Insights into solvent vapor annealing on the performance of bulk heterojunction solar cells by a quantitative nanomorphology study

Abstract

Bulk heterojunctions (BHJ) represent the most promising structures for high efficiency polymer solar cells and their morphologies can be finely tuned by post-treatments such as thermal annealing and solvent vapor annealing. Though extensive studies have shown improved power conversion efficiencies by tuning the treating parameters of both treatments, substantial knowledge of how the BHJ morphologies evolve with various solvent vapors related to photovoltaic characteristics and differ from those with thermal annealing is still limited. Herein we employed simultaneous grazing incidence wide and small angle X-ray scattering (GIWAXS/GISAXS) to systematically investigate the changes in morphology of a poly(3-hexylthiophene)/C61-butyric acid methyl ester (P3HT–PCBM) BHJ manipulated by solvent vapor annealing using different solvents. Solvents with different solubility, i.e. non-solvent, poor solvent and good solvent were studied. Distinctive morphologies were quantitatively resolved among these solvent vapor-annealed BHJs and their evolutions during processing are interpreted. The resolved morphologies can clearly explain the subtle variations in photovoltaic characteristics of open circuit voltage (V_oc), short circuit current (J_sc) and fill factor (FF) related to the working mechanism of the BHJ, i.e. carrier generation, carrier transportation and recombination. This work provides fundamental new insights into how the BHJ morphologies and photovoltaic characteristics can be flexibly tailored by solvent vapor annealing using various kinds of solvent vapors.