The Energy Recycling Of Plasmonic Metal Electrodes And Its Application In Organic Photovoltaics

Transparent conductive electrodes play an important role in fabricating high-performance organic photovoltaics(OPVs).Indium tin oxide(ITO),the most commonly used electrodes,due to its inherent shortcomings such as high-cost,brittleness,and material scarcity,seriously suppresses OPVs commercialization development.Recently,much effort has been put into the research of alternatives to ITO.Compared to ITO,the mutual restriction of conductivity and transmittance hinders most of new and innovative electrodes’ application as transparent conductive electrodes in OPVs.While,plasmonic metal electrodes(PMEs),which can excite surface plasmon polaritons(SPPs)to enhance the transmittance,has become a popular alternative to ITO.However,the non-radiative damping of surface plasmon polaritons(SPPs)during the coupling with the sunlight results in the conversion of the excited hot-electrons to heat,which inevitably limits the light absorption and photocurrent generation.Hence,the new strategies for recycling the SPP energy and restraining the non-radiative damping in PMEs for the optimization of OPVs.Firstly,a periodic single-layer polystyrene(PS)nanosphere array was fabricated through self-assembly method.Then,the size of PS nanospheres was reduced by reactive ion etching as a shadow mask.Silver was deposited on it.After removing PS nanospheres,plasmonic silver nanomesh electrode(A-PME)was obtained.Since A-PMEs can effectively excite SPPs,the transmittance of the electrodes has been significantly increased compared with that of flat silver film.By optimizing the electrode period,the surface coverage and thickness of silver,the A-PMEs with the least SPPs energy loss were obtained,of which the average transmittance and square resistance were more than 70%and less than 40 Ω sq-1.To reduce the SPPs energy loss in A-PMEs and realize energy recycling,inspired by hot-electron injection mechanism,the hot-electron emission was proposed to recycle the SPPs energy trapped in the A-PMEs through the route of "light-SPPshot-carriers-light".Based on this,a yellow-emission organic molecule of 2,8-ditertbutyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene(TBRb)was deposited on the A-PMEs to construct the hot electron emission system as well as A-PMEs with energy recyling(A-PMEs/TBRb).The TBRb was introduced to convert the SPPs energy loss to free electrons energy in A-PMEs,generating excited electrons named hot-electrons.Then,hot-electrons with energy higher than the Schottky barrier(φb)can be injected into the lowest unoccupied molecular orbital(LUMO)of TBRb,and transit to the highest occupied molecular orbital(HUMO),converting into photons.As a result,the transmittance of A-PMEs/TBRb was further enhanced(average value to be 80.3%)while the square resistance still maintained a low value.Furthermore,combined with these A-PMEs/TBRb,the OPVs based on the ternary active material(PM6:N3:PC7iBM)was fabricated with conventional structure.Compared to the reference devices using flat silver film and A-PMEs as the bottom electrodes,devices based on A-PMEs/TBRb obtained higher external quantum efficiency(EQE)in full spectrum range,thus increasing photoelectric conversion ability.The highest power efficiency(PCE)reached 16.1%.Moreover,another active layer(P3HT:PC71BM)with a narrower EQE response was applied to fabricate devices.Compared to A-PMEs-based devices,A-PMEs/TBRb-based OPVs show an obvious EQE enhancement beyond the absorption spectrum range of the P3HT:PC71BM active layer.The extra enhanced EQE further provided a direct evidence of the hot-electron emission for recycling the SPP energy loss in A-PMEs.