Influence of Interface Doping on Charge-Carrier Mobilities and Sub

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Influence of Interface-Doping on Charge Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells Felix Herrmann, Burhan Muhsin, Chetan Raj Singh, Sviatoslav Shokhovets, Gerhard Gobsch, Harald Hoppe, and Martin Presselt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00124 • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 9, 2015

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The Journal of Physical Chemistry

Influence of Interface-Doping on Charge Carrier Mobilities and SubBandgap Absorption in Organic Solar Cells Felix Herrmann1, Burhan Muhsin², Chetan Raj Singh2,3, Sviatoslav Shokhovets ², Gerhard Gobsch², Harald Hoppe², Martin Presselt1,* 1

Institute of Physical Chemistry, Friedrich-Schiller-University Jena, 07743 Jena, Germany

² Institute of Physics, Ilmenau University of Technology, 98693 Ilmenau, Germany 3

Department of Macromolecular Chemistry I, University of Bayreuth, 95440 Bayreuth,

Germany *

Corresponding author: [email protected]

Abstract P3HT:PCBM (poly(3-hexylthiophene-2,5-diyl):([6,6]-phenylC61-butyric acid methyl ester) based bulk heterojunctions were doped by using 4-toluenesulfonic acid (TSA) as dopant. This approach was inspired by the well-known interfacial doping of the active layer via the electron-blocking

layer

PEDOT:PSS

(poly(3,4-ethylenedioxy-thiophene):poly(styrene

sulfonate)) at its interface. TSA is amphiphilic, acidic and structurally very similar to the monomeric building block of PSS. Upon TSA doping a notable increase of the light absorption in the sub-bandgap region of pristine P3HT was observed. These features are assigned to polaron transitions within P3HT. However, the TSA-impact on polaron absorption features in the BHJ is rather small. Though, for small TSA-concentrations and thick active layers (~220 nm) the fill-factor of the solar cells improved dramatically with increasing TSAcontent in the active layer what are discussed in terms of contact resistances at interfaces in the present paper. For 0.5% TSA concentration in the active layer solution the maximum of the power-conversion efficiency was obtained. At the same time, the reproducibility of solarcell performance parameters was considerably improved.

Introduction Recent advances of material properties lead to power conversion efficiencies of bulk heterojunction (BHJ) solar cells exceeding 10 %1-5. For further strategic performance improvements new donor and acceptor6-9 materials are synthesized aiming at proper energy ACS Paragon Plus Environment


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levels, lower band gaps and improved structural as well as charge transport properties10-13. Furthermore, various additives are used14. Low hole mobilities and carrier densities of donor molecules are major issues for bad solar cell performance in terms of electrical characteristics e.g. series resistance, fill factor, s-shaped IV-curves15-17. Therefore molecular doping, which increases mobility and carrier density, might improve the solar cell performance significantly18. In state of the art BHJ solar cells, doping occurs inherently at the interface between holeconducting layer PEDOT:PSS (poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate)) and the active layer due to the acidity of PEDOT:PSS.18 Doping of the active layer can be studied by quantitative evaluation of polaron signatures in sub-bandgap (SBG) absorption spectra, where they might be overlain by other SBG-absorption features19-24. In general, both, doping as well as interfacial properties, have an eminent impact on the electronic properties and consequently on the device performance of the organic solar cell25-27. To study the influence of PEDOT:PSS on the SBG-absorption features and the device performance, the active layer was systematically doped with the small aromatic acid 4toluenesulfonic acid (TSA). As shown in Figure 1, TSA is structurally very similar to the monomeric building block of PSS, styrene sulfonate, and hence should be able to mimic doping of P3HT (poly(3-hexylthiophene-2,5-diyl)) due to PEDOT:PSS. This doping is assumed to be due to the high Lewis acidity of both PEDOT:PSS as well as of the model substance TSA, which is likely to cause positive charges, i.e. hole polarons, on the P3HT backbone. Because TSA is amphiphilic, i.e. it has a non-polar toluene and a highly polar sulfonic acid moiety, TSA is expected to tend to accumulate at heterogeneous interfaces in the solar cell, thus possibly improving electrical parameters.

Figure 1: Chemical structure of PEDOT:PSS (poly(3,4-ethylenedioxy-thiophene):poly(styrene sulfonate)), that serves as a hole-blocking layer in BHJ solar cells, and TSA (4-toluene-sulfonic acid), that is structurally similar to the monomeric building block of PSS. TSA is used as amphiphilic, thus interface-active, dopant in the present work.

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Materials and Methods The active layer of the solar cells were composed of rr-P3HT P200 (BASF) and PCBM ([6,6]phenylC61-butyric acid methyl ester; Solene; purity of 99%). The standard solution for spincasting has a P3HT:PCBM ratio of 12:7 with 1.2% weight in chlorobenzene (CB). Due to the low solubility of TSA (Sigma-Aldrich) in CB, ethanol-dissolved TSA was added to the standard solution. The TSA content mentioned in the following refers to weight percent in comparison to P3HT within the active-layer solution before spin-casting. The ratio of ethanol to CB was set at 2% for all samples to prevent effects of solvents blending. The active area for each solar cell is 1cm 2 . Further details of the solar cell preparation are described elsewhere.19, 28

The optical characterization was done with a photo thermal deflection spectroscopy (PDS) setup29 and a Varian UV-Vis-spectrometer. The IV-characteristics where determined with a Keithley sourcemeter and a solar simulator with an intensity of 1000W/m² (AM 1.5 spectrum). EQE-spectra were measured with a commercially available setup from Bentham Instruments Ltd. The charge carrier mobilities were measured on small area (~ 4 mm2) solar cells devices by the charge extraction by linearly increasing voltage (CELIV) method. The CELIV set-up consisted of a Nd:YAG (532 nm, 10 ns) – laser source, an Agilent 33250A 80 MHz function generator, a Innolas spit light 200 DP and a Tektronix DPO 4104 GHzoscilloscope. the charge carrier mobility was calculated as µ =

2 d 2 30 , where d is the 3 A × t 2 max

active layer thickness and tmax is the maximum current time.

Results and Discussion The influences of TSA-doping on the fill factor and the whole IV-characteristics of nonoptimized devices are shown in Figure 2. As compared to the standard cell, which was not doped with TSA, addition of 0.1 % TSA significantly increased the fill-factor, which could be further increased at a TSA-concentration of 0.5 %. At 2 % TSA-concentration the fill-factor dropped dramatically and a s-shaped IV-curve is obtained31, while no current could be detected anymore at 5 % TSA-concentration. Consequently, the discussion in the following focusses on TSA-concentrations between 0.001 and 0.5%.



10

Current Density (mA/cm2)

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8 6 4

TSA wt.-content: 5% 2% 0.5% 0.1% 0%

2 0 -2 -4 -6 -1.0

-0.5

0.0

0.5

1.0

Voltage (V) Figure 2: IV-curves of solar cells with broadly varied TSA-concentration for doping. A significant enhancement of the fill-factors for concentrations up to 0.5% TSA is clearly visible. For higher TSA contents the fill-factor drops by introducing an s-shape and at 5% TSA the current collapses to zero. The active layers had a film thickness of about 100nm.

For pre-optimized solar cells with thin active layers of around 100 nm the effect of TSA doping was relatively small providing an enhancement in PCE and FF of about 5%, respectively. The optimum concentration was at about 0.05%, while the optimum for nonoptimized process parameters was about 0.5%. Surprisingly, for non-optimized production parameters the increase in PCE was significantly larger. Indeed an increase of the PCE of 50% by adding TSA was found. Generally, the peak solar cell performance is less sensitive to the production parameters when TSA was used as an additive, what is of great importance for large scale production processes. This finding is in good agreement with the works from Trukhanov et al.32, where they predict that doping rises the solar cell performance especially for non-optimized devices. For thicker active layers (~220 nm) the effect of TSA-doping on the solar cell performance was significantly higher. As shown in Figure 3 the fill-factor and the PCE increased strongly with increasing TSA-content. . Furthermore, the rise of Jsc exceeds 10% compared to the standard solar cell with about 100 nm active layer thickness. As seen in the EQE measurements of the thick devices (Figure 4), the EQE rises in regions where mainly P3HT absorbs (2-3 eV). CELIV measurements performed on optimized thin devices (100nm) show a rather constant value for the hole-mobilities (Table 1). The determination of mobilities using CELIV was ACS Paragon Plus Environment

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complicated due to high dark injection currents. These dark currents are most probably caused by very small injection barriers.

4.0

60

PCE (%)

3.5

3.0

2.5

8 6 4 2

40 2.0

0 0 1E-3

0.01

0.1

TSA content (%)

-2

2

Current Density (mA/cm )

TSA-content: 0% 55 0.0025% 50 0.025% 0.1% 45 0.5% FF (%)

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-4

-1.0

-0.5

0.0

-6 1.0

0.5

Voltage (V)

Figure 3: IV-curves, PCE (power-conversion efficiency) and FF (fill-factors) of solar cells with thick (~220 nm) active layers that are doped with TSA. The IV-characteristics clearly show a significant increase in FF with increasing TSA-concentrations. Furthermore, the short-circuit current Jsc was improved due to TSA doping by more than 10% as compared to the standard solar cell.

Table 1: Hole mobilities of thin devices doped with TSA determined with CELIV.

TSA concentration (%) Mobility (10-4 cm2/Vs) 0

2.3

0.05

1.8

0.5

1.9



70 60 50

EQE (%)

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40 30 0.5% TSA 0% TSA

20 10 0 1,5

2,0

2,5

3,0

3,5

4,0

Photon Energy (eV) Figure 4: Comparison of external quantum efficiency (EQE) spectra of thick P3HT:PCBM devices (~220 nm) with and without TSA-doping, respectively. The EQE rises in the regions where mainly P3HT absorbs (2-3 eV).

The optical absorption of doped P3HT and P3HT:PCBM layers was probed by means of photothermal deflection spectroscopy (PDS) as shown in panel A and B of Figure 5, respectively. The PDS spectra of P3HT-films (panel A of Figure 5) show that TSA gives rise to about one order of magnitude more absorption in the sub-bandgap absorption of P3HTfilms. Furthermore, the peak assigned to polaron-transitions of the P2-type18, 19, 21 at 1.25 eV is much more pronounced due to TSA. Around 1.6 eV the absorption is also increased due to TSA, which might be due to trion, P3 and/or bipolaron transitions18,

19, 21

. The doping of

P3HT with molecules similar to TSA is in good agreement with finding of other groups18. Mukherjee et al.18 found an increase in conductivity and an increase in P3HT-polaron absorption by doping the P3HT/PEDOT:PSS interface with DBSA. Furthermore Nam et al.33 observed an increase in polaron absorption and a massive enhancement in PCE of P3HT based polymer:polymer solar cells. In case of the P3HT:PCBM BHJ films, a significantly lower offset signal between 0.8 and 1.1 eV is observed as shown in panel B of Figure 5. The P2-type polaron feature is still clearly pronounced, but additionally a broad absorption feature between 1.4 and 1.8 eV19 is detected, which was debated in-depth in the literature and assigned to a trion transition20, 21, 34 and the weak PCBM transition at 1.8 eV. However, the relative P2-intensity stays rather unchanged with variation of TSA-content. An explanation might be that TSA in the BHJ behaves different than in the pristine P3HT-film, because TSA is amphiphilic and a BHJ offers significantly more and different interfaces. The expected tendency of TSA to


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accumulate on interfaces fits to the weak change in P2-signal with variation of TSAconcentration. Furthermore, PCBM could block doping sites or induce polarons at the P3HT:PCBM interface. The latter would also reduce the effect of TSA doping due to a large density of interface related polarons that superpose the signal of TSA induced polaron signals. Because the CELIV- and the PDS-measurements show that TSA-addition does not affect charge carrier mobility and concentration in the BHJ, the increase in Jsc as well as in the fill factor is attributed to TSA-accumulation at P3HT-PEDOT:PSS interfaces and thereby better charge extraction at the interface. Other groups found similar effects on the performance of solar cells by the incorporation of solution processed interlayers or low work function metals at active layer-electrode interface35,36. These interlayers raise the internal electric field which is required to extract the charges in thick active layer devices. Hence the consistent bulk measurements and significantly improved thick (~220 nm) P3HT:PCBM solar cell devices indicate that TSA doping affects mostly the interface to the PEDOT:PSS, thus giving better solar cell performance via reduced series resistance and enhanced charge extraction.



Photon Energy (eV) 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

100000 pristine P3HT P3HT+TSA

10000 -1

α cm )

P2

1000

A 100 100000

10000

-1

α cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1000

TSA wt.-content: 0.5% 0.1% 0.05% 0.01% 0.005% 0.00%

P2 100

PCBM B

10 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Photon Energy (eV) Figure 5: (A) Comparison of absorption spectra of P3HT with and without TSA as dopant. TSA-doping significantly increases the P2-polaron peak at 1.25 eV. Around 1.6 eV the absorption is also much higher, which may be assigned to the P3- or bipolaron-transition37. (B) The effect of TSA on the sub-bandgap absorption of P3HT:PCBM active layers is rather weak compared to the TSA-influence on absorption spectra of pristine P3HT-films. As the TSA content rises the spectrum alters only slightly in the range of the P2-polaron transition at 1.25 eV.

Summary and Conclusion In the present paper the effects of addition of the small amphiphilic molecule TSA on the efficiency of P3HT:PCBM solar cells and absorption features of P3HT:PCBM thin films is determined. It is shown that TSA can increase fill factors, probably because of TSAaccumulation near the BHJ-PEDOT:PSS interface. ACS Paragon Plus Environment

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The improvement of the solar-cell performance upon TSA-addition is mainly driven by a better FF and slightly increased Jsc. The latter is improved due to reduced extraction barrier at the interface between P3HT and PEDOT:PSS, i.e. interfacial doping. Particularly for thick films, that are desired for commercial OPV devices38, the influence of TSA on FF is very high and additionally decrease the sensitivity of the solar cell peak performance towards variation of production parameters. Consequently, doping with small amphiphilic molecules like TSA is a highly promising approach to improve the performance of BHJ-solar cells that are produced at large scales.

Acknowledgement The authors are grateful for financial support from DFG within the framework of SPP 1355 “Elementary processes in Organic Photovoltaics”. MP gratefully acknowledges financial support by the Carl-Zeiss-Foundation and by the Bundesministerium für Bildung und Forschung.

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Graphical Abstract


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Influence of Interface Doping on Charge-Carrier Mobilities and Sub

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