by Katherine Derbyshire, contributing editor, Solid State Technology

In recent years, inorganic thin-film solar cells have attracted substantial investor interest and have begun to erode the market share of wafer-based silicon cells. Meanwhile, cells based on organic semiconductors have fallen short of some of the more aggressive timetables set by their proponents. Still, organic cells offer an intriguing combination of flexible integration schemes and low cost. P3HT (poly(3-hexylthiophene)), phthalocyanine (Pc) compounds, and other materials can be deposited by low thermal budget methods such as spray coating onto a wide variety of inexpensive plastic substrates. Clothing, curtains, and wall coverings could all incorporate solar cells based on these materials, which could in turn power displays and other organic electronic devices.

Though the potential of organic photovoltaics has been recognized for some time, development of commercially feasible devices has been challenging. Conversion efficiencies have lingered in the 2%-3% range, with lifetimes measured in hours or minutes. Overly optimistic reporting may have raised expectations that the materials were not yet ready to meet. [1]

Still, researchers are making steady progress on organic photovoltaics, to the point where IMEC is targeting 10% champion cell efficiency by 2012, with module average efficiency of 7% and five year operational life, according to senior scientist Tom Aernouts. The targeted efficiency would be comparable to commercial thin-film silicon modules, and would establish organic materials as a viable alternative for cost-sensitive solar cell applications.

Part of the challenge of organic photovoltaics arises from their unique physics. In inorganic semiconductors, incident photons excite electron-hole pairs throughout the bulk. As previously discussed in more detail ("Improved efficiency boosts PV panel prospects," October 2008) these pairs diffuse to a p-n junction, where they are swept apart by the junction potential and proceed to their respective cell electrodes. Recombination of electron-hole pairs before they reach the junction is an important source of efficiency loss. In organic solar cells, in contrast, incident photons generate excitons. The exciton state consists of an excited electron, bound to a site in the molecular structure. Excitons are far less mobile than electron-hole pairs, and will collapse to the ground state unless generated near a heterojunction between donor and acceptor materials. Though it is tempting to view this heterojunction as a type of organic p-n junction, the two involve somewhat different charge separation mechanisms, discussed in more detail in [2]. Because the exciton diffusion length is short, best performance is obtained with 3D interpenetrating networks of donor and acceptor materials. Unfortunately, as Aernouts explained, these bulk heterojunction structures are prone to phase separation, which often occurs in a matter of hours under typical operating conditions.

Research at IMEC and elsewhere has focused on development of materials and process conditions that can achieve a stable nanomorphology. Last March at the Japan Society of Applied Physics meeting, researchers from Osaka University reported that introduction of high purity C₆₀ dramatically improved the performance of their H₂Pc/C₆₀ structures. At IMEC, an evaporated C₆₀/Substituted-Pc bilayer achieved an open circuit voltage (V_oc) of 920 mV and conversion efficiency of 3% (see Fig. 1).

Alternatively, spin-coating of a P3HT/PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester, a fullerene derivative) blend under controlled conditions (see Fig. 2) gave a 4.6% conversion efficiency, which compares favorably to organic champion cells with efficiencies of 5.3%-5.6%. IMEC is partnering with Plextronics to develop a reproducible, scaleable process for large-area manufacturing of high-efficiency organic cells.

Any successful large area manufacturing process will necessarily include improved encapsulation processes. Not only are organic semiconductors especially sensitive to heat-induced degradation in the presence of air and moisture, but the use of flexible substrates brings a need for flexible, yet transparent, barrier layers and sealants. In encapsulation, as in other parts of the process, much of IMEC's work is aimed at transforming laboratory scale techniques to large scale continuous processes. Only with such processes will organic solar cells begin to realize their potential. -- K.D.

References:

[1] Peter Fairley, "Solar Cell Squabble," IEEE Spectrum, April, 2008, p. 37.
[2] Brian Gregg, "Excitonic Solar Cells," J. Phys. Chem. B 2003, 107, pp.4688-4698.