Improvement in Photovoltage and Stability of Porous n-Si Electrodes Coated with Platinum by Regulation of the Thickness of Nanoporous Layers

Abstract

Porous n-Si electrodes, prepared by photoetching in HF under appropriate conditions, have macroporous layers at the surface, consisting of micrometer-sized pores and Si pillars. The wall and top of the Si pillars are further covered with 0.2-0.5-µm-thick nanoporous layers having nanometer-sized pores. The nanoporous layer can be thinned by immersion in HF. The solar cell characteristics (open-circuit photovoltage V OC , fill factor, and stability) for the porous n-Si electrodes with Pt coating in 8.6 M HBr/0.05 M Br 2 were improved by thinning the nanoporous layer to an appropriate thickness, although the electrodes with no nanoporous layers gave only poor characteristics. The maximum solar energy conversion efficiency of 14% (V OC 0.575 V, j SC 34.7 mA‚cm -2 , and fill factor 0.701) was obtained, which is one of the highest for n-Si photoelectrochemical solar cells. A mechanism for the generation of high V OC 's as well as high fill factors in porous Si-based photoelectrochemical solar cells is discussed including a possibility of a low resistivity of the nanoporous layer for hole transport.

Porous silicon formation by HF chemical etching for antireflection of solar cells

“…The photocurrent was converted to illumination intensity by a proportional calculation from the photocurrent under simulated solar illumination (AM 1.5G, 100 mW cm -2 ). The n-Si wafer was fixed to a Teflon holder (effective area: 0.25 cm 2 ) and used to measure the characteristics of solar cells. The n-Si electrode and a Pt plate counter-electrode were immersed in an 8.6 M HBr and 0.05 M Br 2 redox electrolyte aqueous solution.…”

“…This result clearly shows that the porous layer decreased the reflectivity of the n-Si surface, but it did not affect the charge flow between the Pt particles and n-Si. The increase in the open-circuit photovoltage from 0.49 to 0.57 V can be explained by the following three factors, the decrease in the Pt particle density, the decrease in the size of the direct metal-Si contacts, or the effect of nanoporous layer [2,[7][8][9]. The maximum power output reached 12.3 mW cm -2 , corresponding to 12.3% of conversion efficiency.…”

“…Macroporous Si that consists of micrometer-sized pores is expected to be used for the antireflection layer of Si solar cells to improve conversion efficiency [1][2][3]. Porous Si is usually prepared by an electrochemical (p-Si) or photoelectrochemical (n-Si) etching under an anodic bias.…”

“…Recently, we reported that both nanoporous and macroporous layers are produced on n-Si wafers, which are modified with fine Pt particles, by chemical etching in a HF solution without a bias or an oxidizing agent [7]. Photoelectrochemical (PEC) solar cells, equipped with an n-Si electrode that has been modified with fine Pt particles, have been found to generate high open-circuit photovoltages (V OC ) of 0.62-0.68 V and to produce high conversion efficiencies of 14-15% [2,[7][8][9][10][11]. Porous Si effectively improves not only photocurrent density but also V OC and thus the conversion efficiency of such solar cells [2,7,8].…”

“…Photoelectrochemical (PEC) solar cells, equipped with an n-Si electrode that has been modified with fine Pt particles, have been found to generate high open-circuit photovoltages (V OC ) of 0.62-0.68 V and to produce high conversion efficiencies of 14-15% [2,[7][8][9][10][11]. Porous Si effectively improves not only photocurrent density but also V OC and thus the conversion efficiency of such solar cells [2,7,8]. The present work investigates the structural change in porous n-Si with different kinds of deposited metal particles and photoillumination intensity during etching.…”