Tuning of undoped ZnO thin film via plasma enhanced atomic layer deposition and its application for an inverted polymer solar cell

Abstract

We studied the tuning of structural and optical properties of ZnO thin film and its correlation to the efficiency of inverted solar cell using plasma-enhanced atomic layer deposition (PEALD). The sequential injection of DEZn and O2 plasma was employed for the plasma-enhanced atomic layer deposition of ZnO thin film. As the growth temperature of ZnO film was increased from 100 °C to 300 °C, the crystallinity of ZnO film was improved from amorphous to highly ordered (002) direction ploy-crystal due to self crystallization. Increasing oxygen plasma time in PEALD process also introduces growing of hexagonal wurtzite phase of ZnO nanocrystal. Excess of oxygen plasma time induces enhanced deep level emission band (500 ∼ 700 nm) in photoluminescence due to Zn vacancies and other defects. The evolution of structural and optical properties of PEALD ZnO films also involves in change of electrical conductivity by 3 orders of magnitude. The highly tunable PEALD ZnO thin films were employed as the electron conductive layers in inverted polymer solar cells. Our study indicates that both structural and optical properties rather than electrical conductivities of ZnO films play more important role for the effective charge collection in photovoltaic device operation. The ability to tune the materials properties of undoped ZnO films via PEALD should extend their functionality over the wide range of advanced electronic applications

Tuning of material properties of ZnO thin films grown by plasma-enhanced atomic layer deposition at room temperature

“…The resulting GPC is $1.5 Å / cycle, which is comparable to reported values for the GPC of PE-ALD ZnO films using DEZ and O 2 -plasma at the lowest temperatures investigated [28][29][30][31][32] [($2.0 Å /cycle at 100 C, 28 $1.7 Å /cycle at 100 C, 29 $2.5 Å /cycle at 100 C, 30 $1.5 Å / cycle at 100 C, 31 and $1.9 Å /cycle at 75 C (Ref.…”

“…Several groups have been investigating the material properties and growth behavior of ZnO films grown by PE-ALD adopting DEZ as the metal precursor, and oxygen or waterfed plasmas as oxidative coreactants. 1,[28][29][30][31][32][33] A wide range of GPC values has been obtained, spreading from 1.5 to 3 Å / cycle at substrate temperatures of 75-300 C. The GPC values are clues for different growth mechanisms which cannot be associated with pure ALD growth. It has been reported that the reason behind the variation of the ZnO growth characteristics could be attributed to precursor decomposition effects at substrate temperatures above 60 C, leading to nonself-limiting growth.…”

“…In the literature, tuning of structural, optical, and electrical properties has been achieved by modifying process parameters such as substrate temperature, 29,30 O 2 -plasma dose, 30 or introducing additional process steps. 37,38 In this study, we investigated the influence of RF-power on the structural and optical properties of the ZnO films and demonstrate a simple approach to tune the crystallite size, refractive index, extinction coefficient, and bandgap for PE-ALD at room temperature.…”

“…36 On glass substrates, polycrystalline films without preferential orientation have been reported from PE-ALD (using DEZ and O 2 -plasma) at 100 C, 29 while depositions on ITO/ glass substrates yielded rather amorphous films. 30 XPS was used to investigate how the process parameters influence the chemical composition. Two different conditions were investigated.…”

Forming-free resistive switching of tunable ZnO films grown by atomic layer deposition

“…The latter is in agreement with the trend observed in Figure 5f as the concentration of the hydrogen related O-H bonds decreases with the increasing plasma time. It is reported that both hydrogen and hydroxyl in ZnO behave as shallow donors and can lead to higher carrier concentration [22,23]. The reduction of the O-H bond concentration also leads to the decrease of the carrier concentrations presented in Figure 6.…”