Side‐Chain Engineering for Enhancing the Properties of Small Molecule Solar Cells: A Trade‐off Beyond Efficiency

Abstract

Three small molecules with different substituents on bithienyl‐benzo[1,2‐b:4,5‐b′]dithiophene (BDTT) units, BDTT‐TR (meta‐alkyl side chain), BDTT‐O‐TR (meta‐alkoxy), and BDTT‐S‐TR (meta‐alkylthio), are designed and synthesized for systematically elucidating their structure–property relationship in solution‐processed bulk heterojunction organic solar cells. Although all three molecules show similar molecular structures, thermal properties and optical band gaps, the introduction of meta‐alkylthio‐BDTT as the central unit in the molecular backbone substantially results in a higher absorption coefficient, slightly lower highest occupied molecular orbital level and significantly more efficient and balanced charge transport property. The bridging atom in the meta‐position to the side chain is found to impact the microstructure formation which is a subtle but decisive way: carrier recombination is suppressed due to a more balanced carrier mobility and BDTT based devices with the meta‐alkylthio side chain (BDTT‐S‐TR) show a higher power conversion efficiency (PCE of 9.20%) as compared to the meta‐alkoxy (PCE of 7.44% for BDTT‐TR) and meta‐alkyl spacer (PCE of 6.50% for BDTT‐O‐TR). Density functional density calculations suggest only small variations in the torsion angle of the side chains, but the nature of the side chain linkage is further found to impact the thermal as well as the photostability of corresponding devices. The aim is to provide comprehensive insight into fine‐tuning the structure–property interrelationship of the BDTT material class as a function of side chain engineering.

Simultaneous enhanced efficiency and thermal stability in organic solar cells from a polymer acceptor additive

“…It is well established that device efficiency is strongly dependent on the bulk heterojunction (BHJ) nanoscale morphology, to effectively achieve optimized charge generation, carrier transport, and carrier collection [18][19][20] . However, the optimized blend morphology, known as a thermally activated metastable state, is generally sensitive to high-temperature aging 13,21 , and easily moves toward a thermodynamic equilibrium state and thus forms a large phase separation, significantly reducing donor-acceptor (D-A) interfacial area and device efficiency 22,23 . For instance, storing the blend film of polymer donor PTB7-Th (poly [4,8-bis (5-(2-ethylhexyl)thiophen-2-yl)benzo [1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno [3,4b]thiophene-)-2carboxylate-2-6-diyl)]) and fullerene acceptor PCBM (phenyl-C61-butyric acid methyl ester) in a 140°C oven for 2 h induces the formation of numerous clusters, reducing the device efficiency by~80% 24 .…”

Developing High‐Performance Electron‐Rich Unit End‐Capped Wide Bandgap Oligomeric Donor by Weak Electron‐Deficient Central Core Strategy

“…To date, the best devices based on A‐D‐A structured small molecules show a high power conversion efficiency (PCE) of 11% . For example, the vast majority of new these materials are based on A‐D‐A structure and incorporate oligothiophene/benzodithiophene or other electron‐rich units as core and capped with Rhodanine/Indene‐1,3‐dione or other electron‐deficient units . However, the search for high‐performance alternative small molecules with electron‐rich units as terminal groups is not notably pioneered and it has drawn little attention so far.…”

Rhodanine-based dyes absorbing in the entire visible spectrum

“…[13][14][15] Once the ability of Q7 small molecules to efficiently perform like polymers was demonstrated, the race to develop new dyes that are able to broadly harvest light in the visible-near infrared region (where most of the solar spectrum is centred) started.…”

“…13,15,[16][17][18] All these systems share A-D-A structures in common, with rhodanines linked to the central core by a Knoevenagel condensation reaction with an aldehyde group. Herein, we report for the first time four new molecules (RFTA-1-4, see Scheme 1) via incorporating rhodanine (R) derivatives into a fluorene (F) backbone (through its C9 position), giving rise to D-A-D structures (employing triarylamine (TA) as electron-donating groups) that feature absorption spectra spanning the entire visible region.…”