Surface Modification of a ZnO Electron-Collecting Layer Using Atomic Layer Deposition to Fabricate High-Performing Inverted Organic Photovoltaics

Abstract

A ripple-structured ZnO film as the electron-collecting layer (ECL) of an inverted organic photovoltaic (OPV) was modified by atomic layer deposition (ALD) to add a ZnO thin layer. Depositing a thin ZnO layer by ALD on wet-chemically prepared ZnO significantly increased the short-circuit current (Jsc) of the OPV. The highest power conversion efficiency (PCE) of 7.96% with Jsc of 17.9 mA/cm2 was observed in the inverted OPV with a 2-nm-thick ALD-ZnO layer, which quenched electron-hole recombination at surface defects of ZnO ripples. Moreover, an ALD-ZnO layer thinner than 2 nm made the distribution of electrical conductivity on the ZnO surface more uniform, enhancing OPV performance. In contrast, a thicker ALD-ZnO layer (5 nm) made the two-dimensional distribution of electrical conductivity on the ZnO surface more heterogeneous, reducing the PCE. In addition, depositing an ALD-ZnO thin layer enhanced OPV stability and initial performance. We suggest that the ALD-ZnO layer thickness should be precisely controlled to fabricate high-performing OPVs.

Interface engineering for efficient fullerene-free organic solar cells

“…This was compensated by surface modification. 23,26,27 In this report, the wrinkled ZnO surface is modified by inserting an ultrathin electron conducting TP layer prior to depositing the active layer. This procedure leads to reduced bimolecular recombination at the electron extracting interface, which is evident from conducting and surface potential mapping measurements.…”

Development of Inverted Organic Photovoltaics with Anion doped ZnO as an Electron Transporting Layer

“…Another optical characteristic like photoluminescence (PL) spectroscopy has been widely used to determine the optical band structure, and also PL spectra in the high wavelength represents the status of the surface structure kinds of surface defects of ZnO thin film [22,25,26,30,31]. In conventional PL spectra of ZnO thin film, one can detect two major peaks, one is strong emission peaks in the UV region of 380 nm 400 nm which is related to the property of optical band gap.…”

“…Another peak in PL spectra shown in the region between 450 nm and 700 nm is giving information about surface defect such as oxygen vacancy and/or point defects of ZnO thin film [22,25,26,30,31]. It is already well known that surface oxygen vacancy of metal oxide film is quite important for the high electrical conductivity of the film.…”

“…The inverted organic photovoltaics (I-OPVs) are using organic polymer materials as photoactive layer, which are cost-effective, portable, and applicable to the flexible substrates [2][3][4][5][6][7][8]. Generally, in I-OPVs, ntype materials such as zinc oxide (ZnO) [9,30,31], titanium dioxide (TiO 2 ) [10,30], and cesium carbonate (Cs 2 CO 3 ) [11,12] have been widely used as hole blocking layer (HBL) [1,13] and electron transporting layer (ETL) [4,[9][10][11][12][13][14]. Among them, ZnO is an intrinsic n-type semiconductor, which has high optical transparency, wide band gap energy (3.2 eV), large exciton binding energy (60 meV), high electron mobility, and conductivity.…”

“…These anion elements can substitute the oxygen atoms in ZnO lattice [20,24,28]. Generally, the oxygen atoms as vacancies can act as an electron-hole recombination sites in I-OPVs [10,30,31], and results in decrease the performance of I-OPV. Among the various methods to reducing the recombination sites of ZnO ETL, anion doping can be one of the ideal concepts, because the atonic radius of group VII anion is almost similar to the oxygen atom as well as their excess electrons can help to achieving the high mobility [23,26] and conductivity [21,24,26] while maintaining high optical transparency [20,21,23,27], compared to group III cation metallic elements.…”