A Critical Review of Pyrite as a Photovoltaic Material

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1 A Critical Review of Pyrite as a Photovoltaic Material Divija Pandel* and Satender Dehiya** *PhD. (The Department of Metallurgical and Materials Engg.), MNIT Jaipur **M.Tech (The Department of Metallurgical & Materials Engg.), MNIT Jaipur Abstract. High production cost of solar cell materials is the major factor that limits the commercial application of photovoltaic cells. Today, the most widely used PV material is crystalline Silicon, but it suffers from the disadvantage of high material consumption and requires high-tech material processing. A relatively new material that has fetched attention in recent times is Iron Pyrite, which promises to be a potential photovoltaic material because of its strong light absorption (α>10 5 cm -1 for hν > ev), sufficient minority carrier diffusion length ( nm), and essential infinite elemental abundance. Although the band gap of Pyrite is narrow for optical photovoltaic application but it can be easily increased from ~1.0 to ev by replacing 10% of sulfur atoms with oxygen atoms. The resulting FeS 2 x O x (x 0.2) alloys are thermodynamically stable, and show better light absorption than Pyrite itself in the near-infrared region of the spectrum. Oxygen-alloyed Pyrite may therefore be one of the highly promising alternatives of Silicon, for fabrication of Pyrite solar- cells with larger V OC values, provided that adequate synthetic schemes that avoid the phase separation of iron oxides can be developed. Keywords: Photovoltaic cell, Iron Sulphide, Band Gap, Solar cell PACS: b Introduction Energy is the key driver of a nation s economy and one of the most important inputs for improving the quality of life. Primarily, it is the gift of the nature to the mankind in various forms. The consumption of this energy is directly proportional to the progress of mankind [1].Conventional energy sources based on oil, coal, and natural gas have contributed to the advancement of mankind so far, but at the same time, being non-renewable and leading to adverse climate change, they are also posing a threat to the very existence of life on the planet. Thus, it becomes imperative to explore the renewable energy resources, having enormous potential, so as to ensure sustainable growth and development for all. It is becoming clear that future growth in the energy sector would primarily be in the new regime of renewable, and to some extent in natural gas-based systems rather than in conventional oil and coal sources. Over the last two decades, vigorous efforts have been made to tap newer sources and techniques to produce green power for a variety of end user applications. It is a well-known fact that sun provides enough energy in one minute to cater to world s energy needs for complete one year. Hence, efficient and economically-viable solar energy appliances are the torch-bearers of the future energy market. Photovoltaic (PV) cells use the potential of sun for generating electrical power by converting 163

2 solar radiation into direct current with the use of semiconductors that exhibits photovoltaic effect. Though the market is currently replete with Silicon based PV cells, extensive research is underway in the PV industry so that new materials can be discovered and synthesized which in turn would contribute to boost up this industry in a better way. PYRITE: A PROMISING PHOTOVOLTAIC MATERIAL About 45 years ago, Pyrite (FeS 2 ) was proposed as a potential candidate for usage as photovoltaic absorber material for thin film solar cells, as it has a very high absorption coefficient and a suitable energy band gap (Eg 0.95 ev). Further its non-toxicity and its composition from abundant natural elements like Fe attributed its projection for a promising element in the solar cell industry. However, the progress was not up to the mark and the dream of Pyrite carving out a place for itself could not be realized properly. Though the quantum efficiencies and the photocurrents at Air Mass (AM) 1.5 were reasonably high for single crystalline Pyrite samples, but its open circuit voltage, which never exceeded about 200 mv at room temperature, and was much lower compared to the band gap of Pyrite was the major cause of concern. The highest efficiency reported so far is about 2.8 %atam1.5 [1]. Recently, a numerical simulation with the program PC-1D was performed by Altermatt et al. that used conservative measured parameters of Pyrite (Eg=0.8 ev, µe 10 cm2/vs). The results confirmed the previous estimate of Iron disulfide being a suitable photovoltaic material (see Figure 1a & 1b.). 164 In a diffusion cell configuration, efficiencies between % should be possible, provided that the cell is governed by Shockley-Read-Hall (SHR) recombination [1]. Pyrite photo-electrochemical and solidstate Schottky solar cells have been found to have large short-circuit current densities (30 42 ma cm 2 ) and quantum efficiencies (up to 90%) [2-3]. However, the open-circuit voltage (V OC ) of Pyrite cells is low [V OC 0.2 ev, which is less than 20% of the band gap (Eg) of 0.95 ev]. The cause of the 1(a) abnormally low V OC of Pyrite devices remains unclear and has been the subject of several recent investigations [4-7]. The band 1(b) gap of Pyrite is also somewhat narrow for optimal photovoltaic applications according to the Shockley Queisser theory [8].

3 FIGURE 1 - Numerical simulation results of thin film pyrite cells, based on realistic material parameters of pyrite. (a) Open circuit voltage as a function of the short circuit current density. Parameters are the pyrite thickness (0.05 to 1 µm) and the minority carrier lifetime (1 ps to 1 µs). (b) Efficiency of pyrite thin film solar cells as a function of the carrier lifetime [1]. To practically increase the band gap of Pyrite, it is essential to understand the nature of the electronic states at its valence band maximum (VBM) and conduction band minimum (CBM). Each Fe ion in Pyrite is coordinated to six S ions, and each S ion is located at the center of a tetrahedron consisting of another S atom and three Fe atoms. Substituting cations or anions with isovalent elements or compensated dimers is a widely used approach for modifying the band gaps of other semiconductors. A recent density functional theory (DFT) study by Sun and Ceder found that the band gap of Pyrite can be increased slightly by replacing some Os (substitution oxygen) to form Fe 1 x Os x S 2 compounds [9]. Os concentrations of 10% increases the band gap to around ev (depending on the exact spatial distribution of the O S centers) without producing electronic states within the band gap. The resulting FeS 2 x O x (x 0.2) alloys are thermodynamically stable, and show better light absorption than Pyrite itself in the near- IR region of the spectrum. Oxygen-alloyed Pyrite may therefore be promising for the fabrication of Pyrite solar cells with larger V OC values, provided that synthetic schemes 165 that avoid the phase separation of iron oxides can be developed [9]. Rational synthesis of pure phase, highly crystalline cubic FeS 2 nanocrystals (NCs) can be done by using a trioctylphosphine oxide (TOPO) assisted hotinjection method. The synthesized Pyrite NC films show excellent air stability for over a year. In contrast, obvious surface decomposition was observed on the surface of FeS 2 NCs synthesized without TOPO. A high carrier mobility of 80 cm 2 / (V s) and a strong photoconductivity were observed for the first time for Pyrite films at room temperature. Results indicate that TOPO passives both iron and sulfur atoms on FeS 2 NC surfaces, efficiently inhibiting surface decomposition [10]. A photo electrochemical solar cell (PEC) based on n-type FeS 2 with high quantum efficiency and high stability in presence of I- /I3-redox couple has also been reported. Figure 3 shows the photocurrent action spectrum across an n-fes 2 /electrolyte (5M KI) interface. The measured quantum efficiency at the maximum of spectral sensitivity exceeds 90%. This indicates the possibility of using this material for solar energy conversion [11].

4 FIGURE 2 - Quantum yield and spectral dependence of photo electrochemical cell using iodine/iodide electrolyte [11] Comparison of Pyrite with Other PV Materials Table 1.1 gives the comparison of Pyrite material with the other known absorbers. In this table the absorption length (1/α) of FeS 2 in comparison with other semiconductor materials is very low, it means very less amount of FeS 2 PV material is used in PV cell. Also its high absorption coefficient, quantum efficiency and low band gap contributes in it being the most suitable material for sensitization type solar cells [12]. TABLE Comparative data of FeS 2 related to few well known absorber materials [12-16 ] C-Si a- CuInSe 2 Si FeS 2 Energy gap (ev) FIGURE 3 - Comparison of absorption lengths L 1/a for different semiconductors materials [11] Here the absorption length shows the thickness of the material required for producing photovoltaic effect. Low absorption length depicts the less amount of PV material used for photovoltaic effect in PV cells. Figure 3 shows that among all the PV materials, FeS 2 material consumption is very low in comparison to the other PV materials. Also Pyrite being easily available and non-toxic proves to be a relatively cheaper photovoltaic material. Absorption co- efficient α (cm) -1 at 2 Ev Absorption length α - 1 (A ) Quantum Efficiency % Conclusion Though, Pyrite has some limitations in terms of efficiency, but with some modifications it can be a very handy and economically-viable option for the solar energy industry. Pyrite is a stoichiometric semiconductor with a homogeneity range far below the atom (%) range. Therefore, volume defects should not limit the photovoltaic properties of Pyrite, which is in agreement with the reported high quantum efficiencies. To successfully increase the band gap, Pyrite can be doped with oxygen atom, thereby forming a 166

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