Gebhard J. Matt

Detection of X-ray photons by solution-processed lead halide perovskites

The evolution of real-time medical diagnostic tools such as angiography and computer tomography from radiography based on photographic plates was enabled by the development of integrated solid-state X-ray photon detectors, based on conventional solid-state semiconductors. Recently, for optoelectronic devices operating in the visible and near infrared spectral regions, solution-processed organic and inorganic semiconductors have also attracted immense attention. Here we demonstrate a possibility to use such inexpensive semiconductors for sensitive detection of X-ray photons by direct photon-to-current conversion. In particular, methylammonium lead iodide perovskite (CH3NH3PbI3) offers a compelling combination of fast photoresponse and a high absorption cross-section for X-rays, owing to the heavy Pb and I atoms. Solution processed photodiodes as well as photoconductors are presented, exhibiting high values of X-ray sensitivity (up to 25 µC mGyair-1 cm-3) and responsivity (1.9×104 carriers/photon), which are commensurate with those obtained by the current solid-state technology.

Nonvolatile organic field-effect transistor memory element with a polymeric gate electret

Organic field-effect transistors with a polymeric electret as gate insulator and fullerenes as a molecular semiconductor were fabricated. We observed an amplification of the drain–source current Ids on the order of 104 upon applying a gate voltage Vg. Reversing the gate voltage Vg features large metastable hysteresis in the transfer characteristics Ids(Vg) with a long retention time. The observation of a switchable channel current Ids is proposed to originate from charge storage in the organic electret. As such, this device is a demonstration of an organic nonvolatile memory element switchable with the gate voltage.

Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors

The fabrication and characterization of high-mobility, n-channel organic field-effect transistors (OFET) based on methanofullerene [6,6]-phenyl C61-butyric acid methyl ester using various organic insulators as gate dielectrics is presented. Gate dielectrics not only influence the morphology of the active semiconductor, but also the distribution of the localized states at the semiconductor-dielectric interface. Spin-coated organic dielectrics with very smooth surfaces provide a well-defined interface for the formation of high quality organic semiconductor films. The charge transport and mobility in these OFET devices strongly depend on the choice of the gate dielectric. The electron mobilities obtained are in the range of 0.05-0.2 cm2 V-1 s-1. Most of the OFETs fabricated using organic dielectrics exhibit an inherent hysteresis due to charge trapping at the semiconductor-dielectric interface. Devices with a polymeric electret as gate dielectric show a very large and metastable hysteresis in its transfer characteristics. The observed hysteresis is found to be temperature dependent and has been used to develop a bistable memory element.

Overcoming the Interface Losses in Planar Heterojunction Perovskite-Based Solar Cells.

UNLABELLED A scalable, hysteresis-free and planar architecture perovskite solar cell is presented, employing a flame spray synthesized low-temperature processed NiO (LT-NiO) as hole-transporting layer yielding efficiencies close to 18%. Importantly, it is found that LT-NiO boosts the limits of open-circuit voltages toward an impressive non-radiative voltage loss of 0.226 V only, whereas PEDOT PSS suffers from significant large non-radiative recombination losses.

Sensitization of low bandgap polymer bulk heterojunction solar cells

For efficiently harvesting the terrestrial solar spectrum in conjugated polymer based solar cells, low bandgap polymers with a bandgap -1.8 eV are needed. The photophysics of such low band gap conjugated polymers as well as their excited state interactions with electron acceptors such as fullerenes are of importance when using them in photovoltaic devices.In this work we present a device structural study on the soluble low bandgap polymer PTPTB, consisting of alternating electron-rich Ndodecyl-2,5-bis (29-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units.The bandgap of this polymer, determined by electrochemistry and by optical absorption, is 1.6 eV. The performance of the photovoltaic devices is discussed in terms of spectrally resolved photocurrent measurements, AM1.5 measurements and temperature dependent I–V spectroscopy. Strategies to utilize this polymer for bulk heterojunction tandem solar cells with either a wide band gap polymer or in conjunction with strongly absorbing small molecular dyes are discussed. 2002 Elsevier Science B.V. All rights reserved.

IR sensitization of an indene-C60 bisadduct (ICBA) in ternary organic solar cells

This article presents a smart strategy to successfully tackle two main limitations of organic solar cells (OSCs) in one step: the narrow absorption window of polymers as well as the Voc limitation related to the mono-PCBM. We demonstrate the high OSC performance of >5% in the combination of a fullerene multiadduct, i.e. indene-C60 bisadduct (ICBA) with a low bandgap polymer employing the concept of ternary OSCs. Solar cells achieving a fill factor (FF) of >60% along with high quantum efficiency in the near IR region are demonstrated for ternary composites consisting of P3HT, the fullerene-bisindene adduct of ICBA and a low bandgap copolymer sensitizer. Using P3HT as an efficient transport matrix with non-Langevin recombination dynamics allowed us to limit and overcome the otherwise dominant recombination losses of the indene-C60 bisadduct – low bandgap copolymer blends. The success of this strategy resulted in a relative efficiency improvement of over 25%.

Overcoming interface losses in organic solar cells by applying low temperature, solution processed aluminum-doped zinc oxide electron extraction layers

Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.

Abnormal strong burn-in degradation of highly efficient polymer solar cells caused by spinodal donor-acceptor demixing

The performance of organic solar cells is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. Here we demonstrate an abnormal strong burn-in degradation in highly efficient polymer solar cells caused by spinodal demixing of the donor and acceptor phases, which dramatically reduces charge generation and can be attributed to the inherently low miscibility of both materials. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. A theoretical calculation of the molecular parameters and construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells.

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals

In the past few years, hybrid organic-inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15-25 nm formamidinium CH(NH2)2PbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415-740 nm, full width at half-maximum (fwhm) of 20-44 nm, and radiative lifetimes of 5-166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals paves the way for perovskite quantum dot photovoltaics.

A solution-processed barium hydroxide modified aluminum doped zinc oxide layer for highly efficient inverted organic solar cells

Inverted organic solar cells (iOSCs) with air stable interface materials and top electrodes and an efficiency of 6.01% are achieved by inserting a barium hydroxide (Ba(OH)2) layer between the aluminum doped zinc oxide (AZO) electron extraction layer and the active layer. A low bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (pDPP5T-2) and phenyl-C61-butyric acid methyl ester (PC61BM) were chosen as the active layer compounds. Compared to the control device without Ba(OH)2, insertion of a few nm thick Ba(OH)2 layer results in an enhanced VOC of 10%, JSC of 28%, FF of 28% and PCE of 80%. Modification of AZO with a solution processed low-cost Ba(OH)2 layer increased the efficiency of the inverted device by dominantly reducing the energy barrier for electron extraction from PC61BM, and consequently, reduced charge recombination is observed. The drastic improvement in device efficiency and the simplicity of fabrication by solution processing suggest Ba(OH)2 as a promising and practical route to reduce interface induced recombination losses at the cathode of organic solar cells.