Jürgen H. Werner

Barrier inhomogeneities at Schottky contacts

We present a new analytical potential fluctuations model for the interpretation of current/voltage and capacitance/voltage measurements on spatially inhomogeneous Schottky contacts. A new evaluation schema of current and capacitance barriers permits a quantitative analysis of spatially distributed Schottky barriers. In addition, our analysis shows also that the ideality coefficient n of abrupt Schottky contacts reflects the deformation of the barrier distribution under applied bias; a general temperature dependence for the ideality n is predicted. Our model offers a solution for the so‐called T0 problem. Not only our own measurements on PtSi/Si diodes, but also previously published ideality data for Schottky diodes on Si, GaAs, and InP agree with our theory.

Quantum efficiencies exceeding unity due to impact ionization in silicon solar cells

Absolute measurements demonstrate internal quantum efficiencies in silicon solar cells to exceed unity for photon energies above the first direct band gap and to show distinct spectral features that correspond to specific points in the Brillouin zone. Ultraviolet radiation can generate hot carriers with sufficient energy to cause impact ionization which results in two electron hole pairs per incident photon.

Flexible solar cells for clothing

Integration of flexible solar cells into clothing can provide power for portable electronic devices. Photovoltaics is the most advanced way of providing electricity far from any mains supply, although it suffers from the limits of ambient light intensity. But the energy demand of portable devices is now low enough that clothing-integrated solar cells are able to power most mobile electronics. We introduce clothing-integrated photovoltaics, their scope and limitations, the status of flexible solar cells, charge controller and system design, as well as prototype solutions for various applications.

Temperature dependence of Schottky barrier heights on silicon

We investigate the temperature dependence of Schottky barrier heights on silicon. The analysis of a large variety of polycrystalline diodes shows that the temperature coefficient of the barrier height depends on the chemical nature of the metal. This observation is in contradiction with models suggesting Fermi‐level pinning at the center of the semiconductor’s indirect band gap. From the analysis of epitaxial NiSi2/Si Schottky contacts, we conclude that there is a direct influence of interface crystallography on both the barrier height and its temperature dependence. Finally, we present some new results on the pressure coefficient of barrier heights. Pressure and temperature coefficients of polycrystalline Schottky contacts are correlated similarly to the pressure and temperature coefficients of the band gap.

Radiative efficiency limits of solar cells with lateral band-gap fluctuations

The radiative recombination limit of photovoltaic power conversion under one sun terrestrial illumination is calculated for solar cells with lateral fluctuations of the band-gap energy. A simple analytical model quantifies the fluctuations by the standard deviation σEg from the mean band gap. The calculated maximum efficiency decreases by 1.7% (absolute) for σEg=50 meV and by 6.1% for σEg=100 meV with respect to a uniform band gap.

Internal voltages in GaInP∕GaInAs∕Ge multijunction solar cells determined by electroluminescence measurements

We analyze electroluminescence spectra of a GaInP∕GaInAs∕Ge triple-junction solar cell at different injection currents. Using the reciprocity theorem between electroluminescent emission and external quantum efficiency of solar cells allows us to derive the current/voltage curves and the diode quality factors of all individual subcells.

Solar cell efficiency and carrier multiplication in Si1−xGex alloys

Crystalline Si1−xGex compounds offer the possibility for tuning the electronic energy band structure with the chemical composition of the alloy in order to adapt the material for devices utilizing the energy of solar photons at an optimum. We concentrate on the efficiency enhancement due to carrier multiplication by impact ionization. We calculate the internal quantum efficiency and the possible solar cell efficiency for this material system. The number of impact-generated charge carriers is obtained by a simulation of the competing carrier–carrier and carrier–photon scattering processes. These calculations show that the wave vector dependence of the scattering processes is unimportant for good agreement between theoretical and experimental quantum efficiencies in Si and Ge. Finally, we calculate solar cell efficiencies under the ideal assumption of unity collection efficiency and radiative recombination only. Impact ionization enhances the theoretical conversion efficiency by 0.5 percentage point; this i...

Influence of barrier inhomogeneities on noise at Schottky contacts

Electronic properties of Schottky diodes depend sensitively on spatial inhomogeneities of the metal/semiconductor interface. We find that, contrary to previous theories for low‐frequency noise, the electronic properties of Schottky contacts cannot be understood if one neglects spatial fluctuations of the Schottky barrier height. Our systematic investigation of several silicide/silicon diodes yields as an empirical law that excess noise increases drastically when the standard deviation σs of the spatial distribution of Schottky barrier heights exceeds the critical threshold value of 2kT.

Influence of Cu content on electronic transport and shunting behavior of Cu(In,Ga)Se2 solar cells

The paper presents the structural and electrical characterizations of Cu(In,Ga)Se2 thin films and thin-film solar cells realized with different Cu contents in the absorber material. It is shown that the bulk resistivity of the Cu(In,Ga)Se2 thin films (measured in coplanar geometry) dramatically increases with decreasing Cu content. Simultaneously, the shunt resistance Rp of the Cu(In,Ga)Se2 solar cells increases with decreasing Cu content in the absorber material. For a wide range of Cu contents, the resistivity of the thin films is directly proportional to Rp of the solar cell made from the same absorber material. We propose that Rp in Cu(In,Ga)Se2 solar cells originates from highly localized shunt regions in the absorber material. The higher Rp of cells made from Cu-poor material is then due to the high resistivity of the embedding Cu-poor material resistively suppressing extensive current flow towards these shunt regions. Further, we observe an increase of the film resistivity by two orders of magnitud...

Recombination mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

This article investigates limitations to the open circuit voltage of n-type amorphous silicon/p-type crystalline silicon heterojunction solar cells. The analysis of quantum efficiency and temperature dependent current/voltage characteristics identifies the dominant recombination mechanism. Depending on the electronic quality of the crystalline silicon absorber, either recombination in the neutral bulk or recombination in the space charge region prevails; recombination at the heterointerface is not relevant. Although interface recombination does not limit the open circuit voltage, recombination of photogenerated charge carriers at the heterointerface or in the amorphous silicon emitter diminishes the short circuit current of the solar cells.