Keithley Instruments’ solar cell testing survey shows priorities and best practices in test methods

A recent survey of solar cell/photovoltaic device researchers andmanufacturers working in government, university-based, and corporatelabs and manufacturing facilities indicates distinct differences intesting methods and priorities among respondents from Asia, NorthAmerica, and Europe.

Keithley Instruments Inc. conducted thestudy during Summer 2009 using a by-invitation-only online surveygenerating 564 responses. A white paper summarizing the results isavailable for downloading at no charge from Keithley’s website.

Acrossall geographies, the survey indicated the industry is focused onimproving device efficiencies as the dominant development priority.“Reducing manufacturing cost” was the second most important among therespondents, as might be expected, although substantially lesssignificant than the need to boost device and panel performance.

Clear priority emerges for top four solar cell test methods
Forthose engaged in solar cell R&D, a large majority of respondentsidentified their “key parameters” for measurement as short-circuitcurrent (ISC), open-circuit voltage (VOC), maximum output power (PMAX)and, to a lesser extent, conversion efficiency.

Respondents inNorth America and Europe showed consistent agreement on whatconstitutes their most important tests, with a well-defined gap betweenthese top four and ten other tests. Respondents from Asia, whileagreeing with the same top four tests, showed much less preference forthem as other tests were cited as key parameters nearly as often as thetop four.

“The top three tests indicated by respondents areconsistent with what customers tell us are important methodologies forthem in the research phase,” noted Keithley Marketing Director MarkCejer.

“We were a bit surprised not to see more priority givento shunt resistance and series resistance tests, as we hear quite a bitabout how important those measurements are to characterizing deviceperformance. Overall, other than a clear preference for the top threetests, there remains a significant amount of variation in the industryabout what tests are most meaningful.

“There may be severalreasons for this variation. For example, the tests used may varydepending on the cell technology or on the market for which they areintended. Also, many of the solar cell technologies are relatively newand therefore there are no industry-standard testing methodologies.”

Thepriority of test parameters characterized as solar cell/photovoltaicsmove into production remained much the same, but there was far lessagreement about the most important tests than was found in the researchlab. And, Asian respondents were far more definitive about the mostvaluable solar cell tests for production than for research applications.

Engineersin general seem to be relatively unfazed by the solar cell industry’srather complicated battery of tests. When asked which tests presentedtechnical challenges, none of the 15 tests appeared to pose a problemfor a large portion of the respondents, either in the research orproduction test phase.

This held true for even the mostdemanding tests included on the list, such as quantum efficiency andconversion efficiency. Maximum power output was identified as the mostdifficult test to perform, with 28% noting it as the “mostchallenging,” perhaps reflecting the difficulty of integrating 50Wpower supplies into a test system for this type of research.

PV device technologies
Surveyrespondents report engagement in a broad range of solar cell/PV devicetechnologies, including established, relatively well-characterizedtechnologies for making PV devices out of monocrystalline,multicrystalline, or amorphous silicon, as well as newer thin-filmtechnologies such as copper indium gallium selenide (CIGS), cadmiumtelluride (CdTe), and gallium arsenide (GaAs).

In general,respondents seemed more active in thin-film materials than in silicon,perhaps due to the improved response rates and capacitive properties ofthin-film materials.

Given the staggering array of devicetechnologies now being explored or developed, the industry seems poisedto become larger and increasingly competitive, with manufacturersmaking significant investments in finding new ways to extract themaximum energy at the lowest possible cost from every photon thatreaches their products. Asian manufacturers appear firmly committed toplaying a major role in the worldwide solar cell industry.