Indium Tin Oxide-Free Tandem Polymer Solar Cells on Opaque Substrates with Top Illumination

Top-illuminated, indium tin oxide (ITO)-free, tandem polymer solar cells are fabricated on opaque substrates in an inverted device configuration. In the tandem cell, a wide band gap subcell, consisting of poly­[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) blended with [70]­PCBM is combined with a small band gap subcell consisting of a mixture of poly­[{2,5-bis­(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-diyl}-<i>alt</i>-{[2,2′-(1,4-phenylene)­bisthiophene]-5,5′-diyl}] (PDPPTPT) and [60]­PCBM. Compared to the more common bottom-illuminated inverted tandem polymer solar cells on transparent ITO substrates, the front and back cells must be reversed when using opaque substrates and a transparent and conductive top contact must be employed to enable top illumination. A high conductive poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) layer in combination with Ag lines surrounding the active area as current collection electrode is used for this purpose. The tandem polymer solar cell on an opaque glass/metal substrate yields a power conversion efficiency of 6.1% when the thicknesses of the photoactive layers are balanced for optimum performance. This is similar to the equivalent inverted tandem device fabricated on a transparent glass/ITO substrate

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

Enhancing the Photocurrent in Diketopyrrolopyrrole-Based Polymer Solar Cells via Energy Level Control

A series of diketopyrrolopyrrole (DPP)-based small band gap polymers has been designed and synthesized by Suzuki or Stille polymerization for use in polymer solar cells. The new polymers contain extended aromatic π-conjugated segments alternating with the DPP units and are designed to increase the free energy for charge generation to overcome current limitations in photocurrent generation of DPP-based polymers. In optimized solar cells with [6,6]­phenyl-C₇₁-butyric acid methyl ester ([70]­PCBM) as acceptor, the new DPP-polymers provide significantly enhanced external and internal quantum efficiencies for conversion of photons into collected electrons. This provides short-circuit current densities in excess of 16 mA cm^–2, higher than obtained so far, with power conversion efficiencies of 5.8% in simulated solar light. We analyze external and internal photon to collected electron quantum efficiencies for the new polymers as a function of the photon energy loss, defined as the offset between optical band gap and open circuit voltage, and compare the results to those of some of the best DPP-based polymers solar cells reported in the literature. We find that for the best solar cells there is an empirical relation between quantum efficiency and photon energy loss that presently limits the power conversion efficiency in these devices

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

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

Toward Practical Useful Polymers for Highly Efficient Solar Cells via a Random Copolymer Approach

Using benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene and two matched 5,6-difluorobenzo­[2,1,3]­thiadiazole-based monomers, we demonstrate that random copolymerization of two electron deficient monomers, alternating with one electron rich monomer, forms a successful approach to synthesize state-of-the-art semiconducting copolymers for organic solar cells. Over a range of compositions, these random copolymers provide impressive power conversion efficiencies (PCEs) of about 8.0%, higher than those of their binary parent polymers, and with little batch-to-batch variation. A PCE over 8% could also be achieved when the active layer was deposited from nonhalogenated solvents at room temperature

Nanoscale Organic Ferroelectric Resistive Switches

Organic ferroelectric resistive switches function by grace of nanoscale phase separation in a blend of a semiconducting and a ferroelectric polymer that is sandwiched between metallic electrodes. In this work, various scanning probe techniques are combined with numerical modeling to unravel their operational mechanism. Resistive switching is shown to result from modulation of the charge injection barrier at the semiconductor–electrode interfaces. The modulation is driven by the stray field of the polarization charges in the ferroelectric phase and consequently is restricted to regions where semiconductor and ferroelectric phases exist in close vicinity. Since each semiconductor domain can individually be switched and read out, a novel, nanoscale memory element is demonstrated. An ultimate information density of ∼30 Mb/cm² is estimated for this bottom-up defined memory device

Water Splitting with Series-Connected Polymer Solar Cells

We investigate light-driven electrochemical water splitting with series-connected polymer solar cells using a combined experimental and modeling approach. The expected maximum solar-to-hydrogen conversion efficiency (η_STH) for light-driven water splitting is modeled for two, three, and four series-connected polymer solar cells. In the modeling, we assume an electrochemical water splitting potential of 1.50 V and a polymer solar cell for which the external quantum efficiency and fill factor are both 0.65. The minimum photon energy loss (<i>E</i>_loss), defined as the energy difference between the optical band gap (<i>E</i>_g) and the open-circuit voltage (<i>V</i>_oc), is set to 0.8 eV, which we consider a realistic value for polymer solar cells. Within these approximations, two series-connected single junction cells with <i>E</i>_g = 1.73 eV or three series-connected cells with <i>E</i>_g = 1.44 eV are both expected to give an η_STH of 6.9%. For four series-connected cells, the maximum η_STH is slightly less at 6.2% at an optimal <i>E</i>_g = 1.33 eV. Water splitting was performed with series-connected polymer solar cells using polymers with different band gaps. PTPTIBDT-OD (<i>E</i>_g = 1.89 eV), PTB7-Th (<i>E</i>_g = 1.56 eV), and PDPP5T-2 (<i>E</i>_g = 1.44 eV) were blended with [70]­PCBM as absorber layer for two, three, and four series-connected configurations, respectively, and provide η_STH values of 4.1, 6.1, and 4.9% when using a retroreflective foil on top of the cell to enhance light absorption. The reasons for deviations with experiments are analyzed and found to be due to differences in <i>E</i>_g and <i>E</i>_loss. Light-driven electrochemical water splitting was also modeled for multijunction polymer solar cells with vertically stacked photoactive layers. Under identical assumptions, an η_STH of 10.0% is predicted for multijunction cells

Universal Correlation between Fibril Width and Quantum Efficiency in Diketopyrrolopyrrole-Based Polymer Solar Cells

For a series of six diketopyrrolopyrrole (DPP)-based conjugated polymers, we establish a direct correlation between their external quantum efficiencies (EQE) in organic solar cells and the fibrillar microstructure in the blend. The polymers consist of electron-deficient DPP units, carrying long branched 2′-decyltetradecyl (DT) side chains for solubility, that alternate along the main chain with electron-rich aromatic segments comprising benzene, thiophene, or fused aromatic rings. The high molecular weight DT-DPP polymers were incorporated in bulk heterojunction solar cells with [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]­PCBM) as acceptor. The morphology of the DT-DPP:[70]­PCBM blends is characterized by a semicrystalline fibrillar microstructure with fibril widths between 4.5 and 30 nm as evidenced from transmission electron microscopy. A clear correlation is found between the widths of the fibrils and the EQE for photon to electron conversion. The highest EQEs (60%) and power conversion efficiencies (7.1%) are obtained for polymers with fibril widths less than 12 nm. For blends with fibrils wider than 12 nm, the EQE is low because exciton diffusion becomes limiting for charge generation. Interestingly, the correlation found here matches with previous data on related DPP-based polymers. This suggests that for this class of materials the relation between fiber width and EQE is universal. The fiber width is largely correlated with the solubility of the polymers, with less soluble DPP-based polymers giving narrower fibrils

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