Electrical and photovoltaic properties of photosensitised ITO/a-Si:H p–i–n/TPyP/Au cells

https://doi.org/10.1016/S0927-0248(99)00127-0Get rights and content

Abstract

The a-Si:H p–i–n photovoltaic cells photosensitized with an organic layer of 5,10,15,20-tetra (4-pyrydil) 21H,23H porphine (TPyP) have been prepared. The action spectrum of these cells was extended by approximately 30 nm to longer wavelengths, with respect to non-sensitized cells. The presence of the organic layer gives some modifications in the electrical and photovoltaic behavior of the cells, that is why an analysis of dark current–voltage characteristics of the cells at room temperature, is also presented.

Introduction

Hydrogenated amorphous silicon, a-Si:H, is of increasing technological interest for applications such as in photovoltaic devices [1], [2], image sensors [3] and xerographic photoreceptors [4]. The technological interest arises from two basic considerations: first, this material can be readily fabricated into large area, low-cost structures, and second the electrical and mechanical properties of a-Si:H are well suited for applications which require high radiation sensitivities and long device lifetimes. However, as regarding the applications in the photovoltaic systems, a fundamental limitation of a-Si:H is that the band gap is large (approximately 1.70 eV). As a result, a-Si:H shows little photoconductivity in the near-infrared region of the spectrum, which is particularly important in the case of the solar radiation. The a-Si:H shows a photoconductive response very similar to the absorption spectrum, having also a photoconducting absorption edge [5]. Since the long-wavelength absorption edge is believed to be determined by structural disorder there have been attempts to extend the absorption by controlling the degree of disorder, and an extension of the wavelength edge of the action spectrum to about 740–760 nm was already obtained in such a manner. On the other hand, there were attempts to obtain a spectral sensitization of a-Si:H by combining it with other elements such as Ge [6] or Sn [7] to form alloys with smaller band gaps. But also these materials have some fundamental limitations such as very low photogeneration efficiencies as compared to a-Si:H and increased dark conductivity. An efficient method of spectral sensitization of the action spectrum was indicated by Borsenberger [8]. He proposed to use the organic dyes or pigments as spectral sensitizers. By this technique the long-wavelength edge of the photoconductive action spectrum was extended by approximately 100 nm into near-infrared using two organic layers: bromoindium phthalocyanine (as spectral sensitizer) and 1,1-bis(4-di-p-tolylaminophenyl)-cyclohexane (as chemical sensitizer).

Despite the interest in extending the action-spectrum of a-Si:H based solar cells to longer wavelengths, there have been no reported attempts to spectrally sensitize these cells by the Borsenberger's technique. This paper describes the results of such an investigation. The aim of the study was to investigate the spectral sensitization of the a-Si:H p–i–n solar cells, using as spectral sensitizer an organic layer of 5,10,15,20-tetra (4-pyrydil) 21H,23H porphine (TPyP). The structural formula of TPyP molecule is shown in Fig. 1. TPyP is known as an n-type organic semiconductor [9] and has been already used as cosensitizer in the two layer and three-layered organic photovoltaic cells [10], [11], [17]. We have been able to prove that TPyP acts as spectral sensitizer for the a-Si:H p-i-n solar cells, giving some gains for the typical cell parameters, but its presence modifies the electrical behavior of the cell. It is known that an organic semiconductor is, generally, characterized by a high resistivity and low mobilities of the charge carriers; these properties of the organic layer result in an increased series resistance Rs, of the cell. Therefore, a simple analysis of the current-voltage characteristics (IU) is also described in this paper, to elucidate the conduction mechanism and to extract the cell parameters.

Section snippets

Sample preparation and experimental procedures

The a-Si:H p–i–n structure has been deposited through RF; Glow-Discharge decomposition of SiH4, onto the indium tin oxide (ITO) coated glass providing the transparent conducting substrate. The p-layer is a 10 nm a-SiN:H window layer doped with Boron [12]. The i-layer is 0.6 μm thick, while the Phosphorous doped n-layer is about 0.15 μm thick.

An organic layer of TPyP, about 0.1 μm thick, was deposited on top of the p–i–n structure by vacuum evaporation at a pressure of 10−5 Torr. The Au electrode of

The electrical characteristics

The dark current-voltage (IdU) characteristics of the ITO/a-Si:H p–i–n/TPyP/Au is shown in Fig. 2. The forward bias direction corresponds to positive voltage on the ITO electrode.

The characteristic is asymmetric having very high rectification ratio (Rr=104, representing the ratio of forward bias current to reversal bias current, at the same voltage of 0.6 V). The behavior of the mentioned structure is due to the energy barrier at the p–i interface, both ITO and Au electrodes giving ohmic

Conclusions

The spectral sensitization of an a-Si:H solar cell using an organic layer was obtained. The action spectrum was extended by 30 nm to longer wavelength range, using a 100 nm thick layer of TPyP. The sensitization is explained by an exciton dissociation process to the TPyP/a-Si:H interface, which give rise to a higher quantum efficiency at longer wavelengths.

The presence of the organic layer induces some modifications in the electrical and photovoltaic behavior of the cell. That is why a model to

References (17)

  • Y. Hamakawa

    J. Non-Cryst. Solids

    (1983)
  • D.E. Carlson

    Sol. Energy Mater.

    (1980)
  • I. Shimizu

    J. Non-Cryst. Solids

    (1985)
  • H. Kakinuma et al.

    Jpn. J. Appl. Phys.

    (1983)
  • A.M. Barnett et al.

    IEEE Trans. Electron Devices

    (1980)
  • A. Matsuda et al.

    Jpn. J. Appl. Phys.

    (1986)
  • A. Morimoto et al.

    Jpn. J. Appl. Phys.

    (1985)
  • P. Borsenberger

    J. Appl. Phys.

    (1987)
There are more references available in the full text version of this article.

Cited by (16)

  • Thermal annealing induced changes in optical spectroscopic properties of tetrapyridyl-porphyrin thin films for energy applications

    2021, Optical Materials
    Citation Excerpt :

    The free base 5,10,15,20-tetra(pyridyl)-21H, 23H-porphyrin, (TPyP), moiety has a planner structure is containing four meso-pyridyl groups interconnected to meso-carbons of porphyrin macrocycle ring [21], which was utilized in the present work (see Scheme 1). Numerous researches about the optimizations and modifications of novel TPyP derivatives and their characterizations have been achieved towards an effective and flexible possibility advanced applications [22–30]. Gross et al. [22] reported a comprehensive study for the crystallographic of TPyP for the first time, while Yoon et al. [23] investigated a single crystal of TPyP nanotubes prepared from the vapour phase.

  • A critical review of photovoltaic cells based on organic monomeric and polymeric thin film heterojunctions

    2017, Thin Solid Films
    Citation Excerpt :

    This behavior can be explained by taking into account a model based on the combined effects of traps and recombination centers [22,31,32,34]. The low fill factors of this structure are comparable to other organic photovoltaic cells [22,31,33,93,94], but generally larger than of single- and two-layer organic cells. These low FF values arise from two separate effects: the high series resistances and the field-dependent nature of the photogeneration of charge carriers [29,35].

  • Enhanced performance of thin-film amorphous silicon solar cells with a top film of 2.85nm silicon nanoparticles

    2016, Solar Energy
    Citation Excerpt :

    The maximum efficiency of a single-junction solar cell is constrained by the Shockley–Queisser limit (Shockley and Queisser, 1961). There are several loss mechanisms that limit the maximum efficiency of the solar cells such as sub-bandgap-energy photon loss (Antohe et al., 2000; Glunz et al., 2010), thermalization of charge carriers due to the absorption of photons higher than the band gap of solar cell etc. Due to these fundamental losses, an efficiency limit of approximately 30% has been set for a material with a bandgap of 1.1–1.3 eV, under non-concentrated AM1.5G illumination.

View all citing articles on Scopus
View full text