Highly conductive, optically transparent, low work-function hydrogen-doped boron-doped ZnO electrodes for efficient ITO-free polymer solar cells

Abstract

In this work, highly conductive, optically transparent and low work-function hydrogen-doped boron-doped ZnO (BZO:H) cathode electrodes were prepared by a hydrogen post annealing treatment of the as-deposited boron-doped ZnO (BZO) samples. It was found that hydrogen post annealing at temperatures around 200 °C resulted in the formation of electrode materials which exhibited higher conductivity and carrier concentration, reduced sheet resistance and significantly increased optical transparency compared with their non-annealed BZO counterparts. In addition, hydrogen incorporation in the material lattice caused a significant reduction in their work function which may be beneficial for device operation. As a result, polymer solar cells using BZO:H films as transparent cathode electrodes exhibited higher efficiencies compared to those obtained for devices using the non-annealed counterparts. In particular, devices based on the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₇₁butyric acid methyl ester (PC₇₁BM) system as the photoactive layer exhibited a PCE of 3.90%, whereas those based on the poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT):PC₇₁BM and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):PC₇₁BM as the active components reached high PCE values of 5.90 and 7.25%, respectively, which are comparable or even higher than reported efficiencies obtained for devices using doped ZnO-based transparent electrodes.