Mots-clés:

p-type semiconductors, MOCVD, transparent electronics, photovoltaics, thin films, devices

Abstract

The breakthrough of transparent electronics is hindered by the lack of p-type semiconducting oxides with adequate optical transparency and electrical conductivity, limiting the conception of see-through optoelectronic devices as well as transparent photovoltaic devices. In this framework, copper chromium oxide (CuCrO2) arises as promising candidate thanks to its optical, electrical and manufacturing properties. Moreover, it is well known that the composition of the film plays a major role in its properties, and that defects generated by the out of stoichiometry can act as dopants. This work was focused on the synthesis of out of stoichiometry CuCrO2 thin films by atmospheric pressure CVD and their integration into see-through optoelectronic devices. The effect of the deposition temperature was investigated to maximize the electrical conductivity and the transparency of the thin films. The stoichiometry of CuCrO2 films was tuned to further enhance the optoelectronic properties of the material. As highlighted result, we report the synthesis of single phase Cu-rich CuCrO2, characterized by low resistivity, around 0.1 Ω.cm, and wide energy gap of 3.15 eV. The synthesis of an oxygen-poor environment has been demonstrated to impede the formation of CuO, detrimental to the properties of the films. This allows the deposition of films composed of Cu2O and CuCrO2. These nanocomposite films are characterized by carrier mobility of 0.65 cm² V-1 s-1, higher than for the Cu-rich CuCrO2 films, resulting in the lowest resistivity value of 0.02 Ω.cm of this study.

Out of stoichiometry CuCrO2 thin films have been successfully integrated as hole transport layer (HTL) in PBDD4T-2F:PC70BM Organic Solar Cells with architecture glass/ITO(anode)/HTL (PEDOT:PSS or CuCrO2.)/ Organic active layer/ETL (LiF)/cathode (Al). The effect of the stoichiometry of the CuCrO2 films, varied between 40% and 100%, on the Power Conversion Efficiency (PCE) was studied. We obtain an increasing PCE for Cu-rich CuCrO2 as HTL. The optimal cationic ratio was achieved for Cu/(Cu+Cr)= 65%, due to the best trade-off between optical and electrical properties, with a corresponding PCE of 3.1%. The stability of these devices in atmospheric conditions was studied and compared to the one of PEDOT:PSS based-OSC. The integration of oxides as HTL improved the stability in atmospheric conditions of the device, with a lifetime for oxides-based solar cells double when compared to polymer-based photovoltaic devices.

The reusability of the functionalized substrates, glass/ITO/CuCrO2, was tested by the elimination of the top stack Aluminum/ETL/Active layer through a simple chemical method. This was followed by the assembly of new cells above the substrate/ITO/ CuCrO2. The properties of the HTL as well as the performances of the devices based on the recycled HTL were unaltered. This will help the development of new performant and sustainable organic photovoltaic devices.

Additionally, out of stoichiometry CuCrO2 thin films were implemented in combination with ZnO in a fully oxides-based transparent planar p-n junction. This transparent diode was entirely synthesized by chemical deposition techniques at low temperature and atmospheric pressure, with no required post-deposition treatment. The device architecture was optimized through the tuning of deposition parameters for the p and the n-type oxides. The optimized diode, corresponding to a nanocomposite Cu2O+CuCrO2 coupled with polycrystalline ZnO, shows an extremely high rectifying behaviour, Ion/Ioff (±2.5V) around 100000, the highest ever reported, with a transmittance in the visible around 75%. The fitting of the diode characteristic led to a turn-on voltage of 1.3 V, an ideality factor of 3.15, a shunt resistance of 640 kohm.cm-2, and a series resistance of 340 kohm.cm-2. The extremely high performances and the manufacturing advantages of these devices are extremely appealing for the transparent electronic industry.