Fabrication and characterisation of hybrid photodiodes based on PCPDTBT–ZnO active layers
Graphical abstract
Introduction
Organic or hybrid photodiodes (HPDs) using metal oxide acceptor materials could have a number of advantages over the inorganic technologies, including potentially low costs, solution processability and flexibility, which could enable photodiodes to be placed onto non-flat surfaces [1], [2]. Previous reports of HPDs possess a reasonable on/off ratio of photocurrent during illumination when compared to the dark current (typically >2 orders of magnitudes) [3]. Another major advantage is that the absorption profile can be ‘tuned’ to absorb most visible wavelengths by altering the semiconductor inside the active layer within the active layer [4].
In recent years, Zinc Oxide (ZnO) has been widely studied due to its intrinsic properties suitable for optoelectronic applications such as in hybrid devices with organic semiconductors such as solar cells and photodiodes [5]. With a wide direct bandgap of ∼3.4 eV at room temperature, ZnO has been regarded as an excellent semiconductor material for UV detection and possesses an absorption profile that compliments many organic semiconductors. ZnO can be easily deposited at room- or relatively low temperatures to form thin layers by standard techniques such as sputtering [6], atomic-layer deposition [7] and pulsed-laser deposition [8]. The electron mobility is generally limited by surface roughness and carrier scatterings at grain boundaries; however, the electron mobility in ZnO has been demonstrated to reach up to 110 cm2/Vs, when using an elevated substrate temperature during the growth step [9]. In most cases where ZnO is deposited onto substrates at room temperature, the mobility is measured to be around 1–5 cm2/Vs [10]. This value still remains much higher than many organic materials which are used as the acceptor material in organic photodiodes. ZnO has also been used widely for the development of hybrid photovoltaics or photodiodes (HPDs). In photovoltaics, performances up to 0.11% have been reported for ‘planar’ devices, where the ZnO is deposited as a flat, uniform surface and up to 0.76% for devices made with ZnO nanowires, which create an interdigitated interface with the organic layer [11]. In addition, up to 2.0% has been reported for devices using ZnO nano-particles/crystals in a bulk-heterojunction configuration with an donor material such as Poly(3-hexylthiophene-2,5-diyl) (P3HT) [11]. Most work on hybrid devices has focused on photovoltaics and utilised organic semiconductors such as polyfluorene [12], P3HT [11] or polyaniline [13]. Whilst the performance as photovoltaics is low, the potential as photo-diodes or detectors has not been fully investigated.
In this paper, the fabrication, development and characterisation of ZnO HPDs is reported using the polymer Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT). Devices are shown to possess one of the lowest levels of dark current for a HPD, though the on–off ratio is limited due to the low photoresponse of the device. However, the main benefit of using these materials to make photodiodes is that multi spectral light sensing is possible from the UV through to the Near Infrared, encompassing wavelengths ∼350–870 nm. To our knowledge, this is one of the widest response ranges reported for a hybrid-photodiode. It is also one of the first reports of using a low band gap co-polymer for a hybrid device. It is shown that using PCPDTBT ensures wide photoresponse, and also enhances air stability when compared to HPDs manufactured using more commonly used materials such as P3HT.
Section snippets
Experimental
The structure of the OPD device is shown in the inset of Fig. 1(a). OPD devices were initially prepared in a clean room environment using an 80 nm thick indium tin oxide (ITO) coated glass substrates (Rs = 16 Ω/square) that were first cleaned using deionised water, acetone and isopropanol in an ultrasonic cleaner, then treated in a UV-ozone reactor. For this work, a bilayer structure was used which is the simplest device geometry for a HPD. In this device architecture, a layer of ZnO is first
Photodiode characterisation
Fig. 2 shows the dark current and performance under 100 mW/cm2 (AM1.5G) incident power for the fabricated PCPDPTBT:ZnO HPD. The solar cell performance under AM1.5G illumination is low, with a Power Conversion Efficiency (PCE) of 0.070%. This is to be expected owing to the planar interface, which usually leads to lower performances than devices based on ZnO nanocrystals or vertically aligned ZnO nanowires, because of the smaller interfacial area between the polymer and acceptor [11]. However,
Photodiode lifetime
Whilst there are many advantages to using organic or hybrid photodiodes, as listed in Section 1, it is important to research the operational lifetime of the photodiodes. In Fig. 4(a), the normalised photocurrent, measured at −1 V, of PCPDTBT:ZnO HPD is plotted as a function of operating hours. This device was non-encapsulated and stored in the dark in between measurements, which were conducted approximately every 24 h. The photocurrent is relatively stable for more than 200 h, but decreases
Conclusion
In this paper we have reported a hybrid photodiode made with structure of ITO–ZnO–PCPDTBT–MoO3–Ag. The benefit of this approach is that multi spectral light sensing is possible from the UV through to the Near Infrared, encompassing wavelengths 350–870 nm, which is one of the widest responses observed for an organic or hybrid photodiode. A dark current at 0 V bias of 1.71 × 10−2 mA/cm2 is observed, which leads to a low on–off ratio of ∼180 at 100 mW/cm2. Devices made with PCPDTBT show good air
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