Effect of Aging and PCBM Content on Bulk Heterojunction Organic Solar Cells Studied by Intensity Modulated Photocurrent Spectroscopy

A series of encapsulated and nonencapsulated bulk heterojunction photovoltaic devices containing poly­(3-hexyl­thiophene) (P3HT) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) with different P3HT:PCBM ratios were investigated using traditional steady-state as well as non-steady-state intensity modulated photocurrent spectroscopy (IMPS) techniques. The steady state <i>J</i>–<i>V</i> measurements showed that PCBM content did not have a significant effect on the efficiency for freshly prepared devices, whereas aged nonencapsulated devices exhibited a strong dependence on PCBM content. IMPS measurements showed a significant contribution of interfacial nongeminate recombination in nonencapsulated devices, which increased with decreasing PCBM content in the photoactive layer and cell aging. It was related to the formation of interfacial states at the P3HT/PCBM interface due to atmospheric contamination, which act as recombination centers. Device encapsulation was found to be effective in preventing the occurrence of interfacial recombination. Our results suggest that IMPS can be used as a diagnostic tool to predict the performance of bulk heterojunction organic solar cells. If a solar cell shows the presence of interfacial states as indicated by semicircle arcs in quadrant I of the IMPS complex plane plots, it is most likely that its performance will deteriorate with time due to enhanced interfacial recombination, even without further exposure to atmospheric contaminations. We conclude that interfacial nongeminate recombination is an important degradation mechanism in organic solar cells, especially in the case of exposure to atmospheric contaminants

Bias Modulated Scanning Ion Conductance Microscopy

Nanopipets are versatile tools for nanoscience, particularly when used in scanning ion conductance microscopy (SICM) to determine, in a noncontact manner, the topography of a sample. We present a new method, applying an oscillating bias between a quasi-reference counter electrode (QRCE) in the SICM nanopipet probe and a second QRCE in the bulk solution, to generate a feedback signal to control the distance between the end of a nanopipet and a surface. Both the amplitude and phase of the oscillating ion current, induced by the oscillating bias and extracted using a phase-sensitive detector, are shown to be sensitive to the probe–surface distance and are used to provide stable feedback signals. The phase signal is particularly sensitive at high frequencies of the oscillating bias (up to 30 kHz herein). This development eliminates the need to physically oscillate the probe to generate an oscillating ion current feedback signal, as needed for conventional SICM modes. Moreover, bias modulation allows a feedback signal to be generated without any net ion current flow, ensuring that any polarization of the quasi reference counter electrodes, electro-osmotic effects, and perturbations of the supporting electrolyte composition are minimized. Both feedback signals, magnitude and phase, are analyzed through approach curve measurements to different surfaces at a range of distinct frequencies and via impedance measurements at different distances from a surface. The bias modulated response is readily understood via a simple equivalent circuit model. Bias modulated (BM)-SICM is compared to conventional SICM imaging through measurements of substrates with distinct topographical features and yields equivalent results. Finally, BM-SICM with both amplitude and phase feedback is used for topographical imaging of subtle etch features in a calcite crystal surface. The 2 modes yield similar results, but phase-detection opens up the prospect of faster imaging