Markus Scharber

Flexible high power-per-weight perovskite solar cells with chromium oxide–metal contacts for improved stability in air

Photovoltaic technology requires light-absorbing materials that are highly efficient, lightweight, low cost and stable during operation. Organolead halide perovskites constitute a highly promising class of materials, but suffer limited stability under ambient conditions without heavy and costly encapsulation. Here, we report ultrathin (3 μm), highly flexible perovskite solar cells with stabilized 12% efficiency and a power-per-weight as high as 23 W g(-1). To facilitate air-stable operation, we introduce a chromium oxide-chromium interlayer that effectively protects the metal top contacts from reactions with the perovskite. The use of a transparent polymer electrode treated with dimethylsulphoxide as the bottom layer allows the deposition-from solution at low temperature-of pinhole-free perovskite films at high yield on arbitrary substrates, including thin plastic foils. These ultra-lightweight solar cells are successfully used to power aviation models. Potential future applications include unmanned aerial vehicles-from airplanes to quadcopters and weather balloons-for environmental and industrial monitoring, rescue and emergency response, and tactical security applications.

Influence of the bridging atom on the performance of a low-bandgap bulk heterojunction solar cell.

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US

Stabilization of the nanomorphology of polymer–fullerene “bulk heterojunction” blends using a novel polymerizable fullerene derivative

within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5‐6% have been certified at NREL and AIST usually on devices with small active areas. [2] The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10‐12%. [3] Several reasons for the power conversion efficiency limitation have been identified. [1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor‐acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4‐6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells. In this work we demonstrate the effect of replacing a carbon atom with a silicon atom on the main chain of the conjugated polymer. The approach has been used previously, and promising materials for field-effect transistors and organic solar cells have been demonstrated. [7‐9] We find that making this simple substitution in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b 0 ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) yields a polysilole, e.g., poly[(4,4 0 -bis(2-ethylhexyl)dithieno[3,2b:2 0 ,3 0 -d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5 0 -diyl] (Si-PCPDTBT), with a higher crystallinity, improved charge transport properties, reduced bimolecular recombination, and a reduced formation of charge transfer complexes when blended with a fullerene derivative. This silole-based polymer is found to form a highly functional nanomorphology when blended with [6,6]-phenyl C71-butyric acid methyl ester (C70-PCBM), and solar cells prepared using this blend gave efficiencies of 5.2%, certified by the National Renewable Energy Laboratory. [1] The presented polymer is the first low-bandgap semiconducting polymer to have a certified efficiency of over 5%. The chemical structure of the subject polymer is shown in Figure 1. The material was synthesized following the procedure described previously. [10] The synthesis and properties of the carbon-bridged polymer have been described before. [11,12] Figure 2a shows the absorbance and photoluminescence (PL) spectra of a thin solid film of the pristine Si-bridged polymer and

Angle dependence of external and internal quantum efficiencies in bulk-heterojunction organic solar cells

The morphological stabilization of donor–acceptor blends for bulk heterojunction solar cells can be achieved by cross-linking of the small molecular phase in the polymer matrix using a polymerizable fullerene derivative. In a comparative study the morphology of polymer–fullerene blend films was investigated using poly(3-hexylthiophene) (P3HT) as the polymer and C61-butyric acid methyl ester (PCBM) or the newly synthesized polymerizable fullerene derivative, C61-butyric acid glycidol ester, PCBG, as the acceptor molecule, respectively. Changes in the nanomorphology due to heat treatment of the films were studied by means of atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL) studies. The polymerization process was monitored with infrared absorption studies. As demonstrated by these comparative studies this newly synthesized fullerene gives considerable stabilization of the solid state morphology in these blends. Such prevention of the long term, high temperature instability of bulk heterojunction morphology displays an important route to increase the operational stability of plastic solar cells in future applications.

Realization, characterization, and optical modeling of inverted bulk-heterojunction organic solar cells

The realization of highly efficient organic solar cells requires the understanding and the optimization of the light path in the photoactive layer. We present in this article our approach to measure and model the optical properties of our bulk-heterojunction devices, and to control them in order to enhance the photovoltaic performances. We report our recent observations on the dependence of the external quantum efficiency (EQE) on the incidence angle of the light, and our results on the determination of internal quantum efficiency based on EQE measurement and optical modeling cross-checked by reflection measurements. We investigate poly(3-hexylthiophene): 1-(3-methoxy-carbonyl) propyl-1-phenyl[6,6]C61 based solar cells with two different thicknesses of the active layer (170 and 880nm), and show that in the thin ones the absorption is enhanced for oblique incident radiation.

On the Efficiency Limit of Conjugated Polymer:Fullerene‐Based Bulk Heterojunction Solar Cells

Inverted bulk-heterojunction organic solar cells (OSCs) using solution-processed layers possess significant advantages compared to the usual noninverted devices. To investigate the full potential of this type of OSC, we have carried out some optical modeling by rigorous coupled wave analysis. The influence of the thickness of several different layers in the device has been quantified, as well as the maximum possible number of photons absorbed in the poly(3-hexyltiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer for both conventional and inverted structures. It appears that the thickness of the hole injecting layer placed in front of the metallic mirror can influence the electromagnetic field distribution in the OSC, but no additional beneficial optical spacer effect is observed. The thickness of the electron injecting layer deposited on the semitransparent electrode also has a negligible influence on the photons absorbed in the active layer for the inverted structure.

Alternating quinoxaline/oligothiophene copolymers—synthesis and unexpected absorption properties

The power conversion efficiency potential of eight high-performance polymer-fullerene blends is investigated. All studied absorbers show the typical organic solar cell losses limiting their performance to ≈13%.

Double injection as a technique to study charge carrier transport and recombination in bulk-heterojunction solar cells

The optical and electronic properties of novel, alternating quinoxaline/oligothiophene donor–acceptor copolymers show an unexpected independence of absorption and photoluminescence, as well as of the HOMO/LUMO energy levels, on the length of the oligothiophene segments in the copolymer main chain.

Double-injection current transients as a way of measuring transport in insulating organic films

Ambipolar charge carrier mobility and recombination in bulk-heterojunction solar cells based on the mixture of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) has been studied using injection current transients. The experimental results demonstrate double injection with bimolecular recombination limiting the injection current. We found that charge carrier bimolecular recombination is significantly reduced compared to Langevin recombination. We have measured the temperature and electric field dependence of the reduced bimolecular recombination coefficient and the results suggest that the electron and hole pathways are different and the recombination is controlled by the probability of the carriers to meet at the polymer/PCBM interface.

Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes

We propose a double-injection current transient technique for the study of charge-carrier transport in thin insulating films of low-mobility materials with reduced carrier bimolecular recombination compared to the Langevin type. This experimentally simple technique, allows us to estimate the sum of the faster carrier and the slower carrier mobility’s (μf+μs) and the slower carrier mobility (μs). Furthermore, in thin films when the RC current overlaps the injection current transients we propose to estimate these transport parameters using the extracted charge as a function of injection pulse duration. The method is applied on bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene. We have experimentally verified the technique by measuring the charge carrier mobility’s and compared them with results obtained using standard time-of-flight and carrier extraction using linearly increasing voltage techniques.