Semi-transparent Inverted Organic Solar Cells

Semi-transparent Inverted Organic Solar Cells H. Schmidta , T. Winklera , M. Tilgnera , H. Fl¨ uggea , S. Schmalea , T. B¨ ulowa , J. Meyera , H.-H. Johannesa , T. Riedl a and W. Kowalskya a

Institut f¨ ur Hochfrequenztechnik, TU Braunschweig, Schleinitzstrasse 22, 38106 Braunschweig, Germany; ABSTRACT

We will present efficient semi-transparent bulk-heterojunction [regioregular of poly(3-hexylthiophene): (6,6)phenyl C61 butyric acid methyl ester] solar cells with an inverted device architecture. Highly transparent ZnO and TiO2 films prepared by Atomic Layer Deposition are used as cathode interlayers on top of ITO. The topanode consists of a RF-sputtered ITO layer. To avoid damage due to the plasma deposition of this layer, a sputtering buffer layer of MoO3 is used as protection. This concept allows for devices with a transmissivity higher than 60 % for wavelengths 650 nm. The thickness of the MoO3 buffer has been varied in order to study its effect on the electrical properties of the solar cell and its ability to prevent possible damage to the organic active layers upon ITO deposition. Without this buffer or for thin buffers it has been found that device performance is very poor concerning the leakage current, the fill factor, the short circuit current and the power conversion efficiencies. As a reference inverted solar cells with a metal electrode (Al) instead of the ITO-top contact are used. The variation between the PCE of top versus conventional illumination of the semi-transparent cells was also examined and will be interpreted in view of the results of the optical simulation of the dielectric device stack with and without reflection top electrode. Power conversion efficiencies of 2-3 % for the opaque inverted solar cells and 1.5-2.5 % for the semi-transparent devices were obtained under an AM1.5G illumination. Keywords: OPV, semi-transparent, inverted, ITO

1. INTRODUCTION Polymer solar cells have gained a lot of interest due to their potential to be used as technology for large-area, flexible, mobile, and low-cost power generation applications.1 Besides flexibility one of the most outstanding features of organic photovoltaics is the opportunity to realize semi-transparent cells. Concerning the pronounced absorption features of various abosorber molecules used in organic solar cells, devices can be designed to transmit light in a specific spectral range. Thereby, colored see-through photovoltaic elements (e.g. windows or colored semi-transparent curtains) are possible. Combined with transparent organic light emitting diodes (OLEDs)2 novel applications are feasible. For the development of semi-transparent organic solar cells different concepts have been presented relating the transparent top electrode. First attempts to realize semi-transparent organic photovoltaics employing semitransparent thin metal films.3–5 Due to the absorption losses in metal films the transparency of the cells is low in the the visible spectrum. Improvement was done by employing multilayers of thin metal films and sputtered ITO layers ,6–8 where the metal layers were used to prevent the organic layers from damage as a result of the sputtered particles. However the main drawback of these devices is again the limited transmissivity in the visible part of the spectrum. Hence, for higher transparency a metal-free top anode and metal-free buffer layer is required. Some efforts concerning metal-free organic solar cells were made by Bailey Salzmanet al.7 or Hanischet al.9 However, the obtained efficiencies were notsatisfying or the devices showed a poor reproducibility owing to the damages introduced by the sputtered top-electrodes. Another approach for semi-transparent organic solar cells is using conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as top-electrode,10 though so far resulting in low cell efficiencies. Further author information: (Send correspondence to H. Schmidt) H. Schmidt: E-mail: [email protected], Telephone: +49 (0) 531 391 2016

Organic Photovoltaics X, edited by Zakya H. Kafafi, Paul A. Lane, Proc. of SPIE Vol. 7416 741611 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.826114

Proc. of SPIE Vol. 7416 741611-1

1.1 Metal-free Semi-transparent Polymer Solar Cells Our approach was to realize semi-transparent organic photovoltaic devices with a sputtered ITO top contact employing a metal-free protecting layer. The latter should feature the following properties. The layer has to withstand all damaging effects during the sputtering process and must not damage the underlying organic films during the deposition on his part. The transparency should be higher than 80 % in the entire visible spectrum, the vertical conductivity should be high enough to not affect the electrical characteristic of the device. Furthermore, good charge extraction through the layer out of the organic layer is essential. In addition, the layer should form an amorphous pin-hole free film, providing optimal growing conditions for ITO layers for a high lateral conductivity and stacking approaches. Recently, for transparent inverted OLEDs a 40 nm thick layer of tungsten oxide (WO3 ) has been evidenced to function as efficient buffer to prevent the organic layers from damages due to the sputter deposition process of the top-electrode.11 For organic solar cells transition metal oxides (TMOs), like WO3 or MoO3 , have been used as interlayers between organic layers and electrodes to allow for hole extraction at the anode side.12 Moreover, thin layers of TMOs have also been applied as interconnect layers for stacked tandem solar cells.13 These highly transparent transition metal oxides can be thermally evaporated on top of organic without introducing damages, which made them suitable as buffer layers. In this paper we show inverted, semi-transparent polymer photovoltaic devices with a sputter deposited indium tin oxide top-electrode. The cell structure is shown in Fig. 1 (b). We used a transparent buffer layer of MoO3 to prevent sputter damage to the organic active layers. Concerning the eletrical properties and the device performace we compared the semi-transparent devices with opaque, inverted polymer solar cells (Fig. 1 (a)).

Figure 1. Inverted, (a) opaque and (b) semi-transparent solar cell device

2. EXPERIMENTAL We used an inverted layer sequence with a bottom-cathode and a top-anode. As substrate, ITO coated borofloat glass was employed. In our devices, we deposited either a 25 nm thick interlayer of titanium dioxide (TiO2 ) or a 25 nm thick interlayer of zinc oxide by atomic layer deposition (ALD) on top of the bottom ITO electrode (substrate temperature 80 ◦ C for TiO2 and 150 ◦ C for ZnO respectively). As precursors we have used Tetrakis(dimethylamino)titanium(IV) (ZnO: diethylzinc) and H2 O in a 200 mm ALD system (Cambridge Nanotech). The work function of the TiO2 surface has been measured by Kelvin probe (McAllister, KP6500) to 4.2 eV and for the surface of ZnO to 4.3-4.4 eV. For the active organic materials, poly(3-hexylthiophene) (P3HT) (supplied by Honeywell Specialty Chemicals GmbH, Seelze, Germany) was used as donor and the methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) was used as acceptor (supplied by American Dye Source). The active layer of blended P3HT:PCBM dissolved in chlorobenze and chloroform (1:1) was spin coated under inert atmosphere resulting in a film with a thickness of 200 nm. Subsequently, the active layer was annealed at 110 ◦ C for 15 minutes inside the glovebox. MoO3 layers with varied thickness were thermally evaporated on top of the active organic layers in a vacuum system (base pressure 10−8 mbar). For the semi-transparent cells, an anode layer of 60 nm ITO was thereafter deposited by RF-sputtering. The sputter deposition was done in two steps. The first layer (20 nm) was deposited at a sputtering-power density of 0.05 W/cm2 followed by a


further 40 nm of ITO prepared at a sputtering power density of 0.3 W/cm2 . The entire ITO top-electrode has a conductivity of 1350 S/cm measured by the van-der-Pauw method. For the opaque reference cells, the anode consisted of 100 nm of aluminum instead of ITO. The active device area was 3 mm2 defined by photolithography. I −V curves were measured by a Keithley 2400 source meter. The photocurrent was measured under illumination from a 300 W solar simulator with an AM1.5G filter (Newport Corp.). Efficiencies were determined without correction for spectral mismatch. A commercial software (ETFOS) was used for the simulation of the dielectric device stack. The optical parameters of the involved materials were measured by spectral ellipsometry.

3. RESULTS 3.1 Inverted Polymer Solar Cells Kuwabara et al. have shown that a thin interlayer of ITO bottom electrode in inverted organic solar cells.14 nanoparticles to get electron selective layers.15, 16 For selective layers deposited on the ITO bottom electrode

TiOx significantly improves electron extraction via the Other groups used and studied solution processed ZnOthe inverted devices we studied ZnO- or TiO2 electron by ALD.

1000

current density (mA/cm²)

100

ZnO TiO2

illuminated illuminated

dark dark

10 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6

-0.8

-0.4

0.0 voltage (V)

0.4

0.8

Figure 2. I-V characteristics of inverted solar cell devices with ZnO- and TiO2 -electron selctive layer

In Fig. 2 the I-V-characteristics of inverted cells with ZnO- and TiO2 -layers with and without illumination are shown. In comparison to devices with TiO2 -layers devices with ZnO-layers achieve higher current densities at the same voltage in forward-bias and without illumination also in reverse-bias, probably due to the better conductivity of ZnO. Though the device characteristics are poor as a result of a low open circuit voltage with a value of 0,42V and low short circuit current respectively. Better results emerge for cells with TiO2 electron selctive layers. The extracted data is summarized in Table 1. device inverted cell - TiO2 inverted cell - ZnO

Voc (V) 0.51 0.42

FF (%) 54 53

ISC (mA/cm2 ) 7.8 5.5

P CE (%) 2.1 1.1

Table 1. Characteristic data of inverted devices with ZnO- and TiO2 -interlayers.

The results of inverted cells with ALD deposited TiO2 -interlayers were more promising than with the ZnOinterlayers. Hence, in the following we used TiO2 -interlayers to realize semi-transparent devices.


3.2 Semi-transparent Polymer Solar Cells: Transparency Fig. 3 (a) exemplary shows the transmittance of a semi-transparent solar cell with a layer sequence according to that Fig. 1 (b) and with a thin metal layer (15 nm Al) instead of ITO (thickness of the MoO3 buffer layer: 40 nm). Fig. 3 (b) shows a picture of a semi-transparent device with different sizes of the active area (1-4) with ITO top contact. The overall transmission and especially the transmission in the red spectral range of the visible spectrum is considerably higher for the device with the sputtered anode. Concerning the device with sputtered ITO electrode, in the spectral region between 500-550 nm, where the absorption of the P3HT:PCBM layer shows a local maximum, the transmissivity of the device is only 10 %. However, a high transparency of around 80 % is found for wavelengths larger than 650 nm, unsurpassed by previously reported results. Note, the typical absorption coefficient of the thermally evaporated MoO3 buffer at 650 nm is around 8×103 cm−1 . Thus, even a 60 nm thick MoO3 buffer would allow for a transmittance of 95 % . Consequently, the transmissivity data given in Fig. 3 (a) are not limited by absorption in the MoO3 buffer.

100

9mm

transmittance (%)

80

semi-transparent cell with thin metal contact sputtered ITO-contact

1

2

60 40

4

20 0

400

500 600 wavelength (nm)

3

700

Figure 3. (a) Transmittance of inverted semi-transparent polymer solar cells with a 15 nm thick thin metal film and a sputtered ITO top elctrode (b) Picture of semi-transparent device with different sizes of the active area (1-4)

3.3 Semi-transparent Polymer Solar Cells: Optical Simulation Optical simulation of the device structure was done by ETFOS-simulation. Specifically, we compare the penetration of the electromagnetic field (at a wavelength of 550 nm) in an opaque device with Al top-electrode and in the semi-transparent devices. For the semi-transparent devices optical simulation was made for illumination through the glass side as well as for illumination through the top (ITO) electrode. Furthermore the thickness of the MoO3 buffer layer was varied and we derive the spatial overlap ζ of the optical mode intensity with the active organic layer (Fig. 4). For devices with an aluminum top contact ζ is significantly larger compared to semi-transparent cells, where a reflecting Al electrode is missing. For the opaque devices ζ is slightly decreasing with increasing MoO3 layer thickness, owing to some small absorption in the MoO3 layer. Semi-transparent devices illuminated from the top side show a similar behaviour. In general, the ζ values are lower for illumination from top side because of higher losses due to increased absorption by the ITO top-electrode. The RF-sputtered ITO showed a somewhat higher absorption than the ITO used as the bottom-contact (which has undergone high temperature annealing to improve the electro-optical properties).


60

z (a.u.)

50

40 opaque cell: bottom illuminated

30 semi-transparent cell: bottom illuminated top illuminated

20

0

10

20

30 40 d [MoO3] (nm)

50

60

Figure 4. optical simulation: spatial overlap of the optical mode intensity with the active organic layer ζ with varying MoO3 layer thickness

3.4 Semi-transparent Polymer Solar Cells: I-V Characteristics

8

reference: bottom illuminated

4

semi-transparent: b) bottom illuminated top illuminated

0



The functionality of the MoO3 buffer layer becomes obvious in the current-voltage characteristics (under simulated AM1.5G at 100 mW/cm2 ) as shown in Fig. 5 for both semi-transparent and opaque cells with a 5 nm MoO3 buffer and a 40 nm MoO3 buffer, respectively.

-4 -8

(a) 5nm MoO3 -0.8

-0.4

0.0 0.4 voltage (V)

0.8

8

opaque cell: bottom illuminated

4

semi-transparent cell: bottom illuminated top illuminated

0 -4 -8

(b) 40nm MoO3 -0.8

-0.4

0.0 0.4 voltage (V)

0.8

Figure 5. Current-Voltage characteristics under simulated AM1.5G with an intensity of 100 mW/cm2 for semi-transparent and opaque cells with a) 5 nm MoO3 and b) 40 nm MoO3

The extracted characteristic data of the devices are summarized in Table 2. While the characteristics of the opaque devices vary only slightly when the MoO3 buffer thickness is changed from 5 nm to 40 nm, there is a significant difference for the semi-transparent cells. Most strikingly, in the case of the 5 nm thick MoO3 buffer layer, the I − V characteristics of the semi-transparent devices exhibit very pronounced S-shapes with significantly reduced filling factors and a substantially increased series resistance (Fig. 5(a)). This implies that the extraction of charge carriers at the interface organic/MoO3 and MoO3 /ITO is substantially deteriorated. We attribute this to damages due to the impact of sputtered ITO particles on the MoO3 layer and the underlying


organic layer. As a result, efficiencies of less than 0.5 % are obtained. In OLED structures, sputter deposition of ITO electrodes on top of organic layers has been evidenced to severely damage the interface between organics and electrode and result in significantly deteriorated charge transport over the interface organics/electrode.17 On the contrary, semi-transparent cells with a 40 nm thick MoO3 buffer exhibit characteristics of well-behaved organic solar cell devices, with fill factors of 55 - 60% similar to that of the reference device and no sign of increased series resistance. Obviously, damage due to the sputter deposition process of the ITO top-electrode is significantly suppressed by the MoO3 buffer layer. For both MoO3 buffer thicknesses the opaque reference device with the Al top-electrode has the highest short-circuit current density ISC , which is due to the reflectance of the Al contact and a resulting larger overlap ζ of the optical mode with the organic active layers.

3.5 Semi-transparent Polymer Solar Cells: Device Characteristics The resulting cell efficiencies vs. thickness of the MoO3 buffer are summarized in Fig. 6 and Table 2.

power conversion efficiency (%)

2.5 2.0 1.5 1.0

opaque cell: bottom illuminated semi-transparent cell: bottom illuminated top illuminated

0.5 0.0

0

10

20

30

40

50

60

d [MoO3] (nm)

Figure 6. Power conversion efficiencies of opaque and semi-transparent polymer solar cells

The reference devices with a Al top-electrode reach efficiencies of about 2.1%, essentially indedependent of the MoO3 buffer thickness. As discussed before, the semi-transparent cells with a very thin MoO3 buffer suffer from damages and deteriorated characteristics. Towards thicker MoO3 buffers the cell efficiencies reach values of 1.5% for illumination through the top-electrode and up to 1.9% for illumination throught the glass substrate. The PCE difference between top and bottom-illumination can be understood attributed to different ζ values (see Fig. 4). For sufficiently thick buffer layers, the difference between the efficiency of the reference device and that of the semi-transparent device illuminated via the substrate side, can also largely be explained by the smaller overlap ζ of the optical mode with the active material in the semi-transparent device. device

illumination

reference semi-transp. semi-transp. reference semi-transp. semi-transp.

bottom bottom top bottom bottom top

MoO3 thickness 5 nm 5 nm 5 nm 40 nm 40 nm 40 nm

Voc (V) 0.51 0.48 0.47 0.51 0.53 0.52

FF (%) 54 23 25 55 57 61

ISC (mA/cm2 ) 7.8 4.8 2.4 7.8 6.3 5.0

P CE (%) 2.1 0.5 0.3 2.1 1.9 1.5

Table 2. Characteristic data of the semi-transparent and reference devices for varied MoO3 buffer layer thickness.


4. SUMMARY AND ACKNOWLEDGEMENTS In conclusion, we have shown semi-transparent polymer solar cells with inverted device structure with efficiencies of around 2% under AM1.5G illumination (100 mW/cm2 ) with a transmittance on the order of 80% in the red spectral region (650 nm). In a preliminary investigation we studied highly transparent ZnO and TiO2 films prepared by Atomic Layer Deposition as the electron selective layer in opaque inverted solar cells and found out that TiO2 is more suitable for our device stack because of higher short circuit currents and higher open circuit voltages and therefore higher power conversion efficiencies. Based on these results we implemented semitransparent devices with a MoO3 buffer on top of the active organic layer. It has been shown to allow for the sputter deposition of a highly transparent ITO top electrode. Without or with a thin MoO3 buffer the organic layer is damaged and the device performance is poor. For a buffer thickness of 40 nm, electrical characteristics similar to inverted opaque control devices with evaporated Al electrodes can be achieved. The authors thank the German Federal Ministry for Education and Research (FKZ 13N10316 - EPIO) for financial support and Honeywell Specialty Chemicals Seelze GmbH for the supply with P3HT.

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