Study of an Amorphous Silicon Oxide Buffer Layer for p-Type Microcrystalline Silicon Oxide/n-Type Crystalline Silicon Heterojunction Solar Cells and Their Temperature Dependence

2.1. Deposition of i-a-SiO:H Films

The i-a-SiO:H films were prepared on soda-lime glass substrates and float-zone (FZ) silicon wafers (100) using n-type 1–5 Ωcm double side polished wafers 260–300 μm thick. Prior to the deposition, the wafers were cleaned (ethanol/acetone/ethanol: 15/15/15 min) and dipped into diluted 5% hydrofluoric acid (HF) to remove any native oxide before depositing the i-a-SiO:H films. The i-a-SiO:H films were deposited via a 60 MHz very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) in a multichamber with a parallel plate configuration. We used silane (SiH₄), hydrogen (H₂), and carbon dioxide (CO₂) as the reactant gases. The substrate temperature, plasma power density, and deposition pressure were held at 180°C, 20 mW/cm² and 300 mTorr, respectively. The deposition conditions are summarized in Table 1. Afterwards, the effects of the CO₂/SiH₄ ratio on the film properties were investigated. The i-a-SiO:H film passivation quality was characterized using the quasi-steady-state photoconductance (QSSPC) lifetime [15, 16]. The lifetime samples were deposited as i-a-SiO:H films on both sides of the n-c-Si wafer, and their passivation quality was measured in terms of the effective lifetime (τ_eff). The optical properties and thickness of these i-a-SiO:H films coated on soda-lime glass substrates were evaluated via spectroscopic ellipsometry (SE).

2.2. Fabrication of Heterojunction Solar Cells

3.1. Properties of the i-a-SiO:H Films

The effects the CO₂/SiH₄ ratio had on the surface passivation quality and optical properties of the i-a-SiO:H films were investigated. CO₂ was used as O source gas during the i-a-SiO:H film deposition. The samples were prepared by increasing the CO₂/SiH₄ ratios from 0.0 to 0.50 with a thickness of approximately 50 nm. The samples exhibited an i-a-SiO:H (CO₂/SiH₄: 0.0–0.50)/n-c-Si/i-a-SiO:H (CO₂/SiH₄: 0.17) structure. The CO₂/SiH₄ ratio affected the τ_eff of n-c-Si wafers passivated with i-a-SiO:H films as shown in Figure 2. The τ_eff increased with increasing CO₂/SiH₄ ratios. For a CO₂/SiH₄ ratio of 0.0, the sample showed a low τ_eff of 51 μs because the i-a-Si:H layer grew epitaxial silicon on the crystalline substrate. The epitaxial interface is a defect and deteriorates the passivation quality. However, the τ_eff increased rapidly from 232 μs to 2500 μs upon increasing the CO₂/SiH₄ ratio from 0.17 to 0.50. Therefore, increasing the CO₂/SiH₄ ratio can suppress the epitaxial silicon growth. Meanwhile, the implied V_oc increased from 583 mV to 708 mV with increasing CO₂/SiH₄ ratio. In addition, the optical bandgap of the i-a-SiO:H films increased gradually with increasing CO₂/SiH₄ ratio. The optical bandgap of a conventional i-a-Si:H film with a CO₂/SiH₄ ratio of 0.0 was 1.72 eV. Increasing the CO₂/SiH₄ ratio from 0.17 to 0.50 increased the optical bandgap of the i-a-SiO:H films from 1.81 eV to 1.95 eV.

3.2. Fabrication of Heterojunction Solar Cells

The effect of the CO₂/SiH₄ ratio for the front i-a-SiO:H buffer layer on the c-Si-HJ solar cell performance was investigated for various CO₂/SiH₄ ratios. First, the front i-a-SiO:H buffer layer thickness in the c-Si-HJ solar cell was optimized. The front i-a-SiO:H buffer layer thickness was varied from 2 nm to 8 nm. Figure 3 shows the PV parameters and τ_eff as a function of the i-a-SiO:H layer thickness. Increasing the front i-a-SiO:H buffer layer thickness increased the V_oc from 655 mV to 691 mV due to the improved surface passivation. The τ_eff increased from 586 μs to 1125 μs with increasing front buffer thicknesses. A high V_oc of 692 mV and solar cell efficiency of 18.4% were obtained using the optimized thickness of 6 nm. A front buffer layer thickness below 6 nm decreased the V_oc of the solar cell. A thicker front buffer layer improved the passivation quality but reduced the FF of the solar cell due to the increased series resistance.

Next, the effect the CO₂/SiH₄ ratio of the front i-a-SiO:H buffer layer had on the solar cell performance was studied. The front i-a-SiO:H buffer layer CO₂/SiH₄ ratio was varied from 0.0 to 0.50 while holding the thickness at 6 nm. Figure 4 shows the PV parameters and τ_eff as a function of the CO₂/SiH₄ ratio for the front i-a-SiO:H buffer layer. The solar cells exhibited a low V_oc of 626 mV at a CO₂/SiH₄ ratio of 0.00 due to epitaxial silicon growth [17, 18]. The V_oc then increased to 699 mV at a CO₂/SiH₄ ratio of 0.17 and reduced to 683 mV at a CO₂/SiH₄ ratio of 0.50. The τ_eff was 1610 μs for a CO₂/SiH₄ ratio of 0.17 and then decreased to 357 μs for a CO₂/SiH₄ ratio of 0.50. However, the τ_eff tended to increase with increasing CO₂/SiH₄ ratio as shown in Figure 2. For the solar cell, increasing the CO₂/SiH₄ ratio in the front i-a-SiO:H buffer layer decreased the V_oc and FF because of the increased series resistance. Figure 5 shows the I-V characteristics of c-Si-HJ solar cells with CO₂/SiH₄ ratio in front i-a-SiO:H buffer layer. Increasing the CO₂/SiH₄ ratio decreased the V_oc and FF of I-V curves and showed S-shape influenced from the increased series resistance with the CO₂/SiH₄ ratio of 0.50. These results indicate controlling the CO₂/SiH₄ ratio can improve the surface passivation quality and solar cell performance. The spectral response of the solar cells improved with increasing CO₂/SiH₄ ratios as shown in Figure 6 because of the increased optical bandgap of the i-a-SiO:H layer.

The effects the CO₂/SiH₄ ratio of the i-a-SiO:H front buffer layer had on the photovoltaic parameters at various operating temperatures were investigated. Figure 7 presents the normalized solar cell parameters for c-Si-HJ solar cells containing i-a-SiO:H layers with various CO₂/SiH₄ ratios measured in the initial state as a function of the operating temperature. The J_sc gradually increased with increasing operating temperature due to the increase in the diffusion length of the minority carriers. The FF and solar cell efficiency decreased with increasing operating temperature due to the significantly reduced V_oc. The V_oc decreased with increasing operating temperature because of an increase in the reverse saturation current. The normalized TC for the V_oc of a solar cell with a CO₂/SiH₄ ratio of 0, 0.08, and 0.17 in i-a-SiO:H were −0.318%/°C, −0.314%/°C, and −0.290%/°C, respectively. Therefore, the normalized TC for η of the solar cell with CO₂/SiH₄ ratios of 0, 0.08, and 0.17 in i-a-SiO:H were −0.312%/°C, −0.306%/°C, and −0.301%/°C, respectively. The experimental results indicate that the i-a-SiO:H film is a promising buffer layer material for c-Si-HJ solar cells operating at high temperatures.