# Quarterly Technical Status Report change as comment in properties10/1/05-12/31/05

NREL Subcontract NDJ-2-30630-24 with *Syracuse University.\
*(Deliverable D4.1). Submitted: November 13, 2023.

Bolko von Roedern, NREL Technical Monitor; Loretta Schmidt, NREL
Subcontract Administrator.

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

## Hole Drift Mobilities in CIGS

We have been making transient photocurrent and capacitance measurements
on CIGS, with the goal of observing photocarrier time-of-flight and
estimating the hole drift-mobility. While the measurements present
significant experimental difficulties, we were sufficiently confident to
submit an abstract for the forthcoming Spring 2006 MRS meeting with our
collaborators R. Noufi and D. Young at NREL, and we expect to submit a
manuscript for publication shortly.

The results are fairly interesting. Our measurements of the hole drift
mobilities for two different depositions by Noufi\'s group at NREL, and
involving many individual sets of contacts on a particular substrate for
each deposition, give typical drift mobilities that are small (0.1 --
0.5 cm^2^/Vs) and essentially temperature-independent. The
temperature-independence is somewhat surprising to us; low
drift-mobilities are often caused by traps which bind a photocarrier.
Release from such a trap requires thermal excitation, and thus this
mechanism gives a strong temperature-dependence to a drift-mobility. The
conclusion of weak-temperature dependence for the hole drift-mobility
was also reached in capacitance-based measurements, as noted in the next
paragraph.

CIGS presents a \"non-standard\" system for these measurements, which
has greatly slowed this work. Our measurements are sensitive primarily
to the traversal of the holes across the \"top\" depletion layer (lying
just beneath the CdS contacting layer). The depletion layer only extends
across about 20% of the CIGS layer; the \"back\" side of the CIGS layer
is not probed by our measurements. Drift-mobility measurements using
capacitance techniques probe the \"back\" portion of the sample.
Measurements by Dave Cohen\'s group at Oregon on IEC (Delaware) samples
give somewhat larger hole mobilities (typically about 10 cm^2^/Vs) than
we find. The two sets of samples may differ, of course. The second
possibility is that the drift-mobility in the \"back\" portion of the
CIGS layer differs from the mobility in the depletion layer. This
ambiguity needs to be resolved in future work, perhaps by doing hole
drift-mobilities on IEC materials.

It may also be that electron drift-mobilities are more important than
hole measurements for solar cells, and practically nothing is known
about them. We shall describe our efforts to make measurements sensitive
to these in the next quarterly report.

## Amorphous Silicon Solar Cell Metastibility

In this quarter our manuscript to *Applied Physics Letters* describing
our temperature-dependent solar cell measurements and modeling was
accepted. The success of a fairly simple model, involving only bandtail
states for as-deposited a-Si:H, has led us to conclude that
light-soaking is essentially a \"perturbation\" of the properties *under
illumination* of the as-deposited state. The hallmark of this statement
is the fact that open-circuit voltages decline only quite modestly
(perhaps 5%) between the as-deposited and light-soaked states for
contemporary a-Si:H solar cells at the light-soaking temperature.

We believe this apparent coincidence, which is that the effects of
light-soaking saturate just as they become noticeable in *V~OC~* or the
cell\'s power density *P*, is evidence for \"self-limitation\" of
light-soaking. To illustrate, if one assumes that the progress of
light-soaking is due to the increasing density of dangling bond defects,
it is remarkable that this increase saturates just when its effects on
the electronic properties become significant.

One model that is useful for purposes of discussion is that only
bandtail recombination is effective for generating defects. When the
defect density rises sufficiently that recombination switches from the
bandtails to the defects, then of course the rate of defect generation
declines.

This \"recombination crossover\" model has its roots in the
light-soaking models studied by Stutzmann, Jackson, and Tsai. However,
our measurements can be used to rule out this original model. If we
\"hybridize\" this picture with the hydrogen-collision model of Branz,
we are able to account for the \"latency\" or the delay between the
start of light-soaking and the observation of a noticeable decline in
*V~OC~*. We are still unable to predict the saturated value for *V~OC~*.

## Polymer p-layers

Our exploration of polyaniline as a \"top\" *p*-layer on c-Si and a-Si:H
structures has been fairly successful in yielding satisfactory diodes,
but the diodes don\'t have values for the open-circuit voltage that are
high enough to be technologically interesting. In addition, the
photocurrent densities of some of these diodes are unexpectedly low.
While we don\'t understand this effect, it again leads us to conclude
that the approach of spin-casting polyaniline as a *p*-layer is
unpromising technologically.

We are working to write up the polyaniline work for publication; these
open-circuit voltage measurements give an insight into transport in
polyaniline itself that is interesting. We anticipate that we shall
survey some other hole-conducting polymers for better performance.