• Energy Research

Optimized spin-coating and blade-coating are found to produce similar performance yet notably different morphologies.

AbstractCompared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14, created by fluorine and chlorine substitution of the original donor molecule H11, are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.

AbstractTernary organic solar cells (OSCs) have attracted much research attention, as they can maintain the simplicity of the single‐junction device architecture while broadening the absorption range of OSCs. However, one main challenge that limits the development of ternary OSCs is the difficulty in controlling the morphology of ternary OSCs. In this paper, an effective approach to control the morphology is presented that leads to multiple cases of efficient nonfullerene ternary OSCs with efficiencies of up to 11.2%. This approach is based on a donor polymer with strong temperature dependent aggregation properties processed from hot solutions without any solvent additives and a pair of small molecular acceptors (SMAs) that have similar surface tensions and thus low propensity to form discrete phases. Such a ternary blend exhibits a simplified bulk‐heterojunction morphology that is similar to the morphology of previously reported binary blends. As a result, an almost linear relationship between VOC and film composition is observed for all nonfullerene ternary devices. Meanwhile, by carefully designing a control system with a large interfacial tension, a different phase separation and VOC dependence is demonstrated. This morphology control approach can be applicable to more material systems and accelerates the development of the ternary OSC field.

Three structurally identical polymers, except for the number of fluorine substitutions (0, 1, or 2) on the repeat unit (BnDT-DTBT), are investigated in detail, to further understand the impact of these fluorine atoms on open circuit voltage (V(oc)), short circuit current (J(sc)), and fill factor (FF) of related solar cells. While the enhanced V(oc) can be ascribed to a lower HOMO level of the polymer by adding more fluorine substituents, the improvement in J(sc) and FF are likely due to suppressed charge recombination. While the reduced bimolecular recombination with raising fluorine concentration is confirmed by variable light intensity studies, a plausibly suppressed geminate recombination is implied by the significantly increased change of dipole moment between the ground and excited states (Δμ(ge)) for these polymers as the number of fluorine substituents increases. Moreover, the 2F polymer (PBnDT-DTffBT) exhibits significantly more scattering in the in-plane lamellar stacking and out-of-plane π-π stacking directions, observed with GIWAXS. This indicates that the addition of fluorine leads to a more face-on polymer crystallite orientation with respect to the substrate, which could contribute to the suppressed charge recombination. R-SoXS also reveals that PBnDT-DTffBT has larger and purer polymer/fullerene domains. The higher domain purity is correlated with an observed decrease in PCBM miscibility in polymer, which drops from 21% (PBnDT-DTBT) to 12% (PBnDT-DTffBT). The disclosed "fluorine" impact not only explains the efficiency increase from 4% of PBnDT-DTBT (0F) to 7% with PBnDT-DTffBT (2F) but also suggests fluorine substitution should be generally considered in the future design of new polymers.

A novel polythiophene derivative, PBT1, was designed, synthesized, and applied in polymer solar cells (PSCs). This work provides a successful example of using molecular structure as a tool to realize optimal photovoltaic performance with high polymer content, thus enabling the realization of efficient photoabsorption in very thin films. As a result, an efficiency of 6.88% was recorded in a PSC with a 75 nm active layer.

A relationship between solvent additive concentration and active layer thickness in small-molecule solar cells is investigated. Specifically, the additive concentration must scale with the amount of semiconductor material and not as absolute concentration in solution. Devices with a wide range of active layers with thickness up to 200 nm can readily achieve efficiencies close to 6% when the right concentration of additive is used.

The treatment of toluene solvent and DTT additive enables the PBQ6:PYF-T-o-based all-PSC devices with PCE up to 17.06%, which is one of the highest value in non-halogenated-processed all-PSCs to date.

Recently, an intermixed phase has been identified within organic photovoltaic (OPV) bulk heterojunction (BHJ) systems that can exist in addition to relatively phase-pure regions, highlighting the need for a refined picture of the solid-state microstructure of donor–acceptor blends and for gaining further understanding of the exact nature and role such intermixed phases play in such devices. Here we manipulate the microstructure of polymer–fullerene systems via processing means and the selection of the molecular weight of the donor polymer. This manipulation is used as a tool to vary the fraction of intermixed phase present and its effects on the structure and subsequently the opto-electronic processes. We find clear relationships between the state of mixing and amount of exciton quenching and number of polarons generated per absorbed photon. Furthermore, we observe that blend systems incorporating higher molecular weight polymer result in a greater yield of dissociated polarons, likely due to the increase of the intermixed fraction.

Substrate heating during co-evaporation of bulk heterojunction organic solar cells aids phase separation and improves performance. While recombination remains unaffected, hole transport improves due to more crystalline donor domains.