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The single-lifetime model is commonly used to describe recombination in photovoltaic materials. Here we describe two non-linear processes which affect the applicability of that model. Photon recycling is observed in direct band gap materials such as GaAs. This self-absorption and secondary emission of photons makes the effective radiative lifetime a function of device geometry. The saturation of recombination centers by minority carriers produces light intensity dependent lifetimes when the former are present. These effects need to be considered in device design and modeling.

Heavy impurity doping perturbs the energy-band structure of GaAs thereby influencing both the optical and electrical properties of devices. This paper examines the effects of heavy doping on material parameters important for GaAs-based bipolar devices. A broad range of experimental work directed at characterizing the doping-dependent equilibrium np product, minority carrier mobility, and minority carrier lifetime is reviewed. To facilitate device modeling, parameteric fits to the measured results are presented when appropriate. Implications for devices are then considered, and shown to be strong. Finally, the significant uncertainties and characterization needs that remain are identified.

A comprehensive study of n-type lifetimes versus doping concentration in selenium doped n-GaAs is detailed. By observing the photoluminescence delay of double heterostructure it is found that for N/sub D/ 10/sup 18/ cm/sup -3/, the authors are able to set an upper limit on the Auger recombination coefficient. The B coefficient in n-GaAs is deduced to about 2.0*10/sup -10/ cm/sup 3//s for lowly doped material, but it decreases with increasing doping concentration. These results are used to project efficiencies of GaAs solar cells using analytical models. >

Minority electron properties in p+‐GaAs doped with beryllium (Be) and with carbon (C) are reported. Measurements of essentially identical responses for structures differing only in dopant element demonstrate that the diffusivity (Dn) and the diffusion lengths (Ln) are the same in p+‐GaAs doped to ∼1019 cm−3 with Be‐ and C‐dopants. Zero‐field time‐of‐flight analysis yields Dn=35 cm2/s and internal quantum efficiency analysis yields Ln=2.4 μm, which implies a lifetime that is approximately at the estimated radiative limit. In addition, the majority Hall mobility was also found to be identical for the Be‐ and C‐doped material.

Polycrystalline thin films of CdTe grown on glass/SnO2/CdS substrates are studied using X‐ray diffraction, atomic force microscopy, and time resolved photoluminescence decay techniques. CdS films were deposited by chemical solution. CdTe films were grown by close‐spaced sublimation at substrate temperatures between 475–625 °C. CdTe thin films deposited at temperatures higher than 525 °C show preferential orientation in the 〈111〉 direction. The Grain size of the films increases with substrate temperature and the films are faceted for all the temperatures. The PL decay constant increases with substrate temperature up to 575 °C for as‐deposited films on soda‐lime substrates. Films on borosilicate substrates show an increase up to the highest temperature used (625 °C). There is systematic increase in the PL decay constant after CdCl2 heat treatment, and the range of values is 1–1.5 nsec for soda‐lime samples and 1–2 nsec for borosilicate samples.

The minority‐carrier lifetime of polycrystalline CdTe has been measured for a variety of samples produced by two different growth techniques. The CdS/CdTe solar cell structures were either grown by a proprietary spray technique at Photon Energy, Inc. or by close‐spaced sublimation at the University of South Florida. The photoluminescence decay lifetime is investigated as a function of grain size, excess carrier density, and temperature. Results are consistent with carrier recombination at grain boundaries, indicate some defects act as traps, and show relatively high efficiency solar cells can be made from material exhibiting sub‐nanosecond lifetimes.

The photovoltaic technology is based on minority-carrier properties of the semiconducting materials under investigation. The improvement of minority-carrier parameters in a cost-effective manner is the focal point of much current research. The minority-carrier properties of III–V compound semiconductors are best characterized by time-resolved photoluminescence in special diagnostic structures. Here we will describe the measurement theory and measurement techniques. Recent developments in the technology relevant to photovoltaic devices will be reviewed.

Minority electron diffusivities in p+-GaAs-doped N A≃1.4×1018 and ∼1019 cm-3 have been measured in zero-field conditions with an extension of the zero-field time-of-flight technique. Extension of the technique to make it applicable to heavily doped p+-GaAs is described and zero-field data are discussed. Unexpectedly, majority carrier drag effects are not evident in a comparison of this data with recently reported high-field data. Low zero-field mobility of electrons in p+-GaAs has important implications for high-speed devices such as heterojunction bipolar transistors.