Photovoltaic Performance of Ultrasmall PbSe Quantum Dots

TLDR: The results demonstrate that even in this simple device architecture, fine-tuning of the nanoparticle size can lead to substantial improvements in efficiency.

Abstract: We investigated the effect of PbSe quantum dot size on the performance of Schottky solar cells made in an ITO/PEDOT/PbSe/aluminum structure, varying the PbSe nanoparticle diameter from 1 to 3 nm. In this highly confined regime, we find that the larger particle bandgap can lead to higher open-circuit voltages (∼0.6 V), and thus an increase in overall efficiency compared to previously reported devices of this structure. To carry out this study, we modified existing synthesis methods to obtain ultrasmall PbSe nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by adjusting the growth temperature. As expected, we find that photocurrent decreases with size due to reduced absorption and increased recombination, but we also find that the open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm, most likely due to reduced collection efficiency. Owing to this effect, we find peak performance for devices made with PbSe dots with a first exciton energy...

Figures

Figure 1. Transmission electron microscope (TEM) images of eight batches of ultrasmall PbSe nanocrystals with diameters from 1~3 nm are shown. The growth temperature (indicated at the top of each panel) was controlled during the synthesis to yield different sized particles. Size analysis data are plotted in histograms for each sample, showing size distributions. All scale bars are 20 nm.

Figure 4. (a) The external quantum efficiency (EQE) spectrum of a solar cell made with PbSe nanocrystals (first exciton peak of 1.6 eV) is compared to the absorption in the PbSe film calculated with FDTD simulations for a film thickness matching that of the device. (b) Fabry-Pérot interference modes cause peaks in the absorption in the PbSe film; the wavelengths where the peaks occur depends on the thickness of the PbSe layer.

Full text

Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
Title
Photovoltaic performance of ultra-small PbSe quantum dots
Permalink
https://escholarship.org/uc/item/2k52x3h3
Author
Ma, Wanli
Publication Date
2011-09-22
eScholarship.org Powered by the California Digital Library
University of California

1
Photovoltaic performance of ultra-small PbSe
quantum dots
Wanli Ma,
1
Sarah L. Swisher,
2
Trevor Ewers,
1
Jesse Engel,
3
Vivian E. Ferry,
4
Harry A.
Atwater,
4
and A. Paul Alivisatos*
1,5
1
Department of Chemistry, University of California, Berkeley, California
2
Department of Electrical Engineering, University of California, Berkeley, California
3
Department of Materials Science and Engineering, University of California, Berkeley,
California
4
Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology,
Pasadena, California
5
Lawrence Berkeley National Laboratory, Berkeley, California
* To whom correspondence should be addressed. Email: alivis@berkeley.edu
We investigated the effect of PbSe quantum dot size on the performance of Schottky
solar cells made in an ITO/PEDOT/PbSe/Aluminum structure, varying the PbSe
nanoparticle diameter from 1-3nm. In this highly confined regime, we find that the larger

2
particle bandgap can lead to higher open-circuit voltages (~0.6 V), and thus an increase in
overall efficiency compared to previously reported devices of this structure. To carry out
this study, we modified existing synthesis methods to obtain ultra-small PbSe
nanocrystals with diameters as small as 1 nm, where the nanocrystal size is controlled by
adjusting the growth temperature. As expected, we find that photocurrent decreases with
size due to reduced absorption and increased recombination, but we also find that the
open-circuit voltage begins to decrease for particles with diameters smaller than 2 nm,
most likely due to reduced collection efficiency. Due to this effect, we find peak
performance for devices made with PbSe dots with a first exciton energy of ~1.6 eV (2.3
nm diameter), with a typical efficiency of 3.5% and a champion device efficiency of
4.57%. Comparing the external quantum efficiency of our devices to an optical model
reveals that the photocurrent is also strongly affected by the coherent interference in the
thin film due to Fabry-Pérot cavity modes within the PbSe layer. Our results demonstrate
that even in this simple device architecture, fine-tuning of the nanoparticle size can lead
to substantial improvements in efficiency.
Keywords: PbSe, quantum dot, solar cell, photovoltaic, quantum size effect
Over the past four years, lead chalcogenide nanoparticles have been increasingly
investigated as a candidate for low-cost, solution-processed photovoltaics.
1
Lead
chalcogenides possess an extremely large bulk exciton Bohr radius (46 nm for PbSe and
20 nm for PbS
2
), which creates strong quantum confinement in colloidal nanocrystals
and allows their bandgap and absorption edge to be tuned across the entire visible

3
spectrum.
3, 4
Fabrication of Schottky junction solar cells is straightforward with lead
chalcogenide quantum dots (QDs): QDs deposited onto indium tin oxide (ITO) form an
ohmic contact, and thermally evaporating metal electrodes form the Schottky junction.
5,
6
Early devices made with this structure typically resulted in a solar power conversion
efficiency ranging from 1-2%, but would lose nearly all rectification after just minutes of
air exposure due to oxidative doping.
5-8
Smaller PbS nanoparticles (<3 nm diameter)
were found to overcome this extreme air sensitivity by forming different surface
oxidation products due to their reduced faceting.
9
Recently, small PbS dots have been
employed in more complex depleted heterojunction solar cell architectures. Photovoltaic
devices with efficiencies reaching 5.1% have been achieved for PbS/TiO
2
nanocrystal
devices,
10
and PbS/ZnO devices have demonstrated excellent stability for 1000 hours of
continuous illumination in ambient air conditions.
11
These demonstrations of improved
efficiency and stability motivate the further study of strongly confined quantum dots.
The power conversion efficiency of Schottky solar cells using PbSe quantum dots has
typically been limited by low open-circuit voltages (<0.3 V),
5
due in part to the relatively
small bandgap of the PbSe quantum dots. In equivalent cells, PbS nanoparticles have
demonstrated higher open-circuit voltages than PbSe, but lower photocurrents.
6
PbSe
devices tend to have higher photocurrent than PbS, most likely due to the 10-fold
increase in the mobility.
12
Using a PbSe
x
S
1-x
alloy proved to be one successful method to
combine the best aspects of both materials, improving the open circuit voltage (V
OC
) from
230 mV to 450 mV while maintaining a short-circuit current (J
SC
) of 14.8
mA/cm
2
.
13
Similar performance (V
OC
= 440 mV) was achieved by incorporating PbSe
QDs into an excitonic solar cell structure,
14
which could theoretically eliminate the

4
constraints on V
OC
that are inherently imposed by the Schottky architecture. It has also
been found that using smaller PbSe QDs with larger bandgaps (E
G
) results in a higher V
OC
in Schottky cells.
5
The improvement scales linearly as ∆V
OC
∝ ΔE
G
/2, as expected for a
Schottky barrier with the Fermi level pinned near the middle of the bandgap. The
smallest PbSe QDs in that study were ~3 nm in diameter (E
G
= 1.1 eV), which produced a
device with V
OC
= 250 mV. While many recent studies of colloidal lead chalcogenide
nanocrystals have focused on carrier multiplication (CM) as a pathway to improving the
efficiency of nanocrystal solar cells, a recent perspective on the experimental and
theoretical work on CM indicates that the main promise of quantum confinement in
colloidal nanocrystals is, in fact, to increase the photovoltage of the cell.
15
This trend
motivates our investigation into the utilization of the good transport properties of PbSe
nanoparticles without sacrificing open circuit voltage through the use of ultra-small
quantum dots with larger bandgaps.
Results and Discussion
We describe here a new synthesis scheme producing strongly confined PbSe quantum
dots with a diameter as small as 1 nm, and the effects of nanocrystal size on the
photovoltaic performance of simple Schottky-type devices. In this study, we found that
by varying the nanocrystal size, we can significantly increase the V
OC
from 480 mV to
600 mV, which is the highest reported for a PbSe Schottky device to the best of our
knowledge. The best device had a power conversion efficiency of 4.57% with AM1.5
illumination, which we achieved using PbSe quantum dots with a bandgap of ~1.6 eV
(~2.3 nm diameter). While increasing the bandgap of the QDs has increased V
OC
, it will
eventually start to reduce J
SC
due to lost absorption in the IR portion of the solar

Frequently asked questions

Q1. What is the effect of the large excess of lead monomer?

Thelarge excess of lead monomer maintains a high reactant concentration post nucleation, which keeps the reaction in a size-focusing regime due to the smaller critical particle size. 

Q2. What is the effect of the recombination in the smaller particles?

Since smaller dots have more surface area and require more interparticle hops per unit length, the authors hypothesize that charge recombination increases in the smaller particles. 

Q3. What was the optical properties of the QDs grown at 140 oC?

Spectroscopic ellipsometry was used to measure the optical properties of the film of QDs grown at 140 ºC immediately after exposure to air. 

Q4. What is the effect of increasing the bandgap of the QDs on the solar?

While increasing the bandgap of the QDs has increased VOC, it will eventually start to reduce JSC due to lost absorption in the IR portion of the solarspectrum. 

Q5. What is the effect of BDT on the nanocrystals?

The BDT increases the coupling between particles allowing for better transport through the film, and allows deposition of additional nanocrystal layers due to the insolubility of the BDT-coated particles in the octane:hexane solution. 

Q6. What is the effect of the recombination in the nanocrystals?

In addition to the reduced absorption, the loss of current may also be attributed to degraded transport through the film of smaller nanocrystals. 

Q7. How many small PbSe nanoparticles have been found to overcome this extreme air?

5-8 Smaller PbS nanoparticles (<3 nm diameter) were found to overcome this extreme air sensitivity by forming different surface oxidation products due to their reduced faceting.9 

Q8. Why is the power conversion efficiency of a schottky solar cell limited?

The power conversion efficiency of Schottky solar cells using PbSe quantum dots has typically been limited by low open-circuit voltages (<0.3 V),5 due in part to the relatively small bandgap of the PbSe quantum dots. 

Q9. What is the main promise of quantum confinement in colloidal nanocrystals?

While many recent studies of colloidal lead chalcogenide nanocrystals have focused on carrier multiplication (CM) as a pathway to improving the efficiency of nanocrystal solar cells, a recent perspective on the experimental and theoretical work on CM indicates that the main promise of quantum confinement in colloidal nanocrystals is, in fact, to increase the photovoltage of the cell. 

Q10. What is the way to optimize the tradeoff between JSC and VOC?

In order to achieve the best device performance, the authors must choose quantum dots with an appropriate size to optimize the tradeoff between JSC and VOC. 

Q11. Why is the photoluminescence of the smallest particles shown in c?

The absorbance and photoluminescence of the smallest particles, grown at 30 °C, is shown in(c); the red color of the nanoparticle solution (c, inset) is due to their extreme quantum confinement. 

Q12. What is the effect of the PbSe quantum dots on the photovoltaic field?

As a result, for two devices of equivalent film thickness, the photovoltaic device utilizing smaller PbSe quantum dots generates less short-circuit current. 

Q13. What is the origin of the large Stokes shift in lead chalcogenides?

One hypothesis from Fernée et al. to explain the large Stokes shift in lead chalcogenides is derived from excitonic fine structure splitting of the eightfold degenerate ground state, arising from inter-valley scattering.30 

Q14. What is the effect of decreased bandgap on photovoltaic performance?

Below this bandgap, higher photocurrent and fill factor can be achieved due to improved charge transport and enhanced absorbance, but the loss of VOC due to decreased bandgap has a more pronounced impact and the overall device performance suffers. 

Q15. How can the authors increase the VOC of PbSe quantum dots?

In this study, the authors found that by varying the nanocrystal size, the authors can significantly increase the VOC from 480 mV to 600 mV, which is the highest reported for a PbSe Schottky device to the best of their knowledge. 

Q16. What is the PL spectrum for the smallest particles used in this study?

Interestingly the photoluminescence (PL) spectrum for the smallest particles used in this study (d = 1.1 nm), shown in Figure 2c, exhibits a peak that is red-shifted approximately 170 nm from the absorption peak (733 nm and 560 nm, respectively). 

Q17. What is the probability of UV photons contributing to photocurrent?

For films thick enough to absorb sufficient amount of light, these UV photons have a low probability of contributing to photocurrent. 

Q18. How much VOC is the open circuit voltage for PbSe quantum dots?

Despite this limitation, the authors obtained an open circuit voltage of 0.60 V which is the highest VOC reported for PbSe quantum dot solar cells to their knowledge.