Printing highly efficient organic solar cells

During the last five years, increasing concern about global warming has led to an intense search for cost-effective alternative energy sources such as photovoltaics.¹ Yet existing commercial silicon-based (i.e., inorganic) solar cells have limited use. Moreover, they require expensive and complicated manufacturing processes involving clean rooms and vacuum chambers. Polymer (i.e., organic) solar cells built from thin films of organic semiconductors—conjugated polymers and small-molecule compounds provide two examples—are potentially versatile sources of cheap renewable energy, as they may ultimately make it possible to print large-area solar cells on lightweight flexible surfaces at room temperature. Future applications of this technology include self-powered electronic newspapers and self-sufficient buildings.²

At present, bulk heterojunction structures—homogeneous blends of p-type (donor) and n-type (acceptor) semiconductors—based on blends of polymer donor and highly soluble derivatives of fullerene as acceptor have been the material system with the highest published power conversion efficiencies (PCEs), measured at standard incident light conditions (AM1.5).³ We at Konarka Technologies have recently demonstrated a 5.21% PCE plastic solar cell with an active area of 1.024cm², showing the potential of bulk heterojunctions. The next step is to identify suitable methods for producing organic solar cells.

Using a novel solvent mixture, we have printed highly efficient polymer:fullerene bulk heterojunction solar cells from a commercially available inkjet printer. The mix resolves the morphological limitations of commonly used pristine solvents.

During the drying and subsequent annealing process, the solvent creates an optimum distance between the polymer and fullerene bulk heterojunction interfaces to achieve efficient charge separation of the formed exciton. At the same time, it forges a network of polymer and fullerenes to transport the holes and electrons produced after charge separation to the electrodes. The result is inkjet-printed organic solar cells with strongly enhanced performance.⁴

Figure 1. Atomic force microscopy image of a polymer:fullerene blend solution deposited by inkjet printing.

Figure 1 shows an atomic force microscopy image of the polymer:fullerene film printed using the new solution. The inkjet-printed active layer shows a uniform mixing of the film components within the blend. With a mean roughness of only 2nm, the ultrasmooth active layer arising from the optimum spreading and wetting properties of the solvent provides intimate morphology and interfaces offering high-PCE printed cells.⁴

Figure 2. Current density (J) plotted against voltage (V) for the inkjetprinted organic solar cell under light. Short-circuit current density (J_SC), open-circuit voltage (V_OC), fill factor (FF), and power conversion efficiency (PCE) values are included.

Figure 2 shows the current density/voltage relation under AM1.5 illumination with 100mWcm^-2. The devices with inkjet-printed active layers have a short-circuit current density of 8.4mAcm^-2, open-circuit voltage of 0.54V, and fill factor of 64%, resulting in a PCE of 2.9%. The photovoltaic performance is four orders of magnitude higher than previous published results.⁵

In summary, based on a newly developed solvent mixture, we have demonstrated a 3% PCE solar cell with an inkjet-printed polymer:fullerene blend photoactive layer.⁴ The high performance of the device indicates the potential of inkjet printing for mass-producing organic photovoltaics.