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Photoelectrical conversion using pn junctions based on transparent

semiconductor oxides

Auteur : Digregorio, Gabriel

Promoteur(s) : Nguyen, Ngoc Duy; Vanderheyden, Benoit

Faculté : Faculté des Sciences appliquées

Diplôme : Master en ingénieur civil physicien, à finalité approfondie

Année académique : 2017-2018

URI/URL : http://hdl.handle.net/2268.2/4682

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Photoelectrical conversion using PNjunctions based on transparentsemiconductor oxidesAuthor: G. Digregorio ————Advisors: N. D. Nguyen; B. Vanderheyden

Master Thesis presented in partial fulfilment of the requirementsfor the master degree in physics engineering

Academic Year: 2017-2018

T his master thesis focuses on thin film solarcells based on Transparent Conductor Oxide(TCO) that are able to generate an electric

current while maintaining their transparency tovisible light for photovoltaic and UV detectors ap-plications. The studied heterostructure is based onNickel Oxide (NiO), which is a new interesting ma-terial that exhibit an p-type behaviour, and Zinc Ox-ide (ZnO), a well-known n-type material subjectedto many scientific researches in many fields. More-over, dip-coating deposition technique is used inthis master thesis to synthesised the heterojunc-tion. This fabrication process is a soft chemicaldeposition technique where sol-gel solutions areused as precursors for the deposition. All the depo-sitions are performed on a transparent glass sub-strate coated by a Fluorine Tin Oxide (FTO) thinfilm that plays the role of an ohmic back contact.Gold pads are deposited by sputtering on the topof the NiO (p-type side) to form the top contacts.The resulting heterostructure is characterised bydifferent methods and the obtained I-V curves arecompared to a simple analytical model.

1 Heterostructure design and fabrication

A sol-gel solution for each metal oxide (NiO and ZnO)has been synthesised for dip-coating. Each solution istaken from the literature and someminor modificationshave been brought to the recipe in order to stabilisethe colloidal solution. Furthermore, the dip-coatingprocess has been optimised several times to reach goodquality thin films. A cross sectional SEM image of thefinal heterostructure ZnO/NiO is given in Figure 1 andreveals a good structural layering of the different TCOs.

During the fabrication process, the thickness ofeach layer and their morphology have been studiedfor different viscosity of the solutions, differentdip-coating withdrawal speeds and different substrates(Glass and FTO). XRD measurements were performed

on different samples to identify the crystallinity. Thetransparency and the resulting energy band gap werededuced from transmittance measurements. AFMmeasurements and SEM images were performed foreach TCO in order to characterise their texture andmorphology.

2 Characterisation and analyticalmodelling

Finally, an analytical model of a PN heterojunctionis developed. Under some assumptions, the modelcan be simplified into Equation 1 of an homojunction,where the complex behaviour of the real junction istaken into account by different factors: Rs and Rsh

[Ω] respectively the series and shunt resistance and n[-] the ideality factor.

J = Jth

(exp

[q(Va − IRs)

nkbT

]− 1

)+

Va − IRs

Rsh, (1)

where J [A/m2] is the current density, Jth [A/m2] thethermionic current density, q the electronic charge [C],Va [V] the applied bias voltage, I [A] the measuredcurrent, kb [m2 kg s−2 K−1] the Boltzmann constantand T [K] the temperature.

From the experimental data, Rs Rsh and n are ex-tracted in order to understand whether or not the sim-ple model can be used to describe the complex het-erostructure. Moreover the temperature dependencyof the junction, shown in Fig.2, is analysed through thismodel, and the influence of UV light on the junction isfinally briefly studied where a weak photo-current isobserved.

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Figure 1: SEM image (cross-section) of theheterostructure.

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5

0

500

1000

1500

2000

2500

3000

3500

4000

ID: F, at 48°C |Ratio3V

| =11.9481

Opening voltage =3.7257 V

ID: F, at 67°C |Ratio3V

| =14.7448

Opening voltage =2.8064 V

ID: F, at 84°C |Ratio3V

| =18.1994

Opening voltage =2.6705 V

ID: F, at 105°C |Ratio3V

| =21.7001

Opening voltage =2.6791 V

ID: F, at 128°C |Ratio3V

| =36.5812

Opening voltage =2.0254 V

ID: F, at 147°C |Ratio3V

| =42.1296

Opening voltage =1.6566 V

Figure 2: Temperature dependency of the designed PN hetero-junction and its rectifying behaviour.

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