Naslov
Structure of the silicon thin films for solar cells

Autori
Gajović, Andreja ; Gracin, Davor ; Djerdj, Igor ; Tomašić, Nenad ; Su, Dangsheng ; Schlög, Robert

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Proceedings, 2nd Croatian Congress on Microscopy with International Participation / Srećko Gajović - Zagreb : Hrvatsko mikroskopijsko društvo, 2006, 142-143

Skup
2nd Croatian Congress on Microscopy with International Participation

Mjesto i datum
Topusko, Hrvatska, 18.05.2006. - 21.05.2006

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Domaća recenzija

Ključne riječi
Silicon thin films; HRTEM; nanostructure

Sažetak
In production of thin films solar cells most common used material are silicon thin films. The efficiency of the solar cells could be considerably improved by applying Si material in different structural forms, since the absorption of light occurred in broader wavelength interval. As a result of various quantum effects the optical energy gaps alter from 2.2 eV for amorphous Si to 1.1 for single-crystalline material, while nano-crystallites cover gaps between these two values. The silicon has cubic structure, space group Fd3m. Thin silicon films with different degree of crystallinity were prepared by decomposition of silane gas, diluted with hydrogen, in radiofrequency discharge. The crystallite sizes and the portion of crystalline phase were investigated by high-resolution transmission electron microscopy (HRTEM), selected-area electron diffraction (SAED), Raman Spectroscopy (RS) and X-ray powder diffraction (XRD). HRTEM images showed crystallites in all investigated samples (Fig. 1), but the observed sizes and shapes of crystallites were different. SAED patterns showed diffractions spots superimposed to amorphous halo (inserts in Fig. 1). Therefore, both amorphous and crystalline materials were present in all studied thin silicon films, although in particular sample RS and XRD results did not reveal any crystalline bands/lines. The filtering of the HRTEM images was made to accomplish better recognition of the crystallites, thus even in RS and X-ray amorphous sample the crystallites were clearly observed (Fig. 2). Raman spectrum of the microcrystalline Si is characterized by one intensive sharp band at 521 cm-1. For crystallites smaller than 30 nm this band is broadened and its position shifted to lower frequencies with appearance of low energy shoulder [1]. The broad band with maximum around 480 cm-1 is attributed to amorphous phase [2]. The volume fractions of the crystalline phase were estimated from the ratio of the integrated intensities of crystalline and amorphous band after deconvolution of the spectra (Table 1). The position and the full with at half-maximum of the crystalline band also varied in our samples indicating different nano-sized crystallites (Table 1). However, quantitative determination of the crystallinity from Raman measurements is doubtful because of different cross-section factors of the Raman scattering for crystalline and amorphous silicon. XRD measurements showed broad amorphous band for all samples. However, in XRD pattern of sample 1 three reflections of silicon were observed, (111), (220) and (311), indicating considerably higher amount of crystalline phase than in the other samples (Fig 3). The weight fraction of amorphous phase and average crystallite sizes were calculated for this sample from XRD patterns using modified Rietveld refinement with SiO2 as internal standard. The calculated weight fraction of crystalline phase was 25.2 % and average crystallite size 11.1 nm. The results calculated from XRD were in approximately in agreement with the results estimated from Raman spectra. Moreover, the high local amount of crystallites observed in HRTEM images has confirmed the presence of crystalline phase in investigated materials.

Izvorni jezik
Engleski

Znanstvena područja
Fizika, Geologija

POVEZANOST RADA