David E. Carlson

Investigation of the hydrogen and impurity contents of amorphous silicon by secondary ion mass spectrometry

Abstract Secondary ion mass spectrometry (SIMS) was used to characterize glow-discharge-deposited hydrogenated amorphous silicon (a-Si:H) as a function of depth. These depth profiles were used to determine hydrogen concentration as well as levels of impurities (carbon, oxygen and nitrogen) unintentionally grown into the films. Concentration profiles of dopants are also shown. Whenever possible, SIMS data are correlated with electrical measurements.

Factors influencing the efficiency of amorphous silicon solar cells

Abstract This paper briefly reviews the status of amorphous silicon solar cells and considers the factors that limit the performance. While poor minority carrier transport is the major factor limiting the efficiency of amorphous silicon solar cells, other properties may be altered so as to achieve efficiencies in excess of 10%. Several specific recommendations are made for improving the performance.

Light-induced effects in amorphous silicon material and devices☆

Abstract We have studied the stability of hydrogenated amorphous silicon (a-Si:H) using several new techniques such as diffusion length measurements, photovoltage profiling and IR absorption via multiple internal reflections. We find that prolonged illumination generally causes a decrease in the diffusion length and an increase in the space charge density of undoped a-Si:H films.

Solar cells: Present status and future prospects

Abstract Some market estimates indicate that a-Si:H based solar cells now account for the majority of sales. Although much of this market is consumer electronics, other conventional power markets may be expected to grow as the performance, reliability and cost of the devices improve. Prospects in each of these areas appear excellent. Although we believe efficiencies of 14% are realistic for single junction devices, multijunction cells will have better efficiencies and stabilities. The high performance in these devices will require improvements in the properties of compatible wide and low bandgap materials. Two of the most promising candidates are a-SiGe and a-SiC:H alloys. In this paper we examine recent developments in these materials and devices.

Hydrogenated amorphous silicon films in palladium Schottky barrier cells

Abstract The electrical and chemical properties of palladium Schottky barrier cells were investigated for hydrogenated amorphous silicon (a-Si:H) films made in d.c. proximity and d.c. cathodic glow discharges in silane. Water vapor adversely affects the performance of palladium Schottky barrier cells made with either type of a-Si:H film. For palladium contacts on proximity films, exposure to humidity causes oxygen and hydrogen to accumulate in the vicinity of the Pd-(a-Si:H) interface. However, for palladium contacts on cathodic films the presence of water vapor promotes a chemical reaction, even at room temperature, to form a new material consisting of palladium, silicon, oxygen and hydrogen.

Progress toward high efficiency multijunction cells and submodules at Solarex

Abstract Conversion efficiencies as high as 11.95% have been obtained in small-area, single-junction amorphous silicon (a-Si) solar cells and as high as 10.51% in a-SiC/a-SiGe stacked-junction cells. These high performance cells use a combination of textured tin oxide as a front contact and In2O3:Sn/Ag as a back contact to ensure efficient light trapping. For large-area submodules (1000 cm2), active-area efficiencies of 8.7% have been obtained in single-junction structures and 7.03% in stacked-junction structures.

Measurements of the diffusion length in a-Si:H for varying deposition and post-deposition conditions

Abstract The diffusion length and space charge width of undoped a-Si:H has been measured by means of the constant surface photovoltage technique for films grown at different substrate temperatures and in the presence of various gaseous impurities.

Improving the performance of amorphous silicon photovoltaic modules

Abstract High performance amorphous silicon solar cells require high quality undoped hydrogenated amorphous silicon (diffusion lengths greater than or equal to 0.5 μm), a conductive p layer or n layer window ( σ p > 10 −6 ω −1 cm −1 , ϵ opt ⪸ 2.0 eV ), an effective light trapping geometry such as textured tin oxide, a reflective back contact (e.g. silver) and low contact resistance (less than 0.5 ω cm2). Requirements for high module performance require low interconnect resistance (e.g. less than 5 × 10−3ω cm2 for the AlSnO2 contact), large percentage of active area and good uniformity of material properties over large areas. New developments such as superlattice doped layers and improved tin oxide texturing have led to efficiencies as high as 10.9% in small cells (1 cm2). Processing improvements have led to efficiencies of 8.1% in 1 ft2 modules patterned entirely by laser scribing.