Peter G. Hugger

Relationship of deep defects to oxygen and hydrogen content in nanocrystalline silicon photovoltaic materials

We report measurements of the structural and compositional properties of a range of hydrogenated nanocrystalline films. We employed Raman spectroscopy for crystallinity and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) for impurity characterizations. The crystalline volume fractions and impurity levels are correlated with the deep state densities determined by drive level capacitance profiling. Those defects were found to have a thermal emission energy of 0.65±.05 eV. We found that the overall crystallinity correlated reasonably well with the density of such defect states and also found a strong correlation between the defect density and the levels of oxygen impurities. Possible origins of these defects are discussed.

Study of Large Area a-Si:H and nc-Si:H Based Multijunction Solar Cells and Materials

Solar cells based on hydrogenated nanocrystalline silicon (nc-Si:H) have demonstrated significant improvement in the last few years. From the standpoint of commercial viability, good quality nc-Si:H films must be deposited at a high rate. In this paper, we present the results of our investigations on obtaining high quality nc-Si:H and a-Si:H films and solar cells over large areas using high deposition rate. We have employed the modified very high frequency (MVHF) glow discharge technique to realize high-rate deposition. Modeling studies were conducted to attain good spatial uniformity of electric field over a large area (15”×1”) MVHF cathode for nc-Si:H deposition. A comparative study has been carried out between the RF and MVHF plasma deposited a-Si:H and nc-Si:H single-junction and a-Si:H/nc-Si:H double-junction solar cells. By optimizing the nc-Si:H cell and the tunnel/recombination junctions, we have obtained an initial aperture-area (460 cm 2 ) efficiency of 11.9% for a-Si:H/nc-Si:H double-junction cells using conventional RF (13.56 MHz) plasma deposition. The deposition rate was 3 A/sec. Results on solar cells made with MVHF will also be presented.

Insights and challenges toward understanding the electronic properties of hydrogenated nanocrystalline silicon

Hydrogenated nanocrystalline silicon (nc-Si:H) is a complex mixed-phase material containing regions of silicon nanocrystallites interspersed with amorphous silicon. It is an important material in efforts to advance the production of more economical multijunction thin-film silicon-based photovoltaic technologies. We have applied the junction capacitance methods of transient photocapacitance spectroscopy and drive-level capacitance profiling to understand its fundamental electronic properties. We compare results in both the annealed and light-soaked states for nc-Si:H samples having a wide range of amorphous-to-crystallite volume fractions. Significant differences between samples with lower and higher amorphous fractions are observed and we propose a tentative microscopic model that may account for these differences.

A contactless photoconductance technique for the identification of impact ionization

A new, contactless, microwave photoconductance based technique for the direct measurement of the spectral dependence of free carrier generation efficiency in semiconductors is described and demonstrated. The technique is applied to the search for hetero-junction assisted impact ionization (HAII) with promising initial results. A strong photo-dielectric effect is also revealed for a ZnS:i-Si interface demonstrating the ability of the technique to characterize results other than the enhanced photoconductivity we seek. Such characterization will help to inform the next step in hetero-junction preparation.

Material Properties of a-SiGe:H Solar Cells as a Function of Growth Rate

We have examined a series of a Si,Ge:H alloy devices deposited using both RF and VHF glow discharge in two configurations: SS/n + /i ( a-SiGe:H ) /p + /ITO nip devices and SS/n+/i (a-SiGe:H)/Pd Schottky contact devices, over a range of deposition rates. We employed drive-level capacitance profiling (DLCP), modulated photocurrent (MPC), and transient junction photo-current (TPI) measurement methods to characterize the electronic properties in these materials. The DLCP profiles indicated quite low defect densities (mid 10 15 cm -3 . to low 10 16 cm -3 depending on the Ge alloy fraction) for the low rate RF (∼1A/s) deposited a-SiGe:H materials. In contrast to the RF process, the VHF deposited a-SiGe:H materials did not exhibit nearly as rapid an increase of defect density with the deposition rate, remaining well below 10 17 cm -3 . up to rates as high as 10A/s. Simple examination of the TPI spectra on theses devices allowed us to determine valence band-tail widths.. Modulated photocurrent (MPC) obtained for several of these a-SiGe:H devices allowed us to deduce the conduction band-tail widths. In general, the a-Si,Ge:H materials exhibiting narrower valence band-tail widths and lower defect densities correlated with the best device performance.