**3.13 Photoelectron yield spectroscopy of the solar cell heterojunction**

The photoemission yield (PYS) of electrons is a function of the electronic properties of the interface. As shown in **Figure 18**, each PYS shows two thresholds near 5.0 ± 0.2 and 4.2 ± 0.2 eV. The higher energy band, i.e., near the 5.0 ± 0.2, corresponds to the valence band, while the lower band 4.2 ± 0.2 eV corresponds to the defects in the band gap [50].

#### **Figure 18.**

*Photoelectron yield Y(hν) spectral and spectral density of states of SiO2*▬*Si NW, H*▬*Si NW, and CH3*▬*Si NW.*

*Heterojunction-Based Hybrid Silicon Nanowires Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.84794*

To compare the quality of the surface, we normalized the valence emission at 0.76 eV below the valence band maximum where they should be strongly dominated by the valence band emission only. Therefore, all the three samples show identical PYS. To this end, we can clearly see that the SiO2▬Si NWs show the highest defect density in the bandgap, while the CH3▬Si NWs show the lowest defect density.

## **3.14 I-V curves of solar cells**

The three samples were assembled at photovoltaic cells together with polymer (PEDOT:PSS). The polymer is considered as a hole conductor, while the Si NW plays the role of light absorber and electron conductor [9]. In this cell configuration, the photo-generated electron-hole pairs are separated at a heterojunction as shown in **Figure 19**.

Four main advantages for this configuration [51, 52]:

i.Efficient light absorption

ii.Short diffusion distance of carriers

iii.Air-stable and robust polymer, PEDOT:PSS, as an efficient hole conductor [9]

iv.Utilizing only 1% of the Si used in other thin-film cells

**Figure 20** shows the current-voltage (*I-V*) characteristics of CH3▬Si NW/ PEDOT:PSS and SiO2▬Si NW/PEDOT:PSS solar cells under AM1.5 illumination. The SiO2▬Si NW/PEDOT:PSS shows low performance: short circuit current (*Jsc*) of 1.6 mA/cm<sup>2</sup> , an open circuit voltage (*Voc*) of 320 mV, a fill factor (*FF*) of 0.53, and a conversion efficiency (*μ*) of 0.28%. However, in the CH3▬Si NW/PEDOT:PSS, the devices show superior performance relative to the CH3▬Si NW/PEDOT:PSS and exhibit improved performance with *Jsc*, *Voc*, *FF*, and *μ* magnitudes of 7.0 mA/cm2 , 399 mV, 0.44, and 1.2%, respectively.

Both samples show low values due to the high contact resistances (Rs 300 Ω). However, the comparative increase in efficiency (by about a factor of four) upon methylation proves that this kind of surface functionalization has very promising prospective.

#### **Figure 19.** *Schematic diagram shows the charge separation near the Si/polymer interface.*

**Figure 20.**

*(a) Tilted view of the heterojunction Si NW/PEDOT:PSS. (b) J–V characteristic under AM1.5 illuminations of the radial heterojunction solar cells from CH3*▬*NWs to SiO2*▬*NWs. Inset: schematic view of solar cell device structure.*

The improved performance of the CH3▬Si NWs is attributed to the removal of the defects on the surface; therefore, the charges can transfer with low recombination rate: hole to the polymer and electron to Si. In addition, efficient charge coupling can improve the performance which will improve the charge transfer causing to an increase in *Voc*. According to the Shockley diode equation, *Voc* = *kBT/q*. ln(*Jsc*/*J0*), where *J0* is the saturation current. It should be mentioned that observed gain in the *Voc* gain cannot be explained by the increase of the current alone. Assuming a similar *J0*, the increase of *Jsc* would lead to a *Voc* gain of 0.037 V. However, we observed a gain of ∆*Voc* = 0.079 V. This can be attributed to the grafting effect which reduces the surface recombinations (as measured by PY) and/or a favorable barrier formation (surface dipole) [50–58].
