New Small Molecules As Hole-transporting Layer In Efficient Polymer Solar Cells

Polymer solar cells (PSCs) have attracted remarkable attention during the past tenyears due to their advantages of low cost, light weight, and easy fabrication. Theinterfacial layers have played an important role in achieving high-quality devices.Commercial PEDOT:PSS is currently the most popular hole-transporting material(HTM) in PSCs. However its intrinsic deficiencies, such as acidic and hygroscopiccharacter, electrical inhomogeneity and insufficient electron-blocking ability, limit itsfurther application.Small molecules TPDA, TPDB, and TPDH, comprising the same backbone TPDunit and different carboxyl side chains were designed and synthesized ashole-transporting materials for polymer solar cell applications. Their structure wereconfirmed by1H NMR,13C NMR, mass spectrometry, and elemental analysis. Thesesmall molecules demonstrated the improved solubility in polar solvents due to theintroduction of the weakly acidic carboxyl groups. The lengths of the side chains canalso influence the film forming ability. These small molecules showed hightransmittance in the visible range compared with conventional PEDOT:PSS as revealedby UV-vis measurements.. Desirable energy-level alignment for efficient hole-transporting and electron-blocking ability was indicated by their absorption and UPSspectra. Without applying any post-treatment on these buffer layers, the new smallmolecules based devices achieved comparable or improved performances comparedwith control devices using PEDOT:PSS as buffered layers. High power conversionefficiency (PCE) of6.51%was realized by a TPDB buffered device employing a low band gap polymer PBDTTPD and PC61BM as the active material. The TPDB buffereddevice showed15%efficiency enhancement over the control devices. Furthermore,improved device stabilities were also demonstrated by using the new materials as thehole-transporting layers.