Distorted IV curve shape

ID: symptom:iv-curve-distortion
IRI: https://solarlattice.org/kb/symptom/iv-curve-distortion
Status: active
Also known as: curve shape variations

A normal IV curve is smooth and continuous: a nearly flat leg, a knee, and a steep leg down to open-circuit voltage. Distortion is any departure from that shape compared with the predicted curve. Before you look for a fault, make sure the difference is not caused by measurement error, instrument setup, or wrong module or string data entered in the tracer. For shape tests, use irradiance above 100 W/m2. On stable days, overlay the curves of identical strings, which should normally agree within about 5 %. Steps or notches mean bypass diodes are conducting. Likely causes are partial shading, soiling or snow, damaged cells, a shorted bypass diode, or several cell interconnection failures that block current. A rounded knee with higher series resistance points to failing redundant cell interconnects or to aging. A shallower slope near open-circuit voltage points to resistive joints or corrosion. A lower short-circuit current with an otherwise normal shape suggests uniform glass corrosion or loss of transparency. Patchy glass corrosion can also slope the curve near short-circuit current and add inflection points. Confirm the cause with infrared thermography, electroluminescence and visual inspection. These help separate interconnect failures from cracked cells or diode failures.

<!-- review notes

sentence 5: support check: partial - Rounded knee with series resistance from redundant interconnect failure and aging is supported, but the shallower Voc slope is linked to interconnect resistance, corrosion and slack joints, not stated as 'resistive joints' exactly (close), and the rounded-knee/high-Rs link to aging is combined loosely.
sentence 6: support check: partial - Heterogeneous glass corrosion sloping near Isc with inflection points is supported, but homogeneous cases are shown only in the table, and 'otherwise normal shape' for uniform corrosion or transparency loss is implied rather than stated. -->

Claims

`observed_as`

c1 drafted · lead IV curve step

Steps or notches mean bypass diodes are conducting because part of the module or array cannot carry the string current. Causes include partial shading, partial soiling or snow, damaged cells, a shorted bypass diode, heavy cell mismatch, heterogeneous glass corrosion, and several interconnect failures that block current.

IEC 62446-1: Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1 sec=D.2 h=e2d1cc6f1a29
Review of Failures of Photovoltaic Modules sec=5.2.4 h=9ce8dc98ece9
Review of Failures of Photovoltaic Modules tbl=12 h=f12a39856dc6
cell interconnection failure sec=h8 h=9f1fbad653f1

c2 drafted · lead Rounded IV curve knee

A rounder knee can come from aging, or from higher series resistance at failed redundant cell interconnects. Check the slopes of both legs first, because changed slopes can make the knee look similar.

IEC 62446-1: Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1 sec=D.5 h=91494b8229fd
cell interconnection failure sec=h8 h=9f1fbad653f1

c3 drafted · lead Sloped IV curve near Isc

Shunt resistance has dropped because of shunt paths in the cells or interconnects, slight cell mismatch or uneven yellowing. Also listed for heterogeneous glass corrosion.

Review of Failures of Photovoltaic Modules sec=5.2.4 h=9ce8dc98ece9
Review of Failures of Photovoltaic Modules tbl=12 h=f12a39856dc6

c4 drafted · lead Shallow IV slope near Voc

Series resistance has risen in the module, for example from interconnect resistance, corrosion in the junction box or interconnects, or slack joints.

Review of Failures of Photovoltaic Modules sec=5.2.4 h=9ce8dc98ece9

c5 verified · fact Lowered Isc on IV curve

Loss of transparency, glass corrosion or delamination acts like reduced irradiance. Homogeneous and heterogeneous cases change the curve shape differently.

Review of Failures of Photovoltaic Modules sec=5.2.4 h=9ce8dc98ece9
Review of Failures of Photovoltaic Modules tbl=12 h=f12a39856dc6

verified by: review-workflowverified on: 2026-10-20

c6 drafted · lead Low open-circuit voltage

Caused by failed cell interconnects, cell-to-cell shorts, bypass diode failure, LID or PID.

Review of Failures of Photovoltaic Modules sec=5.2.4 h=9ce8dc98ece9

c7 drafted · lead Reduced fill factor

A sloped Isc region and a shallower slope near Voc both reduce fill factor and Pmax. Metallization corrosion shows a lower fill factor from higher series resistance.