Nanophotonic Metasurfaces to Enhance the External Radiative Efficiency of Organic Solar Cells

Lightweight, low-cost organic photovoltaics (OPVs) are considered one of the next generations of solar cells. Metasurfaces possesses distinct advantages of ultrathin thickness and wide wavelength-response, which make them suitable for applications in OPVs to improve light harvesting and power conversion efficiency (PCE). Plasmonic metasurfaces, in particular, can interact in three ways with the OPV active layer to improve light-matter interactions: increased light scattering and surface plasmon polaritons can couple more incident sunlight into the plane of the active layer. Additionally, localized surface plasmon resonances can be used to improve light absorption cross-section (when resonant with active layer absorption) or create a Purcell effect (when resonant with active layer emission) that alters exciton decay rate (radiative and non-radiative). The latter is less well studied and the aim of this work is to understand the impact of plasmonic metasurfaces on engineering radiative decay in OPVs and the effect on external radiative efficiency (ERE) of OPVs.<br/>ERE quantifies the radiative loss in a solar cell and is also related to the performance of the solar cell. The theoretical Shockley-Queisser limit assumes an ERE of 100%; however, in practice, ERE values are much lower than this upper limit. In the realm of high-efficiency solar cells, such as GaAs and silicon solar cells, the recorded ERE increases with improved PCE. For example, the highest reported ERE values are 1.6% for silicon solar cells and 35.7% for GaAs solar cells. However, the reported ERE values for OPVs are still very low (&lt;10^-3%). We hypothesize that improving the radiative efficiency inside the organic semiconductor active layer in OPVs, using plasmonic metasurfaces for example, improves the PCE of OPVs closer to the theoretical limit predicted by Shockley-Queisser.<br/>The photoluminescence of OPV active layers (typically bulk-heterojunctions (BHJs)) is rarely studied due to its extremely weak signal intensity, and its signal is challenging to decouple from background noise and weak substrate fluorescence. In this work, we carry out a systematic study with various substrates, like quartz, planar silver, and hemispherical silver nanoparticle metasurfaces with ~75 nm diameter and ~155 nm diameter nanoparticles. For devices, the nanoparticle metasurfaces are incorporated between the transparent electrode and the BHJ active layer. OPV BHJ materials poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) are selected to form the BHJ OPV active layer. Our preliminary results show improved photoluminescence quantum yield (PLQY) from the BHJ active layer on certain substrates. By optimizing the size and distribution of the metasurface, the BHJ shows different extents of PLQY enhancement. The PLQY data shows 18.4% for the BHJ on metasurfaces with average silver nanoparticle diameter of 155 nm, 23.3% for the BHJ on metasurfaces with smaller nanoparticles (~75 nm), and 13.7% for the BHJ on a planar silver. The metasurfaces with 75 nm diameter silver nanoparticles show the highest improved PLQY, attributed primarily to the Purcell effect. For larger silver nanoparticles (diameter of ~155 nm), the PLQY enhancement is partially suppressed mainly due to excessive metal-induced quenching of excited excitons and a weak Purcell Effect. We have demonstrated the capability to modify the radiative efficiency of organic semiconductors OPV active layers with silver nanoparticle metasurfaces. The performance of PBDB-T/ITIC OPV devices is also studied in both conventional and inverted device structures to verify the relationship of PCE to ERE with and without the plasmonic metasurfaces.