**3. Organic-inorganic hybrid nanowire solar cells**

As a platform for cost-effective crystalline Si (c-Si) solar cells, an organic-inorganic hybrid structure has been proposed to replace the conventional p−n junction. The hybrid structure is composed of a transparent conducting polymer and c-Si [48, 49]. A Schottky junction at the conducting polymer/c-Si interface can easily be created using a simple spin coating process at room temperature, while a conventional Si p−n junction is formed via a high-temperature doping process [50]. In order to enhance the light-trapping efficiency, Si nanostructures such as nanopyramids [49] and nanowires [51] have been applied, leading to an increase in the short-circuit current density (JSC).

Han-Dom Um et al. demonstrate an embedded metal electrode for highly efficient organicinorganic hybrid nanowire solar cells as shown in **Figure 8** [52]. The electrode proposed here is an effective alternative to the conventional bus and finger electrode, which leads to a localized short circuit at a direct Si/metal contact and has a poor collection efficiency due to a nonoptimized electrode design. In this design, an Ag/SiO<sup>2</sup> electrode is embedded into a Si substrate while being positioned between Si nanowire arrays underneath poly(3,4-ethylenedi oxythiophene):poly(styrenesulfonate) (PEDOT:PSS), facilitating suppressed recombination at

**Figure 8.** Embedded metal electrode for highly efficient organic-inorganic hybrid nanowire solar cells, and I-V characteristic of the solar cell [52].

However, SiO<sup>2</sup>

172 Emerging Solar Energy Materials

quantum confinement effect.

short-circuit current density (JSC).

alone should not give rise to the PL emission; rather, defect states would con-

hotoelectron spectroscopy (XPS) signatures, which may originate from dangling bonds and

**Figure 7.** (a) HRTEM and (b) EFTEM analyses of the top section of a PSiNW etched using 4.8 M concentration of HF [33].

Therefore, the emissions from surface/defect states in oxide may contribute to the red PL emission, in agreement with the results obtained in [47], in addition to the contribution from

As a platform for cost-effective crystalline Si (c-Si) solar cells, an organic-inorganic hybrid structure has been proposed to replace the conventional p−n junction. The hybrid structure is composed of a transparent conducting polymer and c-Si [48, 49]. A Schottky junction at the conducting polymer/c-Si interface can easily be created using a simple spin coating process at room temperature, while a conventional Si p−n junction is formed via a high-temperature doping process [50]. In order to enhance the light-trapping efficiency, Si nanostructures such as nanopyramids [49] and nanowires [51] have been applied, leading to an increase in the

Han-Dom Um et al. demonstrate an embedded metal electrode for highly efficient organicinorganic hybrid nanowire solar cells as shown in **Figure 8** [52]. The electrode proposed here is an effective alternative to the conventional bus and finger electrode, which leads to a localized short circuit at a direct Si/metal contact and has a poor collection efficiency due to a

substrate while being positioned between Si nanowire arrays underneath poly(3,4-ethylenedi oxythiophene):poly(styrenesulfonate) (PEDOT:PSS), facilitating suppressed recombination at

O) and Si3+ (Si2

O3

electrode is embedded into a Si

) using X-ray

tribute to PL emission. Indeed, Najar et al. observed Si1+ (Si2

**3. Organic-inorganic hybrid nanowire solar cells**

nonoptimized electrode design. In this design, an Ag/SiO<sup>2</sup>

volumetric stress that distorted the PSiNWs forming localized defect states.

the Si/Ag interface and notable improvements in the fabrication reproducibility. With an optimized microgrid electrode, the 1-cm<sup>2</sup> hybrid solar cells exhibit a power conversion efficiency of up to 16.1% with an open-circuit voltage of 607 mV and a short-circuit current density of 34.0 mA/cm<sup>2</sup> . This power conversion efficiency is more than twice as high as that of solar cells using a conventional electrode. The microgrid electrode significantly minimizes the optical and electrical losses. This reproducibly yields a superior quantum efficiency of 99% at the main solar spectrum wavelength of 600 nm. In particular, the solar cells exhibit a significant increase in the fill factor of 78.3% compared to that of a conventional electrode (61.4%), this is because of the drastic reduction in the metal/contact resistance of the 1 μm-thick Ag electrode. Hence, the use of this embedded microgrid electrode in the construction of an ideal carrier collection path presents an opportunity in the development of highly efficient organicinorganic hybrid solar cells.

Liu et al. fabricated a hybrid silicon nanowire/polymer heterojunction solar cell combined with a polypyrrole-based supercapacitor as shown in **Figure 9**, to harvest solar energy and store it [53]. By efficiency enhancement of the hybrid nanowire solar cells and a dual-functional titanium film serving as a conjunct electrode of the solar cell and supercapacitor, the integrated system is able to yield a total photoelectric conversion to storage efficiency of 10.5%, which is the record value in all the integrated solar energy conversion and storage system. This system may not only serve as a buffer that diminishes the solar power fluctuations from light intensity, but also pave its way toward cost-effective high-efficiency self-charging power unit. Finally, an integrated device based on ultrathin Si substrate is demonstrated to expand its feasibility and potential application in flexible energy conversion and storage devices.

Organic-inorganic hybrid solar cells based on n-type crystalline silicon and poly(3,4 ethylenedioxythiophene)-poly(styrenesulfonate) exhibited promising efficiency along with a low-cost fabrication process (**Figure 10**). Zhang et al. fabricated ultrathin flexible silicon substrates, with a thickness as low as tens of micrometers, which were employed to fabricate hybrid solar cells to reduce the use of silicon material [54]. To improve the light-trapping ability,

**Figure 9.** (a) Schematic of the integrated hybrid device containing a SiNW-based heterojunction solar cell and a polypyrrole-based supercapacitor. (b) SEM image of as-fabricated SiNW. (c) SEM image of SiNW after 2 h of PCl5 treatment. (d) TEM image of a single as-fabricated Si nanowire. (e) TEM image of a single Si nanowire after 10 min of methylation treatment. The scale bars are 500 nm in the insets of (b) and (c) [53].

nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solutionprocessed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge carrier lifetime. As a result, a power conversion efficiency of 9.1% was achieved in the flexible hybrid silicon solar cells, with a substrate thickness as low as ∼14 μm, indicating that interface engineering was essential to improve the hybrid junction quality and photovoltaic characteristics of the hybrid devices.

As shown in **Figure 10(b)**, the leakage current of the TMAH-treated nanostructured silicon devices is much lower than that of the untreated silicon devices, suggesting that the junction quality is improved. Furthermore, the JSC of the hybrid devices based on different surface morphologies is consistent with the external quantum efficiency (EQE) spectra, as shown in **Figure 10(c)**. The device based on ∼14-μm thick planar silicon without TMAH treatment gives a JSC of 18.4 mA cm−2, a VOC of 0.56 V, and an FF of 0.75, achieving a PCE of 7.9%. The previously reported ultrathin planar silicon device with a thickness of 8.6 μm yields a PCE of 5.2% [55]. Here, the reduced JSC for the device based on ∼14-μm thick planar silicon is ascribed to the high light reflection of the planar silicon surfaces, which resulted in poor light absorption compared to that of a thicker device. In comparison with planar devices, nanostructured silicon solar cells exhibit a larger JSC value, indicating that the incident light could be effectively absorbed by nanostructured silicon. The absorption spectra of the planar and nanostructured silicon with or without TMAH treatment are shown in **Figure 10(e)**. According to the absorption spectra, the nanostructured silicon exhibits an obviously better light absorption over a broad spectrum from the visible to the near-infrared range compared to that of planar silicon.

**Figure 10.** Electric-output characteristics of the PEDOT:PSS/Si devices under different surface conditions. (a) J–V curves under illumination at 100 mW cm−2, (b) J−V curves in the dark, and (c) EQE spectra of devices based on silicon substrates with a planar surface (black), an as-prepared nanostructured silicon surface (red), and a TMAH-treated nanostructured silicon surface (blue). (d) Schematic device structure of the flexible PEDOT:PSS/Si hybrid solar cell. (e) Absorption spectra of silicon substrates with a planar surface, an as-prepared nanostructured silicon surface, a TMAH-treated

Hybrid Silicon Nanowires for Solar Cell Applications http://dx.doi.org/10.5772/intechopen.74282 175

nanostructured silicon surface, and upon Yablonovitch limit simulation [54].

nanostructures were built on the thin silicon substrates by a metal-assisted chemical etching method (MACE). However, nanostructured silicon resulted in a large amount of surface-defect states, causing detrimental charge recombination. Here, the surface was smoothed by solutionprocessed chemical treatment to reduce the surface/volume ratio of nanostructured silicon. Surface-charge recombination was dramatically suppressed after surface modification with a chemical, associated with improved minority charge carrier lifetime. As a result, a power conversion efficiency of 9.1% was achieved in the flexible hybrid silicon solar cells, with a substrate thickness as low as ∼14 μm, indicating that interface engineering was essential to improve the

**Figure 9.** (a) Schematic of the integrated hybrid device containing a SiNW-based heterojunction solar cell and a polypyrrole-based supercapacitor. (b) SEM image of as-fabricated SiNW. (c) SEM image of SiNW after 2 h of PCl5 treatment. (d) TEM image of a single as-fabricated Si nanowire. (e) TEM image of a single Si nanowire after 10 min of

As shown in **Figure 10(b)**, the leakage current of the TMAH-treated nanostructured silicon devices is much lower than that of the untreated silicon devices, suggesting that the junction quality is improved. Furthermore, the JSC of the hybrid devices based on different surface morphologies is consistent with the external quantum efficiency (EQE) spectra, as shown in **Figure 10(c)**. The device based on ∼14-μm thick planar silicon without TMAH treatment gives a JSC of 18.4 mA cm−2, a VOC of 0.56 V, and an FF of 0.75, achieving a PCE of 7.9%. The previously reported ultrathin planar silicon device with a thickness of 8.6 μm yields a PCE of 5.2% [55]. Here, the reduced JSC for the device based on ∼14-μm thick planar silicon is ascribed to the high light reflection of the planar silicon surfaces, which resulted in poor light absorption compared to that of a thicker device. In comparison with planar devices, nanostructured silicon solar cells exhibit a larger JSC value, indicating that the incident light could be effectively absorbed by nanostructured silicon. The absorption spectra of the planar and nanostructured silicon with or without TMAH treatment are shown in **Figure 10(e)**. According to the absorption spectra, the nanostructured silicon exhibits an obviously better light absorption over a broad spectrum from the visible to the near-infrared range compared to that of planar silicon.

hybrid junction quality and photovoltaic characteristics of the hybrid devices.

methylation treatment. The scale bars are 500 nm in the insets of (b) and (c) [53].

174 Emerging Solar Energy Materials

**Figure 10.** Electric-output characteristics of the PEDOT:PSS/Si devices under different surface conditions. (a) J–V curves under illumination at 100 mW cm−2, (b) J−V curves in the dark, and (c) EQE spectra of devices based on silicon substrates with a planar surface (black), an as-prepared nanostructured silicon surface (red), and a TMAH-treated nanostructured silicon surface (blue). (d) Schematic device structure of the flexible PEDOT:PSS/Si hybrid solar cell. (e) Absorption spectra of silicon substrates with a planar surface, an as-prepared nanostructured silicon surface, a TMAH-treated nanostructured silicon surface, and upon Yablonovitch limit simulation [54].
