Self-doped n-type small molecular electron transport materials for high-performance organic solar cells

Two naphthalene diimide (NDI) and perylene diimide (PDI) based n-type water/alcohol soluble small molecules (NFN and PFP) are designed and utilized as electron transport layers (ETLs) for organic solar cells (OSCs). NFN and PFP are synthesized by using Sonogashira coupling from alkynyl modified fluorene with mono-bromo substituted NDI and PDI. Density functional theory study results of NFN and PFP show that they possess excellent planarity due to the employment of triple bonds as connection units. Moreover, it was shown by electron paramagnetic resonance study that both NFN and PFP possess obvious self-doping behaviors, which may effectively enhance their charge transporting capability as ETLs in OSCs. Power conversion efficiencies of 8.59% and 9.80% can be achieved for OSCs with NFN and PFP as ETLs, respectively. The higher power conversion efficiency (PCE) of PFP based photovoltaic device is originated from the stronger doping property and higher mobility of PFP.