Nathan Stoddard

Casting Single Crystal Silicon: Novel Defect Profiles from BP Solar's Mono2 TM Wafers

A novel crystal growth method has been developed for the production of ingots, bricks and wafers for solar cells. Monocrystallinity is achievable over large volumes with minimal dislocation incorporation. The resulting defect types, densities and interactions are described both microscopically for wafers and macroscopically for the ingot, looking closely at the impact of the defects on minority carrier lifetime. Solar cells of 156 cm2 size have been produced ranging up to 17% in efficiency using industrial screen print processes.

Evaluating BP Solar's Mono 2 ™ material: Lifetime and cell electrical data

Single crystal Silicon has been produced using a cast-in-place process usually used for multicrystalline ingot production. This Mono2 ™ material produces higher cell efficiencies compared to multi crystalline Silicon material with the same average minority carrier lifetime. Profiles of lifetime through the height of the brick and across the ingot demonstrate rough equivalency in bulk lifetime between Mono2 ™ and multi material. However, the standard deviation of lifetime in wafers is smaller for the Mono2 ™ material. This quality difference accounts for about half of the potential efficiency gain in Mono2 ™ material, while the rest of the gain, primarily in Jsc, is derived from the ability to form pyramidal light trapping texture on the (100) surface. Minority carrier lifetime data is compared with cell electrical data from the same collection of ingot positions. Open circuit voltage and short circuit current correlate well to some degree with bulk lifetime, while fill factor is independent both of bulk lifetime and position in the ingot.

In Situ Point Defect Generation and Agglomeration during Electron-Beam Irradiation of Nitrogen-Doped Czochralski Silicon

Samples of Czochralski silicon were observed after irradiation by a convergent electron beam in a transmission electron microscope. In a nitrogen-doped sample, the 200 keV electrons induced a vacancy-rich region containing point-defect clusters, surrounded by a ring rich in self-interstitials. No comparable effect existed in nitrogen-free reference samples. It is proposed that Frenkel pairs, created by electron collisions, are separated and stabilized by nitrogen or related complexes. Some interstitials become free to diffuse while the nitrogen, vacancies and oxygen agglomerate. This study demonstrates that the initial formation of voids and precipitate nuclei from point defects can be observed at low temperatures.

Segregation and enhanced diffusion of nitrogen in silicon induced by low energy ion bombardment

A sample of nitrogen-doped, single crystal Czochralski silicon was subjected to several different surface preparations. Secondary ion mass spectrometry depth profiling has shown that prolonged glancing-angle bombardment by 3–5kV Ar+ ions significantly increases the nitrogen concentration in the near surface by up to an order of magnitude over the bulk value. Concentrations are observed to be elevated over the bulk value to a depth up to 25μm. Nitrogen-implanted samples and samples with a 1nm surface nitride did not exhibit nitrogen segregation under the same conditions, but a sample with 100nm of surface nitride did exhibit ion bombardment induced drive-in. In nitride-free samples, the source of the nitrogen is indicated to be a nitrogen-rich layer in the first micron of material. The diffusion behavior of nitrogen in silicon is discussed and the Crowdion mechanism for diffusion is suggested as the enabling mechanism for the enhanced low temperature diffusion.

Polycrystalline Silicon Photovoltaic Manufacturing Technology Development and Commercialization

This paper will report on the BP solar/solarex efforts on polycrystalline silicon PV modules under the NREL/DOE PV manufacturing technology program. BP solar (previously solarex) has been working on polycrystalline silicon PV technology under the NREL sponsored photovoltaic manufacturing technology program since 1993. This paper will review some of the major achievements of these programs. Results from a recently completed contract will be presented with emphasis on the development of processes and handling procedures for ultra-thin silicon wafers and cells. Finally the paper will conclude with a discussion of the future work planned in the present program

On the recombination centers of iron-gallium pairs in Ga-doped silicon

Gallium (Ga) doped silicon (Si) is becoming a relevant player in solar cell manufacturing thanks to its demonstrated low light-induced degradation, yet little is known about Ga-related recombination centers. In this paper, we study iron (Fe)-related recombination centers in as-grown, high quality, directionally solidified, monocrystalline Ga-doped Si. While no defect states could be detected by deep level transient spectroscopy, lifetime spectroscopy analysis shows that the minority carrier lifetime in as-grown wafers is dominated by low levels of FeGa related defect complexes. FeGa pairs have earlier been shown to occur in two different structural configurations. Herein, we show that in terms of recombination strength, the orthorhombic pair-configuration is dominant over the trigonal pair-configuration for FeGa. Furthermore, the defect energy level in the band gap for the orthorhombic defect center is determined to be EV + 0.09 eV, and the capture cross-section ratio of the same defect center is determined to be 220.Gallium (Ga) doped silicon (Si) is becoming a relevant player in solar cell manufacturing thanks to its demonstrated low light-induced degradation, yet little is known about Ga-related recombination centers. In this paper, we study iron (Fe)-related recombination centers in as-grown, high quality, directionally solidified, monocrystalline Ga-doped Si. While no defect states could be detected by deep level transient spectroscopy, lifetime spectroscopy analysis shows that the minority carrier lifetime in as-grown wafers is dominated by low levels of FeGa related defect complexes. FeGa pairs have earlier been shown to occur in two different structural configurations. Herein, we show that in terms of recombination strength, the orthorhombic pair-configuration is dominant over the trigonal pair-configuration for FeGa. Furthermore, the defect energy level in the band gap for the orthorhombic defect center is determined to be EV + 0.09 eV, and the capture cross-section ratio of the same defect center is determin...

The impact of the FeGa complex on directionally solidified, mono-crystalline, Ga-doped silicon

In this work we show that the high minority carrier lifetime in as-grown Ga-Si wafers is dominated by low levels of iron contamination incorporated during silicon growth. Upon phosphorous diffusion iron is however effectively removed, increasing the bulk carrier lifetime from a few hundred microseconds to well above one milli-second. Lifetime spectroscopy in combination with Shockley Read Hall theory was used to determine the concentrations of Fei and FeGa complexes in the course of the FeGa association. Finally, we use the estimated concentrations of FeGa as a function of time of storage in the dark to validate that FeGa association follows the laws of coulombic attraction similar to FeB.