Suppressing the Surface Recombination and Tuning the Open Circuit

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Organic Electronic Devices

Suppressing the Surface Recombination and Tuning the Open Circuit Voltage of Polymer/Fullerene Solar Cells by Implementing an Aggregative Ternary Compound Diana Galli, Nicola Gasparini, Michael Forster, Anika Eckert, Christian Widling, Manuela Sonja Killian, Apostolos Avgeropoulos, Vasilis Gregoriou, Ullrich Scherf, Christos L. Chochos, Christoph J. Brabec, and Tayebeh Ameri ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09174 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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ACS Applied Materials & Interfaces

Suppressing the Surface Recombination and Tuning the Open Circuit Voltage of Polymer/Fullerene Solar Cells by Implementing an Aggregative Ternary Compound Diana Galli,1# Nicola Gasparini,1# Michael Forster,2 Anika Eckert,2 Christian Widling,2 Manuela S. Killian,3 Apostolos Avgeropoulos,4 Vasilis G. Gregoriou,5 Ullrich Scherf,2 Christos L. Chochos,4,5 Christoph J. Brabec,1,6 and Tayebeh Ameri1,7* 1

Institute of Materials for Electronics and Energy Technology (I-MEET), FriedrichAlexander-University Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany

2

Macromolecular Chemistry Group, Bergische Universität Wuppertal, Gaußstraße 20, 42119 Wuppertal, Germany

3

Department of Materials Science and Engineering, Chair for Surface Science and Corrosion, Friedrich-Alexander-University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany

4

Department of Materials Science Engineering, University of Ioannina, Ioannina 45110, Greece

5

Advent Technologies SA, Patras Science Park, Stadiou Street, Platani-Rio, 26504, Patra, Greece

6

Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058 Erlangen, Germany 7

Chair of Functional Nanosystems, Research area of Physical Chemistry, Department of Chemistry, University of Munich (LMU), Butenandtstr. 11 (Haus E), 81377 Munich, Germany E-mail: [email protected] Keywords: ternary organic solar cells, small molecules, sensitizers, open circuit voltages, recombination losses #

These authors contributed equally to this work.

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ACS Applied Materials & Interfaces 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

ABSTRACT:

In

this

work,

we

present

a

novel

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small

molecule

based

on

dithienylthienothiadiazole units (named SM1) acting as an efficient component in ternary blend organic solar cells to modify the hole extraction at the interface. Our findings shows that the SM1 suppresses the surface recombination and enhances the open circuit voltage (Voc). By introducing SM1 in a host system composed of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61-butyric acid methyl ester (PC61BM), we obtained Voc values of up to 0.75 V and fill factors (FFs) larger than 70 % for the ternary blends. As a consequence, the power conversion efficiency (PCE) is improved about 30% compared to P3HT:PCBM binary devices. Interestingly, external quantum efficiency (EQE) and absorption spectra in the NIR region do not show any contribution of SM1 in dried films. Instead, the addition of the small molecule improves the Voc by reducing the surface recombination losses. In order to shine light on the recombination processes, we carried out Fourier-transform photocurrents spectroscopy (FTPS) and impedance spectroscopy measurements. This work shows that the ternary concept can also have other functionalities than photosensitization and even act as morphology-directing agent or interface modifier.

1. Introduction Photovoltaics with the potential of converting solar energy to electricity has been categorized as one of the most promising technologies.1 In particular, organic photovoltaics (OPVs) have attracted much interest for the production of solar energy due to its form free, low cost, lightweight, flexible and large area production.2–11 The most effective OPV architecture is the so-called bulk-heterojunction (BHJ), where donor and acceptor materials are combined to form an intimate mixture in the active layer. To date, power conversion efficiencies (PCEs) of BHJ solar cells over 13% have been reported.12–15 By absorbing light, an excited state, which is known as exciton, is generated.16–18 These excitations are dissociated into free charge carriers at the donor:acceptor interface and then transported and 2 ACS Paragon Plus Environment

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collected at their corresponding electrodes. Despite the strong absorption coefficient of polymers, usually they offer narrow absorption windows of only a few 100 nm width in contrast to the broad absorption spectra of inorganic semiconductors.19,20 One approach to improve the light harvesting of organic solar cells and to extend the absorption spectra to the near-IR region is the introduction of an appropriate content of dyes, low bandgap polymers or small molecules to a host system of large bandgap polymer:fullerene blends, the so called ternary blends.21–33 Recently PCEs over 12% have been reported for ternary solar cells.34,35 Different mechanisms have been proposed for exciton dissociation and charge transport in ternary blends, namely the energy transfer, the parallel-like transfer, the cascade charge transfer, and the alloy effect.36–39 In addition to absorption extension, the morphology of the active layer is often improved, where the sensitizer serves also as a morphology-directing agent.40,41 As a result, the fill factor (FF) of the devices is improved in parallel with the short circuit current density (Jsc).21 Most interestingly, the ternary compounds may influence the open circuit voltage (Voc) of the reference device also, depending on which aforementioned charge transfer / transport mechanism governs the ternary system.42,43 Many research efforts are directed in understanding the composition-dependent tunability of Voc in organic ternary solar cells. i.e. many groups attributed the changes in Voc to an organic alloy formation either between the two donors (D1:D2:A) or two acceptors (D:A1:A2), owing to the effective electronic interaction between the components.39,42,44 In some other reports, this phenomenon is explained based on the two charge transfer (CT) states model, where the blend serves as two separate parallel-like binary cells and free charges diffuse on three separate percolation networks corresponding to the three materials present in the cell.37 Salleo et al proposed that in ternary systems Voc is affected by the components surrounding in the nanoscale, since the CT energy depends on the intermolecular distance and relative orientation of the molecules at the D:A interface.43 3 ACS Paragon Plus Environment


Here, we report the Voc tunability of solar cells based on poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) host system by implementing a novel dithienylthienothiadizole-based small molecule as the ternary compound. Although small molecule showed complementary absorption spectra compared to the host donor materials, no notable IR-sensitization is observed in the ternary blends. At high contents of up to 70 wt% sensitizer, the efficiency is increased compared to the binary reference (e.g. P3HT:PCBM), mainly due to a concomitant increase of Voc and FF. Various optoelectronic characterizations such as current-voltage characteristics, Fourier-transform photocurrents spectroscopy (FTPS) and impedance spectroscopy are employed in combination with surface characterization to provide insights into the origin of these improvements.

2. Results and Discussion Figure 1a shows the molecular structures of the materials used in this work. Ternary solar cells based on the P3HT:PCBM host system and the small molecule SM1 as near-IR sensitizer were fabricated in an inverted structure in air under ambient conditions with zinc oxide (ZnO) and poly(3,4-ethylenedioxythiophene) doped with polylstyrene sulfonate (PEDOT:PSS) as interfacial layers (Figure 1b). The P3HT to PCBM weight ratio was kept at 1:1 and the small molecule were added with different weight concentrations relative to P3HT into the host system. Figure 1c shows the complementary absorption of SM1 and P3HT. As depicted in Figure 1d, the HOMO and LUMO energy levels of SM1 are located between the corresponding energy levels of P3HT and PCBM, respectively. Therefore, charge carrier transfer is energetically allowed between P3HT and PCBM, between SM1 and PCBM as well as between P3HT and SM1 which may be beneficial to exciton dissociation, charge carrier transport and collection.29,36


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Figure 1. a) Chemical structures of utilized components; b) schematic illustration of the inverted architecture of the binary/ternary solar cells.; c) Absorbance spectra of P3HT, PCBM, and SM1 films; d) Energy level diagram of used components.

Figure S1a on the one hand shows the J-V characteristics of binary and ternary solar cells with different SM1 mixing ratios under AM1.5G (100 mW cm-2), and the photovoltaic parameters are presented in Figure 2. Interestingly, ternary devices with 5-50 wt% SM1 loading show an enhanced Jsc and then by adding higher sensitizer amount, the Jsc is decreased as compared to the reference P3HT:PCBM binary devices. Notably, a similar trend is observed for FF with a remarkable peak at 50 wt% ratio reaching to values greater than 70%. It is worth mentioning that the high FF obtained is among the highest reported for P3HT:PCBM based solar cells. Finally, the Voc shows a continuous increase up to 0.75 V for the SM1 ratio of 100 wt% (1:1:1 blend ratio). As a result, the ternary device with 50 wt% SM1 concentration exhibited the highest performance of 3.7%, with a Jsc of 8 mA/cm², a FF of 70% and a Voc of 0.65V. Comparing the results with the reference P3HT:PCBM, we obtain a power conversion efficiency enhancement of 30%. On the other hand, Figure S1b shows 5 ACS Paragon Plus Environment


the external quantum efficiency (EQE) spectra of binary and ternary devices. Interestingly, the addition of SM1 in the P3HT:PCBM blend does not contribute in current generation since no signal is observed in the near-IR region (600-800 nm).

a)

0.8

b)

10 8

-2

Jsc (mA cm )

0.7

Voc (V)

0.6 0.5 0.4

6 4 2 0 90 10 bil 0 ay er

80

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40

Sensitizer Content (wt%)

Sensitizer Content (wt%) 80

c)

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90 10 Bi 0 lay er

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0.3

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PCE (%)

FF (%)

50 40

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Figure 2. Presentation of the device performance with increasing SM1 content; a) Voc, b) Jsc, c) FF, and d) PCE. The presented performance parameters are collected from 36 devices.

To prove the SM1 presence and contribution, we measured the absorption spectra of ternary blends in solution as well as in film for different sensitizer contents. Upon increasing SM1 concentration, the ternary blend solutions show an obvious growing signal in the 550800 nm region related to the SM1 absorption (Figure 3a). Moreover, the films absorption spectra show a less pronounced sensitization effect in the absorption range of the dithienylthienothiadizole additive (Figure 3b). Reduced absorption intensity in the 400-550 nm region is due to the less amount of P3HT in a constant solution volume / film thickness upon adding higher contents of SM1. The absorption data suggest that the small molecule


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sensitizer strongly aggregates in the thin film state, and it is partly removed and redistributed during doctor blade coating and film formation.

1,6

SM1

1,4

1,0 0,8

P3HT:SM1:PCBM 1:0.3:1 Center 1:0.5:1 Center 1:0.8:1 Center 1:0.3:1 End edge 1:0.5:1 End edge 1:0.8:1 End edge

0.6

P3HT:SM1:PCBM 1: 0.2 :1 1: 0.3 :1 1: 0.4 :1 1: 0.6 :1 1: 0.8 :1 1: 1 :1

1,2

0.7

b) Absorbance (a. u.)

a) Absorbance (a. u.)

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


0,6 0,4

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0,2 0,0

0.0 400

500

600

700

800

400

Wavelength (nm)

500

600

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800

Wavelength (nm)

Figure 3. Absorbance spectra of a) solutions; and b) bladed films measured in the center (where the photoactive layer of the devices is located) and at the end edge of the substrate (where the residual solution from doctor blading is dried and is not involved in the active area of photovoltaics).

Thus, we measured the absorption at the edges of the films, where the residual solution during coating is concentrated and dried, and should, in principle, include all the SM1 material that is redistributed during coating (Figure 3b). The significant absorption contribution in the near-IR region at the film edges proves our hypothesis. In order to quantify the amount of residual SM1 sensitizer in the photoactive layer, we dissolved the P3HT:SM1:PCBM film with composition ratio of 1:0.5:1 (avoiding film edges) in chlorobenzene and recorded a UV-spectrum and compared it with the absorption spectra of chlorobenzene solutions with different P3HT:SM1:PCBM ratios. As presented in Figure S2, the comparison of the absorption spectra suggests that just around 20 wt.% of SM1 is remained in the ternary photoactive layer of P3HT:SM1:PCBM 1:0.5:1. Indeed, the previous results from the device and absorption spectroscopy imply that the SM1 does not form an intimate mixture within the P3HT:PCBM blend, but rather a BHJbilayer ternary composition. Therefore, we fabricated a bilayer ternary device with structure of Glass / ITO / ZnO / P3HT:PCBM (1:1) / SM1 / PEDOT:PSS / Ag. This device show a Voc of 0.75 V, which is equal to the one obtained for ternary devices at high SM1 concentrations 7 ACS Paragon Plus Environment


(Figure 2). This suggests that in our regular ternary blends, the SM1 tends to aggregate as a thin film or islands, on top of P3HT:PCBM, forming a structure similar to the bilayer ternary device. The Voc of ternary systems is most likely limited by the SM1:PCBM open circuit voltage. It is worthwhile mentioning that we could not directly fabricate and characterize a SM1:PCBM device, owing to the poor film formation. The P3HT:PCBM (1:1) / SM1 bilayer ternary film showed a clear contribution in the absorption spectra in the near-IR region (600800 nm) analogous to the absorption spectra of ternary blend films taken near the substrate edge (Figure S3). This indicates that the residual SM1 on top of the P3HT:PCBM layer in ternary blend films is much less than the bilayer ternary structure. We prove the accumulation of SM1 on top of the P3HT:PCBM bulk by analyzing the surface of pristine materials as well as binary and ternary blends. It has been previously shown that the location of a component in a ternary blend system can be predicted by observing the interactions between a solvent droplet and the surface of each components.45 Figure 4 depicts the photographs of water droplets on different composition surfaces. Notably, by increasing the amount of SM1 in the ternary blends the contact angle decreases dramatically from 86° to 67°. In agreement with the aforementioned assumption, with high load of SM1 the contact angles reach values similar to the pristine SM1 layer, suggesting that in the ternary P3HT:SM1:PCBM blend just few amount of the small molecule is located mainly on top of the P3HT:PCBM blend and the rest are removed during film coating.

Figure 4. Photographs of water droplets on the top surfaces of P3HT:PCBM, P3HT:SM1:PCBM with different loading amount of SM1 and pristine SM1 and the respective contact angle values. The contact angle measurement is conducted in the center of the substrates, avoiding the edges, reflecting the properties of the effective working area in the devices. 8 ACS Paragon Plus Environment

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The atomic force microscopy (AFM) images, presented in Figure S4, also reveal the presence of SM1 on top of P3HT:PCBM active layer, consistent with achieved results from the contact angle measurement. The P3HT:PCBM surface shows an average root mean square (RMS) roughness of 0.85 nm, while the RMS roughness of ternary blend film including optimized ratio of 50 wt% SM1 is increased to 2.21 nm owing to the formed SM1 islands on the surface of active layer. Furthermore, inhomogeneous distribution of the SM1 across the active layer thickness was assessed qualitatively by employing depth-profile time-of-flight secondary ion mass spectrometry (ToF-SIMS) method for P3HT:PCBM reference and 1:0.5:1 P3HT:SM1:PCBM ternary films (Figure 5). All ToF-SIMS depth profiles, depicted in Figure 5, initially show a large signal of C2HO‒, which is attributed to PEDOT:PSS, which was applied as a sacrificial over layer in order to ensure a constant concentration of the sputter species (Cs) throughout the depth profile. SN‒ is caused by fragmentation of the SM1 (see Figure 1a) and the signal is more prominent at the sample surface for the SM1 containing sample, however, not on the SM1 free reference. PCBM, characterized by the fragment C15H‒, follows an opposing trend, the signal increases gradually from sample surface to sample bulk. Significantly, the PCBM shows stronger depletion at the surface and slower gradual increase from surface to bulk in the ternary film compared to the P3HT:PCBM reference. The Si signal indicates the transition into the glass substrate. The increased signal at the beginning of the depth profile can be attributed to the presence of PDMS in the sacrificial layer.



a)

b)

Figure 5. TOF-SIMS depth profiles of a) P3HT:PCBM; and b) P3HT:SM1:PCBM (1:0.5:1) devices. The layers’ structure is indicated at the top of the figure and the gray dot lines are for eye guide. The thin PEDOT:PSS layer is defined by the C2HO‒ profile, the PCBM compound in the active layer with C15H‒ profile, the SM1 compound in the active layer with SN‒ profile, and the glass substrate by Si profile.

In order to further investigate the effect of SM1 sensitizer on the P3HT:PCBM device behavior, we performed impedance spectroscopy measurements. Impedance spectroscopy is a useful method to analyze the interfacial properties, such as charge carrier transport and 10 ACS Paragon Plus Environment

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recombination, in solar cells.46–48 Figure S5 depicts the Nyquist plots of binary and ternary devices taken under 1 sun condition. We fitted the obtained spectra with the commonly used equivalent circuit shown in the inset of Figure S5.49 The fitting parameters are presented in Table S1. We calculated the lowest recombination resistance (Rrec) i.e. highest recombination rate, for P3HT:PCBM reference, while we obtained the highest Rrec for the best ternary cells, in agreement with the enhanced FF. Accordingly, the charge carrier lifetime (τ=Cµ*Rrec) has a similar trend: the lowest lifetime is measured for the binary reference cell (4.9 µs), while for ternary devices with the SM1 concentrations of 30% and 50%, the lifetime is increased to 7.5

µs and 7.6 µs, respectively. At higher SM1 loading (80% SM1), the lifetime is dropped to 5.4 µs. Such decrement of the recombination resistance for the e.g. P3HT:PCBM reference system can be attributed to an increase in the charge accumulation at the interface caused by the reduced internal potential as the forward bias is increased.[50] On the basis of results discussed so far, we attribute the difference seen in the recombination rate to the difference in the bulk composition at the interface with the top contact. The lower recombination rate of ternary solar cells with SM1 suggests that the remained SM1 on top of the P3HT:PCBM layer effectively modifies the interface of photoactive layer / PEDOT:PSS and allows less charge accumulation at the interface. As a result, the SM1 reduces the interface recombination and enables easier extraction of photo-generated charges. Finally, we elucidate the Voc evolution in ternary blends by employing Fourier transformation photocurrent spectroscopy (FTPS) (Figure

6).51–55 In fact, FTPS

measurements could identify the sub-bandgap signals as absorption from charge transfer (CT) states.[51] As demonstrated by Vandewal et al, a direct correlation of the open-circuit voltage with the onset of charge transfer absorption was found in accordance with detailed balance theory.54 Therefore, we investigated the radiative open circuit voltage , which represents the maximum achievable Voc, as given by: 11 ACS Paragon Plus Environment


, =

ln(

+ 1)

(1)

,

where kT/q is the thermal voltage and the saturation current density. In order to obtain

the saturation current density can be calculated from the external quantum efficiency by using FTPS measurements.51–55 Obtained results are summarized in Table 1. The Voc difference ∆Voc = , . -

!"#!

is caused by irreversible losses by non-

radiative recombination. We calculate for 80 wt% SM1 (1:0.8:1) the highest radiative Voc and the smallest ∆Voc. The P3HT:PCBM binary blend showed the smallest Voc measured and the highest ∆Voc, indicating of more non-radiative recombination occurrence. With increasing SM1 content, the ∆Voc decreased which points to reduced non-radiative recombination and a higher Voc. 1000

Spectral Response [a.u.]

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100 10 1 P3HT:PCBM 0,1 P3HT:SM1:PCBM 1:0.3:1 1:0.5:1 1:0.8:1

0,01 1E-3 1E-4

1,00 1,25 1,50 1,75 2,00 2,25 2,50 2,75 3,00

Energy [eV]

Figure 6. FTPS spectra of P3HT:PCBM reference and three different ternary solar cells with 30, 50 and 80 wt% SM1.

Table 1. Calculated radiative Voc, measured Voc and ∆Voc for P3HT:PCBM reference solar cell and ternary solar cells with three different ratios. P3HT:PCBM P3HT:SM1:PCBM 1: 0.3 :1

1: 0.5 :1

1: 0.8 :1

Voc,rad (V) calc.

1.320

1.331

1.346

1.348

Voc (V) measured

0.560

0.640

0.680

0.720

∆Voc (V)

0.760

0.691

0.666

0.628

760

691

666

628

∆Voc (mV)


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Therefore, the achieved results from Fourier transformation photocurrent spectroscopy (FTPS) measurements reveal that ~ 130 mV lower Voc losses occur by increasing the SM1 content, owing to non-radiative recombination suppression at the interface. This is also in agreement with the onset shift of diode current observed in the dark J-V characteristics in Figure S6. The reduced non-radiative recombination at the interface can be attributed to either direct hole extraction from the SM1 domains or modified interface of P3HT/SM1/PEDOT:PSS. Recombination at the photoactive layer / PEDOT:PSS layer interface can origin from the fact that PEDOT:PSS is not an optimum electron-blocking layer.56,57 Therefore, accumulation of SM1 at the PEDOT:PSS / P3HT:PCBM interface causes the depletion of PCBM at this interface (as observed in ToF-SIMS results), suppresses the electron injection from PCBM into the anode and reduces, consequently, the interface parasitic recombination. It is in agreement with ToF-SIMS results, where implementing SM1 into P3HT:PCBM caused more PCBM depletion at the active layer / PEDOT:PSS interface. In order to verify our findings, we change the ternary sensitizer SM1 with two other novel small molecule materials (SM2 and SM3) from the same material class. The chemical structures, the values for the HOMO and LUMO energy levels of SM2 and SM3, atmospheric pressure photoelectron spectra (AC-2), and absorption spectra of these compounds are presented in Scheme S1, Figure S7 and Figure S8, respectively, in Supporting Information. In agreement with the results obtained with SM1, ternary devices with the incorporation of SM2 and SM3 in a P3HT:PCBM host system deliver higher Voc and FF compared to the binary blend (Figure S9 and Table S2). Finally, the last parameter that has to be changed for validating the method was to add the small molecule in a different host matrix. Thus, we designed a novel ternary solar cells based on an indacenodithieno[3,2-b]thiophene-based semiladder-type polymer, namely PIDTT-DFBT blended in [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). We found that upon introduction of the SM1 with an optimized concentration of 5-10 wt.% relative to polymer concentration the ternary device shows around 13 ACS Paragon Plus Environment


15% performance improvement mainly owing to Voc and FF enhancement rather than near-IR sensitization and Jsc increase (Figure S10 and Table S2). 3. Conclusion In conclusion, by implementing a dithienylthienothiadiazole-based small molecule additive (SM1) into the P3HT:PCBM host matrix, we achieved about 30% improvement in power conversion efficiency accompanied with a high FF (>70 %) and notably increased Voc. In contrast to the conventional sensitization where the third component acts to improve the solar harvesting, here SM1 mainly facilitates the hole extraction of the photoactive layer. With the combination of device, surface energy, and depth-profile ToF-SIMS investigations, we realized that aggregative SM1 is dominantly located on top of the photoactive layers instead of being homogeneously mixed in the P3HT:PCBM matrix. Therefore, a proportional contribution of P3HT and SM1 HOMO levels to Voc can partially explain the observed tunable Voc. According to our findings, ternary devices show a decreased non-geminate recombination at interface as compared to the reference system, and consequently the Voc as well as FF are improved. Significantly, a Voc of 0.7-0.75 V was achieved for the P3HT:PCBM devices modified with SM1 without any negative effect on bulk microstructure, which could not be reached practically e.g. for an unmodified binary P3HT:PCBM solar cell. Overall, by implementing a strongly aggregating ternary small molecule into polymer / fullerene solar cells, this work presents a general and efficient strategy to modify the hole extracting interface in inverted solar cells, tuning the open circuit voltage in a single photoactive layer coating process. The universality of our approach was verified by employing other aggregating small molecules as ternary blend additives and / or by using a different donor/acceptor host matrix.

4. Experimental Section Materials: P3HT (P 200) was provided by BASF, PCBM and PC71BM with 99% Purity were

$$$$n ) = purchased from Solenne BV and PIDTT-DFBT [average molecular weight per number (M


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59100 g/mol and dispersity (Đ) = 2.73] was provided by Advent Technologies SA. The synthesis of SM1, SM2 and SM3 is reported in the supporting information. Instrumentation: The HOMO energy level was determined by the photoelectron spectroscopy in air (PESA) with a Rieken Keiki AC2 spectrometer. The optical band gap was determined from the onset of the absorption spectrum in film. The LUMO was calculated from the HOMO and the optical band gap. The J-V characteristics were measured with a source measurement unit from BoTest. Illumination was provided by an OrielSol 1A solar simulator with AM1.5G spectra at 100 mW/cm². The external quantum efficiency (EQE) spectra of the solar cells were measured using a system from Enlitech, Taiwan. The FTPS was carried out using a modified Vertex 70 FTIR spectrometer from Brucker optics, equipped with QTH lamp, quartz beam splitter and external detector. A low noise current amplifier (Femto, DLPCA-200) was employed to amplify the photocurrent produced upon illumination of the photovoltaic device with light modulated by the FTIR. The output voltage of the current amplifier was redirected to the external detector port of the FTIR to use the FTIR’s software to collect the photocurrent spectrum. For the absorbance measurements a UV-VIS spectrometer (Lambda 950 from Perkin) was used. All the devices were tested in ambient air. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was performed on a ToF-SIMS 5 spectrometer (ION-TOF; Münster, Germany) using 25 keV Bi+ bunched down to

Suppressing the Surface Recombination and Tuning the Open Circuit

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