Simple Structured DPP-based Small Molecules for High Efficient Organic Photovoltaics

Abstract

It has recently been reported that a family of solution-processable polymer-based solar cells (PSCs) achieved a high PCE up to 8%. However, conjugated polymers are subjected to inevitable drawbacks such as batch-to-batch variation of molecular weight, broad polydispersity and complicated purification. As a consequence, organic photovoltaics based on small molecules (SMs) have emerged as an alternative to PSCs. Recently, BHJ solar cells based on SMs have achieved a PCE of 6.5%. In developing new conjugated organic materials for BHJ solar cells, a well-known architecture, alternatively composed of weak electron-donating and strong electron-accepting units, has been utilized to manipulate their electronic properties for high VOC and efficient charge separation. Electron-donating units such as benzo[1,2-b:4,5-b']dithiophene, 2,7-carbazole, and 2,7-fluorene are considered as weak electron-donating molecules, in which phenylene rings are fused with hetero-aromatic rings. Since phenylene ring has high aromatic resonance stabilization energy and π-electrons are relatively localized within the phenylene ring in the molecule, molecular units containing phenylene rings can be classified as weak electron-donating units. Therefore, when the weak electron-donating unit is used as a building block for SM, the SM is expected to have a deep HOMO energy level and as a result to afford high VOC in OPVs. This synthetic strategy can effectively be used to tune the frontier molecular orbital energy levels for high efficiency OPV solar cells. Diketopyrrolopyrrole (DPP), which has high electron negativity and molar absorptivity, is a promising building block of organic donor materials due to its easy synthesis. A common feature of DPP-based compounds is their low-lying HOMO energy level leading to relatively high open-circuit voltage (VOC). Moreover, since highly planar structure of DPP unit encourages π-π stacking, DPP-based materials exhibits high crystallinity and resulted in high charge carrier mobility in electronic devices. We have postulated that the introduction of electron-donating units with different aromatic resonance stabilization energy (thiophene (T) vs phenylene (Ph)) into the A-D-A type SM, would precisely control the HOMO energy levels of SMs for efficient OPVs. To examine this approach, a series of SMs based on thiophene-capped diketoppyrolopyrrole, (TDPP), (TDPP)2, T(TDPP)2, and Ph(TDPP)2, were synthesized. As expected, the HOMO energy level of T(TDPP)2 (?5.17 eV) is slightly higher than that of (TDPP)2 (?5.19 eV) due to a strong electron-donating thiophene bridge, while the HOMO energy level of Ph(TDPP)2 (?5.28 eV) is deeper than that of (TDPP)2 due to the introduction of weak electron-donating phenylene bridge between two TDPPs. Consequently, the solar cell based on Ph(TDPP)2 achieved a PCE of 4.01% with a high VOC of 0.93 V. This performance is one of the highest ones among OPVs fabricated from DPP-based SMs.