In this work, we have demonstrated that the electric field assisted poling of the donor polymer possessing conformationally labile nitrile groups increases dipolar polarization and dielectric permittivity. We find that the enhanced dielectric permittivity of the donor polymer reduces non-geminate recombination losses in bulk-heterojunction (BHJ) solar cells (SCs), resulting in increased open circuit voltage (VOC) compared with unpoled devices. This result reveals the importance of dielectric permittivity of polymers and also signifies the promising applicability of electric field assisted poling for high dielectric polymers in BHJ SCs.

본 연구에서는 나이트릴 그룹을 작용기로 가지는 고분자에 전기장을 가해주어 고분자의 분극도를 높여 고분자의 유전상수를 증가시키는 연구를 진행하였다. 전자주게 고분자의 유전상수를 증가시켜 재결합에 의한 손실을 효과적으로 감소시킬 수 있었고 이로 인해 고분자 태양전지의 개방전압을 증가시킬 수 있음을 보고하였다. 이러한 결과로부터 고분자 태양전지에서 고분자의 유전상수가 가지는 중요성 및 외부 전기장에 의한 고분자의 배열을 이용한 응용분야에 대한 중요성을 밝혔다.

Keywords: polymer solar cell, dielectric permittivity, electric field assisted poling, charge dynamics, charge recombination

Suppressing recombination of charge carriers, caused by Coulombic attraction between electron and hole, is critical for efficient photocurrent generation in organic solar cells.^1-4 Since the dielectric permittivity of conventional organic semiconducting polymers and small molecules is relatively low (ε_r ~3), photon absorption leads to excited states (excitons) with high binding energies on the order of E_b~0.5-1.0 eV. These excitons require more than the thermal energy available at room temperature (kT~0.025 eV) to ionize into free charges. This is in contrast to excitations in inorganic semiconductors where the dielectric permittivity is typically higher (ε_r > 10) and the exciton binding energies are smaller than kT. Nevertheless, since excitons of conjugated organic materials can be potent oxidizing or reducing agents, heterojunction (HJ) architectures that pair appropriate excited state charge donors with complementary ground state charge acceptors have been introduced to achieve efficient photoinduced charge transfer.⁵ Unfortunately, photogenerated electrons and holes in HJ experience a strong Coulombic attraction to one another due to poor charge screening in these active materials having low dielectric permittivity. Thus, increasing the dielectric properties of the active layer can be a potentially compelling strategy to reduce the Coulombic attraction of charge carriers and concomitantly suppress recombination losses. Ideally, suppressing exciton binding energy in semiconducting donor polymers by increasing dielectric permittivity can rule out the need of acceptor materials, which can be the most potent gamechanger for solar cells over the next decades. Recently, few attempts have been reported to increase the dielectric permittivity of semiconducting polymers by introducing non-ionic polar side-chains such as nitrile⁶ or oligo(oxyethylene) groups.^7,8
The permanent dipoles in these polar side-chains should be able to freely rotate and reorient owing to their flexibility and the rapid motion of polar groups in the GHz - MHz frequency domain.⁷ Since non-geminate recombination occurs within ns~μs regime, increasing dielectric constant by introducing permanent dipoles can be used to control non-geminate recombination dynamics. While increasing dielectric permittivity by incorporating polar side-chains remains very attractive, further control of dipolar polarization with reorientation of dipole moment by applying external stimuli still remains unexplored.
Here, we demonstrate the viability of using electric field assisted poling on a model donor polymer possessing conformationally labile polar groups to increase dipolar polarization and dielectric permittivity. We show that electric field assisted poling of the donor polymer enhances dielectric permittivity and reduces non-geminate recombination in bulk-heterojunction solar cells (BHJ SCs), resulting in longer carrier lifetimes and increased open circuit voltage (V_OC) compared to unpoled devices.