Abstract
The efficiency of thin-film solar cells with a Cu( Gax)Se2 absorber is limited by nanoscopic inhomogeneities and defects. Traditional characterization methods are challenged by the multi-scale evaluation of the performance at defects that are buried in the device structures. Multi-modal x-ray microscopy offers a unique tool-set to probe the performance in fully assembled solar cells, and to correlate the performance with composition down to the micro- and nanoscale. We applied this approach to the mapping of temperature-dependent recombination for Cu( Gax)Se2 solar cells with different absorber grain sizes, evaluating the same areas from room temperature to . It was found that poor performing areas in the large-grain sample are correlated with a Cu-deficient phase, whereas defects in the small-grain sample are not correlated with the distribution of Cu. In both samples, classes of recombination sites were identified, where defects were activated or annihilated by temperature. More generally, the methodology of combined operando and in situ x-ray microscopy was established at the physical limit of spatial resolution given by the device itself. As proof-of-principle, the measurement of nanoscopic current generation in a solar cell is demonstrated with applied bias voltage and bias light.
Highlights
In the last several years, the photovoltaic industry has grown dramatically, and large factories have achieved economies of scale capable of producing modules with power conversion efficiency in the range of 17–24.4% based on silicon (Si), cadmium telluride (CdTe), and copper indium gallium diselenide (Cu(In1−xGax)Se2, CIGS) at a cost less than $0.5/W [1, 2].CIGS-based solar cells have emerged as one of the most promising candidates for high-efficiency low-cost thin-film solar cells, and R&D efficiencies as high as η = 23.35% have been reported [3]
Prior to the investigation of the temperature dependence of recombination at nanoscopic defects, we evaluated the pristine state of the solar cells at a larger scale
Based on multi-modal scanning X-ray microscopy, we have studied the performance variations in two types of CIGS solar cells with different grain sizes upon annealing
Summary
In the last several years, the photovoltaic industry has grown dramatically, and large factories have achieved economies of scale capable of producing modules with power conversion efficiency in the range of 17–24.4% based on silicon (Si), cadmium telluride (CdTe), and copper indium gallium diselenide (Cu(In1−xGax)Se2, CIGS) at a cost less than $0.5/W [1, 2]. The methods include X-ray beam induced current (XBIC) [19, 20, 17, 21] and voltage (XBIV) [22] to probe the electrical performance, X-ray fluorescence (XRF) to probe the composition [23, 24], X-ray excited optical fluorescence (XEOL) to probe the optical performance [25, 26], ptychography to probe the structure [27] including voids at CIGS grain boundaries [28], and X-ray diffraction to probe the lattice constant [29, 30, 31]. For the temperature control during this experiment, a heating stage [45] developed for the in-situ growth of CIGS [46, 47] has been adapted for electrical XBIC & XBIV measurements in combination with XRF measurements. For the XBIC measurement §, the solar cell was operated with 100 mV forward bias applied by a current pre-amplifier (SR570 from Stanford Research Systems), whose output was sent to the lock-in amplifier with identical demodulator and low-pass filter settings as for XBIV measurements
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