Concepts of inorganic solid-state nanostructured solar cells

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Abstract

The development of inorganic solid-state nanostructured solar cells over the last years has been reviewed with respect to concepts and materials. Major attention has been paid to solar cells with extremely thin absorber, solar cells with ultra-thin nano-composite absorber and solar cells with quantum dot absorber layers. The focus has been set to structured transparent electron conductors and absorber materials prepared by mainly low-temperature and wet chemical deposition methods. The great potential of inorganic solid-state nanostructured solar cells is discussed.

Graphical Abstract

The development of inorganic solid-state nanostructured solar cells has been reviewed with respect to concepts and materials.

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Section snippets

Strategic potential of inorganic semiconductors in photovoltaics

High solar energy conversion efficiencies (η) have been achieved with thin film solar cells (19.9% for Cu(In,Ga)Se2 [1], 16.5% for CdTe [2]) and thin film photovoltaics (PV) have the highest cost reduction potential within the PV sector of solar energy conversion technologies. Among the three classes of materials commonly used for thin film absorbers in PV modules (amorphous and microcrystalline Si, CdTe, chalcopyrite family) production costs below 1 $/Wp were reached for the first time with

Principles of inorganic solid-state nanostructured solar cells

Most semiconductors of the highest strategic potential as well as semiconductors produced at low temperatures have very low values of Ldiff. The local absorber layer thickness has to be reduced to dlocal<Ldiff in order to minimize recombination losses in the solar cell absorber. However, usually only a small fraction of the sun light can be absorbed at very low values of dlocal since the optical absorption length (α−1) can be much larger than dlocal at a given wavelength (λ). As a consequence,

Nanostructures based on TiO2

Nanoporous layers of TiO2 are traditionally prepared by screen printing or by the so-called doctor blade technique [26]. The network of interconnected TiO2 nanoparticles is formed by subsequent sintering in air at temperatures at about 450 °C. Organic molecules are burned out and necks are formed between the TiO2 nanoparticles during the sintering process. However, the nanoporous structure characterized, for example, by the number of nearest neighbors of nanoparticles and by the preferential

Absorbers on structured TiO2 substrates

Numerous absorber materials have been tested on TiO2 substrates. Table 1 summarizes solar cell parameters obtained in selected works.

CdTe: The proof of concept of eta-SC is not trivial since adequate materials, interfaces and morphologies have to be combined. A first concept was based on CdTe absorbers (band gap of CdTe about 1.5 eV [54]) deposited electrochemically from an aqueous CdSO4 with TeO2 solution on mesoporous TiO2 [55]. In this case, open circuit voltages (VOC) of up to 0.67 V were

Tested absorber materials

a-Si:H: The first conformal coating of ZnO nanorods was reported for the deposition of hydrogenated amorphous silicon (a-Si:H) by capacitively coupled plasma-assisted chemical vapor deposition (CVD) [78]. The optical band gap of a-Si:H is about 1.7 eV [79] and well suitable for solar energy conversion. However, inorganic nanostructured solar cells were not realized with a-Si:H to our knowledge, probably due to the fact that ZnO substrates textured by plasma based processes are already excellent

Sensitization and charge separation with quantum dots

The exciton wave function can be confined in semiconductor nanocrystals of low dimension so that the energy levels of the electron and hole are shifted and the band gap increases (quantum confinement) [105]. Especially PbS and PbSe are of great interest due to their large electron and hole radius. Semiconductor nanocrystals of low dimension are also named quantum dots (QD). The tunability of the electronic states of QDs together with their solution based preparation make them very interesting

Conclusions

The concepts of eta-SC, solar cells with ultra-thin nanocomposite absorber and solar cells based on QD layers are very promising for the development of inorganic nanostructured solar cells. Energy conversion efficiencies between 2% and 5% have been demonstrated for different concepts of inorganic nanostructured solar cells. Besides homogeneous extremely thin absorbers with very low diffusion length layers of QDs are becoming very important as strong absorbers with effective thicknesses even

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