An ethanolamine-functionalized fullerene as an efficient electron transport layer for high-efficiency inverted polymer solar cells

An ethanolamine (ETA)-functionalized fullerene (C₆₀-ETA) was synthesized by using a facile one-pot addition reaction, and applied as an efficient electron transport layer (ETL) for inverted polymer solar cells (iPSCs) with the efficiency exceeding 9.5%, which represents the highest power conversion efficiency (PCE) reported so far for iPSC devices with independent fullerene derivative ETLs. The chemical structure of C₆₀-ETA was studied by FT-IR and XPS spectroscopies and elemental analysis, and the average molecular formula of C₆₀-ETA is estimated to be C₆₀(NHC₂H₄OH)₈(H)₈ with an average ETA addition number of eight. C₆₀-ETA was applied as an ETL for bulk heterojunction (BHJ) iPSC devices based on different photoactive layers including poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl C₇₁-butyric acid methyl ester (PTB7-Th:PC₇₁BM), poly(4,8-bis-alkyloxybenzo(l,2-b:4,5-b′)dithiophene-2,6-diylalt-(alkyl thieno(3,4-b)thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C):PC₇₁BM and poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC₆₁BM), leading to the best PCE of 9.55%, 6.50% and 4.18%, respectively, which are all higher than those of the corresponding reference devices based on the ZnO ETL. The PCE improvement of the C₆₀-ETA device relative to that of the ZnO device primarily originates from the increase of short-circuit current density (J_sc), which is due to the smoothened ETL surface and the increase of electron mobility, facilitating electron transport from the active layer to the ITO cathode.