• Energy Research

AbstractDiketopyrrolopyrrole (DPP)‐conjugated polymers are a versatile class of semiconductors for application in organic solar cells because of their tunable optoelectronic properties. A record power conversion efficiency (PCE) of 9.4% was recently achieved for DPP polymers, but further improvements are required to reach true efficiency limits. Using five DPP polymers with different chemical structures and molecular weights, the device performance of polymer:fullerene solar cells is systematically optimized by considering device polarity, morphology, and light absorption. The polymer solubility is found to have a significant effect on the optimal device polarity. Soluble polymers show a 10–25% increase in PCE in inverted device configurations, while the device performance is independent of device polarity for less soluble DPP derivatives. The difference seems related to the polymer to fullerene weight ratio at the ZnO interface in inverted devices, which is higher for more soluble DPP polymers. Optimization of the nature of the cosolvent to narrow the fibril width of polymers in the blends toward the exciton diffusion length enhances charge generation. Additionally, the use of a retroreflective foil increases absorption of light. Combined, the effects afford a PCE of 9.6%, among the highest for DPP‐based polymer solar cells.

AbstractTwo conjugated polymers based on diketopyrrolopyrrole (DPP) in the main chain with different content of perylene bisimide (PBI) side chains are developed. The influence of PBI side chain on the photovoltaic performance of these DPP‐based conjugated polymers is systematically investigated. This study suggests that the PBI side chains can not only alter the absorption spectrum and energy level but also enhance the crystallinity of conjugated polymers. As a result, such polymers can act as electron donor, electron acceptor, and single‐component active layer in organic solar cells. These findings provide a new guideline for the future molecular design of multifunctional conjugated polymers.

The influence of the chemical structure of conjugated polymers on the nanophase separation and device performance in fullerene-based solar cells has been widely studied, while this is less investigated in non-fullerene solar cells. In this work, we design three conjugated polymers with different length of side chains, and we find that the length of side chains has little influence on the quantum efficiencies of non-fullerene solar cells. As a comparison, the length of side chains has a significant effect on the quantum efficiencies of fullerene-based solar cells. This indicates that morphology of the blended thin films in non-fullerene solar cells is rather independent of the length of the donor side chains, and the mechanism for morphology evolution in the non-fullerene system is completely different from that in the fullerene system. Our conclusion is confirmed by a variety of advanced characterization techniques. The studies reveal that in blended thin films based on the non-fullerene material the donor polymers with different side chains have a similar coherence length of π-π stacking, crystal size and domain purity, giving rise to similar internal quantum efficiency and power conversion efficiency of the solar cells.

AbstractA new method is presented to fabricate bilayer organic solar cells via sequential deposition of bulk‐heterojunction layers obtained using spontaneous spreading of polymer–fullerene blends on a water surface. Using two layers of a small bandgap diketopyrrolopyrrole polymer–fullerene blend, a small improvement in power conversion efficiency (PCE) from 4.9% to 5.1% is obtained compared to spin‐coated devices of similar thickness. Next, bilayer–ternary cells are fabricated by first spin coating a wide bandgap thiophene polymer–fullerene blend, followed by depositing a small bandgap diketopyrrolopyrrole polymer–fullerene layer by transfer from a water surface. These novel bilayer–ternary devices feature a PCE of 5.9%, higher than that of the individual layers. Remarkable, external quantum efficiencies (EQEs) over 100% are measured for the wide bandgap layer under near‐infrared bias light illumination. Drift‐diffusion calculations confirm that near‐infrared bias illumination can result in a significant increase in EQE as a result of a change in the internal electric field in the device, but cannot yet account for the magnitude of the effect. The experimental results indicate that the high EQEs over 100% under bias illumination are related to a barrier for electron transport over the interface between the two blends.

A series of "double-cable" conjugated polymers were developed for application in efficient single-component polymer solar cells, in which high quantum efficiencies could be achieved due to the optimized nanophase separation between donor and acceptor parts. The new double-cable polymers contain electron-donating poly(benzodithiophene) (BDT) as linear conjugated backbone for hole transport and pendant electron-deficient perylene bisimide (PBI) units for electron transport, connected via a dodecyl linker. Sulfur and fluorine substituents were introduced to tune the energy levels and crystallinity of the conjugated polymers. The double-cable polymers adopt a "face-on" orientation in which the conjugated BDT backbone and the pendant PBI units have a preferential π-π stacking direction perpendicular to the substrate, favorable for interchain charge transport normal to the plane. The linear conjugated backbone acts as a scaffold for the crystallization of the PBI groups, to provide a double-cable nanophase separation of donor and acceptor phases. The optimized nanophase separation enables efficient exciton dissociation as well as charge transport as evidenced from the high-up to 80%-internal quantum efficiency for photon-to-electron conversion. In single-component organic solar cells, the double-cable polymers provide power conversion efficiency up to 4.18%. This is one of the highest performances in single-component organic solar cells. The nanophase-separated design can likely be used to achieve high-performance single-component organic solar cells.

Conjugated polymers have been extensively studied for application in organic solar cells. In designing new polymers, particular attention has been given to tuning the absorption spectrum, molecular energy levels, crystallinity, and charge carrier mobility to enhance performance. As a result, the power conversion efficiencies (PCEs) of solar cells based on conjugated polymers as electron donor and fullerene derivatives as electron acceptor have exceeded 10% in single-junction and 11% in multijunction devices. Despite these efforts, it is notoriously difficult to establish thorough structure-property relationships that will be required to further optimize existing high-performance polymers to their intrinsic limits. In this Account, we highlight progress on the development and our understanding of diketopyrrolopyrrole (DPP) based conjugated polymers for polymer solar cells. The DPP moiety is strongly electron withdrawing and its polar nature enhances the tendency of DPP-based polymers to crystallize. As a result, DPP-based conjugated polymers often exhibit an advantageously broad and tunable optical absorption, up to 1000 nm, and high mobilities for holes and electrons, which can result in high photocurrents and good fill factors in solar cells. Here we focus on the structural modifications applied to DPP polymers and rationalize and explain the relationships between chemical structure and organic photovoltaic performance. The DPP polymers can be tuned via their aromatic substituents, their alkyl side chains, and the nature of the π-conjugated segment linking the units along the polymer chain. We show that these building blocks work together in determining the molecular conformation, the optical properties, the charge carrier mobility, and the solubility of the polymer. We identify the latter as a decisive parameter for DPP-based organic solar cells because it regulates the diameter of the semicrystalline DPP polymer fibers that form in the photovoltaic blends with fullerenes via solution processing. The width of these fibers and the photon energy loss, defined as the energy difference between optical band gap and open-circuit voltage, together govern to a large extent the quantum efficiency for charge generation in these blends and thereby the power conversion efficiency of the photovoltaic devices. Lowering the photon energy loss and maintaining a high quantum yield for charge generation is identified as a major pathway to enhance the performance of organic solar cells. This can be achieved by controlling the structural purity of the materials and further control over morphology formation. We hope that this Account contributes to improved design strategies of DPP polymers that are required to realize new breakthroughs in organic solar cell performance in the future.