Elsevier

Applied Surface Science

Volume 432, Part B, 28 February 2018, Pages 262-265
Applied Surface Science

Full Length Article
Towards maximizing the haze effect of electrodes for high efficiency hybrid tandem solar cell

https://doi.org/10.1016/j.apsusc.2017.07.204Get rights and content

Highlights

  • Current matching is required to maximize efficiency in tandem solar cells.

  • Haze effect helps in trapping the light within the active layer.

  • Light scattering due to rough electrode surface helped in current matching by balancing light absorption.

  • Simulation shows an improvement in ideal short-circuit current density by 7.6% due to haze effect.

  • Active layer thickness combination of 500 nm (a-Si:H) and 150 nm (OPV) exhibited the best optimized structure.

Abstract

In this study, we executed optical simulations to compute the optimum power conversion efficiency (PCE) of a-Si:H/organic photovoltaic (OPV) hybrid tandem solar cell. The maximum ideal short circuit current density (Jsc,max) of the tandem solar cell is initially obtained by optimizing the thickness of the active layer of the OPV subcell for varying thickness of the a-Si:H bottom subcell. To investigate the effect of Haze parameter on the ideal short-circuit current density (Jsc,ideal) of the solar cells, we have varied the haze ratio for the TCO electrode of the a-Si:H subcell in the tandem structure. The haze ratio was obtained for various root mean square (RMS) roughness of the TCO of the front cell. The effect of haze ratio on the Jsc,ideal on the tandem structured solar cell was studied, and the highest Jsc,ideal was obtained at a haze of 55.5% when the thickness of the OPV subcell was 150 nm and that of the a-Si:H subcell was 500 nm.

Introduction

With fossil fuels depleting at a faster rate due to increase in energy demand, one of the potential alternatives to the conventional energy is solar energy. Solar cell research has bloomed in the past few decades [1], [2]. There are three main categories into which solar cell technology can be divided under: Inorganic solar cells, organic solar cells, and hybrid solar cells. Among the three, hybrid solar cells use the high absorption coefficient of organic solar cells, which allows it to be thin and yet be able to absorb significant photon energy, and the good electron transport properties of inorganic materials [3], [4]. Although hybrid solar cells have shown promising efficiencies, it still lags behind other device structures. To utilize the advantages of both organic solar cells and inorganic solar cells without the draw backs of a hybrid solar cell, hybrid tandem solar cell structure with inorganic front subcell and organic back subcell was designed [5], [6]. In this study, we have used Hydrogenated amorphous silicon (a-Si:H) as the inorganic front subcell and poly[[2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[3′,3′′-dimethyl-2,2′:5′,2′′-terthiophene]-5,5′′-diyl] (PMDPP3T): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the back OPV subcell. PMDPP3T has shown to be a good polymer acceptor with complementary absorption spectrum to a-Si:H [7], [8].

Methods to improve the PCE of the tandem solar cells includes using intermediate charge transport layers, optimizing the tandem structure using optical modelling, and using novel materials [9], [10]. This study focused on increasing the absorption of light in each subcell’s active layer by maximizing the electric field due to light trapped inside them. We have implemented haze effect which can increased the diffused transmission through a layer due to light scattering off rough surfaces [11], [12]. Finite-difference-time-domain (FDTD) simulations can trace the pathway of light through a material for multiple frequencies at the same time [13]. This makes it extremely useful to simulate optimized structures that can trap in as much photon energy as possible. We used an optical solver software called Lumerical, FDTD solutions. It encompasses a technology computer aided designing (TCAD) module which helps to build the solar cell structure. It can also be integrated with MATrix LABoratory (MATLAB) for calculations.

Using the TCAD platform, we implemented triangle ridges in the front transparent conductive oxide (TCO) electrode. This caused light to get scattered and lead to diffused transmission. We have optimized the thickness of the active layers of the front subcell and that of the back subcell such that maximum light is absorbed in these layers. Complementary absorption of the two active layers is the key factor to increase the overall PCE of the solar cell. We have chosen a-Si:H as the front subcell and PMDPP3T:PCBM as the back OPV subcell due to their non-overlapping absorption spectrum. Using optical simulation, we optimized the ideal short-circuit current density extractable from the solar cell structure.

Section snippets

Simulated solar cell structure

The structure of the tandem solar cell is shown in Fig. 1. It is constructed in Lumerical, FDTD solutions using its TCAD platform. The structure was designed as Indium Tin Oxide (ITO)/a-Si:H (p-i-n)/ITO/Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/PMDPP3T:PCBM/Titanium dioxide (TiO2)/Aluminum (Al). The top ITO electrode, through which the light was illuminated, had a thickness of 100 nm. The inter-subcell ITO electrode had a thickness of 5 nm while the PEDOT:PSS layer was 5 

Results and discussion

Spectral haze was found to increase with surface roughness and was dominant in the short wavelength regions. Fig. 2 shows the variation of haze with respect to wavelength for different RMS surface roughness. The spectral haze is high over a wide spectrum for very high surface roughness. Optimizing the haze at wavelengths is useful for trapping light into the active layer of the solar cell to increase its PCE. Fig. 3 depicts the relationship between RMS roughness of the TCO and the haze at 550 nm

Conclusion

We inspected the improvement in Jsc,ideal of the hybrid tandem solar cell due to haze effect. We computed optical simulations on structures with and without rough TCO electrodes. It was examined that at a haze ratio of ∼55.5%, the Jsc,ideal improved from 7.25 mA/cm2 to 8.75 mA/cm2. This maximum short-circuit current density was obtained when the active layer thickness of the front a-Si:H subcell was 500 nm and that of the back OPV subcell was 150 nm. Due to haze effect there was an increase in PCE

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01057942) and by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program (No. 10063474) and has been conducted with the support of the Korea Institute of Industrial Technology as “Research Source Technique Project (KITECH EO170020/170045).

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