Humberto Rodriguez-Alvarez

Effect of Na presence during CuInSe2 growth on stacking fault annihilation and electronic properties

While presence of Na is essential for the performance of high-efficiency Cu(In,Ga)Se2 thin film solar cells, the reasons why addition of Na by post-deposition treatment is superior to pre-deposition Na supply—particularly at low growth temperatures—are not yet fully understood. Here, we show by X-ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu-poor/Cu-rich transition of low temperature 3-stage co-evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental effect on electronic properties if Na is present during growth.

Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells

In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron–hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of – partially electronically active – planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu–Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu–Se at the surface of the Cu(In,Ga)Se2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.

Co-evaporation of Cu(In, Ga)Se 2 at low temperatures: An In-Situ x-ray growth analysis

Cu(In, Ga)Se2 thin films have been coevaporated onto Mo coated soda-lime glass using a multi-stage approach and two different maximum growth temperatures (420°C and 530°C). In order to investigate principal differences in the growth dynamics for the two temperature regimes, the growth has been monitored by in-situ energy dispersive X-ray diffraction, performed at the EDDI beamline at the Helmholtz-Zentrum Berlins BESSY II synchrotron facility. During the in-diffusion of Cu-Se into the In-Ga-Se precursor a signature that points towards a possible Cu-deficient defect phase or a chalcopyrite phase that incorporates stacking faults in the [221] direction is observed to be visible throughout a wider compositional range for the low temperature process. It disappears once the film becomes Cu-rich and thus highlights the critical role of Cu-excess for the growth of chalcopyrite thin films, particularly at low growth temperatures.

Pressure Dependent Rapid Thermal Processing of CuInS 2 Thin Films Investigated by In-Situ Energy Dispersive X-ray Diffraction

The rapid thermal processing (RTP) of Cu-rich Cu/In precursors for the synthesis of CuInS 2 thin films is possible within a broad processing window regarding leading parameters like top temperature, heating rate, and Cu excess. The key reaction pathway for the CuInS 2 phase formation has already been investigated by in-situ energy dispersive X-ray diffraction (EDXRD) for various precursor stoichiometries, heating rates and top temperatures at sulphur partial pressure conditions which are typical for physical vapour deposition processes. According to the phase diagrams of the binary sulphide phases, the sulfur partial pressure strongly determines the occuring crystalline phases. However, a controlled variation of the maximum sulphur partial in a typical RTP experiment has not been carried out yet. In order to study the influence of this parameter a special RTP reaction chamber was designed suitable for in-situ EDXRD experiments at the EDDI beamline at BESSY, Berlin. In a typical in-situ RTP/EDXRD experiment sulphur and a Cu/In/Mo/glass precursor are placed in an evacuated graphite reactor. The amount of sulphur determines the maximum pressure available at the top temperature of the experiment. As the RTP process proceeds a complete EDXRD spectrum is acquired every 10 seconds and thus the various stages of the reaction path and the crystalline phases can be monitored. The first experiments show already a significant change in the reaction pathway and the secondary Cu-S phases which segregate on top of the CuInS 2 thin film during the reaction.