Photoconductive atomic force microscopy

Photoconductive atomic force microscopy (PC-AFM) is a variant of atomic force microscopy that measures photoconductivity in addition to surface forces. Photoconductive atomic force microscopy (PC-AFM) is a variant of atomic force microscopy that measures photoconductivity in addition to surface forces. Multi-layer photovoltaic cells have gained popularity since mid 1980s. At the time, research was primarily focused on single-layer photovoltaic (PV) devices between two electrodes, in which PV properties rely heavily on the nature of the electrodes. In addition, single layer PV devices notoriously have a poor fill factor. This property is largely attributed to resistance that is characteristic of the organic layer. The fundamentals of pc-AFM are modifications to traditional AFM and focus on the use of pc-AFM in PV characterization. In pc-AFM the major modifications include: a second illumination laser, an inverted microscope and a neutral density filter. These components assist in the precise alignment of the illumination laser and the AFM tip within the sample. Such modifications must complement the existing principals and instrumental modules of pc-AFM so as to minimize the effect of mechanical noise and other interferences on the cantilever and sample. The original exploration of the PV effect can be accredited to research published by Henri Becquerel in 1839. Becquerel noticed the generation of a photocurrent after illumination when he submerged platinum electrodes within an aqueous solution of either silver chloride or silver bromide. In the early 20th century, Pochettino and Volmer studied the first organic compound, anthracene, in which photoconductivity was observed. Anthracene was heavily studied due to its known crystal structure and its commercial availability in high-purity single anthracene crystals. The studies of photoconductive properties of organic dyes such as methylene blue were initiated only in the early 1960s owing to the discovery of the PV effect in these dyes. In further studies, it was determined that important biological molecules such as chlorophylls, carotenes, other porphyrins as well as structurally similar phthalocyanines also exhibited the PV effect. Although many different blends have been researched, the market is dominated by inorganic solar cells which are slightly more expensive than organic based solar cells. The commonly used inorganic based solar cells include crystalline, polycrystalline, and amorphous substrates such as silicon, gallium selenide, gallium arsenide, copper indium gallium selenide and cadmium telluride. With the high demand of cheap, clean energy sources persistently increasing, organic photovoltaic (OPV) devices (organic solar cells), have been studied extensively to help in reducing the dependence on fossil fuel and containing the emission of green house gases (especially CO2, NOx, and SOx). This global demand for solar energy increased 54% in 2010, while the United States alone has installed more than 2.3 GW of solar energy sources in 2010. Some of the attributes which make OPVs such a promising candidate to solve this problem include their low-cost of production, throughput, ruggedness, and their chemically tunable electric properties along with significant reduction in the production of greenhouse gases. For decades, the researchers have believed that the maximum power conversion efficiency (PCE) would most likely remain below 0.1%. Only in 1979 Tang reported a two-layer, thin-film PV device, which ultimately yielded a power conversion efficiency of 1%. Tang’s research was published in 1986, which allowed others to decipher many of the problems which limited the basic understanding of the process involved in the OPVs. In later years, the majority of the research focused on the composite blend of poly(3-hexylthiopehene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). This, along with the research performed on fullerenes, dictated the majority of studies pertaining to OPV for many years. In more recent research, polymer-based bulk heterojunction solar cells, along with low band-gap donor-acceptor copolymers have been created for PCBM-based OPV devices. These low band-gap donor-acceptor copolymers are able to absorb a higher percentage of the solar spectrum as compared to other high efficiency polymers. These copolymers have been widely researched due to their ability to be tuned for specific optical and electrical properties.To date, the best OPV devices have a maximum power conversion efficiency of approximately 8.13%. This low power conversion efficiency is directly related to discrepancies in the film morphology on the nano-scale level. Explanations of film morphology include recombination and/or trapping of charges, low open circuit voltages, heterogeneous interfaces, grain boundaries, and phase-separated domains. Many of these problems arise from the deficient knowledge of electro-optical properties on the nano-scale level. In numerous studies, it has been observed that heterogeneities in the electrical and optical properties influence device performance. These heterogeneities which occur in OPVs are a result the manufacturing process, such as annealing time, which is explained below. Research has mainly consisted of discovering exactly how this film morphology affects the device performance. Until recently, microscopy methods used in the characterization of these OPVs consisted of atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). These methods are very useful in the identification of the local morphology on the film surface, but lack the ability to provide fundamental information regarding local photocurrent generation and ultimately on the device performance. To obtain information which links the electrical and optical properties, the use of electrical scanning probe microscopy (SPM) is an active area of research. Electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) have been utilized in the studies of electron injection and charge trapping effects, while scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM) have been used to investigate electron transport properties within these organic semiconductors. Conductive AFM has been widely used in characterizing the local electric properties in both photovoltaic fullerene blends and organic films, but no reports have shown the use of c-AFM to display the distribution of photocurrents in organic thin films. The most recent variation of SPM devices include (tr-EFM) and photoconductive AFM (pc-AFM) . Both these techniques are capable of obtaining information regarding photo-induced charging rates with nano-scale resolution. The advantage of pc-AFM over tr-ERM is present in the maximum obtainable resolution by each method. pc-AFM can map photocurrent distributions with approximately 20 nm resolution, whereas tr-EFM was only able to obtain between 50–100 nm resolution at this time. Another important factor to note is although the tr-EFM is capable of characterizing thin films within organic solar cells, it is unable to provide the needed information regarding the capacitance gradient nor the surface potential of the thin film. The origin of PC-AFM is due to the work performed by Gerd Binnig and Heinrich Rohrer on STM for which they were awarded the Nobel Prize in physics in 1986. They fabricated an instrument called scanning tunneling microscope (STM) and demonstrated that STM provides surface topography on the atomic scale. This microscopy technique yielded resolutions which were nearly equal to scanning electron microscopy (SEM). The fundamental principles of photoconductive atomic force microscopy (pc-AFM) are based on those of traditional atomic force microscopy (AFM) in that an ultrafine metallic tip scans the surface of a material to quantify topological features. The working premises for all types of AFM techniques are largely dependent on the fundamentals of the AFM cantilever, metallic tip, scanning piezo-tube and the feedback loop that transfers information from lasers that guide the motion of the probe across the surface of a sample. The ultra-fine dimensions of the tip and the way the tip scans the surface produces lateral resolutions of 500 nm or less. In AFM, the cantilever and tip functions as a mass on a spring. When a force acts on the spring (cantilever), the spring response is directly related to the magnitude of the force. k is defined as the force constant of the cantilever. Hooke's law for cantilever motion: f = − k d {displaystyle f=-kd}