Photovoltaic absorber materials

Introduction to photovoltaic cells

Solar power is a renewable energy source with huge theoretical potential, making it one of the most attractive alternatives to fossil fuels in terms of sustainability, scalability, and environmental impact. Developing efficient, cost effective and eco-friendly solar cells is a critical goal in meeting the demands of an increasingly energy dependent global society. One way in which solar power can be harnessed is through the use of photovoltaic (PV) technologies, which convert light into electrical energy via the photovoltaic effect. In a PV cell, photons are absorbed by a semiconductor material, and their energy is imparted to valence band electrons to create electron-hole pairs. If the absorber material is placed in contact with a second semiconductor of opposite polarity, a pn junction is created, which when inserted into a circuit, allows current to flow.

The first layer of a PV cell incident photons encounter is the anti-reflective coating. This is comprised of a dielectric material of a particular thickness such that waves reflected from this layer destructively interfere with reflected waves from the semiconductor surfaces below. The next layer is the front electrode, which typically constitutes a transparent conducting oxide (TCO) such as doped In₂O₃, SnO₂, or ZnO. A discussion of TCOs and our research on them can be found here. The central heterojunction comprises an absorber layer and a window layer of opposite polarity. Typically, the window layer is n-type and the absorber layer is p-type, as the wide band gap conductive materials required for the window layer are only viable on a widespread scale as n-type materials. In a silicon-based PV device, the window and absorber materials consist of P-doped n- and B-doped p-type Si, respectively, while CdS typically serves as the window layer in CdTe and Cu(InGa)Se₂ (CIGS) PV cells. Behind the pn junction, the back contact is usually made of a metal such as Mo, while the final glass layer is the substrate on which the rest of the layers are deposited in thin film solar cells.

One of the most important properties of an absorber material is a band gap of appropriate magnitude. For semiconductors with larger band gaps, fewer photons will possess sufficient energy to excite electrons from the valence band to the conduction band. However, as any excess photon energy greater than the band gap magnitude is lost due to thermalisation, neither is it desirable to have a material with a very small band gap as the inherent energy of the electron-hole pairs is low. Therefore, there exists an optimum band gap of ~1.5 eV for a photovoltaic absorber. Ideally, this band gap should be direct so that transitions need not be phonon assisted. In addition, the absorber material should have good electronic conductivity and high optical absorption.

For decades, the solar cell industry has been dominated by silicon-based technologies, with solar cells based on single-crystal silicon materials having achieved energy conversion efficiencies of 25.6%. Due to silicon's fundamental band gap of 1.1 eV, its maximum potential efficiency is 30% according to the Shockley-Queisser limit. As current Si devices are approaching this limit, research on PV materials has diversified to develop alternative materials suited to solar cell application. In recent years, the thin film PV semiconductor industry has focussed on CIGS and CdTe, which with band gaps of 1.0-1.7 eV (depending on the Ga/In ratio) and 1.5 eV respectively, have both achieved efficiencies of 21.0%. However, the toxicity of Cd and the scarcity and expense of In and Te make these materials ill-suited to large-scale use. Hybrid halide perovskites have also garnered widespread attention, having achieved efficiencies of over 20% in just a few years, although there are significant challenges to overcome with regard to their long-term stability and the toxicity of Pb. There is huge interest in developing materials that contain abundant, non-toxic, low-cost elements, with one notable promising example being Cu₂ZnSn(S/Se)₄ (CZTSS), possessing band gaps in the range 1.0-1.6 eV and having a current efficiency of up to 12.6%.

Ternary copper chalcogenide materials

Ternary Cu-based chalcogenide materials have enjoyed much attention as candidate solar cell absorbers, although the vast majority of studies having centered on CIGS, the utility of which is limited by the scarcity and expense of In. In our research, post-DFT methods have been used to investigate the geometric and electronic structure in three Cu_xMCh_y series, namely CuMCh₂ (M = Sb, Bi; Ch = S, Se), Cu₃MCh₃ (M = Sb, Bi; Ch = S, Se), and Cu₃MCh₄ (M = V, Nb, Ta; Ch = S, Se, Te). The utility of these materials as potential PV absorbers has been investigated with respect to various properties, including band gap nature and magnitude, optical absorption, relative valence band position, and effective mass, which should be low for holes in the valence band to have high mobility and result in good p-type conductivity.

In the case of the Sb and Bi materials, the distorted geometric structures are rationalised in terms of the lone pairs on these ions, and the effect of this anisotropic density on the electronic properties of the systems is also considered. More information on the revised lone pair theory can be found here and in this Chemical Society Reviews tutorial review. For the two series in which all materials are isostructural, CuMCh₂ (M = Sb, Bi; Ch = S, Se) and Cu₃MCh₄ (M = V, Nb, Ta; Ch = S, Se, Te), a valence band maximum alignment is performed to assess the doping limits of the series. The valence band offsets and the lattice constants are also used to propose potential alloys of materials in the series to tune properties such as band gap. The wide range of structural and electronic properties calculated for these three ternary copper chalcogenide series evidences how selective combination of elements in ternary systems can significantly alter the behaviour of a material and can thus potentially be used to tune particular properties and target ideal values for specific applications. Of particular interest from our results are Cu₃BiSe₃, which has an optical band gap of 1.41 eV and a hole effective mass of 0.256 m^*_h but has yet to by synthesised experimentally, and a potential alloy of Cu₃NbTe₄ and Cu₃TaTe₄, which could allow optical band gap tuning between 1.46 eV and 1.69 eV.

         

Related references:

196. Kehoe A.B., Scanlon D.O. and Watson G.W.
     Modelling potential photovoltaic absorbers Cu₃MCh₄ (M = V, Nb, Ta; Ch = S, Se, Te) using density functional theory
     J. Phys. Condens. Matter 28, 175801 (2016)
     


192. Kehoe A.B., Scanlon D.O. and Watson G.W.
     The electronic structure of sulvanite structured semiconductors Cu₃MCh₄ (M = V, Nb, Ta; Ch = S, Se, Te): prospects for optoelectronic applications
     J. Mater. Chem. C, 3, 12236-12244 (2015)
     


173. Kehoe A.B., Temple D.J, Watson G.W and Scanlon D.O.
     Cu₃MCh₃ (M = Sb, Bi; Ch = S, Se) as candidate solar cell absorbers: insights from theory
     Phys. Chem. Chem. Phys. 15, 15477-15484 (2013)
     


161. Temple D.J, Kehoe A.B., Allen J.P., Watson G.W and Scanlon D.O.
     Geometry, electronic structure, and bonding in CuMCh₂ (M = Sb, Bi; Ch = S, Se): alternative solar cell absorber materials?
     J. Phys. Chem. C 116, 7334-7340 (2012)
     

Hybrid halide perovskites

Hybrid organic-inorganic perovskites have emerged as highly promising potential light-harvesting materials for next-generation solar cells. Recently, the lead based hybrid material methyl-ammonium lead iodide (CH₃NH₃PbI₃, or MAPI) has been shown to have very high energy conversion efficiency. Materials of this class have also been shown to have potential for applications in other areas including water splitting, light-emitting diodes, photo-detectors, and lasers.

One of the major challenges in the progress of optoelectronic devices based on hybrid organic-inorganic perovskites is to achieve a complete theoretical understanding of their electronic and optical properties. Methyl-ammonium lead halide (CH₃NH₃PbX₃; X = Cl, Br, I) perovskites have gained much attention for photovoltaic devices due to their long-term charge life, light absorption properties, and cost effective processing techniques. However, the instability of the materials and the toxicity of lead are substantial drawbacks. We are therefore currently investigating alternative hybrid perovskites through substitution of the cation and/or the halide ions in MAPI, such as CH₃NH₃GeX₃, to identify ones which might be lead free and stable and which might warrant further experimental investigation. The result of this study is expected to improve the present understanding of the potential role of these materials in solar cell applications.