Effect of Systematically Tuning Conjugated Donor Polymer Lowest Unoccupied Molecular Orbital Levels via Cyano Substitution on Organic Photovoltaic Device Performance

Abstract

We report a systematic study into the effects of cyano substitution on the electron accepting ability of the common acceptor 4,7-bis(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT). We describe the synthesis of DTBT monomers with either zero, 1 or 2 cyano groups on the BT unit, and their corresponding co-polymers with the electron rich donor dithienogermole (DTG). The presence of the cyano group is found to have a strong influence on the optoelectronic properties of the resulting donor-acceptor polymers, with the optical band gap red-shifting by approximately 0.15 eV per cyano substituent. We find that the polymer electron affinity is significantly increased by ca. 0.25 eV upon addition of each cyano group, whilst the ionization potential is less strongly affected, increasing by less than 0.1 eV per cyano substituent. In organic photovoltaic (OPV) devices power conversion efficiencies (PCE) are almost doubled from around 3.5% for the unsubstituted BT polymer to over 6.5% for the mono-cyano substituted BT polymer However, the PCE drops to less than 1% for the di-cyano substituted BT polymer. These differences are mainly related to differences in the photocurrent, which varies by one order of magnitude between the best (1CN) and worst devices (2CN). The origin of this variation in photocurrent was investigated by studying the charge generation properties of the photoactive polymer:fullerene blends using fluorescence and transient absorption spectroscopic techniques. These measurements revealed that the improved photocurrent of 1CN in comparison to 0CN was due to improved light harvesting properties whilst maintaining a high exciton dissociation yield. The addition of one cyano group to the BT unit optimized the position of the polymer LUMO level closer to that of the electron acceptor PC71BM, such that the polymer's light harvesting properties were improved without sacrificing either exciton dissociation yield or device V OC . We also identify that the drop in performance for the 2CN polymer is caused by very limited yields of electron transfer from the polymer to the fullerene LUMO levels, likely caused by poor orbital energy level alignment with the fullerene acceptor (PC71BM). This work highlights the impact that small changes in chemical structure can have on the optoelectronic and device properties of semiconducting polymer. In particular this work highlights the effect of LUMO-LUMO offset on the excited state dynamics of polymer:fullerene blends.

Performance and Matching Band Structure Analysis of Tandem Organic Solar Cells Using Machine Learning Approaches

“…This feature importance analysis can be used to support the physical phenomena associated with the tandem OSCs. In previous articles, the LUMO differences between these donor materials and acceptors were optimized to form an electron transfer path. In addition, the driving force of charge transfer was affected by tuning the LUMO differences of donor and acceptor materials .…”

“…In previous articles, the LUMO differences between these donor materials and acceptors were optimized to form an electron transfer path. In addition, the driving force of charge transfer was affected by tuning the LUMO differences of donor and acceptor materials . Most of the tandem OSCs reported using PC 60 BM (the LUMO level of −3.7 eV) in this dataset; therefore, the LUMO levels of donor materials should be further considered to achieve the high efficiency of tandem OSCs.…”

Preferential Face‐on and Edge‐on Orientation of Thiophene Oligomers by Rational Molecular Design

“…The electrochemical band gaps were calculated as 1.64 eV for OT1 and 1.79 eV for OT2 ; the lower band gap of OT1 could be attributed to the presence of cyano groups on the rhodanine derivative. Literature reports suggest that the cyano group can reduce the energy level of the LUMO by up to ∼0.25 eV . Since their effect on the HOMO is relatively less, molecules with cyano substitution show a decrease in band gap when compared to that of molecules without cyano group…”

Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors

“…There is a nascent interest in polymeric materials with electronic absorption in the short‐wave infrared (SWIR), where recent advances in synthetic techniques have pushed band gaps of solution‐processable materials as low as ≈0.5 eV . Although further development of structure‐function relationships is required to reach even lower band gaps, thus far the successful engineering of narrow gap polymers has, in large part, followed a “push–pull” design strategy in which the energetic difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is controlled by varying the electron affinity of alternating electron‐rich and electron‐deficient comonomer units along the polymer backbone . As an alternative strategy to chemical control of the molecular orbital energies, band gaps have been further extended into the infrared through morphological control, creating low‐energy aggregate and crystallite states .…”