J-V CharacteristicsOptical Properties

Above-11%-Efficiency Organic–Inorganic Hybrid Solar Cells with Omnidirectional Harvesting Characteristics by Employing Hierarchical Photon Trapping Structures

Wan-Rou Wei,1,2 Meng-LinTsai, 1 Shih-Shiang Tai ,1 Cherng-Rong Ho,1 Shin-Hung Tsai,1

Ren-Jei Chung,2* and Jr-Hau He1*

1 Institute of Photonics and Optoelectronics & Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan , R.O.C. *Email: jhhe@ntu.edu.tw

2 Graduate Institute of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan, R.O.C. *Email: rjchung@ntut.edu.tw

Abstract

The study of silicon nanostructure, such as silicon nanowires (SiNRs)[1] or silicon micropyramids (SiMPs) for solar cell applications is attractive due to excellent light harvesting and good anti-refractive capability. Nowadays, inorganic-organic hybrid solar cell is widely investigated such as GaAs NRs hybrid P3HT, CdTe NRs hybrid P3OT, and TiO2 NRs hybrid P3HT. In this paper, we create a hierarchical structure consisting of SiNRs and SiMPs and hybrid the hole conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The schematic of the fabricated structure is shown in Figure 2. Hierarchical structures consisting of micropyramids and nanowires are demonstrated in Si/PEDOT:PSS hybrid solar cells to achieve a power conversion efficiency up to 11.50% with excellent omnidirectionality. The structure provides a combined concepts of superior light trapping ability, significant increase of p-n junction areas, and short carrier diffusion distance, improving the photovoltaic characteristics including JSC, FF, and PCE. The structure provides a combined concepts of superior light trapping ability, significant increase of p-n junction areas, and short carrier diffusion distance, improving the photovoltaic characteristics including JSC, FF, and PCE. The enhancement of power generation is up to 253.8% at high incident angles, showing the outstanding omnidirectional operation ability of hybrid cells with hierarchical Si surfaces. This properly designed hierarchical-structured device paves a promising way for developing low cost, high efficiency, and practical solar applications in the future.

Conclusions

The hierarchical Si surface combining nano- and micro-structures leads to increased internal multiple reflection and provides an intermediate refractive index between air, PEDOT:PSS, and Si, broadbandly and omnidirectionally suppressing the undesired surface reflection.

PEDOT:PSS/Si hybrid solar cells exhibit VOC of 0.52 V, JSC of 34.50 mA•cm-2, and FF of 64.40% by employing the hierarchical Si structures constructed with micro-pyramid and NWs, giving rise to very high PCE of 11.50%. The PCE improves up to 11.50% due to their strong light harvesting capability and optimized charge transport pathways.

More pronounced enhancement of generated power density by hybrid cells with hierarchical Si surfaces is observed at high AOIs (from 51.9% at normal AOI to 253.8% at 75o), showing the excellent omnidirectionality characteristics of the hierarchical Si surfaces for practical PV applications.

These hierarchical hybrid solar cells with omnidirectional absorption will display an guidance to the future high-efficency and low material consumption solar cells.

Figure 8. NW length-dependent photovoltaic characteristic trends ((a) Voc, (b) Jsc, (c) FF and (d) PCE) of hierarchical-structured Si devices.

(b)

Fabrication and Characterization of PV Devices

Figure 2. SEM images of MP-textured Si substrates (a) without NWs, with (b) 30 nm, (c) 185 nm, (d) 234 nm, (e) 357 nm, and (f) 602 nm NWs, respectively.

Figure 3. Schematic illustration of the fabrication process for a hierarchical /polymer solar cell. (a) pyramid Si (b) after SiNWs by metal assisted etching, (c) after a spin-coating of PEDOT:PSS, with a thickness of 30 nm and (d) after deposition of a top electrode, Ag, with a thickness of 500 nm.

Figure 4. (a) Total reflectance spectra of flat Si, pyramid Si with various lengths of 0, 30, 185 and 234 nm SiNWs coated with PEDOT:PSS. Specular reflectance spectra of (b) flat Si, pyramid Si with (c) 0 and (d) 234 nm SiNWs coated with PEDOT:PSS.

Figure 5. Simulated optical field using FDTD (a) flat Si (b) pyramid Si (C) pyramid Si with 234 nm SiNWs coated with PEDOT:PSS (d) Optical absorption detected at the interface between PEDOT:PSS and Si at 550 nm.

Figure 1. Device structure

Figure 9. (a) Schematic of incident angle-dependent powergenerated over a day. (b) Incident angle dependence ofgenerated maximum power (c) enhancement of generatedmaximum power, and (d) estimated average daily powerdensity generated from the devices.

Total Reflectance can be lowered to 8 % in the spectral range of 400-1000 nm.

Figure 4(b)-4(d) clearly show that the hierarchical surfaces can broadbandly and omnidirectionally suppress the undesired Fresnel reflection.

The insets of Figure 3(a)-3(c) show the high-magnification images in the interface region of air and PEDOT:PSS/Si to highlight the light trapping effect.

We show that the FF can be radically improved by introducing an ultrathin SiOx layer at the interfaces.

Enhanced fill factor results from enhanced carrier separation due to increased junction area and short diffusion distance of minority carriers to the junction.

Normal incident angle the Pmax enhancement is 51.9% while Pmax enhancement is increased to 253.8% as the AOI increases to 75o for hierarchical surfaces, implying that light-harvesting capability for hybrid cells with hierarchical structures is distinguished particularly at high AOIs.

Figure 7. IPCE spectra of flat Si cell, pyramid Si andpyramid Si with 234 nm SiNWs.

Figure 6. Current density-voltage (J-V) characteristics of planar, micropyramid and hierarchical structured Si with various Si NW lengths with and without hot DI water treatments.

Finite-difference time-domain (FDTD) analysis was carried out to gain insight into the light propagation across the hierarchical structure interfaces, through which the light harvesting effect of hierarchical structures and the absorption behaviors of the devices can be correlated.