Successful incorporation of optical spacer and additive solvent for enhancing the photocurrent of polymer solar cell

An optical spacer and a processing additive (PA) offer various advantages in the enhancement of the photocurrent in a polymer solar cell, but successful incorporation of both in one device has yet to be demonstrated. Indeed, an optical spacer is considered to conflict the use of a PA in the solar cell system. Here we demonstrate a significant enhancement of the photocurrent in a polymer solar cell via successful incorporation of a solution-processable titanium sub-oxide (TiOx) optical spacer and a PA. By using a simple, non-material-specific, and effective vacuum treatment before deposition of the TiOx optical spacer, the conflict between the optical spacer and PA was resolved and remarkable enhancement in photocurrent was attained without any loss in the fill factor and open-circuit voltage. When the TiOx optical spacer was introduced together with the 1,8-diiodooctane additive, the power-conversion efficiencies were significantly improved from 7.63% to 9.33% for the PBDTTT-EFT:PC70BM device and from 5.5% to 6.46% for the P(BDT-TDPPDT-TPD)/PC70BM device, where PBDTTT-EFT stands for poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)], PC70BM stands for [6,6]-phenyl-C71-butyric acid methyl, and P(BDT-TDPPDT-TPD) stands for poly[benzodithiophene-4,6-bis(thiophen-2-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione-5-octylthieno[3,4-c]pyrrole-4,6-dione].