Donor-Acceptor-Donor (D-A-D) Architectures

• Donor-Acceptor-Donor (D-A-D) architectures are defined by a central electron acceptor flanked by donor units that promote intramolecular charge transfer and tunable optical properties.
• Molecular design, including end-cap selection and planarity control, directly influences absorption profiles, bandgaps, and charge mobility in these systems.
• Optimizing donor/acceptor miscibility and thin-film crystallinity is crucial for enhancing performance in bulk-heterojunction solar cells and organic field-effect transistors.

Donor–Acceptor–Donor (D–A–D) architectures define a prominent motif in small-molecule organic semiconductors, in which a central electron-deficient (acceptor, A) moiety is flanked symmetrically by electron-rich (donor, D) units. This arrangement supports efficient intramolecular charge transfer (ICT), modulates optical and electronic properties, and enables precise tuning of key parameters—especially relevant for organic optoelectronic applications such as bulk-heterojunction (BHJ) solar cells and organic field-effect transistors (OFETs). EBI-based D–A–D systems, constructed around 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) as the electron-accepting core, have been investigated by Le Borgne et al. for their optoelectronic function and structure–property correlations (Borgne et al., 2017).

1. Molecular Design and D–A–D Motif

The molecular design of D–A–D architectures critically determines their optoelectronic performance. In the referenced systems, the central EBI (3,3'-(ethane-1,2-diylidene)bis(indolin-2-one)) unit serves as the electron acceptor, conjugated to donor end-caps via π-linkers. Three variants have received detailed paper:

• EBI-T: Thiophene end-capped,

$\chemfig{[:0]S*5(-=-=[:-72]C(-[:36]EBI)-[:108]C(-[:36]EBI))}$

• EBI-2T: Bithiophene end-capped,

$\chemfig{[:0]S*5(-=-=[:-72]S*5(-=-=[:-72]C(-[:36]EBI)-[:108]C(-[:36]EBI))))}$

• EBI-BF: Benzofuran end-capped,

$\chemfig{[:0]O*6(-=--(-[:90]*5(=-=O)-[:270]EBI)-=)}$

The D–A–D motif promotes an intramolecular charge-transfer (ICT) process, where the donors inject electron density toward the central acceptor, supported by extended π-conjugation. The spatially separated HOMO (across the D–A–D backbone) and LUMO (localized on A) enhance photoinduced charge separation. Under photoexcitation, these characteristics provide both a red-shifted optical gap (lower $E_g^\text{opt}$) and increased ICT band intensity.

2. Conformational and Electronic Structure

2.1 Planarity and Dihedral Control

Molecular planarity, quantified via D–A dihedral angles as determined through DFT (B3LYP/6-31G*) calculations, directly influences π–π stacking propensity:

Compound	D–A Dihedral (°)
EBI-T	26
EBI-2T	22
EBI-BF	0 (coplanar)

A smaller dihedral angle increases planarity, resulting in enhanced intermolecular π–π stacking and reduced distances, which are essential for charge-transport and aggregation control.

2.2 Absorption and Bandgap

The degree of conjugation and electronic coupling modulate the absorption profiles and band gaps:

Compound	Solution $\lambda_\text{max}$ (nm)	Film $\lambda_\text{onset}$ (nm)	$E_g^\text{opt}$ (eV)
EBI-T	531	660	1.88
EBI-2T	549	704	1.76
EBI-BF	565	670	1.85

Bithiophene extension (EBI-2T) causes the broadest, most red-shifted absorption, minimizing the bandgap.

2.3 Frontier Orbital Energies

Frontier energies, as measured via cyclic voltammetry and calculated from optical gaps, dictate energetic alignments:

Compound	$E_\text{HOMO}$ (eV)	$E_\text{LUMO}$ (eV)
EBI-T	–5.71	–3.83
EBI-2T	–5.66	–3.90
EBI-BF	–5.73	–3.89

These values place the EBI derivatives within favorable energy windows for photovoltaic donor materials.

3. Charge-Transport and Thin-Film Electronic Properties

Charge carrier mobilities, measured via OTFT devices on spin-coated films, reflect the influence of both molecular structure and post-deposition treatments. Maximum and average hole mobilities ($\mu_h$) are summarized:

Compound	Treatment	$\mu_h$ (cm$^2$ V$^{-1}$ s$^{-1}$)
EBI-T	as-cast (2500 rpm)	$6.4 \times 10^{-4}$ (avg), $2.3 \times 10^{-4}$ (max)
EBI-2T	as-cast	$1.2 \times 10^{-3}$ (avg), $3.2 \times 10^{-3}$ (max)
EBI-BF-C12	as-cast	$4.5 \times 10^{-4}$
EBI-BF-C12	annealed 100 °C	$4.4 \times 10^{-3}$
EBI-BF-C16	as-cast	$2.9 \times 10^{-4}$
EBI-BF-C16	annealed 150 °C	$2.1 \times 10^{-2}$

Annealing substantially increases $\mu_h$ for benzofuran-capped derivatives, reaching up to $2.1 \times 10^{-2}$ cm$^2$ V$^{-1}$ s$^{-1}$—the highest among these EBI compounds. Current was extracted from OTFT transfer curves using

$$I_{D,\text{sat}} = \frac{W}{2L} C_i \mu_h (V_G-V_\text{th})^2$$

where $W$, $L$, and $C_i$ denote channel width, length, and gate insulator capacitance.

4. Photovoltaic Function in Bulk-Heterojunction Architectures

BHJ solar cell devices employ the structure: ITO/PEDOT:PSS/EBI:PC$_{61}$BM/Ca/Al with ca. 100 nm active layers, AM1.5G irradiation at 100 mW cm$^{-2}$. Device parameters:

Donor	Blend ratio	$J_{sc}$ (mA/cm$^2$)	$V_{oc}$ (V)	FF (%)	PCE (%) avg/max
EBI-T	40:60	0.82	0.73	30	0.18 / 0.21
EBI-2T	40:60	5.51	0.87	34	1.65 / 1.92
EBI-BF	50:50	0.87	0.23	27	0.06 / 0.08

Power conversion efficiency ($\mathrm{PCE}$) was evaluated as

$$\mathrm{PCE} = \frac{J_{sc} \times V_{oc} \times \text{FF}}{P_{in}}$$

Results confirm EBI-2T as the most efficient, attributed to its broad absorption and appropriately intermixed BHJ morphology.

5. Correlations of Molecular Structure, Film Morphology, and Device Function

5.1 Thermal and Miscibility Behavior

Differential scanning calorimetry (DSC) reveals:

• EBI-T exhibits a eutectic composition near 55 wt% donor at $T_{\text{eut}} ≈ 210$ °C, indicating moderate miscibility.
• EBI-2T forms a eutectic at ~50 wt%, $T_{\text{eut}} ≈ 245$ °C, suggesting a well-intermixed blend phase.
• EBI-BF shows no clear eutectic and significant melting-point depression, implying limited miscibility and propensity for demixing.

5.2 Nanoscale Morphology (AFM)

• EBI-T and EBI-2T/PC$_{61}$BM blends show low RMS roughness (<4 nm) and nanodomains (20–30 nm), conducive to efficient exciton splitting and charge transport.
• EBI-BF blends display coarse grains (100–250 nm) and vertical segregation, correlating with poor percolation pathways and device inefficiency.

5.3 Crystallinity (XRD)

Film	(100) d-spacing (nm)	Crystallite size (nm)
Neat EBI-T	weak/broad	–
Neat EBI-2T	1.6	39.4
Neat EBI-BF-C12	2.0	32.2
Neat EBI-BF-C16	2.3	24.0
EBI-2T:PC$_{61}$BM	1.6 (weakened)	33.8

EBI-T shows limited crystallinity, while EBI-2T provides a moderate degree, balanced miscibility, and the optimal compromise between purity and connectivity for charge transport. EBI-BF presents high crystallinity in neat films but forms suboptimal blends with fullerenes, resulting in strong segregation.

6. Guidelines for Rational Design of D–A–D Semiconductors

The paper establishes several structural and processing criteria that maximize performance in D–A–D systems:

• Maximize backbone planarity (small D–A dihedral) to strengthen π–π stacking, supporting higher hole mobility.
• Promote balanced donor/acceptor miscibility (based on eutectic behavior) to build an interpenetrated phase network for exciton splitting, but retain phase purity for carrier percolation.
• Select end-caps to tune $E_\text{HOMO} ≈ -5.6$ to $-5.8$ eV and $E_g^\text{opt} ≈ 1.7$–$1.9$ eV to achieve both optimal $V_{oc}$ and broad spectral absorption.
• Suppress excessive phase segregation (e.g., seen with planar benzofuran caps) by matching end-caps for both π–π stacking ability and fullerene compatibility.
• Control thin-film crystallinity, via side-chain optimization and thermal annealing, to target domain sizes (20–50 nm) favoring both efficient dissociation and carrier transport.

A plausible implication is that further increases in PCE (towards 5–10%) in next-generation D–A–D small molecules require the integrated optimization of molecular planarity, donor/acceptor phase behavior, and post-processing conditions.

7. Summary and Outlook

EBI-based D–A–D materials exemplify the capacity of this architecture to integrate tunable optoelectronic properties, solution processability, and precise control over thin-film microstructure. The explicit relationship between donor structure, planarity, miscibility, nanoscale morphology, and BHJ photovoltaic performance provides a template for rational materials design. There remain challenges corresponding to phase segregation and crystallization in blends, especially with highly planar end-caps, but the established structure–property–function correlations create a robust framework for the continued advancement of D–A–D organic semiconductors with target properties for high-performance solar energy conversion.