Single junction binary and ternary polymer solar cells-based D–A structured copolymer with low lying HOMO energy level and two nonfullerene acceptors

Abstract

A donor–acceptor (D–A) conjugated copolymer denoted as P(DTB-BDD) consisting of benzodithiophenedione (BDD) as a strong acceptor and dithienobenzene (DTB) as a weak donor was prepared, and its optical and electrochemical properties were analyzed. P(DTB-BDD) exhibited a deeper highest occupied molecular orbital and an optical bandgap of 1.74 eV. Polymer solar cells (PSCs) were fabricated through blending P(DTB-BDD) with two non-fullerene acceptors, i.e., narrow bandgap Y6 and medium bandgap DBTBT-IC. The power conversion efficiencies (PCEs) of the PSCs using the bulk heterojunction active layer based on P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6 were 13.16% and 12.62%, respectively. The larger open circuit voltage of the DBTBT-IC based PSCs as compared with that of the Y6 counterpart was due to the up-shifted LUMO energy level of DBTBT-IC, and the high short circuit current for the Y6 based PSCs may be associated with the extended absorption profile of Y6. Taking advantage of the high open circuit voltage and short circuit current of the devices based on DBTBT-IC and Y6, the ternary PSCs were prepared by optimizing the weight ratios between two acceptors and maintaining a constant amount of P(DTB-BDD); the resulting ternary PSCs based on P(DTB-BDD) : DBTBT-IC : Y6 (1.0 : 0.2 : 1.0) showed an improved PCE of 16.32%, which is greater than those for the binary PSCs. The enrichment in the PCE of the ternary device may be concomitant with the effective exploitation of excitons via energy transfer from DBTBT-IC to Y6 and increased the D/A interfacial area for more effective charge transfer. The open circuit voltage of the PSC based on ternary lies in between that for the Y6 and DBTBT-IC-based PSCs, demonstrating the formation of an alloy for the two acceptors.

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Design, System, Application

In this manuscript, we have designed and synthesized a wide bandgap D–A conjugated polymer and used it as a donor, along with two non-fullerene acceptors (medium bandgap and small bandgap). After the optimization of active layer engineering, the ternary polymer solar cells attained a power conversion efficiency of 16.32%. Our results demonstrated that this donor polymer might be suitable for high efficiency polymer solar cells for commercial applications.

1. Introduction

Polymer solar cells (PSCs), which feature large area production and compatibility with flexible substrates at low cost, eco-friendly processing, and semitransparency, have attracted considerable attention in the last few years.^1–6 In PSCs, a bulk heterojunction (BHJ) active layer consisting of a polymer or small molecule as a donor material (D) and an acceptor material (A) is generally used, and it affords sufficient D/A interfaces to dissociate the photogenerated excitons into free charges, which are subsequently collected by the electrodes, generating photocurrent.^7,8 With the uninterrupted invention in designing donor and acceptor materials, engineering the device architecture, optimization of BHJ active layers, and the development of the non-fullerene acceptors (NFAs), particularly ITIC series^9,10 and Y-series NFAs,^11–14 the power conversion efficiencies (PCEs) more than 18% have been attained.^15–19 NFAs have more degree of freedom, allowing higher electron affinity tunability and absorbing incident visible NIR radiation more strongly and are more stable and easier to synthesize as compared to fullerene counterparts. The recent progress in low bandgap NFAs has unlocked a new opportunity for designing new wide-bandgap polymer donors for PSCs.^20–22 The PCE values of BHJ-PSCs are determined by multiplying the open circuit voltage (V_OC) with short circuit current (J_SC), and fill factor (FF), and then, the whole product is to be divided by the incident power. The light-absorbing efficiency of the BHJ active layer is directly linked to the J_SC of PSC. The energy difference between the HOMO level of the donor and the LUMO level of the acceptor is directly linked with the value of V_OC. To improve the above-mentioned photovoltaic parameters, new efficient wide-bandgap polymer donors with the deep-lying HOMO level and extraordinary hole-mobility but with complementary absorption spectra to the narrow bandgap acceptor are to be designed.

The concept of the D–A tactic has been effectively employed to develop efficient organic semiconducting materials. The optical bandgap and the HOMO and LUMO levels of D–A organic semiconducting materials could be easily adjusted via the intermolecular charge transfer (ICT) by selecting the appropriate combination of D and A units^23–26 Moreover, the D–A copolymers also exhibit high hole mobility due to the intermolecular D–A interactions. In 2010, Zhou et al. introduced a concept of a strong acceptor–weak donor approach for synthesizing ideal copolymers for PSCs.²⁷ The weak donor helps to maintain the deep HOMO level and the strong acceptor assists in reducing the bandgap via the ICT process. Many D–A copolymers have been designed by different research groups and reported PCE in the range of 16–17% for OSCs using them as donors along with NFSMAs as acceptors.^28–32

In D–A wide bandgap copolymers, benzothiadiazole (BT),³³ isoindigo (IID),³⁴ and diketopyrrolopyrrole (DPP)³⁵ are used as strong acceptor units and benzodithiophenedione (BDD) as a relatively weak acceptor. The BDD acceptor unit has an enormous planar arrangement and PSC-based polymers centered on BDD attained high PCE.^36–40 The combination of the weak donor and strong acceptor is used to develop the D–A polymer, which exhibits a low-lying HOMO level and thus helps to obtain a high V_OC of the photovoltaic devices.^27,41 The typical approach for designing a weak donor might be realized by reducing the electron donating ability of the thiophene unit, which can be realized by mixing it with a comparatively electron-deficient unit. This approach has been effectively adopted to achieve the deep HOMO level of polymers.⁴²

Herein, taking advantage of the above concept, we have synthesized a D–A copolymer denoted as P(DTB-BDD) comprising BDD as a strong acceptor and dithienobenzene (DTB) as a weak donor. P(DTB-BDD) attained a strong absorption in the wavelength range from 300 nm to 700 nm with optical bandgap and HOMO levels of about 1.74 eV and −5.46 eV, respectively. As the absorption profile is complementary with the well-known narrow bandgap A–DA′D–A non-fullerene acceptor Y6, we have fabricated PSCs using P(DTB-BDD) as the donor and Y6 as the acceptor; the optimized PSCs based on P(DTB-BDD):Y6 exhibited an overall PCE of 12.62% (V_OC = 0.83 V, J_SC = 23.76 mA cm⁻², and FF = 0.64). Although the J_SC of the PSC is quite high, the low value of PCE is due to the low value of V_OC and FF as compared to other Y6 based PSCs. In order to enhance the efficiency of the PSCs, a medium bandgap acceptor DBTBT-IC⁴³ with a high lying LUMO energy level (−3.88 eV) as a second acceptor is used to fabricate the ternary PSCs. Our group has also fabricated the binary PSCs and the improved P(DTB-BDD):DBTBT-IC-based PSCs showed overall PCE of 13.16% (V_OC = 1.05 V, J_SC = 19.06 mA cm⁻², and FF = 0.68). We have incorporated DBTBT-IC as a guest acceptor in the host binary P(DTB-BDD):Y6 active layer to take advantage of the high V_OC and FF for PSCs based on P(DTB-BDD):DBTBT-IC layer, and the optimized ternary PSCs achieve PCE of 16.32%, is greater than the binary counterparts. The increased PCE for ternary PSCs is associated with the improved interfacial area of D/A in the ternary BHJ layer for more effective exciton dissociation and also with efficient utilization of excitons through the transfer of energy from DBTBT-IC to Y6. The V_OC of ternary PSCs lies in between that for Y6 and DBTBT-IC-based PSCs, demonstrating the development of an alloy between these two acceptors.

2. Results and discussion

2.1. Synthesis of P(DTB-BDD)

In Scheme 1, the synthetic process for preparing P(DTB-BDD) is shown, and their characterization is given in the ESI.† The monomers (4,5-diundecylbenzo[1,2-b:6,5-b′]dithiophene-2,7-diyl)bis(trimethylstannane) (M1)⁴⁴ and 1,3-bis(5-bromothiophen-2-yl)-5,7-dioctylbenzo[1,2-c:4,5-c′]dithiophene-4,8-dione-2 (M2)⁴⁵ were synthesized as reported in the literature. The polymerization based on Stille cross-coupling between M1 and M2 was used to prepare the target copolymer P(DTB-BDD). The catalyst was palladium (Pd(Ph3P)4). P(DTB-BDD) was extracted as black solid powders with 86% yields and refined further using Soxhlet extraction to eliminate the catalyst and oligomers.

Scheme 1 Preparation route of P(DTB-BDD).

Elemental analysis and ¹H NMR spectroscopy were used to examine the composition and structure of P(DTB-BDD) (Fig. S1, ESI†). P(DTB-BDD) was soluble in a wide range of organic solvents. For determining the polydispersity index (PDI) and molecular weight of P(DTB-BDD), gel permeation chromatography (GPC) was used at 60 °C with 1, 2, 4-trichlorobenzene and polystyrene as the eluent and standard, respectively. P(DTB-BDD) has a number average molecular weight (Mn) of 32.3 kDa and a PDI of 1.96, respectively.

The thermal property of P(DTB-BDD) was examined using thermo-gravimetric (TGA) analysis (Fig. S3, ESI†). P(DTB-BDD) has a thermal decomposition temperature T_d of 395 °C (5% weight loss) and thus possesses high thermal stability.

2.2. Optical characterization

The absorption spectra of P(DTB-BDD) are shown in Fig. 1, where a dilute chloroform solution was used and the film was cast on a quartz substrate, and related data are summarized in Table 1. It is seen that P(DTB-BDD) exhibited wide absorption spectra covering the 300–700 nm wavelength region (Fig. 1). P(DTB-BDD) exhibited two main absorption bands in the dilute chloroform solution. The band that lies in the wavelength range of 300–450 nm (absorption maxima at 366 nm) is associated with the π–π* transition in the polymer backbone, while the absorption band seen in the 500–700 nm region with absorption maxima at 548 nm may be accredited to intramolecular charge transfer (ICT) among DTB and BDD. In contrast to that in solution, the absorption maxima of the P(DTB-BDD) thin film shifted to the red region of the solar spectrum, which is usual in conjugated D–A organic semiconducting materials. The redshifted ICT absorption band is due to a number of intermolecular interactions and their effects on the energy states and therefore absorption features.⁴⁶ The optical bandgap of P(DTB-BDD) estimated from the absorption edge was 1.74 eV. The absorption profiles of DBTBT-IC and Y6 are also shown in Fig. 1, which are complementary to P(DTB-BDD).

Fig. 1 Solution and film absorption spectra of P(DTB-BDD), thin absorption spectra DBTBT-IC and Y6.

Table 1 Optical and electrochemical properties of P(DTB-BDD)

Copolymer	λ max (nm)	λ max (nm)	E HOMO (eV)	E LUMO (eV)	E ^echg (eV)	E ^optg (eV)
a Dilute CHCl3 solution. b Thin film.
P(DTB-BDD)	366, 548	384, 593	−5.46	−3.53	1.93	1.74

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The HOMO and LUMO levels of P(DTB-BDD) were determined using electrochemical cyclic voltammetry (CV) (Fig. S4†), and the related data are shown in Table 1. To determine the HOMO/LUMO levels using the onset of the oxidation (E^ox_onset) and reduction (E^red_onset) potentials, the following expressions were taken into consideration:

E_HOMO = −q(E^ox_onset + 4.43) eV and E_LUMO = −q(E^red_onset + 4.43) eV.

The values of HOMO/LUMO were found to be −5.46/−3.53 eV. The deep-lying HOMO of the copolymer donor is beneficial for both the steadiness of the PSC in ambient conditions and achieving high V_OC.

2.2.1. Theoretical calculations. A model of the P(DTB-BDD) monomer unit was used to examine the structural and energy parameters. For the simplification of calculations, methyl groups are used to replace the long hydrocarbon radicals since the absorption band and the energy depend largely on the aromatic heterocycles (conjugated). The front and side views of model P(DTB-BDD) are presented in Fig. S4.† The DFT/ωB97X-D3(BJ)/6-31G(d,p)++ method was used to estimate the model geometry and dipole moment in the ground and transition states. All these results are summarized in Fig. S5.† In the ground state, the torsion angles are 12.8°, 39.6°, 37.3°, while in the transition state, they are 19.4°, 60.6°, 35.5°. In the ground and transition states, the dipole moment has magnitudes of 1.60 D and 1.80 D, respectively. The DFT/M11-L/def2-SVPD approach was used to calculate the HOMO and LUMO levels for P(DTB-BDD) in steady state conditions (Fig. 2). The computed HOMO/LUMO values are around −5.46/−3.53 eV, which are very close to the observed values (Table 1). The nitrogen and oxygen atoms are in the center of the negative electrostatic potential values.

Fig. 2 Molecular orbital surfaces for the P(DTB-BDD) model from calculations done using the DFT/M11-L/def2-SVPD method in acetonitrile (PBF solvent model).

2.2.2. Photovoltaic performance. The chemical structure and energy level diagram of P(DTB-BDD), DTBT-IC, and Y6 are shown in Fig. 3. The charge transfer occurs after the photogeneration of excitons and the subsequent charge separation at the D/A interface in the BHJ film. The donor and acceptor's HOMO and LUMO offsets determine the hole transmission from the acceptor's HOMO to the donor and electron transfer from the donor to the acceptor, respectively. The LUMO offset between the donor P(DTB-BDD) and the acceptor (DBTBT-IC or Y6) was more than 0.3 eV, specifying that electron transfer from P(DTB-BDD) to an acceptor was efficient (DBTBT-IC or Y6).

Fig. 3 Chemical structure and energy levels of P(DTB-BDD), DBTBT-IC, and Y6.

The photoluminescence (PL) spectra of neat Y6 film and its blend is studied with P(DTB-BDD) since the HOMO offset P(DTB-BDD)/Y6 is about 0.18, which is not as much of the threshold value required for hole transfer from Y6 to P(DTB-BDD) (Fig. 4). We found that when Y6 is blended with P(DTB-BDD), the intensity of its PL peaks is dramatically reduced, suggesting the efficient hole transfer from Y6 to P(DTB-BDD), with even the HOMO offset below 0.3 eV and even zero, as seen in most non-fullerene-based PSCs.^47–49 As the HOMO offset between DBTBT-IC and P(DTB-BDD) is roughly 0.31 eV, hole transfer from DBTBT-IC to P(DTB-BDD) is possible.

Fig. 4 PL spectra of pure Y6 and Y6:P(DTB-BDD) films.

Using the conventional PSCs, we investigated the photovoltaic functioning of P(DTB-BDD) as a donor in combination with non-fullerene acceptors (DBTBT-IC and Y6) (detailed device fabrication is given in ESI†). Initially, we adjusted the performance by assessment of the donor–acceptor weight ratio and discovered that P(DTB-BDD) : Y6 (1 : 1.2) and P(DTB-BDD) : DBTBT-IC (1 : 1.2) had the best photovoltaic performance (Tables S1 and S2†). To further enhance the photovoltaic performance of the PSCs using a solvent vapor annealing (SVA) treatment, the active layer was exposed to SVA for 40 s in a THF atmosphere. Fig. 5a represents the J–V plots of the binary PSCs under illumination, and Table 2 lists the photovoltaic performance statistics.

Fig. 5 (a) J–V characteristics (AM1.5G, 100 mW cm⁻²) under illumination and (b) EQE response of binary and ternary PSCs.

Table 2 Photovoltaic performance of binary and ternary PSCs

Acceptor	J SC (mA cm^−2)	V OC (V)	FF	PCE (%)
a Average of eight identical devices.
DBTBT-IC	19.06	1.05	0.68	13.61 (13.43)^a
Y6	23.76	0.83	0.64	12.62 (12.46)^a
DBTBT-IC : Y6 (0.3 : 0.9)	24.64	0.92	0.72	16.32 (16.15)^a

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The optimized PSCs based P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6 active layers attained PCE values of 13.61% and 12.62%, respectively. DBTBT-IC-based PSCs showed higher V_OC as compared to Y6 counterparts that are correlated to the upshifted LUMO level of DBTBT-IC (−3.88 eV) relative to Y6 (−4.08 eV) since the V_OC is straightforwardly proportional to the HOMO–LUMO energy offset of the donor and acceptor. Since the J_SC is connected to the photon harvesting efficiency of the BHJ active layer, the trend in the J_SC values agrees with the absorption profile of the acceptors utilized in the BHJ active layer. The external quantum efficiency (EQE) response of the PSCs was also measured to obtain further information about the changes in J_SC values (Fig. 5b). The EQE spectra of P(DTB-BDD):Y6 are wider and extend up to 910 nm, whereas P(DTB-BDD):DBTBT-IC is confined to 810 nm, which accords with the absorption profile of acceptors. The J_SC values computed from the assimilation of EQE spectra for P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6 are 18.81 and 23.48 mA cm⁻², respectively, which are in accordance with the J–V characteristics data (Table 2).

The ternary active layer technique via mixing three organic semiconducting materials in common solvents,^50–57 which uses the simplicity of BHJ as in binary counterparts, is commonly employed to boost the PCE of PSCs. In this method, the complementary absorption spectra of either two donors and one acceptor or two acceptors and one donor were mixed together to enhance the light-harvesting efficiency and well-matched energy levels to increase the D/A interfacial area for exciton dissociation or energy transfer between two acceptors or two donors. The ternary photoactive layer-based PSCs have achieved a PCE of about 19%.^58,59 In our binary BHJ PSCs, the J_SC for Y6 is higher as compared to DBTBT-IC, whereas the FF and V_OC for the DBTBT-IC device were superior to that of the Y6 counterpart. We have incorporated DBTBT-IC as the third component in the host P(DTB-BDD):Y6 to form the ternary BHJ active layer. To fabricate ternary PSCs, weight ratios between the two acceptors were altered while keeping the amount of P(DTB-BDD) and overall concentration of the ternary mixture constant. Table S3† lists the photovoltaic properties for ternary PSCs with various weight ratios (ESI†). The optimized ternary film was then treated with SVA, as is done with binary active layers. Fig. 5a shows the J–V under optimum ternary PSC illumination, with the accompanying photovoltaic data reported in Table 2. The J_SC of the ternary PSCs (24.64 mA cm⁻²) is higher than the binary counterparts, which is also corroborated by the EQE response of ternary PSCs (Fig. 5b). Fig. 5b shows that the EQE values for ternary PSC are greater than those for Y6-based PSC in the 610–800 nm range, where DBTBT-IC absorption is higher than that of Y6, implying that more excitons are created due to DBTBT-IC absorption in the ternary active layer, enhancing the J_SC. According to the EQE spectra, the J_SC value is around 24.43 mA cm⁻², which is consistent with J–V characteristics (Table 2).

The ternary PSC's V_OC value is around 0.92 V, which is higher than the Y6-based PSC but lower than that of DBTBT-IC. We used cyclic voltammetry to determine the HOMO/LUMO values of mixed DBTBT-IC : Y6 (0.3 : 0.9), which are around −5.71 eV/−3.95 eV. The increase in the V_OC of the ternary PSCs may be due to the LUMO level of the blend acceptor being pushed upwards, forming an alloy between two acceptors and behaving as a mixed acceptor.⁶⁰

We have investigated the energy transfer between two acceptors via thin film photoluminescence (PL) measurements (Fig. 6a and b). The DBTBT-IC generated a significant PL in the 700–850 nm range, with a peak at 764 nm (excited at 648 nm), which was almost quenched in the DBTBT-IC:Y6 blend, but the PL strength corresponding to Y6 was increased in the blend compared to pristine Y6 (Fig. 6a). Furthermore, as represented in Fig. 6b, the PL emission band of DBTBT-IC overlaps considerably with the absorption profile of Y6. As demonstrated in Fig. 7, these measurements demonstrate efficient energy transmission from DBTBT-IC to Y6.^61,62 Furthermore, we built acceptor-only devices to learn more about the possibilities of charge transfer between two acceptors. We found that the J_SC value of the DBTBT-IC:Y6 device is similar to that of the pristine acceptor device, showing that charge transfer is not occurring between the two acceptors. Fig. 7 depicts the energy and charge transmission in the ternary device. The excitons are dissociated at the P(DTB-BDD)/Y6 and P(DTB-BDD)/DBTBT-IC D/A interfaces after light absorption by the active layer, and the electrons from the P(DTB-BDD) are transported to both acceptors and finally collected by the cathode, whereas the holes from both Y6 and DBTBT-IC are transferred to P(DTB-BDD) and finally collected by the anode.

Fig. 6 (a) PL response of pristine DBTBT-IC, Y6, and DBTBT-IC:Y6 excited at 648 nm, and (b) PL and absorption response of DBTBT-IC and Y6.

Fig. 7 A schematic of the mechanism of the energy and charge transfer in the ternary PSC.

Charge transport plays a deciding role in efficient PSCs. Thus, we have estimated the hole (μ_h) and electron (μ_e) mobility in the active layers using the single carrier devices and fitting the dark J–V characteristics with the space charge limited current (SCLC) model (Fig. 8a and b).⁶³ The values of μ_h and μ_e are compiled in Table 3. It can be seen that the μ_h/μ_e ratio for the P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6 are about 1.54 and 1.86. This value for P(DTB-BDD):DBTBT-IC is lower than that for P(DTB-BDD):Y6, specifying that the charge transport is further balanced in the former device relative to the latter. The trend in the ratio μ_h/μ_e is well in agreement with the FF values of PSCs (Table 2). It can be seen from the table that both the μ_h and μ_e have increased for ternary devices and the degree of enhancement is more for μ_e and compared to μ_h leading the μ_h/μ_e is about 1.16, demonstrating further balanced charge transportation in the ternary devices and reliable with the increased value of FF.

Fig. 8 Dark J–V plots for (a) hole only and (b) electron only devices and their fitting with SCLC model for binary and ternary devices.

Table 3 Charge carrier mobilities, G_max, P_diss, and P_coll for the binary and ternary PSCs

Acceptor	μ h (cm^2 V^−1 s^−1)	μ e (cm^2 V^−1 s^−1)	μ h/μe	G max (10^28 m^−3 s^−1)	P diss	P coll
DBTBT-IC	3.78 × 10^−4	2.45 × 10^−4	1.54	1.36	0.962	0.814
Y6	3.68 × 10^−4	1.98 × 10^−4	1.86	1.53	0.943	0.798
DBTBT-IC : Y6 (0.2 : 1.0)	4.03 × 10^−4	3.48 × 10^−4	1.16	1.62	0.982	0.854

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We examined the photocurrent density (J_ph)-effective voltage (V_eff) results to learn more about the maximal exciton generation rate (G_max), exciton generation probability (P_diss), and charge collection probability (P_coll) in these devices⁶⁴ (Fig. 9). After an initial linear increase in J_ph as V_eff increased, the J_ph achieved a saturation value J_sat that was independent of V_eff, showing that the majority of photogeneration excitons are efficiently separated into charge carriers. The G_max can be estimated as G_max = J_sat/qL, where q is the elementary charge and L is the active layer thickness. In Table 3, the G_max values for these devices are compiled. The values of G_max for P(DTB-BDD):DBTBT-IC, P(DTB-BDD):Y6 and P(DTB-BDD):DBTBT-IC:Y6 are 1.36 × 10²⁸, 1.53 × 10²⁸, and 1.62 × 10²⁸ m⁻³ s⁻¹, respectively, and is the highest for P(DTB-BDD):DBTBT-IC:Y6 and are consistent with the EQE response as well as J_SC of the PSCs. The values of P_diss and P_coll calculated from the J_ph/J_sat at the short circuit and maximum power point conditions, respectively, are given in Table 3. The values of P_diss/P_coll are higher for P(DTB-BDD):DBTBT-IC as compared to P(DTB-BDD):Y6 and which are in line with the values of FF, as the FF is directly related to the charge collection probability. The values of P_diss/P_coll are further increased for the ternary PSCs, demonstrating improved exciton dissociation owing to the increased interfacial area and charge collection due to the well-adjusted charge transport in the ternary film.

Fig. 9 Dependence of J_ph with V_eff for different PSCs.

In the study of charge recombination kinetics in our PSCs, we looked at the relationship between J_SC/V_OC and illumination intensity (P_in)⁶⁵ (Fig. 10a and b). The power law J_SC ∝ (P_in)^α, follows the dependency of J_SC with P_in (Fig. 10a), where α is the exponent factor and provides information about the extent of recombination (bimolecular). Optimized P(DTB-BDD):DBTBT-IC, P(DTB-BDD):Y6, and P(DTB-BDD):DBTBT-IC:Y6 based PSCs have α values of 0.958, 0.943, and 0.981, respectively. The ternary device's value of α is higher than its binary counterparts, directing that bimolecular recombination is reduced for ternary BHJ films with higher J_SC and FF values.

Fig. 10 (a) J_SC and (b) V_OC–P_in curves for different PSCs.

The formula, V_OC (nkT/q)ln(P_in), describes V_OC's dependency on P_in (Fig. 10b), where the absolute temperature is represented by T, Boltzmann's constant by k, and electronic charge by q. When the number n is approximately 2, under the open circuit condition, the monomolecular or trap-assisted recombination of charges is the primary recombination process, whereas when the value of n is near unity, the bimolecular recombination is dominant. The ternary device's value of n is around 1.13, which is lower than the binary counterpart's values of 1.26 and 1.34 for P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6, respectively, indicating the suppression of trap-assisted recombination in the ternary device and elevate the value of FF.

The X-ray diffraction (XRD) curves of pristine P(DTB-BDD), optimized P(DTB:BDD):Y6, and P(DTB-BDD):DBTBT-IC:Y6 films were recorded to get information about the molecular ordering and crystallinity of binary P(DTB-BDD):Y6 and P(DTB-BDD):DBTBT-IC:Y6 films. Fig. S7† depicts the XRD pattern of pure P(DTB-BDD). The thin film of P(DTB-BDD) revealed two different diffraction heights at 2θ = 5.06° and 24.64° respectively, corresponding to lamellar (100) with d-spacing of 1.714 nm and stacking diffraction (010) with a stacking distance of 0.354 nm. Fig. 11 shows the XRD patterns of binary P(DTB-BDD):Y6 and ternary P(DTB-BDD):DBTBT-IC:Y6. Both binary and ternary films displayed two unique diffraction peaks, namely (100)/(010) peaks at 2θ −5.42°/24.34° and 5.74°/24.78° for binary and ternary films, respectively.

Fig. 11 X-ray diffraction curves of optimized binary and ternary active layers.

The ternary film has a higher intensity of both diffraction peaks than its binary counterparts. For binary/ternary films, the d-spacing for lamellar/stacking is about 1.582/0.358 nm and 1.48/0.351 nm, respectively. The binary/ternary film's crystal coherence length (CCL) in lamellar and π–π stacking directions is about 5.87/2.63 nm and 7.08/3.65 nm, respectively. The ternary film's lamellar d-spacing and stacking distances were found to be lower than their binary equivalents. The CCL values for the ternary film are superior to their binary counterparts, which is beneficial for phase separation in the active layer and improves the charge transport. The charge transport is aided by the small stacking distance and large CCL value near the exciton diffusion length, resulting in a greater FF for ternary PSC compared to its binary ones.⁶⁶

The morphology of the ternary films is also helpful in overcoming the morphology limitations of the binary BHJ active layer, and transmission electron microscopy (TEM) pictures were used to analyze the phase separation and morphology in the photoactive layers (Fig. 12). The photoactive layer's domain size should not be either tiny or too large for an efficient PSC. The charge transport is hampered by a small domain size, whereas the exciton dissociation is restricted by a large domain size. As seen in these images, the ternary films attained better phase separation than that of the binary counterpart, which is advantageous for efficient exciton dissociation and charge transport, causing a high FF and overall greater PCE for the ternary PSC as compared with binary ones.^67,68

Fig. 12 The images of TEM for (a) binary and (b) ternary photoactive layers (scale bar is 100 nm).

3. Conclusion

We have created a D–A copolymer P(DTB-BDD) with a weak DTB (donor) unit and a relatively weak acceptor BDD and investigated its optical and electrochemical properties. With a deep HOMO level of −5.46 eV, P(DTB-BDD) showed strong absorption spectra in the wavelength region of 300–700 nm. Using the middle bandgap acceptor DBTBT-IC and the narrow bandgap acceptor Y6, the photovoltaic performance of P(DTB-BDD) as the donor was examined. The PCE of the BHJ-PSCs based on P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6 was 13.16% and 12.62%, respectively. The V_OC and FF values for P(DTB-BDD):DBTBT-IC are greater than those of the P(DTB-BDD):Y6 device, but the J_SC value for P(DTB-BDD):Y6 is larger compared to that for P(DTB-BDD):DBTBT-IC. Using the benefits of DBTBT-IC and Y6, we generated ternary PSCs by mixing a small quantity of DBTBT-IC into the P(DTB-BDD):Y6, attained a PCE of 16.32%, higher than binary equivalents. The higher exciton generation rate is due to the broader absorption pattern from 300 to 900 nm, efficient exciton utilization through energy transmission from DBTBT-IC to Y6, and a reduction in both bimolecular recombination and trap-assisted recombination. The increase in the V_OC of the ternary PSCs also indicates the formation of an alloy between the two acceptors.

Conflicts of interest

There are no competing interests to be declared by the authors.

Acknowledgements

The present study was supported by RFBR (No. 21-53-53037). We gratefully acknowledge the financial support from the Ministry of Science and Higher Education of the Russian Federation using the equipment of the Center for molecular composition studies of INEOS RAS for NMR, DSC studies and elemental analysis. GDS and his research group are grateful to the Department of Science and Technology for financial support through INDO-Russia and BRICS projects.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2me00166g

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