Edward
New
a,
Ian
Hancox
a,
Luke A.
Rochford
a,
Marc
Walker
b,
Chloe Argent
Dearden
a,
Chris F.
McConville
b and
Tim. S.
Jones
*a
aDepartment of Chemistry, University of Warwick, Coventry, CV4 7AL, UK. E-mail: t.s.jones@warwick.ac.uk
bDepartment of Physics, University of Warwick, Coventry, CV4 7AL, UK
First published on 6th October 2014
In this work, a thin ZnSe layer was deposited in a vacuum and then thermally annealed in air to provide an efficient electron extraction layer for an inverted organic photovoltaic (OPV) cell. Annealing the ZnSe film at 450 °C (ZnSe(450 °C)) increased the device performance and gave an efficiency of 2.83%. X-ray photoelectron spectroscopy (XPS) measurements show that the increased device performance upon annealing at 450 °C is due to the thermal conversion of ZnSe to ZnO. ZnO has a wider band gap than ZnSe, which allows for more light to reach the photoactive layer. The electronic structures of the treated ZnSe films were explored by ultraviolet photoemission spectroscopy (UPS) which showed that the ZnSe(450 °C) films had a Fermi level close to the conduction band edge, allowing for efficient electron extraction compared to the energetic barrier for extraction formed at the ZnSe(RT)/organic interface.
Hole extraction layers typically consist of either solution processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or high work function metal oxides such as MoOx, V2Ox, WOx, which can be deposited either by vacuum deposition or solution methods.5–11 For electron extraction, most small molecule OPV devices use an organic exciton blocking layer such as bathocuproine (BCP), which transports charge through metal defects caused by cathode deposition.12,13 Other electron extraction layers used include low work function metal oxides such as TiOx and ZnO, which have high stability and high electron mobility.14–18 These can be processed by many different methods including; solution from nanoparticles, sol–gel, electrodeposition, pulsed laser deposition and atomic layer deposition.19–24 Whilst these approaches are all capable of producing efficient electron extraction layers for OPVs, they each have drawbacks. Control and reproducibility to sub nanometre accuracy is hard to achieve with layers prepared from nanoparticles and sol–gels. Additionally, these solution processed methods require further optimisation for changes in laboratory temperature and humidity, which may also vary during a single day, as well as with substrate surface conditions. Pulsed laser deposition can suffer from small areas of uniformity and requires careful control of several parameters during growth. Thermal evaporation would be an alternative method to deposit such interfacial layers with sub nanometre accuracy, with film homogeneity across a wide area and without dependence on laboratory conditions. Thermal evaporation of interlayers would also allow reproducible scale-up of layer fabrication as well as the ability to deposit through shadow masks, for more complex modular structures. However, the high evaporation temperatures of both ZnO and TiOx prevent common use by vacuum deposition in conjunction with small molecule vacuum evaporated OPVs. Using other chalcogenide derivatives of Zn, such as ZnS and ZnSe which have lower evaporation temperatures, would allow for a wide band gap, low work function material to be thermally evaporated. By using the same process as the organic layers, a high degree of control over layer thickness and homogeneity can be obtained. Recent work has used vacuum deposition of ZnS as an electron extraction layer, although it requires the use of an Al2S3 dopant for application in OPV cells.25
In this paper we have investigated the deposition of thin ZnSe layers by thermal evaporation in vacuum. The layers were subsequently heat treated under ambient conditions at a range of temperatures before inclusion in inverted small molecule OPV cells. The variation of cell performance with heat treatment temperature was investigated in terms of layer stoichiometry, electronic structure and surface morphology. The favourable change in electronic structure with heating at 450 °C was due to the thermal conversion of the ZnSe interlayer to ZnO, resulting in a highly transparent, low work function electron extraction layer. This method of producing a thin, homogeneous ZnO layer led to improved efficiency for inverted device structures and can potentially be applied to other solar and organic electronic applications.
The current–voltage (J–V) characteristics of the OPV cells were measured under simulated AM1.5G solar illumination at 100 mW cm−2 from a Newport Oriel solar simulator using a Keithley 2400 sourcemeter for current detection.
The X-ray photoelectron spectroscopy (XPS) measurements were carried out following transfer through air and loading into a UHV system with a base pressure of ∼2 × 10−11 mbar. The sample was excited with X-rays from a monochromated Al kα source (hν = 1486.6 eV), with the photoelectrons being detected at normal emission using an Omicron Sphera 7 channel concentric hemispherical electron analyser. The combined energy resolution of the experiment was calculated to be 0.47 eV based on calibration scans with a polycrystalline Ag sample.
Ultra-violet photoemission spectroscopy (UPS) measurements were carried out using a custom multi-chamber ultra-high vacuum (UHV) system with a base pressure of ∼1 × 10−10 mbar. UPS spectra were recorded using a SPECS PHOIBOS 100 hemispherical electron energy analyser with excitation at 21.21 eV from a He I gas discharge source.
Atomic force microscopy (AFM) images were obtained from an Asylum Research MFP-3D (Santa Barbara, USA) in AC mode, using AC240TS cantilevers.
Fig. 1 (a) A schematic of the OPV cell design. (b) J–V characteristic plots for the different ZnSe annealing temperatures. |
ZnSe layer treatment | J SC (mA cm−2) | V OC (V) | FF | η (%) |
---|---|---|---|---|
a ZnSe(450 °C) cell after 2 min AM1.5G 100 mW cm−2 light soak.28,29 | ||||
ZnSe(RT) | 0.53 (±0.13) | 0.33 (±0.04) | 0.19 (±0.01) | 0.03 (±0.01) |
ZnSe(250 °C) | 3.22 (±0.23) | 0.44 (±0.04) | 0.27 (±0.01) | 0.39 (±0.07) |
ZnSe(350 °C) | 3.75 (±0.13) | 0.50 (±0.12) | 0.34 (±0.02) | 0.64 (±0.16) |
ZnSe(450 °C)a | 4.59 (±0.03) | 1.02 (±0.00) | 0.59 (±0.01) | 2.83 (±0.04) |
The transmittance spectra of 80 nm ZnSe(RT), ZnSe(250 °C), ZnSe(350 °C) and ZnSe(450 °C) films on quartz substrates are displayed in Fig. 2, with thicker layers used to obtain the bandgap (Eg) of each. There is a large difference in transmittance of the films, with those treated at room temperature to 350 °C absorbing strongly below 500 nm and more weakly in the red part of the spectrum, whereas the ZnSe(450 °C) film is highly transmissive in the visible range above 350 nm. This change in transmittance with thermal annealing of the ZnSe can also be seen in the inset photograph in Fig. 2, with the ZnSe(RT), ZnSe(250 °C) and ZnSe(350 °C) films displaying a green colour but the ZnSe(450 °C) film being highly transparent. The optical band gap of the two films can be estimated from the transmittance plot from the onset in absorption. This gives values of 2.4 eV for the ZnSe(RT) film and 3.1 eV for the ZnSe(450 °C) film, signifying a change in the film composition.
Fig. 2 Transmittance spectra of 80 nm ZnSe(RT), ZnSe(250 °C), ZnSe(350 °C) and ZnSe(450 °C) films on quartz substrates. Inset shows a photograph of the samples. |
The Se 3d region shown in Fig. 3 displays two features, one centred at 59.0 eV binding energy and the other at 54.0 eV. Each feature comprises two peaks due to the spin orbit splitting of the Se 3d level. At every treatment temperature the magnitude of the feature at 59.0 eV remains similar, with components at 59.6 eV and 58.7 eV. These peaks can be attributed to the Se 3d1/2 and 3d3/2 levels in SeO2.31 However, the contribution from the feature centred at 54.0 eV varies greatly with film treatment. The ZnSe(RT) and ZnSe(250 °C) layers show a large feature of similar dimensions. The peaks at 54.7 eV and 53.8 eV binding energy are due to the 3d1/2 and 3d3/2 splitting of the Se 3d attributed to ZnSe.31 For the ZnSe(350 °C) layers the magnitude of the feature is noticeably smaller, and for ZnSe(450 °C) layers the feature is almost negligible. This would suggest that the films treated at higher temperatures have significantly decreased ZnSe content.
In order to further investigate this change in stoichiometry on thermal processing, a spectrum of the O 1s region for each layer was obtained, and is shown in Fig. 4. The ZnSe(RT) and ZnSe(250 °C) layers show only two major components in the O 1s region, centred at 531.3 eV and 531.1 eV, which are due to contaminants and SeO2 respectively.31 A further smaller component is shown at ∼534.0 eV for each case due to water on the surface. For ZnSe(350 °C) layers, an additional peak is shown at 530.2 eV which is attributed to the presence of ZnO. For the ZnSe(450 °C) layers, the ZnO feature at 530.2 eV becomes dominant. Since the Se 3d region indicated only a small fraction of the ZnSe remained within the films treated at 450 °C, this would suggest that the ZnSe(450 °C) layer has been nearly fully converted to ZnO (we will continue to refer to the ZnO layer formed as ZnSe(450 °C) for consistency). Thermal conversion of ZnSe to ZnO has been shown by annealing in air at 400–600 °C.32 The reaction follows the equation:
The SeO2 is gaseous so is driven off, leaving behind ZnO on the substrate.
To quantify the relative stoichiometry after each treatment condition, the relative sensitivity factors (rsf) for each component were utilised. Table 2 displays the relative composition found for each environment from the XPS spectra. The percentage fraction of SeO2 is also shown. The ZnSe(RT) and ZnSe(250 °C) layers contain negligible ZnO, with ZnSe in a Zn:Se ratio of 0.86:1 and 0.93:1 respectively. This indicates that the ZnSe layers are slightly Se rich. Treatment at 350 °C results in a layer composed of both ZnSe and ZnO. The ratio of Se 3d (ZnSe):O 1s (ZnO) is shown to be 1.27:1. Therefore, nearly half of the ZnSe in the layer is converted to ZnO. The ZnSe(450 °C) layer only contains 0.6% ZnSe content from the Se 3d region. However, ZnO is present in a Zn:O ratio of 0.83:1. Thus, the heat treatment at this temperature produces an oxygen rich ZnO film.
ZnSe layer treatment | Se 3d (ZnSe 54.7, 53.8 eV) % | Se 3d (SeO2 59.6, 58.7 eV) % | O 1s (ZnO 530.2 eV) % | Zn 2p (Zn 1044.7, 1021.6 eV) % |
---|---|---|---|---|
ZnSe(RT) | 49.9 | 4.1 | 0.0 | 43.0 |
ZnSe(250 °C) | 48.2 | 3.5 | 0.0 | 44.0 |
ZnSe(350 °C) | 23.3 | 5.1 | 18.4 | 49.4 |
ZnSe(450 °C) | 0.6 | 2.7 | 51.5 | 42.7 |
These changes in layer composition with heat treatment help explain the widening of the bandgap, Eg, from 2.4 eV for ZnSe(RT) to 3.1 eV for ZnSe(450 °C). However, ZnSe is known to exhibit a much smaller Eg than ZnO when each material is deposited via other methods, hence the widening of the Eg on heat treatment due to conversion from ZnSe to ZnO is to be expected.3,33 The additional transparency of the wide band gap ZnSe(450 °C) film is beneficial for use as an electron extracting layer, allowing the increase in cell JSC shown in Table 1.
UPS measurements were obtained in order to probe the electronic structure of the interface between the ZnSe layers and the organic acceptor material. The secondary electron cut-off and valence band UPS spectra shown in Fig. 5 were used to determine the electronic properties of the ZnSe(RT) and ZnSe(450 °C) layers. Valence band spectra for ZnSe(250 °C) and ZnSe(350 °C) layers are shown in ESI Fig. S4.† In Fig. 5a, the secondary electron cut-off region indicates the work function of each sample. The work function of the ZnSe(RT) layer was found to be 3.6 eV, whilst ZnSe(450 °C) layers possess a work function of 3.4 eV. This indicates that both layers have a favourable work function for use as electron extracting materials.
A more significant difference between the treatments is expressed in the valence band (VB) UPS spectra, shown in Fig. 5b. The untreated ZnSe layer (black line) has a valence band onset 1.1 eV below the Fermi level, yielding an ionization potential (IP) of 4.7 eV. With the position of the VB determined, we can use the optical band gap measurement to predict the position of the CB of the material. Therefore, the band gap of 2.4 eV indicates the electron affinity (EA) of the ZnSe(RT) is 2.3 eV. Due to the positioning of the valence band in relatively close proximity to the Fermi level (EF) in ZnSe(RT), the layer will be unable to block holes from the HOMO of the organic layers. This would lead to detrimental quenching of excitons at this interface. More significantly, the CB of ZnSe(RT) is 1.3 eV above the EF of the layer. Therefore, a large energetic barrier to electron extraction from the organic LUMO is formed at the interface. This is shown with comparison to literature values for the electronic structure of C60 in Fig. 6a, indicating a barrier of 0.8 eV.40 This energetic barrier to electron extraction explains the poor electron extracting characteristics of the ZnSe(RT) layer, and consequently is the cause of the inefficient OPV cell performance shown in Fig. 1b.
Fig. 6 Schematic of hole extraction routes at the C60/electron extracting interface for ZnSe(RT) (a) and ZnSe(450 °C) (b) layers. Values for C60 electronic structure taken from measurements reported in literature.40 |
In stark contrast, the valence band onset of the layer treated at 450 °C (blue line) is 3.3 eV below the EF of the ZnSe(450 °C) layer (Fig. 5a). This indicates the material possesses an IP of 6.7 eV, forming a large energetic barrier to hole extraction at the interface with the organic acceptor. This favourable ability to block hole extraction is shown schematically in Fig. 6b, with an energetic barrier of ∼1.8 eV when compared to the literature values for the electronic structure of C60.40 The measurements also show a highly n-type character, with the CB of ZnSe(450 °C) close to the EF of the layer. This is unsurprising given previous reports of the electronic structure of ZnO films deposited via other methods.3,18 The position of the CB in ZnSe(450 °C) layers allows energetically favourable electron extraction with C60 (Fig. 6b), hence the excellent OPV performance shown for deposition on ZnSe(450 °C) layers in Fig. 1.
Fig. 7 2.0 μm AFM topographical images of (a) ITO/ZnSe(RT) and (b) ITO/ZnSe(450 °C), with the same height scale (±6 nm). |
Footnote |
† Electronic supplementary information (ESI) available: Additional cell data, UPS valence band spectra and XPS spectra. See DOI: 10.1039/c4ta04459b |
This journal is © The Royal Society of Chemistry 2014 |