Showing papers by "Jürgen H. Werner published in 2007"

High quality baseline for high efficiency, Cu(In1- x,Gax)Se2 solar cells

Abstract: We report on the progress that we have made in the quality of our baseline process for the production of high efficiency soda lime glass/Mo/Cu(In,Ga)Se2 (CIGS)/CdS/i-ZnO/ZnO:Al/MgF2 solar cells. The enhancement of the average performance level has enabled us to reach conversion efficiencies of up to 19·3% (internal measurement). The new quality initiative uses process control, optical and electrical modelling, and the critical revision of all process steps as tools for the attainment of the 19% efficiency level. Our experiments show that the compositional process window for CIGS solar cells that have an efficiency of η ≈ 19% is very wide. Accordingly, we suggest that an efficiency of 19·0–19·5% is achievable in the following compositional process windows: 0·69 ≤ Cu/(Ga + In) ≤ 0·98 and 0·21 ≤ Ga/(Ga + In) ≤ 0·38. In addition, our results show that large CIGS grains are not a necessary requirement for high efficiencies up to 19%. These findings and the partly lacking ability to correlate certain aspects of our progress with experimental parameters lead us to the conclusion that there are still some important process variables undiscovered. From this conclusion and from the evaluation of the available data we infer that there is a potential for the enhancement of CIGS solar cell efficiencies beyond 20%. Copyright © 2007 John Wiley & Sons, Ltd.

Light absorption and emission in semiconductors with band gap fluctuations—A study on Cu(In,Ga)Se2 thin films

Abstract: This work investigates the influence of lateral fluctuations of the fundamental band gap on the macroscopic light absorptance and emission spectra of spatially inhomogeneous semiconductors. A model assuming a Gaussian distribution for the local band gaps yields closed-form expressions for the spectral absorptance and emission. Band gap fluctuations broaden the absorption edge of the fundamental band gap, as well as the associated emission peak. The spectral position of the photoluminescence emission peak depends on the length scale of the fluctuations in relation to the characteristic charge carrier transport length. We apply the model to experimental results from Cu(In1−x,Gax)Se2 thin films routinely used as photovoltaic absorbers in thin-film ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. The films feature band gap fluctuations with standard deviations between 15 and 65 meV which would lead to losses in the range of 5–80 mV for the open circuit voltage of solar cells made from these films. The pure ternary compounds CuInSe2 and CuGaSe2 exhibit smaller standard deviations than their quaternary alloys. This experimental finding indicates alloy disorder as one possible source of band gap inhomogeneities. The length scale of the observed fluctuations turns out to be much smaller than the minority carrier diffusion length. Hence, the band gap fluctuations occur on a length scale below 100 nm.

The effect of solar irradiance on the power quality behaviour of grid connected photovoltaic systems

Abstract: Photovoltaic systems have been increasingly used in the generation of electrical energy, either as a means of providing electricity in areas where there is no grid connection (stand alone systems), or by providing electricity to the grid (grid connected systems). In a deregulated energy market, the cost of energy produced from fossil fuels is rising and the photovoltaic energy becomes a promising alternative source. Yet it still suffers from problems that need to be resolved in order to be widely accepted as an equivalent alternative to fossil fuels. Firstly the more direct problem of efficiency and cost needs to be overcome and secondly the indirect problem of the quality of power provided by photovoltaic technology has to be addressed. In this work 14 different grid connected PV systems have been considered and power quality measurements have been taken. Results for low and average irradiance cases are presented and analyzed in order to determine how power quality quantities are affected by changes in solar irradiance.

Photocurrent analysis of a fast Ge p-i-n detector on Si

Abstract: Germanium vertical p-i-n photodetectors grown on silicon with molecular beam epitaxy are investigated. The photocurrent of a high speed detector structure is analyzed at infrared wavelengths of around 1.3μm. The dark current and photocurrent were measured for reverse and forward biased detectors. It is clearly shown that the photocurrent is proportional to the width of the depletion layer. This means that the fast carriers from the depletion layer generate mainly the photocurrent and by this the high speed operation of the germanium detector is explained. Nearly abrupt junctions and low background doping allow zero bias operation of these detectors.

Pulsed laser-doped selective emitter for silicon solar cells

Abstract: Selective emitter formation for industrial silicon solar cells rises in interest due to increase of efficiency �� . This work presents a simple process where a single laser irradiation permits adjustment of emitter properties without necessity of using masks, additional doping precursor or photolithography. As processing tool, we use a pulsed Nd:YVO4 laser with wavelength �� = 532 nm for irradiation of furnace diffused emitters. The laser irradiation reduces the emitter sheet resistance �! by over 50 %. The phosphosilicate glass layer from the furnace diffusion, plays a major role during laser irradiation of emitters with higher sheet resistances of �! = 150 � /�e .

Leakage current reduction in 80 nm biaxially strained Si nMOSFETs on in-situ doped SiGe virtual substrates

Abstract: We present a comprehensive study of biaxially strained (up to ~3 GPa stress) Si nMOSFETs down to 80 nm gatelength Well behaved 80 nm devices with expected strain-induced electrical enhancement were demonstrated Special emphasis was put on investigation of substrate junction leakage and source to drain leakage In-situ doped wells and channel profiles demonstrated superior substrate junction leakage for the relaxed SiGe substrates compared to conventional implantation The source to drain leakage in 80 nm devices was effectively reduced by increment of channel doping and rotation of the channel direction