Erik Ahlswede

Comparative study of the influence of LiF, NaF, and KF on the performance of polymer bulk heterojunction solar cells

Based on the well-known beneficial effect of a thin LiF layer underneath Al contacts for organic solar cells, a comparative study of interlayers made from the alkaline fluorides LiF, NaF, and KF is presented for polymer bulk heterojunction solar cells. The overall suitability of these materials and the underlying mechanisms are discussed. While an improvement in cell efficiency up to a factor of 2 can be reached with all three fluorides, the necessary thickness of the interlayer for maximum improvement is smallest for NaF and largest for LiF, suggesting the alternative use of NaF instead of LiF.

Highly efficient organic solar cells with printable low-cost transparent contacts

Highly efficient organic solar cells with all-solution-processed low-cost transparent front contacts based on highly conductive formulations of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) are reported, demonstrating the feasibility of replacing the costly conventional vacuum-deposited indium-tin-oxide contacts by means of simple printing techniques. For small cell areas, the relatively high sheet resistances were found to be less detrimental to the photovoltaic performance than the deficiencies in the transparency of the contact layer.

Dithienopyrrole-based oligothiophenes for solution-processed organic solar cells

Isomeric dicyanovinylene-terminated oligothiophenes 1 and 2 comprising a central dithienopyrrole (DTP) unit have been developed for solution-processed small molecule organic solar cells (SMOSCs) giving the highest power conversion efficiency of 4.8% for DTP-based oligomeric materials.

Influence of cathode sputter deposition on organic solar cells

The sputter deposition technique is usually avoided for organic solar cells, despite its technological advantages, because it may potentially damage the organic absorber layer. The consequences of sputtering on device performance-presumably related to changes in the absorber morphology and defect distribution near the interface-are discussed. Despite its negative influence on as-deposited samples, sputtering in combination with thermal annealing can actually be beneficial for pure aluminum contacts, leading to efficiencies even better than those of cells with thermally evaporated cathodes, mainly due to an enhanced open-circuit voltage.

ZnO:Al cathode for highly efficient, semitransparent 4% organic solar cells utilizing TiOx and aluminum interlayers

We report on the fabrication of highly efficient, semitransparent bulk heterojunction solar cells comprising poly[[9-(1-octylnonyl)-9h-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) blended with [6,6]-phenyl-C71-butyric acid methyl (PC71BM) esters as active layer. As semitransparent cathode sputtered aluminum doped ZnO was used in combination with a sputter damage preventing, thin (8 nm) TiOx layer processed from solution and a sputtered aluminum interlayer (ALI). The short circuit current improves for thicker ALIs due to increased reflectance at the cathode leading to average efficiencies of 4.0% for semitransparent solar cells. Comparable results (3.9%) were achieved for devices comprising thick absorbers and thin ALIs.

Inverted organic solar cells comprising a solution-processed cesium fluoride interlayer

We investigate the influence of solution-processed cesium fluoride (CsF) interlayers on the performance of inverted polymer solar cells comprising a blend of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61-butyric acid methyl ester. The thickness of the CsF layer is optimized in terms of current-voltage characteristics by a variation of the solid content in solution. Capacitance-voltage characteristics reveal a shift of the built-in voltage at the cathode interface by about 0.3 V as compared to devices without a CsF layer, giving rise to an increase in open-circuit voltage by the same value. The vertical distribution of Cs+ and F+ ions is studied by secondary ion mass spectroscopy, indicating a strong diffusion of the alkaline fluoride into the organic layer stack.

CIGS Cells and Modules With High Efficiency on Glass and Flexible Substrates

Thin-film solar cells based on Cu(In,Ga)(Se,S) 2 (CIGS) have demonstrated both high efficiencies and a high cost-reduction potential in industrial production. This way, future CIGS module production lines can be profitable even for scales below the GW range. Among the different technologies, only the coevaporation method has demonstrated efficiencies above 20%, approaching the record values of polycrystalline Si cells. The main focus of this contribution is on the new results of the ZSW cell line with efficiencies above 20%, as well as on the mini-module line on glass substrates. Mini modules (10 cm × 10 cm) with efficiencies in the range of 17% give a proof of concept for industrial-sized modules. ZSW is also developing flexible cells and modules, transferring the processes from the glass-based technology. We achieved 18.6% cell efficiency on metal substrates and a 15.4% efficient mini module could be demonstrated with adapted methods of module patterning. In order to develop industrially relevant processes for foils, we are running a roll-to-roll deposition plant. Additionally, we have improved CIGS cell efficiencies with alternative buffers to certified 19.0% for solution-grown Zn(O,S), to 16.4% for sputtered Zn(O,S), and 17.1% for evaporated In 2S 3. Our cells deposited by vacuum-free methods exhibit an efficiency of 8.5% with a nanoparticle-based process.

Reversible order-disorder related band gap changes in Cu2ZnSn(S,Se)4 via post-annealing of solar cells measured by electroreflectance

We report on order–disorder related band gap changes in Cu2ZnSn(S,Se)4 solar cells which are induced by post-annealing. The band gap changes of the absorber are detected utilizing electroreflectance and analyzed by comparison with predictions of the stochastic Vineyard model. This yields a critical temperature of TC=195 °C above which the Cu2ZnSn(S,Se)4 absorber layer is entirely disordered within the Cu–Zn layers of the kesterite unit cell. The temporal evolution of the band gap during annealing shows that the equilibrium value is reached on a timescale in the order of hours, depending on the annealing temperature. In contrast to other experimental techniques, electroreflectance precisely measures the band gap and is not influenced by defect-mediated radiative recombination.

Understanding the effect of solvent vapor annealing on solution-processed A–D–A oligothiophene bulk-heterojunction solar cells: the role of alkyl side chains

Solution-processed bulk heterojunction solar cells consisting of the previously developed dithienopyrrole containing A–D–A oligothiophenes (A = acceptor, D = donor unit) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) with power conversion efficiency up to 7.1% after solvent vapor annealing (SVA) are demonstrated. The influence of the position of the alkyl side chains attached to the thiophene units on the SVA, and the usage of either PC61BM or PC71BM as acceptor, is investigated in more detail by negative secondary ion mass spectrometry (SIMS), Kelvin probe force microscopy (KPFM), photoluminescence (PL), and grazing-incidence X-ray diffraction spectroscopy (GIWAXS). It was found that besides increased crystallinity and domain sizes, the active layers consisting of two different isomers which we will refer to in the following as isomer 1 or isomer 2 had different compositions after SVA treatment. In the former, a more or less homogeneously-mixed D:A blend was observed, whereas the latter showed a vertical gradient of PCBM in the active layer and much stronger phase segregation on the surface. These findings correlate well with the differences in solar cell performance of both isomers, before and after SVA.

Solution-Based Preparation of Cu 2 ZnSn(S,Se) 4 for Solar Cells—Comparison of SnSe 2 and Elemental Se as Chalcogen Source

Thin-film solar cells with a Cu2ZnSn(S,Se)4 absorber were prepared in a two-step process based on a metal salt solution that is deposited onto a molybdenum-coated substrate and subsequently annealed in a selenium-containing nitrogen atmosphere. The solar cell performance could be improved to efficiencies above 10% without usage of the highly toxic hydrazine or H2Se gas. A crucial step for obtaining high-quality absorbers is the incorporation of Se during the annealing process. Therefore, a detailed investigation of elemental Se and SnSe2 as chalcogen sources with various process parameters was performed. SnSe2 resulted in absorbers with improved morphology but could achieve only medium efficiencies, whereas elemental Se forms a triple-layer structure with efficiencies of up to 10.3%.