Tuning hole charge collection efficiency in polymer photovoltaics by optimizing the work function of indium tin oxide electrodes with solution-processed LiF nanoparticles

Official URL: http://dx.doi.org/10.1007/s10854-015-3613-z

By varying the density of solution-processed lithium fluoride (sol-LiF) nanoparticles at the interface between tin-doped indium oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), we have demonstrated that the electronic hole collection efficiency of an organic photovoltaic cell can be optimized through tuning the energy level alignment at the ITO/PEDOT:PSS interface. We synthesized the LiF nanoparticles in solution and deposited them onto ITO electrodes with increasing surface coverage up to 13.2 %. The surface work function of the nanostructured ITO increased linearly from 4.88 to 5.30 eV. When the sol-LiF-modified ITO electrodes were incorporated into polymer solar cells based on a bulk heterojunction of poly(3-hexylthiophene) polymer and methanofullerene, a maximum power conversion efficiency was recorded for a device with an ITO anode modified by 5.3 % of sol-LiF coverage, which corresponded to a measured work function of 5.07 eV. The improvement in short circuit current density by 87 % and power conversion efficiency by 74.3 % suggest that the sol-LiF interlayer density enabled work function tuning of the ITO anode to better match the highest occupied molecular orbital level of PEDOT:PSS, facilitating hole charge collection. The increase in electronic hole collection efficiency is attributed to both a lowered resistance at the ITO modified by sol-LiF and faster hole transport, although these gains are offset by an associated increase in contact polarization. Our findings suggest that the surface work function of ITO can be tuned to improve energy level alignment with other contact layers via the surface density of sol-LiF particles. More efficient hole transport, due to higher recombination resistance, offset by an increased charge extraction barrier presented by contact polarization; the two effects combined give rise to an optimum in sol-LiF nanostructuring of the ITO surface properties.