Alexander Hinderhofer

Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20

Transporter layers improve stability Although perovskite solar cells can have power conversion efficiencies exceeding 20%, they can have limited thermal and ultraviolet irradiation stability. This is in part because of the materials used to extract the charge carriers (electrons and holes) from the active layer. Arora et al. replaced organic hole transporter layers with CuCSN to improve thermal stability. Device lifetime was enhanced when a conducting reduced graphene oxide spacer was added between the CuSCN layer and the gold electrode. Science, this issue p. 768 Thin CuSCN films can replace organic hole-transporting layers that limit thermal stability of devices. Perovskite solar cells (PSCs) with efficiencies greater than 20% have been realized only with expensive organic hole-transporting materials. We demonstrate PSCs that achieve stabilized efficiencies exceeding 20% with copper(I) thiocyanate (CuSCN) as the hole extraction layer. A fast solvent removal method enabled the creation of compact, highly conformal CuSCN layers that facilitate rapid carrier extraction and collection. The PSCs showed high thermal stability under long-term heating, although their operational stability was poor. This instability originated from potential-induced degradation of the CuSCN/Au contact. The addition of a conductive reduced graphene oxide spacer layer between CuSCN and gold allowed PSCs to retain >95% of their initial efficiency after aging at a maximum power point for 1000 hours under full solar intensity at 60°C. Under both continuous full-sun illumination and thermal stress, CuSCN-based devices surpassed the stability of spiro-OMeTAD–based PSCs.

Optical properties of pentacene and perfluoropentacene thin films.

The optical properties of pentacene (PEN) and perfluoropentacene (PFP) thin films on various SiO(2) substrates were studied using variable angle spectroscopic ellipsometry. Structural characterization was performed using x-ray reflectivity and atomic force microscopy. A uniaxial model with the optic axis normal to the sample surface was used to analyze the ellipsometry data. A strong optical anisotropy was observed, and enabled the direction of the transition dipole of the absorption bands to be determined. Furthermore, comparison of the optical constants of PEN and PFP thin films with the absorption spectra of the monomers in solution shows significant changes due to the crystalline environment. Relative to the monomer spectrum, the highest occupied molecular orbital to lowest unoccupied molecular orbital transition observed in PEN (PFP) thin film is reduced by 210 meV (280 meV). A second absorption band in the PFP thin film shows a slight blueshift (40 meV) compared to the spectrum of the monomer with its transition dipole perpendicular to that of the first absorption band.

Organic–Organic Heterostructures: Concepts and Applications

We review basic concepts as well as recent examples and applications of organic-organic heterostructures. We organize the different types of heterostructures according to material A deposited on material B (A/B), A co-deposited with B (A:B), heterostructures in the monolayer regime including nanostructuring concepts and systems involving self-assembled monolayers, as well as various other architectures, including superlattices. While most examples are related to small-molecule organic semiconductors, many of the ideas can be applied to other systems. The central theme is growth and structure as well as optical and electronic properties. Finally, we comment on implications for device applications.

High-mobility copper-phthalocyanine field-effect transistors with tetratetracontane passivation layer and organic metal contacts

We investigate ambipolar charge transport in organic field-effect transistors (OFETs) with copper-phthalocyanine (CuPc) as active material. It is shown that charge carrier mobilities can be increased by at least one order of magnitude using the long-chain alkane tetratetracontane (TTC) as a passivation layer on top of silicon dioxide. TTC and CuPc films are characterized by atomic force microscopy and x-ray diffraction. TTC forms a highly crystalline layer that passivates electron traps on the SiO2 surface very efficiently and serves as a template for the growth of CuPc films with significantly improved crystallinity. High electron mobilities comparable to the values reported on single crystals are reached. We show that the contact resistance for hole transport as determined by the transmission line method can be reduced considerably by using organic charge-transfer complexes as top contacts in OFETs based on CuPc.

Charge Separation at Molecular Donor–Acceptor Interfaces: Correlation Between Morphology and Solar Cell Performance

Blends of organic electron and hole conductive materials are widely used for ambipolar charge-carrier transport and donor/acceptor (DA) photovoltaic cells. Thereby, the efficiency of these excitonic solar cells is correlated to the morphology of the interface between the donor and the acceptor materials, which in turns depends on the preparation conditions, the crystallization of the particular materials, and the interaction between the donor and acceptor molecules. In this contribution, the influence of the morphology on the solar cell architecture and performance will be discussed using different molecular DA combinations.

Structure and morphology of coevaporated pentacene-perfluoropentacene thin films.

The structural properties of coevaporated thin films of pentacene (PEN) and perfluoropentacene (PFP) on SiO(2) were studied using x-ray reflectivity and grazing incidence x-ray diffraction. Reciprocal space maps of the coevaporated thin films with different volume fractions reveal the coexistence of two different molecular mixed PEN-PFP phases together with the pure PEN and PFP crystallites. The crystal structure of PEN:PFP blends does not change continuously with volume fraction, instead the proportion of the appropriate phases changes, as seen from the diffraction analysis. Additional temperature dependent experiments reveal that the fraction of the two mixed PEN-PFP phases varies with growth temperature. The λ-phase (molecular plane parallel to the substrate) is metastable and induced by low growth temperature. The σ-phase (molecular plane nearly perpendicular to the substrate) is thermally stable and nucleates predominantly at high growth temperatures.

Optical evidence for intermolecular coupling in mixed films of pentacene and perfluoropentacene

We present optical absorption spectra of mixed films of pentacene (PEN) and perfluoropentacene (PFP) grown on SiO2. We investigated the influence of intermolecular coupling between PEN and PFP on the optical spectra by analyzing samples with five different mixing ratios of PFP:PEN with variable angle spectroscopic ellipsometry (VASE) and differential reflectance spectroscopy (DRS). The data show how the spectral shape is influenced by changes in the volume ratio of the two components. By comparison with the pure film spectra an attempt is made to distinguish transitions due to intermolecular coupling between PEN and PFP from transitions caused by interactions of PEN (PFP) with other molecules of the same type. We observe a new transition at 1.6 eV which is not found in the pure film spectra and which we assign to coupling of PFP and PEN.

Smoothing and coherent structure formation in organic-organic heterostructure growth

We use in situ real-time X-ray reflectivity and complementary atomic force microscopy to monitor crystallinity and roughness evolution during growth of organic heterostructures, i.e. perfluoropentacene (PFP) on diindenoperylene (DIP) and pentacene (PEN) on PFP. For both systems, surface smoothing during thermal evaporation of the second material on top of the first is observed. The smoothing can be rationalized by a, compared to homoepitaxy, lowered step edge barrier for one species diffusing on the other. In addition, we find an exceptionally well-ordered interface for PEN-on-PFP growth and PEN growth with anomalously low roughening, along with coherent scattering over the entire thickness.

Simultaneous in situ measurements of x-ray reflectivity and optical spectroscopy during organic semiconductor thin film growth

Simultaneous in situ real-time measurements of x-ray reflectivity and differential reflectance spectroscopy were conducted during deposition of perfluorinated copper-phthalocyanine thin films on SiO2/Si. We found a continuous spectral change coinciding with structural changes from submonolayer coverage, to standing βbilayer-phase and to β-phase for thicker films. This combined measurement enables us to study the relationship between structural and optical properties of organic semiconductor thin films.

Quantitatively identical orientation-dependent ionization energy and electron affinity of diindenoperylene

Molecular orientation dependences of the ionization energy (IE) and the electron affinity (EA) of diindenoperylene (DIP) films were studied by using ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The molecular orientation was controlled by preparing the DIP films on graphite and SiO2 substrates. The threshold IE and EA of DIP thin films were determined to be 5.81 and 3.53 eV for the film of flat-lying DIP orientation, respectively, and 5.38 and 3.13 eV for the film of standing DIP orientation, respectively. The result indicates that the IE and EA for the flat-lying film are larger by 0.4 eV and the frontier orbital states shift away from the vacuum level compared to the standing film. This rigid energy shift is ascribed to a surface-electrostatic potential produced by the intramolecular polar bond (>C−-H+) for standing orientation and π-electron tailing to vacuum for flat-lying orientation.