Comparative study of photovoltaic performance and stability of polymer/fullerene and polymer/polymer solar cells

Abstract

The commercialization of polymer solar cells (PSCs) is still limited even though the successful advances of performances over ~11% have been achieved. One of the main hurdles is originated in fullerene derivatives used as an electron acceptor in PSCs. Fullerene derivatives have high crystallization behavior and diffusion kinetic, which cause micron-sized fullerene crystals diffused out from fullerene phases and macro-phase separation of polymer/fullerene blends in a very short time scale when the blends are heated. The phase-separated blends significantly reduces the long-term stability of PSCs. Fullerene derivatives also have high brittleness, resulting in the mechanical failures of polymer/fullerene blends against even low level of mechanical deformations. In addition, the fullerene-based PSCs are fragile against solar irradiation. When fullerene derivatives are irradiated, fullerene dimerization occurs from cycloaddition reaction between fullerene balls, forming fullerene dimers. The fullerene dimers in polymer/fullerene blends reduce electron mobility and efficiency of PSCs. Here, a new high performance polymer/polymer solar cell (all-polymer solar cell, all-PSC) system was developed and their stabilities were examined to establish a highly efficient and stable PSC devices. All-PSCs composed of n-type conjugated polymers can possess a variety of advantages in terms of both efficiency and long-term stability. For example, polymer acceptors have much higher light absorption ability in visible ranges, which mainly results in the improvement of short circuit current density $(J_{SC})$, and have variety energy levels, which mainly leads to the enhancement of open circuit voltage $(V_{OC})$, because their molecular structure can be easily modified. Furthermore, much lower diffusion kinetic, high ductility and photo-chemical stability in a $N_2$ condition of polymer acceptors can induce excellent mechanical, thermal and photo-stabilities of all-PSCs, demonstrating the great potential of all-PSCs in applications for flexible and portable devices.