Amorphous Ternary Charge-Cascade Molecules for Bulk Heterojunction Photovoltaics

Abstract

Ternary bulk heterojunctions with cascade-type energy-level configurations are of significant interest for further improving the power conversion efficiency (PCE) of organic solar cells. However, controlling the self-assembly in solution-processed ternary blends remains a key challenge. Herein, we leverage the ability to control the crystallinity of molecular semiconductors via a spiro linker to demonstrate a simple strategy suggested to drive the self-assembly of an ideal charge-cascade morphology. Spirobifluorene (SF) derivatives with optimized energy levels from diketopyrrolopyrrole (DPP) or perylenediimide (PDI) components, coded as SF-(DPP)₄ and SF-(PDI)₄, are synthesized and investigated for application as ternary components in the host blend of poly(3-hexylthiophene-2,5-diyl):[6,6]phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM). Differential scanning calorimetry and X-ray/electron diffraction studies suggest that at low loadings (up to 5 wt %) the ternary component does not perturb crystallization of the donor:acceptor host blend. In photovoltaic devices, up to 36% improvement in the PCE (from 2.5% to 3.5%) is found when 1 wt % of either SF-(DPP)₄ or SF-(PDI)₄ is added, and this is attributed to an increase in the fill factor and open-circuit voltage, while at higher loadings, the PCE decreased because of a lower short-circuit current density. A comparison of the quantum efficiency measurements [where light absorption of SF-(DPP)₄ was found to give up to 95% internal conversion] suggests that improvement due to enhanced light absorption or to better exciton harvesting via resonance energy transfer is unlikely. These data, together with the crystallinity results, support the inference that the SF compounds are excluded to the donor:acceptor interface by crystallization of the host blend. This conclusion is further supported by impedance spectroscopy and a longer measured charge-carrier lifetime in the ternary blend.

Keywords: cascade energy levels; morphology control; self-assembly; ternary organic solar cell.

Figure 1

Chemical structure of the ternary small molecules used in this work: (a) SF-(DPP)₄ with R₁ = 2-ethylhexyl; (b) SF-(PDI)₄ with R₂ = 2-hexyldecyl. (c) Energy levels and band gap of the active molecules used in this work.

Figure 2

(a) DSC second heating curves of neat P3HT and PCBM with varying amounts of SF-(DPP)₄ (solid lines) and SF-(PDI)₄ (dotted lines). Scale bars represent 0.2 mW g^–1. (b) Relative crystallinity (based on melting enthalpies) of PCBM (dots) and P3HT (triangles) as a function of the additive content.

Figure 3

Photovoltaic device performance with added amorphous cascade molecules. The average figures of merit (PCE, J_SC, V_OC, and FF) are shown as a function of the SF-(DPP)₄ and SF-(PDI)₄ content in P3HT:PCBM BHJ OPVs.

Figure 4

(a) IQE of the host blend and containing 1, 2.5, and 10 wt % SF-(DPP)₄. (b) Recombination lifetime obtained from fitting the IS response (dots, inset) at open circuit under 1 sun for increasing content of SF-(DPP)₄ (0, 1, 2.5, 5, and 10 wt %). Corresponding data as a function of the SF-(PDI)₄ content are shown in Figure S15.