Organic solar cells (OSC) offer a promising alternative to achieve highly efficient solar cells with low cost and easy fabrication. However, light absorption efficiency in OSC is limited due to the low carrier diffusion length and low exciton mobility. Plasmonic nanostructures have the ability to localize light in the organic active layer and thus increasing the light path length without increasing its physical thickness. Here, we exploited a new type of plasmonic nanostructures to achieve high and broadband absorption enhancement in organic solar cells. Zirconium Nitride (ZrN), as an example of refractory plasmonics, has one of the highest localized surface plasmon resonance quality factor. In this work, several new ZrN nanostructures such as spherical nanoshells and nanodisks are incorporated in organic solar cells. A theoretical analysis using finite difference time domain (FDTD) simulations is implemented to thoroughly analyze and compare these structures. Mie scattering and absorption efficiencies are calculated to analyze these spherical nanoshells in a polymer environment. A high and broadband absorption enhancement is achieved after their incorporation in the organic solar cell.
One of the key issues limiting the efficiency of organic solar cells is the narrow absorption band of the polymer active layer. Thus, a huge amount of the incident sunlight is lost. Here, a new structure is theoretically proposed achieving wide band absorption in organic solar cells using multifunctional TiN nanowires. In addition to the plasmonic properties of TiN, it was reported that TiN has the capability to produce free carriers upon light absorption. Thus, the structure is based on the ability to collect these photo-generated carriers.
Using the combination of TiN and polymer significantly broadened the absorption band due to the ability of TiN to localize light inside P3HT:PC70BM in addition to its ability to absorb light at longer wavelengths. The optimized structure enhanced the absorbed power by 95% and the optimal short circuit current by 123% over the same structure without the TiN nanowires. Electric field distribution is studied at different wavelengths to gain further insight on the localization of light inside the structure.
The efficiencies of thin film amorphous silicon (a-Si) solar cells are restricted by the small thickness required for efficient carrier collection. This thickness limitations result in poor light absorption. In this work, broadband absorption enhancement is theoretically achieved in a-Si solar cells by using nanostructured back electrode along with surface texturing. The back electrode is formed of Au nanogratings and the surface texturing consists of Si nanocones. The results were then compared to random texturing surfaces. Three dimensional finite difference time domain (FDTD) simulations are used to design and optimize the structure. The Au nanogratings achieved absorption enhancement in the long wavelengths due to sunlight coupling to surface plasmon polaritons (SPP) modes. High absorption enhancement was achieved at short wavelengths due to the decreased reflection and enhanced scattering inside the a-Si absorbing layer. Optimizations have been performed to obtain the optimal geometrical parameters for both the nanogratings and the periodic texturing. In addition, an enhancement factor (i.e. absorbed power in nanostructured device/absorbed power in reference device) was calculated to evaluate the enhancement obtained due to the incorporation of each nanostructure.
Three dimensional optical simulations are performed to assess the design requirements for obtaining highly efficient tapered Si nanowires (TSiNWs)/polymer hybrid solar cells. To avoid the complex fabrication processes of Si p-n junctions, the TSiNWs are coated with a conductive polymer forming a large junction area between both materials and making the charge separation more efficient. The addition of PEDOT:PSS has been reported previously where the absorption occur in the Si only. P3HT:PCBM has been also used on top of Si nanostructures to enhance the absorption. However, the maximum absorption of P3HT and Si are in the same range resulting in competence between the absorption of each material. Thus, thick Si substrates are still needed to achieve decent absorption in these devices. We report a broadband absorption spanning the whole visible and near infra-red range of the solar spectrum with only 5 Microns TSiNWs coated with a low band gap polymer. The tapered structure provides efficient light trapping for the incident light enhancing the absorption in the short wavelengths. The addition of the low band gap polymer (pBBTDPP2:PCBM) significantly enhanced the absorption at long wavelengths (700-900nm). Thus, broadband absorption is attained without the need of thick Si substrates. Full 3D optical simulations were performed to optimize the polymer thickness and compare between the enhancements in absorption for different polymers.
We demonstrate absorption improvement in organic solar cells due to the incorporation of TiN nanopatterned back electrode. Organic solar cells (OSC) have already reached 10% power conversion efficiency (PCE), which made them comparable to commercial solar cells. Localizing light using plasmonic nanostructures has the potential to overcome OSC absorption limitations and thus further improve their PCE. Using a C-MOS compatible, cheap and abundant material for light trapping could facilitate the commercialization of OSC. This work theoretically shows that the replacement of Ag nanopatterned back electrode with TiN in plasmonic OSC gives enhanced performance. In addition, the incorporation of TiN nanoparticles inside the active layer has been studied and analyzed.