Contenido, forma y calidad

1.6.1 Background

Whilst planar bilayer configurations have been effectively used to produce solar cells from SubPc derivatives, the number of reports describing bulk heterojunction devices employing SubPc 1 or SubPc derivatives as active materials is still scarce. Work on the performance of SubPc-based BHJ OPV cells has indeed been focused on the optimization of the SubPc 1/fullerene D/A heterojunction. SubPc 1 is sparingly soluble in most organic solvents and therefore is incorporated into BHJ devices by vapor deposition as opposed to solution processing.

Film morphology optimization for cells containing SubPc 1 as a donor and fullerene C60 as an acceptor has been accomplished by continuously graded D/A heterojunction (GHJ).²³⁸ The GHJ permits the formation of a network of donor and acceptor materials for optimized exciton diffusion and charge carrier transport, leading to a significant improvement in device performance relative to that of planar and uniformly mixed OPVs. Using this architecture, an efficiency of 4.2% has been demonstrated. OPV cells constructed using mixed films of SubPc 1:C60 with an optimum ratio of 80 wt.% C60, provided a PCE of 3.7%.²⁰¹ This optimum composition also coincides with a peak in the FF and the JSC density. The performance of BHJ cells based on the electron D/A pairing of SubPc 1 and C70 fullerene has been also reported.²³⁹ C70, due to its red shifted absorption edge and larger extinction coefficient compared to C60, substantially increased the JSC density. Recently, PCEs up to 6% have been claimed in planar-mixed OPV devices prepared from SubPc 1 and C70.²⁴⁰ The effect of a SubPc interfacial layer on the performance of inverted polymer solar cells based on P3HT/PC71BM has been also reported.²⁴¹

While uncommon, a few examples of solution deposition of SubPc derivatives in BHJ solar cells can be found in the literature. SubNc has been used as an electron donor, combined with a

238Pandey, R.; Holmes, R. J. Adv. Mater. 2010, 22, 5301.

239a) Pandey, R.; Zou, Y.; Holmes, R. J. Appl. Phys. Lett. 2012, 101, 033308. b) Zheng, Y. -Q.; Potscavage, W. J.;

SubPcs as acceptor materials in BHJs: background

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PC71BM acceptor in BHJ cells.²⁴² In spite of the limited solubility of SubNc in organic solvents, the solution processed device exhibited an efficiency of 4.0% under 1 sun, AM 1.5G solar irradiation at room temperature. First report of solution processable SubPcs in BHJ solar cells described the use of perfluorinated SubPc 8c as acceptor and typical polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) as donor, leading to fully solution processed OPVs with efficiencies over 0.1%.²²⁴ Bender et al. have tested the effect of a highly soluble tri-n-hexylsilyl oxide boron SubPc derivative and other three different phenoxy-SubPcs as additives in a P3HT:PC61BM cascade ternary BHJ OPV,²⁴³ and, while the SubPc chromophores had the correct frontier orbital energies to produce a cascade electron transfer between P3HT and PC61BM phases, none of them was proven to facilitate the desired electron transfer effect.

Very recently, the fabrication of solution-processed BHJ devices from halogenated SubPc units, namely F6-SubPc-OPhF5 and Cl6-SubPc-OPh, as the acceptor component and conventional polymeric donor materials such as MEHPPV, P3HT and PTB7 has been demonstrated (Figure 50).²⁴⁴ The high solubility of the SubPc derivatives facilitated the formation of efficient D/A networks and provided PCEs of 0.4% with MEH-PPV, 1.1% with P3HT and 3.5% with PTB7. A clear contribution of photon harvesting by the acceptor was identified from the EQE spectra.

Analysis of the J-V characteristics and photoluminescence quenching revealed trap-assisted and geminate recombination as a loss mechanism. These results show that solution-processable SubPcs are a promising alternative to fullerenes for polymer solar cells.

242Chen, G.; Sasabe, H.; Sano, T.; Wang, X. -F.; Hong, Z.; Kido, J.; Yang, Y. Nanotechnology 2013, 24, 484007.

243Lessard, B. H.; Dang, J. D.; Grant, T. M.; Gao, D.; Seferos, D. S.; Bender, T. P. ACS Appl. Mater. Interfaces 2014, 6, 15040.

244Ebenhoch, B.; Prasetya, N. B. A.; Rotello, V. M.; Cooke, G.; Samuel, I. D. W. J. Mater. Chem. A 2015, 3, 7345.

Chapter I

131 Figure 50. a) Molecular structure (left) of SubPc derivative F6-SubPc-OPhF5, EQE spectra (middle) and J-V characteristics (right) of MEH-PPV (squares), P3HT (circles) and PTB7 (diamonds) blended with F₆-SubPc-OPhF₅. b) Molecular structure (left) of SubPc derivative Cl₆-SubPc-OPh, EQE spectra (middle) and J-V characteristics (right) of MEH-PPV (squares), P3HT (circles) and PTB7 (diamonds) blended with Cl6-SubPc-OPh.

b) a)

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1.6.2 Results and discussion

In the previous section of this chapter, Cl6-SubPc-Cl 29 has demonstrated great potential as the best performing n-type SubPc derivative in combination with diverse small-band-gap donor materials in PHJ OSCs. For this reason, five hexachloro-substituted SubPcs bearing distinct axial substituents, i.e. chlorine, fluorine and differently substituted phenoxy groups, were selected as electron acceptors in BHJ solar cells. While modification of the axial substituent does not alter the absorption spectra and results in a slight variation of its electron-accepting character, it can greatly affect its aggregation and crystallization behavior. On the other hand, the novelty and good results obtained in the use of compound 31 in PHJ devices prompted us to include chloro-tricyano-SubPc derivative 26 in this study. This work has been developed in collaboration with the research group of Professor René Janssen in the Technische Universiteit Eindhoven.

1.6.2.1 Synthesis and characterization of subphthalocyanines 32-35

Five axially substituted Cl6-SubPc-X derivatives were used in this study. Synthesis of Cl6 -SubPc-Cl 29 has been previously described.^109b,²⁰⁶ Reaction with selected phenol derivatives with electron-donating or electron-withdrawing substituents provided Cl6-SubPc-OPh(OMe)3 32, Cl6-SubPc-OPh^tBu 33 and Cl6-SubPc-OPhF5 34 in good yields (Scheme 16). Finally, Cl6-SubPc-F 35 was prepared starting from SubPc 33 by adapting the method of M. V. Fulford et al.^42b,64