J. M. Olson

Modeling of two‐junction, series‐connected tandem solar cells using top‐cell thickness as an adjustable parameter

Theoretical efficiencies are calculated for two‐junction, series‐connected solar cells using air mass 1.5 global and direct irradiance spectra. For band‐gap combinations previously limited by a low bottom‐cell current, thinning of the top cell is shown to result in significant increases in the theoretical efficiencies. The increases are primarily due to increased short‐circuit currents, since current matching is achievable. Smaller gains are also seen in the open‐circuit voltages of the thinner cells when a low surface‐recombination velocity is assumed. Thus, a number of material combinations which previously could only be used in four‐terminal configurations can now be considered for series‐connected two‐junction solar cells.

Heterojunction Solar Cell

This paper gives a qualitative description of semiconductor-semiconductor heterojunction solar cells. The two groups of heterojunctions of greatest economic potential, very highly efficient cells for concentrator applications and moderately efficient thin film cells for fiat plates, are described with examples. These examples illustrate the role of heterojunctions in surface passivation, monolithic multijunction devices, devices with semiconductors of only one conductivity type and low-temperature fabrication techniques.

THE INFLUENCE OF SPECTRAL SOLAR IRRADIANCE VARIATIONS ON THE PERFORMANCE OF SELECTED SINGLE-JUNCTION AND MULTIJUNCTION SOLAR CELLS

Abstract The sensitivities of selected single-junction and multijunction cells to variations in solar irradiance are presented. The one-sun spectral irradiance is varied as a function of air mass, optical aerosol depth (turbidity) and amount of precipitable water vapor for direct-normal and global-normal geometries. Several devices, including one-, two- and three-junction devices with low and high bandgaps and either series- or independent-connection schemes, were investigated. The effects of air mass and turbidity on the consistency of high-bandgap device performance are shown to be greater than the effect of precipitable water vapor. Low-bandgap devices are less affected by variations in air mass and turbidity, but are more sensitive to high water-vapor conditions. The efficiency gained by redesigning a multijunction device for the latitude at which it is expected to be used is shown to be less than about 3% (relative).

Effect of growth rate on the band gap of Ga0.5In0.5P

The band gap of Ga0.5In0.5P is reported as a function of growth rate and growth temperature. The Ga0.5In0.5P is grown lattice matched to 2°‐off (100)u2009GaAs substrates by atmospheric pressure organometallic chemical vapor deposition using an inlet group V/III ratio of 65. The variation of the band gap is surprisingly complex, taking five different functional forms within the two‐dimensional parameter space. These include regions in which the band gap (1) increases with growth rate, (2) decreases with growth rate, (3) is independent of both growth rate and temperature, (4) is independent of growth rate, but dependent on growth temperature, and (5) is not measurable since three‐dimensional, instead of two‐dimensional, growth is observed. The behavior can only be explained by a theory involving competing processes. One such theory is described.

Carbon doping and etching of MOCVD-grown GaAs, InP, and related ternaries using CC14

Abstract Pure carbon tetrachloride is shown to be an excellent carbon-doping source for MOCVD-grown GaAs, but an etchant, rather than a doping source, for InP. Similar to the equilibria used in chloride vapor phase epitaxy, the etching (or reduced growth rate) is believed to occur because of the formation of InC1. In the case of Ga 0.5 In 0.5 P, this etching is manifested by a change in the alloy composition, since the InP etches faster than the GaP. The composition of Ga 0.5 In 0.5 As is not changed significantly for comparable carbon tetrachloride flows. The doping efficiency of CC1 4 for GaAs is shown to decrease sharply with the growth temperature and to be inversely proportional to the V/III ratio. The hole mobilities for carbon-doped GaAs are shown to be as higher than for zinc-doped GaAs. Compensation is not a problem, even at hole concentrations exceeding 10 19 cm -3 . However, the quality of carbon-doped Ga 0.5 In 0.5 P films is significantly lower than that of zinc-doped Ga 0.5 In 0.5 P, and hole concentrations could not be increased above 10 17 cm -3 . The carrier concentration of InP films is unaffected by the flow of carbon tetrachloride.

High efficiency GaAs solar cells using GaInP/sub 2/ window layers

GaAs single-junction solar cells using Ga/sub 0.5/In/sub 0.5/P and having 1 sun, an air mass (AM) of 1.5, and global efficiencies of 25.0-25.7% are reported. The open-circuit voltage (V/sub oc/), short-circuit current (J/sub sc/), and fill factor (ff) for the 25.7% efficient cell were 1.039 V, 28.5 mA/cm/sup 2/, and 86.8%, respectively. The devices were grown at 700 degrees C using conventional atmospheric pressure metalorganic chemical vapor deposition (MOCVD). The antireflection coating is a double layer of MgF/sub 2/ and ZnS. V/sub oc/s as high as 1.055 V were obtained for some of the devices. This high V/sub oc/ is attributed to the low interface recombination velocity of the Ga/sub 0.5/In/sub 0.5/P-GaAs heterointerface. Factors that affect the efficiency of this device, including the thickness and composition of the Ga/sub 0.5/In/sub 0.5/P window layer, are presented and discussed.<<ETX>>

The difference between standard and average efficiencies of multijunction compared with single-junction concentrator cells

Abstract The theoretical performances of ideal single- and multijunction cells are compared at 100 × concentration under a range of cloudless-sky conditions. The sensitivities of device performance to cell temperature and spectral variations are shown to depend on the number of junctions (one, two or three), the way in which the junctions are connected (series, parallel or independent), and the band gaps of the devices. The average performances of all of the multijunction devices surpass that of a single-junction GaAs device, but the inconsistency in performance of some of the multijunction devices is significant for large variations in cell temperature and incident spectrum. The choice of band gap and connection scheme is more important than the number of junctions in determining the consistency of device performance.

Transmission electron microscopy study of the effect of selenium doping on the ordering of GaInP 2

Selenium doped Ga0.51In0.49P films have been grown by metalorganic chemical vapour deposition at 600, 670 and 740° C. The extent of ordering of the Group III sublattice has been monitored by transmission electron microscopy. Ordering disappears at carrier concentrations on the order of 1018 cm−3 for samples grown at 600 and 740° C although a small degree of ordering persists in the samples grown at 670° C up to a carrier concentration of 1019 cm−3. At each growth temperature, the ordering observed decreased and the bandgap measured increased with increasing Se doping.

MOCVD of Ga 0.52 In 0.48 P Using Tertiarybutylphosphine

Tertiarybutylphosphine was explored as a safer alternative to phosphine for the atmospheric pressure chemical vapor deposition of Ga0.52In0.48P on GaAs. For V/III ratios greater than or equal to 50 and growth temperatures between 630 and 700° C, the minority carrier diffusion length of Ga0.52In0.48P grown from t-butylphosphine, approached that of Ga0.52In0.48P grown from phosphine under similar conditions. Compared to deposition with phosphine, with t-butylphosphine the surface morphology was rougher, the carbon and sulfur contamination was greater, and the loss of indium by parasitic reaction was greater. The band gap of the Ga0.52In0.48P at fixed composition varied differently with V/III ratio.

The effect of selenium doping on the optical and structural properties of Ga 0.5 In 0.5 P

Selenium doping at an electron concentration of 1018 – 1019 cm−3 is shown to cause an increase in both the band gap and the disorder of Ga0.5In0.5P films grown by metalorganic chemical vapor deposition on GaAs substrates. The effect of selenium is shown to be very similar to that of the p-type dopants, zinc and magnesium. Selenium doping is also shown to have a dramatic smoothing effect on the surface morphology of Ga0.5In0.5P films.