Efficiency enhancement of polymer solar cells by applying an alcohol-soluble fullerene aminoethanol derivative as a cathode buffer layer

Abstract

Cathode buffer layer (CBL) introduced between the active layer and cathode is crucial for selectively transporting electrons and blocking holes for polymer solar cells (PSCs). Calcium (Ca) is the most commonly used CBL in conventional-structure bulk heterojunction (BHJ) PSC devices, but is prone to oxidation due to its high reactivity, inhibiting its practical applications. Herein, we applied an alcohol-soluble fullerene aminoethanol derivative (C60-ETA) as an efficient CBL surpassing Ca in conventional-structure BHJ-PSC devices, leading to obvious efficiency enhancement with the best power conversion efficiency (PCE) reaching 9.66%. C60-ETA CBL was applied in PSC devices based on three different photoactive layer systems, 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), polythieno[3,4-b]thiophene-co-benzodithiophene (PTB7):PC71BM and poly(4,8-bis-alkyloxybenzo(l,2-b:4,5-b′)dithiophene-2,6-diylalt-(alkylthieno(3,4-b)thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C):PC71BM, affording the best PCE of 9.66%, 8.51% and 7.19%, respectively, which are all higher than those of the corresponding devices based on the commonly used Ca CBL. The mechanism of efficiency enhancement of C60-ETA CBL relative to Ca is studied, revealing that C60-ETA CBL may induce improvements on both the interfacial contact between the active layer/cathode and electron transport, facilitating electron extraction by the Al cathode, and consequently leading to the increase of short-circuit current density (Jsc), which contributes primarily to the PCE improvement.