Abstract
The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells with bandgaps (Eg) > 1.40 eV is studied on a large sample set. It is confirmed that moving away in composition from ternary AgGaSe2 by simultaneous reduction in Ga and Ag content widens the chalcopyrite single‐phase region and thereby reduces the amount of ordered vacancy compounds (OVCs). As a consequence, a distortion in current−voltage characteristics, ascribed to OVCs at the back contact, can be successfully avoided. A clear anticorrelation between open‐circuit voltage (VOC) and short‐circuit current density (JSC) is detected with varying absorber stoichiometry, showing decreasing VOC and increasing JSC values for [I]/[III] > 0.9. Capacitance profiling reveals that the absorber doping gradually decreases toward stoichiometric composition, eventually leading to complete depletion. It is observed that only such fully depleted samples exhibit perfect carrier collection, evidencing a very low diffusion length in wide‐gap ACIGS films. The results indicate that OVCs at the surface play a minor or passive role for device performance. Finally, a solar cell with VOC = 0.916 V at Eg = 1.46 eV is measured, which is, to the best of our knowledge, the highest value reported for this bandgap to date.
Highlights
Today, highest conversion efficiencies (η) of Cu(In,Ga)Se2 (CIGS) solar cells are reached for absorber bandgap energies (Eg) below 1.2 eV, peaking at η 1⁄4 22.6%[1] and η 1⁄4 23.4%[2] if sulfur is incorporated
A substantial formation of ordered vacancy compound (OVC) was found for group-I (I)-deficient ACIGS within the compositional window of GGI > 0.5 and AAC > 0.5.[29,30] Further investigations revealed that these OVCs are present in the form of isolated patches located at the interfaces, as a consequence of the applied three-stage deposition process.[24,28]
CdS is a mature and stable buffer layer, alternative materials like (Zn,Sn)O are suggested for wide-gap chalcopyrite solar cells, as parasitic absorption in the buffer leads to increasing relative JSC losses for larger Eg
Summary
Highest conversion efficiencies (η) of Cu(In,Ga)Se2 (CIGS) solar cells are reached for absorber bandgap energies (Eg) below 1.2 eV, peaking at η 1⁄4 22.6%[1] and η 1⁄4 23.4%[2] if sulfur is incorporated. A likely explanation is that the potentially forming OVC is not fully covering the absorber surface, but rather appears in separated patches This would leave a certain area fraction, exhibiting a direct and detrimental, cliff-like CIGS/CdS interface. A substantial formation of OVCs was found for group-I (I)-deficient ACIGS within the compositional window of GGI > 0.5 and AAC > 0.5.[29,30] Further investigations revealed that these OVCs are present in the form of isolated patches located at the interfaces, as a consequence of the applied three-stage deposition process.[24,28] Recently, we studied the effect of absorber stoichiometry (i.e., [I]/[III] value) on ACIGS with constant GGI 1⁄4 0.80 and AAC 1⁄4 0.85, resulting in Eg 1⁄4 1.61 eV (optimum for top cell in 2T tandem).[28] For a large off-stoichiometry ([I]/[III] < 0.85), (Ag,Cu)(In,Ga)3Se5 (1:3:5) patches were observed at the front and back contact (see top of Figure 1).
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