Available on-demand - F.EN02.01.06
A Scanning Nonlinear Dielectric Microscopic Investigation of Potential-Induced Degradation in Monocrystalline Silicon Solar Cells
Yasuo Cho1,Sachiko Jonai2,Atsushi Masuda2
Tohoku University1,National Institute of Advanced Industrial Science and Technology2
Crystalline silicon (Si) solar cells currently occupy an important place in the solar cell market. However, potential-induced degradation (PID) of crystalline Si photovoltaic (PV) modules has recently been observed in large systems comprising significant numbers of PV modules. In these PV systems, numerous such modules are interconnected in series, such that potential stress is imparted to some of the modules with consequent high power losses. PID in Si PV modules based on p-type crystalline Si solar cells has been previously investigated in detail -.
About estimated changes in solar cell during PID, for example, it has been reported that, due to the penetration of Na into the solar cell, a deep level is formed in the depletion layer or the base layer (p layer) and, as a result, carrier recombination through the level occurs . Na is known to create a level in Si at a position 0.35 eV from the valence band. It also has been suggested that current shunt path is formed by intrusion of Na into stacking faults penetrating the pn junction (or that Na invading Si forms stacking faults) .
Even so, there is still a lack of understanding of the PID mechanism in solar cells, and obtaining more information regarding this phenomenon will require measurements of carrier distributions on a microscopic scale. One of the authors previously succeeded in quantitatively analyzing such distributions in monocrystalline Si solar cells using scanning nonlinear dielectric microscopy (SNDM) .
In the present study, the microscopic carrier distributions in monocrystalline Si solar cells exhibiting PID were investigated using this same technique. In addition, an enhanced version of this technique referred to as super-higher-order (SHO)-SNDM was employed to visualize depletion layer distributions in such cells both with and without PID.
As a result, reductions in carrier density in accordance with the extent of PID were clearly observed, and quantitative measurements demonstrated that the electron concentration was reduced by serval order of magnitude due to PID. Depth profile measurements showed that the solar cells were affected by PID to a significant depth of approximately 90 μm, equal to almost half the cell thickness, suggesting that Na+ migration proceeded easily upon the application of a high voltage. The SHO-SNDM data showed that the depletion layer of the non-PID sample was thinner than that of a PID sample, indicating that the carrier concentration in the former was greater.
This work demonstrates that SNDM is a very useful means of investigating the PID effect through the measurement of carrier distributions in monocrystalline silicon solar cells.
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