W. Kusian

Ambipolar field-effect transistor

The results of measurements performed on an amorphous-silicon thin-film transistor structure are presented and interpreted. The device characteristics show a continuous alternation between n-channel and p-channel operation, an “ambipolar” effect that is made possible by the provision of ohmic source and drain contacts.

Electron and hole μτ products of slightly doped a-Ge:H films

We measured the spectral photoconductance of slightly boron doped amorphous germanium (a-Ge:H) films through the wavelength interval from 450 to 950 nm, and determined the products ( μτ ) n of electrons and ( μτ ) p of holes by means of an own two-carrier photoconductance model. The doping was achieved by adding diborane (B 2 H 6 ) to the germane (GeH 4 ) gas in the deposition chamber, in concentrations up to 0.3%. In nominally undoped films the product ( μτ ) n exceeds the product ( μτ ) p by two orders of magnitude. A 150 ppm admixture of diborane yields “compensation”, i.e. adjusts the equation ( μτ ) n = ( μτ ) p = 5*10 −8 cm 2 /V. A pin solar cell with compensated i-layer is more red efficient than a cell with undoped i-layer.

Stacked a-SiC:H/a-Si:H heterostructures for bias-controlled three-colour detectors

Abstract A family of three-terminal three-colour detectors based on stacked a-SiC:H/a-Si:H heterostructures is presented. The detectors have the assembly TCO/PIN/TCO/PINIP/metal and TCO/PINIP/TCO/PIN/metal and are sensitive for the fundamental chromatic components. These two structures are mutually compared with regard to the calculated spectral responsivity using our simulation program. They both exhibit a linear photocurrent/generation-rate relationship with high rejection ratios for blue, green and red colours at peak wavelengths 430, 530, 630 nm, applying ± 1 V or more. Based on the optimisation analysis of their geometrical dimensions TCO/PIN/TCO/PINIP/metal structures were fabricated. In the fabricated assembly, due to the stringent thickness condition, the top PIN diode does not yet provide satisfactory results, but the PINIP structure renders an excellent spectral separation for green and red colour.

Spectral photocurrent-voltage characteristics of crystalline silicon and amorphous silicon solar cells

Abstract Crystalline silicon (c-Si) and amorphous silicon (a-Si) solar cells represent complementary cell types. The generation of photocarriers takes place in the semiconductor bulk (c-Si cell) or is confined to the barrier layer (a-Si cell). Carrier collection hence relies on diffusion (c-Si cell) or drift (a-Si cell). We explain the different behavior of the two cell types by measuring photocurrent-voltage characteristics. The photocurrent of the a-Si barrier cell changes its sign at a certain transition voltage U T . We find that U T depends slightly on the light wavelength λ but remains invariant with respect to alterations in the light intensity. It is possible to interpret both features in terms of a suitable uniform-field picture for a-Si p-i-n cells. The field through the i layer reverses its sign at the flat-band voltage U F . A vanishing field reveals the small contribution of diffusion to the photocarrier transport. The function U T (λ) stems therefore from diffusion.

Stacked a-Si:H-based three-colour detectors

Abstract A new a-Si:H based three colour-detector with the assembly transparent conducting oxide ( tco )/PINIP/ tco /PIN/metal is theoretically and experimentally investigated. This detector assembly overcomes the problems with a thin PIN diode for the detection of the blue colour in the tco /PIN/ tco /PINIP/metal assembly. The theoretical results demonstrate the advantages of the new detector type. The three colours could be detected with full width half magnitude

The wave nature of light in computer analysis of solar cells

Abstract In this work, the accurate light-generation model which accounts for the wave nature of light is compared to the classical model which assumes the simple exponential decay of absorbed light and considers only the two most important reflections. Both models are used to calculate both internal and external chectrical characteristics of the illuminated amorphous silicon sola cell. The results are compared to find out when the classical formula can be used, and when accounting for light interference is necessary.

Light and current degradation of a-Si:H pin, nin and pip diodes detected with CPM

Abstract Light and current degradation of nin, pip and pin diodes were investigated using CPM and SCLC. We found that degradation can be induced by recombination as well as by hole injection. Excess electrons do not cause any degradation.

Spectral photocurrent-voltage characteristics of a-Si pv and pπ diodes☆

Abstract Standard a-Si pin-solar cells were modified by incorporating small amounts of boron or phosphorous into the “i-layer”, thus producing various psn-diodes. The photocurrent Ip of these diodes decreases with increasing voltage U and vanishes at the transition voltage UT, separating primary and secondary photocurrents. The s-layer fine doping amplifies the dispersion UT(λ) and leads to very strong secondary photocurrents. After a degradation procedure under AM1-light the photocurrent behavior is characterized by a decrease in primary photocurrent and transition voltage but an increase in secondary photocurrent.

Flat-band voltage of a-Si pin solar cells from spectral characteristics

Experimental and simulation results on a-Si p-i-n solar cells are presented. The internal collection efficiency is shown to be a function of wavelength and voltage. The function shows a certain kind of symmetry that is not perfect, but reveals a plateau voltage U/sub p/. A simple photocurrent model is used to explain the experimental results. The model assumes a uniform field through the i-layer. It neglects bulk recombination of photocarriers, but considers surface recombination at the p-i and i-n barriers. A particular voltage, the flatband voltage U/sub F/, causes the field to vanish. A dominant front barrier yields U/sub p/<U/sub F/. It is suggested that a symmetric cell should show a sharp center and U/sub p/=U/sub F/.<<ETX>>

Equivalence of electrons and holes in a-Si p-s-n diodes

Amorphous‐silicon (a‐Si:H) solar cells have the structure of short p‐i‐n diodes, with an electric field extending through the i layer. The photocurrent is voltage dependent. Primary and secondary photocurrents occur. We modify the diode structure by low‐level doping of the i layer in order to obtain p‐s‐n diodes. The s layer has the type ν or π, i.e., is slightly n or p doped. The transition p‐i‐n→p‐s‐n deforms the associated spectral photocurrent‐voltage characteristics. Certain asymmetries build up. A type transfer of π→ν inverts the symmetry pattern. The reversal of the illumination direction has almost the same effect. Electrons and holes therefore play equivalent roles in p‐s‐n diodes and p‐i‐n solar cells. An elementary photocarrier‐collection model provides a basic understanding of all the observed phenomena.