Lorelle M. Mansfield

Enhanced Performance in Cu(In,Ga)Se

Cu(In,Ga)Se2 (CIGS) solar cells fabricated with twostep selenization processes commonly suffer from low open-circuit voltage (Voc). We found that the Voc of solar cells made from selenized Cu/Ga/In stacked metal precursors can be increased by employing a potassium fluoride (KF) postdeposition treatment (PDT). This study presents a comparison of films and resulting devices with and without the KF PDT. By including the KF PDT, an 18.6%-efficient CIGS device with a Voc of 0.709 V was fabricated using a two-step selenization process.

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Potential measurements on Cu(In,Ga)Se2 thin films using scanning Kelvin probe force microscopy have been reported extensively to address grain-boundary (GB) recombination by examining GB charging. However, the results are highly inconsistent. We revisit this issue by measuring high- and low-quality wide-bandgap films and using a complementary method of scanning capacitance microscopy. Our results show consistent positively charged GBs in our high-quality films with minimal surface defects, except for the Σ3[112] GBs, which are charge neutral. We discuss possible artifacts due to surface defects when examining the GB charging and the role of GBs in the device performance.

Solar Cells Fabricated by the Two-Step Selenization Process With a Potassium Fluoride Postdeposition Treatment

This report outlines improvements to the energy conversion efficiency in wide bandgap (Eg>1.2 eV) solar cells based on CuIn1−xGaxSe2. Using (a) alkaline containing high temperature glass substrates, (b) elevated substrate temperatures 600°C-650°C and (c) high vacuum evaporation from elemental sources following NRELs three-stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2<Eg<1.45 eV. Initial results of this work have led to efficiencies >18% for absorber bandgaps ∼1.30 eV and efficiencies ∼16% for bandgaps up to ∼1.45 eV. In comparing J-V parameters in similar materials, we establish gains in the open-circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures may be due to reduced recombination at the grain boundary of such absorber films. Solar cell results, absorber materials characterization, and experimental details are reported.

How grain boundaries in Cu(In,Ga)Se2 thin films are charged: Revisit

Zn(O,S) thin films were deposited by chemical bath deposition (CBD), atomic layer deposition, and sputtering. Composition of the films and band gap were measured and found to follow the trends described in the literature. CBD Zn(O,S) parameters were optimized and resulted in an 18.5% efficiency cell that did not require post annealing, light soaking, or an undoped ZnO layer. Promising results were obtained with sputtering. A 13% efficiency cell was obtained for a Zn(O,S) emitter layer deposited with 0.5%O2. With further optimization of process parameters and an analysis of the loss mechanisms, it should be possible to increase the efficiency.

Improved energy conversion efficiency in wide bandgap Cu(In, Ga)Se 2 solar cells

The deliberate introduction of K and Na into Cu(In, Ga)Se2 (CIGS) absorbers was investigated by varying a combination of an SiO2 diffusion barrier, coevaporation of KF with the CIGS absorber, and a KF postdeposition treatment (PDT). Devices made with no diffusion barrier and KF coevaporation treatment exhibited the highest photovoltaic conversion efficiency with the smallest overall distribution in key current density-voltage (J-V) performance metrics. Out-diffusion of Na and K from the substrate, KF coevaporation, and KF PDT all increased carrier concentration, open-circuit voltage, fill factor, and power conversion efficiency. Quantum-efficiency analysis of devices highlighted the greatest loss in the short-circuit current density due to incomplete absorption and collection. Secondary ion mass spectrometry illustrated the efficacy of the SiO2 film as a sodium and potassium diffusion barrier, as well as their relative concentration in the absorber. Introduction of KF appeared to enhance diffusion of Na from the substrate, in agreement with previous studies.

A comparative study of Zn(O,S) buffer layers and CIGS solar cells fabricated by CBD, ALD, and sputtering

Fiber-fed time-resolved photoluminescence is demonstrated as a tool for immediate process feedback after deposition of the absorber layer for CuInxGa(1-x)Se2 and Cu2ZnSnSe4 photovoltaic devices. The technique uses a simplified configuration compared to typical laboratory time-resolved photoluminescence in the delivery of the exciting beam, signal collection, and electronic components. Correlation of instrument output with completed device efficiency is demonstrated over a large sample set. The extraction of the instrument figure of merit, depending on both the initial luminescence intensity and its time decay, is explained and justified. Limitations in the prediction of device efficiency by this method, including surface effect, are demonstrated and discussed.

Effects of Sodium and Potassium on the Photovoltaic Performance of CIGS Solar Cells

The efficiency of Cu(In, Ga)Se2 (CIGS) solar cells is enhanced when Na is incorporated in the CIGS absorber layer. This work examines Na incorporation in CIGS utilizing Na-doped Mo sputtered from targets made with sodium molybdate-doped (MONA) powder. Mo:Na films with varying thicknesses were sputtered onto Mo-coated borosilicate glass (BSG) or stainless steel substrates for CIGS solar cells. By use of this technique, the Na content of CIGS can be varied from near-zero to higher than that obtained from a soda-lime glass (SLG) substrate. Targets and deposition conditions are described. The doped Mo films are analyzed, and the resulting devices are compared to devices fabricated on Mo-coated SLG as well as Mo-coated BSG with NaF. Completed devices utilizing MONA exceeded 15.7% efficiency without anti-reflective coating, which was consistently higher than devices prepared with the NaF precursor. Strategies for minimizing adhesion difficulties are presented.

Fiber-fed time-resolved photoluminescence for reduced process feedback time on thin-film photovoltaics

We report on a direct nm-resolution resistance mapping on the Cu(In,Ga)Se2 photovoltaic thin films, using scanning spreading resistance microcopy. We found a conductance channel along the grain boundaries (GBs) of the polycrystalline materials, which is consistent with the argument that carrier polarity of the GB and the space charge region around it is inverted. To minimize the probe/film contact resistance, so that the local spreading resistance beneath the probe is measured, the probe must be adequately indented to the film and a bias voltage larger than the onset value of the probe/film barrier should be applied.

Sodium-doped molybdenum targets for controllable sodium incorporation in CIGS solar cells

We present a near-field scanning microwave microscope (NSMM) that has been configured for imaging photovoltaic samples. Our system incorporates a Pt-Ir tip inserted into an open-ended coaxial cable to form a weakly coupled resonator, allowing the microwave reflection S(11) signal to be measured across a sample over a frequency range of 1 GHz - 5 GHz. A phase-tuning circuit increased impedance-measurement sensitivity by allowing for tuning of the S(11) minimum down to -78 dBm. A bias-T and preamplifier enabled simultaneous, non-contact measurement of the DC tip-sample current, and a tuning fork feedback system provided simultaneous topographic data. Light-free tuning fork feedback provided characterization of photovoltaic samples both in the dark and under illumination at 405 nm. NSMM measurements were obtained on an inhomogeneous, third-generation Cu(In,Ga)Se(2) (CIGS) sample. The S(11) and DC current features were found to spatially broaden around grain boundaries with the sample under illumination. The broadening is attributed to optically generated charge that becomes trapped and changes the local depletion of the grain boundaries, thereby modifying the local capacitance. Imaging provided by the NSMM offers a new RF methodology to resolve and characterize nanoscale electrical features in photovoltaic materials and devices.

Electrical conduction channel along the grain boundaries of Cu(In,Ga)Se2 thin films

We report for the first time production-quality P2 and P3 scribes in CIGS based solar cells using a nanosecond-domain industrial pulsed laser We also show how the same laser can be used to produce the P1 scribe, and report what we believe to be the first all-laser-scribed monolithically-integrated CIGS solar cells in which all scribes were made using the same laser, at the same 1064nm wavelength. This paper reports the results of a collaborative effort involving two national laboratories and two private companies, and focuses on the laser scribing processes. A paper describing the design and characterization of the solar cells has been published elsewhere in this journal [1]. The new laser-based P2 and P3 processes rely on a “brittle fracture material removal” mechanism whose material removal characteristics are somewhat similar to those of mechanical scribing in that the material is ejected from the surface in fragments. However, unlike mechanical scribing, the laser process produces highly regular and deterministic edges. The first cells produced showed poor conversion efficiency, mostly attributed to high series resistance. These cells then had additional AZO deposited on them, and the results improved dramatically. Finally, we show results obtained using conventional mechanical scribing for the P2 and P3 processes on cells which are in other respects almost identical. Initial results are similar to the laser-scribed results.