Fabrication optimization improves thermal stability and efficiency of polymer solar cells

Solar cells based on conjugated polymer-fullerene composites are a promising potential source of renewable energy.^1–3 In particular, because electronic devices fabricated from semiconducting polymers are lightweight, mechanically flexible and have the potential for low-cost production, high-efficiency polymer solar cells could have a major impact on the energy needs of our society. Although encouraging progress has been made in recent years, with results such as 3–4% power conversion efficiencies under AM1.5 illumination (AM1.5 denotes the standard spectrum of sunlight at the Earth's surface),⁴ this efficiency is not yet sufficient for large-scale implementation. Moreover, concerns about instability at elevated temperatures have hindered the path toward commercialization. The need to improve the device efficiency and thermal stability requires state-of-the-art fabrication and treatment under more rigorously-defined conditions.

One approach toward improving the device efficiency is through post-production heat treatment. Recent studies have demonstrated improved power conversion efficiencies (η_e) after thermally annealing polymer-fullerene composite solar cells at elevated temperatures (50 – 130^°C).^4–6 In these earlier studies, the best results (η_e ≈ 3.5%) were obtained after post-production thermal annealing at 75^°C for 5 minutes.⁴ Recently by applying rigorously-optimized device fabrication conditions and post-production annealing at 150^°C, we have developed a new fabrication architecture for high-efficiency bulk heterojunction solar cells with power conversion efficiencies approaching 5% under AM1.5 illumination at 80mW/cm². These results were obtained using poly(3-hexylthiophene) (P3HT) as the electron donor and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) as the electron acceptor.⁷

Figure 1(a) summarizes the results of a comprehensive study of characteristic device parameters (fill factor FF and short circuit current I_sc) as a function of the annealing temperature. As shown in the inset of Figure 1(a), the efficiency increases with annealing temperature, and peaks at an optimum value of around 150^°C. The increased efficiency results from two effects: higher nanoscale crystallinity and improved microstructure with demixing between the two components (P3HT and PCBM) in the bulk heterojunction films after thermal annealing. Moreover, we have observed stable solar cell performance even after annealing for one hour at 150^°C, as shown in Figure 1(b). This remarkable stability also indicates the formation of thermally stable nano-scale interpenetrating donor-acceptor networks as evidently shown in Figure 2 This nano-scale control of the morphology of the P3HT/PCBM interpenetrating networks results in optimized and stable phase separation for efficient charge separation and transport. In turn this leads to thermally stable, high efficiency solar cells.

Although the improved morphology and crystallinity of the active layer are evident from the data, postproduction annealing might also be expected to influence the interface between the active layer and the upper Al electrode. The effect of thermal annealing on the interface between the bulk heterojunction layer and the Al electrode can be directly imaged by atomic force microscopy (AFM) tapping mode height images as shown in Figure 3 (a) and (b); for Figure 3(a) the annealing was carried out prior to Al deposition whereas for Figure 3(b) the annealing was carried out on the completed solar cell. The rougher surface in Figure 3(b) suggests sufficiently strong interaction between the bulk heterojunction material and the Al metal that “clumps” of the former were pulled off, whereas the smoother surface in Figure 3(a)implies weaker interfacial adhesion. At elevated temperatures, processes such as Al diffusion or chemical reactions (e.g. the formation of C-Al or C-O-Al bonds) could lead to stronger contacts and increased contact area.

In summary, careful optimization of post-production heat treatment processes has resulted in polymer solar cells with power conversion efficiencies approaching 5%. These devices are promising potential sources for sources of renewable energy.