Kenneth Lord

High quality amorphous silicon materials and cells grown with hydrogen dilution

Hydrogen dilution of the active gas during deposition has been found to be a very effective way to improve the quality of amorphous silicon-based materials and solar cells. With increasing hydrogen dilution, the material is characterized by an improved order, and at a certain threshold dilution, the amorphous to microcrystalline transition takes place. The best material is grown just below the threshold and is heterogeneous consisting of tiny crystallites embedded in an amorphous matrix of improved order. In this paper, we discuss the effects of hydrogen dilution on the material and cell properties of amorphous silicon-based alloys and provide an explanation for their improved stability against light-induced degradation. We also discuss some special properties of the on-the-edge materials that are not seen in the conventional amorphous or microcrystalline alloys.

Electronic states of intrinsic layers in n-i-p solar cells near amorphous to microcrystalline silicon transition studied by photoluminescence spectroscopy

Thin film n-i-p solar cells were prepared using decomposition of disilane-hydrogen mixtures by plasma-enhanced chemical vapor deposition. By increasing either the H dilution ratio or the thickness, the i-layer structure showed a transition from amorphous to microcrystalline silicon characterized by x-ray diffraction. The electronic states of the i layer were examined by photoluminescence (PL) spectroscopy, which showed that: (a) below the onset of microcrystallinity, a blueshift of the 1.4 eV PL peak energy along with a decrease of the band width occur as the structural order is improved; (b) above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4–5 and the PL peak energy is redshifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to...

Light-induced increase in the open-circuit voltage of thin-film heterogeneous silicon solar cells

We observe a significant light-induced increase in open-circuit voltage, Voc, of solar cells whose intrinsic (i) layer consists of an amorphous and microcrystalline mixed phase. The increase depends on the i-layer thickness and light-soaking intensity. An increase of as large as 150 mV or 20% of the original Voc is observed. The original Voc is restored after subsequent thermal annealing. The possible mechanism for the Voc increase is discussed.

Amorphous Silicon Alloy Solar Cells Near the Threshold of Amorphous-to-Microcrystalline Transition

A systematic study has been made of amorphous silicon (a-Si) alloy solar cells using various hydrogen dilutions during the growth of the intrinsic ( i ) layer. We find that the open-circuit voltage (V oc ) of the cells increases as the dilution increases; it then reaches a maximum before it decreases dramatically. This sudden drop in V oc is attributed to the transition from amorphous silicon to microcrystalline inclusions in the i layer. We study i -layer thicknesses ranging from 1000 A to 5000 A and find that the transition occurs in all thicknesses investigated. Based on this study, a-Si alloy p i n solar cells suitable for use in the top cell of a high efficiency triple-junction structure are made. By selecting an appropriate dilution, cells with V oc greater than 1 V can be achieved readily. Solar cells made near the threshold not only exhibit higher initial characteristics but also better stability against light soaking. We have compared top cells made near the threshold with our previous best data, and found that both the initial and stable efficiencies are superior for the near-threshold cells. For an a-Si/a-Si double-junction device, a V oc value exceeding 2 V has been obtained using thin component cells. Thicker component cells give rise to an initial active-area efficiency of 11.9% for this tandem structure.

Hydrogenated Microcrystalline Silicon Solar Cells Made with Modified Very-High-Frequency Glow Discharge

Hydrogenated microcrystalline silicon (μc-Si:H) solar cells are made using modified veryhigh-frequency (MVHF) glow discharge at deposition rates ∼3-5 A/s. We find that the solar cells made under certain conditions show degradation in air without intentional light soaking. The short-circuit current drops significantly within a few days after deposition, and then stabilizes. We believe that post-deposition oxygen diffusion along the grain boundaries or cracks is the origin of the ambient degradation. By optimizing the deposition conditions, we have found a plasma regime in which the μc-Si:H solar cells do not show such ambient degradation. The best a-Si:H/μc-Si:H double-junction solar cell has an initial active-area efficiency of 10.9% and is stable against the ambient degradation. The stability data of the solar cells after light soaking are also presented.

Investigation of Shunt Resistances in Single-Junction a-Si:H Alloy Solar Cells

Dark current-voltage characteristics of one hundred twenty single-junction a-Si:H alloy solar cells were studied. Parametric Modelling of dark I-V Measurements was used to determine some of the cell parameters. Average shunt resistances were 31 to 1200 kΩ for intrinsic layer thicknesses of 200 to 800 nm, respectively. Current switching was observed during dark I-V Measurements; the current in the reverse-bias region varies approximately as the square of the voltage following the onset of switching. The I-V characteristics in the switched mode are not understood. Voltage history and scan rate play a role in dark I-V characteristics.

Investigation of light and dark I-V characteristics of a-Si:H alloy solar cells, irradiated with 1.0 MeV protons

Light and dark I-V characteristics of both virgin and irradiated solar cells with the same history differ; the objective of this work is to elucidate the mechanisms responsible for the observations. Thirty-seven triple-junction and 120 single-junction hydrogenated amorphous silicon alloy cells were investigated. Triple and single-junction cells degrade similarly with 1.0 MeV proton irradiation; the power density degrades for fluences above 1E12 cm/sup -2/. The fill-factor degrades initially; above 5E13 cm/sup -2/ the short-circuit current density dominates the degradation. Dark I-V measurements show changes in the shunt resistance and injection current. High fluences tend to decrease both the shunt and injection currents. Shunt resistances of virgin cells are unstable under reverse bias. Annealing at 200/spl deg/C for two hours increases and stabilizes shunt resistances. A parametric model was used to fit light and dark I-V measurements. The effect of irradiation and light bias on the parameters is reported.<<ETX>>

Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy

a-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.

Effects of 40 keV electron irradiation on dark I-V characteristics of single-junction a-Si:H solar cells

The effects of 40 keV electron irradiations and anneals on dark I-V characteristics of a single-junction a-Si:H solar cell have been investigated. A parametric fitting model was used to identify the current mechanisms by comparing the measured and calculated I-V. A single current injection term fits the dark I-V characteristics before irradiation. Two additional terms had to be introduced following 40 keV electron irradiations with fluences of about 1E17 cm/sup -2/: one is a shunt term and the other is a term of the form CV/sup m/. The current mechanism characterized by the CV/sup m/ term was removed by annealing for two hours at 140/spl deg/C. Subsequent irradiation restores the current mechanism and it is readily removed by another annealing cycle. Initial work is reported on the use of a device simulator to determine the effect of irradiation on the material properties.

Status of amorphous silicon alloy solar cells and modules made near the onset of microcrystallinity

Amorphous silicon (a-Si) alloy solar cells made near the onset of micro-crystallinity exhibit superior characteristics. The amorphous to microcrystalline transition depends critically on the hydrogen dilution used during the film growth and the cell thickness. Cell open-circuit voltage (V/sub oc/) was used to determine the transition threshold for various hydrogen dilutions. Devices made on the transition edge exhibit a significant reduction in V/sub oc/ and a dispersion of its values due to nonuniform microcrystallite inclusions in an amorphous matrix. The best cells are made in the amorphous region just below the threshold. Status of a-Si and a-SiGe alloy component cells suitable for use in high efficiency triple-junction devices is presented. Using near threshold conditions, a large-area (/spl sim/922cm/sup 2/) fully encapsulated triple-junction module with an initial aperture-area efficiency of 11.9% has been achieved and confirmed by NREL.