Jacobus J. van Franeker

A real-time study of the benefits of co-solvents in polymer solar cell processing

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. Controlling the degree of phase separation between these components is crucial to reach efficient solar cells. In solution-processed polymer-fullerene solar cells, small amounts of co-solvents are commonly used to avoid the formation of undesired large fullerene domains that reduce performance. There is an ongoing discussion about the origin of this effect. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large fullerene-rich domains form via liquid-liquid phase separation at ~20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studied.

Polymer Solar Cells : Solubility Controls Fiber Network Formation

The photoactive layer of polymer solar cells is commonly processed from a four-component solution, containing a semiconducting polymer and a fullerene derivative dissolved in a solvent-cosolvent mixture. The nanoscale dimensions of the polymer-fullerene morphology that is formed upon drying determines the solar cell performance, but the fundamental processes that govern the size of the phase-separated polymer and fullerene domains are poorly understood. Here, we investigate morphology formation of an alternating copolymer of diketopyrrolopyrrole and a thiophene-phenyl-thiophene oligomer (PDPPTPT) with relatively long 2-decyltetradecyl (DT) side chains blended with [6,6]-phenyl-C71-butyric acid methyl ester. During solvent evaporation the polymer crystallizes into a fibrous network. The typical width of these fibers is analyzed by quantification of transmission electron microscopic images, and is mainly determined by the solubility of the polymer in the cosolvent and the molecular weight of the polymer. A higher molecular weight corresponds to a lower solubility and film processing results in a smaller fiber width. Surprisingly, the fiber width is not related to the drying rate or the amount of cosolvent. We have made solar cells with fiber widths ranging from 28 to 68 nm and found an inverse relation between fiber width and photocurrent. Finally, by mixing two cosolvents, we develop a ternary solvent system to tune the fiber width. We propose a model based on nucleation-and-growth which can explain these measurements. Our results show that the width of the semicrystalline polymer fibers is not the result of a frozen dynamical state, but determined by the nucleation induced by the polymer solubility.

Wide-Bandgap Benzodithiophene-Benzothiadiazole Copolymers for Highly Efficient Multijunction Polymer Solar Cells.

Novel wide-bandgap semiconducting polymers are designed and synthesized for multijunction polymer solar cell (PSC) applications. In single-junction PSCs, BDT-FBT-2T exhibits efficiencies exceeding 6.5% for active layer thicknesses between 90 and 250 nm, with the highest efficiency of 7.7% at 100 and 250 nm. This enables tandem PSCs to be created with an efficiency of 8.9%.

Effect of side chain length on the charge transport, morphology, and photovoltaic performance of conjugated polymers in bulk heterojunction solar cells

The effect of side chain length on the photovoltaic properties of conjugated polymers is systematically investigated with two sets of polymers that bear different alkyl side chain lengths based on benzodithiophene and benzo[2,1,3]thiadiazole or 5,6-difluorobenzo[2,1,3]thiadiazole. Characterization of the photovoltaic cells reveals a strong interdependency between the side chain length of conjugated polymers and photovoltaic performances (power conversion efficiency, short-circuit current, and fill factor) of the resulting bulk-heterojunction (BHJ) solar cells. Charge carrier transport and external quantum efficiency (EQE) measurements in combination with morphology characterization suggest that too long side chains lead to deteriorated charge transport, suboptimal BHJ morphology, considerable bimolecular recombination, and consequently poor photovoltaic performances. On the other hand, when the side chains are too short, they cannot afford a high enough solubility and molecular weight for the resulting polymers and produce poor solar cell performance as well. This study shows that side chain optimization is of significant importance to maximize the potential of photovoltaic active conjugated polymers, which indicates the fruitful molecular design rules toward highly efficient BHJ polymer solar cells.

Dichotomous Role of Exciting the Donor or the Acceptor on Charge Generation in Organic Solar Cells

In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.

Structure–property relationships for bis-diketopyrrolopyrrole molecules in organic photovoltaics

The design of small organic molecules for efficient solution-processed organic solar cells is hampered by the absence of relationships that connect molecular structure via processing to blend morphology and power conversion efficiency. Here we study a series of bis-diketopyrrolopyrrole molecules in which we systematically vary the aromatic core, the solubilizing side chains, and the end groups to achieve power conversion efficiencies of 4.4%. By comparing the morphology and performance we attempt to identify and rationalize the structure–property relationships. We find that the tendency to aggregate or crystallize are important factors to control and that these require a subtle balance.

Transition dipole moment orientation in films of solution processed fluorescent oligomers: Investigating the influence of molecular anisotropy

The low light-outcoupling efficiency of organic light emitting diodes (OLEDs) is limiting their performance. Orientation of the transition dipole moment of the emitting molecules in the plane of the diodes can improve the luminance of OLEDs. While the orientation of evaporated small-molecule materials has been studied in the past few years, not much is known about solution processed small molecules and short oligomers, and it is not clear yet which parameters influence their orientation in the film. In this work we study a series of short conjugated p-phenylene vinylene oligomers (OPVn), consisting of an increasing number of repeating phenyl rings (n from 2 to 7), which are introduced into a small-molecule host matrix. By measuring the angular distribution of p-polarised fluorescence intensity from thin solution processed films, we determine the average orientation of the transition dipole moment of the emitters in the host matrix. We find that for longer oligomers (n = 6, 7), the transition dipole moments align more horizontally, with ratios of horizontally to vertically oriented dipoles up to 80:20. The preferential horizontal alignment is related to the aggregation of the emitter molecules.

2-Methoxyethanol as a new solvent for processing methylammonium lead halide perovskite solar cells

Methylammonium lead halide perovskites used in photovoltaic devices are generally deposited from high boiling point solvents with low volatility such as N,N-dimethylformamide. The slow drying causes the formation of relatively large perovskite crystallites that enhance surface roughness and lead to pin holes between the crystallites. We show that the use of 2-methoxyethanol, which is a more volatile solvent, results in smaller crystals that still span the entire layer thickness. This improves the surface coverage of perovskite films, reduces the leakage current and increases the open-circuit voltage and fill factor of solar cells. P–I–N configuration solar cells, processed under ambient conditions from a triple anion (iodide, chloride, and acetate) lead precursor salt, provide an increase in the power conversion efficiency from 14.1% to 15.3% when N,N-dimethylformamide is replaced by 2-methoxyethanol as the solvent.

Energy Level Tuning of Poly(phenylene-alt-dithienobenzothiadiazole)s for Low Photon Energy Loss Solar Cells

Six poly(phenylene‐alt‐dithienobenzothiadiazole)‐based polymers have been synthesized for application in polymer–fullerene solar cells. Hydrogen, fluorine, or nitrile substitution on benzothiadiazole and alkoxy or ester substitution on the phenylene moiety are investigated to reduce the energy loss per converted photon. Power conversion efficiencies (PCEs) up to 6.6% have been obtained. The best performance is found for the polymer–fullerene combination with distinct phase separation and crystalline domains. This improves the maximum external quantum efficiency for charge formation and collection to 66%. The resulting higher photocurrent compensates for the relatively large energy loss per photon (E loss = 0.97 eV) in achieving a high PCE. By contrast, the polymer that provides a reduced energy loss (E loss = 0.49 eV) gives a lower photocurrent and a reduced PCE of 1.8% because the external quantum efficiency of 17% is limited by a suboptimal morphology and a reduced driving force for charge transfer.