Integrated optical and electrical modeling of plasmon-enhanced thin film photovoltaics: A case-study on organic devices

Abstract

The nanoscale light control for absorption enhancement of organic photovoltaic (OPV) devices inevitably produces strongly non-uniform optical fields. These non-uniformities due to the localized optical modes are a primary route toward absorption enhancement in OPV devices. Therefore, a rigorous modeling tool taking into account the spatial distribution of optical field and carrier generation is necessary. Presented here is a comprehensive numerical model to describe the coupled optical and electrical behavior of plasmon-enhanced polymer:fullerene bulk heterojunction (BHJ) solar cells. In this model, a position-dependent electron-hole pair generation rate that could become highly non-uniform due to photonic nanostructures is directly calculated from the optical simulations. By considering the absorption and plasmonic properties of nanophotonic gratings included in two different popular device architectures, and applying the Poisson, current continuity, and drift/diffusion equations, the model predicts quantum efficiency, short-circuit current density, and desired carrier mobility ratios for bulk heterojunction devices incorporating nanostructures for light management. In particular, the model predicts a significant degradation of device performance when the carrier species with lower mobility are generated far from the collecting electrode. Consequently, an inverted device architecture is preferred for materials with low hole mobility. This is especially true for devices that include plasmonic nanostructures. Additionally, due to the incorporation of a plasmonic nanostructure, we use simulations to theoretically predict absorption band broadening of a BHJ into energies below the band gap, resulting in a 4.8% increase in generated photocurrent.

Integrating nanostructured electrodes in organic photovoltaic devices for enhancing near-infrared photoresponse

“…19,22 We have previously shown by a combined optical and electrical simulation of the device performance that NIR photocurrent enhancement can be achieved by using a metal nanograting as the back electrode. 16 Our simulations showed that excitation of SPPs in the grating enhances the optical field in the active layer and that the grating can be designed so that the enhancement occurs in the near-infrared where the normal absorption in the active layer is low. 16 Here we experimentally realize this device and demonstrate that enhancement of the EQE is indeed observed in the spectral region predicted by the simulation.…”

“…16 Our simulations showed that excitation of SPPs in the grating enhances the optical field in the active layer and that the grating can be designed so that the enhancement occurs in the near-infrared where the normal absorption in the active layer is low. 16 Here we experimentally realize this device and demonstrate that enhancement of the EQE is indeed observed in the spectral region predicted by the simulation. This confirms the power of simulation to design gratings that target optical field enhancement in a specific wavelength range of interest that can be focused in the active layer.…”

“…23 Here we use the same lamination technique to integrate Ag gratings and flat Ag controls onto OPV devices, albeit with a significant modification compared to previous work. Our simulations indicated that the mismatch in the refractive index between the PEDOT:PSS and the P3HT:PCBM layers 16,28,29 would result in optical losses owing to undesirable light confinement in the PEDOT:PSS layer. The simulated resonance SPP mode shown in Figure 2, which assumes direct contact of the Ag grating and the active layer, will essentially collapse into the PEDOT:PSS layer if such a low index layer is added between the grating and the active layer.…”

“…The simulation takes as inputs the n & k spectra of each layer and the solar spectrum, and numerically solves Maxwell's equations for the steady-state electric field profile within the active layer. 16 The thickness of each of the flat layers on the device stack (ITO, ZnO and active layer) is fixed to the experimental values and the parameters that are allowed to vary to tune the position of the SPP resonance are the depth, line-width or duty ratio, and period of the grating. Periodic boundary conditions are imposed (along the x-direction in Figure 1) in such a way that the twodimensional simulation cell has width equal to the nanostructured grating periodicity.…”

“…More recently, research has focused on the use of patterned metal electrodes, which provide a greater degree of control / flexibility and can be designed to couple incident solar radiation into surface plasmon polariton (SPP) modes or waveguide modes that propagate laterally through the active layer. [3][4][5] There have been several pioneering theoretical 15,16 and experimental [17][18][19][20][21][22] studies of the incorporation of random and periodic patterned metal electrodes into OPV devices, several of which aim to extend the spectral response of the active layer further into the near-infrared (NIR). [18][19][20][21][22] Early, pioneering efforts to include such structures demonstrated the viability of such a strategy, but the sub-bandgap EQE enhancement was limited either by use of a non-optimal grating period that resulted in excitation of higher SPP modes 18,20 or the reduced transparency of the nanostructured electrode.…”