Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Abstract

An approach is presented to modify the WF of solution-processed sol-gel derived ZnO over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using P3HT:PCBM as the active layer. These devices compete with or even exceed the performance of the reference cell on ITO/PEDOT:PSS. Our finding challenges the current view that bottom electrodes in inverted solar cells need to be electron-blocking for good device performance.Transparent metal oxides (TMOs) are integral parts of today's optoelectronic devices, either as electron conducting electrodes or intrinsically-doped semiconductors. Among these, zinc oxide (ZnO) is becoming increasingly important, as it is composed of earth-abundant elements. Also, a wide range of vapor-and solution based methods is available for ZnO deposition, ranging from chemical or physical vapor deposition (CVD, PVD) for the preparation of epitaxially grown single crystalline ZnO films to sol-gel procedures for low cost solution-based deposition. ZnO is already intensively used as a low-cost, transparent electrode in inorganic devices, [1][2] but more recently also as a charge selective interlayer material in efficient organic solar cells (OSCs) or organic light-emitting diodes (OLEDs). [3][4] However, the moderate work function (WF) of untreated ZnO of about 4.3 eV causes injection barriers to almost all conventional organic semiconductors [5][6][7][8][9] and the WF of such ZnO layers is poorly reproducible due to the physisorption of contaminations. [10] Finally, its poor chemical stability in an acidic environment [11][12][13] makes the application particularly in organic devices challenging. Therefore, for some of the recent organic record solar cells, the WF of the ZnO forming the bottom electrode was

Dynamically Switching the Electronic and Electrostatic Properties of Indium–Tin Oxide Electrodes with Photochromic Monolayers: Toward Photoswitchable Optoelectronic Devices

“…[1][2][3] As a result of SAM formation, molecules are covalently bonded to the surface in an ordered manner and form a homogenous coating with controlled molecular-height thickness. When the deposited molecules possess a permanent dipole moment or chemisorption leads to the change of the surface electrostatic potential, this 2-D array of molecules allows tuning the surface potential, 1,4 and hence the optimization of the interfacial energy level alignment with regard to charge injection as well as extraction.…”

Self‐Assembled Quasi‐3D Nanocomposite: A Novel p‐Type Hole Transport Layer for High Performance Inverted Organic Solar Cells

“…In the field of organic solar cells (OSCs), an inverted structure, which discards both PEDOT:PSS and easily oxidized low work function (WF) cathodes, has been widely adopted to achieve prolonged stability . The most commonly used HTL in inverted OSCs, on top of organic active layer (hereafter named as p‐type top HTL), is thermally evaporated molybdenum oxide (MoO 3 ) MoO 3 , once mistakenly identified as p‐type semiconductor, is actually n‐type semiconductor. Alternatively, like vanadium oxide (V 2 O 5 ), tungsten oxide (WO 3 ), and copper iodide (CuI), have also been evaporated onto organic active layer to function as top HTL.…”

Electronic and Optical Excitations at the Pyridine/ZnO(101¯0) Hybrid Interface

“…[1][2][3] These properties together with a type-I level alignment at the hybrid interface would be ideally suited for light-emitting applications. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The adsorption of a molecular layer on inorganic surfaces has also been exploited to tune the work function of the inorganic component. [6][7][8][9][10] Prerequisite for that is the lightinduced creation of hybrid or charge-transfer excitons, which exhibit the hole to a large extent on one side of the interface while the excited electron would reside on the other side.…”