Concept and Nanostructure Control of Plasmonic Porous Silicon Solar Cells

[Introduction]A multi-junction solar cell can generate high open current voltage by each layer absorbing a different wavelength light, although lattice mismatch between materials is one of its problems. A possible new type of solar cell is an all-Si tandem solar cell, which will have no mismatch. Our laboratory noticed that porous silicon is a light-absorbing material for short wavelengths. Its nano-scale holes on its surface increases its band gap due to the quantum size effect. Because this porous structure, however, leads to high resistivity and low absorption coefficient, the porous layer must be made thinner by enhancing light absorption.

To improve light absorption, we propose using localized surface plasmon of metal nanoparticles [1, 2]. By such introduction of these particles into the holes of porous silicon, a thinner porous layer can be also achieved.

Here, we proposed this plasmonic porous silicon solar cell (or PPSSC), and aimed to control the structure of porous silicon. To determine guidelines for fabricating PPSSCs, we created solar cells consisting of porous silicon, and then measured their I-Vcharacteristics. By simulation, we estimated the enhancement of light absorption of porous silicon with metal nanoparticles.

[Experimental]1. Preparation of porous silicon: Here, p-type monocrystalline silicon wafers with three different resistivities were anodized for different times and current density. The surfaces and cross-sections were then observed by field emission scanning electron microscopy (FE-SEM) and evaluated using UV-vis spectrum. Several wafers were also treated with Ar dry etching, and its effect on their surface structure observed.

2. Fabrication of solar cells: An n-type silicon layer was deposited on top of the porous silicon by chemical vapor deposition (CVD) using SiH₄ and PH₃ in H₂. Silver electrodes were fabricated using RF sputtering, and then their rectification property and quantum efficiency were measured.

3. FDTD simulation: With the Finite-Different-Time-Domain (FDTD) method, we created a simulation model in which silver nanoparticles exist in the holes on the silicon surface. The simulation showed how the light absorption in this combined material is enhanced by adding silver nanoparticles to the porous silicon. Then, the necessary layer thickness of porous layer was estimated by the model, and the influence of localized surface plasmon was verified by investigating the strength of the electrical field near the interface between the silicon and silver nanoparticles.

[Results and Discussion] 1.Structure control of porous silicon: By changing the anodization time and current density, we successfully fabricated porous silicon of various pore size, depth, and porosity. Figure 1 shows the relationship between current density I and pore diameter. In addition, results reveal that dry etching expands the pore size of a porous silicon surface (Fig. 2), indicating that dry etching might facilitate both the introduction of metal nanoparticles and the control of band gaps.

2.Evaluation of porous silicon solar cells: Figure 3 shows that a thin n-type silicon layer was successfully deposited on the porous silicon by CVD. Figure 4 shows that p-n junctions were also formed by this deposition. However, compared with a monocrystalline silicon solar cell (Fig.5), porous silicon solar cells tended to have a smaller increase rate of I than that of V. One factor in this trend is the high resistivity of porous silicon, indicating that making the porous silicon layer thinner is important to increase the efficiency of PPSSCs. Figure 6 compares the quantum efficiency of porous silicon and monocrystalline silicon solar cells, revealing that porous silicon solar cells have lower quantum efficiency. One explanation is that insufficient deposition of n-type silicon onto the porous silicon accelerates surface recombination. This surface recombination can be improved by performing passivation.

3.Evaluation of combined materials by simulation: FDTD simulation indicated that the light absorption of the combined materials can be improved when the silver nanoparticles exist in the holes of porous silicon, and that a strong electrical field is injected into the silicon around the interface between silicon and the silver nanoparticles. These results indicate that localized surface plasmon might improve light absorption and enhance thinner porous silicon as a light absorption layer.

[Conclusion] Current density, resistivity of wafers, and dry etching can be factors in controlling the structure of porous silicon. We successfully formed p-n junctions with porous silicon, and based on the measured I-Vcharacteristics and quantum efficiency of porous silicon solar cells, the porous silicon needs to be thinner and needs passivation. The simulations suggest that metal nanoparticles enhance the light absorption.

References

[1] M. Ihara et al, Physica E, 42, (2010), 2867-2871

[2] N. Loew et al, JSS, 3(2), (2014), Q1-Q10