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.
Time-Resolved Electrostatic Force Microscopy (trEFM)
While conventional EFM has been useful in the characterization of a variety of static or quasi-static processes in organic electronic devices, parameters such as surface potential and capacitive gradient fail to provide direct information about the local efficiency of a thin-film solar cell. To address this limitation, we have extended the capability of EFM to enable the study of time-dependent phenomena at sub-ms time scales using time-resolved EFM (trEFM). With trEFM, we can measure the transient behavior in the electrostatic force gradient, for instance, from the rapid accumulation of photogenerated charge in a solar cell following illumination, or the fast trapping and detrapping kinetics of charge carriers on subms time scales.
Figure 1a depicts the operation of a trEFM experiment to measure photogenerated charge. In the dark, the semiconductor slab is, ideally, mostly depleted of charge carriers. The sample is then illuminated with a light pulse; the photoexcitation of the OPV material generates charge carriers. Due to the applied voltage on the tip (in our experiments typically 5-10 V), these photogenerated charge carriers migrate to opposite sides of the active layer. The resulting accumulation of charge changes the capacitance and electrostatic force gradient, in turn causing a resonance frequency shift. By continuously measuring Δf (the shift in resonance frequency) with ~100μs time resolution, we are able to record a charging curve and determine the local charging rate in the material (Figure 1b).
As one example of the capabilities of this technique, we have used trEFM to explore the photoinduced charging behavior in all-polymer OPV blends, in this case poly(9,9'-dioctylfluorene-cobenzothiadiazole) (F8BT) and poly(9,9'- dioctylfluorene-co-bis-N,N'-phenyl-1,3-phenylenediamine) (PFB). We chose PFB:F8BT blends as a model system because of the wide literature discussing the effects of processing and blend morphology on their performance. By comparing the topography (Figure 1c) with the charging rate image (Figure 1d), we can analyze the relationship between charging behavior and the local PFB:F8BT film composition. We have confirmed the utility of trEFM as an analytical technique by showing that the spatially-averaged local charging rate and the measured external quantum efficiency (EQE) are correlated for a wide range of blend ratios (Figure 1e).
This is an exciting result — with only a single calibration factor, a trEFM image of a polymer blend can be used to accurately predict the efficiency of the polymer solar cell that will be fabricated from a particular film. One can imagine using such a method both to screen new materials in the lab, or as a rapid quality control diagnostic in a production facility. Additionally, we note that it is possible to use trEFM to monitor other quantities of interest, such as spatially-correlated charge trapping and detrapping, and work is underway to possibly explore sub-100μs time-dependent charging processes.
Photoconductive Atomic Force Microscopy (pcAFM)
Macroscopic characterization of device parameters such as open circuit voltage, short circuit current, and fill factor provide information about overall device performance; however, on the microscopic level, it can be difficult to explain how these parameters are affected by various processing conditions and blend morphologies without direct measurements that can correlate the local electronic properties of the film with local structural features.
Thus, in addition to trEFM, we have used photoconductive AFM (pcAFM) as a complementary tool for the microscopic characterization of heterogeneous OPV films. A relative of conductive AFM (cAFM), pcAFM records local photocurrents directly in contact mode, essentially by using a metalized AFM probe as the top contact to form a nanoscale solar cell. In pcAFM, we typically use focused laser illumination to photoexcite the sample. The small collection area leads to a small photocurrent and, even for high-quality devices with external quantum efficiencies over 50%, we find it beneficial to use high-intensity illumination to improve signal to noise.
With pcAFM, the photocurrent measured at a given location reflects the local charge generation properties. Using pcAFM, we were able to directly observe the relationship between photocurrent distribution and annealing, namely the increase in both average and peak photocurrent with increased annealing time. For example, in Figure 2a and 2b we show the topography and corresponding short-circuit photocurrent for a P3HT:PCBM film annealed for 10 minutes. As with the MDMO-PPV:PCBM samples, local variations in photocurrent are evident within topographically featureless areas.
As with trEFM, we can assess the quantitative relationship between the pcAFM current information in characterizing OPV efficiency by correlating the spatially-averaged photocurrent in pcAFM data with EQE measurements on the same materials, similar to that shown in Figure 1e. As can be seen in Figure 2c, photocurrent measurements derived via pcAFM follow the same qualitative trend as the efficiencies obtained from the macroscopic devices. This result suggests that pcAFM can probe the microscopic underpinnings of macroscopic device performance. The pcAFM data acquired can then be useful to extract electron and hole current and mobility from OPV devices and could even be used as a tool to select optimal blends and processing conditions.