In a p-i-n type thin film amorphous silicon (a-Si) solar cell, optoelectronic properties of the window layer are of significant importance. For high performance, the p-layer of the solar cell should have a high optical bandgap to minimize optical absorption, high dark conductivity and photoconductivity to reduce series resistance, low activation energy to obtain higher open circuit voltage (V_oc), and a narrow valence band tail in order to obtain higher short circuit current density (J_sc). As the wide optical gap window layer ensures less light lost in absorption at the p-layer, higher J_sc can be achieved [1].

Boron doped hydrogenated amorphous silicon carbide (p-type a-SiC:H) films have been mostly used for this purpose. However, the incorporation of carbon in the p-type a-SiC:H films give rise to a void structure, and caused an increased defect density. In contrast, p-type a-SiO_x:H films have a lower defect density, Urbach energy and comparatively high doping efficiency [2]. Therefore, the p-type a-SiO_x:H films provide a wide bandgap and higher photoconductivity compared to other wide bandgap hydrogenated amorphous silicon materials, which could be useful as a window layer for the solar cells [1,3-4].

In this paper, we prepared a-SiO_x:H films by varying N₂O gas as a p-layer window for a-Si solar cells. We also reported the electrical and optical properties of silicon oxide for different N₂O and B₂H₆ gas ratios as well as current-voltage characteristics for a-Si

solar cells with p-type a-SiO_x:H films as a window layer.

P-type a-SiO_x:H films were deposited by the RF PECVD technique on a glass (Eagle 2000 glass, size 5 × 5 cm², 1 mm-thick) and Si wafer with (100) orientation as substrates. The glass substrates were ultrasonically cleaned by dipping in acetone, isopropyl alcohol, and de-ionized water for 5 min. The 0.2 μm film thickness was fixed for each sample at a substrate temperature (T_s) of 175℃ through the glow discharge decomposition of silane (SiH₄), hydrogen (H₂), nitrous oxide (N₂O 90% diluted in He), and diborane (B₂H₆ 99% diluted in H₂) gas mixtures, with gas flow rates of SiH₄=10 sccm, N₂O(+He)=3(+27), 5 sccm, B₂H₆(+H₂)=0.05(+4.95), 0.1 (+9.90) sccm, H₂=95, and 90 sccm. The RF power density of 50 mW/cm², pressure of 0.2 Torr and an electrode separation of 4.0 cm was used for the whole process.

The Fourier transform infrared spectroscopic (FTIR) (Prestige-21 spectrometer, Shimadzu, in 7,800 ~ 350 cm^-1) system was used to estimate the concentration of oxygen (C(O) at.%) within the film measurements on Si samples. The asymmetric stretching vibration of oxygen in the Si-O-Si bond was detected at a wave number of approximately 1,000 cm^-1 and was used to calculate the oxygen content with the help of a calibration constant from reference [30]. The spectroscopic ellipsometry (SE), (VASE®, J.A. Woollam, 240 nm < λ < 1,700 nm) was used in order to measure the sample thickness (d), refractive index (n) and optical absorption coefficient α at an incident angle of 65° in the spectral wavelength range of 240 nm to 1,700 nm. The electrical characteristics (dark conductivity activation energy (E_a)) were studied by using programmable (Keithley 617 electrometer) at 25 to 125℃ temperature range.

We fabricated the a-Si solar cell with p-type a-SiO_x:H window layer while the i and n-type (1% phosphine, PH₃, doped) layers are a-Si:H based. The quantum efficiency (QE) of a-Si solar cell was measured by the QEX7 system (PV measurement Inc.QEX7).

Figure 1 shows the absorption coefficient between 600 and 2,200 cm^-1 attained by FT-IR spectra of p-type a-SiO_x:H films with different oxygen contents. By neglecting reflectance, the absorption coefficient α is directly defined from the relation [5].

where d is film thickness in centimeters and T is the transmittance by measuring the FT-IR spectra. The spectra exhibits absorption peaks corresponding to an Si-O-Si rocking at 650 cm^{- 1}, overlap of a local bond configuration with mixed bending and stretching character for oxygen and hydrogen atoms (H-Si(Si₂O)) at 790 cm^-1, HSi-O₃ bending at 880 cm^-1, Si-O-Si stretching at 1,000 cm^-1, Si-H stretching at 2,000 cm^-1, and Si-H₂ stretching at 2,100 cm^-1[10-14]. The integrated absorption band centered at around 1,000 cm^-1 mainly represents oxygen content by the following relation [6].

where C(O) is the oxygen concentration (units of at.%), A(O) = 0.156 at.%/eV cm^-1 and I(940-1080) is the integrated absorption. Considering this mathematical relation, the oxygen content depends critically on N₂O and B₂H₆ gases ratio. With increasing N₂O gas ratio (30 and 50%) and decreasing B₂H₆ gas ratio (0.5 and 1%), the oxygen content increased from 24.4 to 36.1 at.%. The peak position was shown to shift slightly to higher wave numbers with increasing oxygen content. The presence of more electronegative neighboring atoms shifts the peak toward higher wave numbers and this shift is due to the reduction of the bond length caused by the transfer of valence electrons to more electronegative neighboring atoms [6]. The properties of p-type a-SiO_x:H films such as I(940-1080), oxygen content, E_opt and refractive index are summarized in Table 1.

In Fig. 2 the absorption coefficient (α) measured by spectroscopic ellipsometry (SE) of p-type a-SiO_x:H films for oxygen content between 24.4 and 36.1 at.% displayed. The absorption area is divided into three regions - the defect region (0.8~1.6 eV), the band tail region (1.6~2.0 eV) and the band to band absorption region (2.0 eV~). In regards to the defect region, absorption occurred by defect states located in the middle of the gap for the defect region [7]. We can deduce defect density (N_D) from the integrated absorption in the defect region by using the below mathematical expression.

where N_D is the defect density, α_means is the measured defect absorption, c is the velocity of light, n is the refractive index, m_e is the electron mass and E_max, of approximately 1.6 eV, after subtracting the Urbach edge contribution. For E_opt = 1.91-2.03 eV, the N_D increased from 1.91 × 10¹⁸ cm^-3 to 5.55 × 10¹⁸ cm^-3 (see inset of Fig. 2). In general, N_D increased with increasing oxygen content as well as E_opt at the same B₂H₆ gas ratio in the p-tpye a-SiO_x:H [8]. However, our deposition condition of gas ratio is a variable factor for both the N₂O and B₂H₆ gas ratio. For this case, the absorption in the defect region increased with increasing the N₂O and B₂H₆ gas ratio. Therefore the value of N_D is highest at the condition regarding N₂O of 50% and the B₂H₆ 1% gas ratio. Therefore, the variation of these gas ratios will be acting as dopants for the role of increasing conductivity, E_opt and defect.

Figure 3 contains the room-temperature dark conductivity (σ_d), photo conductivity (σ_ph) [(a)] and activation energy [(b), derived from the temperature dependence of σ_d] for p-type a-SiO_x:H films as a function of oxygen content. It provides a correlation between the effect of dopant- B₂H₆ and oxygen and conductivity. The value of σ_d and σ_ph increased with increasing the B₂H₆ ratio at the same oxygen ratio. The value of σ_d and σ_ph also increased with the rising N₂O ratio at the same B₂H₆ ratio. The p-type a-SiO_x:H film with N₂O 50% and B₂H₆ 1% has σ_d and σ_ph of 9.79×10^-7 S/cm and 1.71×10^-6 S/cm. From the conductivity between the variable B₂H₆ and the N₂O ratio, we can deduce that the B₂H₆ ratio is a superior factor. One possible reason for this may be that boron doping can compensate the donor like states created by oxygen in boron doped a-SiO_x films [8]. As the oxygen atoms with three fold coordination have extra electrons so the oxygen-induced donor like states move the Fermi-level causing the increase of σ_d and σ_ph. The activation energy is the energy distance between EF and the valence band edges as shown in Fig. 3(b). The activation energy is 0.49-0.57 eV for the p-type a-SiO_x:H films with E_opt=2.03~1.91 eV. The activation energy increases with the in-

crease of B₂H₆ and N₂O ratios. The shifting of Fermi level towards the valence band due to the incorporation of oxygen and boron may be the reason for the increase of activation energy [9,10].

As a result the E_opt decreased with the increase of the B₂H₆ ratio. Figure 4 also represents the variation of the refractive index (n) with different oxygen content. The refractive index is expected to depend on the density of films and their electronic band structure [10]. We also observed that the refractive index decreases from 3.62 to 3.18 at the beginning of O-incorporation into the p-type a-SiO_x:H film network. The decrease in regards to the refractive index may be due to an increase of voids and disorder in films through the incorporation of oxygen [14].

For the high conversion efficiency of a-Si solar cells, the player should have high E_opt, low E_a, high conductivity, and low absorption. We further apply p-type a-SiO_x:H films (N₂O 30% and B₂H₆ 0.5%) and (N₂O 30% and B₂H₆ 1%) to a-Si solar cells.

Figure 5 shows the current - voltage (I-V) characteristics of the a-Si solar cells with 0.5 and 1% of B₂H₆ ratios. The cells with V_oc = 0.853 and 0.842 V, J_sc = 13.87 and 15.13 mA/cm², fill factor = 0.664 and 0.656 and a conversion efficiency of 7.54 and 8.36% have been obtained. The V_oc and FF slightly decreased while J_sc increases with higher a B₂H₆ ratio. The higher E_opt of the B₂H₆ 0.5% condition compared to the B₂H₆ 1% condition will result in a higher V_oc. The higher E_opt has a higher built-in potential energy in the solar cell. The lower J_sc and a higher FF coincides with the tendency of theoretical efficiency limits for homojunction solar cells [15].

The quantum efficiency of a-Si solar cells for different p-type window materials is presented in Fig. 6. We used three different devices with the following (p-type a-SiO_x:H with E_opt of 1.75 eV), (a-SiO_x:H(p) with E_opt of 1.94eV) and (p-type a-SiO_x:H with E_opt of 1.91 eV) conditions. The effect of E_opt has been represented in the short wavelength region and by increasing E_opt, whereby the intensities of the spectral response increases.

We prepared p-type a-SiO_x:H films by optimizing the optical bandgap, electrical conductivity, and activation energy with the PECVD process of SiH₄, H₂, N₂O and the B₂H₆ gas mixture. The E_opt was found to increase with an increase of the N₂O gas ratio through the incorporation of oxygen but decreased with an increase of the B₂H₆ ratio. There is an increase in σ_d, σ_ph and Ea with increasing both the B₂H₆ and N₂O ratio. Here, we selected two ptype a-SiO_x:H films with good optical bandgap, dark conductivity and activation energy for the applications regarding a-Si solar cells. We employed these two p-type a-SiO_x:H films for the a-Si silicon solar cell. The increase in J_sc of the device with B₂H₆ (1%) as compared to B₂H₆ (0.5%) was related to E_opt. A conversion efficiency of 7.54 and 8.56% at the B₂H₆ (0.5 and 1%) ratio was achieved demonstrating the effectiveness of the silicon oxide (SiO_x) material.