The constant photoconductivity measurement (CPM) technique has been widely used to determine the density of defects in thin films of materials where only a small absorption coefficient exists for these defects, particularly in determining the dangling‐bond density in hydrogenated amorphous silicon. Interpretation and modeling in the literature assume that CPM gives an accurate value for the density of defects present. The limitations of this method are examined as a function of several defect parameters, and particularly as a function of the Fermi energy, in models consisting of one level, a Gaussian distribution of levels, two independent levels, and the levels typical of a multivalent defect, applying the results of the latter to actual data on optical degradation kinetics in amorphous silicon. It is concluded that a prerequisite for reliably relating CPM‐determined densities to actual defect densities is a knowledge of the relative location of the equilibrium Fermi level and the energy level of the defect.

Corrections to the constant photoconductivity method for determining defect densities, with application to amorphous silicon

In situ manipulation of the sub gap states in hydrogenated amorphous silicon monitored by advanced application of Fourier transform photocurrent spectroscopy

a-SiC:H alloys are promising candidates for the top junction alloy in triple-junction solar cells. In order to understand the limitations on electronic transport in the alloys, the photoconductivity and optical absorption were measured using photothermal deflection spectroscopy (PDS). Thin-film transistors have also been fabricated and studied. Taken together, the material and device measurements provide strong evidence that electron transport in the material is restricted by declining mobility in the alloys above a bandgap of approximately 1.9 eV. p-i-n devices based on the alloys have exhibited open-circuit voltages as high as 1.05 V, but the fill factors of the devices decline at these high voltages. The open-circuit voltage of the devices scales with optical bandgap, and measurements of V/sub oc/ versus temperature indicate that generation-recombination controls the open-circuit voltage. Triple-junction devices containing 1.85-eV a-SiC:H and 1.45-eV SiGe:H have shown conversion efficiencies of 10.2%. >

a-Si/sub 1-x/C/sub x/:H alloys for multijunction solar cells

A simple deconvolution procedure has been applied to photothermal and photoconductive spectra of a‐Si:H films The method requires no assumption about the midgap density of states, thus obtaining results in an unambiguous way The analysis of the distortion occurring in photothermal spectra due to surface states is shown to be an effective way to separate between surface and bulk absorption, thus enabling a direct comparison between photothermal and photoconductive spectroscopies

A deconvolution procedure for photothermal and photoconductive spectra of amorphous silicon films

Amorphous silicon based solar cells have been developed which have both improved initial conversion efficiency and greater resistance to light induced degradation. The improved initial efficiency arises from higher open circuit voltage which is a result of depositing the i-layer at lower temperatures from SiH/sub 4/ diluted in H/sub 2/. The improvement in open circuit voltage is substantially greater than would be expected from the small increase in optical bandgap that is observed as deposition temperature is lowered and there are indications that charge transport across the cell might change as the deposition temperature is lowered. By optimizing the deposition parameters the authors were able not only to reduce the total light-induced degradation, but to affect a qualitative change in the nature of the functional dependence of the conversion efficiency on light soaking time, so that after a few hundred hours of light soaking time the efficiency asymptotically approaches a limiting value ("saturation"). >

http://ieeexplore.ieee.org/iel2/1061/8044/00347111.pdf

Amorphous silicon based solar cells deposited from H/sub 2/-diluted SiH/sub 4/ at low temperatures

Photothermal deflection spectroscopy (PDS) is a sensitive method used to measure weak optical absorption of sub-bandgap radiation. A method utilizing Fourier transformation is presented which allows removal of optical interference effects and noise which is typically present in PDS data taken on a-Si:H, yet leaves the underlying PDS spectra undistorted. The method greatly facilitates comparison of PDS spectra taken on different samples, and simplifies further analysis. Examples using a-Si samples produced under different deposition conditions are presented to demonstrate the utility of the method.

Method for the Reduction of Photothermal Deflection Spectroscopy Data Taken on Amorphous Silicon (a-Si:H)

Absorption measurements of as deposited and photodegraded intrinsic amorphous hydrogenated silicon films were made using photothermal deflection spectroscopy (PDS). The films were light-soaked in situ using HeNe laser light to simulate AM1 illumination. An increase in subbandgap absorption occurred predominantly near energies of 1.2eV. A simple model was developed in which a density of states function is hypothesized and the resulting optical absorption at subgap energies is calculated. The measured absorption could be well matched in all cases by assuming a single peak of defect states at or slightly below the Fermi level. Further, the observed changes in optical absorption due to degradation could be modeled by increasing the density of the single peak of defect states and moving the Fermi level towards the valence band.

An Investigation of Photo-Induced Degradation by Means of Photothermal Deflection Spectroscopy

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Method for the Reduction of Photothermal Deflection Spectroscopy Data Taken on Amorphous Silicon (a-Si:H)

An Investigation of Photo-Induced Degradation by Means of Photothermal Deflection Spectroscopy