The drive-level capacitance profiling technique has been applied to ZnO/CdS/CuIn1−xGaxSe2/Mo solar cell devices, in order to study properties of defects in the CuIn1−xGaxSe2 film. Properties studied include the spatial uniformity, bulk defect response, carrier density, and light-induced metastable effects. These results indicate that previous estimates of carrier densities, from C–V profiling, may be significantly overestimated. In addition, a defect response previously thought to be located at the interface is observed to exist throughout the bulk material. Finally, an infrared light-soaking treatment is demonstrated to induce metastable changes in the bulk CuIn1−xGaxSe2 film. Hence, the drive-level capacitance profiling technique provides valuable insights into these films. Herein, the technique itself is fully explained, compared to other junction capacitance methods, and its utility is demonstrated using numerical simulation.

Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling

Thin film solar cells with a Cu(In,Ga)Se2 (CIGS) absorber layer achieved efficiencies above 20%. In order to achieve such high performance the absorber layer of the device has to be doped with alkaline material. One possibility to incorporate alkaline material is a post deposition treatment (PDT), where a thin layer of NaF and/or KF is deposited onto the completely grown CIGS layer. In this paper we discuss the effects of PDT with different alkaline elements (Na and K) on the electronic properties of CIGS solar cells. We demonstrate that whereas Na is more effective in increasing the hole concentration in CIGS, K significantly improves the pn-junction quality. The beneficial role of K in improving the PV performance is attributed to reduced recombination at the CdS/CIGS interface, as revealed by temperature dependent J–V measurements, due to a stronger electronically inverted CIGS surface region. Computer simulations with the software SCAPS are used to verify this model. Furthermore, we show that PDT with either KF or NaF has also a distinct influence on other electronic properties of the device such as the position of the N1 signal in admittance spectroscopy and the roll-over of the J–V curve at low temperature. In view of the presented results we conclude that a model based on a secondary diode at the CIGS/Mo interface can best explain these features.

Unveiling the effects of post-deposition treatment with different alkaline elements on the electronic properties of CIGS thin film solar cells.

We present a detailed study of admittance spectroscopy and deep level transient spectroscopy on CuInSe2/CdS/ZnO thin film solar cells. The admittance spectra reveal an emission from a distribution of hole traps centered at an activation energy of 280 meV and a shallower level with a sharp activation energy of ∼ 120 meV. After repetitive annealing of the device in air at 200 °C, the activation energy of the latter level increases continuously from 120 to 240 meV, while the 280 meV hole traps remain unaffected. Deep level transient spectroscopy with optical excitation reveals an emission of minority carriers with time constants comparable to those observed for the shallow level in admittance spectroscopy. The shift of the activation energy after annealing also occurs in deep level transient spectroscopy and ascertains that the emissions observed in both techniques have the same origin. The magnitude and continuous shift of the activation energy of the minority carrier emission indicates a distribution of le...

Distinction between bulk and interface states in CuInSe2/CdS/ZnO by space charge spectroscopy

A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a non-Ohmic back-contact acting as a second junction in the device. The model, which is substantiated with numerical device simulations, allows a unified explanation of characteristic admittance, CV, and IVT features commonly observed in CIGS solar cells.

Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells

A detailed analysis of the radiative recombination processes in CuxGaySe2 epitaxial layers is presented aiming at an investigation of the intrinsic defect levels as a function of chemical composition CuxGaySe2 is grown by metalorganic vapor phase epitaxy to allow a precise control of composition Temperature and excitation intensity dependent photoluminescence is used to identify different recombination mechanisms and to determine the ionization energies of the defect levels involved Defect-correlated optical transitions in Cu-rich epilayers are described in a recombination model consisting of two acceptor and one donor levels showing ionization energies of (60±10) meV, (100±10) meV, and (12±5) meV, respectively The identification of a shallow compensating donor in CuxGaySe2 and the assignment of the 100 meV state to an acceptor are the most important new aspects in this model Photoluminescence properties of layers showing Ga-rich compositions are discussed in a model of highly doped and highly compen

Radiative recombination via intrinsic defects in CuxGaySe2

Heterojunctions, such as ZnO/CdS/CuGaSe2, were fabricated for photovoltaic applications. Optimization of device structures based on monocrystalline CuGaSe2 led to the highest-to-date power conversion efficiencies for CuGaSe2 solar cells. At room temperature under 100 mW/cm2 AM1.5 illumination a maximum cell efficiency of 9.7% was achieved, given by an open-circuit voltage of 946 mV, a short circuit current density of 15.5 mA/cm2, and a fill factor of 66.5%. Preparation and performance of the optimum device are described. Current voltage characteristics dependent on illumination intensity and temperature, spectral response and electron-beam-induced current measurements were performed to determine the device parameters as well as to analyse the current transport and loss mechanisms. Tunneling, assisted by defect levels in the CdS layer, seems to play a major role. High injection effects are observed at forward bias ofV > 0.5 V or an illumination level ofP > 10 mW/cm2. Under such conditions, as well as at low temperatures, the non-zero series resistance comes into play. Effects of the shunt resistance, however, are negligible in all cases.

CuGaSe2 solar cells with 9.7% power conversion efficiency

Admittance spectroscopy was measured on Cu(In,Ga)(S,Se)2 thin film and single crystal heterojunctions. The emission rates of defects for various near‐stoichiometric compositions follow a Meyer–Neldel rule, showing increasing attempt‐to‐escape frequencies with increasing defect depth. Defects in highly (In,Ga)‐rich material showed lower attempt‐to‐escape frequencies and follow a separate Meyer–Neldel relation. Repetitive air annealing of a CuInSe2 heterojunction revealed a shift of the depth and capture cross section of an observed defect.

Meyer–Neldel behavior of deep level parameters in heterojunctions to Cu(In,Ga)(S,Se)2

Chemical bath deposited Zn(Se,OH)x on Cu(In,Ga)(S,Se)2 for high efficiency thin film solar cells: growth kinetics, electronic properties, device performance and loss analysis

Ag-doped CuGaSe2 as a precursor for thin film solar cells

For the development of a CuGaSez-based solar cell CuGaSe2 epitaxial layers were grown on GaAs(001) substrates by low-pressure metalorganic vapour phase epitaxy (MOVPE) exclusively with metalorganic precursors. X-ray diffraction (XRD) measurements revealed predominantly c[001]-oriented growth. The mean surface roughness was determined to be less than 6nm by atomic force microscopy (AFM). Low-temperature photoluminescence (PL) at 10K was dominated by a defect-correlated emission at 1.627eV. Moreover, it was possible to observe near band-edge emission. All CuGaSe2 layers showed p-type conductivity with net carrier concentrations in the order of 1017cm” and Hallmobilities of approximately 30cm2Ns.

M. Saad

Papers

CuGaSe2 solar cells with 9.7% power conversion efficiency

Meyer–Neldel behavior of deep level parameters in heterojunctions to Cu(In,Ga)(S,Se)2

Chemical bath deposited Zn(Se,OH)x on Cu(In,Ga)(S,Se)2 for high efficiency thin film solar cells: growth kinetics, electronic properties, device performance and loss analysis

Ag-doped CuGaSe2 as a precursor for thin film solar cells

MOVPE of CuGaSe/sub 2/ for photovoltaic applications