# Polymer Photovoltaic Cell UsingTiO2/G-PEDOT Nanocomplex Film as Electrode

Authors: F. X. Xie; C. J. Liang; Z. Q. He; Y. L.  Tao
Journal: International Journal of Photoenergy
 (2008)

---

## Abstract

UsingTiO2/G-PEDOT (PEDOT/PSS doped with glycerol) nanocomplex film as a substitute for metal electrode in organic photovoltaic cell is described. Indium tin oxide (ITO) worked as cathode and TiO2/G-PEDOT nanocomplex works as anode. The thickness of TiO2  layer in nanocomplex greatly affects the act of this nonmetallic electrode of the device. To enhance its performance, this inverted organic photovoltaic cell uses another TiO2 layer as electron selective layer contacted to ITO coated glass substrates. All films made by solution processing techniques are coated on the transparent substrate (glass) with a conducting film ITO. The efficiency of this solar cell is compared with the conventional device using Al as electrode.


---

## Body

## 1.
Introduction

Organic photovoltaic  devices offer  great technological potential as a
renewable and alternative source of electrical energy. Efforts focusing on
improving the efficiency of solar cell and reducing their
manufacturing cost are going on. Among
various thin film techniques, solution-processed organic photovoltaic cells
have considerable potential for becoming a true low-cost technology since their
production demand  solution-based deposition methods. And
it brings a brilliant  prospect that large-scale
plastic photovoltaic device
will be produced by industrial technology, such as screen-printing, ink-inject,
spin coating, or dip coating. In order to prepare organic
solar cell by solution process, many methods are attempted to replace one of the
electrodes by using polymer electrode
[1–3]. Commonly, polymer solar cells are constructed by sandwiching an active
layer between two coplanar electrodes [4–6]. The bottom electrode
(anode) is usually made of a transparent substrate (glass) coated with a
conducing firm such as indium tin oxide (ITO). The upper electrode (cathode) is
usually formed by vacuum deposition of low work function (WF) metals like
aluminum (Al) and calcium (Ca) on top of the active layer.Many efforts
have been made to investigate polymer anode. So far, PEDOT/PSS is widely
accepted as a good hole-conducting layer when used in photovoltaic devices.
Moreover, various treatments can be employed to enhance the quality of the
surface morphology and conductivity of PEDOT/PSS layer
[7, 8]. The
conductivity of PEDOT/PSS could be enhanced by doping glycerol, sorbitol, DMSO
and so on
[9–11]. And these modified PEDOT/PSS have been
successfully applied in polymer light-emitting devices and organic photovoltaic
cells as electrodes to replace ITO to reduce  the cost of devices
[3, 12–14].
In this paper, we have explored another method to form organic
photovoltaic cells. We use TiO2/G-PEDOT
nanocomplex film in photovoltaic cells to displace
metal electrode. The interests of using conductive polymer as electrode
originate from its simple film-forming techniques. The conductive polymer in
solution can be easily shaped into different geometric drawings, and
after baking it becomes conductive film. The manipulation
and equipment could be simplified since, rather than high-vacuum evaporation system, and the facility we
need is an ordinary vacuum-annealing oven equipped with mechanism pump and
heater only. Moreover, using polymer electrode to replace metal film is a
useful attempt towards flexible plastic large-scale photovoltaic cells.


## 2.
Experimental

The device architectures are shown in Figure1. The structure of our
solution-processed  photovoltaic cell is ITO/TiO2/P3HT+PCBM/TiO2/G-PEDOT.
TiO2 sol-gel solution, which is described at the back of this part,
was spin coated on a precleaned ITO glass substrate at 7000 rpm to form about
40 nm layer. Subsequently, during 5 minutes in wet air of about 80% relative
humidity at room temperature, the precursor converted to TiO2 by
hydrolysis. Thin films of a blend of P3HT: PCBM with a weight ratio of 1:0.8
were spin coated at 1000 rpm from a 10 mg/mL methylbenzene solution and the
thin film of TiO2 is deposited again by spin TiO2 sol-gel
solution. Solid films are formed after 1-hour  vacuum annealing at 80°C. At last, G-PEDOT is dropped on the device in order to form thick film.
The layer of G-PEDOT is about 20μm, which is semitransparent and conductive.
The photovoltaic device was finished by vacuum annealing at  120°C  for half an hour to remove solvent.(a) The device structures of the solution
processed photovoltaic cell. (b) The energy levels of the single components of
photovoltaic cell. (c) Molecular structures of the active
materials and conductive polymer: P3HT, PCBM, PEDOT:PSS.
(a)(b)(c)Current-voltage curves were recorded by Keithley 2410 source-measure unit.
The photocurrent was obtained with Keithley 2410 unit by illumination with an
intensity of 80 mW/cm2 from an Xe
lamp (71LX150). The photocurrent spectral responses were corrected according to
the spectral distribution of the illuminating light.The sol-gel procedure for
producing TiO2  is as follows: titanium isopropoxide
(Ti[OCH(CH3)2]4, 10 mL) was prepared as a
precursor and mixed with ethanol (CH3CH2OH, 40 mL). Put
the mixture into a conical flask. And then acetic acid (CH3COOH,
4 mL) mixed with pure water (H2O, 2 mL) was dropped into conical flask
slowly. The mixed solution was under magnetic stirring for 2 hours. The
sol-solution was formed after two days placed in a hermetic cuvette.


## 3. Results and Discussion

### 3.1. TiO2/G-PEDOT Nanocomplex Anode

The conductivity of PEDOT/PSS used in our device is enhanced by doping 6 wt% glycerol [15]. The surface sheet resistance of a 20-μm thick
PEDOT:PSS film fabricated with this addition of 6 wt% glycerol is decreased to
nearly  30 (Ω/ □). In our experiment process, there is a problem that G-PEDOT can hardly spread
on P3HT: PCBM layer because of P3HT: PCBM layer’s hydrophobic. TiO2 layer was added to connect the two layers. It changes the P3HT: PCBM layer’s surface into hydrophilic. The testing results show that PEDOT/PSS layer is used as anode. G-PEDOT
layer is supposed to be cathode because TiO2 is a perfect hole-blocking layer
[16]. But  the fact proves to be opposite. In order to testify
the function of TiO2 layer used in this place, a series of tentative
experiments and tests have been done and a model of anode was elicited.The surface of TiO2 can be seen from Figure 2. Nano-size TiO2 grains have been arranged compactly in a neat plane with porosity, which enables the electrolyte
(G-PEDOT) to soak into the films slowly. The size of TiO2 grain is about 20 nm. A
schematic diagram (Figure 3), which is based on the SEM photo of TiO2, presented to demonstrate the forming of the nanocomplex electrode
procedure. Firstly, G-PEDOT solution was laid down on TiO2-coated
sheets to form the nanocomplex electrodes. G-PEDOT molecules then intruded in TiO2 layer through the interstices slowly while solution was removed by vacuum
heating. If the thickness of TiO2 layer is appropriate, G-PEDOT
molecules would finally go to the bottom of TiO2 layer by the time
the solution was completely removed and attach the surface of P3HT+PCBM film. Otherwise,
if the thickness of TiO2 layer is too thick, the interstices of TiO2 layer would be partly filled by G-PEDOT.SEM image of TiO2 film on ITO substrate.
(a)(b)Figure 3
Illustration showing the forming process of TiO2/G-PEDOT
nanocomplex electrode.A series of experiments showed that the appropriate thickness of TiO2 layer is 90 nm. Accordingly, the device with appropriate thickness showed the
highest performance (figure can be seen in Figure 4):
Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33  η = 1.38%. On the other hand, the TiO2 layer is partly filled by G-PEDOT
and  exhibits n-type character for some extend when the TiO2 layer is
too thick. The performance of device is reduced; Jsc =  3.8 mA/cm2 Voc = 0.44 V  FF = 0.30   η = 0.64%. In addition,
with decreasing the TiO2 layer, performance of devices were then
lessened at last, because the active layer is affected by G-PEDOT solution,
when the TiO2 layer is too thin.(a) I–V characteristics
of device in dark condition used different thickness of TiO2 to
construct the TiO2/G-PEDOT nanocomplex anode. (b) I–V characteristics
measured under white illumination (Xe lamp, 80 mW/cm2). Devices’
performance: (square) Jsc =  3.8 mA/cm2 Voc = 0.44 V  FF = 0.30   η = 0.64%,
(circles) Jsc =  4.1 mA/cm2 Voc = 0.59 V  FF = 0.26  η = 0.78%, (up-triangle)
Jsc = 5.7 mA/cm2 Voc = 0.59 V
FF = 0.33   η = 1.38%, and (down-triangle) Jsc =  1.78 mA/cm2 Voc = 0.60 V  FF = 0.26  η = 0.35%.
(a)(b)In
conclusion, the thickness of TiO2 layer
in nanocomplex anode greatly affects the
performance of device, since the thickness of
TiO2 layer determined the form of TiO2/G-PEDOT
nanocomplex. The introduced TiO2 can connect two different
components firmly just like ligament connecting bone and muscle. In other
words, TiO2 grains were embedded G-PEDOT and anchored it on the
active layers.


### 3.2. Function of TiO2 Contacted to ITO

It is known that TiO2 layer is used as electron selective film to enhance the performance of organic
photovoltaic cell
[16]. And  the TiO2 layer which is
contacted to ITO in inverted device also has the function of electron selection,
which is the same case in our device
[17].From the data showed in Figure5, it can be seen that without the TiO2 layer connected to ITO, the device performance is low; Jsc =  0.03 mA/cm2 Voc = 0.22 V  FF = 0.25  η = 0.0016%. By contrast, the device performance, which is using another TiO2 layer contact to ITO anode, proved to be Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33  η = 1.38%. Low performance is mainly due to the inefficiency of electron
collection, since the work function of ITO is not suitable (the energy levels of the single
components of photovoltaic cell are shown in Figure 1(b)). By adding TiO2 film, the energy levels (demonstrated in
Figure 1(b)) satisfy the electronic structure requirements. From what has been
discussed above, we draw the conclusion that TiO2 layer contacted to
ITO is working as electron selective layer.I–V characteristics
of ITO/TiO2/P3HT+PCBM/TiO2/G-PEDOT (square) and
ITO/P3HT+PCBM/TiO2/G-PEDOT (circles). (a) I–V characteristics
of device in dark condition. (b) I–V characteristics
measured under white illumination (Xe lamp, 80 mW/cm2). Devices’
performance: (square) Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33  η =  1.38%, (circles) Jsc =  0.03 mA/cm2 Voc = 0.22 V  FF = 0.25  η = 0.0016%.
(a)(b)


### 3.3. Devices Performance Comparison

To compare with performance of the all-solution-processed solar cell, the
conventional device is prepared (structure: ITO/PEDOT/P3HT: PCBM/TiO2/Al,
the active layer is annealed 1 hour at 80°C and then at 120°C for half an hour in vacuum). The open-circuit voltage (Voc) and the
short-current density Jsc measured from the conventional device were 0.58 V and
6.5 mA/cm2. In contrast, I–V characteristics
of device with TiO2/G-PEDOT
nanocomplex  anode is Voc = 0.58 V, Jsc = 5.7 mA/cm2 (as shown in
Figure 6). And
the efficiency  η is 1.38%, compare to the efficiency of conventional device 1.5%. In addition,
the result of IPCE (incident photon to collected electron efficiency) (showed in
Figure 7), measurements of the device used TiO2/G-PEDOT nanocomplex
anode shows a maximum of 32.4% at wavelength of 400 nm.I–V characteristics
of ITO/PEDOT/P3HT+PCBM/TiO2/Al (square), and ITO/TiO2/P3HT+PCBM/TiO2/G-PEDOT
(circles). (a) I–V characteristics
of device in dark condition, (b) I–V characteristics
measured under white illumination (Xe lamp, 80 mW/cm2). Devices’
performance: (square) Jsc =
6.5 mA/cm2 Voc = 0.58 V  FF = 0.32  η = 1.5%, (circles) Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33.
(a)(b)Figure 7
IPCE of device used TiO2/G-PEDOT
nanocomplex as anode.From the data discussed above, we can see that the efficiency of our
inverted device almost reaches the level of the
conventional device. And the performance of this kind of solar cells is
comparable to the device using Al as electrode.


## 3.1. TiO2/G-PEDOT Nanocomplex Anode

The conductivity of PEDOT/PSS used in our device is enhanced by doping 6 wt% glycerol [15]. The surface sheet resistance of a 20-μm thick
PEDOT:PSS film fabricated with this addition of 6 wt% glycerol is decreased to
nearly  30 (Ω/ □). In our experiment process, there is a problem that G-PEDOT can hardly spread
on P3HT: PCBM layer because of P3HT: PCBM layer’s hydrophobic. TiO2 layer was added to connect the two layers. It changes the P3HT: PCBM layer’s surface into hydrophilic. The testing results show that PEDOT/PSS layer is used as anode. G-PEDOT
layer is supposed to be cathode because TiO2 is a perfect hole-blocking layer
[16]. But  the fact proves to be opposite. In order to testify
the function of TiO2 layer used in this place, a series of tentative
experiments and tests have been done and a model of anode was elicited.The surface of TiO2 can be seen from Figure 2. Nano-size TiO2 grains have been arranged compactly in a neat plane with porosity, which enables the electrolyte
(G-PEDOT) to soak into the films slowly. The size of TiO2 grain is about 20 nm. A
schematic diagram (Figure 3), which is based on the SEM photo of TiO2, presented to demonstrate the forming of the nanocomplex electrode
procedure. Firstly, G-PEDOT solution was laid down on TiO2-coated
sheets to form the nanocomplex electrodes. G-PEDOT molecules then intruded in TiO2 layer through the interstices slowly while solution was removed by vacuum
heating. If the thickness of TiO2 layer is appropriate, G-PEDOT
molecules would finally go to the bottom of TiO2 layer by the time
the solution was completely removed and attach the surface of P3HT+PCBM film. Otherwise,
if the thickness of TiO2 layer is too thick, the interstices of TiO2 layer would be partly filled by G-PEDOT.SEM image of TiO2 film on ITO substrate.
(a)(b)Figure 3
Illustration showing the forming process of TiO2/G-PEDOT
nanocomplex electrode.A series of experiments showed that the appropriate thickness of TiO2 layer is 90 nm. Accordingly, the device with appropriate thickness showed the
highest performance (figure can be seen in Figure 4):
Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33  η = 1.38%. On the other hand, the TiO2 layer is partly filled by G-PEDOT
and  exhibits n-type character for some extend when the TiO2 layer is
too thick. The performance of device is reduced; Jsc =  3.8 mA/cm2 Voc = 0.44 V  FF = 0.30   η = 0.64%. In addition,
with decreasing the TiO2 layer, performance of devices were then
lessened at last, because the active layer is affected by G-PEDOT solution,
when the TiO2 layer is too thin.(a) I–V characteristics
of device in dark condition used different thickness of TiO2 to
construct the TiO2/G-PEDOT nanocomplex anode. (b) I–V characteristics
measured under white illumination (Xe lamp, 80 mW/cm2). Devices’
performance: (square) Jsc =  3.8 mA/cm2 Voc = 0.44 V  FF = 0.30   η = 0.64%,
(circles) Jsc =  4.1 mA/cm2 Voc = 0.59 V  FF = 0.26  η = 0.78%, (up-triangle)
Jsc = 5.7 mA/cm2 Voc = 0.59 V
FF = 0.33   η = 1.38%, and (down-triangle) Jsc =  1.78 mA/cm2 Voc = 0.60 V  FF = 0.26  η = 0.35%.
(a)(b)In
conclusion, the thickness of TiO2 layer
in nanocomplex anode greatly affects the
performance of device, since the thickness of
TiO2 layer determined the form of TiO2/G-PEDOT
nanocomplex. The introduced TiO2 can connect two different
components firmly just like ligament connecting bone and muscle. In other
words, TiO2 grains were embedded G-PEDOT and anchored it on the
active layers.


## 3.2. Function of TiO2 Contacted to ITO

It is known that TiO2 layer is used as electron selective film to enhance the performance of organic
photovoltaic cell
[16]. And  the TiO2 layer which is
contacted to ITO in inverted device also has the function of electron selection,
which is the same case in our device
[17].From the data showed in Figure5, it can be seen that without the TiO2 layer connected to ITO, the device performance is low; Jsc =  0.03 mA/cm2 Voc = 0.22 V  FF = 0.25  η = 0.0016%. By contrast, the device performance, which is using another TiO2 layer contact to ITO anode, proved to be Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33  η = 1.38%. Low performance is mainly due to the inefficiency of electron
collection, since the work function of ITO is not suitable (the energy levels of the single
components of photovoltaic cell are shown in Figure 1(b)). By adding TiO2 film, the energy levels (demonstrated in
Figure 1(b)) satisfy the electronic structure requirements. From what has been
discussed above, we draw the conclusion that TiO2 layer contacted to
ITO is working as electron selective layer.I–V characteristics
of ITO/TiO2/P3HT+PCBM/TiO2/G-PEDOT (square) and
ITO/P3HT+PCBM/TiO2/G-PEDOT (circles). (a) I–V characteristics
of device in dark condition. (b) I–V characteristics
measured under white illumination (Xe lamp, 80 mW/cm2). Devices’
performance: (square) Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33  η =  1.38%, (circles) Jsc =  0.03 mA/cm2 Voc = 0.22 V  FF = 0.25  η = 0.0016%.
(a)(b)


## 3.3. Devices Performance Comparison

To compare with performance of the all-solution-processed solar cell, the
conventional device is prepared (structure: ITO/PEDOT/P3HT: PCBM/TiO2/Al,
the active layer is annealed 1 hour at 80°C and then at 120°C for half an hour in vacuum). The open-circuit voltage (Voc) and the
short-current density Jsc measured from the conventional device were 0.58 V and
6.5 mA/cm2. In contrast, I–V characteristics
of device with TiO2/G-PEDOT
nanocomplex  anode is Voc = 0.58 V, Jsc = 5.7 mA/cm2 (as shown in
Figure 6). And
the efficiency  η is 1.38%, compare to the efficiency of conventional device 1.5%. In addition,
the result of IPCE (incident photon to collected electron efficiency) (showed in
Figure 7), measurements of the device used TiO2/G-PEDOT nanocomplex
anode shows a maximum of 32.4% at wavelength of 400 nm.I–V characteristics
of ITO/PEDOT/P3HT+PCBM/TiO2/Al (square), and ITO/TiO2/P3HT+PCBM/TiO2/G-PEDOT
(circles). (a) I–V characteristics
of device in dark condition, (b) I–V characteristics
measured under white illumination (Xe lamp, 80 mW/cm2). Devices’
performance: (square) Jsc =
6.5 mA/cm2 Voc = 0.58 V  FF = 0.32  η = 1.5%, (circles) Jsc = 5.7 mA/cm2 Voc = 0.59 V  FF = 0.33.
(a)(b)Figure 7
IPCE of device used TiO2/G-PEDOT
nanocomplex as anode.From the data discussed above, we can see that the efficiency of our
inverted device almost reaches the level of the
conventional device. And the performance of this kind of solar cells is
comparable to the device using Al as electrode.


## 4. Conclusion

In summary, compared
with common conductive metal,
TiO2/G-PEDOT nanocomplex can be an interesting alternative electrode
applied in organic solar cell. The thickness of TiO2 layer in nanocomplex greatly effects the
performance of device. The other TiO2 layer contacted to ITO works
as an electron collection layer to improve
the performance of device. The efficiency of photovoltaic devices with polymer
anode is comparable to that of conventional devices using Al as electrodes.



---

*Source: 415861-2009-03-05.xml*