Charge Transfer Dynamics and Device Performance of

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Cite This: ACS Appl. Energy Mater. 2018, 1, 4776−4785

Charge Transfer Dynamics and Device Performance of Environmentally Friendly Processed Nonfullerene Organic Solar Cells Luana Cristina Wouk de Menezes,†,‡ Yingzhi Jin,‡ Leandro Benatto,† Chuanfei Wang,‡ Marlus Koehler,† Fengling Zhang,*,‡ and Lucimara Stolz Roman*,† †

Department of Physics, Federal University of Paraná, Curitiba 81531-990, Paraná, Brazil Department of Physics Chemistry and Biology (IFM), Linköping University, Linköping SE-581 83, Sweden

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‡

S Supporting Information *

ABSTRACT: In the last years, one of the pursuits has been to replace the use of halogenated solvent during the processing of organic photovoltaic (OPV) devices. Herein, we investigate the nonhalogenated solvent, o-methylanisole (o-MA) and the wellstabilized o-dichlorobenzene (o-DCB) to process the bulk heterojunction (BHJ) based on PTB7-Th donor (D) and 3,9-bis(2methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2b:5,6-b′] dithiophene) (ITIC) acceptor (A). The formation of D−A interfaces with different (solvent-dependent) characteristics was verified by steady-state photoluminescence and morphological and electrical measurements. These measurements show a rather comparable device efficiency of the PTB7-th:ITIC BHJ processed by o-MA (compared to the device processed using o-DCB) despite the lower absorption of the films and the lower VOC. Also, the charge-transfer (CT) state formation was investigated and the reasons behind the VOC losses were correlated to the interface variations when processed by different solvents. Some experimental results are then discussed in light of the electronic structure of the molecules calculated using the density functional theory (DFT) method. The comparison between the experimental data and the theoretical calculations give some insights about the microscopic processes involved in the variation of the devices properties processed using the o-DCB and o-Ma solvents. We concluded that the D−A distance clearly affects the CT state energy and consequently the VOC. Furthermore, higher air stability is observed when the active layer is processed using o-MA instead of oDCB. The better stability was observed in self-lifetime measurements and air-processed devices. KEYWORDS: green solvents, nonfullerene solar cells, voltage losses, charge transfer state, density functional theory, air stability of nontoxic and sustainable resources,8 the suitable preparing conditions and a device with high-stability.9 A safe and environmentally friendly process is the main requirement for the industrial manufacturing.10 A good photovoltaic response of organic solar cells is achieved with a delicate and extremely experimental relationship between the solvents, the additive, the substrate, and the processing methods. The appropriate solvent selection is one of the

1. INTRODUCTION The efficiency of solution-processable bulk heterojunction (BHJ) organic photovoltaic (OPV) devices based on nonfullerene (NF) acceptors has been increased over the years,1−3 reaching the milestone power conversion efficiency of 14%.4 These great efforts give a new perspective to OPV community, transiting the device toward commercialization as a promisingattractive source of renewable energy.5 To take full advantage of the outstanding advantages of OPV, such as light weight and flexibility,6 improvements should be done in the tools and process necessary for large-area printing.7 For instance, the use © 2018 American Chemical Society

Received: June 1, 2018 Accepted: August 22, 2018 Published: August 22, 2018 4776

DOI: 10.1021/acsaem.8b00884 ACS Appl. Energy Mater. 2018, 1, 4776−4785

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ACS Applied Energy Materials

Figure 1. (a) Chemical structure of the polymer donor, PTB7-Th, and the small molecule acceptor, ITIC. Absorption coefficient of the films composed by pristine (b) PTB7-Th, (c) ITIC, and (d) the blend (D:A = 1:1.3) in different solvents.

solvent processed nonfullerene OPV (11.34%) were reported.19 Other types of solvent have been reported for preparing highly efficient nonfullerene solar cells, for example, xylene,20−22,24 toluene,23 trimethylbenzene (TMB),22,24 mesitylene,22,23 and tetrahydrofuran derivatives.24−26 However, those solvents are still classified as environmentally hazardous in the chemical safety database.27 Several reports have investigated the use of additives such as 1,8-diiodooctane (DIO), N-methyl pyrrolidone (NMP), or diphenyl ether (DPE) in a nonhalogenated solvent. Although the binary combination can enhance the miscibility of the BHJ, it can also reduce reproducibility when applied on a large scale, along with severe oxidative degradation.28 A single solvent is still the main pursuit to process the materials. o-Methylanisole (o-MA) can be a good choice to replace the use of halogenated solvent in all polymer29 and fullerene-based solar cells.30 The attractive advantage of o-MA is its use in the industry as a flavor and food additive.27 The replacement of fullerenes by small-molecule NF acceptors is a promising approach to reduce the voltage losses through radiative and nonradiative recombination. However, a larger reduction in the Voc is observed in some eco-friendly NF devices than in their corresponding halogenated one.21−23 It was proven that the donor (D)− acceptor (A) material interface plays an important role in the operation of OPV, and the charge-transfer (CT) state formation is directly related to the Voc values.31−34 The CT states are first populated by transfer from photoinduced charges, which lead to a significant offset compared to the optical bandgap (Egap). This energy offset is one of the main factors behind the large Voc loss13 in OPVs. Hence minimizing the difference between the Egap and ECT can be a possible strategy to improve the open circuit voltage in those devices. The components used to form the active-layer film are factor responsible for the device stability.35 From the commercializa-

prior parameters to polymer semiconductor solutions. Poor solubility can lead to aggregate formation and changes in the viscosity during the coating process. The halogenated solvents are generally good solvents.11 In large-mass production, a high quantity of solvent is necessary to process the active layer, which currently uses solvents based on hazardous components such as chloroform, chlorobenzene, or o-dichlorobenzene.12−14 The use of halogenated solvents and/or halogenated additives derives from the good solubility of organic materials in those kinds of solvents, which leads to a fine intermix into the BHJ of solar cell device. On the other hand, the highly toxic and carcinogenic effect of these common solvents give an unsuitable setting, remaining an obstacle between the lab results and upscaling. Even with the use of individual protection, the wrong disposal of halogenated solvent leads to a long-term source of groundwater contamination.15 Therefore, it is essential to develop OPV using an environmentally friendly solvent that achieves performance comparable to or even higher than that obtained from halogenated solvents. In addition, emphasis on green solvent processing has the potential to make the OPV a solar-energy alternative that does not use toxic compositions.10 The most environmentally friendly solvent is water. Some studies have shown the possibility of diluting the organic materials by adding hydrophilic side groups in an organic semiconductor.16 Another attempt is the use of mini-emulsion technique to provide both water solubility17 and tune the BHJ mixture into the nanoparticle.18 However, in both cases, the low efficiency still limits further application. Although several studies have been successfully replacing the use of hazardous solvents by lesser harmful, few such studies are reported to the nonfullerene small molecules.19−24 Combining the tetrahydrofuran (THF) and the isopropanol (IPA), tuning the energy level of the new donor-polymer (PBQ-4F) with the ITIC, the highest PCE of halogen-free 4777


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Figure 2. (a) Schematic structure of the solar cell used in this study. (b) J−V characteristic curves (under illumination) for the device with the PTB7-Th:ITIC BHJ processed from o-MA and o-DCB. (c) Photocurrent density versus effective voltage (Jph−Veff) curves of devices. (d) Jph/Jsat− Veff curves of the PTB7-Th:ITIC BHJ processed from o-MA and o-DCB.

2. RESULTS AND DISCUSSION The chemical structure of fluorinated polymer donor PTB7-Th is given in Figure 1a. The PTB7-Th is a low-band-gap donor polymer that has high performance and is commonly used in OPV devices.37 The absorption spectrum of PTB7-th is slightly red-shifted when processed by o-MA in comparison with the oDCB (Figure 1b). Different film density will directly affect their optical properties, as also observed in the references.38,39 This difference in the film density will lead to an absorption intensity reduction, because lower light will interact with the film as there are lower polymer chains. Observing the optical properties of PTB7-Th film, there is a reduction in the absorption intensity when the film is processed by the green solvent compared to the halogenated one. As shown in Figure 1c, the small-molecule acceptor ITIC can induce intramolecular-charge transfer to extend the absorption spectra. The ITIC has been intensively investigated together with their derivatives to replace the commonly used fullerene as electron acceptor in OPVs. The absorption spectra of ITIC, displayed in Figure 1c, exhibits the first peak slightly more intense to film processed by o-Ma than that processed by o-DCB. This modification suggests some molecular self-organization with different solvent. The absorption profile of the PTB7-Th:ITIC (1:1.3) films processed by different solvent in the Figure 1d, displays higher absorption to the BHJ film processed by o-DCB than the o-Ma.

tion point of view, it is crucial to avoid (or delay) the degradation process of OPV devices, which has intrinsic features depending on the nature of the interface and materials involved.28 Considering this issue, few studies in the literature explore the device shelf-stability interconnection with the kind of solvent applied to process the active layer. The relationship between morphological and photovoltaic properties of NF with different solvent processing remains elusive and unexplored for the films processed by nonhalogenated solvent. In this work, we attempted to investigate the differences on the photoactive layer comprising a polymer donor, PTB7-th, and an NF acceptor, ITIC, when processed using a solvent that is currently used to process the active layer of OPV and has already been reported in the PTB7-Th:ITIC system (o-DCB) or when processed using the new o-MA solvent. The BHJ using the PTB7-th and ITIC give a PCE of 6.80% in solution processed by o-DCB with the structure ITO/ PEDOT:PSS/PTB7-Th:ITIC/PDIN/Al.36 A binary solution of toluene and DPE (diphenylether) to form the BHJ film in the ITO/ZnO/PTB7-TH:ITIC/MoO3/Ag structure, report 7.09% of PCE. Even with toluene being a nonhalogenated solvent, as discussed above, its toxic level is still high. Our findings here point out that the o-MA is a new nonhalogenated solvent, which can be used to replace hazardous solvents in OPV based on small-molecule NF acceptors. 4778


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Table 1. Photovoltaic Parameters of the OPVs Based on PTB7-Th:ITIC BHJ, with D:A Ratio of 1:1.3 (Average Values of 20 Devices Are Provided in Parentheses) devices

thickness (nm)

o-MA o-DCB o-DCB

120 ± 5 100 ± 5 120 ± 5

Jsc (mA cm−2) 13.31 13.91 14.65

13.40 ± 0.40 13.71 ± 1.00 14.0 ± 1.00

Voc (V) 0.79 ± 0.01 0.82 ± 0.01 0.83 ± 0.01

0.79 0.82 0.83

FF (%) 53.7 54.7 45.7

PCE (%)

52.2 ± 1.0 53.7 ± 0.5 44.3 ± 1.0

5.69 6.21 5.58

5.36 ± 0.30 5.90 ± 0.20 5.12 ± 0.30

average E is the same for both devices, this result indicates that the product μe(h)τe(h) is higher for the layer processed using the o-Ma. This effect generates a different balance of free charge carriers between the two devices. To improve our understanding of OPV performance upon different solution processing, we investigated the maximum exciton generation rate (Gmax) of characterized devices. The exciton generation rate Gmax value can be deduced using the equation JSat = qLGmax, with Jsat as the saturated photocurrent density, q the electron charge, and L the thickness of the active layer. The value of saturation current is limited by the amount of photons absorbed by the active layer.45 Thus, the Gmax and the Jsat of o-DCB are about 30% higher than those of the o-Ma under AM 1.5G illumination. This result can be attributed to higher absorption coefficient of the PTB7-TH:ITIC blend when processed by o-DCB (Figure 1d). The exciton dissociation probability P(E,T), which is the probability that an electron and hole generated at a temperature T and field E will escape recombination, can be calculated using the equation Jph = qGmaxP(E,T)L and the expression for the saturated photocurrent density showed above (P(E,T) = Jph/Jsat).46 The P(E,T) is represented in terms of the Veff in the Figure 2d. At the short-current densitycondition, the photocurrent is 92, 87, and 90% of the saturation current density for devices with o-Ma, o-DCB at 120 nm, and o-DCB at 100 nm, respectively. Furthermore, in the low effective voltage range (Veff < 0.3 V), there is a larger difference in the Jph − Veff curve characteristics for the devices processed by o-Ma and o-DCB. Because the P(E,T) is basically the efficiency on exciton dissociation and charge collection, a decrease could be due to either a reduction at exciton dissociation efficiency or charge collection efficiency. It is clear from the measurements in Figures 2c, d that the PTB7TH:ITIC blend film processed by o-Ma generates and/or collects more free charge carriers than the blend film processed by o-DCB, which partially compensates its lower absorption coefficient. As a result, the OPV fabricated using o-Ma had a Jsc magnitude comparable to the magnitude in the device fabricated using o-DCB. The results above are in agreement with estimates of the hole and electron motilities obtained by the space-charge-limitcurrent (SCLC) method. The hole and electron mobilities were calculated from the J−V curves (Figure SI3) of the singlecarrier device. The mobilities were measured for film thickness which had the best performance (120 nm to o-Ma and 100 nm to o-DCB). The hole mobilities of 7.9 × 10−4 cm V−1 s−1 and 1.3 × 10−4 cm V−1 s−1 and electron mobilities of 2.4 × 10−5 cm V−1 s−1 and 8.0 × 10−5 cm V−1 s−1 have been deduced for the devices processed from o-DCB and o-Ma solutions, respectively. Those results suggest that a slightly more balanced charge transport in the device can be achieved using the o-Ma solvent, which is in accordance with the better extraction of photogenerated carriers observed in Jpf − Veff curves. This result is also one of the reasons why the FF of the o-Ma device is greater than the FF of the o-DCB with the same thickness

The difference in the absorption coefficient comes from the distinct formation of film when using the nonhalogenated solvent. The use of o-Ma systematically reduces the absorption of the blend. Because the measurements of Figure 1d were taken in films with approximately the same thickness (120 nm, see Discussion below), the reduction of the absorption indicates that the layer processed by o-Ma is less dense than the layer processed by o-DCB.38,39 Conventional solar cells based on a bottom anode composed by PEDOT:PSS-modified ITO and a top LiF/Al bilayer cathode were fabricated. A schematic description of the device used in this work is represented in Figure 2a. As reported, the desired donor−acceptor proportion is 1:1.3 processed by oDCB.36 To observe the device performance in film processed by o-MA, we investigated the donor−acceptor proportion and we found that the best performance is acquired with 1:1.3. Via careful optimization, the thickness of the active layer is 120 nm for o-MA. The details can be found in the Figures S1 and S2. The current density−voltage (J−V) curves of the devices under AM 1.5G (100 mW cm−2) are represented in Figure 2b and the corresponding photovoltaic parameters are summarized in Table 1. OPVs without any treatment processed from oMA exhibited good PCE approaching 6%. To the best of our knowledge, this result was the best performance reported using anisole as a single solvent on nonfullerene-based OPV. The best BHJ OPV using o-MA (120 nm) showed a PCE of 5.69% with Voc of 0.79 V, Jsc of 13.31 mA cm−2, and FF of 53.7%, whereas the best device processed from o-DCB exhibited a PCE of 6.21% with Voc of 0.82 V, Jsc of 13.91 mA cm−2, and FF of 54.7% at 100 nm of thickness. Although both devices are composed of the PTB7-Th:ITIC blend, the optimization of performance results in different film thickness. Considering the same film thickness (120 nm), the performance of o-DCB is a PCE of 5.58% with Voc of 0.83 V, Jsc of 14.65 mA cm−2, and FF of 45.7%. It is observed that, even with the higher values of Voc and Jsc, the o-DCB showed lower FF which was the responsible to decrease in the efficiency. In this case (120 nm), the PCE of o-MA showed slightly higher than o-DCB. Note that lower coefficient absorption of o-MA induced slightly lower current density than that of the device processed from o-DCB at the same thickness. However, the dependence of photocurrent on voltage close to short circuit condition is low, which can be attributed to dissociation efficiency of photogenerated excitons. The Figure 2c shows the plotting of photocurrent density (Jph) as a function of the effective voltage (Veff). Following an established procedure,2,40−43 in a large reverse bias, the photocurrent saturates in both films. Under this condition, all the electron−hole pairs are dissociated and collected in their respective electrodes.44 The electron and hole drift lengths (we(h)), expressed by we(h) = μe(h)τe(h)E (τ is the lifetime of free charges and E is the electric field in the device), at high voltage are then equal or larger than the sample thickness.44 It is clear that at the same thickness, the saturation voltage is lower for o-Ma than for o-DCB. Thus, considering that that 4779


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ACS Applied Energy Materials (120 nm), and even comparable to the FF of the o-DCB device with a thinner active layer (100 nm), that tends to decrease the series resistance. To further investigate the exciton-dissociation efficiency in the blend films, we employed steady-state photoluminescence (PL). The emission quenching in PL measurements indicates the charge transfer efficiency at donor/acceptor interface. In contrast to the behavior observed for the absorption coefficient, the PL spectra of PTB7-th (Figure 3) is more

4, the AFM height and phase images exhibited no crystalline domains in films from both solvents. The phase topography

Figure 4. AFM for the blend films of PTB7-Th:ITIC (1:1.3): topography and phase images of films prepared with (a, b) o-DCB and topography and phase images of films prepared with (d, e) o-MA. TEM images of the BHJ film prepared with (c) o-DCB and (f) o-MA.

Figure 3. Photoluminescence spectra (exc nm) for the pristine ITIC and PTB7-Th, and the PTB7-Th:ITIC heterojunction (1:1.3) films processed from (a) o-DCB and (b) o-MA. In all the cases, the emission is corrected for the absorption in the excitation (635 nm).

exhibits smaller phase separation domains to films processed from the nonhalogenated solvent than the o-DCB solvent, Figure 4e, b, respectively. Transmission electron microscopy (TEM) was used to probe the BHJ processed from o-DCB (Figure 4c) and o-MA (Figure 4f). The result reveals that the active layer processed from o-Ma showed lower phase separation of donor and acceptor, with the number of large domains lower than in films of o-DCB. The PL, AFM, TEM, and Jph−Veff measurements above suggest that the reasonable performance of the device prepared using nonhalogenated solvent is the result of a better miscibility between the materials of the blend. The improved contact between the donor and the acceptor gave rise to a higher exciton dissociation and charge collection efficiency. This effect increases JSC so that it has a magnitude comparable to the device processed by o-DCB (despite lower absorption coefficient of the film). However, the OPV fabricated using oMA has a higher Voc loss relative to the solar cell processed by o-DCB. This loss limits the performance of the device using oMA. It was already reported that the use of nonhalogenated solvents usually reduces Voc relative to devices fabricated using halogenated solvents.19−24 We further examined these energy

intense for films processed from o-Ma than the films processed from o-DCB. This result suggests that because of the reduction in the density of the film, there is more space between the polymeric chains resulting in a reduction on nonradiative selfquenching for the film prepared with o-Ma, reaffirming the hypothesis of different film density. In this way, PTB7TH:ITIC active layer processed from different solvent exhibited a PL quenching compared to the pristine materials (Figure 3a, b). However, in the case of the active layer processed from o-MA (Figure 3b), a PL quenching of 96% is observed. This PL quenching is slightly higher than the observed in films of PTB7-Th:ITIC processed from o-DCB. One possible explanation to this trend maybe found when one assumes that the lower absorption coefficient of the blend processed by o-MA is the result of a lower density of the active layer. The higher free space between the polymer’s chain may allow the penetration of the ITIC molecule to the inners part of the PTB7-Th during the film formation. This performance suggests better molecular miscibility between the materials in the o-MA film, indicating that dissociation probability of excitons in that film is higher than the o-DCB. The film miscibility and surface morphology are investigated using atomic force microscopy (AFM). As displayed in Figure 4780


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ACS Applied Energy Materials losses in our samples by measuring properties related to details of the interaction between the donor and acceptor materials. The CT exciton is weakly bound at the donor/acceptor interface, which can be populated by photoinduced electron transfer after excited donor or acceptor, or direct absorption of photon energy lower than donor−acceptor band gap. In binary solar cells, the energy difference between the fully relaxed CT state and the ground state of the blend is defined as the energy of the CT state.34 It has been widely observed that there is a correlation between the CT state energy (ECT) and the values of Voc. In the BHJ system, the analytical expression of Voc as a function of ECT, considering the losses due to the radiative (ΔVrad) and nonradiative (ΔVnonrad) paths (the equation details are presented in Supporting Information), is given by eq 1:34 Voc =

ECT − ΔVrad − ΔVnonrad q

(1)

From eq 1, it is clear that Voc decreases when ECT is reduced. Radiative decay of the CT excitons can be observed in electroluminescence (EL) spectra using a forward voltage over the photovoltaic device. In this view, we found a red-shift in the peaks of PTB7-Th:ITIC compared to pristine materials (Figure SI4). This is a result of lower emission of CT state. To confirm the theory for the BHJ system based on nonfullerene processed from nonhalogenated devices, we performed Fourier transform photocurrent spectroscopy (FTPS) technique to measure the external quantum efficiency of the CT state absorption. In the region below bandgap, the EQE increase and red-shift when processed by o-Ma. Considering the Marcus theory of the CT absorption cross-section,47 the relation between the EQE of CT absorption and ECT is EQE =

ij −(ECT − λ − E)2 yz zzz expjjj j z 4λkT E (4πλkT ) k {

Figure 5. FTPS-EQE spectrum for the PTB7-Th:ITIC film in (a) oDCB and (b) o-MA to deduce the ECT value and parameters fitting of eq 2 (feλ are 1.1 × 10−3 eV and 0.14 eV for o-DCB and 1.1 × 10−3 eV and 0.15 eV for o-MA, respectively).

relatively the same. Although higher losses can be observed in BHJ system using eco-friendly solvent, it demonstrates that this approach can quantify the system using halogenated and ecofriendly solvent. Finally, it can indicate that the reduction on Voc of the PTB7-th:ITIC solar cell processed by o-Ma is mainly a consequence of low-CT energy. To get some insights about the possible mechanisms behind the variations of ECT induced by different solvents, we performed theoretical calculations using density functional theory (DFT) with M0648 hybrid functional and 6-31G(d) basis sets. In this study, we considered two complexes formed by a 3-mer oligomer of PTB7-Th interacting with one ITIC molecule. We then calculated ECT with different cofacial distances among the units. The idea is to investigate how the energy of the CT state changes with those distances. This is a very simplified approach to simulate microscopic fluctuations in the density of the PTB7-Th:ITIC blend. We found that slightly variations in the separation between PTB7-Th and ITIC can produce changes in ECT. Hence, we considered two distinct systems (numbered 1 and 2) where the distance between the polymer’s oligomer and the ITIC molecule are 3.6 and 3.9 Å (details of DFT calculations in the Supporting Information). The charge transfer state with the lowest energy in both complexes was the one with the HOMO (highest occupied molecular orbital) mainly localized in PTB7-th oligomer and the LUMO (lowest unoccupied molecular orbital) mainly localized in the ITIC molecule (Figure 6). With the simulations, we observe the decrease of charge transfer energy defined by ECT = EHOMO − ELUMO when the distance between molecules is larger (Table 2). The values of ECT calculated using DFT are about 0.5 eV higher compared to the experimental values. Yet this difference was expected since we are considering isolated molecules in the gas phase. Polarization effects of the surrounding molecules in the solid state tend to decrease the values of ECT calculated

f

(2)

The value of the ECT deduced by the EQEPV is 1.41 eV to oDCB and 1.38 eV to o-Ma. ECT of the PTB7-Th:ITIC solar cell processed by o-DCB is slightly higher than the value for the devices processed by o-Ma. From the CT state EQE fitting, we find that the radiative losses (ΔVrad) in eq 1 are 0.248 and 0.249 V for o-DCB and oMa, respectively. In this case, the similar radiative recombination loss is due to logarithmical dependence on the density of CT, so it is possible to assume approximately the same radiative-recombination loss for the films processed from halogenated, o-DCB, and nonhalogenated, o-Ma, solvents. If the device presents only radiative recombination of the charge carrier, i.e., electroluminescence external quantum efficiency (EQEEL) is close to 1, the slightly higher performance would be observed on o-DCB with a Voc of 1.16 V than 1.13 V to oMa. We also investigated the influence of solvent on the Voc losses through nonradiative recombination quantified by measuring EQEEL. A slightly higher EQEEL is obtained for the o-DCB solar cell compared to o-Ma (Figure S5). This results in a slightly lower nonradiative recombination loss when the film is processed by halogenated solvent. As predicted in eq 1, the Voc can be calculated subtracting the energy losses due to radiative and nonradiative (0.342 and 0.341 V for films processed from o-DCB and o-MA, respectively) from CT state energy. As shown in the Figure 5b, c, for both systems, the measured and deduced Voc are 4781


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Figure 6. Molecular orbitals correspondents to charge transfer state of complexes of PTB7-Th:ITIC. The distance between PTB7-Th and ITIC in angstroms was 3.6 for complex 1 and 3.9 for complex 2.

Table 2. HOMO, LUMO, and ECT Energies (eV) of Simulated Complexes in Figure 6 complex

HOMO

LUMO

ECT

1 2

−5.08 −5.07

−3.17 −3.18

1.91 1.89

for the gas phase.49 The longest distance between PTB7-th and ITIC (which simulates a decrease in the density of the film) makes the charge transfer state more spatially separated giving rise to a lower hybridization between the HOMO of the donor with the LUMO of the acceptor. As discussed above, the blend processed using o-MA solvent is less dense and shows a lower ECT compared to the blend processed using o-DCB. The theoretical analysis suggests that those differences in the CT interactions might be induced by microscopic variations in the average distance between the PTB7-Th and ITIC when the blend is processed with the two different solvents. Device Stability. To correlate device performance and stability with nonfullerene BHJ device processed from the halogenated and eco-friendly solvent, we further analyzed an air-tested and an air-processed method. Following the ISOS-D1, the standard consensus to measure the air stability of solar cell devices,50 the devices here studied were kept in dark, at ambient atmosphere without encapsulation and evaluated during 2.5 h. The stability was evaluated by J−V measurements at ambient conditions under AM 1.5G. The Figure 7a reports the OPV efficiency as a function of the storage time. It is observed that the device clearly changed after almost 1 h of storage. In the firsts minutes, both devices exhibited similar decrease compared as beginning values. However, after this time the device made with o-Ma presented better self-stability. Thus, a trend between the solvent processing and device stability is observed, suggesting that devices made with o-Ma displays better stability. The use of iodine additives was already investigated to reduce the device’s stability,28 but the effect of halogen atoms has not been established yet. However, similar results have been reported implying that the halogenated solvents, besides being harmful and costly, they also exhibit lower time of air stability than the eco-friendlier solvent.23 Next, we tested the air-processed OPV to investigate the stability tendency. The devices were made following the same structure as previously reported, save the active-layer deposition step. In this case, PTB7-th:ITIC blends were deposited in air from o-DCB and o-Ma solution. Then, the devices were transferred to the vacuum chamber to electrodes

Figure 7. Stability evaluation of the air-tested and air-processed OPV performance composed by PTB7-Th:ITIC (1:1.3) in o-DCB and oMA. (a) The air-tested devices were made at glovebox and investigated for more than 2 h in environmental conditions. (b) The active layer of the air-processed devices was spin-coated at room conditions.

deposition. Figure 7b illustrates the J−V curves under AM 1.5G illumination of the PTB7-th:ITIC film deposited in air from different solution with the respective parameters represented in Table S1. We clearly observe a decrease in JSC and FF in both devices. This consequently reduces the device efficiency to 4.57 and 4.35% to o-Ma and o-DCB, respectively. However, the air-processed devices fabricated with o-Ma do reduce just 13% in density of current while the o-DCB was 25% compared to devices prepared in glovebox condition. Because there is less material to interact with the oxygen, the better stability can also be a consequence of lesser density of film. However, it can be also a consequence of the better morphology or even the small amount of remaining solvent. 4782


Article

ACS Applied Energy Materials Further studies should be performed to better understand the relationship between the stability and solvent processing of active layer. The better performance of the device suggests that the PTB7-th: ITIC film has greater air stability when the blend was previously diluted in o-Ma.

3. CONCLUSIONS A single eco-friendly solvent has been employed to fabricate PTB7-Th devices with ITIC, resulting in a power conversion efficiency of 5.69%. The question remains: is this result comparable to that of a commonly used halogenated solvent? A profound understanding of the difference using distinct solvent is a significant step to providing future developments of OPVs that are processed by environmentally benign solvents. Here, experimental and theoretical studies were presented to clarify this question. In conclusion, we have demonstrated the connection between solvent and properties of films based on nonfullerene small molecule acceptor. We have elucidated how a nonhalogenated solvent (o-MA) can influence the photovoltaic properties compared to a halogenated solvent (o-DCB). We have probed how the optical, morphological, transport, and electronic properties changes when the BHJ is processed with a halogenated and an eco-friendly solvent. Here we proposed that the film properties are solvent-dependent, exhibiting a less dense and lower ECT of the blend prepared using o-MA compared to the blend using o-DCB. DFT calculations were performed for the complex structure aiming to understand the microscopic variations associated with these observed effects. The theoretical analysis suggests that those differences in the CT interactions might be induced by microscopic differences in the average distance between the PTB7-Th and ITIC when the blend is processed with the two different solvents. The distance between donor and acceptor units clearly affect the ECT energy. A slightly higher distance in films prepared by the o-MA exhibited lower ECT values than the film of o-DCB (which is verified using the experimental technique FTPSEQE) that is visibly impacting the VOC results. The formation of D-A-interfaces with different characteristics show a rather comparable device efficiency of the PTB7-th:ITIC BHJ processed by o-MA despite the lower absorption of the films and the lower VOC (the balance comes from better exciton dissociation and charge collection of films prepared by o-MA than those prepared by o-DCB). We have also observed the air-tested and air-processed devices. The results observed here showed that the o-MA is slightly more stable in air than the oDCB. In our study, we firmly indicate that the morphologyassociated parameters that influence the photovoltaic performance can be investigated combining our approach using experimental and theoretical data. In addition, the findings here clearly exhibit that the o-MA can efficiently replace the use of hazardous solvent to process the active layer of a OPV based on a nonfullerene small molecule acceptor.

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complex; device performance at different D/A ratio or active layer thickness; J−V characteristic curves of singlecarrier devices; electroluminescence and electroluminescence quantum efficiency spectra of solar cell device; photovoltaic parameters of the air-processed OPV (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: lsroman@fisica.ufpr.br. ORCID

Luana Cristina Wouk de Menezes: 0000-0002-1843-0741 Marlus Koehler: 0000-0001-9935-5060 Fengling Zhang: 0000-0002-1717-6307 Lucimara Stolz Roman: 0000-0001-6567-5920 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University [200900971 to F.Z.]. L.C.W.M., L.B., M.K., and L.S.R. also thank CNPq, CAPES, Grant Aneel/ COPEL PD 2866-0470-2017, CAPES-PDSE (process number 88881.134822/2016-01) for partial financial support and CENAPAD for providing the computational facilities. We acknowledge Centro de Microscopia of UFPR for supporting the TEM images.

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REFERENCES

(1) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with Over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734−4739. (2) Kwon, O. K.; Park, J.-H.; Kim, D. W.; Park, S. K.; Park, S. Y. An All-Small-Molecule Organic Solar Cell with High Efficiency Nonfullerene Acceptor. Adv. Mater. 2015, 27, 1951−1956. (3) McAfee, S. M.; Topple, J. M.; Hill, I. G.; Welch, G. C. Key Components to the Recent Performance Increases of Solution Processed Non-Fullerene Small Molecule Acceptors. J. Mater. Chem. A 2015, 3, 16393−16408. (4) Li, S.; Ye, L.; Zhao, W.; Yan, H.; Yang, B.; Liu, D.; Li, W.; Ade, H.; Hou, J. A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells. J. Am. Chem. Soc. 2018, 140, 7159−7167. (5) Zhang, F.; Inganäs, O.; Zhou, Y.; Vandewal, K. Development of Polymer−Fullerene Solar Cells. Natl. Sci. Rev. 2016, 3, 222−239. (6) Salvatierra, R. V.; Cava, C. E.; Roman, L. S.; Zarbin, A. J. G. ITO-Free and Flexible Organic Photovoltaic Device Based on High Transparent and Conductive Polyaniline/Carbon Nanotube Thin Films. Adv. Funct. Mater. 2013, 23, 1490−1499. (7) Carlé, J. E.; Helgesen, M.; Hagemann, O.; Hösel, M.; Heckler, I. M.; Bundgaard, E.; Gevorgyan, S. A.; Søndergaard, R. R.; Jørgensen, M.; García-Valverde, R.; Chaouki-Almagro, S.; Villarejo, J. A.; Krebs, F. C. Overcoming the Scaling Lag for Polymer Solar Cells. Joule 2017, 1, 274−289. (8) Zhang, S.; Ye, L.; Zhang, H.; Hou, J. Green-Solvent-Processable Organic Solar Cells. Mater. Today 2016, 19, 533−543. (9) Wang, S.; Qu, Y.; Li, S.; Ye, F.; Chen, Z.; Yang, X. Improved Thermal Stability of Polymer Solar Cells by Incorporating Porphyrins. Adv. Funct. Mater. 2015, 25, 748−757. (10) McDowell, C.; Bazan, G. C. Organic Solar Cells Processed from Green Solvents. Curr. Opin. Green Sustain. Chem. 2017, 5, 49−54.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00884. Experimental details for the material characterization, device fabrication and characterization, mobility measurements; computational details for the PTB7-Th:ITIC 4783


Article

ACS Applied Energy Materials (11) Gu, X.; Shaw, L.; Gu, K.; Toney, M. F.; Bao, Z. The MeniscusGuided Deposition of Semiconducting Polymers. Nat. Commun. 2018, 9, 534. (12) Wang, C.; Xu, X.; Zhang, W.; Dkhil, S. B.; Meng, X.; Liu, X. Nano Energy Ternary Organic Solar Cells with Enhanced Open Circuit Voltage. Nano Energy 2017, 37, 24−31. (13) Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; Inganäs, O.; Gundogdu, K.; Gao, F.; Yan, H. Fast Charge Separation in a Non-Fullerene Organic Solar Cell with a Small Driving Force. Nat. Energy 2016, 1, 16089. (14) Li, Y.; Qian, D.; Zhong, L.; Lin, J.-D.; Jiang, Z.-Q.; Zhang, Z.G.; Zhang, Z.; Li, Y.; Liao, L.-S.; Zhang, F. A Fused-Ring Based Electron Acceptor for Efficient Non-Fullerene Polymer Solar Cells with Small HOMO Offset. Nano Energy 2016, 27, 430−438. (15) Matteucci, F.; Ercole, C.; Del Gallo, M. A Study of Chlorinated Solvent Contamination of the Aquifers of an Industrial Area in Central Italy: A Possibility of Bioremediation. Front. Microbiol. 2015, 6, 1−10. (16) Subianto, S.; Dutta, N.; Andersson, M.; Choudhury, N. R. Bulk Heterojunction Organic Photovoltaics from Water-Processable Nanomaterials and Their Facile Fabrication Approaches. Adv. Colloid Interface Sci. 2016, 235, 56−69. (17) Wouk de Menezes, L. C.; Renzi, W.; Marchiori, C. F. do N.; de Oliveira, C. K. B. Q. M.; von Kieseritzky, F.; Duarte, J. L.; Roman, L. S. Nonradiative Energy Transfer Between Porphyrin and Copolymer in Films Processed by Organic Solvent and Water-Dispersible Nanoparticles with Photovoltaic Applications. J. Phys. Chem. C 2018, 122, 5796−5804. (18) Yamamoto, N. A. D.; Payne, M. E.; Koehler, M.; Facchetti, A.; Roman, L. S.; Arias, A. C. Charge Transport Model for Photovoltaic Devices Based on Printed Polymer: Fullerene Nanoparticles. Sol. Energy Mater. Sol. Cells 2015, 141, 171−177. (19) Zheng, Z.; Awartani, O. M.; Gautam, B.; Liu, D.; Qin, Y.; Li, W.; Bataller, A.; Gundogdu, K.; Ade, H.; Hou, J. Efficient Charge Transfer and Fine-Tuned Energy Level Alignment in a THFProcessed Fullerene-Free Organic Solar Cell with 11.3% Efficiency. Adv. Mater. 2017, 29, 1604241. (20) Zhao, W.; Ye, L.; Li, S.; Liu, X.; Zhang, S.; Zhang, Y.; Ghasemi, M.; He, C.; Ade, H.; Hou, J. Environmentally-Friendly Solvent Processed Fullerene-Free Organic Solar Cells Enabled by Screening Halogen-Free Solvent Additives. Sci. China Mater. 2017, 60, 697−706. (21) Wu, Y.; Zou, Y.; Yang, H.; Li, Y.; Li, H.; Cui, C.; Li, Y. Achieving over 9.8% Efficiency in Nonfullerene Polymer Solar Cells by Environmentally Friendly Solvent Processing. ACS Appl. Mater. Interfaces 2017, 9, 37078−37086. (22) Wadsworth, A.; Ashraf, R. S.; Abdelsamie, M.; Pont, S.; Little, M.; Moser, M.; Hamid, Z.; Neophytou, M.; Zhang, W.; Amassian, A.; Durrant, J. R.; Baran, D.; McCulloch, I. Highly Efficient and Reproducible Nonfullerene Solar Cells from Hydrocarbon Solvents. ACS Energy Lett. 2017, 2, 1494−1500. (23) Park, G. E.; Choi, S.; Park, S. Y.; Lee, D. H.; Cho, M. J.; Choi, D. H. Eco-Friendly Solvent-Processed Fullerene-Free Polymer Solar Cells with over 9.7% Efficiency and Long-Term Performance Stability. Adv. Energy Mater. 2017, 7, 1−10. (24) Dayneko, S. V.; Hendsbee, A. D.; Welch, G. C. Fullerene-Free Polymer Solar Cells Processed from Non-Halogenated Solvents in Air with PCE of 4.8%. Chem. Commun. 2017, 53, 1164−1167. (25) Dayneko, S. V.; Hendsbee, A. D.; Welch, G. C.; Dayneko, S. V.; Hendsbee, A. D.; Welch, G. C. Combining Facile Synthetic Methods with Greener Processing for Efficient Polymer-Perylene Diimide Based Organic Solar Cells. Small Methods 2018, 1800081, 1−9. (26) Chen, X.; Liu, X.; Burgers, M. A.; Huang, Y.; Bazan, G. C. Green-Solvent-Processed Molecular Solar Cells. Angew. Chem., Int. Ed. 2014, 53, 14378−14381. (27) Kim, S.; Thiessen, P. A.; Bolton, E. E.; Chen, J.; Fu, G.; Gindulyte, A.; Han, L.; He, J.; He, S.; Shoemaker, B. A.; Wang, J.; Yu, B.; Zhang, J.; Bryant, S. H. PubChem Substance and Compound databases. Nucleic Acids Res. 2016, 44, D1202−D1213.

(28) Kwon, S.; Kang, H.; Lee, J.-H.; Lee, J.; Hong, S.; Kim, H.; Lee, K. Effect of Processing Additives on Organic Photovoltaics: Recent Progress and Future Prospects. Adv. Energy Mater. 2017, 7, 1601496. (29) Ye, L.; Xiong, Y.; Li, S.; Ghasemi, M.; Balar, N.; Turner, J.; Gadisa, A.; Hou, J.; O’Connor, B. T.; Ade, H. Precise Manipulation of Multilength Scale Morphology and Its Influence on Eco-Friendly Printed All-Polymer Solar Cells. Adv. Funct. Mater. 2017, 27, 1702016. (30) Ye, L.; Xiong, Y.; Yao, H.; Gadisa, A.; Zhang, H.; Li, S.; Ghasemi, M.; Balar, N.; Hunt, A.; O’Connor, B. T.; Hou, J.; Ade, H. High Performance Organic Solar Cells Processed by Blade Coating in Air from a Benign Food Additive Solution. Chem. Mater. 2016, 28, 7451−7458. (31) Vandewal, K.; Albrecht, S.; Hoke, E. T.; Graham, K. R.; Widmer, J.; Douglas, J. D.; Schubert, M.; Mateker, W. R.; Bloking, J. T.; Burkhard, G. F.; Sellinger, A.; Fréchet, J. M. J.; Amassian, A.; Riede, M. K.; McGehee, M. D.; Neher, D.; Salleo, A. Efficient Charge Generation by Relaxed Charge-Transfer States at Organic Interfaces. Nat. Mater. 2014, 13, 63−68. (32) Benduhn, J.; Tvingstedt, K.; Piersimoni, F.; Ullbrich, S.; Fan, Y.; Tropiano, M.; McGarry, K. A.; Zeika, O.; Riede, M. K.; Douglas, C. J.; Barlow, S.; Marder, S. R.; Neher, D.; Spoltore, D.; Vandewal, K. Intrinsic Non-Radiative Voltage Losses in Fullerene-Based Organic Solar Cells. Nat. Energy 2017, 2, 17053. (33) Pelzer, K. M.; Darling, S. B. Charge Generation in Organic Photovoltaics: a Review of Theory and Computation. Mol. Syst. Des. Eng. 2016, 1, 10−24. (34) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. Relating the Open-Circuit Voltage to Interface Molecular Properties of Donor:Acceptor Bulk Heterojunction Solar Cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 125204. (35) Gevorgyan, S. A.; Heckler, I. M.; Bundgaard, E.; Corazza, M.; Hösel, M.; Søndergaard, R. R.; dos Reis Benatto, G. A.; Jørgensen, M.; Krebs, F. C. Improving, Characterizing and Predicting the Lifetime of Organic Photovoltaics. J. Phys. D: Appl. Phys. 2017, 50, 103001. (36) Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174. (37) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the Bright Future-Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%. Adv. Mater. 2010, 22, 135−138. (38) Xiang, H. F.; Xu, Z. X.; Roy, V. A. L.; Che, C. M.; Lai, P. T. Method for Measurement of the Density of Thin Films of Small Organic Molecules. Rev. Sci. Instrum. 2007, 78, 034104. (39) Lourenco, O. D.; Benatto, L.; Marchiori, C. F. N.; Avila, H. C.; Yamamoto, N. A. D.; Oliveira, C. K.; da Luz, M. G. E.; Cremona, M.; Koehler, M.; Roman, L. S. Conformational Change on a BithiopheneBased Copolymer Induced by Additive Treatment: Application in Organic Photovoltaics. J. Phys. Chem. C 2017, 121, 16035−16044. (40) Elumalai, N. K.; Saha, A.; Vijila, C.; Jose, R.; Jie, Z.; Ramakrishna, S. Enhancing the Stability of Polymer Solar Cells by Improving the Conductivity of the Nanostructured MoO3 HoleTransport Layer. Phys. Chem. Chem. Phys. 2013, 15, 6831. (41) Oh, J.; Kranthiraja, K.; Lee, C.; Gunasekar, K.; Kim, S.; Ma, B.; Kim, B. J.; Jin, S. H. Side-Chain Fluorination: An Effective Approach to Achieving High-Performance All-Polymer Solar Cells with Efficiency Exceeding 7%. Adv. Mater. 2016, 28, 10016−10023. (42) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in PolymerFullerene Bulk Heterojunction Solar Cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 245207. (43) Wu, J.-L.; Chen, F.-C.; Hsiao, Y.-S.; Chien, F.-C.; Chen, P.; Kuo, C.-H.; Huang, M. H.; Hsu, C.-S. Surface Plasmonic Effects of Metallic Nanoparticles on the Performance of Polymer Bulk Heterojunction Solar Cells. ACS Nano 2011, 5, 959−967. (44) Lenes, M.; Morana, M.; Brabec, C. J.; Blom, P. W. M. Recombination-Limited Photocurrents in Low Bandgap Polymer/ Fullerene Solar Cells. Adv. Funct. Mater. 2009, 19, 1106−1111. 4784


Article

ACS Applied Energy Materials (45) Wilken, S.; Wilkens, V.; Scheunemann, D.; Nowak, R.-E.; von Maydell, K.; Parisi, J.; Borchert, H. Semitransparent Polymer-Based Solar Cells with Aluminum-Doped Zinc Oxide Electrodes. ACS Appl. Mater. Interfaces 2015, 7, 287−300. (46) Mihailetchi, V. D.; Koster, L. J. A.; Hummelen, J. C.; Blom, P. W. M. Photocurrent Generation in Polymer-Fullerene Bulk Heterojunctions. Phys. Rev. Lett. 2004, 93, 19−22. (47) Marcus, R. A. On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer. I. J. Chem. Phys. 1956, 24, 966. (48) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other function. Theor. Chem. Acc. 2008, 120, 215−241. (49) Zheng, Z.; Brédas, J.-L.; Coropceanu, V. Description of the Charge Transfer States at the Pentacene/C 60 Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model. J. Phys. Chem. Lett. 2016, 7, 2616−2621. (50) Reese, M. O.; Gevorgyan, S. A.; Jørgensen, M.; Bundgaard, E.; Kurtz, S. R.; Ginley, D. S.; Olson, D. C.; Lloyd, M. T.; Morvillo, P.; Katz, E. A.; Elschner, A.; Haillant, O.; Currier, T. R.; Shrotriya, V.; Hermenau, M.; Riede, M.; Kirov, K. R.; Trimmel, G.; Rath, T.; Inganäs, O.; Zhang, F.; Andersson, M.; Tvingstedt, K.; Lira-Cantu, M.; Laird, D.; McGuiness, C.; Gowrisanker, S.; Pannone, M.; Xiao, M.; Hauch, J.; Steim, R.; Delongchamp, D. M.; Rösch, R.; Hoppe, H.; Espinosa, N.; Urbina, A.; Yaman-Uzunoglu, G.; Bonekamp, J. B.; Van Breemen, A. J. J. M.; Girotto, C.; Voroshazi, E.; Krebs, F. C. Consensus Stability Testing Protocols for Organic Photovoltaic Materials and Devices. Sol. Energy Mater. Sol. Cells 2011, 95, 1253− 1267.

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