important implications for long-term reliability, manufacturing, and future applications of electronic organic thin films. In this work, we show that the molecular weight rr-P3HT in organic solar cells can also significantly change the internal cohesion of the photoactive layer using micromechanical testing techniques. Cohesive values ranged from similar to 0.5 to similar to 17 J m(-2), following the general trend of greater cohesion with increasing molecular weight. Using nanodynamic mechanical analysis, we attribute the increase in cohesion to increased plasticity which helps dissipate the applied energy. Finally, we correlate photovoltaic efficiency with cohesion to assess the device physics pertinent to optimizing device reliability. This research elucidates the fundamental parameters which affect both the mechanical stability and efficiency of polymer solar cells.