Publications

Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells

Hybrid metal halide perovskites have become one of the hottest topics in optoelectronic materials research in recent years. Not only have they surpassed everyone's expectations and achieved similar performance as tried and true polycrystalline silicon photovoltaic devices, but they are also finding applications in a variety of different fields, including lighting. The main advantages of hybrid metal halide perovskites are simple processability, compatible with large‐scale solution processing such as roll‐to‐roll printing, and abundance of ingredients, all coupled to materials properties reminiscent of GaAs. On the road to this remarkable success, a series of challenges have been overcome, while some still remain. In this review, some of these challenges and possible solutions are described. In particular, understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices, how to overcome reproducibility issues and mitigate hysteresis, and the long‐term prospects of the technology in terms of stability and sustainability will all be discussed.

Highly Efficient Solid‐State Dye‐Sensitized TiO₂ Solar Cells Using Donor‐Antenna Dyes Capable of Multistep Charge‐Transfer Cascades

Novel ruthenium dyes with electron- donor antenna groups (triphenyl amine and tetraphenylbenzidine) are developed and applied in solid-state dye- sensitized solar cells. The higher absorption arising from extended conjugation in these dyes leads to efficient light harvesting, which in turn results in higher short-circuit currents; multistep charge-transfer cascades take place, very similar to the photosynthetic cascade process (see figure). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2007/c1872_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Influence of the Excited-State Charge-Transfer Character on the Exciton Dissociation in Donor–Acceptor Copolymers

We synthesize a polytriphenylamine homopolymer and two donor–acceptor copolymers (D–A-copolymers) based on triphenylamine (TPA) as donor in combination with two different acceptor moieties to study the effect of the acceptor unit on the excited-state charge-transfer characteristics (CT-characteristics) and charge separation. The two acceptor moieties are a dicyanovinyl group in the side chain and a thieno[3,4-b]thiophene carboxylate in the main chain. Absorption and photoluminescence studies show new CT-bands for both of the D–A-copolymers. Field-dependent charge extraction studies in bilayer solar cells indicate a stronger CT-character for the copolymer in which the acceptor group is less conjugated with the copolymer backbone. The D–A-copolymer carrying the acceptor unit in the main chain exhibits smaller excitonic CT-character and good conjugation leading to less-bound electron–hole pairs and a better charge separation. This fundamental study gives insight into the interdependence of conjugation, charge carrier mobility, and solar cell performance for two different D–A-copolymers.

NMRP versus “Click” Chemistry for the Synthesis of Semiconductor Polymers Carrying Pendant Perylene Bisimides

The synthesis of well-defined polymers with pendant perylene bisimide (PBI) groups by a combination of nitroxide-mediated radical polymerization (NMRP) of trimethylsilyl propargyl acrylate followed by copper-catalyzed azide−alkyne cycloaddition (CuAAC, "click" chemistry) is described. The kinetics of NMRP of trimethylsilyl propargyl acrylate polymerization was monitored by 1H NMR and size exclusion chromatography (SEC). Almost quantitative conversion in the "click" reaction with an azide functionalized PBI derivative was proven by FTIR and 1H NMR analysis. Thus, semiconductor polymers carrying PBI pendant groups with Mn up to 15800 g·mol−1 and polydispersity indices as good as 1.16 were obtained by this route. These polymers were compared with poly(perylene bisimide acrylate)s, PPerAcr(CH2)11, and PPerAcr(CH2)6, which were synthesized by direct NMRP of PBI acrylates. These samples do not carry any triazol unit and they differ in their spacer length connecting the PBI unit to the main chain. All polymers were comparatively studied by SEC, thermogravimetry, differential scanning calorimetry, polarization microscopy, UV/vis spectroscopy, and photoluminescence measurements. The crystalline structure of the polymers was analyzed by X-ray diffraction. Inductively coupled plasma mass spectrometry analysis confirmed that copper content in the "click" polymer could be reduced down to 126 ppm (w/w).

Polymer Crystallization as a Tool To Pattern Hybrid Nanostructures: Growth of 12 nm ZnO Arrays in Poly(3-hexylthiophene)

Well-ordered hybrid materials with a 10 nm length scale are highly desired. We make use of the natural length scale (typically 10-15 nm) of the alternating crystalline and amorphous layers that are generally found in semicrystalline polymers to direct the growth of a semiconducting metal oxide. This approach is exemplified with the growth of ZnO within a carboxylic acid end-functionalized poly(3-hexylthiophene) (P3HT-COOH). The metal-oxide precursor vapors diffuse into the amorphous parts of the semicrystalline polymer so that sheets of ZnO up to 0.5 μm in size can be grown. This P3HT-ZnO nanostructure further functions as a donor-acceptor photovoltaic system, with length scales appropriate for charge photogeneration.

<title>Synthesis and properties of new hole transport materials for organic light-emitting devices</title>

We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5-tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4-ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between -4.97 and -5.16 eV. By using a polymeric TPD as hole transport layer and tris(8-quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cs?m<SUP>2</SUP> were obtained under ambient conditions, using airstable Al-electrode as cathode and ITO as anode.

Highly Efficient n‐Doping via Proton Abstraction of an Acceptor₁‐Acceptor₂ Alternating Copolymer toward Thermoelectric Applications

Abstract Electron transporting ( n ‐type) polymers are the coveted complementary counterpart to more thoroughly studied hole transporting ( p ‐type) semiconducting polymers. Besides intrinsic stability issues of the doped form of n ‐type polymer toward ubiquitous oxidizing agents (H 2 O and O 2 ), the choice of suitable n ‐dopants and underlying mechanism of doping is an open research field. Using a low LUMO, n ‐type unipolar acceptor 1 ‐acceptor 2 copolymer poly(DPP‐TPD) in conjunction with bulk n ‐doping using Cs 2 CO 3 these issues can be addressed. A solid‐state acid‐base interaction between polymer and basic carbonate increases the backbone electron density by deprotonation of the thiophene comonomer while forming bicarbonate, as revealed by NMR and optical spectroscopy. Comparable to N‐DMBI hydride/electron transfer, Cs 2 CO 3 proton abstraction doping shifts the poly(DPP‐TPD) work function toward the LUMO. Thereby, the anionic doped state is resilient against O 2 but is susceptible toward H 2 O. Based on GIWAXS, Cs 2 CO 3 is mostly incorporated into the amorphous regions of poly(DPP‐TPD) with the help of hydrophilic side chains and has minor impact on the short‐range order of the polymer. Cs 2 CO 3 proton abstraction doping and the acceptor 1 ‐acceptor 2 copolymer architecture creates a synergistic n ‐doped system with promising properties for thermoelectric energy conversion, as evidenced by a remarkable power factor of (5.59 ± 0.39) × µW m −1 K −2 .

Perovskite Solar Cells: Capturing the Sun: A Review of the Challenges and Perspectives of Perovskite Solar Cells (Adv. Energy Mater. 16/2017)

In article number 1700264, Sven Hüttner, Pablo Docampo, and co-workers give an overview of hybrid metal halide perovskites for solar cell applications. The focus is on the series of challenges that have been overcome and those still remaining to be solved. In particular, the authors lay out their understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices; how to overcome reproducibility and hysteresis issues; and the long-term prospects of the technology in terms of stability and sustainability. Image by Criss Hohmann (Nanosystems Initiative Munich).

Donor–acceptor block copolymers for photovoltaic applications

Extensive research activities in polymer synthesis and device engineering have been devoted to the development of donor–acceptor (D–A) bulk heterojunction solar cells in the last years. In such devices, several photophysical processes occur all of which have to be optimized for efficient operation. First, excitons created upon light absorption need to reach the D/A interface within their exciton diffusion length (10– 20 nm), where they may dissociate into holes and electrons. Subsequent charge transport and finally charge collection at the electrodes can occur, given that co-continous pathways of donor and acceptor domains are provided. Owing to the small exciton diffusion lengths and the required optical absorption length of 100–200 nm, vertically aligned pathways with a high aspect ratio of either phase should percolate through the film. The morphologies resulting from this ideal situation resemble those of vertically oriented microphase separated block copolymer thin films, and hence suggest the importance of D–A block copolymers for organic photovoltaics. Furthermore, the covalent bond between the donor and acceptor blocks is not only desired to improve morphology control, but also to enhance long term stability of the device. The potential of block copolymers with electronic functionality to microphase separate into well-defined microstructures with several tens of nanometers in size thus addresses the morphological requirements mentioned above. This article gives an overview of donor–acceptor block copolymers and summarises recent developments of this field.

Perovskite solar cells involving poly(tetraphenylbenzidine)s: investigation of hole carrier mobility, doping effects and photovoltaic properties

A detailed study on the important properties of poly(tetraphenylbenzidine) as hole transport material in perovskite solar cells such as the influence of the molecular weight, the doping effects on charge carrier mobility and the polarity of the material is presented.

Donor–acceptor block copolymers carrying pendant PC₇₁BM fullerenes with an ordered nanoscale morphology

A PC₇₁BM-grafted donor–acceptor block copolymer with enhanced absorption showing a periodic nanostructure of 37 nm both in bulk and in thin films was synthesized by combining KCTP and CRP methods.

Influence of fluorination in π-extended backbone polydiketopyrrolopyrroles on charge carrier mobility and depth-dependent molecular alignment

The degree of fluorination in π-extended polydiketopyrrolopyrroles is correlated with semiconductor properties in transistors and an improved molecular alignment in thin films using depth-dependent grazing incidence X-ray scattering.

High Bulk Electron Mobility Diketopyrrolopyrrole Copolymers with Perfluorothiophene

The question of designing high electron mobility polymers by increasing the planarization using diffusive nonbonding heteroatom interactions in diketopyrrolopyrrole polymers is addressed in this. For this, three different diketopyrrolo[3,4‐ c ]pyrrole (DPP) derivatives with thienyl‐, 2‐pyridinyl‐, and phenyl‐flanked cores are copolymerized with an electron‐rich thiophene unit as well as an electron‐deficient 3,4‐difluorothiophene unit as comonomer to obtain diverse polymeric DPPs which vary systematically in their structures. The crystallinity differs significantly with clear trends on varying both flanking unit and comonomer. The optical gap and energy levels depend more on the nature of the flanking aryl units rather than on fluorination. Additionally, the charge transport properties are compared using different methods to differentiate between interface or orientation effects and bulk charge carrier transport. In organic field effect transistor devices with very high electron as well as hole mobilities (up to 0.6 cm 2 V −1 s −1 ) are obtained and fluorination leads to a more pronounced n‐type nature in all polymers, resulting in ambipolar behavior in otherwise p‐type materials. In contrast, space‐charge limited current measurements show a strong influence of the flanking units only on electron mobilities. Especially, the elegant synthetic strategy of combining pyridyl flanking units with difluorothiophene as the comonomer culminates in a record bulk electron mobility of 4.3 × 10 −3 cm 2 V −1 s −1 in polymers.

Self-Assembly of Semiconductor Organogelator Nanowires for Photoinduced Charge Separation

We investigated an innovative concept of general validity based on an organogel/polymer system to generate donor-acceptor nanostructures suitable for charge generation and charge transport. An electron conducting (acceptor) perylene bisimide organogelator forms nanowires in suitable solvents during gelation process. This phenomenon was utilized for its self-assembly in an amorphous hole conducting (donor) polymer matrix to realize an interpenetrating donor-acceptor interface with inherent morphological stability. The self-assembly and interface generation were carried out either stepwise or in a single-step. Morphology of the donor-acceptor network in thin films obtained via both routes were studied by a combination of scanning electron microscopy and atomic force microscopy. Additionally, photoinduced charge separation and charge transport in these systems were tested in organic solar cells. Fabrication steps of multilayer organogel/polymer photovoltaic devices were optimized with respect to morphology and surface roughness by introducing additional smoothening layers and charge injection/blocking layers. An inverted cell geometry was used here in which electrons are collected at the bottom electrode and holes at the top electrode. The simultaneous preparation of the interface exhibits almost 3-fold improvement in device characteristics compared to the successive method. The device characteristics under AM1.5 spectral conditions and 100 mW/cm(2) for the simultaneous preparation route are short circuit current J(sc) = 0.28 mA cm(-2), open circuit voltage V(OC) = 390 mV, fill factor FF = 38%, and a power conversion efficiency eta = 0.041%.

Avoiding Bias Effects in NMR Experiments for Heteronuclear Dipole–Dipole Coupling Determinations: Principles and Application to Organic Semiconductor Materials

Carbon-proton dipole-dipole couplings between bonded atoms represent a popular probe of molecular dynamics in soft materials or biomolecules. Their site-resolved determination, for example, by using the popular DIPSHIFT experiment, can be challenged by spectral overlap with nonbonded carbon atoms. The problem can be solved by using very short cross-polarization (CP) contact times, however, the measured modulation curves then deviate strongly from the theoretically predicted shape, which is caused by the dependence of the CP efficiency on the orientation of the CH vector, leading to an anisotropic magnetization distribution even for isotropic samples. Herein, we present a detailed demonstration and explanation of this problem, as well as providing a solution. We combine DIPSHIFT experiments with the rotor-directed exchange of orientations (RODEO) method, and modifications of it, to redistribute the magnetization and obtain undistorted modulation curves. Our strategy is general in that it can also be applied to other types of experiments for heteronuclear dipole-dipole coupling determinations that rely on dipolar polarization transfer. It is demonstrated with perylene-bisimide-based organic semiconductor materials, as an example, in which measurements of dynamic order parameters reveal correlations of the molecular dynamics with the phase structure and functional properties.

Controlled Synthesis of Water-Soluble Conjugated Polyelectrolytes Leading to Excellent Hole Transport Mobility

Conjugated polyelectrolytes (CPE) find widespread applications due to their solubility in aqueous systems or highly polar solvents. However, ion reorganization under applied fields and low charge carrier mobility limit their use as active layers in electronic devices. Here, we present a novel controlled synthetic route for CPEs based on polythiophene carrying sulfonate side groups. We prepared three different polymers with varying molecular weights and narrow polydispersity. For the CPE with the highest molecular weight, we observed the formation of small aggregates in aqueous solution which was confirmed by UV–vis absorption and fluorescence spectroscopy. In the UV–vis spectrum, vibrational bands are observed, which are maintained in the thin film. These absorption bands are similar to those of crystalline poly(3-hexylthiophene). The fluorescence signal is almost completely quenched for these aggregates. Adding other polar solvents such as DMSO results in the dissolution of the aggregates indicated by the decrease of the vibrational bands in UV–vis and the increase of the fluorescence signal. This polymer further exhibits a remarkably high hole transport mobility of (1.2 ± 0.5) × 10–2 cm2/(V s) as determined by the space charge limited current method. The underlying transport mechanism was studied by current (J)–voltage (V) measurements and impedance spectroscopy. The former shows a quadratic dependence of J vs V and a fast response within microseconds characteristic for a classical semiconductor, while the latter shows no sign of any ion motion. In contrast to other reported CPEs, the regioregular chain conformation and the narrow molecular weight distribution here promote the formation of aggregates which improve the electronic charge transport throughout the bulk. Additionally, the presence of sterically demanding counterions suppress the ion motion and reorganization, resulting in a water-soluble semiconducting material with high hole transport mobility.

Charge separation and recombination in self-organizing nanostructured donor–acceptor block copolymer films

A series of donor–acceptor diblock copolymers with varying molecular weight are studied in thin film and compared with an ‘equivalent’ blend formed from donor and acceptor homopolymers. Steady-state and transient spectroscopies are used to demonstrate a correlation between low molecular weight block copolymers and increased photoluminescence quenching (up to 99%) leading to higher yields of long-lived free charges. Such block copolymers are shown, by electron microscopy, to exhibit phase-segregated micrdomains whose size and periodicity are determined by their molecular weight. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with our spectroscopic results, subject to a trade-off between charge generation and tranpsort/collection.

Light-emitting diodes based on phenylenevinylene oligomers with defined chain lengths