Development of Fullerene Based Molecules as an Acceptor Partner with P3HT for Opv Devices in a Thin Layer Organic Solar Cell

A fullerene and its derivatives have attracted much attention due to their attractive physical and chemical properties.¹ The most notable features of fullerenes are their optical and electrochemical properties, which have recently been applied to an n-type material in organic photovoltaic (OPV) devices composed with p-type materials typified by p-conjugated polymers such as poly(3-hexylthiophene) (P3HT).² Among fullerene derivatives, methyl [6,6]-phenyl-C₆₁-butylate ([C₆₀]-PCBM)³ has been widely used as the n-type material.⁴

We have reported that some OPV devices using fulleropyrrolidines as acceptor materials with P3HT, in particular, 1-(2-(2-methoxyethoxy)ethyl)-2-(2-methoxyphenyl)fulleropyrrolidine (1)⁵ and 1-([2,2'-bithiophen]-5-yl)-1-(2-(2-methoxyethoxy)ethyl) fulleropyrrolidine (2)⁶ showed higher power conversion efficiency (PCE) compared to those using [C₆₀]-PCBM. Fulleropyrrolidines have a certain advantage over [C₆₀]-PCBM from the standpoint of their stable nature under atmospheric conditions and the ease of producing various types of analogues. However, it was essential to use a special electrode which lacked the poly(3,4-ethylenedioxythiophene) :poly(styrenesulfonate) (PEDOT: PSS)⁷ layer to obtain high PCE. The role of HTL is believed to be to prevent the leak current from the active layer to the ITO electrode, thereby strongly contributing to the enhancement of the OPV performance, especially FF and Voc.⁸ Therefore, we investigated the interaction between the sulfonic acid group of PEDOT:PSS and nitrogen moiety of the fulleropyrrolidine to determine the reason why PEDOT:PSS caused the drop in PCE of fulleropyrrollidine based OPV devices. The study indicated that quaternary ammonium salt was formed on the interface of the active layer with PEDOT:PSS and this prevented the smooth hole transportation.⁶Inspired by the results, we hypothesized that a good OPV device might be produced using fulleropyrolidine derivatives if we could prevent the formation of such quarternary ammonium salt by PEDOT:PSS. We hypothesized that 2,5-diaryl-substiuted fulleropyrrolidine might become a good acceptor material because it might be difficult for the sulfonic acid group of PSS to access the nitrogen atom on the pyrrolidine ring due to enhanced steric hindrance of the N atom, and formation of quarternary ammonium salt would thus be avoided.

Here, we report that the performance of the OPV devices using 2,5-diarylfulleropyrrolidines with P3HT and an ITO electrode with the PEDOT:PSS layer significantly depend on the stereochemistry of two substituents of the pyrrolidine ring.

Three types of 2,5-diarylfulleropyrrolidine derivatives, i. e. 2,5-diphenylfulleropyrrolidine (3), 2,5-di(thiophen-2-yl) fulleropyrrolidine (4), and 2,5-di(thiophen-3-yl)fulleropyrrolidine (5) were synthesized and used as acceptor molecules with P3HT of OPV devices using two types of ITO electrodes. We discovered that PCE values of OPV devices prepared by 2,5-diarylfulleropyrrolidines as an acceptor partner with P3HT depended on both the stereochemistry and nature of the substituent: PCE of the devices using trans-2,5-diphenylfulleropyrrolidine (trans-3), cis-2,5-di(thiophen-3-yl)fulleropyrrolidines (cis-5) and trans-2,5-di(thiophen-3-yl)fulleropyrrolidines (trans-5) were significantly lowered by PEDOT:PSS. On the other hand, cis-3, thiophen-2-yl-substituted compounds cis-4 and trans-4 worked in a different mode: these are independent of the presence of PEDOT:PSS and good PCE values were obtained for both isomers of 2,5-di(thiophen-2-yl) fulleropyrrolidine. Among tested compounds, trans-3 showed the best PCE which was superior to that of [C₆₀]-PCBM. These results clearly indicate a rich possibility of fulleropyrrolidine type OPV devices with high PCE values with further investigation.⁹

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