NREL’s use of potassium fluoride process improves the open-circuit voltage and conversion efficiency.
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Cu(In,Ga)Se2 (CIGS) solar cells that are fabricated using two-step selenization processes often exhibit low open-circuit voltage (Voc). National Renewable Energy Laboratory (NREL) scientists have found a way to improve Voc without using a more
complex three-stage co-evaporation process.
Previously, NREL investigated the two-step selenization process using two different evaporated precursors—stacked metal precursors and Se-containing precursors. These experiments led to a 16.2%-efficient solar cell fabricated from the Cu/Ga/
In stacked metal precursor. But the goal was to increase the efficiency and decrease the time needed for selenization. To accomplish efficiency gains, the Voc needed to be increased.
The two-step selenization process has a few limitations. First, Ga segregation toward the back of CIGS films degrades performance due to lower Voc. This can be alleviated by adding a sulfurization step to widen the bandgap near the CIGS surface. Second, these films lack a precisely engineered surface. A desired Cu-depleted surface region can be created using a
three-stage coevaporation process. The challenge with the two-step selenization process is to produce a similar surface without adding a third stage of Ga and In.
NREL found that the Voc of solar cells made from a Cu/Ga/In stacked metal precursor can be increased by using a potassium fluoride (KF) post-deposition treatment. A two-step selenization process that included this post-deposition treatment led to an 18.6%-efficient CIGS device with a favorable Voc of 0.709 volt.
In addition, efficiency and fill factor values were also improved. For devices made from the same CIGS absorbers, the KF treatment improved efficiencies by 1.5%–1.9%, which resulted from Voc values being boosted by 48–70 millivolts. In one case, fill factor was improved by 2.9%.
The KF post-deposition treatment even improved the Voc and efficiency of previously grown CIGS solar cells that were treated at a later time.
Key Research Results
NREL applied a KF post-deposition treatment to solar cells made from a Cu/Ga/In stacked metal precursor.
A two-step selenization process that included this KF post-deposition treatment led to an 18.6%-efficient CIGS device with a favorable Voc of 0.709 V.
NREL’s approach precludes the need to use a more complex three-stage coevaporation process to improve CIGS solar cell performance.
Technical Contact: Lorelle Mansfield, firstname.lastname@example.org
Reference: Mansfield, L.M.; Noufi, R.; Muzzillo, C.P.; Dehart, C.; Powers, K.; To, B.; Pankow, J.W.; Reedy, R.C.; Ramanathan, K.(2014). “Enhanced Performance in Cu(In,Ga)Se2 Solar Cells Fabricated by the Two-Step Selenizaton Process with a Potassium Fluoride Post-Deposition Treatment.” IEEE J. Photovoltaics, forthcoming.
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