Tobias Eisenbarth

Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells

A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a ...

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.

Characterization of metastabilities in Cu(In,Ga)Se2 thin-film solar cells by capacitance and current-voltage spectroscopy

Metastabilities in Cu(In,Ga)Se2 (CIGS)-based solar cells induced by red light illumination, blue light illumination, and voltage bias treatment are investigated by admittance spectroscopy, capacitance-voltage profiling, thermally stimulated capacitance, and temperature dependent current-voltage (IVT) measurements. It is found that the characteristic N1-admittance response exhibits changes in the activation energy after light soaking and forward bias treatment, whereas no significant change of the activation energy is observed after reverse bias treatment. A roll-over effect is observed in the IVT characteristics for conditions of the sample for which the N1-activation energy is larger than 100 meV. We show that these results are in good agreement with a recently proposed interpretation of admittance spectra, which relates the N1 signature to a back-contact barrier in CIGS solar cells. We also show that, within this model, the changes in the IV characteristics upon light soaking with blue light can be cons...

Effect of Zn incorporation into CuInS2 solar cell absorbers on microstructural and electrical properties

Zn incorporation into CuInS2 absorbers is found to increase the open-circuit voltage but decrease the short-circuit current of the corresponding thin-film solar cells. In this article, we study the effect of Zn incorporation into CuInS2 absorbers with a focus on the mechanisms leading to the measured changes in the electrical properties of the solar cells. Solar cells with varying Zn concentrations in their absorbers are characterized via the application of transmission electron microscopy, quantum efficiency, and current-voltage measurements, as well as admittance, x-ray photoelectron and photoluminescence spectroscopy. A Zn accumulation on the absorber side of the CuInS2–CdS interface and a higher structural defect density within the absorber are found after Zn incorporation. Capacitance, quantum efficiency, and current-voltage measurements in combination with device simulations suggest that Zn incorporation induces or enhances a shallow donor at the CuInS2–CdS interface. The interface defect pins the F...

Aspects for the optimization of CIGSe growth at low temperatures for application in thin film solar cells on polyimide foil

Cu(In,Ga)Se2 (CIGSe) thin film solar cells are the most efficient thin film photovoltaic technology available. Deposited onto the appropriate substrate they are potentially flexible, very light, robust and low cost. Due to their excellent radiation hardness and potentially high specific power, they have also attracted interest for use in space applications. Highest quality CIGSe absorber layers are usually grown at temperatures well above 500°C. So far only metal foils are a suitable choice as flexible substrate material in this temperature range. However, as those are conductive, the use of monolithic integration for solar cell interconnection requires an electrically insulating barrier between substrate and solar cell back contact. A non-conductive alternative to metal is polyimide foil. Commercially available polyimide foils are only tolerant to temperatures of up to around 400°C. It is therefore necessary to identify and understand the influence of main process parameters in order to achieve growth of high quality absorber material at these low temperatures. Former work has already highlighted that the amount of sodium present during film growth is a key parameter regarding optimum growth results. The work that is presented here summarizes previous work and investigates the complex relationship between the growth temperature and the effect of Na on the compositional, structural and electronic properties of CIGSe thin films.