Reiner Klenk

Characterisation and modelling of chalcopyrite solar cells

In this contribution, we have discussed current models of the chalcopyrite-based heterojunction solar cell and their implications for achieving reliable and efficient devices. Numerical simulation of the device is an important tool for assessing the influence of various parameters. It has been used here to provide examples for the topics discussed. The key points for the minimisation of interface recombination are: doping of absorber and window; electrical charge and band line-up at the interface; and interface recombination velocity.

Influence of Na on Cu(In,Ga)Se2 solar cells grown on polyimide substrates at low temperature: Impact on the Cu(In,Ga)Se2/Mo interface

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated.

Modeling plasmonic scattering combined with thin-film optics

Plasmonic scattering from metal nanostructures presents a promising concept for improving the conversion efficiency of solar cells. The determination of optimal nanostructures and their position within the solar cell is crucial to boost the efficiency. Therefore we established a one-dimensional optical model combining plasmonic scattering and thin-film optics to simulate optical properties of thin-film solar cells including metal nanoparticles. Scattering models based on dipole oscillations and Mie theory are presented and their integration in thin-film semi-coherent optical descriptions is explained. A plasmonic layer is introduced in the thin-film structure to simulate scattering properties as well as parasitic absorption in the metal nanoparticles. A proof of modeling concept is given for the case of metal-island grown silver nanoparticles on glass and ZnO:Al/glass substrates. Using simulations a promising application of the nanoparticle integration is shown for the case of CuGaSe(2) solar cells.

Mini-modules from a CuInS2 baseline process

Abstract A baseline process has been established for CuInS 2 solar cells. Stable efficiencies in the range of 10–11% on small area single solar cells are achieved with this technology, the best cell so far has reached 11.4% (total area). This baseline technology serves as base for further improvements at various steps of the whole process. Additionally integrated series connected mini-modules on 5×5 cm 2 substrates have been prepared with the same technology. The majority of these mini-modules are in the range of 8–9% efficiency with 9.7% for the best one. These modules show open circuit voltages per cell that are identical to those achieved with good single cell devices, whereas fill factor and short circuit current density are still somewhat lower. The gap between single solar cells and mini-modules, however, is less than 2% absolute in efficiency, a rather small difference. Analysis of modules with 8–9% efficiency using an infrared camera system reveals hot spots that are due to small shunts. These small shunts are a major reason for non-optimum efficiency. Modules and cells show stable performance throughout 2000 h exposed to dry heat (85 °C).

Combined in-depth scanning Auger microscopy and Raman scattering characterisation of CuInS2 polycrystalline films

Abstract In this work, the combination of in-depth scanning Auger microscopy with Raman microprobe spectroscopy is applied for the detailed microstructural characterisation of CuInS 2 (CIS) thin films. CIS films are used for the fabrication of high efficiency solar cell devices. These films are obtained by sequential sputtering of Cu and In layers on a Mo-coated glass substrate, followed by a sulphurisation step at 500°C in a rapid thermal processing furnace. In order to study this process, samples obtained at intermediate steps are investigated. The obtained data show the formation of the CIS phase already at the first stages of the sulphurisation process, although with a highly disordered structure. Moreover, segregation of CuS towards the surface is observed before sulphurisation is completed. This fact is accompanied by a significant increase of the structural quality of the CIS film, which allows for the fabrication of high efficiency solar cell devices. The performed analysis corroborates the strong complementarity between the used techniques for the detailed microstructural analysis of complex multilayer systems.

Influence of Na on the properties of Cu-rich prepared CuInS2 thin films and the performance of corresponding CuInS2/CdS/ZnO solar cells

Using different glass substrate types the Na content in sequentially and Cu-rich prepared CuInS 2 films and corresponding CuInS 2 /CdS/ZnO thin-film solar cells is varied. The purpose was to investigate the influence of different Na concentrations on absorbers and devices. While the morphology of the absorbers seems not to be affected by this variation, corresponding PL spectra differ significantly. The properties of the solar cells, however, show no dependence on the Na concentration. This implies that even though the defect chemistry of CuInS 2 , sequentially prepared under Cu excess, is changed by the presence of Na this influence has no impact on properties of corresponding solar cells.

Current transport in Cu(In,Ga)S2 based solar cells with high open circuit voltage-bulk vs. interface

Cu(In,Ga)S 2 thin films prepared by rapid thermal sulfurization of metallic precursors yielded solar cells with efficiencies reaching 12.9% [1]. A good short circuit current density was observed together with open circuit voltages up to 850 mV. However, the fill factor was close to, but typically did not exceed 70%. In this contribution we report on the role of junction formation by chemical bath deposition on these parameters. Concentrations in the bath and deposition times were varied. A comparison is made between CdS and Zn(S,O) buffer layers. The influence of the incorporated gallium on surface properties was investigated by ultraviolet photoelectron spectroscopy (UPS) for the valence band edge and near edge X-ray absorption fine structure (NEXAFS) for the conduction band edge. Even in our best cell (13.1%) the activation energy of the saturation current is found to be still smaller than the band gap. High diode ideality factors and voltage dependent current collection prevent higher fill factors.

Optical modeling of chalcopyrite-based tandems considering realistic layer properties

Previous models of chalcopyrite-based tandem solar cells have not taken into account the limited optical transmission of the top cell observed. We use a quantitative model derived from measured optical properties of the different layers of the top cell to re-evaluate the benefits and limitations of the tandems. Guidelines are provided for minimizing optical losses in the structure. Optimization of the bottom absorber band gap and top absorber thickness is carried out. In combination with straightforward assumptions concerning the electronic cell properties, we calculate tandem maximum efficiencies in the range of 26%–28% depending on the degree of nonideal optical absorption.

Enhanced absorption in tandem solar cells by applying hydrogenated In2O3 as electrode

To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm2 V−1 s−1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no sub-bandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide electrode for tandem solar cells in modules.

Band alignment at Sb2S3/Cu(In,Ga)Se2 heterojunctions and electronic characteristics of solar cell devices based on them

Band offsets at Sb2S3/Cu(In,Ga)Se2 heterojunctions have been studied by x-ray and ultraviolet photoemission spectroscopy. The valence and conduction band offset have been estimated to −(0.6±0.3) eV and (0.2±0.3) eV, respectively. This result suggests Sb2S3 as a potential buffer layer material for chalcopyrite based solar cells. However, Cu(In,Ga)Se2/Sb2S3/ZnO solar cells have been investigated. While the open circuit voltage ranged up to ∼0.4–0.5 V, the short circuit current was limited to ∼1.8–4.9 mA/cm2. A photocurrent of about 30 mA/cm2 was found for negative bias. On the basis of bias dependent quantum efficiency measurements and calculations, limiting mechanisms are discussed.