Local Surface Plasmon Resonance-Enhanced Polymer Solar Cells Based On P3HT:PCBM

The power conversion efficiency（PCE）of polymer solar cells（PSCs）keeps increasing in the past two decades with the synthesis of new materials and the improvement of preparation process.However,it should be futher enhanced to satisfy commercial application.Introducing metallic nanoparticles（MNPs）in PSCs has been proved to be one of the most effective methods to improve device performance.As we know,if the distance between MNPs and photoactive layer is close enough（less than 20 nm）,the electromagnetic field near MNPs can be strengthened due to the localized surface plasmon resonance（LSPR）excited by these MNPs,which can improve absorption of photoactive layer,thereby enhancing photocurrent.In this paper,the influences of several kinds of MNPs on performance of poly（3-hexylthiophene）:[6,6]-phenyl-C61-butyric acid methyl ester（P3HT:PCBM）-based PSCs have been investigated,and a series of work has been carried out to maximize the PCE as the goal.Sm nanoparticles（NPs）were introduced by evaporating 0.5 nm-thick Sm on the P3HT:PCBM surface in our PSCs,in which Sm NPs can help to enhance absorption of the P3HT:PCBM photoactive layer owing to the strengthened local field induced by LSPR and the back scattering of Sm NPs,as well as lower the interface energy barrier between the cathode and the photoactive layer due to its low work function.However,because depositing 0.5 nm-thick Sm forms NPs,rather than a continous film in this structure,it is inevitable for diffusing Al atoms（from Al cathode）into the P3HT:PCBM layer,leading to performance degradation of PSCs.To prevent the direct contact between Al and the photoactive layer,a 13 nm-thick Ca film was incorporated on the Sm NPs,that is,the Sm/Ca bilayer of cathode buffer was used in the final PSCs.The experimental results revealed that short-circuit current density（JSC）,open-circuit voltage（VOC）,and fill factor（FF）were apparently enhanced in the plasmonic device with Sm/Ca cathode buffer bilayer.Based on an inverted device using Zn O and Mo O3 as cathode and anode buffer layers respectively,Au nanorods（NRs）with an aspect ratio of about 3.5:1 were embedded into Zn O via spin-coating to obtain improved performance,and the influences of the upper Zn O thickness on Au NRs on device performance were investigated.The experimental results revealed that the best performance of plasmonic PSCs was achieved with an 8 nm Zn O overlayer.Compared with incorporating MNPs into poly（3,4-ethylenedioxythiophene）: poly（4-styrenesulfonate）（PEDOT:PSS）in conventional devices,Au NRs can arouse a stronger plasmon electromagnetic field in our experiments due to a higher refractive index of Zn O,thus bring enhanced absorption in P3HT:PCBM.Au NRs（average aspect ratio of 2.1:1）with 2 nm Si O2 shells（Au@SiO₂ NRs）were synthesized and directly incorporated on P3HT:PCBM in inverted PSCs via a spin-coating technique.Using such ultra-thin Si O2 shells can not only utilize LSPR effect to improve absorption as far as possible,but also avoid the contact between Au NRs and the photoactive layer,thus reduce a direct exciton recombination on Au NR surfaces.Based on above plasmon device,the dual-plasmon device was fabricated to further improve device performance,in which Au@SiO₂ NRs and Ag NPs were incorporated on P3HT:PCBM and into Mo O3,respectively.Ag NPs,obtained through evaporating 1 nm-thin Ag between two layers of Mo O3,have plasmonic absorption peak at 517 nm and the spectra width of about 135 nm,therefore can broden the range of absorption band in P3HT:PCBM.Besides,the insertion of Ag NPs can also improve the conductivity of Mo O3 and enhance hole extraction efficiency.Consequently,both FF and JSC were further enhanced in our dual-plasmon device,resulting in a PCE enhancement factor of 11.4% compared with mono-plasmon one with only Au@SiO₂ NRs.