Tyler Buchanan
Advanced Organic Chemistry
November 2, 2018
Fulleropyrrolidines derivative synthesis and properties in Organic Photovoltaic Performance as a electron acceptor.
Introduction:
In the field of alternative energy, the research of photovoltaic (PV) cell research specifically organic photovoltaic (OPV) cells has been one of the key technologies being researched today. Organic photovoltaic cells are a rapidly developing technology that is an attractive option in the PV cell field due to OPV cells ability to absorb a broader range of wavelengths and in its efficiency to build cheap transparent devices. In current OPV cells, the cells are typically constructed of two types of semiconductors called p-type and n-type semiconducting materials. P-type and n-type materials are semiconducting materials that contain atomic impurities to allow the material to either accept electron (p-type) or electron donators (n-type). In the latest research, laboratories have put a lot of money and time into creating better n-type semiconductors to increase OPV performance in areas such as higher short-circuit current density (Jsc), and higher open circuit voltage (Voc). With ongoing research focused more on electron donor than electron acceptors, new development and synthesis of p-type semiconducting materials have been infrequent. One of the few events in recent years has been in the area of synthesizing fullerene derivatives as an electron acceptor such as functionalized fullerene, Fulleropyrrolidines. Fulleropyrrolidines are considered to have great potential in the work of OPV devices due to the fullerene attributes of being significantly hydrophobic, low reorganization energy and exceptional photophysical effects with the characteristics of functional groups attached. In OPV performance, Fulleropyrrolidines and Fulleropyrrolidines derivatives are one of the key topics being researched to improve electron acceptor performance in OPV cells. In this article, we will review Fulleropyrrolidines and discuss the synthesis and applications of Fulleropyrrolidines in the field of organic photovoltaic devices and material science.
Background Material:
The primary objective of the Solar Energy community is for one day to be able to make solar cells efficient enough where the world can run on solar energy with zero pollutants. Now long before that day comes to light, current researches have constantly been working on improving the efficiency and production methods of solar cells. One of the primary types of solar cells today has been organic photovoltaic (OPV) cells. Organic photovoltaic cells are a rapidly developing technology that is an attractive option in the PV cell field due to OPV cells ability to absorb a wider range of wavelengths and in its efficiency to build cheap transparent devices. In the general structure of organic photovoltaic cells consist of three different structure types: single layer, bilayer and bulk heterojunction as shown in Figure 1:
Figure 1: OPV Structures: (a) Single layer l (b) bilayer and (c) bulk heterojunction OPV cell. Reprinted from Kevin M. Coakley and Michael D. McGehee, Chem. Mater. , 2004, 16 , 4533–4542. Copyright 2004, American Chemical Society.
For single layer OPV cells it consists of an electrode, organic electronic material and an electrode with a lower work function. The difference in the electrode’s work function will cause an electric field in the organic electronic material. The organic electronic material that will excite electrons to the LUMO but not fill holes in the HOMO causing exciton pairs with are separated by pulling electrons to the positive electrodes creating a current. Now for a bilayer structure instead of an organic electronic material there is an electron donor layer and electron acceptor layer. These two layers have different electron affinity creating electrostatic forces so when light hits the electrode it creates excitons at the border between the acceptor and donor layers for more efficient charge separation and collecting. With more efficient charge separation and collecting the bilayer is more efficient then the single layer system. In the final OPV structure, the bulk heterojunction mixes the electron acceptor and electron donor layer at a nanoscale level. With the mixing of these two layers it allows for the excitons pairs to dissociate and interface quicker causing short cycles for the excitons where they can recombine into pairs. In latest research, labartories have put a lot of money and time into creating better electron donator (p-type) semiconductors to increase OPV performance. With current research focused more on electron donor than electron acceptors, new development and synthesis of electron acceptor (p-type) semiconducting materials has been infrequent. One of the few developments in recent years has been in the area of synthesizing fullerene derivatives as an electron acceptor such as functionalized fullerene, Fulleropyrrolidines. Before the functionalization of fullerenes to make compounds such as fulleropyrrolidines, fullerenes already attracted the attention of many researchers in the photovoltaic and material science field. Fullerene received such high attention due to the uncommon ability of being a hydrophobic molecule that contains low reorganization energy and extraordinary photophysical attributes. With fullerene containing these properties, scientist saw it as an ideal candidate for solar cell components such as organic light-emitting diode (OLED) and a organic field-effect transistor (OFET). As researchers began to study more about fullerenes in hopes to discovering the ability to functionalize fullerene to give the possibility of combining fullerenes significant properties to other photoactive materials to create more efficient devices and new applications. In 1996, Maurizo Prato and Michele Maggini discovered the ability to functionalize fullerene through the use of Prato reactions. Prato reaction are a 1,3-dipolar cycloaddition of azomethine ylides to olefins that are electron poor. Now electron poor olefin contains the same chemical reactivity as fullerene allowing them to undergo the same quick reaction. When the Prato reaction is applied to a fullerene, the ylide attacks the double bond contained in a 6,6-ring location on a fullerene creating the functionalized fullerene compounds like fulleropyrrolidines as shown to the right in scheme 1. Looking at the fulleropyrrolidine compound, one can see the multiple R groups that can be attached to the pyrrolidine ring that is now attached to the fullerene. With the multiple locations for the R-groups on fulleropyrrolidine, there are multiple fulleropyrrolidine derivatives that can be created through the combination of different functional groups at the R group locations. With the versatility of the fulleropyrrolidine compound it has now become a frequent topic of research to create fulleropyrrolidine derivatives that are more efficient electron acceptor through the testing of different functional groups. For the next section, we will review the current synthesis and testing methods of fulleropyrrolidine derivatives that currently being researched today.
Discussion:
As stated previously the synthesis of fulleropyrrolidine and its derivatives are made through the use of the use of Prato reactions. In this discussion we will review the general synthesis of fulleropyrrolidine derivatives and few specific examples that are being applied to OPV cells and the field of material science. The original synthesis for fulleropyrrolidines was first discovered through Prato reaction where a 1,3-dipolar cycloaddition of azomethine ylides to fullerene C60. Azomethine ylides one of the main reagents in Prato reaction are nitrogen-based that contain 1,3-dipoles that contain a carbanion beside a iminium ion as displayed on Figure 3.
Figure 3
Azomethine ylides are synthesized by variety of obtainable starting materials such as aziridines, imines, and iminiums. With a significant number of ways to generate reactive intermediates and the easy obtainability of the starting materials make azomethine ylides a versatile reagent to functionalize a reagent product to be used for a Prato reaction with fullerene. One of the most typical way to construct azomethine ylides is through the condensation of amino acids and aldehydes to create iminium salts which undergo decarboxylation to create azomethine ylides. With the azomethine ylides formed a 1,3-dipolar cycloaddition will occur where ylide attacks the double bond contained in a 6,6-ring location on the fullerene creating the fulleropyrrolidines as shown in the general mechanism below in Figure 4:
Knowing the general mechanism of fulleropyrrolidines we will now look at fulleropyrrolidines derivatives that are used by researchers to improve the efficiency of OPV cells. In the research performed by these scientists they all use fulleropyrrolidines derivatives as a electrotropic additivete to electron acceptor layer in a bilayer structure or as electron acceptor in a bulk heterojunction layer in a bulk heterojunction shown in Figure 1. For researchers at the University of Massachusetts, Amherest, they used the fulleropyrrolidines derivative called 2,3,4-tris(3- (dimethylamino)propoxy)fulleropyrrolidine (C60-N), as an additive in a BHJ active layer and test the new OPV cell performance by measuring the power conversion efficiency of the cell and other performance measurement such as photocurrent density (Jph) as a function of internal voltage (Vint). The BHJ layer also consisted of a mixture of two other compounds being 7,7′-(4,4- bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5yl)benzo[c][1,2,5]thiadiazole) and [6,6]-phenyl-C71-butyric acid methyl ester (p-DTS(FBTTh2)2:PC71BM),24,25,26. In testing of three different OPV cells one with fulleropyrrolidines derivative C60-N that undergoes electric field poling, one that contains C60-N wirth no poling and one without the C60-N. In there results they came to the conclusion that when fulleropyrrolidines derivative C60-N is added as an additive to the BHJ active layer that the fulleropyrrolidines derivative C60-N is less efficient then non-additive device at a PCE of 5.18% to 7.717% when the C60-N does not go through electric field poling. Now when the fulleropyrrolidines derivative C60-N under goes electric field poling the PCE values is marked as 7.97% which is more efficient then the optimized reference devices that contained no C60-N additive. For the C60-N addition with electric poling, the OPV showed increased performance in other areas as well. The OPV cell that contain C60-N addition with electric poling had increased performance in fill factor (FF) and short circuit current density (Jsc) meaning that the OPV devices were causing the excitons pairs to dissociate and interface quicker causing short cycles for the excitons meaning that the excitons could not recombine into pairs reducing recombination losses. Also, in the scientist experiments they ran a test to measure the photocurrent density versus internal voltage of the OPV cells. In running this test, the researchers analyzed the photocurrent density versus internal voltage trends and discovered that improved performance of the OPV cell originates from the efficient withdrawal of photogenerated charge carriers. The high efficiency in the extraction of photogenerated charge carriers was caused by an incorporated electric field created from the macroscopic alignment of dipole moments of fulleropyrrolidines derivative C60-N molecules from electric field poling. In conclusion of their research, it shown that the fulleropyrrolidines derivative C60-N increase the overall performance of OPV but has to be configured into specific alignment using electric field poling to be useful. Another researcher that investigated the use of fulleropyrrolidines affecting organic photovoltaic performance but instead of testing how macroscopic alignment of dipole moments using electric field poling, the researchers Makoto Karakawa and Takabumi Nagai testing how the natural basicity of Fulleropyrrolidine derivative plays a significant factor into controlling the OPV performance. In their research, Makoto Karakawa and Takabumi Nagai synthesized basic N-benzyl fulleropyrrolidines derivatives using Prato reaction to create an organic electron acceptor layer in a bilayer structure OPV cell. Three different fulleropyrrolidines derviatives with N-phenyl or bulky substituents were formed to create the organic layer. The three fulleropyrrolidines derviatives chemical structures are shown below:
The researchers originally hypothesized that the basicity of fulleropyrrolidine organic layer may affect the photovoltaic devices performance containing an acidic poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer. To determine the how the basicity of the fulleropyrrolidine derivative affected the OPV performance through the interfacial reaction of PEDOT:PSS and the fulleropyrrolidine derivative organic layer, the researchers tested a OPV device that contained the PEDOT:PSS layer and a OPV device without. In there testing they discovered the chemical reactivity of the PEDOT:PSS layer was affected by the organic layer’s basicity with a organic layer with a high enough pKa value. The results showed that the OPV device that contain the PEDOT:PSS layer contained high resistance causing the performance level to lower of the OPV cell when compared to a OPV cell with no PEDOT:PSS layer. Overall the researchers suggested that in the design of new OPV cells that organic layer needs to have lower pKa values to remain efficient enough to improve the OPV cell performance. Looking at the fulleropyrrolidines derivative findings, fulleropyrrolidines derivative have an enormous potential to increase OPV cell performance but further testing of the fulleropyrrolidines derivative structure and organization in the organic layer is need to improve OPV cells to there maximum performance.
Conclusion:
With the field of alternative energy, the research of photovoltaic (PV) cell research specifically organic photovoltaic (OPV) cells has been one of the key technologies being researched today. The main development has been in the area of synthesizing fullerene derivatives as an electron acceptor such as functionalized fullerene, Fulleropyrrolidines. Fulleropyrrolidines have great potential in the work of OPV devices as provided in the discussion where Fulleropyrrolidines have increased the efficiency of OPV cells by a half percent. Looking at the fulleropyrrolidines derivative findings, fulleropyrrolidines derivative have an enormous potential to increase OPV cell performance but further testing of the fulleropyrrolidines derivative structure and organization in the organic layer is needed to improve OPV cells to their maximum performance. In the review of fulleropyrrolidines derivative synthesis and properties in organic photovoltaic performance, fulleropyrrolidines derivative are a realtively easy compound to manufacture due to Prato reactions with the potential to increase the performance of OPV cells. Therefore fulleropyrrolidines derivative should be continued to be researched to their maximum efficiency by determining the proper functional groups, basicity, layer arrangement to produce the most viable OPV cell.