Formation of Thermally Stable Bulk Heterojunction by Reducing the Polymer and Fullerene Intermixing

Yoonhee Jang, Yun Ju Cho, Minjung Kim, Jeesoo Seok, Hyungju Ahn, Kyungkon Kim
2017 Scientific Reports  
A morphologically stable bulk heterojunction (BHJ) with a large heterojunction area is prepared by reducing the portion of the small band gap polymer (PTB7) and fullerene intermixture through a sequential deposition (SqD) of the nanostructured PTB7 and the fullerene layer. The nanostructured PTB7 layer is prepared using a ternary solvent composed of chlorobenzene, 1,8-diiodooctane (DIO) and 1-chloronaphthalene (1-CN). Adding DIO and 1-CN enhances the ordering of PTB7 chains and results in a
more » ... structured polymer surface. The grazing incidence X-ray diffraction results reveal that the SqD of the nanostructured PTB7 and fullerene layers forms the BHJ with little intermixing between the polymer and the fullerene domains compared to the BHJ formed by the deposition of the blended PTB7 and fullerene solution (BSD). The OPV utilizing the SqD processed BHJ (SqD-OPV) exhibits a power conversion efficiency (PCE) of 7.43%, which is similar to that when the BSD processed BHJ (BSD-OPV) is utilized. Furthermore, the SqD-OPV exhibits an excellent thermal stability. The SqD-OPV maintains its initial PCE even after thermal annealing at 140 °C for 10 days, whereas the BSD-OPV maintains 78% of its initial efficiency under the same condition. The bulk heterojunction (BHJ) is the most widely used photo-active layer system for organic photovoltaic (OPV) devices 1-3 . The BHJ is mostly formed by the blended solution deposition (BSD), where a BHJ is prepared by depositing a blended solution of electron donating polymer and electron accepting organic semiconductors. The fullerene derivatives have been used mainly as the electron accepting organic semiconductors. Nowadays, many efficient non-fullerene acceptors have been reported to achieve excellent solar cell performance in the BHJ OPV 4 . The BSD process is effective in forming the heterojunction with a large surface area. However, delicate controls of the processing conditions (e.g., donor to acceptor ratio, amount of processing additives, types of solvents, processing temperature, and additional annealing process) are required to maximize the solar cell performance by inducing the nano-scale phase separation of the donor and the acceptor materials 5 . An alternative method of forming the BHJ through sequential deposition (SqD) has recently been reported by several researchers 6-12 . The SqD process is more flexible in finding an optimized condition because the system can be optimized by independently controlling the thickness of each layer. However, finding a method of enhancing the heterojunction area and a proper solution for the top-layer that does not dissolve the bottom-layer are required to achieve efficient SqD processed OPVs (SqD-OPV). Yang et al. claimed that the SqD-OPV based on P3HT/PCBM had the potential to show a better performance than a BSD processed OPV (BSD-OPV) because of a significant reduction in the bimolecular charge recombination 5 . The Schwartz group found that the BHJ morphology formed by the SqD process was almost identical to that formed by the BSD process and could be controlled using tailored semi-orthogonal solvent blends 7 . In terms of the BHJ morphology, pure polymer and fullerene domains and polymer and fullerene intermixture were reported to co-exist in the BHJ prepared by the BSD 13-16 . The intermixture domain may cause the morphological instability of the BHJ processed by the BSD 16 . Meanwhile, the polymer crystallinity was not disturbed by the SqD of fullerene, which would help in enhancing the morphological stability 12 . However, as the BSD process does, the SqD-OPVs require an additional thermal annealing step to obtain a large heterojunction area because of the ineffective penetration of fullerenes into the polymer layer. The post thermal annealing process might form polymer and fullerene intermixtures. Published: xx xx xxxx OPEN www.nature.com/scientificreports/ 2 SCiENtifiC RePoRTS | 7: 9690 |
doi:10.1038/s41598-017-09167-4 pmid:28851926 pmcid:PMC5575051 fatcat:fsdlhzqyjjaovdg5omvrkxv53m