Solution-Processable Graphene Oxide as an Efficient Hole Transport Layer in Polymer Solar Cells

Abstract

Polymer solar cells undoubtedly hold tremendous potential for solutionprocessable, inexpensive photovoltaics on flexible plastic substrates. 1Ϫ5 The most promising organic photovoltaics (OPVs) are based on random dispersion of acceptors in the form of organic or inorganic nanostructures within a conjugated polymer matrix, which acts as the donor phase. The interfaces at the acceptor nanostructures and the donor host polymer matrix create bulk heterojunctions (BHJs), which provide a large number of sites for charge separation and bicontinuous pathways for efficient carrier transport. 6Ϫ8 Although numerous combinations of acceptors and donors have been reported, 9Ϫ14 the most popular BHJ polymer solar cells consist of poly(3-hexylthiophene) (P3HT) and fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) blends. The mechanisms for achieving high photovoltaic efficiencies in excess of 6% in P3HT:PCBM BHJ devices are well-established. 15 However, several key components remain unresolved and must be addressed if the theoretical efficiencies of ϳ10% are to be realized. 16 For example, in a simple BHJ device, both the donor and acceptor phases are in direct electrical contact with the cathode and anode electrodes, leading to recombination of carriers and current leakage. To minimize such detrimental effects, electron blocking and hole transport layers (HTLs) are deposited on top of the transparent and conducting indium tin oxide (ITO) anode. HTLs must be wide band gap p-type materials, and several inorganic materials such as V 2 O 5 and MoO 3 17 have been reported with NiO being the most effective, yielding efficiencies greater than 5%. 18 However, inorganic HTLs are deposited using vacuum deposition

Revealing the ultrafast process behind the photoreduction of graphene oxide

“…Such a temperature is far below previously reported thermal treatments leading to any observable reduction 18 . A second possible pathway is the direct photoreduction of GO; if occurring such a process would consist of the photoionization of the GO, as ultraviolet light (4.6 eV) is far from any resonant excitation of C-O or C ¼ O bond but near the work function 40 (4.9 eV). However, photoionization would lead to the creation of an electron-hole pair and consequently further oxidize the sample, not reduce it.…”

Graphene/polymer composites for energy applications

“…12(A)]. 34 The introduction of GO obviously reduced the recombination of electrons and holes in comparison with pristine ITO. A GO film with an optimized thickness of 2 nm exhibited the best results [ Fig.…”

“…To date, various graphene composites with insulating or conducting polymers (CPs) have been prepared through noncovalent 22 or covalent approaches. 24 They have also been applied as electrodes for supercapacitors and lithium ion batteries (LIBs), [25][26][27][28][29] counter electrodes of dye-sensitized solar cells (DSSCs), 15,30,31 and transparent conducting electrodes (TCEs) 32,33 or active layers of organic solar cells, 34 catalytic electrodes, and polymer electrolyte membranes of fuel cells. [35][36][37] This review focuses on summarizing the recent advances in the synthesis of graphene/polymer composites and their energy applications.…”

Origin of size effect on efficiency of organic photovoltaics

“…The phenomenon is fundamental and should take place in any OPV or other excitonic (with voltage-dependent photocurrent) solar cells 15 with ITO front electrode or any other similar transparent electrodes (graphene 46,47 and carbon nanotube electrodes, 48 surface-plasmon enhanced Ag grids, 49 metal nanowire mesh, 50 etc.) in which current flows parallel to the cell surface.…”