Highly efficient fullerene and non-fullerene based ternary organic solar cells incorporating a new tetrathiocin-cored semiconductor

A new dual-chain oligothiophene-based organic semiconductor, EH-5T-TTC , is presented. The molecule contains two conjugated chains linked by a fused tetrathiocin core. X-ray crystallography reveals a boat conformation within the 8-membered sulfur heterocycle core and extensive π - π and intermolecular sulfur-sulfur interactions in the bulk, leading to a 2-dimensional structure. This unusual molecule has been studied as a ternary component in organic solar cell blends containing the electron donor PTB7-Th and both fullerene (PC 71 BM) and non-fullerene acceptors ITIC and EH-IDTBR . By incorporating EH-5T-TTC as a ternary component, the power conversion efficiency of the binary blends containing non-fullerene acceptor increases by 17 % (from 7.8 % to 9.2 %) and by 85 % for the binary blend with fullerene acceptor (from 3.3 % to 6.3%). Detailed characterisation of the ternary blend systems implies that the ternary small molecule EH-5T-TTC functions differently in polymer:fullerene and polymer:non-fullerene blends and has dual functions of morphology modification and complementary spectral absorption. 71 BM:EH-5T-TTC ternary blend compared to PTB7-Th:PC 71 BM can explain the reduced recombination losses of the ternary compared to the binary blend. The difference in morphology observed between fullerene and non-fullerene based blends upon the incorporation of EH-5T-TTC can account for the variation in recombination dynamics detailed above.


Introduction
Organic photovoltaic (OPV) devices based on nanocomposites of π-conjugated semiconductors are a prospective solar cell technology 1 and have attracted considerable attention due to unprecedented attributes such as printability, foldability, portability, wearability, semitransparency and amenability to cost-effective large area fabrication. 2  Despite the high absorption coefficient of π-conjugated organic semiconductors (10 5 -10 6 cm -1 ), due to their bonding properties, the narrow absorption bandwidth of the organic semiconductors limit the spectral overlap of the D/A blend with the solar spectrum, thus adversely restricting the JSC. 9 Furthermore, the low exciton diffusion length (5-15 nm) and low charge carrier mobility (~ 10 -4 cm 2 V -1 s -1 ) of organic semiconductors restrict the active layer thickness of the bulk heterojunction (BHJ) solar cells ~ 80-150 nm, which also results in incomplete absorption of the incident solar spectrum. To circumvent this, homo and heterojunction tandem solar cell device architectures can be introduced to broaden the light harvesting spectral region and to reduce any thermalisation losses. 10 Although the technology is successful in terms of achieving its purpose, the improvement in PCE comes with a high cost and complexity. The design of charge recombination layers, stringent current matching conditions, necessity of solvents with orthogonal solubility properties, complex fabrication steps and low yield, all make the tandem architecture too complicated for a printing process and hence less appealing for adoption by industry. 2 Another very promising and simple strategy is to keep the single junction architecture, but combine multiple donor/acceptor molecules to extend the spectral overlap of the organic solar cells with the incident solar spectrum, resulting in so-called ternary blend organic solar cells. 11 Here, in this work, this latter approach has been used to obtain highly efficient fullerene and non-fullerene based ternary organic solar cells; the light absorbing active layer is composed of two donor molecules and one fullerene/non-fullerene acceptor molecule and such ternary blend systems have been shown to provide significant improvements in power conversion efficiencies when compared to parent binary blends. 11,12 The main donor material considered is the polymer poly [4,8- In the present study, the binary blend systems investigated to enhance the power conversion efficiency are PTB7-Th:ITIC [3,9-  to both fullerene and non-fullerene based organic solar cells is significant and such wide applicability has seldom been reported in ternary blend organic solar cells. Detailed characterisation of the ternary blend systems implies that the ternary small molecule EH-5T-TTC functions differently in polymer:fullerene and polymer:non-fullerene blends and has dual function in terms of morphology modification and complementary spectral absorption. The improvement in PCE of the ternary blend systems due to the incorporation of EH-5T-TTC is a synergistic effect of enhanced light absorption, improved nanoscale and percolative morphology, reduced recombination losses and increased exciton generation and dissociation probability.

Synthesis of EH-5T-TTC and X-ray crystallographic studies
The syntheses of compounds EH-5T-TTC and EH-5T-Ge is accomplished from the 1,3-dithiole-2-one precursor (1 in Scheme 2), which in turn is prepared from a protocol reported in our previous work on a hexyl analogue of EH-5T-Ge 19 Figure 1). Because of the nature of the slip-stacked structure, these S···S contacts perpetuate through the crystal. In combination with the π-π interactions, these contacts give rise to a structure featuring extensive 2-dimensional orbital overlap which is important for good charge transport properties in the bulk. 23 Note that the sheets depicted in

Photophysical and photovoltaic studies of blends
The first series of blends to be studied incorporated PTB7-Th as the donor and ITIC as the acceptor component. The UV-vis absorption spectra of the donor, acceptor and the corresponding binary blend (PTB7-Th:ITIC) films are shown in Figure 3(a) (see also Figure S1 for energy level diagrams of the individual components in the binary and each of the ternary configurations). The absorption of the PTB7-Th:ITIC blend is mainly concentrated in the 550-750 nm spectral range, with poor absorption below the wavelength of 550 nm. Figure 3 Figure S2). For the ternary blend system consisting of P3HT and EH-5T-Ge a drastic drop in PCE is observed. This could be due to the surface energy differences among the components leading to their poor miscibility and/or incompatible crystalline structure or orientation. The PCE comparison of the ternary blends with the binary blends and the corresponding J-V characteristics are shown in Figure S2. Since the ternary blend system consisting of EH-5T-TTC showed an enhancement in PCE, the optimum content of EH-5T-TTC was investigated. Figure  The corresponding photovoltaic performance parameters are listed in Table S1. It was found that the addition of 10 wt % EH-5T-TTC to the PTB7-Th:EH-IDTBR blend increased the PCE by 17%, from 7.82 % to 9.19%. This improvement in PCE is mainly due to the increase in the short circuit current density and the fill factor of the ternary blend solar cells. The EQE spectra shown in (Figure 5 OPVs and acts as a co-solvent and plasticiser to give varying morphologies in blends. 24 The corresponding device performance parameters for the various blends are listed in Table S2  This increase in Jsc is in correspondence with the increase in EQE as shown in Figure S4(d).
Also as seen from Figure S4(d) and S4(a), the absorbance of the ternary blend and the EQE spectra show the enhancement in the similar spectral region, implying the enhances light harvesting by the ternary blend compared to the binary blend. Table 1 Table S1. To understand the role of surface energy as the driving force for the difference in morphology for fullerene and non-fullerene based ternary blends containing EH-5T-TTC, contact angle measurements were performed. Previous studies have shown that the surface energy of different components in a blend determines their segregation and location in the blend system. 27,28 The contact angles of polymer PTB7-Th, fullerene acceptor PC71BM, nonfullerene acceptors EH-IDTBR and ITIC, and the ternary component EH-5T-TTC were measured to evaluate the surface energy ( Figure S8 and Table 2 Table 2.

Equation 1
In   Thus, enhanced exciton generation and exciton dissociation was observed for EH-5T-TTCadded ternary components.
Along with increased exciton generation rate, exciton dissociation probability is also important for enhanced photocurrent. The exciton dissociation probability has now been estimated using the relation: where P(E,T) is the dissociation probability 34,36,37 . In the case of PTB7-Th:ITIC, the incorporation of EH-5T-TTC increased the P(E,T) from 81 to 91%, for PTB7-Th:EH-IDTBR the improvement is from 83 to 90 %, and the highest improvement is obtained for PTB7-Th: PC71BM where the P(E,T) improvement is from 71 to 90% ( Figure S9). This highest dissociation probability for the PTB7-Th:PC71BM blend with the incorporation of EH-5T-TTC can be related to the finer morphology of the ternary blend compared to the binary blend system as shown in Figure 7(c ) and (d).
= e × ℎ = P(E, T) Finally, to investigate the possible energy transfer process between the EH-5T-TTC and PTB7-Th, steady state PL spectra were taken ( Figure S10). The excitation wavelength used is 515 nm. No measurable PL signal could be obtained from EH-5T-TTC. In the case of PTB7-Th, the PL spectra show a broad emission peaking at 760 nm. The excitation wavelength of 515 nm used in this study excited both PTB7-Th and EH-5T-TTC. The change observed in the PL of PTB7-Th with 20% and 40% wt blend may be either due to reduced selfquenching upon dilution with EH-5T-TTC or energy transfer from EH-5T-TTC to PTB7-Th 38 .
In order to prove that the beneficial properties of EH-5T-TTC are unique to the core tetrathiocin structure, three types of fullerene-containing organic solar cells were prepared and characterised. The devices, which were made under the same conditions, were not optimised and the experiments were carried out simply to compare a binary device of PTB7-Th:PC71BM with ternary blend devices incorporating the non-tetrathiocin compound 1 (half-unit of EH-5T-TTC, Scheme 2) and EH-5T-TTC. The highest recorded PCE of the binary device was found to be 3.33% (Table S3). This value increased to 4.26% in PTB7-Th:PC71BM:1 devices, but a much greater increase was observed for the ternary blend containing EH-5T-TTC (5.89%).
The corresponding J-V characteristics and the EQE spectra are shown in Figure S11. These results clearly indicate that the enhancement in device characteristics is not simply a function of the quinquithiophene chains, but is due to the physical properties of the more complex tetrathiocin structure.

Characterisation of the binary and ternary active layer blend
The surface morphology of the PTB7:PC71BM films was characterised using atomic force microscopy (AFM