Textured ZnO films from evaporation-triggered aggregation of
nanocrystal dispersions and their use in solar cells
Junfeng Yana, Qing Liana, Muhamad Z. Mokhtara, Amir H. Milania,
Eric Whittakerb, Bruce Hamiltonb,
Paul O’Briena,c, Nam T. Nguyena and Brian R. Saundersa,*
a School of Materials, MSS Tower, The University of Manchester,
Manchester, M13 9PL, U.K.
b Photon Science Institute, University of Manchester, Alan
Turing Building, Oxford Road, Manchester, M13 9PL, U.K.
c School of Chemistry, The University of Manchester, Manchester,
M13 9PL, U.K.
Due to its high electron mobility, good stability and potential
for low-temperature synthesis ZnO has received considerable
attention for use in solar cells, photodetectors and sensors.
Whilst there have been reports involving the formation ZnO films
with porous morphologies the majority of those have involved
elaborate or time-consuming preparation methods. In this study we
investigate a simple new method for preparing textured porous ZnO
(tp-ZnO) films. We used colloidal instability triggered by the
evaporation of a volatile stabilising ligand during spin-coating of
a ZnO nanocrystal (NC) dispersion to deposit crack-free tp-ZnO
films. The porosity of the tp-ZnO films was 56 % and they could be
prepared using amine-based ligands with boiling points less than or
equal to 78 oC. To demonstrate the ability to use the tp-ZnO films
as electron acceptors they were infiltrated with
poly(3-hexylthiophene) (P3HT) and solar cells prepared. The power
conversion efficiencies of the tp-ZnO/P3HT devices reached values
that were three times higher than a control bilayer ZnO/P3HT device
prepared using a sol-gel derived ZnO film. Because our method used
a low temperature treatment and ZnO films are used in a wide
variety of third-generation solar cells, the new tp-ZnO films
introduced here may offer a low cost method for improving the
efficiency of other solar cells.
INTRODUCTION
ZnO is a wide bandgap semiconductor with a high electron
mobility1 that is of considerable interest in a number of areas
including electronics2, 3, optics4, 5, water-purification6, solar
cells7-10 and photocatalysis11, 12. ZnO can be prepared using
solvothermal synthesis in a range of different morphologies, which
include nanorods13, 14 and 3D superstructures15. Textured or porous
ZnO films are of particular interest as they may provide an
increased surface area and enhanced light scattering16. ZnO films
have received arguably the greatest attention within
optoelectronics2, 17, 18. The synthetic methods for preparing
textured ZnO films include chemical bath deposition11, 19,
electrochemical deposition14, 16, 20 or the sol-gel method21, 22.
However, these approaches are either time-consuming, require
high-temperature sintering or use elaborate equipment. We report
here our unexpected discovery that textured and porous, crack-free,
ZnO (tp-ZnO) films can be simply prepared using layer-by-layer
spin-coating of ZnO nanocrystals (NC) dispersions containing a
volatile stabilising ligand.
Deposited films of inorganic semiconducting NCs have major
fundamental and practical interest23. From the fundamental
viewpoint such films have optoelectronic properties that differ
from the dispersed NCs and depend on the nature of the inter-NC
contacts24. Deposited ZnO NCs are used as key components in at
least three types of third-generation solar cells (quantum dot,
hybrid-polymer and perovskite solar cells)7-9, 25-32 which is
primarily due to the electron accepting properties of ZnO. Usually,
semiconducting inorganic NCs are insoluble in the organic solvents
used for deposition and require stabilisation with ligands33.
Unfortunately, crack formation within films deposited from such NC
dispersions often occurs34-36, which can adversely affect film
electronic properties. Cracks may result from ligand removal when
this causes a major volume decrease with a simultaneous local
stress build-up35. Methods to achieve crack-free film formation
within deposited NC films are urgently needed. There is evidence
that films containing disordered NCs have a lower propensity to
crack35. Here, we study a simple spin-coating approach that uses
evaporation-triggered destabilisation of ZnO NC dispersions under
conditions favouring disordered aggregate formation to provide
crack-free tp-ZnO films.
Whilst ZnO NCs have been studied for many years37 there have
been few systematic studies of their colloidal stability in
non-aqueous solvents. In our previous work38, 39 we reported that
concentrated non-aqueous ZnO NC dispersions could be stabilised
with ligands such as 1-propylamine (PA) or destabilised by adding
bifunctional ligands38. We found that amine-based ligands were
moderately good dispersants for ZnO. However, they could also be
readily displaced by bifunctional thiol-ligands to trigger
aggregation. We recently observed in our laboratory that ZnO NC
dispersion aggregation could also be triggered by evaporation of a
volatile amine ligand, such as PA, and that this resulted in
textured ZnO NC films during spin-coating when films were prepared
using a layer-by-layer approach. Because such films have potential
for optoelectronic applications we conducted a study of this new
approach to preparing tp-ZnO films and the results are presented
herein.
As part of this work bilayer tp-ZnO/poly(3-hexyl thiophene)
(P3HT) films and solar cells were studied. ZnO/P3HT and closely
related poly(3-hexyl selenophene)-based solar cells are well
established and have been prepared using spin-coating of mixed
dispersions21, 40, 41, in-situ ZnO NC formation42 or using
pre-formed textured ZnO films8. The best efficiencies for ZnO/P3HT
solar cells have been achieved to date using in situ ZnO
formation42. However, a difficulty for the latter method is control
of the NC size and distribution. The use of vertical ZnO nanorods
represents an alternative approach where well aligned pre-formed
nanorods were first prepared8. However, infiltration can be
problematic if the acceptor phase has very small pores43. Here, we
study tp-ZnO films with pores that favoured P3HT infiltration.
The approach used to prepare the textured porous ZnO films
(tp-ZnO) is depicted in Scheme 1. A ZnO NC dispersion was dispersed
in CHCl3 containing ligand and then spin-coated at room
temperature. The ligands used were PA, 1-butylamine (BA) and
1-dodecylamine (DA). The latter was used as a high-boiling point,
control. Evaporation of the volatile ligands (BA and DA) resulted
in ZnO aggregation and deposition of a tp-ZnO film which was
heated. The process was repeated a further five times to form a
film with a thickness of ~ 120 nm. The ability of the textured
tp-ZnO to act as an electron acceptor was tested by infiltrating
the film with P3HT (Scheme 1) and constructing tp-ZnO/P3HT bilayer
solar cells.
Scheme 1. Method used to prepare textured porous ZnO (tp-ZnO)
films and their infiltration with P3HT. A concentrated ZnO NC
dispersion in CHCl3 and stabilised with a volatile ligand was
spin-coated to give a porous ZnO film. The film was dried at 80 oC
and the process repeated a further five times. The tp-ZnO film was
immersed in a dilute P3HT solution for 24 h and then P3HT
spin-coated onto the tp-ZnO film using a concentrated P3HT
solution. The ligands used were 1-propylamine (PA), n-butylamine
(BA) and n-dodecylamine (DA).
The study begins by characterising the morphologies of the
tp-ZnO films prepared using PA. The role of evaporation in the
triggering of aggregate formation is then probed. The importance of
ligand boiling point on textured morphology is investigated using
ligands with different boiling points and a mechanism describing
film formation is proposed. P3HT films are then coated on the
tp-ZnO films and the morphologies assessed. The tp-ZnO/P3HT films
are used to construct solar cells and the device performance
studied. The power conversion efficiency is shown to reach at least
three times that of a control bilayer ZnO/P3HT device which
confirms the potential for the tp-ZnO films to act as an improved
electron accepting layer for solar cells. Because our new method
for preparing textured ZnO films used low temperatures and
inexpensive equipment it should be widely applicable.
EXPERIMENTAL DETAILS
Materials
Zn(CH3COO-)2.2H2O (98%), CHCl3 (99.8%), BA (99.5%), DA (>
99%), ethanolamine (99.5%), 2-methoxyethanol (99.8%) and methanol
(MeOH, 99.8%) were purchased from Aldrich. PA (> 99%) and
chlorobenzene (CBZ) were purchased from Alfa Aesar. P3HT (average
molecular weight of 15,000 - 45,000 g/mol and 99.995% trace metals
basis) was purchased from Aldrich and had a regioregularity >
95%. All materials were used as received.
ZnO nanocrystal synthesis
ZnO NCs were synthesised according to the method reported
previously38. Briefly, Zn(CH3COO-)2.2H2O (2.95 g, 13.4 mmol.) was
dissolved in MeOH (135 mL), then reacted with a KOH (1.48 g, 23
mmol) solution in MeOH (65 mL) at 60 oC for 135 min44. The ZnO NC
precipitate was collected and re-dispersed in CHCl3 and PA was
gradually added until the PA volume fraction was 13 vol.% The
dispersion became transparent after agitation and the ZnO
concentration (CZnO) was 18.1 mg/mL. The stock dispersion was
stored at -18 oC. The dispersion stability was assessed before use
by means of dynamic light scattering (below).
tp-ZnO film preparation
A portion of stock ZnO NC dispersion (300 L) was added onto a
pre-cleaned glass substrate (~ 2.5 x 2.5 cm2). Spin-coating was
conducted using a Laurell, Model WS-650Mz-23NPP spin processor. The
latter was programmed to first spin at 300 rpm for 3.0 s (phase 1)
and then at 2,000 rpm for another 10.0 s (phase 2). The ZnO film
was heated in an oven 80 oC for 5 min. [CARE: fumes from 1-PA
should not be inhaled.] The process was conducted a total of six
times to give a total film thickness of 116 nm.
tp-ZnO/P3HT film preparation
P3HT solutions (2 – 20 mg/mL) were prepared by dissolving the
proper mass of P3HT in CBZ and heating the solution at 60 oC
together with ultrasonication. Following the method of Ravirajan et
al.13, the tp-ZnO films were immersed overnight in a solution of
P3HT in CBZ (2 mg/mL) and then briefly dried with a flow of dry
nitrogen gas. Finally, a more concentrated P3HT solution in CBZ (of
various concentrations) was spin-coated at 3,000 rpm. The films
were then heated at 50 °C in air for 5 min.
tp-ZnO/P3HT device fabrication
ITO coated glass was sonicated in water for 30 min, then in NaOH
solution (2.0 M) for 30 min which was followed by exhaustive
washing with high purity water. The substrate was then dried under
N2. A compact hole blocking layer (bl-ZnO) of ZnO (thickness 35 nm)
was prepared via sol–gel method21 which involved spin-coating a
Zn(CH3COO-)2.2H2O solution (0.75 M) with ethanolamine (45 l) in
2-methoxyethanol (100 L) onto ITO at 1000 rpm and annealing in air
on a hotplate at 300 oC for 10 min. The latter process was then
repeated. Subsequently, tp-ZnO/P3HT films were prepared as
described above. The P3HT concentrations (CP3HT) used to prepare
devices were 7, 15 and 20 mg/mL and the total thickness of the
tp-ZnO/P3HT films were, respectively, 130, 133 and 149 nm. The
films used for devices were subjected to an additional annealing
step in order to improve the mobility of the P3HT phase45. For all
devices, the films were annealed for a further 140 oC for 5 min on
a hotplate. An Ag (or Au) layer (80 nm) was deposited onto the
P3HT.
Control bilayer sol-gel ZnO devices were also prepared. In this
case the sol-gel bl-ZnO layer was prepared as described above. The
P3HT film was then deposited onto the bl-ZnO layer using a P3HT
solution concentration of 20 mg/mL and annealed as described above.
The thickness of the sol-gel ZnO layer and P3HT layer were 35 and
30 nm, respectively.
Physical measurements
All SEM measurements were conducted using a Philips FEGSEM XL30
instrument. Atomic force microscopy (AFM) images were taken from an
Asylum Research MFP-3D operating in AC (“tapping”) mode. Imaging
was performed using Olympus high aspect ratio etched silicon probes
(OTESPA) with nominal spring constant of 42 N/m (Bruker AXS S.A.S,
France). The cantilever oscillation frequency varied between 300
and 350 kHz and was determined by the auto-tune facility of the
Asylum software, as was the drive amplitude. The set-point was
adjusted to just below the point at which tip-sample interaction
was lost to minimise sample damage. The thickness of the films was
measured using a Dektak 8 (Bruker) via scratch a line on the film
surface (Stylus Profilometer). UV-visible spectra were obtained
using a Hitachi U-1800 spectrophotometer. Dynamic light scattering
(DLS) data were obtained using a Malvern Zetasizer Nano-2S
instrument.
Porosity determination for tp-ZnO film
The porosity of tp-ZnO film was determined via Inductively
Coupled Plasma Mass Spectrometer following the method of Goux et
al.46 The ZnO film (area of 1.2 cm2) was fully dissolved using
aqueous HCl solution (10 %) to give a Zn2+ solution with total
volume of 2.0 mL. The digested solution was diluted with high
purity water to give a final solution volume of 10 mL. This
solution was analysed using ICP analysis. From the calculated mass
of ZnO and the dimensions of the films, the average porosity (six
samples) was determined.
Device characterization
The current density-voltage (J–V) characteristics were measured
using a Keithley 2420 Sourcemeter and 100 mW/cm2 illumination (AM
1.5G) and a calibrated NREL certified Oriel Si-reference cell. An
Oriel SOL3A solar simulator was used for these experiments. The
active area of the devices was defined using a square aperture
within a mask and fixed at 0.025 cm2. Forward and reverse direction
sweeps were measured with a sweep rate of 50 mV/s.
RESULTS AND DISCUSSION
Porous ZnO film morphology
The ZnO NCs studied here were characterised by TEM (Fig. S1a)
and had a number-average TEM diameter (dTEM) of 4.1 ± 0.4 nm. The
NC diameter was also determined from UV-visible spectra (Fig. S1b)
for NCs dispersed in CHCl3/PA using the value of the wavelength at
half peak height (1/2 = 346 nm). From Meulenkamp’s relationship37
the diameter (dUV-vis) was 3.5 nm. DLS data for the NCs dispersed
in CHCl3/PA (Fig. S1c) gave a z-average diameter (dz) of 7.9 nm.
This result is indicative of good dispersion of the ZnO NCs, which
was due to the PA. The higher dz value compared to dUV-vis and dTEM
was due to the presence of ligand and the stronger scattering of
light by the largest particles.
ZnO films were deposited by repeated layer-by-layer spin-coating
of the ZnO NC dispersions which was conducted a total of six times
(Scheme 1). While control ZnO films deposited from CHCl3 did not
form uniform films and contained very large aggregates (Fig. S2a),
the films deposited from CHCl3/PA had a uniform morphology at the
scale of 10s of m (Fig. S2b). This is because 1-PA acted as a
ligand. The ZnO film thickness increased linearly with cycle number
(See Fig. S3). Unless otherwise stated a total of six spin-coating
cycles were used and the final thickness was 116 ± 12 nm as
measured by surface profilometry. Higher magnification SEM images
of the films showed that they had a textured porous morphology
(Fig. 1a - c). The morphology consisted of ridges with valleys in
between. Whilst many different types of textured morphologies have
been reported in the literature14-16, 47, the present approach was
very simple and only required a spin-coater and an oven. The
textured crack-free morphology extended over the entire film (100s
of m, See Fig. S2b). This is an important observation because
cracking of NC films is an important problem that hinders their
use35, 48.
Figure 1. SEM images of ZnO film deposited from CHCl3/PA (a –
c). (d) and (e) shows a representative tapping mode AFM image and
line profile of the films. (f) shows a higher magnification AFM
image and the yellow arrows highlight the particulate nature of the
ZnO NC clusters. Scale bars: (a) 10 m; (b) - (d): 2 m; (f); 200
nm.
It is instructive to consider the implications of the ligands on
the morphology of the tp-ZnO films because the ligands were removed
after deposition. The ratio of the ligand volume (VL) to NC
particle volume (VNC) for the dispersed ZnO NCs can be estimated
from
(1)
where L, ML, dNC and L are the surface area per ligand adsorbed,
ligand molecular weight , NC diameter and ligand density,
respectively. NA is Avogadro’s number. (The derivation of equation
(1) is given in the ESI.) If we assume a L value of49 22 Å2 (for a
monovalent ligand adsorbed to NCs with a similar diameter to those
studied here) and use ML, dNC and L values of 59 g/mol, 4.0 nm and
0.72 g/cm3 then VL/VNC = 0.92. The latter corresponds to a ligand
volume fraction (L) of ~ 48 vol.% using equation S4 (See ESI and
Table S1). The latter value is slightly lower than the measured ZnO
NC film porosity of 56 ± 4 %
Agreement between the calculated L value and the film porosity
may be expected if all the ligand were removed without significant
overall volume change. The absence of film cracking is also
attributed to a disordered NC arrangement caused by evaporation
triggered aggregation as well as the layer-by-layer (LBL) approach
used for film formation which is known to reduce internal
stresses35. It is proposed that evaporation resulted in diffusion
controlled aggregation which gave disordered aggregates that were
unable to rearrange in to more ordered packing.
AFM data (Fig. 1d) confirmed that the textured morphologies
apparent from SEM (e.g., Fig. 1c) were not an artefact of the high
vacuum using during SEM. Rather, the texture was formed as a
consequence of spin-coating and oven drying. A representative
line-profile (Fig. 1e) shows that the pores were deep and almost
spanned the whole depth of the film. The RMS roughness was 67 nm.
The highest magnification AFM image (Fig. 1f) shows that the film
peaks were aggregates of smaller ZnO NC aggregates with diameters
of ~ 20 to 50 nm (indicated by yellow arrows). 3D AFM images (Fig.
S5) highlight the relatively rough morphology present within the
tp-ZnO films. SEM images of tp-ZnO films prepared using 1, 3 and 6
layers show that porosity was present from the first layer and
became more pronounced with subsequent layers (Fig. S6).
To further probe the morphology the surface was scratched and
the film regions next to the scratch were imaged (Fig. S7). Whilst
the film showed evidence of deformation and flow under shear, the
higher magnification images revealed smaller pores in the lower
layers. These pores support the view that the films were fully
porous in the z-direction as indicated by line profile in Fig. 1e.
We fortuitously found a region of film where the uppermost layer
adjacent to a scratch had been removed to expose the under-layer
(See Fig. S8). Examination of the latter region revealed relatively
small pores with diameters of 60 - 100 nm. These data provided
additional evidence that the pores were connected. Taken together,
these data show that the tp-ZnO films had porosity that extended
through the film depth.
Mechanism for porous ZnO film formation
The most important physical chemistry question regarding the
present system concerns what the mechanism is for the formation of
the textured and porous ZnO NC films. During spin-coating the CHCl3
and ligand (PA) evaporated rapidly from the deposited (wet) film.
PA has a significantly lower boiling point (48 oC) compared to
CHCl3 (61 oC) and would have been relatively rapidly evaporated
during spin-coating. It is proposed that as ligand was
preferentially lost, ZnO colloidal stability was compromised. To
qualitatively probe the effect of solvent evaporation on ZnO
colloidal stability we prepared a stock ZnO NC dispersion (18.1
mg/ml) dispersed in CHCl3/PA and then used UV-visible spectroscopy
and DLS to study the dispersion stability changes that occurred as
the solvent evaporated (See Fig. 2). The dispersion became visibly
turbid as the evaporation proceeded (Fig. 2a) and light scattering
was evident from the UV-visible spectra (Fig. 2b). The numerical
value of the absorbance at 500 nm (A500) is a measure of light
scattering because ZnO does not absorb light at this wavelength
(compare to Fig. S1b). Fig. 2c shows that the value for A500
increased strongly with CZnO. (The latter was estimated from the
dispersion volumes.) This trend is due to aggregates which are
apparent from the DLS data (Fig. 2d) and were triggered by ligand
(and solvent) evaporation. It is likely that as ligand (and
solvent) evaporated during spin-coating aggregation also occurred.
Because the timescale for aggregation was much shorter for
spin-coating the aggregates would have been smaller than those
apparent from Fig. 2d.
Figure 2. (a) Image of a ZnO NC dispersion with CZnO of 18 mg/mL
(far left). The other images show the dispersion as evaporation
proceeded. The calculated CZnO values (mg/mL) are shown. (b)
UV-visible spectra for the dispersions are shown in (a). The
variation of the absorbance at 500 nm with CZnO is shown in (c).
(d) DLS data from selected samples from (a).
It is proposed that the key reason for the formation of porous
ZnO morphologies was the low boiling point of the ligand (PA) which
favoured aggregation during spin-coating. To test this hypothesis
we investigated the importance of ligand boiling point by preparing
ZnO dispersions using BA (boiling point = 78 oC). As a control, we
also prepared ZnO dispersions using a high boiling point ligand
(DA, boiling point = 248 oC). (The structures of the ligands are
shown in Scheme 1.) The dz values obtained for the ZnO NCs
stabilised with BA and DA were 8.5 and 10.6 nm, respectively. (The
DLS data are shown in Fig. S9.) These data confirm that BA and DA
act as stabilising ligands for ZnO NCs. ZnO films prepared using
the CHCl3/BA had a porous morphology (Fig. 3a and b). There was
evidence of smaller pores present for this film compared to tp-ZnO
prepared using 1-PA (Fig. 1c) which is suggestive of morphology
tuneability. In contrast a non-porous film was produced using
DA/CHCl3 (Fig. 3c and d). These data support our hypothesis that
the porous ZnO morphology was due to the low boiling point of the
ligand.
Figure 3. (a and b) SEM images of as-prepared ZnO film deposited
from CHCl3/BA dispersion. (c and d) SEM images of ZnO film
deposited from CHCl3 /DA dispersion. All scale bars are 2 m.
Fig. 4 depicts the mechanism proposed to explain the formation
of the tp-ZnO morphology. This mechanism has some similarities to
that proposed by Snaith et al. for polymer infiltration of
mesoporous scaffolds50. Accordingly, a wet film was initially
present (Fig. 4a) which lost a significant proportion of dispersion
during initial spin-coating (Fig. 4b). Solvent and ligand
evaporation preferentially occurred from the top of the film (Fig.
4c) which concentrated the dispersion. Above a critical CZnO value
the dispersion formed NC aggregates. The aggregates deposited onto
the substrate and residual solvent/ligand was removed by heating.
The process was repeated a further five times. The relatively small
pores formed at the bottom of the films during the early cycles
(Fig. S6a) were not able to be effectively filled by successive ZnO
layers because of aggregate formation. The latter tended to deposit
onto pre-existing ZnO aggregates, which gave rise to the textured
morphology. In the case of high boiling point ligand (e.g., DA) the
ligand was not removed which prevented NC destabilisation and pore
formation. In that case the solvent was removed first. It is well
known that for binary co-solvent blends the less volatile solvent
resides in a spin-coated film for the longest time51. It follows
from this discussion that to prepare textured tp-ZnO films by
spin-coating the ligand boiling point should be less than or equal
to that for BA (78 oC).
Figure 4. Depiction of the proposed evaporation-triggered
aggregation mechanism for tp-ZnO formation. The initial wet film
(a) contained dispersed ZnO NCs. Volatile solvent and ligand were
lost during spin-coating (b – d) and evaporation-triggered
aggregation occurred (c) resulting in deposition of aggregates (e).
The process was repeated to give a textured tp-ZnO film (f). The
components are not drawn to scale.
Morphology and optoelectronic properties of tp-ZnO/P3HT hybrid
films
We next investigated hybrid films consisting of P3HT spin-coated
onto tp-ZnO film. Following Ravirajan et al.8 the ZnO substrate was
first pre-soaked in dilute P3HT solution (2 mg/mL) overnight as a
conditioning step before spin-coating P3HT solutions with a range
of P3HT concentrations (CP3HT). Compared to the unfilled tp-ZnO
film (Fig. 1c) there was a progressive loss of morphological detail
apparent from SEM images as CP3HT increased from 7 (Fig. 5a),
through 15 (Fig. 5b) to 20 mg/mL (Fig. 5c). AFM data for the film
prepared using CP3HT = 15 mg/mL are shown in Fig. 5d and a 3D image
for this film appears in Fig. S10a. The images show the morphology
was less undulating. This suggestion is supported by line profile
(Fig. 5e) and the RMS roughness of 41 nm, which was significantly
less than that for the parent tp-ZnO film (67 nm, above). P3HT
likely filled the pores and coated the ZnO aggregates. The higher
magnification AFM image (Fig. 5f) for the film prepared using 15
mg/mL P3HT is relatively featureless. The 3D image (Fig. S10b)
confirms that much of the texture had been mostly lost and that the
P3HT had covered the ZnO and filled in the pores to a large extent
(especially when compared to Fig.S5b).
Figure 5. Morphologies of tp-ZnO/P3HT films. The films were
coated with P3HT at a concentration of (a) 7 mg/mL (b) 15 mg/mL and
(c) 20 mg/mL. (d) to (f) show AFM images and a line profile for the
film prepared using 15 mg/mL P3HT. Scale bars: (a) – (d): 2 m; (f);
200 nm.
The tp-ZnO/P3HT films absorbed UV-visible light strongly in the
500 – 600 nm region due to vibronic bands52 (Fig. S11a). With an
increase of CP3HT used for spin-coating step the maximum absorption
(~ 520 nm) increased, which is a consequence of increased film
thickness. The photoluminescence (PL) spectra were also measured
for the films (Fig. S11b) and the spectra show similar features to
that reported elsewhere53 with the vibronic transitions from P3HT
evident54. The PL spectra for P3HT films is known to originate from
aggregates55 and is dependent on the extent of chain order56. We
attribute the change in the relative intensities of the main PL
peak with increasing CP3HT to differences in packing of the chains.
It is proposed that the P3HT structure was less influenced by the
tp-ZnO film as CP3HT increased based on the relative similarity of
the PL spectra for the films prepared using CP3HT = 15 and 20
mg/mL. Furthermore, the PL intensity increased with CP3HT which
indicates a greater number of photo-excited charge carriers were
available for charge transport. One might then expect improved
device performance for hybrid tp-ZnO/P3HT devices and this was
explored.
tp-ZnO/P3HT hybrid solar cells were constructed using tp-ZnO
films with P3HT deposited as a light harvesting layer. The CP3HT
values used were 7, 15 and 20 mg/mL. Control bilayer devices were
also prepared using only the sol-gel bl-ZnO layer and a P3HT layer
(CP3HT = 20 mg/mL). The device architectures used are depicted in
Fig. 6a. The sol-gel ZnO layer was relatively featureless as shown
by the 3D AFM images shown in Fig. S12. Representative J-V curves
for all four devices are shown in Fig. 6b. The device parameters
measured for the sol-gel device are similar to those reported by
Monson et al for their bilayer ZnO/P3HT device57. The Jsc values
for the tp-ZnO/P3HT devices (Fig. 6c) were not significantly
affected by CP3HT, whilst the FF value (Fig. 6d) was highest for
the tp-ZnO/P3HT prepared using CP3HT = 20 mg/mL. This result is
presumably because of the greater absorption from P3HT (Fig. S11a)
which increased charge generation (Fig. S11b).
Figure 6. (a) Depiction of device architectures used. The
sol-gel device was a control. (b) J-V data for the tp-ZnO/P3HT and
sol-gel devices. The effect of P3HT concentration used for
deposition on (c) short-circuit current density, (d) Fill factor,
(e) open circuity voltage and (f) power conversion efficiency are
shown.
The Voc values increased with CP3HT (Fig. 6e) which may be due
to less recombination. The latter suggestion is also consistent
with the PL data(Fig. S11b). However, the Jsc values were not
highest for the tp-ZnO/P3HT device prepared using CP3HT = 20 mg/mL
and other explanations for the change in Voc should also be
considered. The value for Voc for P3HT/ZnO solar cells originates
from the offset between the conduction band of ZnO and the HOMO of
P3HT58. For the tp-ZnO/P3HT devices the band gaps of ZnO NCs and
P3HT would be independent of CP3HT. Consequently, Voc should not
depend on CP3HT. Beek et al.44 showed that UV light (380 to 420 nm)
caused Voc to decrease for ZnO based bulk heterojunction devices.
One of the potential advantages of the inverted geometry used for
our cells is improved protection of the ZnO from UV-light. Indeed,
the UV-visible spectrum for tp-ZnO films (Fig. S11a) shows an
increasing absorption due to P3HT in the 380 – 420 nm region as
CP3HT increased. Consequently, an additional contribution to the
increase in Voc with CP3HT (Fig. 6e) is increased UV-protection
afforded by the P3HT films with increasing CP3HT.
Whilst the number of CP3HT values used was limited, it can be
seen that the PCE appeared to increase linearly with CP3HT for our
tp-ZnO/P3HT devices (Fig. 6f). Importantly, the PCE values for the
tp-ZnO/P3HT devices prepared using CP3HT values of 15 and 20 mg/mL
were greater than that measured for the sol-gel control by factors
of 2 and 3, respectively. The superior device performance for these
tp-ZnO/P3HT devices is attributed to the greater ZnO-P3HT
interfacial area (caused by the textured tp-ZnO morphology) which
increased the Jsc values (Fig. 6c) as well as an increase of the
Voc values.
A referee suggested that Au should be used instead of Ag as the
top contact and that a device should be prepared without the bl-ZnO
layer. We therefore constructed bl-ZnO-free, ITO/tp-ZnO/P3HT/Au
cells (Fig. S13 and Table S2). The average PCE was 0.146%. The
latter value was the highest PCE of all the device types studied.
These data further confirm that tp-ZnO was an effective electron
transport layer.
Conclusions
In this study we have investigated a new, simple and potentially
versatile method for preparing textured ZnO NC films. The process
simply used a volatile ligand and a spin-coater. Evaporation was
shown to trigger aggregation of the ZnO NC dispersion stabilised by
volatile ligands, which resulted in deposition of aggregates that
formed the textured porous tp-ZnO films. The tp-ZnO films were
crack-free. It appears that textured morphologies can be prepared
by our LBL film preparation method when the ligand boiling point is
less than or equal to 78 oC using amine-based ligands. The textured
films were infiltrated with P3HT and the hybrid bilayer films
provided improved PCE values that were at least a factor of three
higher than that for a sol-gel control film. These results
demonstrate the good potential for the tp-ZnO films to act as an
effective electron transport layer for hybrid bilayer solar cells.
Because ZnO layers are used for other solar cells the new tp-ZnO
approach may offer benefits for other devices where an increased
surface area compared to bl-ZnO layers is required. Moreover, the
deposition method used relatively low toxicity amine-based ligands
as well as low temperatures which are potentially advantageous for
future studies.
Acknowledgements
BRS and POB would like to thank the EPSRC funding for support
this work (K010298/1). We thank one of the referees for their
experimental suggestions concerning construction of new
devices.
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17
012345678
0
50
100
Height / nm
X / m
(f)(a)(c)(d)(e)(b)
1 10 100 1000
18
22
Intensity / AU
26
Size / nm
33
15 20 25 30 35
0.0
0.2
0.4
0.6
0.8
A
500
C
ZnO
/ (mg/ml)
300400500600700800
0.0
0.5
1.0
1.5
2.0
Absorbance
Wavelength / nm
33
26
24
22
20
18
(c)(d)(b)(a)
182022242633
(d)(a)(c)(b)
ZnONCLigandInitial wet filmExcessdispersionremovedEvaporation of
solvent & ligandTriggeredaggregation80
o
CGlasstp-ZnOtp-ZnO6 cycles1
st
layer(a)(b)(c)(d)(e)(f)
012345678
0
50
100
Height / nm
X / m
(a)(b)(c)(d)(e)(f)
0.0 0.2 0.4 0.6
0.0
0.2
0.4
0.6
20
15
7
Solgel
Voltage / V
J
sc
/
(
mA/cm
2
)
5 10 15 20
0.00
0.05
0.10
Porous
Sol-gel
PCE / %
C
P3HT
/ (mg/mL )
99.0
0054.0
2
3
R
CPCE
HTP
5 10 15 20
0.0
0.2
0.4
0.6
FF
C
P3HT
/ (mg/mL )
5 10 15 20
0.0
0.2
0.4
0.6
V
oc
/ V
C
P3HT
/ (mg/mL )
5 10 15 20
0.0
0.2
0.4
0.6
J
sc
/
(
mA/cm
2
)
C
P3HT
/ (mg/mL )
(a)(b)(d)(c)(f)(e)
tp-ZnO/P3HTSol-gel
tp-ZnO/P3HT
bl-ZnOITO
Ag
bl-ZnOITO
Ag
P3HT
NH
2
NH
2
NH
2
ZnONCdispersionSpin-coatZnONCLigandLigandPADABAtp-ZnO/P3HTGlassGlassGlassSpin-coatP3HT/CBZ6
cyclestp-ZnO
