# Quarterly Technical Status Report change as comment in properties11/11/04-2/10/05

NREL Subcontract NDJ-2-30630-24 with *Syracuse University.\
*(Not a deliverable - ref. mod. No. 9). Submitted: November 13, 2023.

Bolko von Roedern, NREL Technical Monitor; Vicki Riddell, NREL
Subcontract Administrator.

Eric A. Schiff, principal investigator; Steluta Dinca, Jianjun Liang,
Weining Wang, graduate assistants.

## Summary

The main work during this quarter has been on a-Si:H solar cell
characterization & modeling, and in particular on the time-dependence of
the degradation of the open-circuit voltage. We have been fairly
successful in accounting for the measurements of the time-dependence of
degradation with an elementary model. Additional, rather interesting
results emerged from measurements of hole drift-mobilities in CIGS.
Finally, we have found a good tool for exploring *V~OC~* in a-Si:H and
c-Si solar cells with polymer *p*-layers, and are exploring *V*~OC~ as a
function of the resistivity of the polymer layer.

## Amorphous Silicon Solar Cell Characterization & Modeling

The principal experimental work has been on the time-dependence of
*V~OC~* during light-soaking in a-Si:H solar cells. We are trying to
test a specific proposal for defect generation and its effects on
recombination during light-soaking; we suspect that light-soaking is
"self-limiting" in a-Si:H, by which we mean that, once defect
recombination traffic becomes significant, light-soaking ceases to
generate new defects. This basically implies that light-soaking is
associated primarily with samples for which recombination primarily
involves the bandtails.

Experimentally, the degradation under near-solar illumination conditions
(we use a 685 nm laser for this work) is about 0.03 V in 200 hours at
295 K. Interestingly, there is a "latency" of a few hours after the
onset of light-soaking before there is much change in *V~OC~*;
light-soaking then appears to saturate in about 300 hours. We believe
that the latency is the time required for recombination traffic through
defects to become a significant fraction of recombination through
bandtails. We are able to account quite well for the kinetics of *V~OC~*
by assuming that degradation corresponds to a modest crossover of
recombination traffic from bandtails to defects. Oddly, it appears that
the long-term stabilization process (nominally light-induced annealing)
sets in on the same timescale as the crossover.

## Hole Drift Mobilities in CIGS

We obtained a new CIGS sample from the NREL CIGS group (D. Young,
contact), and started drift-mobility measurements. The preliminary
results are very interesting. The hole drift-mobility appears rather low
(less than 0.1 cm^2^/Vs), and nearly temperature-independent. If these
results are confirmed, they would indicate that CIGS solar cells are
actually low-mobility cells and possibly mobility-limited.

As always, we do capacitance measurements in conjunction with the
drift-mobility work. These indicate an apparent "electrical" thickness
of about 0.5 microns, which is much thinner than the physical thickness
(nominally 2 microns) of the CIGS absorber layer. We need to understand
these capacitance measurements better. We hope to obtain additional CIGS
samples from NREL in the next quarter.

## Polymer p-layers

We have continued work with polyaniline *p*-layers on top of *n/i*
a-Si:H and *n-*type c-Si layers. The main parameter has been *V~OC~*
measured under white-light; our present value is about 0.75 V for the
a-Si:H heterostructures. We stumbled onto a useful effect. Polyaniline
comes from its manufacturer (Aldrich) as a dispersion in xylene. We
increased the dilution in an effort to change the resistivity, and
discovered that the resistivity of the films increases essentially
exponentially with dilution. While we don't understand why this happens,
it appears to be a well-known effect. We are thus able to study *V~OC~*
over an enormous range of resistivities. The results for a-Si:H suggest
that *V~OC~* is limited by the Fermi-level position of the polyaniline
film; *V*~OC~ declines as the resistivity of the polyaniline increases,
exactly as one would predict from the idea that the built-in potential
across the structure was limiting *V*~OC~. Interesting, for work on
*n*-type c-Si, Fermi-level limitation applies only for larger
resistivities. For lower resistivities, *V~OC~*, which is about 0.5 V,
is independent of the polyaniline's resistivity, suggesting that *V~OC~*
becomes limited either by our wafer properties or by interface states.