Spin‐Coated Small Molecules for High Performance Solar Cells

Abstract

Organic solar cells (OSCs) have attracted signifi cant attention as a clean and competitive renewable energy source due to their attractive features such as low-cost, light weight, solution processability and high mechanical fl exibility. [1][2][3][4] Benchmark power conversion effi ciencies (PCEs) of 10% or higher have been predicted if a suitable low bandgap donor material can be designed and implemented. [ 5 ] More recently, bulk heterojunction (BHJ) OSCs using solution-processed small molecules as the donor have attracted great attention. [6][7][8] This long-time but recently increased interest lies in the fact that solutionprocessed small molecule based OSCs have numerous advantages, such as relatively simple synthesis and purifi cation methods, monodispersity and well defi ned structures, high open circuit voltage and charge carrier mobilities, and better batch-to-batch reproducibility. [6][7][8] However, solution-processed small molecule OSCs have not met such high expectations as those of their polymeric counterparts due to their limited PCEs. In most cases, small molecule devices using solution processing always seem to have poorer fi lm quality than that of their polymeric counterparts in BHJ OSCs. [ 6 , 7 ] It is thus expected that better PCE could be achieved if the intrinsic poor fi lm quality and morphology in BHJ architecture could be improved. However, in order to achieve this, careful molecule design has to be carried out to address many factors simultaneously, including the material's solar light absorption associated with its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) positions, mobility, fi lm forming quality, electronic band structure and morphology compatibility with the acceptors, and so on. Indeed, several families of solution processed small molecules, such as oligothiophenes, [ 9 ] triarylamines, [ 10 , 11 ] diketopyrrolopyrroles (DPP), [ 12 ] squaraines, [ 13 , 14 ] merocyanine, [ 15 ] and dipyrromethene boron difl uoride (BODIPY) [ 16 ] derivatives, have offered great promise in this direction. Although still lower than the PCE of their polymeric counterparts ( ∼ 6-8%), [17][18][19][20][21] the highest published PCE of 4.4% for solution-processed small molecule/fullerene derivative based BHJ OSCs was recently achieved by a University of California Santa Barbara group using a DPP-thiophene derivative. [ 12 ] Push-pull chromophores involving electron-donating and electron-withdrawing groups have been widely investigated for OSC molecules. [ 8 ] This type of molecular architecture can lower the material bandgap and extend the absorption spectrum toward longer wavelengths, and at the same time, the HOMO and LUMO levels can be tuned effectively. [ 11 , 22 ] Oligothiophenes with well defi ned structures possess highly delocalized π -electrons along the molecular backbone and are well known as one of the most effective hole-transporting materials. Oligothiophenes with a push-pull structure have been widely investigated in solution processed ...

Integrated molecular, morphological and interfacial engineering towards highly efficient and stable solution-processed small molecule solar cells

“…4 Note that all analogs with shorter terminal alkyls possess a shoulder peak in the long wavelength region, indicating the presence of an additional ordering in lms. 33,34 The strongest intensity of the shoulder peak was found for the DTS(Oct) 2 -(2T-DCV-Me) 2 , having the shortest terminal methyl groups and linear octyl groups at the DTS unit. Changing the octyl unit at the DTS core into either more branched 2-ethylhexyl or longer decyl groups further decreases intermolecular interactions in the solid state.…”

π-Linkage effect of push-pull-structure organic small molecules for photovoltaic application

“…A sufficiently longer conjugation bridge like thiophene [22][23][24], oligothiophene derivatives [25,26] or fused-thiophene provides an extension of the absorption towards the red and NIR wavelengths as well as an increase of the absorption coefficient due to strong intramolecular charge transfer (ICT). More detailed discussion on π-bridge of thiophene and its derivatives are depicted in the following part.…”

“…9c based device showed the best PV performance with a PCE of 2.67% after thermal annealing at 100°C for 20 min. Liu and co-workers [25] reported three push-pull-structure OSMs based on oligothiophene backbone with high PCEs (4.46%-5.08%) for solution-processed BHJ-OSCs in 2011. Benefited from highly delocalized π-electrons along the molecular backbone and effective hole-transporting, 10a produced a PCE as high as 5.08% without any special treatment.…”

Resent Progress of Benzodithiophene Based Efficiency Small Molecule Organic Solar Cells

“…Chen 课题组基于对 DCAO7T [50] 的研究, 将 中心的噻吩单元替换为富电子的 BDT 单元, 吸电子能 力较强的氰基乙酸辛酯作为封端基团, 合成了小分子 DCAO3T(BDT)3T [51] . 与 DCAO7T 相比, 可溶液加工小 [56,57] .…”