Grading and metastable effects in admittance spectroscopy of CIGS-based solar cells

Cu(In, Ga)Se2-based (CIGS) solar cells have achieved efficiencies up to 20%. Despite these excellent results, the understanding of the underlying mechanisms and the influence of defects on their performance is still incomplete. The determination of the energetic position of the defects and of their density of states is important. Admittance spectroscopy is an adequate technique for this. By varying the external voltage during the measurement, the spatial position where the defect distribution is sensed can be varied. However, the application of external biases can lead to metastable effects in the absorber and therefore to defect relaxation and changes in the doping distribution. Hence, it is important to separate between the effects caused by metastable changes and the change in sensing position of the admittance spectroscopy measurement. This can be achieved by varying the applied voltage during the creation of the metastable state and the measurement itself independently or simultaneously. Admittance spectroscopy under different bias voltage conditions performed on a flexible CIGS-based solar cell are presented and assessed

Assignment of capacitance spectroscopy signals of CIGS solar cells to effects of non-ohmic contacts

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Triple-cation perovskite solar cells fabricated by hybrid PVD/blade coating process using green solvents

The scalability of highly efficient organic-inorganic perovskite solar cells (PSCs) is one of the remaining challenges of solar module manufacturing. Various scalable methods have been explored to strive for uniform perovskite films of high crystal quality on large-area substrates. However, each of these methods have individual drawbacks, limiting the successful commercialization of perovskite photovoltaics. Here, we report a fully scalable hybrid process, which combines vapor- and solution-based techniques to deposit high quality uniform perovskite films on large-area substrates. This two-step process does not use toxic solvents, and it further allows facile implementation of passivation strategies and additives. We fabricated PSCs based on this process and used blade coating to deposit both charge transporting layers (SnO2 and Spiro-OMeTAD) without hazardous solvents in ambient air. The fabricated PSCs have yielded open-circuit voltage up to 1.16 V and power conversion efficiency of 18.7 % with good uniformity on 5 cm x 5 cm substrates

Monolithic CIGS-Perovskite Tandem Cell for an Optimal Light Harvesting Without Current Matching

We present a novel monolithic architecture for optimal light harvesting in multijunction thin film solar cells. In the configuration we consider, formed by a perovskite (PVK) cell overlying a CIGS cell, the current extracted from the two different junctions is decoupled by the insertion of a dielectric nonperiodic photonic multilayer structure. This photonic multilayer is designed by an inverse integration approach to confine the incident sunlight above the PVK band gap in the PVK absorber layer, while increasing the transparency for sunlight below the PVK band gap for an efficient coupling into the CIGS bottom cell. To match the maximum power point voltages in a parallel connection of the PVK and CIGS cells, the latter is divided into two subcells by means of a standard three-laser scribing connection. Using realistic parameters for all the layers in the multijunction architecture we predict power conversion efficiencies of 28%. This represents an improvement of 24% and 26% over the best CIGS and PVK single-junction cells, respectively, while at the same time outperforms the corresponding current-matched standard tandem configuration by more than two percentage points.Peer ReviewedPostprint (author's final draft

Deep level transient spectroscopy measurements on Mo/Cu(In,Ga)Se2/metal structure

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Combining optical and electrical studies to unravel the effect of Sb doping on CIGS solar cell

A way to lower the manufacturing cost of Cu(In,Ga)Se2 (CIGS) thin film solar cells is to use flexible polymer substrates instead of glass substrates. Because such substrates require a low temperature during absorber deposition, the efficiency of the cells remains slightly lower (18.7%) compared to CIGS on glass substrates (20.3%). Partial compensation of this efficiency loss might be accomplished by Sb doping of the absorber, which is reported to have a positive effect on the morphology of this layer. In this work the defect structure of Sb doped CIGS solar cells is investigated using optical and electrical spectroscopic techniques. Experiments were performed on cells deposited on soda lime glass substrate, adding a thin Sb layer (8, 12 nm) onto the Mo back contact prior to the CIGS absorber deposition. The results are compared with those for cells without Sb doping using the same process. Fourier-Transform near infrared photocurrent measurements in the 10–300K range demonstrate that the band gap of Sb-doped samples is larger than for undoped samples. Photoluminescence spectra in the 5–100K region provide information on shallow-level defects. Deep-Level Transient Spectroscopy spectra of Sb-doped cells exhibit two features not encountered for non-doped cells: 1) a peak at lower temperature than the N1 signal and 2) incomplete charge carrier freeze-out down to 8 K. While the first result appears to be the fingerprint of an extra non-Ohmic contact in the solar cell structure, the second suggests the introduction of a very shallow acceptor by Sb doping. As a salient feature one can accurately monitor the partial hole freeze-out in the 40-60 K range and determine the signature of the intrinsic defects that provide the p-type conductivity of the CIGS absorber using Admittance Spectroscopy

Local Band Gap Measurements by VEELS of Thin Film Solar Cells

This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS. The precision and accuracy of the results are discussed based on the analysis of individual error sources, which leads to the conclusion that the precision of our measurements is most limited by the acquisition reproducibility, if the signal-to-noise ratio of the spectrum is high enough. Furthermore, we simulate the impact of radiation losses on the measured band gap value and propose a thickness-dependent correction. In future work, localized band gap variations will be measured on a more localized length scale to investigate, e.g., the influence of chemical inhomogeneities and dopant accumulations at grain boundarie