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

An ethanolamine (ETA)-functionalized fullerene (C-60-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 C-60-ETA was studied by FT-IR and XPS spectroscopies and elemental analysis, and the average molecular formula of C-60-ETA is estimated to be C-60(NHC2H4OH)(8)(H)(8) with an average ETA addition number of eight. C-60-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-Abenzo[1,2-b:4,5-b']dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl Cm-butyric acid methyl ester (PTB7-Th:PC71BM), poly(4,8-bis-alkyloxybenzo(1,2-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 C-61-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 C-60-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.