Organic photovoltaic (OPV) materials are an emerging alternative technology for converting sunlight into electricity. OPVs are potentially inexpensive to process, highly scalable in terms of manufacturing, and compatible with mechanically flexible substrates. In an OPV device, semiconducting polymers or small organic molecules are used to accomplish the functions of collecting solar photons, converting the photons to electrical charges, and transporting the charges to an external circuit as a useable current.
At present, the most intensely-studied and highest-performing OPV systems are those that employ bulk heterojunction (BHJ) blends as the active layer, with NREL-certified power conversion efficiencies improving seemingly monthly and currently standing at 7.4%. Despite the advances of the last few years, the efficiencies of OPVs are still below the level needed for widespread commercial viability.
The path towards improved OPV efficiency appears straightforward and researchers are actively working on goals such as better coverage of the solar spectrum to increase current, and tailored energy levels of the donors and acceptors to gain higher open circuit voltages. However, these otherwise straightforward problems in materials synthesis are complicated by the fact that the texture, or morphology, of the donor acceptor blend which is sensitive to the exact conditions of how the blend was processed into a thin film has a dramatic effect on the performance of OPVs. In a bulk heterojunction blend, the donor and acceptor material are typically mixed in solution, and the mixture is then coated on the substrate to form the active layer. The donor/acceptor pair can consist of two different conjugated polymers, but it is often a conjugated polymer (donor) and a soluble fullerene derivative (acceptor).
The importance of morphology in such materials arises from the competing demands of a number of microscopic processes. First, when light is absorbed in an organic semiconductor, the energy produces a neutral quasiparticle, or exciton, rather than free charge carriers. In most organic solar cells, the exciton is typically dissociated into free charges at the interface between two different organic semiconductors with different electron affinities, hence the widespread use of donor/acceptor blends. However, while the active layer of an organic solar cell needs to be ~100-200 nm thick to absorb most of the incident light, the diffusion length of an exciton is ~10 nm, and thus the donor and acceptor materials must be mixed on this length scale to yield an efficient device. Therefore, local morphological information necessarily impacts bulk device measurements.
Analysis of this nanoscale morphology requires high-resolution spatial mapping of the active layer, particularly using scanning probe techniques such as atomic force microscopy (AFM) and electrical adaptations. Scanning probe microscopy is especially useful because of the ability to image at resolutions approaching the ~10-100 nm scale of the domains observed in common OPV materials. Several groups have, for example, analyzed OPV systems using AFM, conducting AFM, electrostatic force microscopy (EFM), and scanning Kelvin probe microscopy (SKPM). Optical variations such as near-field scanning optical microscopy (NSOM) and tunneling luminescence-based AFM have also been used to probe OPV blend morphology.
In our research we have reviewed the broad use of scanning probe microscopy and other structural probes in the field of organic electronics and identified specific areas of nanoscale physics that are important to OPV operation. Here we take a more practical turn and discuss the experimental challenges and opportunities associated with two different AFM-based optoelectronic scanning probe techniques — photoconductive AFM (pcAFM) and time-resolved EFM v(trEFM) — that have been used to help understand how morphology impacts OPV performance.
trEFM is a non-contact technique that utilizes time-resolved measurements on OPV layers to analyze the local variations in photoinduced charge generation, collection, and discharge, while pcAFM is a contact-mode method that measures the photocurrent directly to correlate the local morphology with local photoresponsivity.