Reducing Interface Recombination for Cu(In,Ga)Se2 by Atomic Layer
Deposited Buffer Layers
Adam Hultqvist1, Jian V. Li2, Darius Kuciauskas2, Patricia Dippo2, Miguel A.
Contreras2, Dean H. Levi2, Stacey F. Bent1
1Department
2National
of Chemical Engineering, Stanford University, Stanford, CA 94305 USA
Renewable Energy Laboratory, Golden, CO 80401, USA
Supplementary Information
FIG. S1. Energy band positions for the binary passivation layers studied in this work and the CIGS layer 1–3
The interface-trap density distribution (Dit) is a measure of the overall number of trap states per area for a
given energy in the band gap and is used to describe recombination dynamics at hetero junction interfaces such as
the SiOx/Si junction found in CMOS transistors4. Fig. S2 illustrates how Dit generally decreases for increasing ZnS
thickness and that the thicker films have lower Dit than CdS. In addition, there is a significant effect of
pretreatments. One explanation for these variations in the partial stack where there is no heavily doped ZnO to set
the Fermi level position is the possibility that the BB moves the Fermi level position at the interface and through that
shift activates more or less defect states there5, but further studies would be required to verify such a connection.
FIG. S2. Dit vs the energy of the trap level for ZnS with different pretreatments and for the CdS reference.
We motivate the suggested connection between the trend in Q t and the trend in BB as follows. It is possible
that there is an increase of the overall positive charge in the buffer layer for thicker films, if two assumptions are
met. Firstly, if the intrinsic doping ions of the buffer layer, ND, Buffer are assumed to be ionized, a positive background
charge of qND, Buffer is present in the layer. Secondly, as previously mentioned, the negative Q t was found to decrease
with the passivation layer thickness. If Qt is smaller in magnitude than qND, Buffer and if Qt decreases in magnitude,
then the net positive charge of the buffer layer will increase leading to a stronger BB 4. Figure SI3 illustrates the
energy band diagrams and the charge distribution for the two cases Q t = 0 (shown with blue lines) and when Qt is
equal in magnitude to qND, Buffer (shown as purple lines). From Fig. S3. it is clear that a decreasing Qt leads to an
increasing BB. Interestingly, the pretreatments have a bigger effect on the BB than does the film thickness,
indicating once more that pretreatments changes the charge distribution in the stack. With regards to previous
studies the extracted BB values are within the same range 6.
FIG. S3. Schematics of the energy band diagram and charge distribution for the Hg/ZnS/CIGS stack. Blue color identifies the
stack without Qt and the purple color represents the case where Qt is equal to the charge from the ionized donors, qND, Buffer in the
buffer layer.
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