NEAR-IR ABSORBING PHTHALOCYANINE DERIVATIVES AS MATERIALS FOR ORGANIC SOLAR CELLS

Abstract

Phthalocyanines (Pcs) are highly conjugated synthetic porphyrin analogs that exhibit high extinction coefficients and hole mobilities, and strong pi-pi interactions. We have developed a general method for the synthesis of peripherally functionalized Pc chromophores using `click' chemistry, wherein an alkynyl substituted Pc is reacted with an azide, providing an elegant route to the creation of a library of numerous Pcs. We have also developed a simple route to the synthesis of tri- and tetravalent metal Pc derivatives such as titanyl phthalocyanine (TiOPc) involving solvent-free conditions. Solvent-free conditions are environmentally friendly and industrially economical, and in the present context effectively eliminate the formation of non-metallated phthalocyanine (H2Pc), a side product often seen in other routes that interferes with their purification. We have also prepared and characterized thin-films of some of these Pcs, TiOPcs in particular, wherein we have developed an easy route to various TiOPc polymorphs exhibiting different near-IR sensitivities via spin-coating whose optical properties are reminiscent of Phase-I and Phase-II polymorphs of the unmodified TiOPc. Phase-II is particularly interesting as it is photoelectrically active in the near-IR region with a Q-band maximum at ca. 890 nm. We have also fabricated and characterized organic solar cells in both planar heterojunction (PHJ) and bulk heterojunction (BHJ) architectures based on one of these materials, which exhibited good near-IR photoactivity with the absorption spectrum extending up to 1 micrometer in the near-IR. The incident and absorbed photon to current efficiency (IPCE and APCE) spectra showed contributions from the TiOPc in the near-IR region with local maxima around 680 nm and 920 nm, corresponding to the Frenkel and the charge-transfer (CT) bands of the TiOPc, respectively.