Solvent‐Annealed Crystalline Squaraine: PC₇₀BM (1:6) Solar Cells

Abstract

Effi cient bulk heterojunction (BHJ) solar cells are characterized by a large interface area between donor and acceptor materials that ensures effi cient photogenerated exciton dissociation into free charge. The optimal scale of the phase separation between these consistuents is that of the exciton diffusion length ( L D ), and the separated phases must be contiguous to allow for low-resistance charge transport pathways from the photosensitive region to the electrodes. [1][2][3][4][5][6] To realize such a BHJ nanostructure, techniques such as thermal [ 7 ] and solvent-vapor annealing [ 8 ] have been demonstrated. The most successful processing protocols affect the aggregation and morphology in a predictable and and controlled manner. In past work, we have shown that solution-processed squaraine (SQ), followed by vacuum thermally evaporated C 60 donor/acceptor solar cells can have power conversion effi ciencies of η p = 4.6 ± 0.1% when they are fabricated into a lamellar device that is subsequently annealed at high temperature (110 ° C). [ 9 ] It was found that the annealing roughens the SQ surface, thereby creating a highly folded BHJ interface with the C 60 and thus compensating for the very short (1.Although the L D of SQ is very small, this defi ciency is partially compensated by its high absorption coeffi cient compared to that of C 60 . This motivates the use of SQ:fullerene blends, whereby the ratio of materials strongly favors that of the fullerene to take advantage of its large L D and low absorption. In previous work this approach has been partially successful, with the highest external quantum effi ciencies ( EQE ) under low intensity illumination of SQ:PC 70 BM (1:6) blends approaching 50% across the visible spectrum. Unfortunately, devices fabricated using such blends exhibited exceptionally low fi ll factors ( FF ∼ 0.35) due to a large internal series resistance to charge extraction from the low density of SQ in the mixture. Hence, under standard simulated solar illumination conditions (100 mW/cm 2 , AM1.5G spectrum), the effi ciency was limited to only ∼ 3%. [ 10 ] In this work, we explore annealing of these SQ:PC 70 BM (1:6) blends in solvent vapor to create continuous crystalline (and hence low resistance) pathways for hole conduction through the rareifi ed SQ environment. We note that, while spin-casting of these mixtures provides a simple means to prepare homogeneous thin fi lms, rapid solvent evaporation does not allow for suffi cient molecular reorganization, which is needed to achieve an equilibrium, crystalline, and uniformly phase-separated mixture. [11][12][13][14] We fi nd that post-annealing through additional extended exposure of the blend to dichloromethane (DCM) can lead to a more optimized morphology that reduces series resistance, and hence increases the FF to 0.50 ± 0.01 and a power conversion effi ciency of η p = 5.2 ± 0.3% of the resulting cells under AM1.5G, 1 sun simulated solar emission (corrected for spectral mismatch). Indeed, our best cells measured reached effi ciencies of...

Efficiency Improvement of Organic Solar Cells Using Two-step Annealing Technique

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Difluorobenzothiadiazole‐Based Small‐Molecule Organic Solar Cells with 8.7% Efficiency by Tuning of π‐Conjugated Spacers and Solvent Vapor Annealing

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Small molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for solution-processable bulk-heterojunction solar cells

“…Reports of BHJ devices using small organic molecules as donor components have recently emerged that have utilized squaraine [17], oligothiophene [18], merocyanine dyes [19][20], diketopyrrolopyrrole derivatives [21], dibenzochrysene [22], and push-pull organic dyes [23][24][25]. Progress in the field has resulted in devices exhibiting PCEs in the excess of 9% [26][27][28][29][30].…”