Device Strategies Directed to Improving the Efficiency of Solution-Processed Organic Solar Cells

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At the time of archiving, the student author of this dissertation opted to temporarily restrict access to it. The full text of this dissertation became available to the public after the expiration of the embargo on 2019-05-01.

In the last decade, organic photovoltaics (OPVs) have been attracting much attention for their low cost, and feasibility of mass production in large-area modules. Reported power conversion efficiencies (PCE) of organic solar cells have reached more than 10%. These promising PCEs can be realized by uncovering important principles: (1) rational molecular design, (2) matching of the material energy level, (3) favorable morphology of donor-acceptor (D/A) network, (4) higher carrier mobilities, and (5) suppression of charge recombination within the bulk heterojunction (BHJ). Though these key properties are frequently stated, the relationships between these principles remain unclear, which motivates us to fill these gaps. In the beginning, we show that changing the sequence of donor and acceptor units of the benzodithiophene-core (BDT) SM donors critically impacts molecular packing and charge transport in BHJ solar cells. Moreover, we find out that by adding small amount of the external solvent additive, the domain size of the SMFQ1 become relatively smaller, resulting in the FF enhancement of ~70% and thus pushing PCE to >6.5%. To further improve the device performance, we utilize another technique of device optimization: Solvent Vapor Annealing (SVA). Compared with solvent additive, the SVA creates a solvent-saturated environment for SMs to re-arrange and crystalize, leading to PCE of >8%, with nearly-free bimolecular recombination. When the systems are shifted from fullerene acceptors to nonfullerene acceptors, using solvent additives in indacenodithiophene-core (IDT) systems significantly reduces the domain size from >500nm to <50nm and also allows the SM donors to orderly packed, rising the PCE from <1% to 4.5%. Furthermore in a similar IDT-based system, it shows unexpectedly high VOC and low energy loss, and high PCE > 6% can be reached by employing the dimethyl disulfide (DMDS) as the SVA solvent to re-organize the morphology from excessive mixing to ordered phase-separated D/A network. Lastly, taking advantage of the distinct and complementary absorption of fullerene and nonfullerene acceptors, we show that the SM ternary system successfully realizes the high PCE of 11%, good air stability, and scalable property.

Liang, R.-Z. (2018). Device Strategies Directed to Improving the Efficiency of Solution-Processed Organic Solar Cells. KAUST Research Repository.


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