Improved photovoltaic performance of silicon nanowire/organic hybrid solar cells by incorporating silver nanoparticles
© Liu et al.; licensee Springer. 2013
Received: 22 December 2012
Accepted: 26 January 2013
Published: 18 February 2013
Silicon nanowire (SiNW) arrays show an excellent light-trapping characteristic and high mobility for carriers. Surface plasmon resonance of silver nanoparticles (AgNPs) can be used to increase light scattering and absorption in solar cells. We fabricated a new kind of SiNW/organic hybrid solar cell by introducing AgNPs. Reflection spectra confirm the improved light scattering of AgNP-decorated SiNW arrays. A double-junction tandem structure was designed to manufacture our hybrid cells. Both short-circuit current and external quantum efficiency measurements show an enhancement in optical absorption of organic layer, especially at lower wavelengths.
Organic solar cells have emerged as potential energy conversion devices for several advantages, including flexibility, lightweight, semi-transparent characteristics, and ability to large-scale production at low temperature [1–3]. However, their reported efficiencies are still very low even for laboratory cells. The most crucial problems many of these devices face are limited mobility of charge carriers and rapid recombination. To mitigate these problems, some special methods, such as reducing the thickness of the active layer of solar cell and incorporating inorganic materials with high carrier mobility, have been taken for effective charge separation [4–6].
One of these inorganic materials is silicon nanowires (SiNWs) [7–9]. Most recently, some research groups have demonstrated fabrication of SiNW/organic hybrid solar cells [10–16]. These SiNWs can offer at least three advantages for solar energy conversion. First, they provide high-mobility pathway from the active interface to the electrodes for carriers. Second, they can significantly reduce reflection and induce strong light trapping between nanowires, resulting in strong absorption. Finally, they increase the contact area between the two materials.
On the other hand, application of AgNPs in organic photovoltaic devices is of considerable interest . Surface plasmon resonance in AgNPs offers a promising way to enhance the power conversion efficiency (PCE) of organic solar cells as it exhibits strong local field enhancement around the AgNPs, which can increase light scattering and absorption in the organic film [18–21]. In recent years, a simple method for depositing AgNPs on silicon wafers by galvanic displacement has received renewed interests . As a versatile fabrication method, it is well suited to yield films with high purity and substrate adhesion . Thus, it is expected that the integration of AgNP-decorated SiNW array and polymer could lead to a simple process and high-performance solar cells.
In this work, we report an efficient approach for enhancing the PCE of SiNW/poly(3-hexylthiophene) (P3HT):-phenyl-C61-butyric acid methyl ester (PCBM) hybrid solar cells by decorating AgNPs on the SiNW surface. In order to evaluate the performance of the scattering effect of AgNPs, we have prepared different diameters of AgNP-decorated SiNW array samples by varying Ag deposition duration, with a Ag-free SiNW array sample as reference. Some hybrid solar cells with the structure of Al/n-type SiNW/AgNP/P3HT:PCBM/poly(3,4-ethylene-dioxythiophene):poly-styrenesulfonate (PEDOT:PSS)/indium tin oxide (ITO) were fabricated.
N-type silicon wafers with a thickness of 200 μm and a resistivity of 1 to 10 Ω cm were used. Vertically aligned SiNW arrays were prepared by metal-assisted chemical etching [24, 25]. Silicon pieces were first immersed into an aqueous solution of 5 M hydrofluoric (HF) acid and 0.02 M silver nitrate (AgNO3) for 60 s at room temperature to deposit Ag particles. Then, the Ag particle-coated silicon wafers were moved into an etching solution contained in a reactive vessel for 3 min. The etching solution was made of 5 M HF acid and 0.2 M hydrogen peroxide (H2O2). When the etching processes were over, the silicon strips were dipped into an aqueous solution of nitric acid (HNO3) and then rinsed with deionized water to remove any residual silver. After that, the synthesized SiNW array samples were immersed in a plating solution containing HF acid (5 M) and AgNO3 (0.02 M) to deposit AgNPs on SiNWs. The diameter of AgNPs was adjusted by changing deposition times. For comparison, another sample without AgNPs was also prepared. In order to obtain standard spherical particles and decrease defects on the surface, the AgNP-decorated SiNW array was annealed in N2 at 200°C for 90 min before cell fabrication.
Before polymer coating, aluminum (Al) had been attached onto the rear side by thermal evaporation to obtain an ohmic contact. The polymer, P3HT:PCBM (refers to PCBM) with a weight ratio of 1:1, was deposited onto SiNWs by spin coating (2,000 rpm, 1 min), and PEDOT:PSS was deposited onto ITO/glass substrate by spin coating (4,000 rpm, 1 min) in air. Then, PEDOT:PSS/ITO/glass substrate were coated on the P3HT:PCBM and fixed with a clip to complete the hybrid solar cell fabrication. After that, the whole substrates were baked at 110°C in nitrogen for 20 min. A hybrid solar cell without AgNPs decorated was also prepared as a reference device. The active area of all the cells was 16 mm2.
The morphology of SiNWs and AgNPs was characterized using a scanning electron microscope (SEM; JSM-7401F, JEOL Ltd., Akishima-shi, Japan). The crystal structures of the AgNPs were characterized by X-ray diffraction (XRD) using the copper Kα radiation. The reflection spectra were obtained at room temperature using a fiber-optic spectrometer (AvaSpec-2048, Avantes BV, Apeldoorn, The Netherlands) equipped with an integrating sphere. Current density-voltage (J-V) characteristics were measured with assistance of AM 1.5 illumination (100 mW/cm2). The quantum efficiency testing was performed on a DH1720A-1 250-W bromine tungsten arc source (DaHua Electronic, Beijing, China) and a Digikrom DK240 monochromator (Spectral Products, Putnam, CT, USA).
Results and discussion
It is well known that the transmittance of silicon in the wavelength region of 300 to 1,000 nm is almost zero . Therefore, the absorbance of silicon will be directly related to the reflectance. It should also be noticed that the reflected light only contains the part of scattering light which escapes from the structure. Other scattering light from AgNPs will be absorbed by the adjacent SiNWs or experience multiple reflections in the structure. On the other hand, the scattering effect is relative to the dielectric around the particles. That is to say, only after incorporating the polymer into the space of the structure could the scattering light be utilized effectively.
Device performances of SiNW/organic hybrid solar cells
However, we note that the Voc of AgNP-decorated cells decreases lightly. It has been reported that the passivation provided by the polymer and the interface area between the polymer and SiNWs (or AgNPs) could influence the open-circuit voltage of the devices . In other words, increased AgNP diameter will lead to some increased interface area and hence decreased Voc. It should be mentioned that the fill factor of all the hybrid cells are still very low. The series resistance comes from defects in the SiNW array, and poor electrode contact might be responsible for the low value.
Although the efficiencies of our devices are much lower than those of commercial silicon solar cells, the results of our experiments proved good effects of AgNPs in the SiNW/organic hybrid solar cell very well. Several other methods may be used to increase the efficiency of this hybrid solar cell. For example, etching the silicon substrate with an anodic aluminum oxide template could obtain a SiNW array with controlled size and excellent uniform distribution . If we used a small-sized SiNW array to manufacture hybrid solar cells, the organic layer would become thinner, resulting in the improvement of carrier collection efficiency. On the other hand, a gas-phase polymerization method could be introduced in the polymer coating process to form a uniform thin layer on SiNWs, resulting in a core-shell-structured solar cell with lateral heterojunction . Therefore, further efforts should be focused on these issues to improve the properties of SiNW/organic hybrid solar cells.
In summary, AgNP-decorated SiNWs were fabricated by metal-assisted chemical etching and electroless deposition. AgNP-decorated SiNW/organic hybrid solar cells were also demonstrated, treating them as double-junction tandem solar cells. The power performance of cells is enhanced by using AgNPs. The performance is dominated by current enhancement. The short-circuit current increases from Jsc = 10.5 mA/cm2 for the reference cell to 16.6 mA/cm2 for the best AgNP-decorated cell, with an enhancement up to 58%. The current gain gives a rise of the conversion efficiency from η = 2.47% to 3.23%, with an enhancement up to 30%. This enhancement is explained by light trapping effect of SiNWs and surface plasmon resonance scattering of AgNPs.
This work was mostly supported by the National Basic Research Program of China (grant no. 2012CB934200) and the National Natural Science Foundation of China (contract nos. 50990064, 61076009, 61204002).
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