Electrical properties of Al/PCBM:ZnO/p-Si heterojunction for photodiode application

doi:10.1016/j.jallcom.2020.154279

Journal of Alloys and Compounds

Volume 827, 25 June 2020, 154279

https://doi.org/10.1016/j.jallcom.2020.154279 Get rights and content

Highlights

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Electrical characteristics of Al/PCBM:ZnO/Si diode were investigated under the aim of photodiode application.
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PCBM:ZnO hybrid layer was proposed as an alternative thin film layer in the use of lower cost optoelectronic devices.
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PCBM:ZnO interlayer was deposited on p-Si using spin-coating technique.
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Dark and illuminated current, transient photocurrent, photocapacitance and photo-conductance measurements were performed.
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The deviations from ideality were discussed by means of distribution of interface states and series resistance.

Abstract

In this paper, the electrical characteristics of spin-coated PCBM:ZnO interlayered Al/PCBM:ZnO/Si diode are investigated under the aim of photodiode application. Under dark condition, the diode shows about four orders in magnitude rectification rate and diode illumination results in efficient rectification with increase in intensity. The analysis of current-voltage curve results a non-ideal diode characteristics according to the thermionic emission model due to the existence of parasitic resistances and interface states. The measured current-voltage values are used to extract the barrier height and ideality factor under dark and illumination conditions. Under illumination, photo-generated carriers contribute to the current flow and linear photo-conductivity behavior in photo-current measurements with illumination shows the possible use of hybrid PCBM:ZnO layer in Si-based photodiodes. In addition, change in the series and shunt resistance values under illumination is found to be effective in this light-sensing behavior of the diode. This characteristic is also observed from the typical on/off illumination switching behavior for the photodiodes in transient photo-current, photo-capacitance and photo-conductance measurements with the quick response to the illumination. The deviations from ideality are also discussed by means of distribution of interface states and series resistance depending on the applied frequency and bias voltage.

Introduction

Depending on increasing demand for renewable energy sources, optoelectronic devices, that can transform solar light into current, have been extensively investigated with the aim to further develop low cost and efficient technology with alternatives to traditional materials. In fact, most of these devices on the market are dominated by silicon (Si) and compound thin film based heterojunctions [1]. Similar to the process in semiconductor/semiconductor (p-n) junction photovoltaic devices, photodiodes with metal/semiconductor (Schottky) junction has a depletion region to separate photo-generated carriers with a high electric field and it has a potential to be used in solar light conversion to current [2,3]. This type of diodes is also a rectifier at forward biases whereas limit the carrier conduction at reverse bias region. Since these devices contain a semiconducting active layer to direct formation of a barrier between their surface and rectifying metal, insulator/dielectric/oxide materials have been deposited between these metal and semiconductor layer surfaces to improve capacitive response together with tailoring the current flow mechanisms and diode parameters to the field of interest in device applications [4,5]. The most efficient devices in this research field have been fabricated by chalcopyrite absorber layers and their earth-abundant alternatives with promising properties have been still ranked below the results of the reported studies for solar energy generation [[6], [7], [8]]. On the other hand, Si-based technology still a main source of active semiconductor layer in these optoelectronic applications. In the field of photodiodes, instead of inorganic materials, organic semiconductors have also triggered considerable interest in the use of solar power due to the promising direct detection of light in solar spectrum and potential advantages in the application of lightweight, flexible and cheap devices [[9], [10], [11]]. Alternative to these materials, organic materials have been focus of interest to improve light sensing properties together with the large-scale applicability of these type of devices [[12], [13], [14]]. Therefore, in the production of p-n and Schottky junctions, organic materials can offer alternative low-temperature layers having potentials for many applications of customary inorganic materials. They exhibit a potential in the development of optoelectronic applications, however, in the solar cell application, they cannot reach comparable efficiencies with the inorganics [15]. In addition to the response to the solar illumination, photo-stability of [6,6]-Phenyl C61-butyric acid methyl ester (PCBM) based layers have motivated researches to achieve drawbacks of the organic devices based on low electrical characteristics with instabilities in the diode [9,16,17]. From the economic viewpoint of the solar energy industry, these devices offer low cost and fast fabrication with low-temperature solution based techniques [18,19]. PCBM thin films behave as an electron conductor with high mobility, and they have favorable characteristics as high transparency and high flexibility in the device applications [20]. With several photodiode applications, PCBM is also used in perovskite solar cells as an electron transport layer and interfacial modifier [12,16,21]. In literature, based on Si which an abundant, nontoxic and well-known material, PCBM/p-Si heterojunction have been studied from ultra-violet to infrared photo-detection [9,17]. On the other hand, in this device application, material characteristics of this layer results in non-ideal rectifying behavior. The carrier flow through this layer limits the current-voltage characteristics under the effects of charge traps in PCBM and it is still a challenging problem in this technology [17,18]. Therefore, there are several attempts to improve the diode characteristics with interface engineering between PCBM and metal contact; and to enhance the photovoltaic response of the diodes with combination of organic and inorganic materials and new class of materials. These alternative low-temperature layers have been fabricated with inserting additional layers such as LiF, Ca, TiO_x and ZnO in the PCBM/contact interface; incorporation of inorganic nanoparticles such as PbS, ZnO and TiO₂ to the PCBM layer; and hybrid active layer of PCBM [9,15,[20], [21], [22], [23]]. Since organic materials have low mobility when compared to inorganic materials, these alternatives as a combination of organic and inorganic materials have been considered to be used in high effect devices without changing the possible advantages in the fabrication of organic materials [24]. Thus, there are several efforts that have been devoted to utilize the potential of PCBM layer with incorporation of ZnO [21,23]. Due to having direct wide band gap, expectable optical transparency and high exciton binding energy, n-type semiconductor zinc oxide - ZnO has been mostly preferred as an inorganic material for the photodiode and photodetector applications [12,14,25,26]. It is an attractive material as an interface layer in advanced optoelectronic applications with its superior electronic performance. This characteristic due to having a higher electron mobility has been adapted to several electronic and optoelectronic applications. Enhancement device characteristics, mostly power improvement in photodiodes and photovoltaics have been achieved with use of ZnO as an individual layer or incorporation into the interface [14,16,[27], [28], [29]]. Additionally, doping ZnO in a photoactive material has been also point of interest to improve the device performance of organic diodes and development of PCBM:ZnO hybrid layer can offer lower cost optoelectronic devices together with higher mobility characteristic. In this study, Al/PCBM:ZnO/p-Si/Al diode structure was fabricated by solution-processed ZnO incorporation to the PCBM layer. The effects of this interface layer on the electrical properties of the diode have been discussed in detail with the help of current-voltage ( $I - V$ ), capacitance-voltage ( $C - V$ ), conductance voltage ( $G - V$ ) measurements under dark and illuminated conditions.

Section snippets

Experimental details

PCBM doped ZnO diode was synthesized using PCBM organic compound that was supplied from Sigma-Aldrich Ltd. and used without further purification. For this purpose, firstly, PCBM solution was prepared by dissolving in 1–2 dichlorobenzene with a concentration of 25 g/L. Then, the dissolved materials were stirred at 60 °C for 3 h in a dry nitrogen environment. These solutions were stirred for 10 min at 500 rpm by using a magnetic stirrer. The stirring process was followed by the preparation of

Results and discussion

Forward and reverse bias $I - V$ characteristics of the Al/PCBM:ZnO/p-Si/Al diode structure is examined under dark and room temperature conditions, and also photo-response characteristics of the diode are investigated under different illumination intensities in the range of 20–100 mW/cm². As shown in Fig. 3, the diode shows a rectifying behavior in which current can flow in the forward bias region, however the diode blocks the reverse current transport. In the ideal case, the forward biased current

Conclusion

In this work, the electrical characteristics of spin-coated PCBM:ZnO film interlayered Al/PCBM:ZnO/Si diode are discussed in terms of $I - V$ , $C - V$ and $G - V$ measurements under room temperature and dark conditions. In addition, the illumination effects and light-response of the diode are investigated performing these measurements under different illumination intensities. The room temperature $I - V$ characteristics show a good rectifying behavior with about $Φ_{b}$ and $n$ of 0.76 eV and 8.53, respectively. The

CRediT authorship contribution statement

H.H. Gullu: Conceptualization, Writing - original draft, Writing - review & editing. D.E. Yildiz: Conceptualization, Writing - original draft. A. Kocyigit: Methodology, Writing - original draft. M. Yıldırım: Methodology, Writing - original draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work is supported by Selçuk University BAP office with the research Project Number 19401034.

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Electrical properties of Al/PCBM:ZnO/p-Si heterojunction for photodiode application

Highlights

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgment

Mater. Sci. Semicond. Process.

J. Mol. Struct.

Thin Solid Films

Synth. Met.

J. Alloys Compd.

Sci. Bull.

Mater. Sci. Eng. B

Sol. Energy Mater. Sol. Cells

J. Alloys Compd.

J. Alloys Compd.

J. Alloys Compd.

J. Phys. Condens. Matter

J. Alloys Compd.

Mater. Res. Bull.

Mater. Sci. Semicond. Process.

Curr. Appl. Phys.

Solid State Electron.

Appl. Surf. Sci.

Mater. Sci. Semicond. Process.

Appl. Surf. Sci.

Compos. B Eng.

Mater. Sci. Semicond. Process.

Sol. Energy Mater. Sol. Cell.

Thin Sold Films

Microelectron. Eng.

Bull. Mater. Sci.

Solid State Electron.

Solid State Electron.

Physica B

Microelectron. Eng.

Microelectron. Eng.

Prog. Photovoltaics Res. Appl.

Adv. Mater.

J. Phys. Condens. Matter

Chem. Mater.

J. Mater. Sci. Mater. Electron.

J. Appl. Phys.

Science

Sol. RRL