Jung-Hwa Park

An All‐Small‐Molecule Organic Solar Cell with High Efficiency Nonfullerene Acceptor

DOI: 10.1002/adma.201405429 self-assembling behavior of our novel nonfullerene acceptors, polymeric solar cells combined with the most typical donor polymer, poly(3-hexylthiophene), showed a surprisingly high PCE of 2.7%, which is almost comparable to that of the PC 61 BM based solar cell. [ 14 ] To fully assess the unexplored potential of this high-performance nonfullerene acceptor, it is essential to fabricate and evaluate various organic/polymeric solar cells with different kinds of donor materials. In this work, we explored a highly effi cient all-small-molecule organic solar cell consisting of our DCS–NI acceptor, (2E,2′E)3,3′-(2,5-dimethoxy-1,4-phenylene)bis(2-(5-(4-(N-(2-ethylhexyl)1,8-naphthalimide)yl)thiophen-2-yl)acrylonitrile) (NIDCS–MO), and the state-of-the-art high-performance small molecule donor, 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c] [1,2,5]thiadiazole) ( p -DTS(FBTTh 2 ) 2 ) ( Figure 1 a). [ 19–22 ] Thus far, the best PCE value of all-small-molecule solar cells with nonfullerene acceptors is around 3%, which is signifi cantly lower than the 6% given by the polymer donor-based nonfullerene acceptor systems. [ 15,16 ] However, it is apparent that the allsmall-molecule solar cell is more benefi cial than the polymeric one if the PCE levels of both systems were comparable. This is because the small molecule system is free from the batchto-batch variation and molecular weight distribution issues inherent to the polymeric system. [ 5,21–23 ] By using the balanced self-assembling behavior of DCS–NI acceptor in this work, we successfully demonstrate the 5.44% PCE all-small-molecule solar cell and elucidate the rationale of the enhanced effi ciency. Figure 1 b shows individual absorption spectra of the donor and acceptor components in spin-coated fi lms. The small molecule donor, p -DTS(FBTTh 2 ) 2 , has a maximum absorption peak at 680 nm while the nonfullerene DCS–NI acceptor, NIDCS– MO, has that at 498 nm. As shown in the absorption spectra, this donor–acceptor pair has well-matched complementary absorption peaks that completely cover the broad wavelength range from 300 to 750 nm. Furthermore, the individual absorption maximum and peak shape of either donor or acceptor spectra were almost unaltered by thermal annealing below 150 °C (Figure S1, Supporting Information). However, the absorption spectra of the donor–acceptor blend fi lms were signifi cantly altered by thermal annealing, as shown in Figure 1 c. While the 680 nm absorption peak of p -DTS(FBTTh 2 ) 2 was rather obscure and featureless in the as-cast blend fi lm, clear and distinct vibronic peaks were generated at 626 and 681 nm inherent to the crystalline p -DTS(FBTTh 2 ) 2 by thermal annealing, suggesting the formation of ordered molecular structures in the blend fi lm. [ 19,20 ]

A High Efficiency Nonfullerene Organic Solar Cell with Optimized Crystalline Organizations.

A well-organized donor-acceptor crystalline structure is examined for high -performance nonfullerene solar cells. By thermal annealing, nanoscale structures of both donor and acceptor domains are successfully modulated, followed by -significant changes in the resulting -photovoltaic characteristics. When annealed at 90 °C, a maximum power conversion efficiency of 7.64% with a -remarkable open-circuit voltage of 1.03 V is obtained.

Stimuli-Responsive Reversible Fluorescence Switching in a Crystalline Donor-Acceptor Mixture Film: Mixed Stack Charge-Transfer Emission versus Segregated Stack Monomer Emission.

We report on a molecularly tailored 1:1 donor-acceptor (D-A) charge-transfer (CT) cocrystal that manifests strongly red-shifted CT luminescence characteristics, as well as noteworthy reconfigurable self-assembling behaviors. A loosely packed molecular organization is obtained as a consequence of the noncentrosymmetric chemical structure of molecule A1, which gives rise to considerable free volume and weak intermolecular interactions. The stacking features of the CT complex result in an external stimuli-responsive molecular stacking reorganization between the mixed and demixed phases of the D-A pair. Accordingly, high-contrast fluorescence switching (red↔blue) is realized on the basis of the strong alternation of the electronic properties between the mixed and demixed phases. A combination of structural, spectroscopic, and computational studies reveal the underlying mechanism of this stimuli-responsive behavior.

High performance all-small-molecule solar cells: engineering the nanomorphology via processing additives

The use of small volumes of solvent additives (SAs) or little amounts of non-volatile additives is a processing approach that has been implemented in many high/record performing bulk heterojunction (BHJ) organic solar cells (OSCs). Here, the effects of six SA systems and a molecular additive di-2-thienyl-2,1,3-benzothiadiazole (DTBT) were studied with respect to the photovoltaic parameters of solution-processed all small molecule solar cells (all-SMSCs) based on the BDTT-S-TR:NIDCS-MO system. An effective strategy with binary additives has been employed in this all-SM system, where a small amount, 0.75 vol% 1,8-diiodooctane (DIO) and 2 wt% DTBT were added to the casting solution. This efficient SA approach yielded the highest power conversion efficiency (PCE) of 5.33%. The relevant additives facilitate phase separation in the nm domains and improve bulk transport as evidenced by photoluminescence (PL), atomic force microscopy (AFM), X-ray diffraction (XRD) and space charge limited current (SCLC) measurements.