Self-Assembled and Cross-Linked Fullerene Interlayer on Titanium Oxide for Highly Efficient Inverted Polymer Solar Cells

Abstract

b S Supporting Information ' INTRODUCTIONPolymeric solar cells (PSCs) have emerged as a promising alternative technique for producing clean and renewable energy due to their potential for fabrication onto large areas of lightweight flexible substrates by low-cost solution processing. To maximize the donor-acceptor heterojunction interfacial area for efficient exciton dissociation, mainstream PSC devices adopt the concept of a bulk heterojunction (BHJ), where an active layer contains a p-type donor and an n-type acceptor to form an interpenetrating nanoscale network. 1 A conventional BHJ PSC with an active layer sandwiched by a low-work-function aluminum cathode and a holeconducting poly(3,4-ethylenedioxylenethiophene):poly(styrene sulfonic acid) (PEDOT:PSS) layer on top of an indium tin oxide (ITO) substrate is the most widely used and investigated device configuration. On the basis of this device architecture, high powerconversion efficiencies (PCEs) of ca. 4-5% have been achieved for a blend containing a regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative, [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). 2 Along with high performance, long-term stability is a primary area of concern for PSCs. However, it is highly challenging to develop a PSC that can achieve a high PCE while maintaining good ambient stability of the device. Prolonged exposure to air rapidly reduces the performance of unencapsulated conventional devices. Rapid oxidation of the low-work-function metal cathode and etching of ITO by the acidic PEDOT:PSS layer are the most common reasons for instability in conventional unencapsulated devices. An effective approach to solve these problems, and improve device lifetime, is to fabricate inverted PSCs. 3 By reversing the polarity of charge collection in a regular cell, airstable Ag combining with an adjacent PEDOT:PSS layer can substitute for air-sensitive Al as the anodic electrode for efficient hole collection. In such an inverted configuration, it is necessary to insert an inorganic metal oxide (TiO x or ZnO) between ITO and the active layer to function as an electron-selective contact. 4 Despite dramatic improvement in the operational lifetime, inverted solar cells still suffer from a trade-off between stability and performance. Recently reported inverted devices based on P3HT/PCBM composite exhibited PCEs in the range of ca. 2-4%, which is inferior to that of regular solar cells. The relatively lower performance is

Solution-processed cathode interfacial layer materials for high-efficiency polymer solar cells

“…The authors showed that with appropriate choice of SOMs, the device performance of the PSCs can be improved dramatically. Later, Cheng et al [43] reported the synthesis of two oxetane-functionalized cross-linked fullerene derivatives that can form self-assembling layer on the surface of TiOx for inverted type PSCs. It is worth noting that these cross-linked fullerene derivatives can effectively prevent the consequent interfacial erosion, and thus are ideal for the fabrication of all-solution processing PSCs.…”

Enhanced Performance in Inverted Polymer Solar Cells with D–π–A‐Type Molecular Dye Incorporated on ZnO Buffer Layer

“…[7][8][9][10] Therefore, we finally measured the WFs of the ITO substrate, neat ZnO, and d-ZnO by using ultraviolet photoelectron spectroscopy (UPS), as shown in Figure S8 in the Supporting Information. [7][8][9][10] Therefore, we finally measured the WFs of the ITO substrate, neat ZnO, and d-ZnO by using ultraviolet photoelectron spectroscopy (UPS), as shown in Figure S8 in the Supporting Information.…”

“…It is well known that the presence of buffer layers in PSCs affects the WF of electrodes, and thus, provides the ohmic contact at electrodes. [7][8][9][10] Therefore, we finally measured the WFs of the ITO substrate, neat ZnO, and d-ZnO by using ultraviolet photoelectron spectroscopy (UPS), as shown in Figure S8 in the Supporting Information. The WF values of ITO, neat ZnO, and d-ZnO were calculated to be about 4.7, 4.5, and 4.4 eV, respectively.…”

“…[1][2][3] Also, instead of an acidic and hygroscopic poly (3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT:PSS) film as a hole transport layer (HTL), other high WF metal oxides, such as MoO 3 , WO 3 , or V 2 O 5 , were used on top of the active layer of inverted PSCs. [4][5][6] The n-type metal compounds, for instance, TiO x , [7] TiO 2 , [8] titanium diisopropoxide bis(2,4-pentanedionate) (TIPD), [9] or ZnO, [10] are generally deposited on an indium tin oxide (ITO) electrode to effectively transport electrons, but block holes in this inverted architecture. Among these compounds, ZnO is one of the most promising candidates due to its high electron mobility, environmental inertness, and solution processibility, as well as the ease with which it can be used to fabricate various noble nanostructures.…”

Patternable conjugated polymers for organic solar cells

“…Furthermore, there is evidence in the literature that oxetane crosslinking is possible in the presence of fullerenes. 18 The first low-bandgap polymer we have chosen is poly(2,7-(9-(2'-ethylhexyl)-9-hexylfluorene)-alt-5,5-(4',7'-di-3-hexylthien-2-yl-2',1',3'-benzothiadiazole) PFDTBT first synthesized by Svensson et al in 2003. 19 The high mobility and stability of polyfluorene in combination with the benzothiadiazole comonomer DTBT that lowers the bandgap of the polymer render PFDTBT suitable for the use in organic solar cells and lead to a device efficiency of 2.2%.…”