Misión intercultural – ad extra

Squaraine chromophores has been enthusiastically researched for applications in BHJ- OPV devices recently due to its high extinction coefficient in the near infrared region of the solar spectrum as well as the facile synthetic processes.115–117 Among many SQ structures, aniline- and indoline-based molecules are more synthetically accessible and thus are more frequently investigated for OPVs. With solubilizing alkyl groups attached to the nitrogen atoms, these SQ molecules generally exhibit high solubility in conventional organic solvents. Marks et al. have first reported a use of SQ donors in solution processed BHJ solar cells with efficiency above 1%.118 In the report, the authors pointed out that the linear or branched alkyl substituents allow manipulation of the solubility as well as control the crystalline packing structures. They have also observed that the solar cell efficiency is sensitive to SQ:PCBM ratio and thermal annealing treatment, presumably due to the

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changes in BHJ morphology. Later, the same group has compared the alkyl- and alkenyl- substituted SQ donors for BHJ-OPV devices.119 _{The marked effect of different solubilizing} alkyl groups has been further recognized.

Thompson and Forrest et al. have first reported highly efficient, vapor deposited OPV cells by using an aniline-based squaraine donor, later named “DIBSQ”.120 DIBSQ has been a very successful squaraine donor and high device efficiency can be realized by either solution or evaporation processes. In Wei et al.’s report,120 _{when compared to the} previously used, blue-absorbing copper phthalocyanine, DIBSQ-based devices with thinner donor layer (6.5 nm vs. 40 nm) can achieve higher efficiency (3.1% vs. 1.2%), marked its advantages in absorption (i.e. the high extinction coefficient and the NIR absorption peak). Later, the efficiency has been further increased to 4.6% by thermal annealing the donor layer to improve the SQ crystalline structures and subsequently the exciton diffusion length.121 Yet, it has also been recognized that the exciton diffusion length is still short (~5 nm) even in these crystalline SQ structures, which significantly limit the use of a thicker donor layer for more efficient solar photon harvesting. Thus, the same group have explored the potential of DIBSQ in solution-processed BHJ solar cells.40,41 Bulk heterojunction structure alleviates the negative influence of the short exciton diffusion length in DIBSQ donors and an averaged solar cell efficiency of 2.4 % has been obtained with the optimal DIBSQ:PC71BM ratio of 1:6. The lower efficiency can be attributed to the incomplete phase separation between DIBSQ and PC71BM in as-cast blends. Upon solvent annealing the BHJ layer in dichloromethane vapor, the SQ molecules crystallize and phase separate from the fullerenes, leading to a maximum device efficiency of 5.2%.41 _{Later, the} same group has explored various SQ molecules with N-aryl groups attached to the nitrogen

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atoms.122–125 The new squaraines exhibit red-shifted and broader absorption peaks as compared to DIBSQ. In addition, the aromatic groups are expected to improve π-π stacking of the SQ molecules and thus should improve exciton and charge transport. Yet, only small improvements have been seen.

Chen et al. have independently studied DIBSQ in solution processed OPV devices, with reported efficiency of 4.8% under 1-sum illumination.126 Interestingly, the authors reported an improved power conversion efficiency of 6.1% by co-evaporating the DIBSQ and C70 to form the BHJ layer.127 The authors also investigated the effect of alkyl substituents and the hydroxyl groups on the aniline moiety on solar cell performance.59 Dramatic changes in absorption spectra of neat films and the SQ single crystal structures have been realized with small modifications in those functional groups, which are responsible for the different solar cell performances.

Recently, Yang et al. have explored the possibilities of using asymmetrical squaraines in BHJ devices, and device efficiencies similar to that of DIBSQ have been reported.128–132 Noticeably, by binding two asymmetrical squaraine molecules together with a benzodithiophene unit, the hole carrier mobility has been improved significantly, leading to a high OPV efficiency of 6.33%.

Spencer et al. reported the unique aggregation properties of SQ molecules and the aggregates can be controlled by co-solvent methods.133,134 These results highlight that the SQ aggregation can be used to control the thin film morphology and thus the device performance. SQ aggregates yield broader absorption spectra which should be beneficial for photon harvesting. At the same time, aggregates represent more ordered packing of molecules and thus are expected to have higher charge and exciton transport properties.

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However, SQ aggregation does not always result in device improvements, and Spencer et

al. made the efforts to apply Marcus-Hush theory to explain the changes in solar cell

performance due to the SQ aggregation.69

To summarize the above short review, SQ has been utilized in OPV devices only recently, but the power conversion efficiency has been dramatically improved. These achievements were realized by combining the efforts in material design and synthesis, better device structures and fabrications, and deep understanding of these small SQ molecules and their aggregates. In order to further improve the device performance with better molecules, some critical inefficient properties of SQ donors must be overcome; i) the narrow absorption spectra of SQ single molecules, ii) the short exciton diffusion length and iii) the low charge mobility in SQ films. SQ aggregation might provide a solution to all three shortages as aggregates generally have much broader absorption profiles and the crystalline structure in the aggregates is expected to improve the transport properties. In this dissertation, we focus on the effect of squaraine aggregation on solar cell performance and the controlling of squaraine aggregation to further improve the OPV efficiency.