Correlation of Light-induced Enhancement of Open-Circuit Voltage and Structural Change of Heterogeneous Silicon Solar Cells

Abstract

We observe a significant light-induced increase in the open-circuit voltage, Voc, of thin-film silicon solar cells whose intrinsic (i) layer consists of an amorphous and microcrystalline mixed phase. The increase depends on the i-layer thickness, the i-layer deposition temperature, the initial Voc values, and the light-soaking intensity. An increase of as large as 150 mV is observed. The original Voc is restored after subsequent thermal annealing. In-situ photoluminescence (PL) spectroscopy is used to investigate this metastable phenomenon. We find that the PL intensity and peak-energy position associated with the amorphous component of the heterogeneous material increase upon light soaking, suggesting a structural change. We propose that a reduction of microcrystalline volume fraction or size is responsible for the Voc enhancement.

Light induced electrical and macroscopic changes in hydrogenated polymorphous silicon solar cells

“…1), is related to the diffusion of molecular hydrogen, resulting in a reduction of active solar cell area due to delamination. However such delamination can hardly account for the observed increase in V oc , which in the literature has been related to dissociation of B-H complexes [18], hydrogen evolution from nanocrystals [21], and light-induced defect states re-distribution at p/i interface [22].…”

“…Various hypothesis have been proposed to explain the increase in V oc : (i) the activation of boron in the p-layer of the solar cells during light-soaking [4,[13][14][15][16][17][18], (ii) light-induced changes in the intrinsic mixed-phase material [19][20][21] and (iii) decrease in valence band tail and state re-distribution at p/i interface [22]. Most of those studies deal with solar cells based on a-Si:H materials deposited by dissociating silane-hydrogen gas mixtures, while our standard a-Si:H was deposited by dissociating pure silane.…”

High-Efficiency Amorphous Silicon and Nanocrystalline Silicon-Based Solar Cells and Modules: Final Technical Progress Report, 30 January 2006 - 29 January 2008

“…We controlled the hydrogen dilution ratio so that the films were deposited in the amorphous/nanocrystalline transition regime. We were able to obtain various material structures from a single deposition on the same substrate: amorphous (from the center of the substrate), mixed-phase (along the edges of the substrate), and a high nanocrystalline volume fraction (near the four corners), as previously reported [8,9,14,15]. We deposited many samples with different hydrogen-dilution ratios and different P-doping levels and found that adding PH 3 in the gas phase significantly increased the hydrogen-dilution threshold required for reaching the amorphous/nanocrystalline transition.…”

“…We controlled the deposition parameters in the regime of amorphous to nanocrystalline transition to obtain various material structures on one substrate, such as the center of the deposition showing an amorphous structure, edges with a mixed-phase signature, and the corners with a substantially nanocrystalline volume fraction as previously reported [14,15].…”

“…In general, the mixed-phase solar cells show an open-circuit voltage (V oc ) ranging from 0.5 to 1.0 V, which is between the V oc values of typical a-Si:H and nc-Si:H cells. One interesting phenomenon for the mixed-phase solar cells is that the V oc increases after light soaking [14,15], opposite to commonly observed for a-Si:H and nc-Si:H solar cells where the V oc decreases after light soaking due to the Staebler-Wronski effect [16]. The original explanation for the light-induced V oc increase in the mixed-phase solar cell was based on light-induced structural changes from the crystalline phase to amorphous phase [14,15].…”