Kh Koen Hendriks

Efficient tandem and triple-junction polymer solar cells.

We demonstrate tandem and triple-junction polymer solar cells with power conversion efficiencies of 8.9% and 9.6% that use a newly designed, high molecular weight, small band gap semiconducting polymer and a matching wide band gap polymer.

Efficient Small Bandgap Polymer Solar Cells with High Fill Factors for 300 nm Thick Films

A high-molecular-weight conjugated polymer based on alternating electron-rich and electron-deficient fused ring systems provides efficient polymer solar cells when blended with C60 and C70 fullerene derivatives. The morphology of the new polymer/fullerene blend reduces bimolecular recombination and allows reaching high fill factors and power conversion efficiencies for films up to 300 nm thickness.

High Quantum Efficiencies in Polymer Solar Cells at Energy Losses below 0.6 eV

Diketopyrrolopyrrole-based conjugated polymers bridged with thiazole units and different donors have been designed for polymer solar cells. Quantum efficiencies above 50% have been achieved with energy loss between optical band gap and open-circuit voltage below 0.6 eV.

Diketopyrrolopyrrole polymers for 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.

Small-bandgap semiconducting polymers with high near-infrared photoresponse

Lowering the optical bandgap of conjugated polymers while maintaining a high efficiency for photoinduced charge transfer to suitable electron acceptors such as fullerene has remained a formidable challenge in the area of organic photovoltaics. Here we present the synthesis and application of a series of ultra-small-bandgap donor-acceptor polymers composed of diketopyrrolopyrrole as acceptor and pyrrole-based groups as strong donors. The HOMO energy levels of the polymers can be progressively increased by increasing the donor strength while the LUMO level remains similar, resulting in optical bandgaps between 1.34 and 1.13 eV. Solar cells based on these polymers blended with fullerene derivatives show a high photoresponse in the near-infrared (NIR) and good photovoltaic characteristics, with power conversion efficiencies of 2.9-5.3%. The photoresponse reaches up to 50% external quantum efficiency at 1000 nm and extends to 1200 nm. With the use of a retro-reflective foil to optimize light absorption, high photocurrents up to 23.0 mA cm(-2) are achieved under standard solar illumination conditions. These ultra-small-bandgap polymers are excellent candidates for use in multi-junction applications and NIR organic photodetectors.

Effect of the Fibrillar Microstructure on the Efficiency of High Molecular Weight Diketopyrrolopyrrole‐Based Polymer Solar Cells

The nature of the solubilizing alkyl side chains has a strong effect on the performance of semiconducting diketopyrrolopyrrole polymers in organic solar cells with fullerene acceptors. The effect relates to the width of semicrystalline polymer fibrils that form in these blends. If the width of the fibril is wider than the exciton diffusion length, fewer charges form and the efficiency drops.

Homocoupling Defects in Diketopyrrolopyrrole-Based Copolymers and Their Effect on Photovoltaic Performance

We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects.

Comparing random and regular diketopyrrolopyrrole-bithiophene-thienopyrrolodione terpolymers for organic photovoltaics

Isomeric random and regular alternating π-conjugated terpolymers comprising diketopyrrolopyrrole (DPP), thienopyrrolodione (TPD), and bithiophene (2T) were synthesized to study the effect of the sequential distribution of monomeric units on the semiconducting properties. The optical and electrochemical properties and the performance in photovoltaic cells of the random and regular terpolymers are found to be significantly different. DPP2T-rich sections in the random terpolymer cause higher HOMO and deeper LUMO energy levels and a smaller optical band gap compared to the regular terpolymer. The randomization of DPP and TPD units along the chain has a negative effect on the photovoltaic performance, resulting in power conversion efficiencies of merely 1.0% for the random terpolymer while a more favorable efficiency of 5.3% is obtained for the regular terpolymer when combined with a fullerene acceptor.

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.

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.