Uwe Rau

Electronic properties of CuGaSe2-based heterojunction solar cells. Part I. Transport analysis

This article presents a systematic study on the electronic transport mechanisms of CuGaSe2-based thin film solar cells. A variety of samples with different types of stoichiometry deviations, substrates and buffer layers is investigated. We propose two transport models, namely tunneling enhanced volume recombination and tunneling enhanced interface recombination, which allow to explain the observed features for all devices under consideration. The doping level of the absorber layer turns out to be the most decisive parameter for the electronic loss mechanism. The doping is influenced by the type of stoichiometry deviation as well as by the Na content of the substrate. High doping levels result in tunnel assisted recombination. The best solar cells display the lowest tunneling rates. For these devices treatments of the absorber surface by air-annealing and/or the deposition temperature of the CdS buffer layer are decisive for the final device performance. We use the investigation of the open-circuit voltage...

Electronic properties of ZnO/CdS/Cu(In, Ga)Se2 solar cells : aspects of heterojunction formation

Abstract This contribution briefly reviews the present status of our knowledge on the dominant recombination mechanisms in ZnO/CdS/Cu(In,Ga)Se 2 heterojunction devices. Then we discuss the question how the formation and the electronic structure of the heterointerface and the heterojunction partners can influence the electronic properties and even the amount of recombination in the bulk of the absorber material. We examine the role of Cd- and Cu- diffusion during junction formation and air-annealing of the completed device. Furthermore, we explain the role of the intrinsic ZnO layer in the heterostructure routinely used together with a chemical bath deposited CdS layer to produce high efficiency heterojunction solar cells. We propose that these layers prevent electrical inhomogeneities from dominating the open circuit voltage of the entire device. A simple parallel connection model of two diodes demonstrates that 0.1–5% of the cell area with inferior electronic quality could degrade the photovoltaic performance without the buffer layer and have almost no consequences with buffer layer.

A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se2 chalcopyrite semiconductors

High efficiencies in Cu(In,Ga)(S,Se)2 solar cells result from alloying CuInSe2 base material with the corresponding Ga- or S-containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu-depleted absorber surface is essential. We consider this Cu/In grading non-intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.

Interdependence of absorber composition and recombination mechanism in Cu(In,Ga)(Se,S)2 heterojunction solar cells

Temperature-dependent current-voltage measurements are used to determine the dominant recombination path in thin-film heterojunction solar cells based on a variety of Cu(In,Ga)(Se,S)2 alloys. The activation energy of recombination follows the band gap energy of the respective Cu(In,Ga)(Se,S)2 alloy as long as the films are grown with a Cu-poor final composition. Thus, electronic loss in these devices is dominated by bulk recombination. In contrast, all devices based on absorber alloys with a Cu-rich composition prior to heterojunction formation are dominated by recombination at the heterointerface, with activation energies smaller than the band gap energy of the absorber material. These activation energies are independent from the S/Se ratio but increase with increasing Ga/In ratio.

Influence of sodium on the growth of polycrystalline Cu(In,Ga)Se2 thin films

We investigate the influence of Na on the growth of Cu(In,Ga)Se2 thin-films by three model experiments. First, we examine the influence of Na on the Se activity during selenisation of Mo films on soda-lime and borosilicate glass after growth and after thermal treatments. Second, we analyse the location and possible binding partners of Na in polycrystalline Cu(In,Ga)Se2 prior to and after air-exposure. The final experiment focuses on the identification of the chemical state of Na on the surface of as grown and air-exposed films. Our experiments demonstrate that Na influences the growth of CIGS Cu(In,Ga)Se2 due to its interaction with Se. In non-air-exposed films Na is mainly localised in the form of sodium-polyselenides (Na2Sex) at the grain boundaries. We conclude that Na2Sex acts as Se-reservoir during film formation and oxidation.

Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga)Se2 films and devices

Post-deposition air-annealing effects of Cu(In,Ga)Se2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 °C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells shows that the Cu(In,Ga)Se2 surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 °C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent wi...

Model for electronic transport in Cu(In,Ga)Se2 solar cells

Temperature-dependent measurements of the current–voltage characteristics and of the junction admittance of ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells are presented, together with numerical modelling of these experimental results. We explain the cross-over between dark and illuminated current–voltage characteristics currently observed for this type of device by the impact of the defect chalcopyrite layer at the surface of the Cu(In,Ga)Se2 absorber. Our model assumes an illumination-dependent voltage drop across a defect layer with a thickness of 15 nm to explain the cross-over. The voltage drop results from the electrical dipole made up of donor-like states at the interface between the defect layer and CdS and deep acceptor states in the defect layer itself. The illumination dependence of this voltage drop is explained by photogenerated holes trapped by the deep acceptor states in the defect layer. Numerical simulations have been carried out using the program SCAPS-1D in order to verify our model assumptions. From our model, indirect conclusions are derived concerning the maximum conduction band offsets between CdS and the defect layer and between CdS and ZnO. Copyright

Influence of the Ga-content on the bulk defect densities of Cu(In, Ga)Se2

Abstract We fabricate a series of CIGS absorber layers with Ga-contents x =Ga/(In+Ga) ranging from x =0 (CuInSe 2 ) to 1 (CuGaSe 2 ) by single layer coevaporation. The open circuit voltages V oc of the completed solar cells increase with increasing Ga-content but not proportional to the change of the band gap energy E g of the CIGS-layers. In contrast to the behaviour at Ga-contents exceeding x ≈0.3 the difference E g −q V oc decreases from x =0 to x ≈0.3, thus, having a minimum at x ≈0.3. We determine defect concentrations in the absorber of these cells by admittance spectroscopy. These bulk defects also have a minimum concentration at x ≈0.3. This low concentration of deep defects reduces recombination losses and thus, the difference of E g −q V oc . In addition, we fabricated absorber layers with a three stage process and an average Ga content x of approximately 0.3. The resulting solar cells have a lower defect concentration and a higher open circuit voltage than solar cells from single stage processes with x ≈0.3. We find a correlation between volume defect concentrations and the difference E g −q V oc suggesting that volume defects determine the open circuit voltage of CIGS solar cells in the whole composition range from CuInSe 2 to CuGaSe 2 and when using different absorber processes.

Radiative efficiency limits of solar cells with lateral band-gap fluctuations

The radiative recombination limit of photovoltaic power conversion under one sun terrestrial illumination is calculated for solar cells with lateral fluctuations of the band-gap energy. A simple analytical model quantifies the fluctuations by the standard deviation σEg from the mean band gap. The calculated maximum efficiency decreases by 1.7% (absolute) for σEg=50 meV and by 6.1% for σEg=100 meV with respect to a uniform band gap.

Plasmonic reflection grating back contacts for microcrystalline silicon solar cells

We report on the fabrication and optical simulation of a plasmonic light-trapping concept for microcrystalline silicon solar cells, consisting of silver nanostructures arranged in square lattice at the ZnO:Al/Ag back contact of the solar cell. Those solar cells deposited on this plasmonic reflection grating back contact showed an enhanced spectral response in the wavelengths range from 500 nm to 1000 nm, when comparing to flat solar cells. For a particular period, even an enhancement of the short circuit current density in comparison to the conventional random texture light-trapping concept is obtained. Full three-dimensional electromagnetic simulations are used to explain the working principle of the plasmonic light-trapping concept.