We study the photoinduced degradation of hybrid organometal perovskite photovoltaics under illumination and ambient atmosphere using UV-vis absorption, atomic force microscopy, and device performance. We correlate the structural changes in the surface of the perovskite film with changes in the optical and electronic properties of the devices. The photodecomposition of the methylammonium lead triiodide perovskite layer itself proceeds much more slowly than the photodegradation of the performance of devices with fullerene/bathocuproine/aluminum top contacts, indicating that the active layer is more stable than the interface with the electrodes in this geometry. The evolution of the perovskite active layer performance proceeded through several phases: (1) an initial improvement in device characteristics, (2) a plateau with very slow degradation and finally (3) a catastrophic decline in material performance accompanied by marked changes in film morphology. The rapid increase in surface roughness of the active perovskite semiconductor associated with sudden failure also correlates with decreased absorption at the perovskite band edge and growth of a lead iodide absorption feature. We find that degradation requires both light and moisture, is accelerated at increased humidity, and scales linearly with light intensity, depending primarily on total photon dose.

Work function changes of Au were measured by Kelvin probe force microscopy (KPFM) in the nonpolar liquid decane. As a proof of principle for the measurement in liquids, we investigated the work function change of an Au substrate upon hexadecanethiol chemisorption. To relate the measured contact potential difference (CPD) during the chemisorption of alkanethiols to a change of the work function, the influence of physisorbed decane must be taken into account. It is crucial that either the work function of the scanning probe microscope (SPM) tip or the sample surface remains constant throughout the reaction, since both contribute to the CPD. We describe two routes for determining the work function shift of Au coated with a monolayer of alkanethiols: In the first route, the SPM tips were taken as reference surfaces (constant tip work function). For this approach, we used Au(111) surfaces and kept the SPM tip ex situ during the adsorption process. In the second route, structured surfaces with reactive and inert parts were studied by KPFM (constant reference work function). For this route, we prepared nanometer sized Au structures by nanosphere lithography on SiO(x) substrates. Now, the SiO(x) served as the inert reference surface. The shifts in the work function after exposure to the hexadecanethiol (HDT) solution were determined to be ΔΦ(Au+HDT,decane-Au,air) = -1.33 eV ± 0.07 eV (route I) and ΔΦ(Au+HDT,decane-Au,air) = -1.46 eV ± 0.04 eV (route II). Both values are in excellent agreement with the work function shifts determined by ultraviolet photoemission spectroscopy (UPS) reported in literature. The presented procedures of measuring work function changes in decane open new ways to study local reactions at solid-liquid interfaces.

We apply a combination of photoswitch-modified DNA and AFM-based pulling measurements to study the force-induced melting of double-stranded DNA in the unzipping geometry. We measure the differences in peak rupture force for azobenzene-modified DNA, as the incorporated azobenzenes are photoswitched reversibly between the trans and the cis form. Fitting our rupture force versus loading rate data, we obtain off rate (koff) at zero force values in the range of ∼10 s(-1). We show that the change in peak rupture force and koff induced by destabilizing the DNA duplex depends on the position of the destabilizing azobenzene photoswitch relative to the force-loading site. When the azobenzenes are proximal to the unzipping end, the decrease in peak force and koff upon azobenzene photoisomerization is significantly larger than when the azobenzene is distal to the site of force loading. We interpret these results as experimental evidence supporting the picture that the destabilization of a double-stranded DNA by a photoswitch isomerization is localized to a small bubble around the photoswitch.

Performance improvement of advanced polymer electrolyte membranes requires control over the morphology as it plays an important role for mechanical, thermal, and proton transport properties. The ionic domain orientation of films cast from sulfonated block copolymer solutions is generally anisotropic, and to maximize proton conductivity in fuel cell applications, hydrophilic channel alignment is desirable. In this work, a series of multiblock copolymers based on sulfonated copolyimides were synthesized and characterized by NMR, TEM and AFM microscopy, and impedance spectroscopy. For constant ion exchange capacity, the higher the block length, the higher the proton conductivity and water uptake for a given relative humidity. A random copolymer exhibited the lowest performance, in particular at low relative humidity, caused by a reduced phase separation as derived from AFM and TEM measurements. Orientational order probed by 2 H NMR on absorbed D 2 O showed preferential alignment in the through-plane direction. However, proton pulsed-field-gradient (PFG) NMR along the two orthogonal membrane directions revealed water diffusion to be faster in-plane than in the through-plane direction. This difference in diffusion is attributed to a lamella-like structure composed of rather short, through-plane hydrophilic channels in our systems. For the block copolyimide with the highest block length, two distinct diffusion processes could be identified. This is ascribed to a superimposed morphology on the micrometer scale, leading to an opaque appearance of the membrane.

We investigated the photoinduced changes in the surface potential and conductivity for locally degraded active layers of organic solar cell materials using electrical modes of scanning force microscopy. Samples were degraded under different partial pressures of oxygen and humidity in the presence of light. Degraded and nondegraded areas were investigated by Kelvin Probe Force Microscopy (KPFM) and conductive scanning force microscopy (cSFM). The analysis allowed us to quantify the extent of degradation and compensate the contribution of the probe tip. Two typical blends used for organic solar cell, i.e., P3HT:PCBM and PCPDTBT:PCBM, were investigated. We observed that P3HT:PCBM photodegraded significantly more than PCPDTBT:PCBM for an environment containing oxygen. For short photodegradation times (1 h), we verified that changes in the surface potential and conductivity of P3HT:PCBM films were fully reversible after annealing. For individual layers of P3HT and PCBM, we found that only P3HT degrades. However, the blend material of P3HT and PCBM leads to an accelerated degradation supporting the interpretation that PCBM undergoes a series of oxidations in the blend.