Abstract
In a high performance PTB7:PC71BM bulk-heterojunction (BHJ) solar cell, the commonly optimized polymer:fullerene (D:A) weight ratio is about 1:1.5, when PC71BM is used as the acceptor. This report explores alternative D:A weight ratios. We describe how to enrich the polymer contents of these BHJ solar cells to achieve high power conversion efficiencies (PCEs). The concentration of 1,8-diiodooctane (DIO), a solvent additive for the BHJ precursor solutions, is increased in order to re-optimize the BHJ cells. The PCEs of the re-optimized cells are improved for the PTB7 cells. Detailed charge transport measurements were carried out to examine the polymer-rich BHJs. We observed enhanced hole mobilities for the PTB7 BHJs. Additionally, the electron mobilities are preserved due to the dispersion of fullerene domains by increased DIO concentrations. Two other well-known polymer donors PCDTBT and PDTSTPD have been also investigated, and the improvements of hole mobilities and PCEs can be obtained for both polymer-rich BHJ solar cells.
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