Microcrystalline silicon oxides for silicon-based solar cells: impact of the O/Si ratio on the electronic structure

Abstract

Hydrogenated microcrystalline silicon oxide (μc-SiOx:H) layers are one alternative approach to ensure sufficient interlayer charge transport while maintaining high transparency and good passivation in Si-based solar cells. We have used a combination of complementary x-ray and electron spectroscopies to study the chemical and electronic structure of the (μc-SiOx:H) material system. With these techniques, we monitor the transition from a purely Si-based crystalline bonding network to a silicon oxide dominated environment, coinciding with a significant decrease of the material’s conductivity. Most Si-based solar cell structures contain emitter/contact/passivation layers. Ideally, these layers fulfill their desired task (i.e., induce a sufficiently high internal electric field, ensure a good electric contact, and passivate the interfaces of the absorber) without absorbing light. Usually this leads to a trade-off in which a higher transparency can only be realized at the expense of the layer’s ability to properly fulfill its task. One alternative approach is to use hydrogenated microcrystalline silicon oxide (μc-SiOx:H), a mixture of microcrystalline silicon and amorphous silicon (sub)oxide. The crystalline Si regions allow charge transport, while the oxide matrix maintains a high transparency. To date, it is still unclear how in detail the oxygen content influences the electronic structure of the μc-SiOx:H mixed phase material. To address this question, we have studied the chemical and electronic structure of the μc-SiOx:H (0 ≤ x = O/Si ≤1) system with a combination of complementary x-ray and electron spectroscopies. The different surface sensitivities of the employed techniques help to reduce the impact of surface oxides on the spectral interpretation. For all samples, we find the valence band maximum to be located at a similar energy with respect to the Fermi energy. However, for x > 0.5, we observe a pronounced decrease of Si 3s – Si 3p hybridization in favor of Si 3p – O 2p hybridization in the upper valence band. This coincides with a significant increase of the material’s resistivity, possibly indicating the breakdown of the conducting crystalline Si network. Silicon oxide layers with a thickness of several hundred nanometres were deposited in a PECVD (plasma-enhanced chemical vapor deposition) multi chamber system using an excitation frequency of 13.56 MHz with a plasma power density of 0.3 W/cm2. Glass (Corning type Eagle) and mono-crystalline silicon wafer substrates were coated in the same run at a substrate temperature of 185°C. The deposition pressure was 4 mbar and the substrate-electrode distance 20 mm. Mixtures of silane (SiH4), 1% TMB (B(CH3)3) diluted in helium, hydrogen (H2), and carbon dioxide (CO2) gases were used at flow rates of 1.25 - 0.18/0.32/500/0 – 1.07) sccm (standard cubic centimeters per minute) for the deposition of μc-SiOx:H(B) layers. By changing the CO2/SiH4 gas flow rate ratio from 0 to 6, μc-SiOx:H(B) layers with a composition of 0 ≤ x = O/Si ≤ 1 were prepared using a constant sum of SiH4 and CO2. The TMB flow and the H2 flow were kept constant within the series. For more details see Ref. [1]. The oxygen content in the films was determined using Rutherford Backscattering Spectroscopy (RBS). With RBS, the area-related atomic density of oxygen and silicon can be determined (± 2% [2]), and thus x can be calculated. This quantity considers only the number of silicon / oxygen atoms and not the number of atoms of other elements, such as hydrogen, which is also incorporated to a considerable extent: up to 20% in μc-SiOx:H (measured using the hydrogen effusion method). To avoid charging effects, the measurements were performed on films deposited on a substrate of mono-crystalline silicon wafers. The electrical conductivity was measured in the planar direction of the film in a vacuum cryostat, using voltages from - 100 V to + 100 V. For that two co-planar Ag contacts were evaporated on the film with a gap of 0.5 mm 5 mm