Masanobu Azuma

Fabrication of Solar Cells Having SiH2Cl2 Based I-Layer Materials

Intrinsic amorphous silicon films were fabricated using electron cyclotron resonance (ECR) assisted chemical vapor deposition and SiH2Cl2 source gas. Intrinsic layers were used for material characterization and also for the absorber layer of solar cells. The highly reducing atmosphere produced by the high energy ECR hydrogen plasma used to deposit these intrinsic films caused some degradation and/or etching of the previously deposited solar cell doped layers as well as the SnO2-coated glass substrates. The p-layer etching rates were greater than those of the n-layer when these layers were exposed to ECR hydrogen plasma. Optimum photovoltaic performance was achieved when an optimized n/i interfacial buffer layer was used for a solar cell deposited in the n-i-p sequence. Better solar cell performances were obtained when the solar cells were measured under n-side illumination. In part, the buffer layer optimization involved careful consideration of band gap matching to the relatively wide band gap (1.85 eV) intrinsic layers prepared from SiH2Cl2. Further performance gains were possible through transparent conductive oxide/substrate optimization. For example, the open circuit voltage (Voc) increased to ~0.89 V when gallium-doped zinc oxide/glass substrates were used compared to ~0.63 V when tin oxide/glass substrates were used. Interface recombination and minority carrier diffusion lengths were probed by n- and p-side illuminated quantum efficiency measurement and analysis. The electron and hole µτ products were estimated to be 4.4×10-8 cm2/V and 3.5×10-8 cm2/V, respectively. The stability of the solar cells was also examined.

Relation Between Defect Density and Local Structures of a-Si:H

High quality a-Si:H thin films with varied optical gaps in the range from 1.55 to 2.1 eV were fabricated by various methods, i.e., the standard RF glow discharge of silane, “Chemical Annealing” and ECR-H-plasma from SiCl 2 H 2 under in situ monitoring with an ellipsome try. Despite marked differences in the local structure, all these films showed low defect density as low as (3–5) × 10 15 cm 3 . In addition, the stability for light soaking was improved markedly for the films made by promoting intensively structural relaxation with atomic hydrogen.

Structure and Carrier-Transport in a-Si:H, a-Si(Ge): H Films Prepared by Chemical Annealing

The stability and opto-electric properties of a-Si:H films fabricated by “chemical annealing (CA)” with excited states of He (He*) were systematically investigated. The films made by the CA mode showed a high photoconductivity due to a low level of defect density, 2×10 15 cm -3 , and improvement in the stability against light soaking. A marked improvement was also found in the hole-transport in films fabricated by the CA. The structural relaxation on the growing surface was also enhanced by addition of a small amount of Ge.

Propagation Reactions and Structures of Hydrogenated Amorphous Silicon Depending on Plasma Sources with Different Excitation Frequencies.

Chemical reactions related to Si-network formation and its structure using SiHnClm (n+m3) as the precursor decomposed by various plasma sources such as modulated radio frequency (RF), very high frequency (VHF) and electron cyclotron resonance (ECR) plasmas are investigated. The characteristics of each plasmas estimated by diagnostic study are systematically varied depending on the excitation frequency of the plasma source. We confirmed that atomic hydrogen is an effective agent to promote the propagation reaction of the Si network due to its strong reactivity under SiHmCln flow. We also report that high-quality hydrogenated amorphous silicon films with defect density as low as 2×1015 cm-3 are successfully fabricated by ECR hydrogen plasma at growth rates higher than 10 A/s.