OPTICAL ANALYSIS OF EFFICIENCY LIMITATIONS OF CU(IN,GA)SE2 GROWN UNDER COPPER EXCESS

Solar cells made from the compound semiconductor Cu(In,Ga)Se2 reach efficiencies of 22:9 % and are thus even better than multi crystalline silicon solar cells. All world records are achieved using absorber layers with an overall copper deficient composition, but Cu-rich grown samples have multiple favourable properties. However, especially losses in the open circuit voltage limit the device performance. Within this work these efficiency limitations of chalcopyrites grown with copper excess are investigated. The work has been divided into four chapters addressing different scientific questions. (i) Do alkali treatments improve Cu-rich absorber layers? The alkali treatment, which lead to the recent improvements of the efficiency world record, is adapted to CuInSe2 samples with Cu-rich composition. The treatment leads to an improvement of the VOC which originates roughly equally from an improvement of the bulk and the removal of a defect close to the interface. The treatment also improves the VOC of Cu-poor samples. In both cases, the treatment increases the fill factor (FF) and leads to a reduction of copper content at the surface. (ii) Is the VOC limited by deep defects in Cu-rich Cu(In,Ga)Se2? A deep defect, which likely limits the VOC, is observed in photoluminescence measurements (PL) independent of a surface treatment. The defect level is proposed to originate from the second charge transition of the CuIn antisite defect (CuIn(-1/-2)). During the investigation also a peak at 0:9 eV is detected and attributed to a DA-transition involving a third acceptor situated (135 ± 10) meV above the valence band. The A3 proposed to originate from the indium vacancy (VIn). Furthermore the defect was detected in admittance measurements and in Cu(In,Ga)Se2 samples with low gallium content. (iii) Is the diode factor intrinsically higher in Cu-rich chalcopyrites? Cu-rich solar cells exhibit larger diode ideality factors which reduce the FF. A direct link between the power law exponent from intensity dependent PL measurements of absorbers and the diode factor of devices is derived and verified using Cu-poor Cu(In,Ga)Se2 samples. This optical diode factor is the same in Cu-rich and Cu-poor samples. (iv) Is the quasi Fermi level splitting (qFLs) of Cu-rich Cu(In,Ga)Se2 absorber layers comparable to Cu-poor samples? Measuring the qFLs of passivated Cu-rich and Cu-poor Cu(In,Ga)Se2 samples, on average a 120 meV lower splitting is determined for Cu-rich samples. This difference increases with gallium content and is likely linked to a defect moving deeper into the bandgap, possibly related to the second charge transition of the CuIn antisite defect. Overall, samples with Cu-rich composition are not limited by the diode factor. However, a deep defect band causes recombination lowering the qFLs and thus the VOC. This defect is not removed by alkali treatments. A key component to improve Cu-rich solar cells in the future, especially Cu(In,Ga)Se2, will be to remove or passivate this defect level

The Optical Diode Ideality Factor Enables Fast Screening of Semiconductors for Solar Cells

In the search for new materials for solar cells, a fast feedback is needed. Radiative efficiency measurements based on photoluminescence PL are the tool of choice to screen the voltage a material is capable of. Additionally the dependence of the radiative efficiency on excitation density contains information on the diode ideality factor, which determines in turn the fill factor of the solar cell. Both parameters are immediate ingredients of the efficiency of a solar cell and can be determined from PL measurements, which allow fast feedback. The method to determine the optical diode ideality factor from PL measurements and compare to electrical measurements in finished solar cells are discusse

Quasi Fermi level splitting of Cu-rich and Cu-poor Cu(In,Ga)Se2 absorber layers

The quasi Fermi level splitting is measured for Cu(In,Ga)Se2 absorber layers with different copper to (indium + gallium) ratios and for different gallium contents in the range of 20%-40%. For absorbers with a [Cu]/[In + Ga] ratio below one, the measured quasi Fermi level splitting is 120 meV higher compared to absorbers grown under copper excess independent of the gallium content, contrary to the ternary CuInSe2 where the splitting is slightly higher for absorber layers grown under copper excess. Possible explanations are the gallium gradient determined by the secondary ion mass spectrometry measurement which is less pronounced towards the surface for stoichiometric absorber layers or a fundamentally different recombination mechanism in the presence of gallium. Comparing the quasi Fermi level splitting of an absorber to the open circuit voltage of the corresponding solar cell, the difference for copper poor cells is much lower (60 meV) than that for copper rich cells (140 meV). The higher loss in V OC in the case of the Cu-rich material is attributed to tunneling enhanced recombination due to higher band bending within the space charge region

Potassium fluoride postdeposition treatment with etching step on both Cu-rich and Cu-poor CuInSe2 thin film solar cells

Recent progress in the power conversion efficiency of Cu(In,Ga)Se2 thin film solar cells has been achieved by an alkali postdeposition treatment. This treatment has been shown to change the surface composition and structure as well as the bulk properties. To investigate the relative importance of those two effects we study the impact of the treatment on Cu-rich and Cu-poor CuInSe2, which show a different influence of interface recombination without the treatment. We develop a potassium postdeposition treatment that can be applied to Cu-rich material, where an additional etching step is necessary. The same postdeposition treatment with etching step is applied to Cu-poor material. In both cases we observe an increase of the power conversion efficiency and open circuit voltage. Comparing the increase in open circuit voltage to the increase in quasi-Fermi level splitting indicates that the improvement in Cu-poor solar cells is mostly due to changes in the bulk, whereas in Cu-rich solar cells both the bulk and the interface are improved. The improvement of the interface is corroborated by temperature dependent current-voltage characteristics, which show that the dominating recombination path in Cu-rich solar cells moves from the interface to the bulk after treatment and by admittance spectroscopy, which shows that the treatment removes a 200 meV deep defect. Photoluminescence spectroscopy shows that even in Cu-rich material the alkali treatment creates a Cu-poor surface, which in this case cannot be created by diffusion of Cu into the bulk, but is grown during the treatment

The hunt for the third acceptor in CuInSe2 and Cu(In,Ga)Se2 absorber layers

The model for intrinsic defects in Cu(In,Ga)Se2 semiconductor layers is still under debate for the full range between CuInSe2 and CuGaSe2. It is commonly agreed by theory and experiment, that there are at least one shallow donor and two shallow acceptors. Spatially resolved photoluminescence on CuGaSe2 previously revealed a third acceptor. In this study we show with the same method that the photoluminescence peak at 0.94 eV in CuInSe2, previously attributed to a third acceptor, is a phonon replica. However another pronounced peak at 0.9 eV is detected on polycrystalline CuInSe2 samples grown with high copper and selenium excess. Intensity and temperature dependent photoluminescence measurements reveal that this peak originates from a DA-transition from a shallow donor (<8 meV) into a shallow acceptor A3 (135 [Formula: see text] 10) meV. The DA3 transition has three distinct phonon replicas with 28 meV spectral spacing and a Huang Rhys factor of 0.75. Complementary admittance measurements are dominated by one main step with an activation energy of 125 meV which corresponds well with the found A3 defect. The same defect is also observed in Cu(In,Ga)Se2 samples with low gallium content. For [Ga]/([Ga]  +  [In])-ratios of up to 0.15 both methods show a concordant increase of the activation energy with increasing gallium content shifting the defect deeper into the bandgap. The indium vacancy [Formula: see text] is discussed as a possible origin of the third acceptor level in CuInSe2 and [Formula: see text] in Cu(In,Ga)Se2

High‐performance low bandgap thin film solar cells for tandem applications

Thin film tandem solar cells provide a promising approach to achieve high efficiencies. These tandem cells require at least a bottom low bandgap and an upper high bandgap solar cell. In this contribution, 2 high‐performance Cu(In,Ga)Se2 cells with bandgaps as low as 1.04 and 1.07 eV are presented. These cells have shown certified efficiencies of 15.7% and 16.6% respectively. Measuring these cells under a 780‐nm longpass filter, corresponding to the bandgap of a typical top cell in tandem applications (1.57 eV), they achieved efficiencies of 7.9% and 8.3%. Admittance measurements showed no recombination active deep defects. One additional high‐performance CuInSe2 thin film solar cell with bandgap of 0.95 eV and efficiency of 14.1% is presented. All 3 cells have the potential to be integrated as bottom low bandgap cells in thin film tandem applications achieving efficiencies around 24% stacked with an efficient high bandgap top cell

Oxidation as Key Mechanism for Efficient Interface Passivation in Cu (In,Ga)Se2 Thin-Film Solar Cells

Copper-indium-gallium-diselenide (CIGS) thin-film solar cells suffer from high recombination losses at the back contact and parasitic absorption in the front-contact layers. Dielectric passivation layers overcome these limitations and enable an efficient control over interface recombination, which becomes increasingly relevant as thin-film solar cells increase in efficiency and become thinner to reduce the consumption of precious resources. We present the optoelectronic and chemical interface properties of oxide-based passivation layers deposited by atomic layer deposition on CIGS. A suitable postdeposition annealing removes detrimental interface defects and leads to restructuring and oxidation of the CIGS surface. The optoelectronic interface properties are very similar for different passivation approaches, demonstrating that an efficient suppression of interface states is possible independent of the metal used in the passivating oxide. If aluminum oxide (Al2O3) is used as the passivation layer we confirm an additional field-effect passivation due to interface charges, resulting in an efficient interface passivation superior to that of a state-of-the-art cadmium-sulfide (CdS) buffer layer. Based on this chemical interface model we present a full-area rear-interface passivation layer without any contact patterning, resulting in a 1% absolute efficiency gain compared to a standard molybdenum back contact. © 2020 authors. Published by the American Physical Society

Silver Doped Cu2SnS3 Absorber Layers for Solar Cells Application

The p-type semiconductorCu2SnS3 is alloyedwithAg to investigate its effect on absorber layer and solar cell properties. Ag replaces the Cu in (Cu1-xAgx)2SnS3 (ACTS) up to x ≥ 6% at 550 °C. Above this percentage, Ag forms secondary phases.We find a significant increase in grain size, from hundreds of nanometers to severalmicrons, and increased photoluminescence yield with increasing Ag concentration. Low-temperature photoluminescence measurements show that compensation is increased for the ACTS absorber layers, which could be beneficial for CTS, but also that the electrostatic band gap fluctuations are increased. The external quantum efficiency of the solar cells made from ACTS shows an increased carrier collection length from 320 nm for CTS to 700 nm and a thicker buffer layer. We attribute the increase in collection length to both increased depletion width (increased compensation) and diffusion length (larger grains). Overall the ACTS solar cells have a lower power conversion efficiency due to lower shunt resistance and open-circuit voltage, which are attributed to increase in pinholes, electrostatic fluctuation, and changes at the CdS/ACTS interface