An ultraviolet-visible-near infrared spectrometer was used to explore the optical behavior of a-Si:H nanocone. Figure depicts the transmittance of a-Si:H planar and nanocone layer. At the wavelength less than 500 nm, the low transmittance [T0
] indicates the high absorption of a-Si:H. The surface reflectance will affect the amount of light absorption; therefore, the reflectance [R] spectrum was measured and depicted in Figure . For both planar and nanocone a-Si:H, they exhibit similar transmittance and reflectance. Consequently, the absorbance [A] (%), which is calculated through A
and was depicted in Figure , has similar behavior for planar and nanocone structure. There is no light-trapping behavior in the a-Si:H random nanocone with low aspect ratio [14
]. By taking the reflectance [R] and transmittance [T0
] spectra into account, the absorption coefficient can be calculated by the following equation:
Transmittance (a), reflectance (b), absorbance (c) and absorption coefficient (d). Percentage values are for planar (black broken line) and nanocone (red line) a-Si:H layers.
, in which T is the absolute transmission, α is the absorption coefficient and d is the thickness of the intrinsic-a-Si:H layer. Figure depicts the absorption coefficient of planar and nanocone a-Si:H layer. The absorption coefficient for the a-Si:H nanocone is approximately 5 × 105/cm at 500 nm which is slightly higher than the planar structure. The effect of the difference in total amount of light harvesting between planar and nanocone solar cell while exploring the carrier collection efficiency can be minimized by light illumination through the glass side.
The photovoltaic properties of a-Si:H solar cell was measured by solar simulator under air mass 1.5 G condition. Figure shows the photocurrent density-voltage behavior. The planar pin a-Si:H solar cell exhibit short-circuit current density [Jsc] of 5.0 mA/cm2
and power conversion efficiency [PCE] of 1.43%. The detailed photovoltaic properties were listed in Table . With surface nanocone structure, the Jsc increases to 5.7 mA/cm2
which is 14% enhanced. Additionally, the PCE also increases to 1.77% which is 24% enhanced. The short transport path for the electron in the a-Si:H nanocone contributes the additional 0.7 mA/cm2
]. However, the native oxide and defects on the a-Si:H nanocone after RIE etching may either restrict part of photocurrent transport or act as recombination centers that increase the series resistance as high as 160 Ω·cm2
. The H2
plasma was used to remove the native oxide and passivate the defects. After treating the a-Si:H nanocone surface by H2
plasma, the native oxide was removed and the surface defects were passivated [15
]. This could be understood by the dark current as shown in Figure . The reverse leakage current density was largely reduced after plasma treatment. The series resistance also reduces to around 60 Ω·cm2
. Additionally, the Jsc further increases to 5.8 mA/cm2
. The PCE is 2.0%, approximately 40% enhanced over planar solar cell. Another way to reduce the interface state defect density is to cover the surface by a layer of intrinsic a-Si:H. The 10-nm intrinsic a-Si:H was grown after H2
-plasma treatment. With this additional layer, Jsc further increases to 5.9 mA/cm2
. The PCE increases to 2.2%, approximately 54% enhanced over planar solar cell. Further analysis of the nanocone solar cell by input photon-to-electron conversion efficiency [IPCE] spectrum could investigate the photoresponse at each wavelength. Figure shows the IPCE results. At short wavelength, the surface was passivated. The higher electrical field in the nanocone structure [17
] accelerates the carrier transport that reduces recombination. As a consequence, there is an increase in the IPCE. At long wavelength, the better carrier collection efficiency of the nanocone structure ensures the higher IPCE as compared to the recent publication on the a-Si PIN solar cell with nanodome surface which exhibits efficient light management [18
], achieving extensive solar energy conversion efficiency. This work mainly focuses on the transport part. Without light-trapping efficient in nanocone structure, the photocurrent enhancement supports the assumption of efficient carrier collection by the nearly perpendicular light absorption and carrier transportation in nanostructure.
Figure 4 The current density-voltage characteristics. Solar cells with 200-nm thick planar-a-Si:H film, 200-nm thick a-Si:H nanocone, 200-nm thick a-Si:H nanocone with 10-min H2 plasma treatment, and 200-nm thick a-Si:H nanocone and additional 10-nm a-SiH layer (more ...)
The detailed photovoltaic properties of a-Si:H nanocone solar cell
Figure 5 The input photon-to-current conversion efficiency [IPCE] spectrum current density-voltage characteristics. IPCE spectrum current density-voltage characteristics of solar cells with 200-nm thick planar-a-Si:H film, 200-nm thick a-Si:H nanocone, 200-nm (more ...)