Plasmonic Organic Solar Cells Employing Nanobump Assembly

Abstract

Plasmonic organic solar cells (OSCs) have attracted attention in recent years because of their superior optical properties, increasing light absorption inside the device. Plasmonic OSCs are normally consisted with small sized metallic nano particles (NPs) and structures, whose size is smaller than the wavelength of light. These nano structures lead to elongated light paths inside the active layer of OSCs and boosting electrical field near them, thereby resulting in increased light absorption and photocurrent. For generating NPs, two different methods, the wet-based and the thermal evaporation process, have been widely used. In the case of wet based method, NPs are mixed with the solution processed buffer layer. It is very important to prevent NPs from the aggregation of each other, which works as exciton quenching sites and leakage sources, so that the careful handling for the generation and dispersion of NPs is mandatory. On the other hands, the size of thermally deposited NPs is limited because of the trade-off relationship between transparent of layer and plasmonic effects. Therefore, it is imperative to introduce new method to form uniformly sized NPs without aggregation, which is much more beneficial for the fabrication of OSC module. In this thesis, a novel plasmonic OSC incorporating a nanobump assembly (NBA) is demonstrated. Here, the NBA is consisted of aerosol derived silver NPs covered by thermally evaporated hole extraction layer (HEL), MoO3. The size of NPs generated evaporation and condensation method via aerosol process is precisely controlled with small deviation of their size. Additionally, the aggregation between NPs is negligible compared to the conventional NPs, synthesized by wet process. Moreover, thermally evaporated MoO3 follows underlying structure, so that the NBA has oval shaped structure, Thus the NBA provide precisely size controlled plasmonic structure with textured surface. In the polymer based OSC, the device performance of OSC with NBA (6.07%) shows higher power conversion efficiency compared to the device without it (5.20%). This is mainly originated to the enhancement in the photo current without deteriorating other photovoltaic parameters. Especially, the improvement in spectral response to the light at the region from 500 to 600 nm leads to the better device performance of OSC with NBA. This wavelength region is in a good agreement with calculated plasmonic effect from NBA, which is derived from 3D finite domain time difference method. Moreover, the undulated structure arising from NBA provides better carrier transport. Whats more, it is possible to tune the main peak of plasmonic effect by managing the size of NPs. As the size of NP increase, the plasmonic peak wavelength is red-shifted, with increasing the scattering intensity. Compared to the widely used conventional method, where NPs are incorporated to the solution processed PEDOT:PSS, the NBA provides better electrical and optical effect to the device. Since the NPs are covered by oval shaped high refractive index material (n= ~ 2.0 at 550 nm), the plasmonic effect from NBA is higher and occurred at longer wavelength than that of conventional structure, NPs embedded in flat low index material, PCDOT:PSS (n= ~ 1.8 at 550 nm). Besides, fully covered NPs by dielectric layer minimize NP induced exciton annihilation, which facilitate the generation of free carrier. Furthermore, the undulated active layer allows free carrier to extract effectively to the electrode. As a result, the efficiency of OSC with NBA (6.07%) is higher than that of OSC with NPs embedded in PEDOT:PSS (5.63%), while performances of devices without NPs are similar to each other regardless of change of buffer layer (MoO3: 5.20%, PEDOT:PSS: 5.16%). To estimate the effect of NPs encapsulation status, electrical and optical properties of NBA is studied by changing the thickness of dielectric layer, MoO3. The UV-visible absorption spectrum shows that the enhancement in absorption of film with partially encapsulated NPs is higher than that of film with fully encapsulated one. This can be understood by the short penetration length of plasmonic effect induced by NPs. However, the transient photoluminescence and impedance analysis illustrated that the partially encapsulated NPs work as the exciton quenching and the recombination centers, which mitigates the transfer of enhanced absorption to the photo-current. Considering these factors, the maximum enhancement of device performance is achieved, when the thickness of MoO3 is 20 nm for 40 nm Ag NPs. Therefore, it is mandatory for highly efficient plasmonic OSCs to employ fully covered NPs with minimum thickness of dielectric layer. This thesis demonstrates the practical approaches to enhance power conversion efficiency of plasmonic polymer solar cells, employing nanobump assembly. Additionally, the optical and electrical effect of NBA is studied by varying the thickness of encapsulation layer and size of NP. This approach is guideline for plasmonic solar cell for organic device as well as other type one polymer-NC hybrid, dye-sensitized solar and perovskite based solar cells. And this technique can be extended to optoelectronic devices.