Molecular Oligothiophene–Fullerene Dyad Reaching Over 5% Efficiency in Single‐Material Organic Solar Cells

A novel donor–acceptor dyad, 4, in which the conjugated oligothiophene donor is covalently connected to fullerene PC71BM by a flexible alkyl ester linker, is synthesized and applied as photoactive layer in solution‐processed single‐material organic solar cells (SMOSCs). Excellent photovoltaic performance, including a high short‐circuit current density (JSC) of 13.56 mA cm−2, is achieved, leading to a power conversion efficiency of 5.34% in an inverted cell architecture, which is substantially increased compared to other molecular single materials. Furthermore, dyad 4‐based SMOSCs display excellent stability maintaining 96% of the initial performance after 750 h (one month) of continuous illumination and operation under simulated AM 1.5G irradiation. These results will strengthen the rational molecular design to further develop SMOSCs for potential industrial application.


Introduction
Single-material organic solar cells (SMOSCs) came into focus again, because the photoactive molecules and materials have been vastly improved in recent time. [1] The promise of facilitated cell fabrication by deposition of only one sole photoactive component leading to high thermal and light stability and therefore long-lived solar cells, renders SMOSCs to attractive candidates for technological production of large-scale organic (DTP)-based A-D-A-type oligomer as D and fullerene PC 61 BM as A which were linked by a flexible alkyl ester chain with spacer lengths of 8-12 atoms. SMOSCs containing dyad 1 with the shortest spacer in the photoactive layer in a "normal" cell architecture reached PCEs of 4.26% after post-treatment with SVA which is the highest reported to date for molecular single materials. The rational molecular design and precise distance of the oligothiophene D and fullerene A resulted in strongly confined lamellae of nanoscopic dimensions allowing for crucial phase separation as important key to ambipolar charge transport and well performing and highly stable SMOSCs. [9b] Herein, we now report the extension of our molecular design to D-A dyad 4, in which PC 61 BM of formerly best performing dyad 1 is replaced by the larger and less symmetric PC 71 BM as acceptor leading to substantially improved SMOSCs with over 5% efficiency and high short-circuit current density J SC of 13.56 mA cm −2 after SVA. This improvement is due to increased spectral absorption of the films, more efficient charge generation, dissociation, and transport as well as reduced charge recombination. Analysis of the energy losses gave detailed information on radiative and nonradiative recombination. [13,15] Moreover, dyad 4-based SMOSCs showed excellent stability under continuous light exposure retaining 96% of the initial efficiency after 750 h (1 month).

Synthesis of D-A Dyad 4
The synthetic route to D-A dyad 4 is shown in Scheme 1 and detailed synthetic procedures are given in the Supporting Information. Hydroxyethyl-substituted dialdehyde 1 [9b] and fullerene carboxylic acid PC 71 BA 2 were connected in a Steglich esterification furnishing D-A intermediate 3 in 74% yield. This was converted in a Knoevenagel condensation with malononitrile and ammonium acetate to targeted dicyanovinylene (DCV) endcapped dyad 4 in a yield of 90%. Oligomer 5 was accordingly synthesized as a reference molecule and mimics the donor subunit of dyad 4 (see the Supporting Information). The elaborated synthesis route over six reaction steps and starting from commercially available reagents delivered D-A dyad 4 in 36% overall yield which will allow for up-scaling. The structures of D-A systems 3, dyad 4, and reference 5 were confirmed by 1 H-and 13 C-NMR-spectroscopy (Figures S1-S3, Supporting Information) and high resolution mass spectra (HRMS) (Figures S4-S6, Supporting Information). Dyad 4 exhibited good thermal stability with degradation temperature T d > 395 °C (5% weight loss) which was determined by thermal gravimetric analysis (TGA) ( Figure S7, Supporting Information). Differential scanning calorimetry (DSC) first heating thermogram revealed an endothermic peak at 123 °C which we address to a glass transition temperature T g due to the mainly amorphous character of this type of dyad. [9b] The second DSC heating scan only revealed a (shallow) exotherm at 260 °C which most probably arises from the PC 71 BM side chains and reflects cold recrystallization. [16] This finding is in accordance with the corresponding scan of pure amorphous PC 71 BM in which this transition is recovered ( Figure S8, Supporting Information).

Optical and Redox Properties of D-A Dyad 4
The optical properties of well soluble D-A dyad 4 were investigated by UV-vis and fluorescence spectroscopy in dichloromethane (DCM) solutions (Figure 1, left) and in thin films before and after SVA (Figure 1, right). The data is compiled in Table 1 and compared to corresponding PC 61 BM-dyad 1 [9b] and to reference D 5 without pending fullerene unit. The absorption spectrum of dyad 4 (black curve) reflects a superimposition of the absorption of Dsubunit 5 (green curve) and A PC 71 BM (red curve). The strongest main absorption band with a maximum at 552 nm is attributed to the π-π* (HOMO-LUMO) transition with charge-transfer (CT) character coming from the A-D-A structure of the D unit and is slightly blueshifted compared to PC 61 BM-dyad 1 (blue curve) and reference 5. The higher energy band at 400 nm comprises the characteristic pure D π-π* and n-π* transitions and the weaker shoulders at 472 and 378 nm are assigned to the appended PC 71 BM. At the low energy side of the spectrum, the onset of absorption was found at 692 nm leading to an optical energy gap of 1.79 eV which is in accordance with those of dyad 1 and reference 5. For all three molecules, emission maxima were found in the regime of 812-814 nm and arise from the excitation of the oligomeric D unit.
UV-vis spectra of thin films of dyad 4, which were prepared by spin-coating of chloroform solutions on glass slides, were measured before and after SVA with carbon disulfide (CS 2 ) (black curves) in comparison to the film spectra of dyad 1 before and after SVA (blue curves) (Figure 1, right). Compared to the solution spectra, the film spectra showed a typical broadening and bathochromic shift of the absorption bands which are more pronounced for the bands arising from the D moiety indicating π-π interactions in the solid state, whereas the fullerene bands are only slightly shifted. In this respect, in the spectra before SVA, the redshift of the longest wavelength band of dyad 4 (Δλ = 28 nm) is slightly smaller than that for dyad 1 (Δλ = 33 nm) indicating less orientation of the D π-system in the pristine state due to the more perturbing PC 71 BM-unit. However, after SVA a stronger redshift is noticeable for dyad 4 (Δλ = 70 nm) than for 1 (Δλ = 50 nm) finally resulting in a similar maximum of 650 nm and 646 nm. For the first series of PC 61 BM-dyads, we rationalized that SVA mainly affects reorientation of oligomeric D segments by π-stacking and decreased distance between the π-conjugated donors. [9b] Moreover, in the film spectrum of dyad 4 a shoulder on the low energy side (≈700 nm) accounts for an even better packing of the conjugated π-system compared to dyad 1.
The film absorption of dyad 4 covers a broad range from 350 to 900 nm and compared to the solution spectrum, the optical energy gap is decreased from 1.79 to 1.58 eV before and to 1.52 eV after SVA. A comparison of the spectra of dyad 4 and dyad 1 after SVA clearly shows substantial additional absorption in the regime of 350-620 nm which is attributed to the stronger absorption of the PC 71 BM-pendant group compared to PC 71 BM and from 660 to 900 nm due to the slight redshift of the CTband of dyad 4 compared to dyad 1. In both cases, a weak shoulder at lower energies (≈700 nm) indicates an improved organization of the conjugated donor parts.
The redox properties of dyads 4 and 1, reference 5, and PC 71 BM were investigated by cyclic voltammetry (CV). CVs are shown in Figure S9 (Supporting Information) and data is compiled in Table S1 (Supporting Information). The CV profile of dyad 4 reflects the superimposition of the redox processes of the D and A subunits as it was found in accordance with the optical investigation and for dyad 1. [9b] From the onset potential of the first oxidation and reduction wave, HOMO and LUMO energy levels were calculated, respectively, which are well adjusted to the electrodes and interfacing layers and are slightly    Measured in dichloromethane, maxima underlined; b) Thin films by spin-coating from chloroform solution on glass, maxima underlined; c) Taken from onset of absorption and calculated by E g = 1240/λ. sh = shoulder increased with respect to dyad 1 (Figure 2c, bottom, left). Concomitant with the optical data, dyad 4 should therefore represent a good candidate for application in SMOSCs.

Photovoltaic Data
The excellent solubility of dyad 4 in solvents such as chloroform (86 mg mL −1 ), chlorobenzene, or o-xylene allowed the preparation of defect-free thin films by spin-coating under ambient conditions. Sole photoactive SMOSCs layers were optimized using the "normal" architecture glass/ITO/PEDOT:PSS/dyad 4/LiF/Al in terms of processing solvent, SVA solvent and exposure time, concentration of the coating solution, thickness of the photoactive layer by spin-speed variation, and additives (Tables S2-S8, Supporting Information). For advanced investigations SMOSCs with the "inverted" device structure glass/ITO/ZnO/dyad 4/MoO x /Ag were used and compared to dyad 1 in the same device architecture. The best performing SMOSCs were obtained by spin-coating a 15 mg mL −1 solution of dyads 4 and 1 in chloroform at a spin-speed of 1500 rpm at room temperature leading to film thicknesses of 100 ± 5 nm and post-treatment of the films with 30 s of SVA in CS 2 atmosphere ( Table 2). J-V-characteristics under 100 mW cm −2 simulated AM 1.5G illumination of the best performing SMOSC with dyad 4 as cast and after SVA are shown in Figure 2a Table S13, Supporting Information. efficiency mainly arises from the substantially increased photocurrent density J SC of 13.56 mA cm −2 in dyad 4 which is the highest value so far measured in oligomer-based SMOSCs. [9,13] The additional absorption of the pending PC 71 BM compared to dyad 1 (Figure 1, right) contributes to this outstanding J SC and is well reflected in the EQE-spectra, in which dyad 4 showed an increased photon-to-charge carrier conversion of above 50% in the wavelength regime of 350-850 nm. A maximum EQE value of 54.2% is obtained at 660 nm, which is the highest for oligomer-based SMOSCs (Figure 2b).
At slightly lower open circuit voltage V OC , the fill factor FF is identical for dyad 4 indicating a similar molecular organization and nanomorphology in the photoactive film. [9b] The PCE above 5% obtained for dyad 4 outperforms several of the polymer-based SMOSCs [1] and represents a new best value for molecular materials which has been raised from recently 3.22% [13] to 4.26% [9b] and now to 5.34% by advancement of the D-A dyad structures (Figure 2d; and Table S13, Supporting Information).

Long-Term Stability of Dyad 4-Based SMOSCs
In order to further highlight the advantages of SMOSCs, especially for D-A oligomeric systems, we conducted illumination stability test of dyad 4 devices at room temperature in nitrogen atmosphere under continuous LED light exposure with an intensity of 100 mW cm −2 ( Figure S10, Supporting Information). Figure 3 exhibits the evolution of the photovoltaic parameters, in which the PCE showed a tiny decrease in the beginning and then stayed flat afterward, retaining 96% of the initial efficiency after 750 hours (1 month). V OC and FF stayed fairly stable over time, while the slight decrease of J SC is responsible for the total decrease, which we ascribe to the reorganization of fullerene aggregates and the contact with interface layers. This result is in accordance with the previous reported data for dyad 1, which also exhibited excellent illumination stability, indicating a potential generality in high illumination stability for SMOSCs based on D-A molecular systems. [1c,9b]

Photoluminescence (PL) and Time-Resolved Photoluminescence (TrPL) Analyses
To investigate the charge transfer (CT) behavior, photoluminescence (PL) and the time-resolved PL (TrPL) decay were measured accordingly. PL spectra of films are shown in Figure 4a. Apparently, both dyads 4 and 1 show significantly quenched PL emission compared to pure donor 5 and fullerenic acceptors, indicating efficient CT between the two moieties. Moreover, after SVA post-treatment, the emission is further quenched for both dyads 4 and 1 ( Figure S11, Supporting Information). The results from PL quenching indicate effective exciton dissociation and CT. This emission quenching behavior was further precisely investigated through TrPL experiments (Figure 4b,c). For the films of pure donor 5 and fullerenic acceptors, PL intensity shows a triexponential decay behavior with a lifetime of 521.6 ps for D 5, and 973.1 and 721.1 ps for PC 61 BM and PC 71 BM, respectively (Table S9 and Figure S12, Supporting Information). Dyad 4 and 1 exhibited a faster decay than the pure D and A films. Particularly, the decay is even accelerated after SVA treatment (Table S9 and Figure S13, Supporting Information).
For the emission from the D segment, the lifetime of dyad 1 is shortened from 223.9 to 58.1 ps after SVA and the one of dyad 4 from 217.0 to 56.2 ps. For the emission from the A part, the difference in lifetime before and after SVA is less compared with the difference of emission from the D, whereby the lifetime of dyad 1 decreases from 37.6 to 34.2 ps and of dyad 4 from 37.1 to 31.9 ps. Compared to the A segment, this more significant change of lifetime for the D segment reveals that SVA treatment mainly contributes to the improvement of the D morphology, while the fullerenic acceptors are less influenced by SVA. [9b] In particular, dyad 4 shows the fastest decay with a lifetime of 56.2 ps for the emission from the D segment and 31.9 ps for the emission from the A segment. This phenomenon indicates a significantly improved D packing after SVA post-treatment for dyad 4, where excitons generated in the D region are most efficiently dissociated at the D/A interface.

Energy-Loss Analyses
For the determination of the detailed energy loss, we first calculated the effective energy bandgaps (E g ) of dyad 4-and dyad 1-based devices by the method originally proposed by Rau et al. and recently extended by Almora et al. The detailed analysis is described in the Supporting Information ( Figure S14 and Table S10, Supporting Information) and results in an identical bandgap value E g of 1.62 eV for dyad 1 and dyad 4. [17] In order to determine the origin of the voltage losses from the detailed balance theory, electroluminescence (EL) spectra, and Fourier transform photocurrent spectroscopy-external quantum efficiency (FTPS-EQE) measurements were carried out on dyad 4 and dyad 1 devices. [18][19][20] It is well  accepted that the three sources of energy loss (E loss ) obey the following equation [21,22] loss gap C T O C,rad O C,nr The calculated energy loss from bandgap to V OC is similar for SMOSCs with dyad 1 (0.77 eV) and with dyad 4 (0.79 eV).
The CT state energies (E CT ) were estimated by simultaneously fitting EL and EQE PV according to the literature. [23,24] As shown in Figure 4d  loss due to nonradiative charge carrier recombination, ΔE 3 = qΔV OC,nr , reflects the difference between qV OC,rad and the measured qV OC under AM 1.5G simulated solar spectrum. Dyad 4based SMOSCs exhibited slightly higher ΔE 3 value than dyad 1, which could be responsible for the slightly lower V OC from dyad 4. Further research is required to better understand the nature of the nonradiative recombination losses. Geminate recombination is expected to be more dominant in SMOSCs because of their inherent intimate mixture and has to be studied in detail.

Charge Recombination and Transport
In order to display other factors contributing to the large difference in J SC , we investigated the charge recombination process in both SMOSCs based on dyad 4 and dyad 1. J ph -V eff characteristics was first measured and is shown in Figure 5a. Under high reverse bias, the saturated current density J ph,sat was determined and the maximum charge generation rate (G max ) was calculated accordingly. As listed in Table S11 (Supporting Information), dyad 4 showed an increased G max of 1.14 × 10 28 m −3 s −1 compared to 7.89 × 10 27 m −3 s −1 for dyad 1 which is as well enhanced compared to the device without SVA post-treatment ( Figure S15, Supporting Information). Light intensity (P light ) dependent photovoltaic performance was also measured for dyad 4 and dyad 1. From the J SC -P light plots as shown in Figure S16 (Supporting Information), dyad 4-and dyad 1-based SMOSCs showed almost unity dependence of J SC on P light , indicating negligible effect from 2nd order recombination. From the V OC -P light plots (Figure 5b), we observed a dependence of V OC on P light for dyad 4-and 1-based SMOSCs with a slope of 1.21 or 1.49 kT q −1 , reflecting the existence of both, 2nd order recombination and trap-assisted recombination. [11a] Additionally, charge transport was as well investigated for dyad 4 and 1. From the space charge limited current (SCLC) measurements (Figures S17, S18, and Table S12, Supporting Information), dyad 4 showed slightly higher mobilities for holes and electrons (7.58 × 10 −5 cm 2 V −1 s −1 , holes; 1.31 × 10 −4 cm 2 V −1 s −1 , electrons) compared to dyad 1 (6.48 × 10 −5 cm 2 V −1 s −1 , holes; 8.64 × 10 −5 cm 2 V −1 s −1 , electrons). [25]

Film Morphology
In order to obtain information on the impact of SVA on the nanomorphology of the photoactive layer of dyad 4, we investigated the surface by means of tapping mode atomic force microscopy (AFM). Topography images for the as-cast films revealed a tightly packed morphology with smooth surface comprising statistically distributed small holes (d = 25-70 nm, h = 2-10 nm) ( Figure S19, left, Supporting Information). After SVA, a morphological change is visible, whereby diameter and height of the holes are reduced (d = 25-60 nm, h = 2-4 nm) and roughness slightly increased (0.7-1.1 nm) ( Figure S19, right, Supporting Information). Corresponding phase images in both cases lacked relevant contrast suggesting that phase separation should be smalller than the AFM-resolution of 15 nm. In the case of dyad 1, AFM topography images revealed finest morphology with small structural features, which were slightly growing after SVA.
We attempted the investigation of the inner film morphology by grazing-incidence wide-angle X-ray scattering (GIWAXS) of E g was determined by the method reported by Almora; b) V OC was measured under a solar simulator; c) V OC,rad is the radiative limit to V OC , measured and calculated using FTPS; d) ΔV OC,rad is the voltage loss due to non-ideal absorption, calculated from EL and FTPS measurement; e) ΔV OC,nr is the voltage loss due to non-radiative recombination. samples of dyad 4 before and after SVA. The results confirm that the films also after SVA are highly disordered and amorphous due to diffuse reflections in the GIWAXS patterns. According to the 2D images and the z-and y-cuts, unfortunately, not a ring nor other hints for slight π-π stacking of the donor subunits could be identified in each of the samples. In comparison to the symmetrical C 60 moiety in dyad 1, which showed weak ordering of the D units in films after SVA, [9b] the larger and unsymmetrical C 70 moiety in dyad 4 obviously prevents better self-organization. However, these subtle differences did not lead to significant changes in the fill factor FF of SMOSCs, which practically was the same for both derivatives (vide supra).

Conclusion
We have extended our molecular design concept for structurally defined, covalently linked D-A dyads by preparing ambipolar derivative 4, in which a p-type oligothiophene D and a n-type PC 71 BM fullerene A unit are linked by a flexible alkyl ester chain in a "T-shaped" fashion. The exchange of the pending PC 61 BM unit in the previously reported dyad 1 [9b] for the larger and less symmetrical PC 71 BM in 4 led to fairly enhanced photovoltaic performance in SMOSCs. The rise to the highest PCE of 5.34% for molecular materials (see Figure 2) mainly derives from a substantially increased photocurrent density J SC of 13.56 mA cm −2 . The additional absorption of the pending PC 71 BM contributes to the outstanding J SC , which outperforms several of the polymer-based SMOSCs, [1] and is well reflected in the increased photon-to-charge carrier conversion with EQE values over 50% over the visible region (see Figure 2). Moreover, dyad 4-based SMOSCs showed a promising long-term stability under continuous light exposure by retaining 96% of the initial efficiency after 750 h (1 month). With respect to the fundamental photophysics and charge carrier dynamics, SMOSCs based on dyad 4 not only showed an increased efficiency of exciton splitting to overcome the exciton binding energy, but also improved charge transport and led to less trap-assisted recombination of holes and electrons compared to corresponding dyad 1. The detailed energy losses were calculated and analyzed taking the energetics of the CT-states and band gaps of both dyads into account.
The semiquantitative structure-property-device performance relationships obtained in this investigation for our oligomeric D-A dyads will help in joint efforts to further upgrade the rational development of photoactive single materials by chemists and of SMOSC devices by physicists and engineers. Starting points would be to further widen and intensify the absorption range of D-A systems into the NIR by tuning the molecular structure. It is expected that advanced photophysics and a further increase of J SC could be obtained. A further key step, which however is still difficult to achieve, is the control of the structurally imprinted self-organization behavior of the dyads to form less amorphous photoactive supramolecular nanostructures with optimally segregated and oriented D and A domains allowing for ameliorated charge separation and transport.
Despite the advantageous performance of highly optimized binary or ternary bulk-heterojunction solar cells, the plausible simplified concept of SMOSCs might evolve to a follow-up technology for practical application in the near future, if their recently achieved continuously improving photovoltaic performance will persist.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.