News: Suppliers
23 March 2021
LayTec extends PearL to t-PearL for time-resolved PL of thin films
The transfer of copper indium gallium diselenide (Cu(In,Ga)Se2, CIGS) thin-film photovoltaics from R&D to the gigawatt production scale leads to a growing demand for fully automated process control methods. To support this, in-situ metrology system maker LayTec AG of Berlin, Germany has extended its portfolio of photoluminescence (PL) products for thin-film semiconductor characterization.
The spectral PL (sPL) product PearL has been applied for several years for in-line production control of CIGS thin-film photovoltaic modules. Here, sPL allows spectral analysis of the effective CIGS bandgap energy, which is directly correlated to the [Ga]/([Ga]+[In]) atomic ratio (GGI). Furthermore, the PL signal also reveals information about the electronic quality of the CIGS thin films. Now, t-PearL has been designed especially for complementary characterization of semiconductor thin films by means of time-resolved photoluminescence (TRPL). In contrast to sPL, TRPL investigates the time-resolved decay of the PL intensity at a fixed wavelength range, therefore directly delivering key figures on the charge carrier lifetime of the semiconductor.
In particular, for thin-film photovoltaic materials such as CIGS, cadmium telluride (CdTe) and perovskites, this method has been well established in recent years in the research community but, until today, no commercial setup for integrated measurements in deposition chambers or production lines has been available. With t-PearL, this metrology method can now be integrated directly into the deposition environment for the measurement of carrier lifetimes as low as 5ns. By vacuum integration it is also possible to investigate pristine thin films that have not yet been exposed to ambient conditions.
For a most complete sample characterization, PearL and t-PearL can be combined into a single system. Additionally, both methods are available as metrology components in LayTec’s integrated combined in-line metrology stations ILMetro as well as in the stand-alone mapping stations EpiX. Modified versions for longer lifetime materials such as silicon are available on request.
Figure. Left: Production-integrated in-line PearL metrology system for spectral PL measurements and exemplary data for CIGS measurements. Right: Vacuum-integrated combined PearL/t-PearL setup for in-vacuo CIGS analysis at ZSW. Additionally, exemplary transients for CIGS time-resolved PL measurements are shown.
In February, the first combined PearL/t-PearL system was installed and commissioned at Zentrum für Sonnenenergie- und Wasserstoffforschung Baden-Württemberg (ZSW) in Stuttgart, Germany. The materials research team at ZSW will use this fully integrated metrology system in combination with various in-situ and in-line metrology methods to monitor and control the process development of CIGS solar cells.
The target is to further develop and improve the solar energy conversion efficiency of CIGS thin-film solar cells to values above 25% – i.e. beyond the existing record of 23.4% – and to understand the mechanisms governing the performance of these devices.
A new vacuum cluster system combining two MBE 412 molecular beam epitaxy (MBE) chambers made by Riber of Bezons, France and a physical vapor deposition (PVD) chamber made by Von Ardenne of Dresden, Germany has hence been designed. Riber’s vacuum cluster system is a fully automated system that includes a central UHV cluster robot for transferring samples between all chambers.
All LayTec systems have been fully interfaced into Riber’s Crystal XE control software for acquisition control and data recording. That enables for quick and easy access to the relevant analytic parameters of each processed sample in Crystal XE software during or after thin-film deposition for real-time monitoring or for later analysis.
Regarding metrology methods, this cluster system employs LayTec`s InspiRe system for in-situ reflectance measurements in the MBE chamber during CIGS co-evaporation. Moreover, two EpiTT systems are integrated for monitoring the CIGS co-evaporation and the post-deposition treatment in-situ. Additionally, a combined PearL/t-PearL has been integrated into the transfer chamber between the Riber MBE chamber and the Von Ardenne PVD chamber. As a result, samples can be analyzed by both PL methods directly after CIGS deposition or post-deposition treatments as well as before and after transparent conducting oxide (TCO) deposition. The resulting in-depth understanding of the CIGS material should enable further improvement of CIGS solar cells, targeting even more efficient CIGS devices.
