Download Elimination of Potential-Induced Degradation for Crystalline Silicon

School of Electrical, Computer and Energy Engineering
PhD Final Oral Defense
Elimination of Potential-Induced Degradation for Crystalline Silicon Solar Cells
by
Jaewon Oh
7/27/2016
2:00 pm
ENGRC 189
Committee:
Dr. Stuart Bowden (co-chair)
Dr. Govindasamy Tamizhmani (co-chair)
Dr. Christiana Honsberg
Dr. Peter Hacke
Dr. Dieter Schroder
Abstract
Potential-Induced Degradation (PID) is an extremely serious photovoltaic (PV)
durability issue significantly observed in crystalline silicon PV modules due to its rapid
power degradation, particularly when compared to other PV degradation modes. The
focus of this dissertation is to understand PID mechanisms and to develop PID-free cells
and modules. PID-affected modules have been claimed to be fully recovered by high
temperature and reverse potential treatments. However, the results obtained in this work
indicate that the near-full recovery of efficiency can be achieved only at high irradiance
conditions, but the full recovery of efficiency at low irradiance levels, of shunt resistance,
and of quantum efficiency (QE) at short wavelengths could not be achieved. The QE loss
observed at short wavelengths was modeled by changing the front surface recombination
velocity. The QE scaling error due to a measurement on a PID shunted cell was addressed
by developing a very low input impedance accessory applicable to an existing QE
system. The impacts of silicon nitride (SiNx) anti-reflection coating (ARC) refractive
index (RI) and emitter sheet resistance on PID are presented. Low RI ARC cells (1.87)
were observed to be PID-susceptible whereas high RI ARC cells (2.05) were determined
to be PID-resistant using a method employing high dose corona charging followed by
time-resolved measurement of surface voltage. It has been demonstrated that the PID
could be prevented by deploying an emitter having a low sheet resistance (~ 60 /sq)
even if a PID-susceptible ARC is used in a cell. Secondary ion mass spectroscopy
(SIMS) results suggest that a high phosphorous emitter layer hinders sodium transport,
which is responsible for the PID. Cells can be screened for PID susceptibility by
illuminated lock-in thermography (ILIT) during the cell fabrication process, and the
sample structure for this can advantageously be simplified as long as the sample has the
SiNx ARC and an aluminum back surface field. Finally, this dissertation presents a
prospective method for eliminating or minimizing the PID issue either in the factory
during manufacturing or in the field after system installation. The method uses
commercially available, thin, and flexible Corning® Willow® Glass sheets or strips on
the PV module glass superstrates, disrupting the current leakage path from the cells to the
grounded frame.