G. Bilger

Influence of sodium on the growth of polycrystalline Cu(In,Ga)Se2 thin films

We investigate the influence of Na on the growth of Cu(In,Ga)Se2 thin-films by three model experiments. First, we examine the influence of Na on the Se activity during selenisation of Mo films on soda-lime and borosilicate glass after growth and after thermal treatments. Second, we analyse the location and possible binding partners of Na in polycrystalline Cu(In,Ga)Se2 prior to and after air-exposure. The final experiment focuses on the identification of the chemical state of Na on the surface of as grown and air-exposed films. Our experiments demonstrate that Na influences the growth of CIGS Cu(In,Ga)Se2 due to its interaction with Se. In non-air-exposed films Na is mainly localised in the form of sodium-polyselenides (Na2Sex) at the grain boundaries. We conclude that Na2Sex acts as Se-reservoir during film formation and oxidation.

Comparison of structural and electrical properties of Cu(In, Ga)Se2 for substrate and superstrate solar cells

Abstract Cu(In, Ga)Se 2 absorber layers were deposited on glass coated with a ZnO/ZnO:Al double layer for the fabrication of superstrate solar cells. Their photovoltaic performance and material properties were compared with cells in the substrate configuration which were grown on Mo-coated glass. Measurements of the capacitance–voltage characteristics were used to study the carrier density of the absorber material. Compared to substrate cells, superstrate cells revealed a low acceptor concentration. Profiling with secondary mass spectroscopy revealed a low concentration of Na, which is an effective acceptor for Cu(In, Ga)Se 2 . In contrast to the Mo layer in substrate cells, the ZnO/ZnO:Al bilayer acts as barrier against the diffusion of Na from the glass substrate. Furthermore, the carrier density analysis revealed a high concentration of trap states close to the interface in the absorber layer of superstrate solar cells. Light soaking saturates such trap states and increases the net carrier density, which improves the open circuit voltage and the efficiency of the solar cells.

Spatial inhomogeneities in Cu(In,Ga)Se2 solar cells analyzed by an electron beam induced voltage technique

Spatial variations of the local open circuit voltage in Cu(In,Ga)Se2 solar cells are analyzed by an electron beam induced voltage (EBIV) technique. The major pattern visualized by our EBIV measurements are spatial inhomogeneities on a length scale of between 5 and 20μm. Quantitative evaluation of the EBIV signals shows that the loss of open circuit voltage due to the inhomogeneities is about 100mV. Additional analysis of our samples by energy dispersive x-ray analysis excludes fluctuations of the Ga or Cu content as the source of the inhomogeneities. Instead, the spatial inhomogeneous supply of Na from the glass substrate turns out as a possible origin of inhomogeneities. Spatially resolved secondary ion mass spectroscopy measurements show that the Na content of our Cu(In,Ga)Se2 samples varies between 0.03 and 0.15at.% on a length scale of tens of micrometers.

Towards the development of flexible CIGS solar cells on polymer films with efficiency exceeding 15

Development of Cu(In,Ga)Se/sub 2/ (called CIGS) solar cells on polymers is challenging because of the thermo-physical properties of layers and substrates. CIGS layers of suitable structural and opto-electronic properties should be grown at low temperature (< 500/spl deg/C) as polyimides tend to degrade at higher deposition temperatures. Additionally, a method for controlled incorporation of an optimum amount of Na in CIGS is needed for high-efficiency cells since polyimides do not contain Na. Solar cells were developed on commercially available Upilex foils. CIGS layers were grown by evaporation of elemental Cu, In, Ga and Se at different substrate temperatures. Na from a NaF film was incorporated into CIGS layers with a post-deposition diffusion method that is suitable for in-line production of solar cells. Independent measurements have confirmed 14.1% efficiency under simulated AM1.5 standard test conditions. This is the highest efficiency reported to date for any kind of solar cell grown on polymer films. An average reflectance loss of about 13% was measured for these cells. Application of a commonly used anti-reflection coating would enable more than 15% efficiency flexible CIGS solar cells on polyimide foils.

Laser induced lifetime degradation in p-type crystalline silicon

Pulsed, green laser irradiation of uncoated p-type silicon leads to a significant reduction of the effective minority carrier lifetime. The reason for the lifetime drop lies in the introduction of recombination centres into the laser melted and recrystallized surface layer, leading to a low local minority carrier lifetime τ ≈ 10 ns inside this surface layer. The laser treatment introduces the impurities oxygen, carbon and nitrogen into the silicon and further leads to an n-type doping of the surface layer. There are strong indications that these impurities are responsible for the observed n-type doping, as well as the lifetime reduction after irradiation. Both effects are removed by thermal annealing. An estimate shows that the low local lifetime does nevertheless not affect the performance of industrial or contacted selective solar cell emitter structures.

Target to intermediate target sputtering technique for the preparation of continuously composed alloys and for doping of amorphous silicon

Abstract A novel sputtering method for the preparation of the hydrogenated amorphous alloys a-SiGe:H and a-SiC:H and for doping of a-Si:H with good lateral homogeneity is presented: target to intermediate target sputtering. The series of sputtering processes starts with a primary target containing lateral structures of different materials which become increasingly equilibrated and smoothed as they are sputtered to the intermediate target and to the substrate. For homogenous composition of the substrate a minimum spatial frequency and/or maximum spatial distance of structures at the primary target is derived. Amorphous semiconductors (a-SiGe:H, a-SiC:H and a-Si:H doped with boron, gallium, antimony or phosphorus) prepared by target to intermediate target sputtering were analysed with respect to the elecrical conductivity, the position of the Fermi level, the type of charge transport and the optical band gap. A change in the room temperature conductivity of a-Si:H from 10-10 Ω-1 cm-1 for undoped samples to 10-2 Ω-1 cm-1 for doped samples has been achieved; the incorporation of germanium or carbon shifts the optical band gap to 1.1 eV or 2.5 eV respectively. In principle the target to intermediate target sputtering technique is expected to be applicable for any type of compound material.

CIGS solar cells grown by a three-stage process with different evaporation rates

Solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGS) absorber layers have shown high potential for low cost photovoltaic energy conversion. Our group has achieved 18.1% efficient cells on glass substrates and 14.1% on flexible polyimide foils without antireflection (AR) coating. These results were achieved by applying a three-stage evaporation process with common growth rates of about 35 nm/min. Aim of this study was to enhance the growth rates of layers deposited at low substrate temperature (450°C) suitable for polyimide foils and to investigate structural properties of CIGS layers and photovoltaic properties of solar cells. It was found that very high deposition rates of 500 nm/min during the 2nd and 3rd stage of the evaporation process are feasible, maintaining high absorber quality and performance of solar cells.

Optimization of composition grading in Cu(In,Ga)Se2 for flexible solar cells and modules

For the development of high efficiency Cu(In,Ga)Se2 (CIGS) flexible solar cells on polymer films with efficient and robust deposition process the so-called three-stage evaporation processes were modified. CIGS layers were grown by co-evaporation of constituent elements at low substrate temperature compatible with the stability of the polyimide film. The CIGS layers grown with our standard process are compositionally graded, especially the Ga has a strong grading profile across the layer thickness. Different variations of the standard evaporation process were investigated, especially in view of changing the energy band gap profiles in the absorber layer for achieving further improvements in high efficiency solar cells. Further on, the incorporation of sodium during the CIGS deposition was optimized. Structural and chemical composition properties of CIGS layers were characterized with SEM, XRF, SIMS and the photovoltaic properties were characterized with I–V and quantum efficiency measurements. A flexible CIGS solar cell on polyimide film with a record efficiency of 17.6% has been developed.

Properties of plasma-produced amorphous silicon governed by parameters of the production, transport and deposition of Si and SiHx☆

Abstract Amorphous silicon thin films, which are a fundamental material for photovoltaic applications, can be prepared by plasma chemical processes. Four types of gas discharges, namely r.f. sputtering, d.c. glow discharge, arc and corona discharge, have been investigated to determine their particular characteristics as plasma chemical processes for the deposition of amorphous silicon. By determining the parameters of the plasmas and the conditions of the specific discharge by optical emission spectroscopy and by mass spectroscopy we can control the reproducibility of the plasma state and obtain defined variations in the generated species Si and SiH which constitute the layer. Analyses of film properties show high reproducibility. Moreover, the properties of the films reflect the plasma parameters as well as the deposition conditions.

Field quenching effects in polycrystalline SrS:Pb and SrS:Ce,Pb thin films for electroluminescence devices

The luminescence properties of lead ions and their sensitizing effect on cerium activators in strontium sulfide thin‐film electroluminescence (TFEL) devices have been investigated. Polycrystalline SrS:Pb and SrS:Ce,Pb thin active layers for such TFEL devices have been prepared by multi‐source deposition. Emission spectroscopy under application of a high electric field (EL), under e‐beam excitation [cathodoluminescence (CL)], and under CL conditions with additional EL drive has been carried out. It has been found that the luminescence of SrS:Pb under EL drive is very weak, while the CL signal is considerably higher. This CL signal has shown strong quenching effects upon application of an additional ac voltage. The Pb2+ emission dropped to some 5% at an applied voltage of 100 V0p, being roughly 40 V below EL threshold. More than half of this drop occurred between 0 and 30 V0p. A remarkably lower quenching effect on the Ce3+ activator luminescence has been observed. Such luminescence quenching is attributed ...