Structural analyses of seeded thin film microcrystalline silicon solar cell

Abstract

This contribution investigates the effect of seeding the growth of thin film microcrystalline silicon (µc‐Si : H) deposited by radio frequency plasma‐enhanced chemical vapor deposition on the material properties of µc‐Si : H film and the device performance of p‐i‐n and n‐i‐p µc‐Si : H solar cells. By means of Raman measurement, x‐ray diffraction (XRD) and transmission electron microscopy (TEM), we investigate the structure of seeded µc‐Si : H. In particular, the effect of seed layers on the crystallinity development is investigated. Measurements of the depth profile of the crystalline mass fraction using Raman spectroscopy show that seed layers lead to a more rapid and uniform crystallinity development in growth direction. The amorphous incubation layer is suppressed and crystallization begins directly from onset of film growth without evolving through the intermediate growth phases. From TEM analyses, we observe that crystal sizes are not affected by seed layers. Horizontal cracks are however observed to dominate the early growth of µc‐Si : H in p‐i‐n solar cell and this is reduced upon seeding. For the n‐i‐p cells, these cracks are not affected by seeding. XRD results also indicate that the use of seed layers does not affect the crystal sizes but affects the direction of preferential orientation. Solar cell external parameters show that seeding of p‐i‐n solar cells leads mainly to increase in short‐circuit current density, Jsc with a slight drop in open‐circuit voltage, Voc. For the n‐i‐p cells, a reverse effect is observed. In this case, the Voc increases and the Jsc decreases. Copyright © 2012 John Wiley & Sons, Ltd.

Interplay between crystallinity profiles and the performance of microcrystalline thin-film silicon solar cells studied by in-situ Raman spectroscopy

“…1): It is well known that an increase of I RS C for the initial growth phase induces an overall increase of I RS C for the later layer growth. 12 Increasing SC avoids strongly deviating crystallinity from advantageous I RS C of $60%-70% 1,12,15,[18][19][20] ( Fig. 1).…”

“…1 It is common practice to estimate the crystalline volume fraction by the Raman crystallinity. 9,10 The growth of lc-Si:H is influenced by a large number of external and internal process parameters (substrate morphology, 11 seeding effect of doping layers, 12 plasma excitation frequency, 6 reactor geometry, 13 etc.). Therefore, the reproducibility of Raman crystallinity profiles in the growth direction is challenging and optimized Raman crystallinities throughout the absorber layer are difficult to realize.…”

“…21 To achieve these Raman crystallinities throughout the entire absorber layer process settings have to be adapted during the deposition of lc-Si:H. Since this requires an adequate monitoring of the Raman crystallinity, various in-situ and ex-situ measurement techniques have been applied to characterize the lc-Si:H absorber layer deposition. [8][9][10]12,13,19,[21][22][23][24][25][26][27][28] To monitor the layer growth with adequate accuracy, diagnostic methods with high depth and temporal resolution are required. For example, in-situ ellipsometry features high surface sensitivity.…”

“…High depth resolution of the Raman crystallinity in the growth direction was achieved by KOH wet-etching or reactive ion etching. 12,21,24 Scanning the etched craters by microscopic Raman spectroscopy, the Raman crystallinity is estimated in the growth direction of the lc-Si:H film ex-situ. 24 In the present paper, we investigate the microcrystalline silicon growth by the technique of in-situ Raman spectroscopy.…”

“…1,17 The highest solar cell performances were detected for absorber layers with Raman crystallinities of about 60%-70%. 1,12,15,[18][19][20] The ideal evolution of the Raman crystallinity throughout the absorber layer is still a matter of debate. Therefore, this is one of the questions that will be addressed here.…”