Benzothiadiazole Substituted Semiconductor Molecules for Organic

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 13782−13789

Benzothiadiazole Substituted Semiconductor Molecules for Organic Solar Cells: The Effect of the Solvent Annealing Over the Thin Film Hole Mobility Values Cristina Rodríguez-Seco,† Subhayan Biswas,‡ Ganesh D. Sharma,*,‡ Anton Vidal-Ferran,*,†,§ and Emilio Palomares*,†,§

Downloaded via IOWA STATE UNIV on January 29, 2019 at 07:34:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Institute of Chemical Research of CataloniaThe Barcelona Institute of Science and Technology (ICIQ-BIST), Avda. Països Catalans, 16, Tarragona E-43007, Spain ‡ Department of Physics, The LNM Institute of Information Technology, Jamdoli, Jaipur 302031, India § ICREA, Passeig Lluís Companys, 23, Barcelona E-08010, Spain S Supporting Information *

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 a 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 a 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 in-depth analysis reveals that the solvent annealing led to a better hole mobility, but the electron mobility remains similar.

■

INTRODUCTION Although at present silicon based solar cells are the leading photovoltaic technology, it is desirable to look at other photovoltaic materials that are more flexible, have low cost, are light weight, are transparent, and are 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 nonfullerene as acceptor, meet all these criteria.1−4 To date, the OSCs based on BHJ with conjugated polymers showed overall power conversion efficiency of 11% and 12−13% for fullerene5−8 and nonfullerene acceptors,9,10 respectively. OSCs prepared using low molecular weight molecules are attracting interest due to their small losses in Voc in comparison with other organic semiconductor materials. 11 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 difficult purification processes for cleaning the metal catalysts12 used in the crosscoupling chemical reactions.13−17 In fact, it is of utmost importance that the synthesis of the small molecules is straightforward with only a few synthetic steps and an easy to © 2018 American Chemical Society

scale process. Moreover, to achieve efficient light harvesting, it is also desirable that the processed organic molecules extend their absorbance as far into the IR as possible.18 Such a narrow bandgap property has to come without compromising the efficient charge transfer to the electron acceptor organic molecule, which is often a fullerene derivative, although recently nonfullerene based electron acceptors have also displayed high efficiencies in the conversion of sunlight into electrical current. The state of the art of OSCs based on SMs had made great strides with PCE more than 11% based on fullerene derivatives as acceptors,19−24 which is comparable to the polymer counterpart. Therefore, the design of SM donors for OSCs has been a promising alternative for polymers and has attracted a 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 devices.25−27 Because of their versatility as a Special Issue: Prashant V. Kamat Festschrift Received: January 24, 2018 Revised: February 25, 2018 Published: February 26, 2018 13782

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

Article

The Journal of Physical Chemistry C

human body. The thickness of the active layer is about 90 ± 5 nm. Subsequently, a thin film of PFN was spin-coated from the PFN solution in methanol and dried in a 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 incident-photon-to-electron conversion efficiency (IPCE) spectra were collected illuminating the device using a monochromator, and the resultant current at short circuit condition was measured using the Keithley electrometer. The hole and electron mobility measurements were conducted by 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.

chemical core, it is relatively easy to design suitable semiconductor molecules through the appropriate substitution of the benzo[c][1,2,5]thiadiazole rings. We report here the synthesis and photophysical and electrochemical properties of two novel benzo[c][1,2,5]thiadiazole derivatives (as shown in Scheme 1) that differ Scheme 1. Chemical Structures of CS01 and CS03

■

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

RESULTS AND DISCUSSION Absorption and PL Emission Spectra. The absorbances of CS01 and CS03 in solution and in a thin film (thickness 80 nm) processed from chloroform solution are shown in Figure 1.

■

EXPERIMENTAL SECTION 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 alcoholsoluble conjugated polymer, poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9−dioctylfluorene)] (PFN), was used as a cathode interfacial layer. The ITOcoated 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(3,4ethylenedioxythiophene):(polystyrenesulfonate) (PEDOT:PSS) was spin-coated onto the precleaned ITO-coated glass substrate at 3500 rpm for 40 s, subsequently baked 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 of carbon disulfide (CS2) for 30 s. Special care has been taken during the SVA treatment in terms of masking the noise, so that the vapors of CS2 do not affect the

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

The absorption spectra of CS01 and CS03 in solution (concentration 1 × 10−5 M in chloroform) show a complete (panchromatic) absorption from 350 to 650 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 which may originate from the ordered SM π−π packing.28 This absorption shift is due to the formation of J-type aggregates in the solid state. In fact, for 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 (λonset) 13783

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

Article

The Journal of Physical Chemistry C in the thin films using Egapopt = 1240/ λonset, with values of 1.72 and 1.65 eV for CS01 and CS03, respectively. The PL emission properties of the thin films were also measured. Figure 2 illustrates the PL spectrum upon excitation

Figure 3. 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/def2-SVP level of theory. The theoretical energy values are shown between parentheses. Figure 2. Fluorescence emission spectra for CS01 and CS03 and the CS01:PC71BM and CS03:PC71BM thin films.

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. Electrochemical Properties and Relative Energy Levels. The electrochemical properties of CS01 and CS03 were analyzed in dichloromethane solvent and by 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 0.22 V for CS01 and 0.03 V 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 and −5.13 eV by applying eq 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 eq 2.

at λex = 530 nm for CS01 and λex = 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 bulkheterojunction thin films, together with the PC71BM, in organic solar cells. Computational Studies. Computational studies were performed in order to simulate the UV−vis 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 Supporting Information 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 Supporting Information. The geometry optimized coordinates with the computational details already mentioned for CS01 and CS03 were used in the calculation of UV−vis data employing a hybrid exchangecorrelation functional (CAM-B3LYP32), since this methodology has proven to be reliable in UV predictions33 (see the Supporting Information for details). 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 and 4.33 eV, respectively), as was also observed experimentally (see the following section). The calculated UV−vis spectra of compounds CS01 and CS03 are

E HOMO (eV) = [−q(Eox + 5.1)] eV

(1)

Here, q is the electron charge, and Eox is the oxidation potential of the first wave of the molecule measured using Fc/Fc+ as a reference using cyclic voltammetry. E LUMO (eV) = Eg opt − E HOMO

(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. 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− 13784

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

Article

The Journal of Physical Chemistry C

Figure 4. Calculated UV−vis spectra of compounds CS01 and CS03 at the CAM-B3LYP/def2-SVP level of theory in THF.

value (−5.32 eV) in comparison with that of 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 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 those measured experimentally. The CS01 and CS03 based OSCs without any treatment showed low PCE mainly due to the low values of both Jsc and FF which may be due to poor nanoscale morphology of the active layer which led to charge recombination during their transportation toward electrodes. In order to improve the morphology of the active layer, we have adopted the solvent vapor annealing (SVA) as reported in the literature.34−37 First, we have used THF for solvent vapor annealing and found that the PCEs 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 the literature35,38 due to its high vapor pressure as compared to those of other treatments, and these small molecules have medium solubility in CS2. Moreover, PC71BM exhibits better solubility in CS2 as compared to other solvents such as tetrahydrofuran. Therefore, the combination of high vapor pressure and medium solubility of the small molecule donor implies fast vapor 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 that of as cast OSCs. With

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

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 Jsc = 7.13 mA/cm2, Voc = 0.84 V, and FF = 0.34. The devices made with CS03:PC71BM also showed an efficiency close to 2.4% with Jsc = 8.61 mA/cm2, Voc = 0.73 V, and FF = 0.38. As described before, the higher Voc observed for CS01 is due to the deeper relative HOMO energy 13785

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

Article

The Journal of Physical Chemistry C

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), and PCE (Solar Cell Efficiency at 1 sun) for the Different Solar Cells Using CS01 and CS03 as Donor and PC71BM as Acceptor Jsc (mA/cm2) CS01 CS03 CS01 CS03 a

(as cast) (as cast) (SVA) (SVA)

7.13 8.61 10.48 12.81

(±0.08) (±0.06) (±0.08) (±0.10)

Voc (V) 0.84 0.73 0.79 0.67

FF

(±0.03) (±0.02) (±0.03) (±0.02)

0.34 0.38 0.58 0.62

(±0.023) (±0.024) (±0.05) (±0.07)

Jsc (mA/cm2)b

PCE (%) 2.03 2.39 4.80 5.32

a

(1.93) (2.32)a (4.73)a (5.25)a

7.01 8.49 10.37 12.73

(±0.05) (±0.07) (±0.09) (±0.08)

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

SVA treatment for 30 s, the OSC38 based on CS01 and CS03 showed overall PCEs 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 30 s, both Jsc and FF drastically drop, while the Voc remains almost constant. Since the SVA treatment, the crystallites of donor materials have grown; with the SVA treatment for a longer time, these crystallites are overgrown (>exciton diffusion length) and as large as the film thickness such that the active layer no longer forms the BHJ networks. The large domains are not desirable for charge generation, leading to the reduction in both Jsc and FF and the resulting reduction in the overall PCE. The increase in the Jsc for the OSCs after the SVA treatment has been also confirmed from the IPCE spectra of the 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 have been improved by the SVA treatment of the active layer, related to the improved nanoscale morphology and phase separation. The values of Jsc estimated from the integration of IPCE spectra (Table 1) of the devices are very close to the experimental values from J−V characteristics. The hole mobilities of CS01 and CS03 in the blend films were measured by the J−V characteristics on the hole only devices (Figure 7) and by 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 mobilities in the as cast and SVA treated active layers were measured using electron only devices in a manner similar

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.

to that for hole mobility and compiled in the table. There is a slight change in the electron mobility values after the SVA treatment. The improved hole mobility and better balance between the electron and hole mobility after the SVA treatment could promote charge transport and reduced charge recombi13786

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

Article

The Journal of Physical Chemistry C Table 2. Solar Cell Carrier Mobilities Measured Using the Space Charge Limited Current (SCLC) Model active layer CS01:PC71BM CS03:PC71BM CS01:PC71BM CS03:PC71BM

(as cast) (as cast) (SVA) (SVA)

hole mobility (cm2/(V s)) −5

2.56 (±0.09) × 10 4.19 (±0.11) × 10−5 7.89 (±0.13) × 10−5 9.87 (±0.15) × 10−5

nation, 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, and may be one of the reasons for high values of FF and PCE for former OSCs. Nonetheless, the solvent annealing process was indeed beneficial for the carrier collection. As illustrated in Figure 8

electron mobility (cm2/(V s))

ratio

2.13 (±0.11) × 10−4 2.28 (±0.09) × 10−4 2.42 (±10.3) × 10−4 2.48 (±0.12) × 10−4

8.32 5.44 3.06 2.51

values at maximum power output correspond to the charge transport and collection efficiency (Pcc)18 and are 0.73 and 0.78 for CS01 and CS03 based OSCs, respectively, suggesting 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 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 do 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. In order to get the information about the difference in the photovoltaic performance for 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 are shown in Figure 9. Both active layers showed a strong (100)

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.

the photocurrent density (Jph) plotted as a function of the effective voltage (Veff) shows significant differences between the solvent annealed thin films and the cast ones.39,40 The photocurrent density (Jph) can be expressed as the difference between the photogenerated current (Jlight) and the current in the dark (Jdark). The effective voltage (Veff) can also be defined as the difference between the voltage, in which the Jph is zero (V0), and the external voltage applied (Vext). It 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 it completely saturates at a 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 the CS03 based device than its CS01 counterpart due to the better phase separation. The Jph/Jphsat

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

diffraction peak at 2θ = 4.89°, which correspond to the lamellar distance of 1.98 nm; however, the (010) diffraction peak at 2θ = 21.14° and 22.04° for CS01 and CS03 corresponds to the π−π stacking distance of 0.46 and 0.43 nm, respectively. This suggests that CS03 forms a denser molecular packing than do CS01 and CS03, 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 corresponding to (100) and (010) planes, which indicates that the degree of crystalline nature is higher for the CS03:PC71BM blend film than that for CS01:PC71BM which is beneficial for charge transport and collection. In addition to these two peaks in 13787

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

The Journal of Physical Chemistry C

■

XRD, a weak diffraction peak around 2θ = 18° is also observed in all the blend films, and corresponds to the PC71BM.41 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, shown in Figure 10

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00840. Detailed synthesis of CS01 and CS03, as well as their full characterization, and theoretical methodology employed to obtain the UV−vis spectra of CS01 and CS03 along with the theoretical energy values of the HOMO and LUMO for CS01 and CS03 (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 10. TEM images of as cast and CS2 SVA treated CS03:PC71BM (1:2) blended thin films. Scale bar is 100 nm.

Emilio Palomares: 0000-0002-5092-9227 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

(only for CS03:PC71BM active layers). Similar TEM images were also observed for CS01:PC71BM. As can be seen from the TEM images, the as cast CS03:PC71BM thin film did not show any clear phase separation which limits the charge transport within the active layer toward the electrodes, resulting in low values of both Jsc and FF. However, for the sample shown after the SVA treatment with CS2, the blend film showed a larger domain (∼20 nm) and clear phase separation compared to those of the as cast film, revealing an interpenetrating path for the electrons and hole transportation toward cathode and anode, respectively, leading to the increase in both Jsc and FF and resulting in an improvement in PCE of the corresponding OSCs.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to acknowledge ICIQ-BIST (Severo Ochoa Excellence Accreditation 2014-2018 SEV-2013-0319) and ICREA for economic support. E.P. and A.V.-F. are also grateful to Spanish MINECO and the Agencia Estatal de Investigació n (AEI) for the CTQ2016-80042-R/AEI, CTQ2017-89814-P, and CTQ2014-60256-P/AEI projects and Generalitat de Catalunya for the AGAUR funding. E.P. and C.R.-S. also thank the MINECO and the AEI for the FPI grant. G.D.S. and S.B. are thankful to the Department of Science and Technology for financial support.

■

CONCLUSIONS In summary, we have synthesized two new light 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’s spectrum. These molecules show an 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 process between hole and electron mobility. While the hole mobility property of the thin bulk-heterojunction film improves noticeably, the electron mobility characteristics remain almost identical indicating improved balanced charge transport. Nevertheless, the solvent annealing has positive effects on the film morphology that lead to better charge collection as measured by monitoring the changes in Jph versus 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 mobilities are being carried out. These SMs can be used as donors with the low bandgap nonfullerene acceptors, and we are working in this direction.

■

REFERENCES

(1) Espinosa, N.; Hosel, M.; Angmo, D.; Krebs, F. C. Solar Cells with One-Day Energy Payback for the Factories of the Future. Energy Environ. Sci. 2012, 5, 5117−5132. (2) Liu, C.; Wang, K.; Gong, X.; Heeger, A. J. Low Bandgap Semiconducting Polymers for Polymeric Photovoltaics. Chem. Soc. Rev. 2016, 45, 4825−4846. (3) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153. (4) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666−12731. (5) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y. Single-Junction Polymer Solar Cells with High Efficiency and Photovoltage. Nat. Photonics 2015, 9, 174. (6) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (7) Liao, S.-H.; Jhuo, H.-J.; Yeh, P.-N.; Cheng, Y.-S.; Li, Y.-L.; Lee, Y.H.; Sharma, S.; Chen, S.-A. Single Junction Inverted Polymer Solar Cell Reaching Power Conversion Efficiency 10.31% by Employing Dual-Doped Zinc Oxide Nano-Film as Cathode Interlayer. Sci. Rep. 2015, 4, 6813. (8) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nature Energy 2016, 1, 15027.

13788

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789

Article

The Journal of Physical Chemistry C (9) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (10) Ma, D.; Feng, S.; Zhang, Z.; Li, C.; Yan, S.; Bo, Z. The Design of Highly Efficient Polymer Solar Cells with Outstanding Short-Circuit Current Density Based on Small Band Gap Electron Acceptor. Dyes Pigm. 2018, 150, 363−369. (11) Tuladhar, S. M.; et al. Low Open-Circuit Voltage Loss in Solution-Processed Small-Molecule Organic Solar Cells. ACS Energy Lett. 2016, 1, 302−308. (12) Ku, J.; Song, S.; Park, S. H.; Lee, K.; Suh, H.; Lansac, Y.; Jang, Y. H. Palladium-Assisted Reaction of 2,2-Dialkylbenzimidazole and Its Implication on Organic Solar Cell Performances. J. Phys. Chem. C 2015, 119, 14063−14075. (13) Roncali, J.; Leriche, P.; Blanchard, P. Molecular Materials for Organic Photovoltaics: Small Is Beautiful. Adv. Mater. 2014, 26, 3821− 3838. (14) Yuan, L.; Lu, K.; Xia, B.; Zhang, J.; Wang, Z.; Wang, Z.; Deng, D.; Fang, J.; Zhu, L.; Wei, Z. Acceptor End-Capped Oligomeric Conjugated Molecules with Broadened Absorption and Enhanced Extinction Coefficients for High-Efficiency Organic Solar Cells. Adv. Mater. 2016, 28, 5980−5985. (15) Deng, D.; Zhang, Y.; Yuan, L.; He, C.; Lu, K.; Wei, Z. Effects of Shortened Alkyl Chains on Solution-Processable Small Molecules with Oxo-Alkylated Nitrile End-Capped Acceptors for High-Performance Organic Solar Cells. Adv. Energy Mater. 2014, 4, 1400538. (16) Yuan, L.; Zhao, Y.; Zhang, J.; Zhang, Y.; Zhu, L.; Lu, K.; Yan, W.; Wei, Z. Oligomeric Donor Material for High-Efficiency Organic Solar Cells: Breaking Down a Polymer. Adv. Mater. 2015, 27, 4229− 4233. (17) Shen, X.-X.; Han, G.-C.; Yi, Y.-P. Multiscale Description of Molecular Packing and Electronic Processes in Small-Molecule Organic Solar Cells. Chin. Chem. Lett. 2016, 27, 1453−1463. (18) Varotto, A.; Nam, C.-Y.; Radivojevic, I.; P. C. Tomé, J.; Cavaleiro, J. A. S.; Black, C. T.; Drain, C. M. Phthalocyanine Blends Improve Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2010, 132, 2552−2554. (19) Wu, R.; Yin, L.; Li, Y. Π-Linkage Effect of Push-Pull-Structure Organic Small Molecules for Photovoltaic Application. Sci. China Mater. 2016, 59, 371−388. (20) Collins, S. D.; Ran, N. A.; Heiber, M. C.; Nguyen, T.-Q. Small Is Powerful: Recent Progress in Solution-Processed Small Molecule Solar Cells. Adv. Energy Mater. 2017, 7, 1602242. (21) Cai, Y.; Huo, L.; Sun, Y. Recent Advances in Wide-Bandgap Photovoltaic Polymers. Adv. Mater. 2017, 29, 1605437. (22) Deng, D.; et al. Fluorination-Enabled Optimal Morphology Leads to over 11% Efficiency for Inverted Small-Molecule Organic Solar Cells. Nat. Commun. 2016, 7, 13740. (23) Wan, J.; Xu, X.; Zhang, G.; Li, Y.; Feng, K.; Peng, Q. Highly Efficient Halogen-Free Solvent Processed Small-Molecule Organic Solar Cells Enabled by Material Design and Device Engineering. Energy Environ. Sci. 2017, 10, 1739−1745. (24) Kan, B.; et al. A Series of Simple Oligomer-Like Small Molecules Based on Oligothiophenes for Solution-Processed Solar Cells with High Efficiency. J. Am. Chem. Soc. 2015, 137, 3886−3893. (25) Mohammad, L.; Chen, Q.; Mitul, A.; Sun, J.; Khatiwada, D.; Vaagensmith, B.; Zhang, C.; Li, J.; Qiao, Q. Improved Performance for Inverted Organic Photovoltaics Via Spacer between Benzodithiophene and Benzothiazole in Polymers. J. Phys. Chem. C 2015, 119, 18992− 19000. (26) Zeng, J.; Zhang, T.; Zang, X.; Kuang, D.; Meier, H.; Cao, D. Da-P-a Organic Sensitizers Containing a Benzothialzole Moiety as an Additional Acceptor for Use in Solar Cells. Sci. China: Chem. 2013, 56, 505−513. (27) Ku, J.; Lansac, Y.; Jang, Y. H. Time-Dependent Density Functional Theory Study on Benzothiadiazole-Based Low-Band-Gap Fused-Ring Copolymers for Organic Solar Cell Applications. J. Phys. Chem. C 2011, 115, 21508−21516.

(28) Badgujar, S.; Lee, G.-Y.; Park, T.; Song, C. E.; Park, S.; Oh, S.; Shin, W. S.; Moon, S.-J.; Lee, J.-C.; Lee, S. K. Organic Solar Cells: High-Performance Small Molecule Via Tailoring Intermolecular Interactions and Its Application in Large-Area Organic Photovoltaic Modules. Adv. Energy Mater. 2016, 6, 1600228. (29) Wong, M. W.; Wiberg, K. B.; Frisch, M. J. Solvent Effects. 3. Tautomeric Equilibria of Formamide and 2-Pyridone in the Gas Phase and Solution: An Ab Initio Scrf Study. J. Am. Chem. Soc. 1992, 114, 1645−1652. (30) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (31) Balakrishna, B.; Bauzá, A.; Frontera, A.; Vidal-Ferran, A. Asymmetric Hydrogenation of Seven-Membered C = N-Containing Heterocycles and Rationalization of the Enantioselectivity. Chem. - Eur. J. 2016, 22, 10607−10613. (32) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange− Correlation Functional Using the Coulomb-Attenuating Method (Cam-B3lyp). Chem. Phys. Lett. 2004, 393, 51−57. (33) Kupfer, S.; Guthmuller, J.; González, L. An Assessment of Rasscf and Tddft Energies and Gradients on an Organic Donor−Acceptor Dye Assisted by Resonance Raman Spectroscopy. J. Chem. Theory Comput. 2013, 9, 543−554. (34) Wang, J.-L.; Wu, Z.; Miao, J.-S.; Liu, K.-K.; Chang, Z.-F.; Zhang, R.-B.; Wu, H.-B.; Cao, Y. Solution-Processed DiketopyrrolopyrroleContaining Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing. Chem. Mater. 2015, 27, 4338−4348. (35) Más-Montoya, M.; Janssen, R. A. J. The Effect of H- and JAggregation on the Photophysical and Photovoltaic Properties of Small Thiophene−Pyridine−Dpp Molecules for Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2017, 27, 1605779. (36) Kumar, C. V.; Cabau, L.; Viterisi, A.; Biswas, S.; Sharma, G. D.; Palomares, E. Solvent Annealing Control of Bulk Heterojunction Organic Solar Cells with 6.6% Efficiency Based on a Benzodithiophene Donor Core and Dicyano Acceptor Units. J. Phys. Chem. C 2015, 119, 20871−20879. (37) Fernandez, D.; et al. Understanding the Limiting Factors of Solvent-Annealed Small-Molecule Bulk-Heterojunction Organic Solar Cells from a Chemical Perspective. ChemSusChem 2017, 10, 3118− 3134. (38) Li, Y.; Lee, D. H.; Lee, J.; Nguyen, T. L.; Hwang, S.; Park, M. J.; Choi, D. H.; Woo, H. Y. Two Regioisomeric Π-Conjugated Small Molecules: Synthesis, Photophysical, Packing, and Optoelectronic Properties. Adv. Funct. Mater. 2017, 27, 1701942. (39) Yu, J.; Yin, X.; Xu, Z.; Deng, P.; Han, Y.; Zhou, B.; Tang, W. Bisalkylthio Side Chain Manipulation on Two-Dimensional Benzo[1,2-B:4,5-B′]Dithiophene Copolymers with Deep Homo Levels for Efficient Organic Photovoltaics. Dyes Pigm. 2017, 136, 312−320. (40) Lin, Y.; et al. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138, 4955−4961. (41) Chen, W.; et al. Hierarchical Nanomorphologies Promote Exciton Dissociation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Nano Lett. 2011, 11, 3707−3713.

13789

DOI: 10.1021/acs.jpcc.8b00840 J. Phys. Chem. C 2018, 122, 13782−13789