Effect of alkyl-chain branching position on nanoscale morphology and performance of all-polymer solar cells

Abstract

Manipulating the alkyl-chain branching position afforded two naphthalene diimide-based polymers (C1 and C3). The optoelectronic properties, including the absorption spectra and electronic energy levels, conformations and photovoltaic properties of the two polymers, were fully studied and compared. The effect of alkyl-chain branching position on their optoelectronic properties and crystallinity was investigated in detail. Moving the branching position away from the backbone resulted in superior crystallinity of C3. All-polymer solar cells (all-PSCs) based on C1 or C3 as the acceptor and polymer PBDTTT-C-T as the donor were fabricated and optimized. The power conversion efficiencies (PCEs) of the optimized all-PSCs based on C1 and C3 reached 1.68% and 1.92%, respectively. The improved photovoltaic performance may be due to the clear nanoscale morphology of C3-based blend film. The blend film exhibits favorable phase separation and well-ordered structure of polymer C3. This substantially increased the electron mobility (up to 1.0 × 10⁻⁴ cm² V⁻¹ s⁻¹), which contributes to balancing the charge transport in C3-based blend film. This is the first report on the realization of favorable phase separation and high electron mobility through improvement of acceptor crystallinity in all-PSCs.