Long exposure to light decreases the photoconductivity and dark conductivity of some samples of hydrogenated amorphous silicon (a‐Si : H). Annealing above ∼150 °C reverses the process. The effect occurs in the bulk of the films, and is associated with changes in density or occupation of deep gap states. High concentrations of P, B, or As quench the effect. Possible models involving hydrogen bond reorientation at a localized defect or electron‐charge transfer between defects are discussed. An example is shown where these conductivity changes do not affect the efficiency of an a‐Si : H solar cell.

Optically induced conductivity changes in discharge‐produced hydrogenated amorphous silicon

The correlation between the density of dangling-bond defects and the slope of the Urbach tail in hydrogenated amorphous silicon is examined. It is shown that this correlation can be explained quantitatively by a spontaneous decay of the weakest bonding orbitals into non-bonding defects during deposition or annealing of a sample and that the same correlation holds for all types of disorder affecting the slope of the Urbach edge. The temperature dependence of the defect density as well as the creation of metastable defects are discussed, and quantitative expressions are derived which can be used to estimate the quality and the stability of a given sample on the basis of the slope of its Urbach tail alone. Possible ways for future improvement of the material are indicated.

The defect density in amorphous silicon

The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed. >

Amorphous silicon solar cells

Quantitative depth profiling in surface analysis: A review

Hydrogen diffusion in doped and compensated a-Si:H has been measured by secondary-ion mass spectrometry profiling in the temperature range 155-300°C. Doping reduces the activation energy and enhances the diffusion coefficient by up to three orders of magnitude at 200°C, and a correlation between the diffusion coefficient and the dangling-bond density is found. An analysis of three different diffusion models indicates that the breaking of weak Si – Si bonds by hydrogen may be an important process. The relation between the diffusion results and the thermal equilibration of the electronic structure is discussed. The hydrogen diffusion coefficient in a-Si:H has been measured over the temperature range 155-300°C, with particular emphasis on the effects of doping and compensation. In all cases D H is thermally activated with an energy 1[sdot]2-1[sdot]5 eV. The diffusion coefficient decreases slowly with time which is attributed to the disorder-induced variation in site energies. We find that D H is gre...

Hydrogen diffusion in amorphous silicon

Secondary ion mass spectroscopy (SIMS) has been used to measure the diffusion of deuterium in hydrogenated amorphous silicon. For a film deposited in a dc glow discharge in SiH4 at a substrate temperature of 315 °C, the diffusion data fits D (T) =1.17×10−2 exp(−1.53 eV/kT) cm2/s. This result implies that degradation of these films due to hydrogen out‐diffusion at 100 °C will not be significant until after more than 104 years.

D. E. Carlson

Papers

A SIMS analysis of deuterium diffusion in hydrogenated amorphous silicon