Challenges for Low-Cost Roll-to-Roll Organic Solar Cells: A Review of their Physics, Limitations, and a Comparison with Inorganic PV Technologies

#Organic #solar #cells #energy

Key advantages of organic photovoltaic (OPV) technology is that organic based PV, versus traditional inorganic semiconductors, are inherently inexpensive; typically have very high optical absorption coefficients (>10⁵ cm^-1); are compatible with plastic and flexible substrates; and can be fabricated using high-throughput low temperature processes for low-cost roll-to-roll (R2R) manufacturing. Organic solar cells have improved rapidly over 20 years from very low efficiencies (~3.5%) to moderately high efficiencies of 15-18.2%, with the overall performance of OPVs poised for commercial opportunities. The bulk heterojunction topology launched this revolution.

Fundamental limitations for bulk heterojunction OPVs traditionally have been (i) narrow absorption spectral region, (ii) difficulty to dissociate excitons and (iii) fully collect liberated charged carriers. The baseline model used poly(3-hexylthiophene-2,5-diyl) (P3HT) interspersed with phenyl-C61-Butyric-Acid-Methyl Ester (PCBM), as a mixture of conjugated polymers with functionalized fullerenes, and only the P3HT absorbs sunlight, while missing the infrared regions of the solar spectrum. PCBM is optically inert, and does not collect sunlight due to its soccer-ball symmetry of the C₆₀ molecule. With new developed low bandgap absorbers via new conjugated polymer chemistries, the current platform is expanding its narrow collection window to be more spectrally matched to the incident solar spectrum. However, this can create additional issues that will be discussed.

Among recent advances towards improving the efficiency in organic PV devices, various interface and surface engineering techniques to the anode have demonstrated improved efficiencies for organic PV devices by an optimization of the short-circuit current, the open-circuit voltage and the fill factor. Although direct surface modifications to the anode have been successfully implemented for organic light emitting diodes (OLED) to improve their device performance by creating an interface energy step between the anode and the hole-transporting layer, thereby enhancing hole injection by effectively lowering the hole injection barrier, this approach had not been fully explored for organic PV yet. In this talk, I will also present efficient PV devices through tailored modifications to electrode surfaces.

In order to improve the efficiency of polymer PV devices, another approach, addressed here, will be to yield increased optical absorption and photocurrent generation in the photoactive layer over a broad range of visible wavelengths by inducing surface plasmons through careful control of metallic nanoparticle’s properties. In this work, a unique colloidal silver nanoparticle solution with the presence of suitable organic capping groups that stabilize the nanoparticles and inhibit their propensity to agglomerate is applied to organic bulk heterojunction PV devices. An improved optical absorption and photocurrent for PV devices was demonstrated due to the increased electric field in the photoactive layer by excited localized surface plasmons of Ag nanospheres.

For completeness, time permitting, testing issues specific to OPV, their packaging and economics will also be reviewed