Note that in the wavelength region from 500 to 580 nm, the absorption curve of P3HT/Si NWA (T = 40 and 80 nm) overlaps with that of bare Si NWA. This is due to the fact that the bare Si NWA exhibits the absorptance close to 1 in this wavelength region. Thus, although the absorptivity is increased as the P3HTs are coated on the surface of NWA, the absorption curves do not exhibit obvious enhancement. When the incident wavelength is above 650 nm, P3HT becomes transparent and only Si absorbs incident light.
At this region, despite the size of photoactive Si NW is fixed, a certain amount of absorption enhancement can still be observed as the thickness of organic coating is increased. For example, at the wavelength of 700 nm, we note that the absorption at T = 80 nm has a Blasticidin S order factor of 1.81 higher than the case of the uncoated NWs. This can be understood Selleckchem GDC-0068 by electrostatic approximation. The absorption in Si NW is proportional to the factor of |E core / E inc|2, where E core and E inc are the electric field intensity in the core and incident light of Si NW, respectively . In the absence of the organic coating, |E core / E inc|2 = |2ϵ ext
/ (ϵ ext + ϵ core)|2 = 0.0169, where ϵ ext = 1 is the dielectric function of the vacuum exterior to AG-881 purchase the NW, and ϵ core ≈ 14.34 + 0.0985i is the dielectric function (for λ = 700 nm) of the Si NW. When an organic coating is added, |E core / E inc|2 = |2ϵ ext / (ϵ ext + ϵ coat)|2|2ϵ coat / (ϵ coat + ϵ core)|2 = 0.030, where ϵ coat = 3.75 is the dielectric function (for λ = 700 nm) of P3HT. About 1.78 times enhancement can be obtained at organic coating T = 80 nm than that of uncoated NWs, which is close to the absorptance enhancement at this wavelength Sclareol (as shown in Figure 2c). Obviously, above the cutoff of P3HT, the organic coating can serve as a non-absorbing dielectric shell, which drastically increased the absorption in vertical semiconductor NWs. Moreover, at the wavelength larger than
650 nm, the extinction coefficient of silicon is small and interference effects exist, resulting in the oscillation of reflectance and transmittance . Figure 2 Optical characteristics of the hybrid solar cells with various P3HT coating thicknesses. (a) Reflection. (b) Transmission. (c) Absorption. In order to understand the propagation of light in the hybrid solar cells, we simulated the electrical field intensity and calculated the optical generation rates within the arrays from where ϵ″ is the imaginary part of the complex permittivity and E is the electric field . We give the optical generation rates for conformal coating hybrid structure with 80-nm P3HT at three typical wavelengths of 400, 600, and 700 nm. The optical generation rates of the uncoated Si NWs are used as comparison.