Solu%on processed metal oxide interlayers for small molecule photovoltaic devices Sarah Orsborne1, Tim Jones1 , Neil Wilson2, Ian Hancox1, Edward New1 1Department of Chemistry, University of Warwick, Coventry, United Kingdom Sarah Orsborne s.r.e.orsborne@
warwick.ac.uk 2Department of Physics, University of Warwick, Coventry, United Kingdom Background Information
Experimental
Presently, the majority of the world’s energy is produced from the combustion of
fossil fuels such as coal, gas and oil but there is an increasing need for renewable
and environmentally friendly sources of energy. Commercially, the inorganic silicon
wafer based devices are the most commonly used photovoltaic devices, typically
exhibiting efficiencies in the range of 15 to 20%. However, they are expensive, rigid
and require a large input of energy to produce. Organic photovoltaics (OPVs), on
the other hand, have huge potential due their low cost, flexibility and tunability.
However, they are currently limited by their poor performance relative to inorganic
counterparts.
Cerium acetate layers were deposited onto
ITO substrates by spin coating. Other
layers were vacuum deposited onto the
substrate to give the inverted structure
shown in figure 4. The devices were then
tested and analysed in a number of ways.
This was then repeated by adding
monoethanolamine (MEA) to the cerium
acetate solution and annealing.
Figure 2: Spin-­‐
coa;ng Figure 1: How an OPV device works: 1) Light absorp;on and exciton forma;on, 2) Exciton diffusion, 3) Exciton dissocia;on, 4) Charge collec;on Interlayers are usually introduced into the OPV’s structure to prevent damage to
the active layer and to ensure good alignment of the energy levels for efficient
charge collection. For an electron transport layer (ETL), a low work function is
desirable for good ohmic contact.
Figure 3: Vacuum deposi;on Figure 4: Inverted device Results
Prior to the addition of MEA to the cerium acetate solutions, the cells
performed poorly. The cerium acetate was not completely soluble in
isopropanol (IPA) but was soluble in water. However, water layers did not
spin coat well.
ITO
Ce-ac in IPA (1
mgml-1) + 0.05 ml
MEA
Zr-acac in IPA
Ce-ac in IPA (1
mgml-1)
Upon addition of MEA, the solutions changed from colourless to yellow and
also the cerium acetate fully dissolved in IPA. The performance of the cells
also significantly improved.
Work Function
4.87
Jsc
4.74
Voc
0.95
FF
0.47
Efficiency
2.09
4.68
4.64
4.73
4.70
1.02
0.85
0.52
0.47
2.49
1.90
4.52
4.05
0.61
0.29
0.72
Table 1 (above): The work functions of various layers spin coated onto ITO substrates. The
Jsc, Voc, Fill factor (FF) and Efficiencies are also shown to give an indication of the
performance of the cells.
The work function of each layer was lower that that of blank ITO, showing
the layers have potential as electron transport layers (ETLs).
2
3
4
5
6
Figure 5 (left): Cerium acetate solutions with MEA
1) 10 mgml-1 Ce-ac + 0.05 ml MEA,
2) 10 mgml-1 Ce-ac + 0.20 ml MEA,
3) 5 mgml-1, Ce-ac + 0.05 ml MEA,
4) 5 mgml-1 Ce-ac + 0.05 ml MEA,
5) 1 mgml-1 Ce-ac + 0.05 ml MEA,
6) 1 mgml-1 Ce-ac + 0.20 ml MEA.
0.3
EQE
1
0.2
ITO
Ceac_ipa_MEA
Zracac_ipa
0.1
350
450 500 550 600
Wavelength (nm)
650 700nm
-2
-2
Current Density (mA cm )
4mA cm
400
Figure 6 (above): UV-Vis data of the six Ce-ac based
layers pictured in figure 5 spin coated onto ITO
against an air and ITO background scan
Figure 7 (left):
AFM images of
solutions 2 and 5
(top and middle
images
respectively)
coated onto an
ITO substrate
and a blank ITO
substrate (bottom
image).
This shows that
the more
concentrated
solutions, and
hence the thicker
layer, is
smoother.
2
Ceac_ipa_MEA
Zracac_ipa
Ceac_ipa
ITO
0
-2
-4
-1.5V
-1.0
-0.5
0.0
Voltage (V)
0.5
1.0
Figure 8 (top right): J-V curves of devices with 1 mgml-1 Ce-ac + 0.05 ml MEA, 1
mgml-1 Ce-ac, no ETL (labelled ITO) and 1 mg ml-1 Zr-acac.
Figure 9 (bottom right): EQE curves of devices with a 1 mgml-1 Ce-ac + 0.05 ml MEA,
1 mg ml-1 Zr-acac and with no ETL.
Figure 10 (left): Degradation measurements taken over 5 mins showing that the
cerium ETL device was stable in comparison to an ITO device which, in contrast,
rapidly degraded with time.
Conclusions
The cerium based interlayers work effectively in small molecule
photovoltaic devices when MEA is added to cerium acetate. The
devices achieve efficiencies of 2.49%. As desired, the work function of
the ITO surface is lowered upon addition of the cerium derivative,
suggesting a good ohmic contact is formed with the acceptor, C60.
The colour change upon addition of MEA suggests that cerium has been
oxidised from Ce (III) to Ce (IV). Perhaps there are a mixture of Ce (IV)
complexes in the yellow solution. Further investigation would be required
to determine whether or not this is true.