Benzothiadiazole Based Cascade Material to Boost the Performance of Inverted Ternary Organic Solar Cells

A conjugated, ladder-type multi-fused ring 4,7-dithienbenzothiadiazole:thiophene derivative, named as compound ‘T’, was for the first time incorporated, within the PTB7:PC71BM photoactive layer for inverted ternary organic solar cells (TOSCs) realization. The effective energy level offset caused by compound T between the polymeric donor and fullerene acceptor materials, as well as its resulting potential as electron cascade material contribute to an enhanced exciton dissociation, electron transfer facilitator and thus improved overall photovoltaic performance. The engineering optimization of the inverted TOSC, ITO/PFN/PTB7:Compound T(5% v/v):PC71BM/MoO3/Al, resulted in an overall power conversion efficiency (PCE) of 8.34%, with a short-circuit current density (Jsc) of 16.75 mA cm−2, open-circuit voltage (Voc) of 0.74 V and a fill factor (FF) of 68.1%, under AM1.5G illumination. This photovoltaic performance was improved by approximately 12% with respect to the control binary device.


Introduction
The prevalent research rush of the scientific community towards cost-effective and reliable alternative energy sources involves the whole spectrum of research on photovoltaic technologies and corresponding material science. Recent progress in the subfield of organic solar cells (OSCs) highlights and even updates their potential as ideal, low-cost alternatives to the conventional inorganic silicon technology. This is attributed to their advantages, such as light-weight and solution-processability, as well as their compatibility with flexible substrates and upscaling techniques [1][2][3]. The emergence of novel materials [4][5][6][7], advancements of device's engineering [1,8,9] for enhanced light harvesting and trapping by extending the absorption spectrum or introducing optical cavities [10,11], and additionally leading theoretical studies [12,13], have so far skyrocketed their efficiency over 13% [8].
PTB7 was purchased from Solaris Chem, while PC 71 BM and PFN were both purchased from Solenne BV. CSM was synthesized according to a multi-step procedure, successively containing donor-acceptor moieties seriatim. More specifically, two phenyl or thiophene rings were linked to the 4,7-dithienbenzothiadiazole core unit, through a cyano-substituted vinylene bond. The chemical structures of PTB7, compound T and PC 71 BM, employed in the ternary BHJ, are illustrated in Figure 1. Such an approach is popular and is widely utilized to reduce the band gap of conjugated semiconductors [71,72]. The exact synthetic procedure of compound T can be found in another publication of our research team and is analyzed in Figure S1 [70].

Materials and Instruments
PTB7 was purchased from Solaris Chem, while PC71BM and PFN were both purchased from Solenne BV. CSM was synthesized according to a multi-step procedure, successively containing donor-acceptor moieties seriatim. More specifically, two phenyl or thiophene rings were linked to the 4,7-dithienbenzothiadiazole core unit, through a cyano-substituted vinylene bond. The chemical structures of PTB7, compound T and PC71BM, employed in the ternary BHJ, are illustrated in Figure  1. Such an approach is popular and is widely utilized to reduce the band gap of conjugated semiconductors [71,72]. The exact synthetic procedure of compound T can be found in another publication of our research team and is analyzed in Figure S1 [70]. UV-vis absorption spectra were taken using a Shimadzu UV-2401 PC spectrophotometer to a wavelength range of 300-800 nm. The photoluminescence (PL) spectra of the ternary active layers were conducted at room temperature (RT) and set through a UV sensitive, calibrated CCD camera, liquid N2 cooled-over a wavelength range of 600-950 nm. The excitation source employed was a He-Cd CW laser at 325 nm with a full power of P0 = 35 mW. The morphologies of the surfaces were examined with an atomic force microscope (Park Systems XE7). The OSC device photovoltaic parameters (Jsc, Voc, FF and PCE) were measured at room temperature within glove box (MBRAUN) conditions ((O2 < 0.1 ppm), moisture-free (H2O < 0.1 ppm)) and under standard illumination conditions (at AM 1.5 G and an intensity of 1000 Wm −2 ). The external quantum efficiency (EQE) curves of the devices were extracted under short-circuit conditions immediately, in order to avoid any device degradation, using an integrated system (Enlitech, Taiwan) equipped with a lock-in amplifier with a current preamplifier. To further enhance the credibility of our measurements, a monocrystalline photodetector of known spectral response was used to calibrate the spectrum of the solar simulator. OSC devices were assessed using a xenon (Xe) lamp, as well as an optical chopper at low frequencies (200 Hz) in order to get the maximum sound/noise (S/N) ratio. To ensure the reproducibility of the J-V characteristics, ten same devices consisting of six photovoltaics cells each were fabricated and evaluated.

Device Fabrication
A conventional procedure was followed for OSC devices fabrication [16]. More specifically, OSCs were fabricated onto glass substrates sputtered with indium-tin-oxide (ITO) that exhibited a 20 Ω sq −1 sheet resistance. To remove any impurities from the ITO glass, ultrasonic bath (Elma S 30 H Elmasonic) for 10 min was applied using three different washing components, namely Hellmanex III detergent, acetone and isopropanol in subsecutive steps, and then dried in an oven. Finally, a UV-ozone treatment was performed for 15 min. The inverted structure was preferred for fabricating the OSC devices. PFN solution, with concentration 0.5 mg/mL, was dispersed in 1 mL of methanol (MeOH) and 2 μL of acetic acid and was stirred overnight. Then PFN layer (ETL) was spin-cast at UV-vis absorption spectra were taken using a Shimadzu UV-2401 PC spectrophotometer to a wavelength range of 300-800 nm. The photoluminescence (PL) spectra of the ternary active layers were conducted at room temperature (RT) and set through a UV sensitive, calibrated CCD camera, liquid N 2 cooled-over a wavelength range of 600-950 nm. The excitation source employed was a He-Cd CW laser at 325 nm with a full power of P 0 = 35 mW. The morphologies of the surfaces were examined with an atomic force microscope (Park Systems XE7). The OSC device photovoltaic parameters (J sc , V oc , FF and PCE) were measured at room temperature within glove box (MBRAUN) conditions ((O 2 < 0.1 ppm), moisture-free (H 2 O < 0.1 ppm)) and under standard illumination conditions (at AM 1.5 G and an intensity of 1000 Wm −2 ). The external quantum efficiency (EQE) curves of the devices were extracted under short-circuit conditions immediately, in order to avoid any device degradation, using an integrated system (Enlitech, Taiwan) equipped with a lock-in amplifier with a current preamplifier. To further enhance the credibility of our measurements, a monocrystalline photodetector of known spectral response was used to calibrate the spectrum of the solar simulator. OSC devices were assessed using a xenon (Xe) lamp, as well as an optical chopper at low frequencies (200 Hz) in order to get the maximum sound/noise (S/N) ratio. To ensure the reproducibility of the J-V characteristics, ten same devices consisting of six photovoltaics cells each were fabricated and evaluated.

Device Fabrication
A conventional procedure was followed for OSC devices fabrication [16]. More specifically, OSCs were fabricated onto glass substrates sputtered with indium-tin-oxide (ITO) that exhibited a 20 Ω sq −1 sheet resistance. To remove any impurities from the ITO glass, ultrasonic bath (Elma S 30 H Elmasonic) for 10 min was applied using three different washing components, namely Hellmanex III detergent, acetone and isopropanol in subsecutive steps, and then dried in an oven. Finally, a UV-ozone treatment was performed for 15 min. The inverted structure was preferred for fabricating the OSC devices.
Energies 2020, 13, 450 4 of 12 PFN solution, with concentration 0.5 mg/mL, was dispersed in 1 mL of methanol (MeOH) and 2 µL of acetic acid and was stirred overnight. Then PFN layer (ETL) was spin-cast at 1000 rpm for 45 s but, in order to achieve layer thickness of approximately 10 nm, a multilayer approach was used. Specifically, four layers of PFN solution were developed with intermittent annealing steps at 150 • C for 30 s between each spin-coating stage. PTB7 and PC 71 BM, were blended (25 mg/mL, ratio 1:1.5) in chlorobenzene (CB):DIO (97:3 vol%) solvents' mixture. The said photoactive layer was subsequently developed by spin-coating of the blend solutions at 1500 rpm on top of the PFN layer with a thickness of 85 nm. Thereafter, a MoO 3 layer (8 nm) and the Al electrode (100 nm) were thermally evaporated through a shadow mask hence defining an active area of 4 mm 2 for each device.

Photophysical Properties
Compound T was characterized using UV-vis, FT-IR and PL spectroscopy. The UV-vis absorption spectrum of a thin film pristine compound T exhibited two main peaks at 398 nm and 485 nm, respectively ( Figure 2), which were both due to the π→π* electron transition.
Energies 2020, 13, x FOR PEER REVIEW 4 of 12 1000 rpm for 45 s but, in order to achieve layer thickness of approximately 10 nm, a multilayer approach was used. Specifically, four layers of PFN solution were developed with intermittent annealing steps at 150 °C for 30 s between each spin-coating stage. PTB7 and PC71BM, were blended (25 mg/mL, ratio 1:1.5) in chlorobenzene (CB):DIO (97:3 vol%) solvents' mixture. The said photoactive layer was subsequently developed by spin-coating of the blend solutions at 1500 rpm on top of the PFN layer with a thickness of 85 nm. Thereafter, a MoO3 layer (8 nm) and the Al electrode (100 nm) were thermally evaporated through a shadow mask hence defining an active area of 4 mm 2 for each device.

Photophysical Properties
Compound T was characterized using UV-vis, FT-IR and PL spectroscopy. The UV-vis absorption spectrum of a thin film pristine compound T exhibited two main peaks at 398 nm and 485 nm, respectively ( Figure 2), which were both due to the π→π* electron transition. According to the FT-IR spectrum of compound T [70] ( Figure S2) a characteristic peak at 2214 cm −1 appears, due to the CN-stretching bond. The peak at 3103 cm −1 is ascribed to aromatic C-H stretching, while the absorption bands at 1652 cm −1 , 1577 cm −1 , 1533 cm −1 , 1520 cm −1 , 1479 cm −1 and 1441 cm −1 correspond to C=C aromatic stretching bonds, respectively. Finally, peaks at 1267 cm −1 , 1252 cm −1 , 1243 cm −1 , 1224 cm −1 , 1203 cm −1 , 1189 cm −1 , 1097 cm −1 , 1080 cm −1 and 1048 cm −1 are attributed to the benzene ring of benzothiazole. On the other hand, the PL spectrum of compound T in solid state was recorded in the range of 600−900 nm, upon photoexcitation at 485 nm, as depicted in Figure S3. Compound T exhibited a broad peak at 700 nm.
In Figure 3 the absorption spectra of the ternary active layers are illustrated. It is observable that the absorption intensity slightly increased as the concentration of compound T got 5% across the full spectrum range and especially between 400 and 560 nm, where the host PTB7 and compound T spectra overlapped. Moreover, the common peak of compound T and fullerene derivative PC71BM got higher values, while the absorption peak of PTB7, decreased. According to relevant existing literature [73], two broad absorption peaks at around 614 and 682 nm are attributed to the characteristic π-π* transition of the PTB7 polymer. According to the FT-IR spectrum of compound T [70] ( Figure S2) a characteristic peak at 2214 cm −1 appears, due to the CN-stretching bond. The peak at 3103 cm −1 is ascribed to aromatic C-H stretching, while the absorption bands at 1652 cm −1 , 1577 cm −1 , 1533 cm −1 , 1520 cm −1 , 1479 cm −1 and 1441 cm −1 correspond to C=C aromatic stretching bonds, respectively. Finally, peaks at 1267 cm −1 , 1252 cm −1 , 1243 cm −1 , 1224 cm −1 , 1203 cm −1 , 1189 cm −1 , 1097 cm −1 , 1080 cm −1 and 1048 cm −1 are attributed to the benzene ring of benzothiazole. On the other hand, the PL spectrum of compound T in solid state was recorded in the range of 600−900 nm, upon photoexcitation at 485 nm, as depicted in Figure S3. Compound T exhibited a broad peak at 700 nm.
In Figure 3 the absorption spectra of the ternary active layers are illustrated. It is observable that the absorption intensity slightly increased as the concentration of compound T got 5% across the full spectrum range and especially between 400 and 560 nm, where the host PTB7 and compound T spectra overlapped. Moreover, the common peak of compound T and fullerene derivative PC 71 BM got higher values, while the absorption peak of PTB7, decreased. According to relevant existing literature [73], two broad absorption peaks at around 614 and 682 nm are attributed to the characteristic π-π* transition of the PTB7 polymer. In the latter, the photoluminescence (PL) spectrum of PTB7:compound T was quenched as an optimum concentration value of the third element was approaching its optimum value. The PL quenching is clear evidence of the improved exciton dissociation, between compound T and PTB7, as the emission of the latter quenched ( Figure 4). The best charge extraction was observed after the addition of 5% v/v Compound T, compared to the reference binary system (PTB7:PC71BM). This is a possible reason for the enhanced PV parameters in the case of 3% and 5% ternary devices, as demonstrated below in Table 1. This fact proves that compound T could act as an electron cascade material that facilitates the higher electron extraction from PTB7 towards PC71BM compared to the binary reference system.  In the latter, the photoluminescence (PL) spectrum of PTB7:compound T was quenched as an optimum concentration value of the third element was approaching its optimum value. The PL quenching is clear evidence of the improved exciton dissociation, between compound T and PTB7, as the emission of the latter quenched ( Figure 4). The best charge extraction was observed after the addition of 5% v/v Compound T, compared to the reference binary system (PTB7:PC 71 BM). This is a possible reason for the enhanced PV parameters in the case of 3% and 5% ternary devices, as demonstrated below in Table 1. This fact proves that compound T could act as an electron cascade material that facilitates the higher electron extraction from PTB7 towards PC 71 BM compared to the binary reference system. In the latter, the photoluminescence (PL) spectrum of PTB7:compound T was quenched as an optimum concentration value of the third element was approaching its optimum value. The PL quenching is clear evidence of the improved exciton dissociation, between compound T and PTB7, as the emission of the latter quenched ( Figure 4). The best charge extraction was observed after the addition of 5% v/v Compound T, compared to the reference binary system (PTB7:PC71BM). This is a possible reason for the enhanced PV parameters in the case of 3% and 5% ternary devices, as demonstrated below in Table 1. This fact proves that compound T could act as an electron cascade material that facilitates the higher electron extraction from PTB7 towards PC71BM compared to the binary reference system.

Photovoltaic Performance
A schematic representation of the inverted ternary OSC structured as ITO/PFN/PTB7:compound T:PC 71 BM/MoO 3 /Al is depicted in Figure 5, while the respective configuration and energy diagram of Due to the energy level offset between PTB7 [74], compound T [70] and PC 71 BM [74] (3.31 eV, 3.58 eV and 4.3 eV in LUMO level and 5.15 eV, 5.48 eV and 6.0 eV in the HOMO level, respectively) the charge transfer is enhanced. This is clear evidence, that the addition of compound T up to an optimum concentration, promoted the electron-cascade effect. More specifically, excitons are dissociated at the D:A interfaces throughout the blend layer. Since the HOMO levels of compound T and the conjugated polymer PTB7 perfect match, holes' transport was facilitated through the compound T en route to the anode electrode. In Figure 5, current-to-voltage characteristics based on the PTB7:PC 71 Figure 5, while the respective configuration and energy diagram of the device with the HOMO and LUMO energy levels of the materials selected are depicted in Figure 1. Due to the energy level offset between PTB7 [74], compound T [70] and PC71BM [74] (3.31 eV, 3.58 eV and 4.3 eV in LUMO level and 5.15 eV, 5.48 eV and 6.0 eV in the HOMO level, respectively) the charge transfer is enhanced. This is clear evidence, that the addition of compound T up to an optimum concentration, promoted the electron-cascade effect. More specifically, excitons are dissociated at the D:A interfaces throughout the blend layer. Since the HOMO levels of compound T and the conjugated polymer PTB7 perfect match, holes' transport was facilitated through the compound T en route to the anode electrode. In Figure 5, current-to-voltage characteristics based on the PTB7:PC71BM and the ternary PTB7:compound T:PC71BM active layers, are also depicted. The corresponding PV performance parameters are listed in Table 1 As shown, Jsc did not follow the rising trend of compound T concentration, but instead it increased from 3% to 5% and then started to decay as the concentration of compound T got higher values. This was attributed to the degradation of the interfaces between compound T and the polymer donor material. On the other hand, the Voc obtained in the ternary cells was almost identical to the binary PTB7:PC71BM, reflecting that charge transfer state energy (ECT) does not change upon the addition of Compound T [75].  As shown, J sc did not follow the rising trend of compound T concentration, but instead it increased from 3% to 5% and then started to decay as the concentration of compound T got higher values. This was attributed to the degradation of the interfaces between compound T and the polymer donor material. On the other hand, the V oc obtained in the ternary cells was almost identical to the binary PTB7:PC 71 BM, reflecting that charge transfer state energy (E CT ) does not change upon the addition of Compound T [75].

Charge Transfer Properties
In view of further investigating the origin of the J sc increase upon the addition of the compound T up to an optimum concentration, the incident photon to electron conversion efficiency was measured (IPCE) for each device ( Figure 6). From EQE curves, it was deduced that the 5% concentration (v/v) had the best response at both main absorption peaks, which indicates that this ratio might yield the most efficient charge transportation and collection due to a better domain engineering within the active layer of the ternary system.
Carriers' mobilities of the reference device (PTB7:PC 71 BM) and the ternary one, incorporating 5% compound T, were also obtained by space charge limited current (SCLC) method. To this end, hole and electron only devices of the structure ITO/PEDOT:PSS/active layer/MoO 3 /Au and ITO/PFN/active layer/Ca/Al were respectively fabricated and J-V 2 characteristic curves are reported ( Figure S5). Calculations were based on the Mott-Gurney equation [76]: where ε r is the relative dielectric constant, ε 0 is the permittivity of free space, µ is the charge carrier mobility, V is the applied voltage, V bi is the built-in potential and d is the thickness of the active layer.

Charge Transfer Properties
In view of further investigating the origin of the Jsc increase upon the addition of the compound T up to an optimum concentration, the incident photon to electron conversion efficiency was measured (IPCE) for each device ( Figure 6). From EQE curves, it was deduced that the 5% concentration (v/v) had the best response at both main absorption peaks, which indicates that this ratio might yield the most efficient charge transportation and collection due to a better domain engineering within the active layer of the ternary system.
Carriers' mobilities of the reference device (PTB7:PC71BM) and the ternary one, incorporating 5% compound T, were also obtained by space charge limited current (SCLC) method. To this end, hole and electron only devices of the structure ITO/PEDOT:PSS/active layer/MoO3/Au and ITO/PFN/active layer/Ca/Al were respectively fabricated and J-V 2 characteristic curves are reported ( Figure S5). Calculations were based on the Mott-Gurney equation [76]: where εr is the relative dielectric constant, ε0 is the permittivity of free space, μ is the charge carrier mobility, V is the applied voltage, Vbi is the built-in potential and d is the thickness of the active layer. The μh and μe values for holes and electrons mobilities of the reference and 5% compound T ternary device are summarized in Table 2, respectively. It is obvious that the incorporation of compound T into the binary solution resulted to the improvement of hole mobility, an even higher increase in electron mobility and most important the more balanced ratio between hole and electron mobilities. Moreover, the fact that the μh/μe ratio is closer to one for the case of the ternary device, is a key prerequisite in avoiding charge accumulation in the device and thus higher photovoltaic performances. The aforementioned result confirms the previous assumption for the electron-cascade-role of compound T. The µ h and µ e values for holes and electrons mobilities of the reference and 5% compound T ternary device are summarized in Table 2, respectively. It is obvious that the incorporation of compound T into the binary solution resulted to the improvement of hole mobility, an even higher increase in electron mobility and most important the more balanced ratio between hole and electron mobilities. Moreover, the fact that the µ h /µ e ratio is closer to one for the case of the ternary device, is a key prerequisite in avoiding charge accumulation in the device and thus higher photovoltaic performances. The aforementioned result confirms the previous assumption for the electron-cascade-role of compound T.

Morphology
In order to study the effect of compound T on active layer's morphology, a reference and a ternary blend layer of 5% (v/v) concentration were subjected to AFM measurements. Subsequent results ( Figure S4) indicate a better morphology together with a reduction in roughness (RMS) for the ternary device containing 5% of compound T, by approximately 9.1%. In particular, the RMS value for the reference binary layer was 1.4 nm, whereas the ternary layer demonstrated an RMS value of approximately 1.28 nm. This is proof that the addition of compound T enhances the interface quality between the active layer and hole transport layer, thereby indicating an improvement in FF and other PV parameters [77].

Conclusions
In conclusion, simple ternary inverted organic solar cells incorporating the soluble compound T, into the photoactive layer of the binary PTB7:PC 71 BM blend were fabricated by differentiating additive's concentration from 3% to 15%. The incorporation of the compound T led to a favorable energy alignment between the energy levels of PTB7 donor and PC 71 BM acceptor, thus facilitating the electron-cascade effect. The champion ternary blend device based on the ITO/PFN/PTB7:compound T:PC 71 BM/MoO 3 /Al structure, with a 5% (v/v) concentration of compound T, resulted in a PCE of 8.34%, with an enhancement of 12% compared to the reference device. Therefore, since compound T had a wide optical bandgap and low HOMO level, it demonstrated the potential for boosting device performance of other visible, as well as the near-infrared non-fullerene blends like, which are of great interest for the OSC community.
Supplementary Materials: The synthetic procedure of compound T and its intermediate stages, as well as explanations of experimental details to understanding and reproducing the research are available online at http://www.mdpi.com/1996-1073/13/2/450/s1, Figure S1: Five-step synthesis of compound T, Figure S2: FT-IR spectrum of compound T, Figure S3: Normalized PL spectrum of compound T in thin film, Figure S4: AFM images of (a) binary and (b) ternary bulk heterojunction film with 5% compound T content, Figure S5: J-V 2 characteristics of the fabricated (a) electron-only and (b) hole-only devices, upon the addition of 5% compound T, for the determination of carriers' mobilities.