Solution-Processed Small-Molecule Bulk Heterojunctions: Leakage

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Solution processed small molecule bulk heterojunctions: leakage currents and the dewetting issue for inverted solar cells Elodie Destouesse, Sylvain Chambon, Stéphanie Courtel, Lionel Hirsch, and Guillaume Wantz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06964 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 20, 2015

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ACS Applied Materials & Interfaces

Solution processed small molecule bulk heterojunctions: leakage currents and the dewetting issue for inverted solar cells Elodie Destouesse†,‡, Sylvain Chambon†, Stéphanie Courtel‡,Lionel Hirsch† and Guillaume Wantz†*

†

Univ. Bordeaux, IMS, UMR 5218, F-33400 Talence, France. CNRS, IMS, UMR 5218, F-33400 Talence, France. Bordeaux INP, IMS, UMR 5218, F-33400, Talence, France

‡

ARMOR, 20, rue Chevreul - BP 90508 44105, Nantes, France

*

Author to whom correspondence should be addressed; electronic mail: [email protected]

ABSTRACT In organic photovoltaic (PV) devices based on solution-processed small molecules, we report here that the physico-chemical properties of the substrate are critical for achieving high performances organic solar cells. Three different substrates were tested: ITO coated with 1 ACS Paragon Plus Environment


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PEDOT:PSS, ZnO sol-gel and ZnO nanoparticles. PV performances are found to be low when the ZnO nanoparticles layer is used. This performance loss is attributed to the formation of many dewetting points in the active layer due to a relatively high roughness of the ZnO nanoparticles layer compared to the other layers. We successfully circumvented this phenomenon by adding a small quantity of polystyrene (PS) in the active layer. The introduction of PS improves the quality of film forming and reduces the dark currents of solar cells. Using this method, high efficiency devices were achieved even in the case of substrates with higher roughness.

KEYWORDS: organic photovoltaics, p-DTS(FBTTh2)2, solution processable small molecule, filming properties, dewetting I.

INTRODUCTION Bulk heterojunction (BHJ) organic solar cells (OSC) are currently being investigated by

both scientists and industries1–3. These solar cells can be produced in large quantities via rollto-roll coating or ink-jet printing in order to provide cost-effective energy4,5. A BHJ is a 3D structure with an interpenetrated network between two components: an electron donor and electron acceptor materials. These two materials, dissolved in a common solvent, are subsequently deposited onto a substrate. During this step, the two materials self-organize and phase separate6. The arrangement of the two phases plays a major role in the performances of the resulting device. In the literature, efficient OSCs were mainly fabricated using fullerenebased materials such as 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PC61BM)7,8 as acceptor and many components can be used as donor. Because of the very interesting filming properties of polymers, the scientific community focused for years on synthesizing efficient polymer donors. One of the most widely studied polymer donor is the so-called poly(3hexylthiophene) (P3HT) (Figure 1) which demonstrated power conversion efficiency (PCE) of 2 ACS Paragon Plus Environment

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3.5%9. P3HT is not the most efficient polymer but it has a relatively long stability and is easy to process, which makes it an interesting candidate for large scale production10. However, the limited light absorption spectrum of this polymer inspired the community to develop a new class of materials the low band gap polymers (LBG) 11,12 which are able to absorb above 600 nm13. This class of donor polymers mixed with the right acceptor material can achieve performances up to 9.5% in single junction lab cells14. Despite the interesting film forming properties, polymers present several drawbacks such as batch to batch reproducibility in large production scale, in terms of polydispersity, molecular weight or impurity level. That is the reason why some groups of researchers focused on designing solution-processable small molecule donors, insuring a good batch-to-batch reproducibility and relatively easy purification. In 2009, Walker et al.15 showed it was possible to fabricate efficient BHJ OSC with a small molecule as donor. They introduced a family of small molecule donors with a diketopyrrolopyrrole (DPP) core and recently performances up to 4.96% were obtained16. Many other well-performing small molecules have been described performances

was

achieved

with

17–20

and one of the best

7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-

b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5yl)benzo[c][1,2,5]thiadiazole) commonly abbreviated as p-DTS(FBTTh2)2 (Figure 1). PCE as high as 9.02% with a p-DTS(FBTTh2)2: PC71BM blend21 in single junction cell was reported, making thus small molecule-based BHJ perform as well as polymer-based BHJ22. In this study, we compared the filming properties of two types of BHJ: one made with P3HT as a polymer donor and another with p-DTS(FBTTh2)2 as a small molecule donor. We chose PC61BM as the acceptor for both donors. We evaluated each BHJ in direct structures with PEDOT: PSS as hole transport layer (HTL), but also in inverted structures with zinc oxide (ZnO) as electron transport layer (ETL) (Figure 2). Concerning the inverted structures, two types of ZnO layers were investigated: a ZnO deposited through a sol-gel process (ZnO 3 ACS Paragon Plus Environment


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sol-gel) and a ZnO deposited from nanoparticles dispersion (ZnO NPs). We examined the nano and microstructures of each interface and their wetting properties. We also showed that the small molecule BHJ OSC always presents higher leakage currents, i.e. dark currents in reverse bias, than a polymer BHJ OSC. This phenomenon gets even worse when a small molecule BHJ active layer is spin-casted onto layers with an increased roughness such as ZnO NPs, leading to poor photovoltaic (PV) performances.

Figure 1. Chemical structure of P3HT and p-DTS (FBTTh2)2. R1=n-hexyl R2=2-ethylhexyl.

Figure 2. Structure of the three different solar cells studied. (A) Direct structure cell using PEDOT:PSS has HTL . (B) Inverted structure cell using ZnO Sol-gel as ETL (C) Inverted structure cell using ZnO NPs as ETL II.

MATERIALS AND METHODS

The structures used in this study are presented on Figure 2. We used 15×15 mm2 ITOcoated glass sheets (10 Ω/sq, Visiontec) successively cleaned in acetone, ethanol and isopropanol in an ultrasonic bath and exposed to UV-ozone for 20 min. ZnO sol-gel was prepared as followed: zinc acetate dihydrate (ZnAc, Sigma-Aldrich) was diluted in absolute 4 ACS Paragon Plus Environment

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ethanol at a concentration of 0.18 M, to which ethanolamine was added in order to have a 0.45 molar ratio compared to ZnAc. The precursor solution was first stirred for 2 h at 60°C and then 48 h at room temperature. We then diluted by 2 the solution obtained. 40 µL of ZnO sol-gel solution was spin-casted on the substrate in air (ambient atmosphere) at 2000 rpm for 60 s leading to a thickness of 40 nm in average. The layer was then annealed at 140°C for 1 min. ZnO NPs were prepared according to Sun et al.23 with particles average diameter of 30 nm. Parameters of spin coating were adjusted to obtain a 40 nm thick layer in average. PEDOT: PSS (Clevios PH) was purchased from Heraeus and filtered with a 0.2 µm RC (Regenerated Cellulose) before being used. 60 µL of the solution was deposited on the ITO substrate in air (ambient atmosphere) at 4000 rpm for 60 s and then placed into an oven at 110°C under dynamic vacuum for 20 min. Subsequently the substrates were transferred to a nitrogen-filled glovebox (O2 and H20 < 0.1 ppm). Starting from this point, the rest of the fabrication process and all the electrical characterizations were carried out under inert atmosphere. P3HT (Plexcore OS2100) and PC61BM (99.5%) were purchased from Plextronics and Solaris-Chem Inc. respectively and used as received. Solutions were prepared in odichlorobenzene, at a 1:1 weight ratio and a concentration of 20 mg/mL. Solutions were first stirred at 90 °C for 10 min and subsequently at 50 °C for 24 h. P3HT: PCBM was spin coated at 1000 rpm during 25 s. Immediately after spin coating, the substrates were individually placed in small closed Petri dishes for several hours at room temperature for solvent vapor annealing. Solvent vapor annealing is a well-known technique to promote the phase segregation of bulk-heterojunction materials by keeping the layers under a saturated solvent atmosphere24.

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p-DTS(FBTTh2)2 was purchased from 1-material and used as received. The solution with pDTS(FBTTh2)2:PC61BM was prepared in chlorobenzene (CB) containing 0.4% v/v of 1,8diiodooctane (DIO), at a 6:4 weight ratio and a total concentration of 35 mg/mL. Solutions were first stirred at 80°C overnight and then kept at this temperature before use. pDTS(FBTTh2)2:PC61BM solution was spin cast at 1000 rpm during 60 s and then thermally treated at 90°C for 5 min. Concerning the active layers made with the different percentages of polystyrene (PS), CB solutions with 0.4% v/v of DIO were first prepared and stirred for two hours at 80°C. Then using this mixture, the different solutions with p-DTS(FBTTh2)2:PC61BM were prepared at a 6:4 weight ratio and a total concentration of 35 mg/mL. Finally, solutions were stirred at 80°C overnight and kept at this temperature before use. p-DTS(FBTTh2)2:PC61BM solutions were spin cast at 1000 rpm during 60 s and then thermally treated at 90°C for 5 min. For the inverted structures, a 7 nm-thick MoO3 (Neyco) layer followed by 100 nm-thick silver electrodes were successively thermally-evaporated under high vacuum (10-6 mbar) onto both types of BHJ. For direct structures a 20 nm-thick Ca layer followed by 100 nm-thick silver electrodes were successively thermally-evaporated under high vacuum onto both types of BHJ. Layers were evaporated through a shadow mask to define a 10.5 mm2 active area. Experiments were repeated on 8 individual cells to evaluate the standard deviation. The devices were characterized using a K.H.S. SolarCelltest- 575 solar simulator with AM1.5G filters set at 100 mW/cm2 with a calibrated radiometer (IL 1400BL). The Atomic force microscopy was carried out using a Veeco di Innova V1.0 in tapping mode with a silicon probe (Aluminium reflex coating). The dimensions of the probe were 160 µm x 40 µm. The optical profilometer used to capture images of the dewetting points in the active layer was a Veeco NT9000. The interferences were generated using a green light. The contact potential difference (CPD) of the ZnO layers was determined using Besocke Delta Phi Kelvin 6 ACS Paragon Plus Environment

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Probe (Kelvin probe S and Kelvin control 07). Highly Ordered Pyrolytic Graphite (HOPG) was used to calibrate the equipment (Wf(HOPG) = 4.65 eV). The contact angle measurements were performed optically with a OCA 40 Micro with ES Picodrop. Three different solvents were used for the experiment: the thiodoglycol, the diiodomethane and the ethyleneglycol. The polar and disperse components of the surface energies were obtained using OwensWendt-Rabel-Kaelbe (OWRK) method25. In order to obtain the wetting envelopes, the Dupré equation and the Young equation were coupled. III.

RESULTS AND DISCUSSION A.

Comparison polymer/SM

The PV performances obtained with the different BHJs for each structure are presented on Figure 2. With the P3HT:PC61BM BHJ blend, similar efficiencies were obtained in direct and inverted structures. Performances around 3.5% (Table 1 and Figure 3B) were achieved, corresponding to the state-of-the-art26. With the p-DTS(FBTTh2)2: PC61BM BHJ blend, we obtained an average photo conversion efficiency (PCE) of 5.57% in direct structure with a Fill Factor (FF) around 69%. This result is in agreement with the literature27. In the case of inverted structures, the PV efficiencies are lower. Inverted solar cells made using ZnO sol-gel as the ETL showed a drop of the FF down to 57% and a decrease in open-circuit voltage (Voc) of 40 mV compared to the direct architecture. This results in a lower PCE of 4.73%. Surprisingly, these losses became even more pronounced in the inverted structure made with the ZnO NPs as ETL. Indeed, we measured a FF of 44% and an average Voc of only 0.45 V. These two parameters, much lower than expected, led to an average efficiency of 2.14%. A decrease in both the FF and the Voc can be attributed to an increase in the leakage currents28. Table 1. PV parameters summary of devices based on P3HT:PC61BM or pDTS(FBTTh2)2:PC61BM active layers spin cast on different types of interfacial layers.



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Active layer type

P3HT:PC61BM

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Interfacial

Jsc

Voc

FF

PCE

Rs

Rsh

layer type

[mA/cm²]

[V]

[%]

[%]

[Ω]

[kΩ]

PEDOT:PSS

8.0 ± 0.2

0.57 ± 0.01

69 ± 1

3.2 ± 0.1

89

1370

ZnO sol-gel

11.1 ± 0.7

0.53 ± 0.01

60 ± 1

3.5 ± 0.3

76

4952

ZnO NPs

11.8 ± 0.4

0.52 ± 0.01

56 ± 1

3.4 ± 0.1

75

387

PEDOT:PSS

10.3 ± 0.1

0.78 ± 0.01

69 ± 1

5.6 ± 0.1

55

52

11.3 ± 0.3

0.74 ± 0.03

57 ± 1

4.7 ± 0.3

120

17

10.8 ± 1.0

0.45 ± 0.16

44 ± 1

2.1 ± 0.9

172

1.4

p-DTS(FBTTh2)2:PC61BM ZnO sol-gel ZnO NPs

In order to understand the origin of these low efficiencies, the leakage currents for both BHJ on the three different substrates were examined (Figure 3). First of all, we observed that small molecules based devices exhibit a two-order of magnitude higher dark current compared to polymer based devices. More interestingly, devices made with ZnO NPs present higher leakage currents with at least one order of magnitude difference compared to PEDOT:PSS devices, impacting therefore the shunt resistance (Rsh). This is especially true for devices made with ZnO nanoparticles as ETL which exhibited a very low Rsh of 1.4 kΩ. These results strongly suggest that the decrease in performances observed for the small molecule basedBHJ devices in inverted structures is caused by high leakage currents. However we did not encounter such troubles with the polymer-based BHJ, as comparable performances were obtained with the different type of interfacial layers (PEDOT:PSS, ZnO NPS, ZnO sol-gel). In order to understand the influence of the interlayers on the appearance of leakage currents in small-molecule based solar cells, the properties of the different interlayers were studied in detail.


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Figure 3. (A) Comparison of the dark currents obtained for a P3HT or a p-DTS(FBTTh2)2 based BHJ spin cast on three different interfaces: ZnO sol-gel, ZnO NPs and PEDOT:PSS. (B) Comparison of the JV curves under illumination obtained for a P3HT or a pDTS(FBTTh2)2 based BHJ spin cast on three different interfaces: ZnO sol-gel, ZnO NPs and PEDOT:PSS.

B.

Interfacial layers analysis 1.

Roughness, work function and wettability

First of all, the work functions of both types of ZnO layers were determined using the Kelvin Probe technique. We found comparable values for both ZnO NPs and sol-gel layers (Table 2) in accordance with previously reports29. Based on these results, one can rule out non-selective interlayer which could explain the presence of shunts. The homogeneity of the small molecule based BHJ was studied by optical microscopy. Significant differences were observed between the two active layers. A larger number of dewetting points were found in the active layer deposited onto the ZnO NPs (see Figure 4A and Figure S2). Figure 5A shows a typical image of the small-molecule based BHJ obtained with an optical profilometer in which several dewetting points can be observed. For every interlayer the dewetting points



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were counted and sized with the Image J software (Table 2). In the case of a BHJ deposited onto a ZnO NPs layer, the density of dewetting points was multiplied by 2.5 compared to a PEDOT:PSS layer. Furthermore, these points have an average area about four times larger. Thus, the fraction area covered by the dewetting points in the case of a small molecule BHJ deposited onto a ZnO NPs appears to be ten times bigger than in the case of the direct cell. We could also observe the same phenomenon in the case of the ZnO sol-gel but with only a factor four. This increase in the number of dewetting points is related to leakage current. Thus, lower performances were obtained with ZnO interlayers. In the following part, the causes of these dewetting spots formation are investigated. First of all, the surface energy of each interfacial layer (Table 2) was measured and from these values the corresponding wetting envelopes were drawn (Figure 6). The surface tension of the small molecule based solution was also measured. The values are shown in Table 2. It clearly shows that the solution is wetting every surface very well. This observation was experimentally verified with a contact angle measurement of the active layer solution on the different surfaces lower than 10°. As the surface tension was not the cause of the dewetting points observed, the three surfaces were examined using Atomic Force Microscope (AFM) (Figure 7). The images clearly show that each surface presents major differences. The ZnO NPs layer showed a very fine structure with nanoparticle sizes reaching 10 to 20 nm. From the AFM imaging, the root mean square roughness and the average peak valley roughness (RRMS and RPV) are determined and reported in Table 2. The results show that the interlayer ZnO NPs exhibits the highest value of RRMS together with the highest value of RPV when compared to PEDOT: PSS and ZnO sol-gel. These two sets of data clearly suggest that the ZnO Nps interlayer demonstrates a higher roughness compared to the others. These results show that an increase in the surface roughness leads to an increase in the number and the size of the dewetting points. While observing the dewetting points, one can 10 ACS Paragon Plus Environment

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notice that they are formed around a small aggregate (Figure 5). The radius of the aggregate was also directly proportional to the size of the dewetting point. The bigger the aggregate the larger the dewetting point. It is well known that an aggregate can be generated by heterogeneous nucleation30. The ZnO NPs interface is the best candidate for this kind of generation with its 10-20 nm nanoparticles, acting as nucleating points. In order to avoid the generation of too many nucleating points we decided to add an inert polymer in the small molecule based BHJ.

Table 2. Physical and chemical properties summary of the different interfacial layers used for

Dewetting points of

Surface tension Work Interfacial

function

layer type Wf

PEDOT:PSS

the active layer

Roughness

RRMS

Dispersive

Polar

Number

component

component

of points

γd

γp

RPV

Np

Area

Fraction area

S

F

[mm ]

[µm²]

[%]

-2

[eV]

[nm]

[nm]

[mN/m]

[mN/m]

4.95 ± 0.01

1.4 ± 0.2

1.8 ± 0.3

41.8 ± 0.5

8.0 ± 0.3

7

32

0.02

ZnO sol-gel

4.50 ± 0.01

1.69 ± 0.04

2.3 ± 0.7

36.4 ± 1.9

6.9 ± 0.1

10

100

0.08

ZnO NPsa

4.48 ± 0.01

1.92 ± 0.03

3.9 ± 0.6

39.2 ± 0.9

4.6 ± 0.1

17

113

0.19

11

102

0.11

10

59

0.06

ZnO NPs

b

-

-

-

-

c

-

-

-

-

ZnO NPs a

b

The active layer deposited on top of the ZnO NPs layer contains 0% of PS. The active layer deposited on top

of the ZnO NPs layer contains 2.5% of PS. cThe active layer deposited on top of the ZnO NPs layer contains 15% of PS

solar cells fabrication and measurements of the dewetting points of a p-DTS(FBTTh2)2 based BHJ deposited on top of the different interfacial layers.



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Figure 4. Optical microscope pictures of the p-DTS(FBTTh2)2 based BHJ deposited on top of the ZnO NPs layer. (A) a p-DTS(FBTTh2)2 based BHJ on a ZnO NPs with 0% of HMPS. (B) a p-DTS(FBTTh2)2 based BHJ on a ZnO NPs with 2.5% of HMPS. (C) a p-DTS(FBTTh2)2 based BHJ on a ZnO NPs with 15% of HMPS

Figure 5. Optical profilometer images of (A) a p-DTS(FBTTh2)2 based BHJ on a ZnO NPs layer (B) Details of a dewetting point morphology.


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Figure 6. Wetting envelopes of the different interfaces: ZnO sol-gel, ZnO NPs and PEDOT:PSS and the position of the surface energy of the SM based solution (

) compared

to these wetting envelopes.

Figure 7. AFM topography images of (A) ZnO NPs (B) ZnO sol-gel (C) PEDOT:PSS.

2.

Increase of the small molecule filming properties

Regarding small molecule-based BHJ, it is now well-known that co-solvents additives, such as DIO31, have a positive effect on devices performances, especially in the case of pDTS(FBTTh2)232. However, in some cases, DIO can also be inefficient33 and then the addition of some macromolecular additives can be used to improve small molecule-based device 13 ACS Paragon Plus Environment


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performances. Mainly, polydimethylsiloxane (PDMS)

17,22,33

was shown to be efficient by

slightly improving the interpenetration of the donor and acceptor phases. The use of highdensity polyethylene and polystyrene has also already been reported in P3HT:PC61BM by the Imperial College London group34. They showed it was possible to add up to 50 % of inert polymer without decreasing drastically the cell performance. Other groups also reported the usefulness of insulating polymers in transistors35–37. Recently, Bazan et al. showed that adding a high molecular weight PS (2·107 M) could increase the PV performances of the devices by favoring the crystallization of p-DTS(FBTTh2)2 and improving the phase separation38. As we were facing bad filming properties, we studied the influence of two different types of PS on the BHJ: one with a relatively low molecular weight (LM PS) (280 000 M, Aldrich) and one with a higher molecular weight (HM PS) (750 000 M, Aldrich). We investigated different loadings of PS, from 1% to 20% of the total active materials mass in the active layer solution. PV devices were fabricated using various concentrations and the resulting PV properties are displayed in Figure 8. Only 1% of PS loaded in the small molecule BHJ is sufficient to significantly decrease the leakage currents in the solar cells made with ZnO NPs layer as ETL. When 2.5% of PS is added to the BHJ, a decrease of one order of magnitude of leakage current is observed. Further increasing the proportion of PS leads to a decrease in the leakage current of nearly two orders of magnitude. In the case of ZnO sol-gel, a slight decrease in leakage currents is also observed. As a consequence, for solar cells made with the ZnO NPs as ETL, an average PCE of 4.7% is reached. This value is comparable to those obtained in inverted structure with the ZnO sol-gel ETL. The decrease in leakage currents is attributed to the presence of PS in the BHJ which seems to improve significantly the small molecule filming properties. To confirm this hypothesis, the dewetting points of the active layer deposited on top of the ZnO NPs layer were counted. This was done for the active layer containing 2.5% PS and 15% PS (Table 2). A 2.5% of PS addition decreases significantly the 14 ACS Paragon Plus Environment

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number of dewetting points observed in the active layer and reached a level comparable to what is obtained with ZnO sol-gel layer (Figure 8). Increasing the proportion of PS up to 15% leads to an homogenous film, comparable with small molecules BHJ deposited on PEDOT:PSS. Unfortunately, such high amount of inert polymer also leads to a decrease in the performances of the solar cells. As a result, comparable performances were obtained for the different interlayers using an intermediate proportion of PS (2.5%). Concerning the effect of the PS molecular weight, in the range of 1 to 10% of PS in the BHJ, a similar effect on the leakage currents and the performances was observed. One can underline that no change in the viscosity of the solutions was observed until 10% of PS in both cases. This also explained why no significant increase in the thickness of the BHJ was detected until 10% of PS was added (see Table S1 and Table S2). However, above 10% of PS in the BHJ, a different behavior can be noticed. In the case of the addition of HMPS, the PCE is significantly altered. Small molecules BHJ prepared with 20% LMPS exhibited PCE of 3.4% whereas a PCE of only 2.1% was reached with the same amount of HMPS. For instance, with 20% of PS, active layer thicknesses of 165 nm and 235 nm were measured for LMPS and HMPS respectively. As a result, BHJ made with 20% of HMPS present lower performances due to a decrease in Jsc and FF values. The loss in PV performances may be caused by a non-optimum phase separation (see Figure S3 and Figure S4) and weak charge transport along the thick layer. Several works have been carried out on the addition of inert polymers in polymer-based BHJ39–41 and this phenomenon has already been observed in the case of MDMO-PPV:PCBM embedded in a PS matrix39. It has been shown that it was possible to add 10% of PS in the BHJ without altering the PV performances of the devices. However, above 10% of PS in the BHJ the performances dropped. It has also been recently published that a 15% addition of poly(4-vinylpyridine) in an APFO-3:PC61BM 15 ACS Paragon Plus Environment


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blend radically altered the blend morphology41. Another work done on tuning the viscosity of halogen free P3HT:PC61BM BHJ showed that a too large quantity of PS could lead to a decrease in performances40.

Figure 8. Current density measured in the dark at -0.9V for small molecule based-inverted devices respectively made with ZnO NPs (A) and ZnO sol-gel (C) as ETL. PCE for small molecule-based inverted devices respectively made with ZnO NPs (B) and ZnO sol-gel (D) as ETL

IV.

CONCLUSIONS

It was found that the poor filming properties of the small molecule originate from dewetting issues especially on rough surfaces. The morphology of the interface on top of which the small molecule-based BHJ is deposited, is critical. Nanoparticle-based interfaces enhance the 16 ACS Paragon Plus Environment

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formation of dewetting points in the active layer leading to very high leakage currents and poor photovoltaic performances. Improving the filming properties of the small moleculebased BHJ is a way to overcome this issue. A simple addition of an insulating polymer such as polystyrene is an efficient way to enhance the filming properties and photovoltaic performances. However, above 10% of PS amount a significant decrease of photovoltaic parameters is noticed. This work showed that the poor filming properties of solutionprocessed small molecules for OPV requires careful optimization of formulation and coating processes for future scale up of printed large-area OPV modules.

ASSOCIATED CONTENT Supporting information. Dynamic viscosity in mPa.s of the p-DTS(FBTTh2)2:PC61BM solutions with different percentages of LMPS and HMPS. Thickness of the active layer versus the type and the amount of polystyrene added. Optical microscope pictures of the pDTS(FBTTh2)2 based BHJ deposited on top of the ZnO NPs layer. Photovoltaic parameters of solar cells with different ratio of PS, in inverted structure with the ZnO NPs layer and the ZnO Sol-gel layer as ETL.

ACKNOWLEDGEMENTS This work has been supported by ARMOR Company, the French CNRS and the ANRT for the CIFRE PhD fellowship awarded to E.D. The authors thank Dr. G. Mattana, Dr. T. Gorisse, Dr. U. Vongsaysy, M. Bertrand, E. Michaud, and S. Delhommeau for their support and their helpful guidance. Authors are also grateful to ANR-13-PRGE-0006 “HELIOS” for funding participation.



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Solution-Processed Small-Molecule Bulk Heterojunctions: Leakage

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