The
effects of RbF postdeposition treatment (RbF-PDT) on Cu(In,Ga)Se2, CuInSe2, and CuGaSe2 thin films and
solar cell devices are comparatively studied. Similar to the effect
of the KF postdeposition treatment (KF-PDT), Cu(In,Ga)Se2 and CuInSe2 film surfaces show significant pore formation
resulting in a rough surface morphology with RbF-PDT, whereas this
is not the case for In-free CuGaSe2. The device properties
of the In-containing and In-free Cu(In,Ga)Se2 solar cells
also show contrasting results, namely, Cu(In,Ga)Se2 or
CuInSe2 devices show an increase in the open circuit voltage
(V
oc) and fill factor (FF) values and
almost constant or a slight decrease in the short-circuit current
density (J
sc) values with RbF-PDT, whereas
CuGaSe2 devices show no significant improvements in the V
oc and FF values but a substantial increase
in the J
sc values. These results suggest
that the alkali effects on the Cu(In,Ga)Se2 film and device
properties strongly depend on the group III elemental composition
in the Cu(In,Ga)Se2 films as well as alkali-metal species.
The effects of long-term heat-light soaking on Cu(In,Ga)Se2 (CIGS) solar cells with KF postdeposition treatment (KF-PDT) have been investigated. CIGS solar cells with KF-PDT frequently deteriorate after storage in the dark because of decreasing hole concentrations in the CIGS layers. Although light soaking improves hole concentrations, the resulting increase in conversion efficiency is not significant. In contrast, we found that long-term heat-light soaking effectively increases conversion efficiency, since the saturation current density and ideality factor are clearly improved by passivating the recombination centers at CdS/CIGS interfaces.
Single-crystal Cu(In,Ga)Se2 (CIGS) solar cells were produced with techniques developed for high-efficiency polycrystalline CIGS solar cells. The CIGS layers of a lattice match with GaAs were grown on GaAs(001) substrates by co-evaporation. The presence of a single-crystal CIGS layer without dislocations was confirmed by transmission electron microscopy. Alkaline metal incorporations were achieved by doping and postdeposition treatments. Ga grading structures were fabricated by two-layer deposition with different Ga contents. The Ga grading significantly increased the fill factor and open-circuit voltage. The best efficiency of 20% was achieved after heat–light soaking.
In this study, the influences of bromine-based etching (Br etching) of narrow band gap CuInSe 2 (CIS) absorbers and Cu(In,Ga)Se 2 absorbers with various single Ga gradings (CIS:Ga) on the properties of solar cells were investigated. Absorbers with narrow absorption edge energies (E abs ) of 1.0−1.02 eV, ideal for the application as a bottom cell in a tandem device, were fabricated using a modified three-stage process and subjected to Br etching. The evolution of surface flatness and their optical and electrical properties upon Br etching were investigated. Br etching typically reduced the root-mean-square deviation of the surface roughness height (R q ) for a CIS:Ga absorber from several hundreds to several tens of nanometers, whereas for some CIS absorbers, R q reduction was limited by the remaining voids. Moreover, Br etching reduced the leakage current across the pn junction. The high shunt resistances (R sh ) typically up to >10 kΩ•cm 2 were obtained by introduction of Br etching. However, etching sometimes adversely increased the V OC deficit. The investigation of the minority carrier lifetime and diode parameters revealed that backsurface recombination in CIS and low-Ga CIS:Ga solar cells increased as the absorber layer thickness decreased. A higher Ga grading significantly reduced back-surface recombination. Narrow band gap CIGS solar cells with improved surface flatness and high V OC were achieved by introducing Br etching and proper Ga grading.
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