Olle Inganäs

Modeling photocurrent action spectra of photovoltaic devices based on organic thin films

We have modeled experimental short-circuit photocurrent action spectra of poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophene) (PEOPT)/fullerene (C60) thin film heterojunction photovoltaic devices. Modeling was based on the assumption that the photocurrent generation process is the result of the creation and diffusion of photogenerated species (excitons), which are dissociated by charge transfer at the PEOPT/C60 interface. The internal optical electric field distribution inside the devices was calculated with the use of complex indices of refraction and layer thickness of the materials as determined by spectroscopic ellipsometry. Contributions to the photocurrent from optical absorption in polymer and fullerene layers were both necessary to model the experimental photocurrent action spectra. We obtained values for the exciton diffusion range of 4.7 and 7.7 nm for PEOPT and C60, respectively. The calculated internal optical electric field distribution and resulting photocurrent action spectra were used in or...

Fullerene‐Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability

A nonfullerene-based polymer solar cell (PSC) that significantly outperforms fullerene-based PSCs with respect to the power-conversion efficiency is demonstrated for the first time. An efficiency of >11%, which is among the top values in the PSC field, and excellent thermal stability is obtained using PBDB-T and ITIC as donor and acceptor, respectively.

On the origin of the open-circuit voltage of polymer–fullerene solar cells

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

Electrochromic and highly stable poly(3,4-ethylenedioxythiophene) switches between opaque blue-black and transparent sky blue

Abstract The synthesis and properties of poly(3,4-ethylenedioxythiophene) (PEDOT) are reported. The polymer shows a low band-gap, 0.5 eV lower than that of polythiophene. This leads to very pronounced electrochromic properties since the doped polymer is almost transparent, with a sky-blue tint, and the neutral polymer is blue-black. The low band-gap also leads to the possibility of n-doping by electrochemical methods. The conductivity of the doped polymer is up to 200 S cm−1, and the stability is remarkable, much better than observed in our studies of the polythiophene and polypyrrole families.

Controlled Folding of Micrometer-Size Structures

Several types of microactuators have been fabricated, from simple paddles to self-assembling and -disassembling cubes. Conducting bilayers made of a layer of polymer and a layer of gold were used as hinges to connect rigid plates to each other and to a silicon substrate. The bending of the hinges was electrically controlled and reversible, allowing precise three-dimensional positioning of the plates. The structures were released from the substrate with a technique based on differential adhesion. This method, which avoids the use of a sacrificial layer and allows the actuators to pull themselves off the surface, may have general applications in micromachining. Possibilities include the manufacture, of surfaces whose light reflection or chemical properties can be switched.

An Easily Synthesized Blue Polymer for High‐Performance Polymer Solar Cells

High performance solar cells fabricated from an easily synthesized donor-acceptor polymer show maximum power point up to 6.0 mW cm−2, with an open-circuit voltage of 0.89 V, short-circuit current density of 10.5 mA cm−2 and fill factor of 0.64, making this polymer a particularly promising candidate for high-efficiency low-cost polymer solar cells.

A Planar Copolymer for High Efficiency Polymer Solar Cells

An alternating copolymer, poly(2-(5-(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)-9-octyl-9H-carbazole) (HXS-1), was designed, synthesized, and used as the donor material for high efficiency polymer solar cells. The close packing of the polymer chains in the solid state was confirmed by XRD. A J(sc) of 9.6 mA/cm(2), a V(oc) of 0.81 V, an FF of 0.69, and a PCE of 5.4% were achieved with HXS-1 and [6,6]-phenyl C(71)-butyric acid methyl ester (PC(71)BM) as a bulk heterojunction active layer spin-coated from a solvent mixture of 1,2-dichlorobenzene and 1,8-diodooctane (97.5:2.5) under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW/cm(2).

Thermochromic and solvatochromic effects in poly(3-hexylthiophene)

Abstract We report thermochromic and solvatochromic effects in the soluble conductive polymer poly(3-hexylthiophene). Thin solid films of the polymer change colour at higher temperatures. The colour of solutions of the polymer can be changed by varying the composition of the solvent. Optical absorption in good solvents is similar to that of thin solid films at higher temperatures, while in poor solvents, absorption features similar to those found in solid films at low temperatures are observed. We discuss these effects with reference to the existence of a hitherto disregarded type of conformational defect, conformons. These are rotational defects on the polymer chain, leading to loss of planarity. High vacuum studies of the thermochromic effect in thin films using X-ray photoemission spectroscopy support this interpretation. The standard series of defects in conductive polymers, solitons, polarons and bipolarons, may have to be extended to include conformons.

Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/ fullerene derivative

The photovoltaic parameters of donor/acceptor blend organic solar cells are highly influenced by several parameters, such as the strength of the acceptor species, the morphology of the film due to ...

Field‐effect mobility of poly(3‐hexylthiophene)

A field‐effect transistor structure is used to study the transport properties of the soluble conductive polymer, poly(3‐hexylthiophene). We have measured conductance, mobility, and carrier concentration in undoped polymer thin films. The field‐effect mobility was found to be 10−5–10−4 cm2/V s at room temperature. The mobility decreases with increased temperature. The change is only partly reversible. Possible transport models are discussed.