• Energy Research

Inline printing and coating methods have been demonstrated to enable a high technical yield of fully roll-to-roll processed polymer tandem solar cell modules. We demonstrate generality by employing different material sets and also describe how the ink systems must be carefully co-developed in order to reach the ambitious objective of a fully printed and coated 14-layer flexible tandem solar cell stack. The roll-to-roll methodologies involved are flexographic printing, rotary screen printing, slot-die coating, X-ray scattering, electrical testing and UV-lamination. Their combination enables the manufacture of completely functional devices in exceptionally high yields. Critical to the ink and process development is a carefully chosen technology transfer to industry method where first a roll coater is employed enabling contactless stack build up, followed by a small roll-to-roll coater fitted to an X-ray machine enabling in situ studies of wet ink deposition and drying mechanisms, ultimately elucidating how a robust inline processed recombination layer is key to a high technical yield. Finally, the transfer to full roll-to-roll processing is demonstrated

Abstract With the aid of optical modeling, the internal quantum efficiencies of organic Bulk Heterojunction (oBHJ) photovoltaic devices based on low band gap polymer of poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) blended with the acceptors of 1-(3-Methoxycarbonyl) propyl-1-phenyl [6,6] C61 (PCBM) and bis–adduct (bis–PCBM) are determined. The Si-PCPDTBT:bis–PCBM devices show considerably lower short circuit current density (Jsc) as compared to the Si-PCPDTBT:PCBM devices. The results show that 30% of this smaller Jsc is due to the lower optical absorption of bis–PCBM, while the major losses originate from the electrical losses. It is found that for the best Si-PCPDTBT:bis–PCBM devices with an active layer thickness in the range of 70–100 nm, the inefficient charge generation within the bis–PCBM domains is the major contribution to the whole losses. Increasing the active layer thickness of Si-PCPDTBT:bis–PCBM device significantly enhances recombination losses in polymer/bis–fullerene matrix.

Semi‐transparent organic solar cells (ST‐OSCs) show a unique potential to be integrated in windows due to their outstanding characteristics such as high transparency and color‐adjustability. In order to achieve both, high transparency and high efficiency, the use of dielectric mirrors is an excellent concept. However, such a mirror will not only improve the photocurrent generated by a solar cell but also cause losses in transparency. In this work, a theoretical model is developed that predicts the effect of the dielectric mirror on the balance between photocurrent enhancement and transparency loss depending on the spectral shape of the ST‐OSC absorption. Experimental investigations with three fully printed ST‐OSCs showing different absorption characteristics underline the validity of these studies. It is concluded that ST‐OSCs with broad absorption spectra ranging from the short wavelength region over the visible to at least 950 nm are most suitable for the implementation of a dielectric mirror. A narrower absorption spectrum or a shift of the spectrum toward longer wavelengths makes an increase in photoactive layer thickness more beneficial than the attachment of a dielectric mirror. Moreover, the dielectric mirror approach is an excellent strategy to obtain high photocurrents for materials which cannot be processed at high active layer thicknesses.

Abstract Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO 2 and ZnO. However, the device performance is strongly correlated to the electronic properties of the interfacial materials, and here specifically to their work function, surface states as well as conductivity and mobility. It is noteworthy to say that these properties are massively determined by the crystallinity and stoichiometry of the metal oxides. In this study, we investigated aluminum-doped zinc oxide (AZO) as charge selective extraction layer for inverted BHJ solar cells. Thin AZO films were characterized with respect to their structural, optical and electrical properties. The performance of organic solar cells with an AZO electron extraction layer (EEL) is compared to the performance of intrinsic ZnO or TiO x EELs. We determined the transmittance, absorbance, conductivity and optical band gap of all these different metal oxides. Furthermore, we also built the correlations between doping level of AZO and device performance, and between annealing temperature of AZO and device performance.

We discuss an alternative route to determine the solubility parameters of two prototype organic semiconductors, namely the semi-crystalline polymer poly-(3-hexylthiophene-2,5-diyl) (P3HT) and the methano-fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The HSP (Hansen solubility Parameters) derived by this novel method are compared to the findings derived from the classical multi-solvent method to determine the HSP, and significantly higher accuracy is found. For this novel approach we designed two component solvent blend systems, being composed by mixing a solvent with a non-solvent. Varying the composition of the solvent – non-solvent blends from 0% to 100% gradually converts a solvent into a non-solvent. This very accurate control of the dispersive, polar and hydrogen contributions to the overall solubility now allows determining the Hansen sphere for P3HT and PCBM with much higher accuracy. The transition from a solvent into a non-solvent was further followed by solar cell investigations. Comparing the solubility studies with device investigations allows identifying the processing limits of solvent systems.

The increasing demand for higher quality in solar cell production led to the development of several inline control methods. Beneath the image-guided methods, X-ray is not yet very well investigated for the application in photovoltaic research but shows high potential. In contrast to the ordinary X-ray radioscopic method, the tomosynthesis technique exhibits additional depth information of the solar cells and modules. In this article, several applications of tomosynthesis for the investigation of solar cells and modules are studied. It will be shown what potential the application of X-ray and especially tomosynthesis has as quality control tool for photovoltaics. Copyright © 2013 John Wiley & Sons, Ltd.

Flexible organic tandem solar modules with high geometric fill factors were constructed by utilizing a fully roll-to-roll compatible processing.