Small organic molecules have emerged as promising candidates for thermoelectric applications due to their tunable electronic properties, low thermal conductivity, and potential for solution-processable fabrication. Among these, derivatives of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and related electron-accepting molecules exhibit high charge carrier mobility and controllable doping characteristics, making them suitable for thermoelectric energy conversion. The efficiency of thermoelectric materials is quantified by the dimensionless figure of merit, ZT = (S²σT)/κ, where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity. Optimizing ZT in small-molecule semiconductors requires balancing these interdependent parameters through molecular design, doping strategies, and structural engineering.

**Molecular Design and Charge Transport**
TCNQ derivatives, such as F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), are widely studied for their strong electron-accepting behavior, which facilitates p-type doping in organic semiconductors. The high electron affinity of these molecules enables efficient charge transfer when blended with donor-type hosts, enhancing electrical conductivity. For instance, F4-TCNQ-doped pentacene films have demonstrated electrical conductivities exceeding 100 S/cm, with Seebeck coefficients around 100 µV/K. The planar conjugated structure of TCNQ derivatives promotes π-π stacking, which improves charge carrier mobility by facilitating intermolecular hopping. Modifying side chains or introducing heteroatoms can further fine-tune energy levels and packing motifs to optimize transport properties.

**Doping Strategies**
Controlled doping is critical for achieving high ZT in small-molecule systems. Chemical doping with strong acceptors like F4-TCNQ or iodine vapor can significantly increase charge carrier density. However, excessive doping may degrade the Seebeck coefficient due to the trade-off between σ and S. Charge transfer complexes formed between TCNQ derivatives and host molecules (e.g., rubrene or tetrathiafulvalene) can achieve precise doping levels by adjusting stoichiometry. Electrochemical doping offers another route, enabling dynamic control over carrier concentration through applied potentials. For example, electrochemical doping of TCNQ-based thin films has yielded power factors (S²σ) up to 200 µW/mK² at room temperature. Molecular doping avoids the ion scattering issues common in inorganic systems, preserving carrier mobility at high doping levels.

**ZT Optimization Approaches**
Reducing thermal conductivity is essential for maximizing ZT, as organic materials inherently exhibit low κ values (0.1–1 W/mK). Introducing disorder through mixed compositions or nanostructuring can further suppress phonon transport without severely compromising electrical properties. Binary blends of TCNQ derivatives with insulating polymers have achieved κ values below 0.3 W/mK while maintaining reasonable conductivity. Energy filtering at interfaces between small molecules and dopants can enhance the Seebeck coefficient by selectively scattering low-energy carriers. Graded doping profiles or multilayer structures may also decouple the optimization of S and σ. Recent studies on TCNQ-based composites report ZT values approaching 0.2–0.3 near room temperature, competitive with some inorganic counterparts.

**Challenges and Future Directions**
Long-term stability under thermal cycling and ambient exposure remains a hurdle for small-molecule thermoelectrics. Oxidation of dopants or phase separation in blends can degrade performance over time. Encapsulation strategies and stable dopant-host combinations are under investigation to address these issues. Scalable deposition techniques, such as inkjet printing or roll-to-roll processing, must be adapted for precise doping control in device fabrication. Advances in computational screening of molecular candidates could accelerate the discovery of new high-ZT materials by predicting electronic structures and doping efficiencies.

The versatility of TCNQ derivatives and related small molecules provides a rich platform for thermoelectric research. By systematically engineering doping mechanisms and nanostructures, organic thermoelectrics may soon reach performance benchmarks suitable for wearable energy harvesting or low-grade waste heat recovery. Future work should focus on unifying high power factors with ultralow thermal conductivity while maintaining processability and stability for practical applications.