Organic Photovoltaics
In 2007, IMEC has continued its research of solar cells composed of organic semiconducting materials RP130, RP109, RP113, RP161. The two main classes of organic materials that IMEC researches actively are solution processable polymers, as well as small-molecular weight materials. Currently, there is a large research effort in this field which has produced ever-increasing power conversion efficiencies exceeding 5% P14996. Along with this rapid increase in device efficiency is the number of refereed journal publications related to organic photovoltaic materials and devices, which has increased exponentially for approximately the past three decades, as shown in figure 1.
Figure 1: Timeline of the power conversion efficiency of small molecule-based organic photovoltaic cells (filled circles) and polymer-based cells (open triangles), as well as the number of journal publications related to organic-based photovoltaics (open squares).
Polymer solar cells
The class of organic solar cells based on polymeric materials can be processed from solution owing to their solubility in various organic solvents. The most efficient polymer-based devices fabricated to date have been based upon the bulk heterojunction approach, whereby the two active components of the solar cell are blended together in solution and processed simultaneously. By optimizing the deposition procedure, one can achieve a fine level of control over the nanostructure of the blend, and we can probe this structure through various techniques including but not limited to absorption, x-ray diffraction, and atomic force microscopy.
Among the methods for depositing this bulk heterojunction layer, spin coating is the most common, but is limited to small areas. To realize large-area coverage for practical solar cells, alternative deposition techniques have been utilized, such as ink-jet, screen or gravure printing. Recently, we have realized a new method, spray coating, and show that this method is a valid alternative to other techniques C15855. This high-rate, large-area deposition technique ensures an ideal coating on a variety of surfaces with different morphologies and topographies. Moreover, the fluid waste is reduced to minimal quantities compared to spin coating. To justify the usefulness of this technique, we compared a standard spin coated solar cell based on a mixture of poly(3-hexyl thiophene) (P3HT) and the C60-derivative (6,6)-phenyl C61-butyric acid methyl ester (PCBM) with a spray coated one, where the P3HT:PCBM blend was sprayed by a N2-powered airbrush, as shown in figure 2. Spray-coated solar cells were found to have power conversion efficiencies above 2%, a performance which is comparable to that of the spin coated devices.
Figure 2: (a) Schematic of a spray-coated P3HT:PCBM blend on a PEDOT:PSS coated indium-tin-oxide glass substrate. The cathode is formed by a LiF/Al layer. (b) Comparison of the current density (j) vs. voltage (V) characteristics of solar cells with a P3HT:PCBM blend deposited by either spin coating (red curves) or spray coating (black curves) in the dark (dashed curves) and under 1 sun AM1.5G illumination (solid curves).
Small-molecular weight devices
Thin films of small molecules are deposited by either vacuum thermal evaporation or vapor phase deposition C15147. Photovoltaic device structures have been developed for which a novel donor material, subphthalocyanine (SubPc) was applied P14945. The extraordinary feature of SubPc over other metallophthalocyanines is its high extinction coefficient as well as its high refractive index as determined from ellipsometry (see figure 3). A high refractive index (high screening) may be advantageous to reduce the binding energy of the electron-hole pair at the donor-acceptor interface and as a result generate free charges more efficiently than other donor-acceptor pairs.
Figure 3: Complex refractive index ñ = n - ik for SubPc.
The device structure, shown in figure 4, consists of planar stack of ultrathin layers of donor (SubPc; 13nm) and acceptor (buckminsterfullerene; 32.5nm) material in combination with an additional buffer layer (bathocuproine; 10nm). Indium-tin-oxide and Al are applied as anode and cathode, respectively. The efficiencies on this stack obtained under 1 sun AM1.5G simulated solar radiation yield 3.0% and are among the highest reported for this combination of materials (see figure 4).
Figure 4: Schematic of the SubPc device structure along with layer thicknesses and direction of incident light. Also given is a table of device performance parameters under various incident light intensities.