UV light protection through TiO 2 blocking layers for inverted organic solar cells

UV light protection through Ti0

blocking layers for inverted organic solar cells

Haiyan Sun, Jonas Weickert, Ho\ger Christian Hesse, Lukas Schmidt-Mende *

Ludwig-Muxil1lili,l/Is-Ulliversity Munich. Dep"rtmellt oj Physics ,mel Center Jor NunuSciellce (CeNS). Allwliellstr. 54.80799 MUllic/l. Cerl1lullY

AB ST RACT

Keywords:

TiO, layer Organic solar cell Trap Alling UV Alter effect Stability

Fully organic solar cells (OSCs) based on polymers and fullerenes have attracted remarkable interest during the last decade and high power conversion efficiencies (PCEs) beyond 8% have been realized.

However. air stability of these cells remains poor. The conventional geometry of OSCs utilizes strongly oxidizing metal top contacts like AI or Ca. These metals are easily oxidized in air resulting in rapid decrease of PCE if cells are not perfectly encapsulated. Using a thin electron-selective hole-blocking bottom layer like TiO, enables fabrication of solar cells in a so-called inverted geometry. In this geometry. noble metals like Ag or Au can be used as top contacts. which are less sensitive to ambient oxygen. Thus. air-stability of these inverted solar cells is significantly improved. In this study we investigate inverted polythiophene-methanofullerene solar cells. We find significant influence of the TiO, layer thickness on light absorption and illumination stability of the solar cells. as well as the trap filling by photoinduced carriers. Even though TiO, layers as thick as 500 nm seem not to be detrimental for charge transport, light intensity losses limit the device performance. In turn. illumination stability is better for thicker TiO, layers. which can serve as UV filters and protect the photoactive materials from degradation. when compared to thin TiO, layers. Considering these different effects we state that a thickness of 100 nm is the optimization of the TiO, layer.

1. Introduction

Polymer solar cells (PSCs) have been significantly improved during the last few years. and power conversion efficiencies (PCEs) as high as 8.3% have been reported [l-5J. Even though PSCs hold the promise of low mass production costs when compared to conventional Si-based photovoltaics. much work has to be done to investigate the stability [6-81 and to enhance the lifetime of PSCs [91. The most commonly used geometry of PSCs uses non-nobel metals like AI and Ca as top contacts. which quickly oxidize in air. As a consequence. PCEs decrease rapidly if cells are operated in ambient air without elaborate encapsulation.

PSCs with a so-called inverted structure with a Ti02 layer as hole blocking materials [10]. allow the use of noble metals as top electrodes. thus increasing air stability [11\. As both a photo- active and an interfacial layer (1). Ti02 layers play an important role on the performance of PSCs.

In this work. we investigate the influence of Ti02 blocking layer thickness on the power conversion efficiency and illumina- tion stability of PSCs. Ti02 layers are fabricated by spray pyrolysis

• Corresponding author.

E-l1lail address: L.Schmidt-Mende@physik.uni-muenchen.de (L. Schmidt-Mende).

method. which allows control over layer thickness. Solar cells with a thicker layer of Ti02 show lower short circuit current density Usc) and PCE, but higher stability. We prove that the dependence of performance on Ti02 thickness is caused by a filter effect due to the trap states in our Ti02• rather than its electron transport ability. These findings are important'not only for blend

I

oxide solar cells. but also for hybrid organic solar cells. which have a layer of semiconductor oxides.

2. Experimental 2.1. Device fabrication

Solar cells are produced on ITO coated glass substrates (Kintec, 100./0 ). ITOs are cleaned consecutively by ultrasonication for 30 min in acetone and 30 min in 2-propanol. dried in a nitrogen stream and treated for 7 min in a plasma cleaner. Ti02 is deposited by spray pyrolysis as described by other group earlier 112 J. Briefly we sprayed a 1: 10 solution of di-isopropoxytitanium bis (acetylacetonate) in ethanol (Sigma Aldrich) at 450"C on our ITO substrates and samples are cured at this temperature for 15 min. The heating ancl cooling rates are 30 and 2.5 "C/min . respectively. Ti02 layers are ultrasonicated for 5 min in acetone.

rinsed with ethanol and dried in a nitrogen stream.

http://dx.doi.org/10.1016/j.solmat.2011.08.004

Konstanzer Online-Publikations-System (KOPS) URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-179798

Poly-3-hexyl-thiophene (P3HT, Merck) and [6,6]-phenyl-C61- butyric acid methyl ester (PCBM, Nano-C) solutions are prepared in chlorobenzene (Sigma Aldrich) at 24 and 20 mgjml, respec- tively, mixed at a 1: 1 volume ratio and spin coated at 600 rpm for 1 min, then slow-dried for 20 min covered with a Petri dish [13].

A thin layer of PEDOT:PSS was deposited on top of the active layer from a 1: 10 dilution in 2-propanol by spray-spin coating as described elsewhere [14]. Samples were pre-annealed at 10S"C for S min in ambient air before 100 nm thick silver top contacts are DC sputtered by KS7SX (Emitech) sputter coater onto the organic material through a shadow mask. During sputtering the vacuum in the chamber is 6 x 10-³ mbar, and tile whole process takes 30 min. After top contact deposition solar cells are annealed at 140 "C for 10 min.

Cell fabrication and all electrical and spectral measurements are carried out in ambient air at a humidity of approximately 30%

and room temperature (between 22 and 2S ·>C).

2.2, Device characterization

Current density-voltage UV) characteristics are acquired with a Keithley 2400 Source-Meter using a self-made LabView pro- gram. Curves are measured in dark or under illumination with an AM l.5G solar simulator. Light intensity is calibrated with a Fraunhofer Institute certified silicon reference cell with a KGS filter and adjusted to 100 mWjcm² Solar cells are illuminated through a shadow mask, yielding an active area of 0.12S cm².

Shunt and series resistance (Rsh and Rs) have been estimated using a fit to the slope of the lV-curves at high reverse and fOlward bias, respectively, as shown previously [lS]. For external quantum efficiencies (EQEs) alSO W Xe lamp is focused onto a monochromator. Curves are recorded using a Keithley 2400 Source-Meter. Absorption curves are acquired with an Agilent 84S3 UV-vis spectrometer in transmission mode. Ti02 layer thicknesses are probed with a Dektak profilometer. Carrier densities are measured by Hall effect using an Ecopia system of HMS3000.

3. Results and discussion

3.1. Optimized Ti02 layer thickness

In this work, we fabricate solar cells of the type ITOjTi02j P3HT:PCBMjPEDOT:PSSjAg. We used the same thickness of blend layer (lS0nm), Ag top contact (100nm) and PEDOT: PSS (about 10 nm). Variations made on the solar cells are on the thickness of the Ti02 oxide layer. Four different thickness ofTi02 were studied:

SO, 100, 200 and SOO nm. Current-voltage curves acquired under simulated illumination of AM ·1.5G solar light (100 mWjcm²) for different thicknesses of Ti02 layer are shown in Fig. 1.

Data are shown in Table 1 and represented in Fig. 1 for devices with different thicknesses ofTi02 layer (black squares: SO nm; red circles: 100 nm; green up triangles: 200 nm; blue down triangles: SOO nm). Among the four types of solar cells, best performance is shown on the one with the thinnest Ti02 layer of only SO nm.

While the thickness ofTi02 increases, both efficiency and shortjsc are linearly reduced, but open circuit voltage (Vocl and fill factor (FF) keep almost the same. The sample with SO nm Ti02 gives 2.4% efficiency, while the SOO nm one gives only 1.1 %. This 60%

loss in efficiency is mainly caused by reduced 15c,

Since the FF and series resistance, as shown in Table 1, are barely changed depending on Ti02 thickness, we can infer that the lsc 01' the solar cells should not be limited by the resistance of the Ti02 layer. A possible reason for reduce of lsc might be light absorption in the Ti02 reducing the light intensity inside of' the

N -2

~

.s

-4 '00 c

Q) '0

C -6

~ ~ u

-8

0.0 0.1 0.2 0.3

Bias (V) 0.4

- 50nm

- 100nm

---A- 200nm ---T- 500nm 0.5 0.6

Fig. 1. Current-voltage characteristics under illumination with simulated AM 1.5G solar light (100 mW/cm²). Data is shown for devices with 4 different thicknesses of TiO,. (black squares: 50 nm; red circles: 100 nm; green up triangles: 200 nm; blue down triangles: 500 nm) Devices with thin TiO, exhibit higher PCE and Jsc. (For interpretation of the references to color in this figure legend. the reader is referred to the web version of this article.)

Table 1

Photovoltaic parameters of a representative solar cell frolll current-voltage data acquired under simu\(ltcd solar illumination. Values arc given for solar cells witl1 different thicknesses of TiO,.

Thicknesses PCE voc Jsc FF Rsh R,

(nm) (%) (V) (mA/cm') (%) (o./ cm') (o./ CI\1,)

50 2.41 0.59 7.44 55 5546 2.18

100 2.06 0.58 6.38 55.4 2961 2.27

200 1.84 0.58 5.74 55.5 1967 2.01

500 1.19 0.57 3.86 54.6 1622 6.62

photoactive organic layer. To investigate this effect we measure the absorption of the different Ti02 layers.

3.2. Absorption

Ti02 layers show a light yellow to brown color with increasing thickness, suggesting their ability to absorb light in the visible region. As shown in Fig. 2, the absorption peak of Ti0₂ layer at 33S nm increases with its thickness. A slight visible light absorp- tion suggests that there are sub-band gap states in the Ti0₂,

probably due to impurities [161. Compared to pure Ti02, which has transitions from the valence band to the conduction band, Ti02 introduced with dopants and traps exhibits additional extrinsic electronic levels, which can be located in its energy band gap. As our Ti02 layers are fabricated by spray pyrolysis method in air we assume that the precursor is directly decom- posed at an elevated temperature of 4S0 'C. Although great care has been taken, it is expected that at such a dispersion and slow application of the solution precursor it is impossible to avoid impurities present in the Ti02. Therefore some trap states due to impurities will form in the Ti0₂ layers [17].

3.3. Photoinduced tmp filling of Ti02

During the lV measurements for all the samples with different thicknesses of Ti02 layers, we find a trap filling effect in photo- induced Ti02, depending on the time of illumination. Fig. 3 summarizes the dependence of PCE and FF on illumination time.

Upon first measurement under solar illumination, solar cells show

obvious S-shape on the JV curves, with quite low FF and peE. As being illuminated longer, the performances of all solar cells are improved and reach their maximum points after a certain time.

This illuminating time is proportional to the thickness of Ti02

layers. Devices with thicker Ti02 take longer time to get their best performances. For all the parameters of solar cell during the improvement, the FF and efficiency increase significantly, how- ever the Jsc and Voc stay almost constant.

As Weidmann reported, defects in Ti02 layers can reduce the electron drift mobility [181. Therefore we propose that traps limit electron transport in Ti0_{2 .} The absorption shift of Ti0₂ to the visible light, caused by the trap states in Ti02 layer, is clearly shown in Fig. 2, so the trap filling effect of solar cells could be explained as follows.

Photo-excited electrons and holes in Ti02 are firstly obtained to fill the traps close to the conduction bands [191. and the trap becomes inactivated for further electron capture [201. This causes the low FF and efficiency, when a first current-voltage character- istic is recorded without illumination prior to the measurement.

Even a lower Jsc of SCs can be observed with the thickest Ti02

layer, which probably contains the highest number of traps that

limited the charge transport. As light induced filling of electron traps can cause an increase of the electron drift mobility, assist electron transport through Ti02 [211, and improve the quantum yield of carriers at higher light intensities [22]. The performances of SCs are improving with constant illumination in solar spectrum.

3

0' Q. ²

c .Q i5. 0

.n If)

<{

o

300 400 500

Wavelength (nm)

- .- 50nm _ - 100nm

-.... - 200nm

-,,-500nm

600 700

Fig. 2. Absorption spectra or dirrerent thicknesses or TiO,. (For interpretation or the rererences to color in this figure legend. the reader is rci'erred to the web version or this article.)

~

60

50

40

30

20

~

--+-100nm

- A -200nm

~ 500nm

2 3 4

Illumination Time (min)

SCs with thicker Ti02 layers contain more traps, and thus they take longer time to be filled and to reach the best charge transport state.

3.4. Filter effect

We fabricate a group of semitransparent solar cells with different Ti02 layers, by sputtering a 20 nm thin Ag top contact.

Thus, devices can be illuminated from the backside with a light path through Ag/PEDOT:PSS/P3HT:PCBM/Ti02 , Without the Ti02

absorption of both UV and partly visible light. we can confirm that the light intensity inside the active layer is the same for all the SC, thus the excited charge carries are supposed to be generated independently of the thickness of Ti02 as well.

As shown in Fig. 4, when illuminated from the front side, i.e.

solar light is absorbed by the Ti02 layer first, the EQE results of SCs are obviously in inverse proportion to Ti02 thickness (addi- tional plots shown in Fig. Sl supporting information). With thickness of Ti02 increasing, EQE of SCs decreases, following the same trend that was found for the PCE, While illuminated from the backside, both PCE and EQE appear independent on Ti02 layer.

During this investigation of reverted illumination, we could confirm that the power conversion of solar cells depending on their Ti02 thickness is caused by a filter effect of the Ti02 layer.

Light intensity is reduced when illuminated through Ti02 layer.

As apparent from Fig. 2, the absorption band of Ti02, will be observed between 340 nm and 450 nm, and thicker Ti02 layers absorb stronger at this wavelength. Then the light intensity is reduced reaching the active layer, thus theJsc and peE of SCs with thicker Ti02 layer are reduced. Thicker Ti02 layers absorb and filter more solar light and lower light reduces the light intensity at the active layers, which contributes to charge generation. Accord- ingly, less light participates in power conversion in SCs.

Hall effect measurements on our Ti02 layers show a charge carrier density in the range of 2 x 10⁶ for all different thicknesses. This is well in accordance with the series resistances for different solar cell types, which was found to be relatively constant among the different Ti02 thicknesses. Since the PCE of solar cells are independent of Ti02 thickness when illuminated from backside, we assume that the Ti02 layer does not limit the charge transport in organic solar cells even with a thickness of 500 nm.

3.5. Illumination stability

We investigated the illumination stability of the solar cells with different Ti02 thickness. In ambient condition within 100 min solar illumination, the 50 nm Ti02 layer solar cell has a 33.5% degradation of PCE, while the thicker ones are much more stable, 0.13% for 100 nm Ti02 , 0.10% and 0.16% for 200 nm and 500 nm Ti02 , respectively.

A~~ '--'--"

~ .

~~

-m- 50nm --+-100nm

..

~500nm

2 3 4

Illumination Time (min)

w u a..

E

o

z

Fig. 3. Lert: FF, right: peE illumination time trap filling erfect.

1.75 - , - - - ,

1.50

~ 1.25

~ o

w ()

a.. 1.00

0.75

~ Back illuminated

-+--

0.50 -t---,----.--~--r-~---,-~-_.__~-___.__----l

o

40

30

0 ~

w ₂₀

a

10

0 300

~'

" ^I I I I I I

"

"

\

100 200 300 400 500

Thickness (nm)

- - - 500nm Back - -500nm Front - -- 50nm Back - -50nm Front

____ -_01,,,. .. ,_,, ... ---_______ _

400 500

Wavelength (nm)

"

600

"

" ,--

700

Fig. 4. peE (up) and EQE (down) dependence of the TiO, thicknesses in front (solid) and back (dash) illuminated devices.

The degradation of polymer solar cells has many reasons, and the most common one is that oxygen is readily activated by UV illumination on the interface between titanium oxide and the organics. As the Ti02 layers are fabricated on ITO substrates and coated with blend, we think that air, including moisture, will affect on the blend layer rather than the Ti02 layer. In this case the innuence of oxygen on solar cells should be independent on the thickness of Ti02 layers. This leaves us with the possible explanation that, if the TiOz layer is not thick enough to filter out all the UV light, the super-oxide or hydrogen peroxide formed, is lil<ely to aggressively attack the organic compounds [6].

However, filtering of UV light results in more stable solar cells making devices with thicker TiOz more durable to solar irradia- tion. TiOz layers show a significant absorption in the UV-spectral region. However, as already discussed, their filter effect reduces PCE of SCs. As shown in Fig. 2, the thinnest TiOz layers under investigation (50 nm) absorb only about 90% of high energetic photons (). $; 300 nm). As such, the degradation process caused by UV-light incident to the organic materials cannot be entirely ruled out for the thinnest TiOz layers. This competition between efficiency and stability depending on the thickness of TiOz layers gets in balance at 100 nm, which has a reasonable slow degrada- tion and high efficiency (Fig. 5).

2.7 2.4 2.1

~ 1.8

0 () w 1.5 a..

1.2

0.9 0.6

20 40 60

Time (min) 80

_ _ 50nm _ _ 100nm --4-200nm -"f'-500nm

100

Fig. S. Illumination stability of different thickness ofTiO,.

4. Conclusion

The trap states in TiOz layers fabricated by standard spray pyrolysis technique caused slight absorption in the visible. This reducesjsc and PCE of solar cells with thicker TiOz layers, because light intensity is reduced when solar cells are illuminated through TiOz. However, according to series resistance analysis, EQE of backside illuminated devices and Hall effect measurements, charge transport seems not to be limited even for 500 nm thick Ti02 . The trap states in TiOz layers fabricated by spray pyrolysis, not only cause the filter effect, but also demand for extended photodoping times. During jV measurements under solar illumi- nation, solar cell performances initially improve, due to filling of traps in TiOz. However, upon long-term illumination solar cell performance decreases, because of the degradation of organic materials caused by UV illumination. The illumination stability is increased with thickness of TiOz layer, while the performance decreases. Considering both competing effects, 100 nm is the optimal thickness of TiOz for our inverted solar cells.

Aclmowledgment

H.S. thanks the Chinese Scholarship Council (CRC) for funding.

This work has been supported by the German Excellence Initiative of the Deutsche Forschungsgemeinschaft (DFG) via the "Nanosys- tems Initiative Munich (NIM)" and the German Research Founda- tion (DFG) in the program "SPP 1355: elementary processes of organic photovoltaics".

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