Additives have been known to play an important role for improving the photovoltaic performance of organic solar cells. However, the reasons why additives improve the performance have not been clearly known yet. We employed an electrical modulated optical spectroscopy to investigate the relationship between the photovoltaic performance and additive concentration in organic photovoltaic devices. Our measured modulation spectra of a sample without additive showed both first- (-α’) and second-derivative (α”) components of the absorption spectrum. The second-derivative (α”) component in the modulation spectrum increased with the additive concentration. These results indicate that the sample without additive contains both localized (or Frenkel type) and relatively delocalized excitons and the added additive results in a significant increase of the relatively delocalized excitons. We conclude that the increased delocalized excitons lead to a significant additive-induced improvement of the photovoltaic-device performance.

已知添加剂在改善有机太阳能电池的光伏性能中起重要作用。但是，尚不清楚添加剂改善性能的原因。我们采用电调制光谱法研究有机光伏器件中光伏性能与添加剂浓度之间的关系。我们测量的不含添加剂的样品的调制光谱显示了吸收光谱的一阶（-α '）和二阶导数（α '' ）分量。二阶导数（α”）调制谱中的成分随添加剂浓度的增加而增加。这些结果表明，不含添加剂的样品既包含局部（或Frenkel型）激子，也包含相对离域的激子，添加的添加剂导致相对离域的激子显着增加。我们得出的结论是，增加的离域激子会导致添加剂引起的光伏器件性能的显着改善。