The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk‐Heterojunction Solar Cells

Abstract Four hexachlorosubphthalocyanines SubPcCl6‐X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution‐processed bulk‐heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone‐shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7‐Th as the electron donor exhibit a power conversion efficiency up to 4 % and an external quantum efficiency approaching 60 % due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non‐fullerene acceptors based on subphthalocyanines and structurally related systems.

Abstract: Four hexachlorosubphthalocyanines SubPcCl 6 -X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution-processed bulkheterojunction organic solar cells.S ubphthalocyanines are aromatic chromophoric molecules with cone-shaped structure, good solution processability,i ntense optical absorption in the visible spectral region, appropriate electron mobilities,a nd tunable energy levels.S olar cells with subphthalocyanines as the electron acceptor and PTB7-Th as the electron donor exhibit apower conversion efficiency up to 4%and an external quantum efficiency approaching 60 %d ue to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating ap romising prospect of non-fullerene acceptors based on subphthalocyanines and structurally related systems.
Solution-processedbulk-heterojunction(BHJ)organicsolar cells (OSCs) are ap romising renewable-energy technology towards future efficient, large-area, flexible photovoltaic modules. [1] Them ain component of an OSC is its BHJactive layer, consisting of an electron-donor and an electronacceptor phase separated into abicontinuous interpenetrating network morphology. [2] Power conversion efficiencies (PCEs) exceeding 11 %have been achieved recently. [3] While numerous electron donors,i ncluding semiconducting polymers and small molecules,h ave been assessed, [4] electron-acceptor components are still dominated by fullerene derivatives because of their high electron mobility,i deal frontier orbital energy levels,a nd isotropic charge-transport properties. [5] However,f ullerene derivatives have intrinsic shortcomings, such as high cost of synthesis,l ow absorption coefficients in the visible spectral region, limited variability in the energy levels,a nd morphological instability in the blended films. [6] Thedevelopment of new electron acceptors which overcome the drawbacks associated with fullerene-based acceptors is thus vital for further advancing OSCs. [7] Encouragingly, several studies have reported BHJ solar cells with PCEs > 8% based on non-fullerene acceptors. [8] Subphthalocyanines (SubPcs) are aromatic chromophoric molecules with ab oron atom at their inner cavity,i ntense optical absorption in the 460-580 nm spectral region, [9] and relatively high electron mobilities. [10] Traditionally,t hey have been used as electron donors in vacuum-deposited planarheterojunction solar cells. [9a] However,the electronic properties of SubPcs can be easily adjusted by introducing axial and/ or peripheral substituents. [9a] Hence,b yr ational molecular design, that is,i ntroducing peripheral electron-withdrawing groups,SubPcs have been transformed into electron-acceptor molecules. [11] Forexample,non-fullerene vacuum-evaporated solar cells containing SubPc molecules achieved aP CE of 8.4 %. [12] Thec one-shaped structure of SubPcs prevents excessive aggregation in solution and in the solid state, providing good solution processability even without the assistance of electrically insulating alkyl chains.Nevertheless, only very recently,SubPc molecules have been demonstrated as electron acceptors in solution-processed BHJ solar cells. [13] Herein, we report on four hexachlorosubphthalocyanines (SubPcCl 6 -X) bearing different axial substituents,t hat is, chlorine and differently substituted phenoxy groups,a s electron acceptors in BHJ solar cells.S ubPcCl 6 -Cl has demonstrated great potential as n-type material in planarheterojunction OSCs. [11] Modification of the axial substituent results in aslight variation of the electron-accepting character of the molecule and can greatly affect its aggregation and crystallization behavior. Amaximum PCE of 4.0 %has been achieved for SubPcCl 6 -Cl, which is the highest value for solution-processed solar cells based on SubPc derivatives. This work paves the way to an ew class of non-fullerene acceptors with tunable optoelectronic properties and microstructures for efficient BHJ OSCs.
All four SubPc compounds feature almost identical absorption spectra in toluene,s ignifying as mall influence of the axial substituents.T he Q-band peaks are found at wavelengths around 570 nm with absorption coefficients of 4.0 10 4 -4.5 10 4 m À1 cm À1 ( Figure S2, Table 1). In films,t he absorption spectra of SubPcCl 6 -X (Figure 2a)s how ab athochromic shift of about 10 nm compared to the spectra in solution (Table 1). Remarkably,t he SubPcCl 6 -Cl absorption spectrum in film displays abroader Qband with an additional intense maximum at 558 nm, which could be attributed to the formation of head-to-tail columnar stacks (H-type-like aggregates) in the solid state,favored by the presence of the small chlorine atom in the axial position. Axial substitution with bulky phenoxy groups in the other three SubPcCl 6 -X deriv-atives precludes this behavior.
Thefluorescence quantum yield f F of SubPcCl 6 -Cl, SubPcCl 6 -OC 6 H 4 t Bu and SubPcCl 6 -OC 6 F 5 in toluene is around 0.35-0.64, but it is only 0.003 for SubPcCl 6 -OC 6 H 2 (OMe) 3 .T he strong quenching in the latter is attributed to an intramolecular photoinduced electron-transfer process from the electron-rich axial phenoxy substituent to the SubPc acceptor.
Thephotovoltaic properties of the SubPc molecules were evaluated in solar cells with an ITO/ZnO/PTB7-Th:SubPcCl 6 -X/MoO x /Ag device architecture under simulated AM1.5G illumination (100 mW cm À2 ). Thecurrent density-voltage (J-V)c haracteristics and external quantum efficiency (EQE) spectra of the optimized devices are shown in Figure 3a nd summarized in Table 2. To accurately determine the PCEs of the solar cells,t he short-circuit current density (J sc )w as obtained by integrating the EQE with the AM1.5G spectrum. Results from different fabricating conditions and device statistics can be found in Table S1 and S2. Solar cells with as tructure that uses either ITO/PEDOT:PSS or ITO/MoO x   as bottom contacts all produced very poor results,p ossibly caused by an unfavorable vertical phase segregation (Figure S5). Thehighest PCE of 4.0 %was found for SubPcCl 6 -Cl, which offered a J sc of 10.7 mA cm À2 ,a no pen-circuit voltage (V oc )of0.77 V, and afill factor (FF) of 0.48. In each case both the polymer donor and the SubPc acceptor contribute substantially to the photocurrent (Figure 3b). TheE QE maxima located at about 710 nm originate from PTB7-Th, while the peaks at about 570 nm result from the SubPc molecules.T he lack of light absorption of PTB7-Tha nd SubPcCl 6 -X between 350 and 450 nm results in ar ather uncommon shape in the EQE spectra, with av alley in this region. At higher wavelengths,animpressive EQE maximum of 0.58 was achieved for SubPcCl 6 -Cl, while the other SubPc molecules produced significantly lower EQE and J sc values. As ar esult, these devices all afforded PCEs below 2.0 %.
Notably,e xcept for SubPcCl 6 -OC 6 F 5 ,t he SubPc solar cells exhibit asimilar V oc value (about 0.8 V) as PTB7-Th:fullerene devices.T able 2also shows that acommon limitation of these solar cells is their low FF (< 0.5), which is considerably lower than that of the state-of-the-art fullerene-based devices.
Understanding the origin of the low FF of these devices is crucial to further develop SubPc-based acceptors.
Hence,t he charge-carrier transport properties and the bimolecular charge recombination of the PTB7-Th:SubPcCl 6 -Xb lend films were investigated. Electron mobilities (m e ) ( Table 2) were estimated from electron-only PTB7-Th :SubPcCl 6 -X devices by fitting the J-V data ( Figure S6) to as pace-charge-limited current model, resulting in m e % 10 À6 cm 2 V À1 s À1 for all four acceptors.K eeping in mind that PTB7-Th:fullerene blends exhibit a m e of 10 À3 -10 À2 cm 2 V À1 s À1 , [14] the considerably lower m e values of the PTB7-Th:SubPcCl 6 -X films are likely causing the low FF.
As ar esult of the low m e values,P TB7-Th:SubPcCl 6 -X solar cells exhibit substantial bimolecular charge-recombination losses.T hese are evidenced by the large difference between the EQE measured with light bias (EQE bias )a nd without light bias (EQE nobias )( Figure S7). Theaverage values for 1 = EQE bias /EQE nobias (Figure 4) can be used to estimate the bimolecular recombination efficiency via h BR = 1À1. [15] Hence al ow 1 value indicates considerable bimolecular recombination. Fors tate-of-the-art polymer:fullerene solar cells 1 approaches unity.T he low 1 values of these PTB7-Th :SubPcCl 6 -X devices evidence severe bimolecular recombination losses,e ven at short circuit. Theh ighest 1 value is  found for PTB7-Th:SubPcCl 6 -Cl, which is consistent with the higher m e value,FF, and PCE of the SubPcCl 6 -Cl solar cell. We speculate that the higher performance originates from the Htype-like aggregates of SubPcCl 6 -Cl in the solid state,w hich are favorable for charge transport.
Next to bimolecular recombination, also geminate recombination can result in al ow FF.T he current density of illuminated solar cells shows as ubstantial increase under reverse bias.T his can be ac onsequence of the enhanced internal electric field promoting charge separation from the donor-acceptor interface.
Themorphology of PTB7-Th:SubPcCl 6 -X blend films was investigated by transmission electron microscopy (TEM). The blends based on SubPcCl 6 -OC 6 H 4 t Bu and SubPcCl 6 -Cl show homogeneous films without noteworthy phase separation (Figure 5b and c). In intimately mixed blends,c harge separation is prevented because of the lack of pure domains. High domain purity is beneficial for dissociating photogenerated charges from the donor-acceptor interface,w hile low domain purity often causes serious geminate recombination. [16] In the films based on SubPcCl 6 -OC 6 H 2 (OMe) 3 and SubPcCl 6 -OC 6 F 5 ,t here is as lightly increased contrast between the light and dark regions,indicating amore distinct phase separation, but these films lack long-enough fibrillary structure (Figure 5a and d). Charge-carrier transport in such films may thus be impeded because of ap oor interconnectivity between neighboring domains.C ombined, the low electron mobility,t he charge recombination, and the morphology explain why PTB7-Th:SubPcCl 6 -X BHJ solar cells show low FF.
In conclusion, hexachlorosubphthalocyanines bearing different axial substituents have been synthesized and used as electron acceptors in BHJ polymer solar cells.APCE up to 4.0 %w as achieved, which is the highest reported value for solution-processed SubPc-based solar cells.Both the polymer donor and the SubPc acceptor contribute significantly to the photocurrent, indicating promising acceptor properties of subphthalocyanines.T he main limitation of these SubPcbased solar cells is their low fill factor,w hich is ac ollective result of low electron mobility,s erious bimolecular recombination, and suboptimal BHJ morphology.F urther research involving subphthalocyanines and structurally related systems such as subnaphthalocyanines should therefore focus on improving electron mobility,a voiding geminate recombination, and controlling microstructure in solid state through rational molecular design.