Dynamics of time-resolved photoluminescence in GaInNAs and GaNAsSb solar cells
© Gubanov et al.; licensee Springer. 2014
Received: 18 November 2013
Accepted: 21 January 2014
Published: 17 February 2014
We report a time-resolved photoluminescence study for GaInNAs and GaNAsSb p-i-n bulk solar cells grown on GaAs(100). In particular, we studied the extent to which the carrier lifetime decreases with the increase of N content. Rapid thermal annealing proved to significantly increase the decay times by a factor of 10 to 12 times, for both GaInNAs and GaNAsSb heterostructures, while for the 1-eV bandgap GaNAsSb structure, grown at the same growth conditions as the GaInNAs, the photoluminescence decay time remained slightly below 100 ps after annealing; the approximately 1.15-eV GaInNAs p-i-n solar cell exhibited a lifetime as long as 900 ps.
78.47.D; 78.55.Cr; 88.40.hj
In recent years, multijunction III-V semiconductor solar cells have experienced remarkable improvements, not only for space applications but also for terrestrial concentrated photovoltaic systems. The highest photovoltaic conversion efficiency reported so far is 44.7% and has been obtained with four junction solar cell . A very promising way to further improve the performance of solar cells is to utilize dilute nitride and dilute antimonide materials, which can be grown lattice matched onto GaAs and Ge substrates . These materials provide suitable absorption bands to harvest photons down to 1 eV and even below. Recently, a conversion efficiency of 44% was reported for a triple junction solar cell including a bottom junction based on GaInNAs(Sb) grown by molecular beam epitaxy (MBE) . Adding antimony to ternary GaAsN to form GaAsNSb compounds can be also used to lower the bandgap beyond the 1-eV limit, serving as an alternative to quinary alloys, which are somewhat more difficult to grow due to the presence of three elements of group V [4, 5]. The drawback in using dilute nitrides/antimonides is related to challenges in material fabrication  and formation of defects [7, 8]. Careful growth parameter optimization and thermal annealing are known to increase the material quality and carrier lifetimes . Carrier lifetime correlates with solar cell performance via the minimum diffusion length required for the carriers to travel without recombination, and it should be maximized in order to harvest efficiently the photogenerated carriers . Time-resolved photoluminescence (TRPL) using up-conversion technique  is commonly used for estimating carrier lifetimes of optoelectronic heterostructures and has been extensively used in connection with optimization of GaInNAs heterostructures [2, 12–14]. However, most of the studies have been concerned with analyses of quantum wells . Studies on GaInAsN epilayers have reported a wide variety of lifetimes in the range of 70 to 740 ps [8, 16]. In this paper, we report TRPL values for bulk GaInAsN and GaNAsSb p-i-n solar cells. In particular, we focus on correlating the effects of thermal annealing and the nitrogen composition.
TRPL measurements were carried out with an up-conversion system . For instrumentation details, see . The excitation source was an 800-nm mode-locked Ti-sapphire pulsed laser, which delivered 50-fs pulses enabling a final time resolution of approximately 200 fs (FWHM). The excitation density was approximately 3 × 10-4 J/cm2, with a 20-μm diameter spot on the sample.
This model ignores thermalization of carriers after excitation, which is typically a very fast process and was not time-resolved in these measurements. To account for limited time resolution of the instrument, emission decays were fitted using deconvolution with the instrument response function. The monoexponential fits gave satisfactory results for all measured decays.
Results and discussion
The spectral dependence of carrier lifetime in GaInNAs can be explained in terms of interplay between the radiative recombination and hopping energy relaxation of localized excitons as described by Rubel et al.  and references therein. According to Takahashi et. al , the increased nitrogen concentration, as in samples 1 to 3, merely increases the band bowing and reduces the dipole interaction by involving more non-Γ states, which has a direct effect on the radiative lifetime when larger N concentrations are used. Moreover, nitrogen increases the density of nonradiative recombination centers in the bandgap which strongly contributes to the carrier lifetime.
We investigated the carrier lifetime dynamics in lattice-matched GaInNAs and GaNAsSb p-i-n solar cells using TRPL. The increase of nitrogen content decreases the carrier lifetime owing to increase of defect densities. An increase of the lifetime by at least tenfold was observed after thermal annealing of bulk GaInNAs layers. Thermal annealing was also found to affect the carrier energy relaxation process in GaNAsSb. Further growth and annealing parameter optimization is needed to improve the quality of GaNAsSb to make it an effective subjunction material in high-efficiency terrestrial and space solar cells.
The authors acknowledge the Finnish Funding Agency for Technology and Innovation, Tekes, via projects “Solar III-V” (40120/09) and “Nextsolar” (40239/12). Alexander Gubanov and Ville Polojärvi acknowledge the National Doctoral Programme in Nanoscience (NGS-NANO). Joel Salmi and Wenxin Zhang are acknowledged for their support in sample processing.
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