Journal
SOLAR ENERGY MATERIALS AND SOLAR CELLS
Volume 111, Issue -, Pages 90-96Publisher
ELSEVIER
DOI: 10.1016/j.solmat.2012.12.038
Keywords
Thin film silicon; Nanocrystalline silicon; Optical bandgap; Mobility bandgap; Solar cell; Diode characteristic
Funding
- US Department of Energy through the Solar America Initiative Program [DE-FC36-07 GO 17053]
- US Department of Defense through US Army [W9132T-11-C-0007]
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We present optical and electrical measurements of bandgap of hydrogenated nanocrystalline silicon (nc-Si:H). The absorption coefficient (alpha) does not yield a linear relation on the Tauc plot, indicating that the concept of the Tauc bandgap is no longer valid. The energy at the absorption coefficient alpha=10(4) cm(-1) increases with crystallinity, but it does not imply that the optical bandgap increases with crystallinity. Instead, it reflects the characteristics of mixed-phase material, where the crystalline phase has lower absorption than the amorphous phase. Dark current versus voltage measurements of nc-Si:H solar cells as a function of temperature showed that the products of ideality factor and the activation energy of the pre-factor are the same for nc-Si:H solar cells with different crystallinities. A mobility bandgap at 0 K of 1.06 eV was derived from these studies. Finally, the temperature dependence of open circuit voltage (V-oc) of nc-Si:H solar cells with different V-oc values at 25 degrees C shows the same intersection of 1.04 eV at 0 K. It provides additional evidence that the mobility bandgap is independent of crystallinity and is similar to that of c-Si. The values obtained from both the methods are similar to but smaller than the bandgap of c-Si at 0 K; however, the difference could result from experimental errors. (C) 2013 Elsevier B.V. All rights reserved.
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