The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed dependence of the paramagnetic defect density on the Doppler S parameter indicates that porous, nanosized void-rich materials exhibit higher spin densities, while dense, divacancy-dominated materials show smaller spin densities. However, after light soaking more similar spin densities are observed, indicating a long-term defect creation process in the Staebler-Wronski effect that does not depend on the a-Si:H nanostructure. From ESE decays it appears that there are fast and slowly relaxing defect types, which are linked to various defect configurations in small and large open volume deficiencies. A nanoscopic model for the creation of light-induced defects in the a-Si:H nanostructure is proposed. The light-induced degradation (LID) of hydrogenated amorphous silicon (a-Si:H), also known as the StaeblerWronski effect (SWE) [1,2], has been extremely thoroughly investigated in the past decades [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Although the origin of the SWE and the nature of native and metastable defects is still poorly understood, impressive progress has been made in, for instance, the development of thin-film silicon (TF Si) solar cells. Record initial and stable conversion efficiencies of 16.3%[22] and 13.4%-13.6% [23,24], respectively, have been reported for small area (∼1 cm 2 ) solar cells, all in triple-junction configuration. However, the amorphous junction produces most of the power in such solar cells, which means that fundamentally understanding the SWE is still important when aiming to increase the conversion efficiency of TF Si solar cells. Successfully applied LID-reduction methods include hydrogen (H 2 ) dilution of the silane gas (SiH 4 ) used during the plasma-enhanced chemical vapor deposition (PECVD) [25,26] and the use of a triode PECVD reactor [27,28]. However, the film growth follows such a complex interplay of deposition, etching, and hydrogen (H) effusion that the precise role of H in the a-Si:H nanostructure and the SWE remains obscured, although various growth models have been proposed [29].

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In recent years, Thulium in its 2+ oxidation state has been identified as candidate dopant in halide hosts for luminescent solar concentrators. Yet, some of its luminescent properties with regard to these applications remain unexplored. In this study we report on the temperature dependent photo-luminescent behaviour of NaCl:Tm 2+ , NaBr:Tm 2+ , and NaI:Tm 2+ . These monohalide materials demonstrate up to five distinct emission peaks which can be attributed to the 4f 12 →4f 12 and 4f 11 5d 1 →4f 12 transitions of Tm 2+ . Their time-and temperature dependent luminescence intensity behaviours are explained by a qualitative model describing the thermally stimulated radiative-and non-radiative relaxation dynamics. The behaviour of Tm 2+ in these monohalides proves to be similar to earlier reported findings on Tm 2+ -doped trihalide perovskites of the form CsCaX3 (X=Cl, Br, I), however, the 4f-4f emission remains by far the most dominant emission between 10-300K.

The prospect of using Tm 2+ -doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm 2+ excited states dynamics that determines the internal quantum efficiency (QE) and thereby the efficiency of the LSC. In this study we investigated the dynamics in CaX 2 :Tm 2+ (X = Cl, Br, I) by temperature-and time-resolved measurements. At 20 K up to four distinct Tm 2+ emissions can be observed. Most of these emissions undergo quenching via multi-phonon relaxation below 100 K. At higher temperatures, only the lowest energy 5d-4f emission and the 4f-4f emission remain. Fitting a numerical rate equation model to the data shows that the subsequent quenching of the 5d-4f emission is likely to occur initially via multi-phonon relaxation, whereas at higher temperatures additional quenching via interband crossing becomes thermally activated. At room temperature only the 4f-4f emission remains and the related QE becomes close to 30%. Possible reasons for the quantum efficiency not reaching 100% are provided.

In this study, we systematically vary the Cl/Br and Br/I ratios in CsCaX3:Tm2+ (X = Cl, Br, I) and hereby gradually shift the positions of the Tm2+ 4f125d1-levels as relative to the two 4f13 levels. At low temperatures up to five distinct Tm2+ 4f125d1→4f13 emissions and the 4f13→4f13 emission can be observed. As the temperature increases, most of the 4f125d1→4f13 emissions undergo quenching via multi-phonon relaxation (MPR) and at room temperature only the lowest energy 4f125d1→4f13 and the 4f13→4f13 emission remains. For all compositions a 4f13→4f13 risetime phenomenon is then observed whose duration matches the 4f125d1→4f13 decay time. It shows the feeding of the 4f13 state after 4f125d1 excitation. Surprisingly, the feeding time becomes longer from Cl→Br→I, while the related 4f125d1-4f13 energy gap becomes smaller. The temperature dependence of the 4f125d1→4f13 and 4f13→4f13 emission intensity shows a anticorrelation as earlier observed in other systems and confirms that the feeding process is thermally stimulated. However, the thermally stimulated activation energies that control the feeding process, increase from Cl→Br→I despite our observation that the 4f125d1-4f13 energy gap becomes smaller. An analysis reveals that the unexpected behaviour in risetime and activation energy, as a function of composition, cannot be explained by 4f125d1→4f13 feeding via interband crossing, but more likely via MPR where the electron–phonon coupling strength decreases from Cl→Br→I. No strong relation was found between composition and the quantum efficiency (QE) of the 4f13→4f13 emission, due to the presence of fluctuations that are likely caused by intrinsic differences in sample quality. Nevertheless, a 4f13→4f13 QE of up to 70% has been observed and the materials can therefore be used in luminescence solar concentrators.