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Benzothiadiazole Substituted Semiconductor Molecules for Organic Solar Cells: The Effect of the Solvent Annealing Over the Thin Film Hole Mobility Values Cristina Rodriguez-Seco, Subhayan Biswas, Ganesh D. Sharma, Anton Vidal-Ferran, and Emilio Palomares J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00840 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Benzothiadiazole Substituted Semiconductor Molecules for Organic Solar Cells: The Effect of the Solvent Annealing Over the Thin Film Hole Mobility Values Cristina Rodríguez-Seco1, Subhayan Biswas2, Ganesh D. Sharma*2,Anton Vidal-Ferran1,3*, Emilio Palomares*1,3. 1. Institute of Chemical Research of Catalonia-The Barcelona Institute of Science and Technology (ICIQ-BIST). Avda. Països Catalans, 16. Tarragona. E-43007. Spain. 2. Department of Physics, The LNM Institute of Information Technology, Jamdoli, Jaipur 302031, India 3. ICREA. Passeig Lluís Companys, 23. Barcelona, E-08010. Spain ABSTRACT We have synthesized and characterized two low molecular weight organic molecules, namely CS01 and CS03 having the benzo[c][1,2,5]thiadiazole-4,7-diamino core but differing in the number of aromatic rings at the amino groups. The molecules, when processed to make thin organic films, display absorbance up to the near IR region (750 nm) and good hole mobility values. Upon mixing each organic semiconductor molecule with the fullerene derivative PC71BM we monitored a strong quenching of the fluorescence emission. We assigned such process to efficient charge transfer from the CS01 and CS03 molecules to the fullerenes. Moreover, fueled by this observation, we prepared organic solar cells and obtained, as a first attempt, efficiencies over 2% under 1 sun light simulated solar radiation. Furthermore, the film optimization through careful solvent annealing process increased further the efficiencies up to 4.80 % for CS01 and 5.12 % for CS03. The observed increase in efficiency is due to a better morphology obtained through solvent annealing of the thin films. However, an analysis in depth reveals that the solvent annealing leaded to a better hole mobility but the electron mobility remains alike.

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INTRODUCTION Although, at present silicon based solar cells are the leading photovoltaic technology but it is desirable to look at other photovoltaic materials that are more flexible, low cost, low weight, transparent and suitable for roll-to-roll fabrication, as compared to silicon. Organic solar cells (OSCs) based on the solution processed bulk heterojunction (BHJ) consisting of conjugated polymer as donor and fullerene and non-fullerene acceptor meet all these necessities1-4. To date the OSCs based on BHJ with conjugated polymers showed overall power conversion efficiency of 11 % and 12-13 % for fullerene5-8 and non-fullerene acceptor9-10, respectively. OSCs prepared using low molecular weight molecules are attracting interest due to their small losses in Voc in comparison with other organic semiconductor materials11. Indeed, compared to the their organic counterparts, the semiconductor polymers, the so called “small molecules (SMs)” have the advantage of not being dependent on regioregularity issues, differences in molecular weight and the difficult purification processes to clean the metal catalysts12 used in the cross-coupling chemical reactions13-17. In fact, it is of utmost importance that the synthesis of the small molecules results straightforward with only a few synthetic steps and an easy to scale process. Moreover, to achieve efficient light harvesting, it is also desirable that the processed organic molecules extend their absorbance as far as in the IR as possible18. Such narrow bandgap property has to come without compromising the efficient charge transfer process to the electron acceptor organic molecule, which is often a fullerene derivative although recently non-fullerene based electron acceptor have also display high efficiencies in the conversion of sun-light into electrical current. The state of art of OSCs based on SMs had made great strides with PCE more than 11 % based on fullerene derivatives as acceptors19-24, which is comparable to the polymer counterpart. Therefore, the design of SM donors for OSCs has been a promising alternative for polymers and attracted lot of attention in recent years. Benzo[c][1,2,5]thiadiazole-based molecules have been used recently to prepare organic solar cells and other organic optoelectronic devices25-27. Because of their versatility as chemical core, it is relatively easy to design suitable semiconductor molecules through the appropriated substitution of the benzo[c][1,2,5]thiadiazole rings. We report here the synthesis, photophysical and electrochemical properties of two novel benzo[c][1,2,5]thiadiazole derivatives (as shown in Scheme 1) that differ only in the number of aromatic rings at the amino groups. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) estimated from the cyclic


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voltammetry indicates that these SMs are suitable as donor along with PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) as acceptor for the solution processed BHJ OSCs. After the optimization of the active layer, the OSCs based on CS01 and CS03 exhibit overall PCE of to 4.80 % and 5.12 %, respectively. The higher value of PCE for CS03 is attributed to the denser - stacking distance, more crystalline as inferred from the X-ray diffraction, beneficial for better hole transport and broader absorption spectra of CS03: PC71BM active layer.

Scheme 1 Chemical structures of CS01 and CS03 EXPERIMENTAL The synthetic details for the preparation of CS01 and CS03 and their characterization are described in the Supporting Information (SI). Solar cell fabrication and characterization The OSCs were fabricated with a conventional structure of glass/ITO/PEDOT:PSS/ CS01 or CS03:PC71BM/ PFN/Al. The alcohol-soluble conjugated polymer, poly [(9,9-bis(3-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) was used as an cathode interfacial layer. The ITO coated glass substrates were cleaned by ultrasonic treatment in detergent, deionized water, acetone and isopropanol for 15 min each and subsequently dried in vacuum oven for 15 min at a temperature of 40° C. Then a thin layer of poly (3,4-ethylenedioxythiophene):(polystyrene sulfonate) (PEDOT:PSS) was spin-coated onto the pre-cleaned ITO coated glass substrate at 3500 rpm for 40 s and subsequently baked 3 Environment ACS Paragon Plus


at 110° C for 20 min and allowed to cool to room temperature. In order to deposit the thin film of the active layer, a mixture of CS01 or CS03 with PC71BM with different weight ratios in chloroform (total concentration16 mg/mL) was spin coated at 2000 rpm for 20 s on the top of the PEDOT:PSS substrate and then dried at room temperature. After the active spin coating, SVA (solvent annealing) was used to tune the morphology of the blend film. For SVA, the films were placed in a closed glass petri-dish containing 1 mL carbon disulphide (CS2) for 30s. Special care has been taken during the SVA treatment in terms of masking the noise, so that the vapors of CS2 does not affect the human body. The thickness of the active layer is about 90 nm 5 nm. Subsequently, a thin film of PFN was spin coated from the PFN solution in methanol and dried in vacuum oven at room temperature for 90 s. Finally, an aluminum (Al) layer was deposited by thermal evaporation under vacuum (ca. 10 -5 Pa) through a mask, yielding four individual devices with 16 mm2 effective area. The devices for hole mobility measurement were fabricated with an architecture of ITO/PEDOT:PSS/SMsdonor:PC71BM/Au. The current density-voltage (J-V) characteristics of OSCs were recorded with a Keithley 2400 Source Measure Unit under the simulated AM 1.5 G illumination with an intensity of 100 mW/cm2, calibrating with the standard silicon solar cell. The monochromatic incidentphoton-to-electron conversion efficiency (IPCE) spectra were collected illuminating the device using a monochromator and resultant current at short circuit condition was measured using the Keithley electrometer. The hole and electron mobility measurements were conducted y measuring the J-V characteristics in the dark with a computer-controlled Keithley 2400 Source Measure Unit system using the hole only and electron only devices. RESULTS AND DISCUSSION Absorption and PL emission spectra The absorbance of CS01 and CS03 in solution and in thin film (thickness 80 nm) processed from chloroform solution are shown in Figure 1. The absorption spectra of CS01 and CS03 in solution (concentration 1x10-5 M in chloroform) show a complete (panchromatic) absorption from 350 nm to 650 nm and 700 nm, respectively. Moreover, an important red shift of the main absorption band with a noticeable shoulder can be monitored when the molecules are deposited in thin films attributed to the strong interactions may be originated from the ordered SM - packing28. This absorption shift is due to the formation of J-type


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aggregates in the solid state. In fact, for the CS03 the formation of J-aggregates seems favored and a higher bathochromic shift is observed and the absorption edge extended up to 750 nm. The optical gap (Egapopt) was calculated from the onset of the absorption wavelength (onset) in the thin films using Egapopt=1240/onset , with values of 1.72 eV and 1.65 eV for CS01 and CS03, respectively. 1.2

1

Absorption (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


CS01 (sol) CS03 (sol) CS01 (film) CS03 (film)

0.8

0.6

0.4

0.2

0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 1 Normalized absorption spectra of CS01 and CS03 in chloroform solution and thin films.

The PL emission properties of the thin films were also measured. Figure 2 illustrates the PL spectrum upon excitation at ex=530 nm for CS01 and ex= 540 nm for CS03. Both PL spectra show the expected specular image of the absorption spectra. More interesting is the fact that, when the organic thin films contain the fullerene derivative PC71BM in a ratio 1:2 the CS01 and CS03 fluorescence is totally quenched. This observation indicates efficient electron transfer from the CS01 and CS03 excited state to the fullerene and it is a first indication that both CS01 and CS03 can be used in bulk-heterojuction thin films, together with the PC71BM, in organic solar cells.



1.2

1

PL intensity (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

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CS01 CS03 CS01:PC71BM CS03:PC71BM

0.8

0.6

0.4

0.2

0 500

600

700

800

900

Wavelength (nm)

Figure 2 Fluorescence emission spectra for CS01 and CS03 and the CS01:PC71BM and CS03:PC71BM thin films

Computational studies Computational studies were performed in order to simulate the UV-Visible spectra of compounds CS01 and CS03 and obtain an estimation of the HOMO and LUMO energy values. Initially, a full level DFT geometry optimization of these compounds was carried out using the M06-2X29 functional and the def2-SVP basis set30 (see the SI for details). This level of theory offers a good compromise between the size of the system (up to 129 atoms for compound CS01) and the accuracy of the results.31 Solvent effects (tetrahydrofuran) were incorporated as indicated in the SI. The geometry optimised co-ordinates with the computational details already mentioned for CS01 and CS03 were used in calculation of UV-Visible data employing an hybrid exchange– correlation functional (CAM-B3LYP32), since this methodology has proven to be reliable in UV predictions33 (see the SI for details).


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The graphical representation for the HOMO and LUMO orbitals of compounds CS01 and CS03 is shown in Figure 3. The theoretical HOMO-LUMO energy gap is slightly lower in CS03 than in CS01 (4.32 eV and 4.33 eV, respectively), as it was also observed experimentally (see the following section). The calculated UV-Visible spectra of compounds CS01 and CS03 are shown in Figure 4. The good agreement between the experimental Visible absorption maxima (Figure 1) and the calculated Visible absorption maxima for CS01 and CS03 (Figure 4) is noticeable.

HOMO CS01 (-5.89 eV)

LUMO CS01 (-1.56 eV)

HOMO CS03 (-5.77 eV)

LUMO CS03 (-1.45 eV)



Figure 3. The HOMO and LUMO graphical representation and the theoretical energy values of the molecular orbitals in compounds CS01 (top) and CS03 (bottom) at the M06-2X/def2SVP level of theory. The theoretical energy values are shown between parentheses.

Figure 4. Calculated UV-Visible spectra of compounds CS01 and CS03 at the CAMB3LYP/ def2-SVP level of theory in THF.

Electrochemical properties and relative energy levels The electrochemical properties of CS01 and CS03 were analyzed in dichloromethane solvent and using cyclic voltammetry (CV). Figure 5 shows the cyclic voltammograms obtained at room temperature. As can be seen, both compounds show a first reversible oxidation wave at 0.22V for CS01 and 0.03V for CS03 versus Fc/Fc+. From the first oxidation potential values, the relative HOMO energy level (Highest Occupied Molecular Orbital) for CS01 and CS03 can be elucidated to be -5.32 eV and -5.13 eV by applying Equation 1, respectively. The deeper value for the CS01 HOMO should lead to higher Voc in the solar cells compared to the CS03. The relative LUMO energy values (Lowest Unoccupied Molecular Orbital) are -3.6 eV for CS01 and -3.48 eV for CS03 as calculated using Equation 2.


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Equation 1

EHOMO (eV) = [-q(Eox+5,1)]eV

, where q is the electron charge, Eox is the oxidation potential of the first wave of the molecule measured using Fc/Fc+ as reference using cyclic voltammetry.

ELUMO (eV) = Egopt- EHOMO

Equation 2

The experimental results show good correlation with the theoretical values with the HOMO energy value for CS01 being deeper than the HOMO energy value of CS03.

1,2x10-5

Current (A)

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


(a)

-6

8,0x10

CS01 CS01 + Ferrocene

4,0x10-6 0,0 -4,0x10-6 -8,0x10-6 -2,0

-1,5

-1,0

-0,5

0,0

Voltage (V)


0,5

1,0


6,0x10-6

(b)

-6

Current (A)

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

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4,0x10

CS03 CS03 + Ferrocene

2,0x10-6 0,0 -2,0x10-6 -4,0x10-6 -2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

Voltage (V) Figure 5 Cyclic voltammograms for CS01 (a) and CS03 (b) using ferrocene (Fc/Fc+) as internal reference in dichloromethane solvent measured at room temperature.

Photovoltaic measurements In order to investigate the photovoltaic performance of these SMs, OSCs were fabricated with the following device structure ITO/PEDOT:PSS/CS01 or CS03:PC71BM/ PFN/Al. First of all, we have optimized the performance of OSCs through varying the D:A weight ratios in chloroform solution. It was found that the optimized weight ratio is 1:2 for both the CS01 and the CS03. The current-voltage (J-V) curves under illumination for the devices are shown in Figure 6a and photovoltaic parameters are listed in Table 1. The solar cells based on CS01:PC71BM showed an overall efficiency of 2.03% with a Jsc = 7.13 mA/cm2, Voc= 0.84V and a FF= 0.34. The devices made with CS03:PC71BM showed also an efficiency close to 2.4% with a Jsc= 8.61 mA/cm2 , Voc= 0.73 V and a FF= 0.38. As commented before, the higher Voc observed for CS01 is due to the deeper relative HOMO energy value (-5.32 eV) in comparison with the CS03 (-5.13 eV). As demonstrated previously, for equal recombination kinetics the Voc of the organic solar cells can be correlated with the energy difference between the HOMO of the electron donor molecule and the LUMO energy level of the electron acceptor molecule. The differences in Jsc can be understood in terms of better light


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harvesting in the case of the CS03 thin films as already observed in Figure 1. Nonetheless, to reinforce our hypothesis we measured the incident photon to current conversion efficiency (IPCE) spectra for the OSCs (Figure 6b). The IPCE spectrum is in perfect agreement with the absorption data and showed two bands, indicating that both PC71BM and small molecule donors are contributing the exciton generation and subsequent dissociation in free charge carriers after the absorption of photons by the active layers. Moreover, the integration of the IPCE spectrum with respect to the 1.5 AM G sun spectra leads to values of Jsc very close (see Table 1) to the measured experimentally.

Figure 6 (a) Photocurrent –voltage characteristics under 1 sun simulated illumination and (b) IPCE spectra of the organic solar cells based on CS01 and CS03 as donor and PC71BM as acceptor.

Table 1. Photovoltaic parameters (Jsc (photocurrent density), Voc (open Circuit Voltage), FF (fill factor), PCE (solar cell efficiency at 1 sun) for the different solar cells using CS01 and CS03 as donor and PC71BM as acceptor.



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Jsc (mA/cm2)

Voc (V)

FF

CS01 (as cast)

7.13 (0.08)

0.84 (0.03)

0.34 (0.023)

2.03 (1.93)a

7.01 (0.05)

CS03 (as cast)

8.61 (0.06)

0.73(0.02)

0.38 (0.024)

2.39 (2.32)a

8.49 (0.07)

CS01 (SVA)

10.48 (0.08)

0.79 (0.03)

0.58 (0.05)

4.80 (4.73)a

10.37 (0.09)

CS03 (SVA)

12.81(0.10)

0.67 (0.02)

0.62 (0.07)

5.32 (5.25)a

12.73 (0.08)

a b

PCE (%)

Jsc (mA/cm2 )b

Average of 10 devices Calculated after the integration of IPCE spectra with respect to the 1.5 AM G solar spectrum.

The CS01 and CS03 based OSCs without any treatment showed low PCE mainly due to the low values of both Jsc and FF and may be due to poor nanoscale morphology of the active layer which lead to charge recombination during their transportation towards electrodes. In order to improve the morphology of the active layer, we have adopted the solvent vapor annealing (SVA) as reported in literature34-37. Firstly, we have used THF for solvent vapor annealing and found that the PCE of the devices are enhanced up to 4.13 % and 5.01 %. In order to improve the PCE further, we have used CS2 for SVA treatment as reported in literature35, 38 due to its high vapor pressure than other and these small molecules have medium solubility in CS2. Moreover, PC71BM exhibits better solubility in CS2 as compared to other solvent such as tetrahydrofuran. Therefore, the combination of high vapor pressure and medium solubility of the small molecule donor implies fast vapour penetration in the active layer. The current-voltage characteristics of the OSCs based on SVA treated active layer are shown in Figure 6a and the corresponding photovoltaic parameters are listed in Table 1. The photovoltaic performance of the OSCs has been significantly enhanced compared to as cast OSCs. With SVA treatment for 30 s, the OSC38s based on CS01 and CS03 showed overall PCE of 4.80 % (Jsc = 10.48 mA/cm2, Voc = 0.79 V and FF=0.58) and 5.32 % (Jsc = 12.81 mA/cm2, Voc = 0.67 V and FF=0.62), respectively. When the active layers are subjected to SVA treatment beyond 30s, the both Jsc and FF drastically drops, while the Voc remain almost constant. Since the SVA treatment the crystallite of donor materials are grown, the SVA treatment of the longer time, these crystallites are over grown (> exciton diffusion length) and as large as the film thickness and the active layer no longer form the BHJ networks. The large domains are not desirable for charge generation, leading to the reduction in both Jsc and FF and resulting reduction in the overall PCE.


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The increase in the Jsc for the OSCs after the SVA treatment has been also confirmed from the IPCE spectra of devices (as shown in Figure 6b). The values of IPCE for the SVA treated OSCs are higher than those for the as cast counterparts, indicating that the exciton generation and their dissociation into free charge carriers has been improved by the SVA treatment of active layer, related to the improved nanoscale morphology and phase separation. The value of Jsc estimated from the integration of IPCE spectra (Table 1) of the devices are very close to the experimentally values form J-V characteristics. The hole mobility of CS01 and CS03 in the blend films were measured by the J-V characteristics on the hole only devices (Figure 7) and employing the space charge limited current (SCLC) model. The hole mobility values for as cast and SVA treated active layers are summarized in Table 2. The electron mobility in the as cast and SVA treated active layers were measured using electron only devices in similar manner as for hole mobility and complied in table. There is a slight change in the electron mobility values after the SVA treatment. The improved hole mobility and better balance between the electrons and holes mobility after the SVA treatment could promote charge transport and reduced charge recombination, leading to improvement in Jsc and FF, and thereby PCE. Moreover the hole mobility in CS03:PC71BM is higher than that of CS01:PC71BM for as cast and SVA processing conditions, respectively) irrespective of processing conditions, may be one the reasons for high values of FF and PCE for former OSCs. 10

Current density (mA/cm2)

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


1

0.1 CS01 (as cast) CS03 (as cast) CS01 (SVA) CS03 (SVA) 0.01

0.001 0

0.5

1

1.5

2

V-Vbi (V)


2.5

3


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Figure 7 Current-voltage characteristics of hole-only devices based on CS01:PC71BM and CS03:PC71BM photoactive thin films with and without the solvent annealing process.

Table 2. Solar cells carriers mobility measured using the Space Charge Limited Current (SCLC) model. Active layer

Hole mobility (cm2/Vs)

Electron mobility

Ratio

2

(cm /Vs) CS01:PC71BM (as cast)

2.56 (0.09) x10-5

2.13 (0.11) x10-4

8.32

CS03:PC71BM (as cast)

4.19 (0.11) x10-5

2.28 (0.09) x10-4

5.44

CS01:PC71BM (SVA)

7.89 (0.13) x10-5

2.42 (10.3) x10-4

3.06

CS03:PC71BM (SVA)

9.87 (0.15) x10-5

2.48 (0.12) x10-4

2.51

Nonetheless, the solvent annealing process was indeed beneficial for the carriers collection. As illustrated in Figure 8 the photocurrent density (Jph) plotted as a function of the effective voltage (Veff) shows significant differences between the solvent annealed thin films and the casted ones39-40. The photocurrent density (Jph) can be expressed as the difference between the the photogenerated current (Jlight) and the current in the dark ( Jdark). The effective voltage (Veff) also can be defined as the difference between the voltage, in which the Jph is zero (V0) and the external voltage applied (Vext). As can be seen from Figure 8 that Jph increases rapidly at low voltages and starts to saturate at an effective voltage at different values of Veff and completely saturate at high value of Veff. The early onset saturation of the Jph observed for the CS03 based device indicates that the internal electric field plays a minor role during the charge extraction and the charges are efficiently extracted by the electrodes. The ratio of the Jph to the saturation current density (Jphsat) i.e. Jph/Jphsat, under short circuit condition of the OSCs represents the exciton dissociation efficiency (Pdiss) and the values of Pdiss for the OSCs based on CS01 and CS03 are about 0.89 and 0.93, respectively indicating exciton dissociation is more efficient for CS03 based device than CS01 counterpart due to the better phase separation. The Jph/Jphsat value at maximum power output corresponds to the charge transport and collection efficiency (Pcc)18 and are 0.73 and 0.78 for CS01 and CS03 based OSCs, respectively suggesting that efficient charge transport and collection in the CS03 based devices, as also confirmed from the mobility measurements. As shown in Figure 8 the as cast OSCs do not show any saturation region and the Jph keeps on increasing with 14 Environment ACS Paragon Plus

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Veff. At high Veff, the Jph for the as cast OSCs is almost same for the respective SVA treated OSCs indicating that the low values of Jsc measured for the as cast OSCs does not originate from the charge generation issues but rather from the poor charge extraction at the electrodes. The enhanced value of Pcc is also well supported by the increased value of FF. 100

10

Jph (mA/cm2)

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


1 CS01 (as cast) CS03 (as cast) CS01 (SVA) CS03 (SVA)

0.1

0.01 0.01

1

0.1

10

Veff (V)

Figure 8 Variation of Jph with the effective voltage (Veff) for OSCs CS01:PC71BM and CS03:PC71BM photoactive thin films with and without the solvent annealing process. In order to get the information about the difference in the photovoltaic performance OSCs based on CS01 and CS02 as donor, X-ray diffraction measurements were applied to explore the crystallinity and molecular ordering in the optimized active layers ( SVA treated active layers) and shown in Figure 9. Both active layers showed strong (100) diffraction peak at 2=4.89, corresponds to the lamellar distance of 1.98 nm and however, the (010) diffraction peak at 2= 21.14 and 22.04 for CS01, and CS03, corresponds to the - stacking distance of 0.46 nm and 0.43 nm, respectively. This suggests that CS03 forms a denser molecular packing than CS01 and M1, which may induce a better nanoscale phase separation between donor and acceptor in the blend film. These factors may be responsible for the higher Jsc and FF. It can be seen from the XRD patterns that the CS03:PC71BM blend film showed stronger diffraction peaks correspond to (100) and (010) planes, which indicates that degree of crystalline nature is higher for CS03:PC71BM blend film than that for CS01:PC71BM which is beneficial for charge transport and collection. In addition to these 15 Environment ACS Paragon Plus


two peaks in XRD, a weak diffraction peak around 2= 18 also observed in all the blend films, corresponds to the PC71BM.41

Figure 9. X-ray diffraction patterns of the optimized CS01:PC71BM and CS03:PC71BM thin films

In order to get information about the influence of SVA (CS2)on phase separation in the active layer, we have measured the transmission electron microscopy (TEM) images of the active layers before and after the SVA treatment and shown in Figure 10 (only for CS03:PC71BM active layers. Similar TEM images were also observed for CS01:PC71BM As can be seen from the TEM images that as cast CS03:PC71BM thin film did not showed any clear phase separation which limits the charge transport within the active layer towards the electrodes, resulting low values of both Jsc and FF. However, the after the SVA treatment with CS2, blend film showed larger domain (20 nm) and clear phase separation compared to as cast, which for an interpenetrating path ways for the electrons and holes transportation towards cathode and anode, respectively, leading to the increase in both Jsc and FF, resulting an improvement in PCE of the corresponding OSCs.


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Figure 10. TEM images of as cast and CS2 SVA treated CS03:PC71BM (1:2) blended thin films. Scale bar is 100nm. CONCLUSIONS. In summary, we have synthesized two new low weight organic molecules that can be processed to form semiconductor organic thin films. The molecules show excellent absorption from the visible to the near IR region of the sun spectra. These molecules show excellent interfacial electron transfer process when mixed in thin bulk-heterojunction films with the fullerene derivate PC71BM. The optimization through solvent annealing of the photoactive films in solar cells with the standard configuration ITO/PEDOT:PSS/ Photoactive film/PFN/Al leads to efficiencies of 4.80 % and 5.1 % for CS01 and CS03, respectively. However, a deeper analysis of the photovoltaic parameters leads to the finding that the solvent annealing processing implies the formation of an unbalanced processes between holes and electrons mobility. While the hole mobility property of the thin bulkheterojunction film improves noticeably, the electron mobility characteristics remains almost identical indicating the improved balanced charge transport. Nevertheless, the solvent annealing has positive effects on the film morphology that leads to better charge collection as measured by monitoring the changes in Jph vs. Veff leading to higher solar-to-energy conversion efficiencies over 5 % in the case of the CS03 molecule. Further efforts to reduce



the inequity between hole and electron mobility are being carried out. These SMs can be used as donor with the low bandgap non-fullerene acceptors and we are working in this direction. ACKNOWLEDGMENTS The authors would like to acknowledge ICIQ-BIST (Severo Ochoa Excellence Accreditation 2014-2018 SEV-2013-0319) and ICREA for economical support. EP and AV are also grateful to Spanish MINECO and the Agencia Estatal de Investigación (AEI) for the CTQ2016-80042-R/AEI, CTQ2017-89814-P and CTQ2014-60256-P/AEI projects and Generalitat de Catalunya for the AGAUR funding. EP and CRS thanks also the MINECO and the AEI for the FPI grant. GDS and SB are thankful to Department of Science and Technology (project number) for financial support. ASSOCIATED CONTENT Supporting Information. The SI contains the detailed synthesis of CS01 and CS03, as well as their full characterization. Moreover, the SI also contains the theoretical methodology employed to obtain the UV-Visible spectrum of CS01 and CS03 along with the theoretical energy values of the HOMO and LUMO for CS01 and CS03.

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AUTHOR INFORMATION Corresponding Author The author(s) to whom correspondence should be addressed Prof. Emilio Palomares ( email: [email protected]) and Prof. Ganesh D. Sharma (email: [email protected] and [email protected] ) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.



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