Two terminal (2T) perovskite/copper-indium-gallium-selenide (CIGS) tandem solar cells combine high conversion efficiency with lightweight flexible substrates which can decrease manufacturing and installation costs. In order to improve the power conversion efficiency of these tandem solar cells, the use of advanced simulation tools is crucial to estimate the loss mechanisms. In this regard, most of the available simulation works on tandem solar cells are oriented to minimize optical losses and assuming simplifications for the electrical simulations in particular in the top and bottom cell interconnection at the so-called tunnel recombination junction (TRJ) neglecting the inner physics of the complete tandem device. Therefore, the effect of charge exchange mechanism between top and bottom soler cells on the external parameters of a tandem devices is not fully understood yet. In this work, we present an experimentally validated opto-electrical model based on the fundamental semiconductor equations for the study of loss mechanisms of a reference perovskite/CIGS solar cell. Different from other numerical works, because our simulation platform includes the fundamental working mechanisms of the layers comprising the TRJ, we can properly calculate the losses related to it. We firstly present the calibration and validation of our opto-electrical model with respect to three fabricated reference solar cells: top cell only, bottom cell only and tandem device. Then, we use the calibrated model to evaluate main loss mechanisms affecting the baseline tandem device. Finally, we use the model to propose a roadmap for the optimization of monolithic perovskite/CIGS tandem solar cells.

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One of the key approaches to slow down and eventually prevent dramatic climate change is direct electricity generation from sunlight. Thin-film copper indium gallium (di)selenide (CIGS) is an excellent candidate for highly efficient and stable solar cells. A tuneable and direct bandgap as well as a high absorption coefficient allow for CIGS solar cells to be nearly 100 times thinner than their crystalline silicon (c-Si) counterparts; a feature suitable for flexible photovoltaic (PV) applications. In this thesis, light management for sub-micron CIGS solar cells is studied with the help of opto-electrical simulations. In Chapter 2, the theoretical optical limits for CIGS solar cells as well as the various available opto-electrical modelling platforms are briefly discussed. We study the Green absorption benchmark as a function of thickness and bandgap. Our modelling tools of choice, namely Ansys HFSS for the optical simulations, and Sentaurus TCAD for the electrical simulations are introduced in more details. The interface between CIGS and molybdenum (Mo) back contact is subject to a considerable amount of optical and electrical loss. This issue is investigated in Chapter 3, where we firstly discuss the plasmonic nature of the optical losses. Later, we introduce a double-layer dielectric spacer consisting of MgF2 and Al2O3 with periodic point contacts to quench the Mo-associated losses. We optimize the spacer thickness and the point contact area coverage for maximal photo-current density (Jph) in a CIGS solar cell with 750-nm thick absorber. The front reflection losses, contributing to roughly 10% of optical losses, are addressed in Chapter 4. We show that an MgF2-based double-layer porous-on-compact anti-reflection coating (ARC) allows for gradual refractive index change from air to CIGS and, therefore, according to the Rayleigh effect leads to a wideband antireflection effect. This is done by means of Bruggemann’s effective medium approximation and sequential nonlinear programming (SNLP) for the optimization process. Our models suggest that the proposed ARC surpasses the conventional single-layer ARC in resiliency against angle of incidence. A hybrid light management, employing both the suggested ARC at the front side and MgF2 / Al2O3 dielectric spacer at the rear side, proves to increase Jph of a 750-nm thick CIGS solar cell beyond that of a 1600-nm thick absorber (without light managment). In the rest of the thesis, we take an approach beyond the state-of-the-are architecture of CIGS solar cells and, for the first time, introduce the inter-digitated back-contacted (IBC) structure for CIGS technology. This structure, which no longer suffers from parasitic absorption (associated with the buffer and window layers), is optically studied in Chapter 5. We compare the results with a reference front- and back-contacted (FBC) solar cell with the same absorber volume, and take the Green limit as the benchmark. Two ARC schemes are studied; (i) high-aspect ratio features at the front side of the absorber and, (ii) the as-grown CIGS morphology with optimized MgF2 / Al2O3 layers. Once the optical potential of the IBC CIGS solar cells is realized, we continue our studies with an opto-electrical analysis in TCAD Sentaurus environment (Chapter 6). We not only optimize the geometry of electron- and hole-contacts, the gap between them and the contacts’ period, but also, study the CIGS bandgap grading and its defect density. The electric field map around the gap region is used to highlight the importance of electrical passivation in achieving a high performance. Our models (calibrated with real FBC solar cells fabricated at Solliance at the High-tech campus in Eindhoven) show the high potential of IBC CIGS solar cells for high efficiency PV applications@en

An interdigitated back-contacted (IBC) configuration is proposed for submicron copper indium gallium (di)selenide (CIGS). In a modelling platform, the structure was opto-electrically optimized for maximum efficiency. The results are compared with a reference front/back-contacted (FBC) solar cell with similar absorber thickness and exhibiting 11.9% efficiency. The electrical passivation at the front side is accomplished by an Al₂O₃ layer, which is endowed with negative fixed charges. The results indicate that with an optimal geometry and engineered bandgap grading, the efficiency of the new IBC structure can reach 17%. Additionally, with a reasonably low defect density in the absorber layer, efficiencies as high as 19.7% and open-circuit voltage comparable with that of the record solar cell are possible with the IBC structure.

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The optical losses associated with sub-micron absorbers in CIGS solar cells can be reduced by light management techniques. 3-D optical modelling was used to optimize light in-coupling and internal rear reflectance in a 750-nm thick CIGS reference solar cell. At the front side, an effective medium approximation (EMA) approach for describing optical properties of a MgF₂-based anti-reflection coating (ARC) was applied. Taking reflectance as the cost function and sequential nonlinear programming as the optimization algorithm, an optimal porous-on-compact double-layer ARC was determined. This led to a wideband light in-coupling with a 6.8% improvement in the photo-current density (J_ph) with respect to the reference solar cell without ARC. Considering the variation of the sunlight direction due to day and seasonal changes, different light incidence angles were investigated. The results indicate that in this case, our designed double-layer ARC outperforms the standard compact MgF₂ single-layer ARC. By using the EMA approach, the amount of computational memory can be reduced by a factor of 30, shortening the simulation time from four days to one hour. At the rear side of the cell, a point-contacted MgF₂/Al₂O₃ reflector, in combination with our proposed front ARC, enhances the J_ph by 11.3% considering the same reference solar cell. Compared to a much thicker cell (1600-nm thick absorber) with no light management applied, our front-and-rear optical approaches more-than-compensate optical losses resulting from using thinner absorbers. This design is suitable for industrial uptake and practical to realize. Additionally, the approach of using EMA for double-layer ARC optimization is innovative with respect to other ARC approaches applicable to not only chalcopyrite photovoltaic technologies.

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An optical investigation of ultra-thin CIGS solar cells and guidelines for elimination of optical losses is presented. Then, a novel back contacted structure for CIGS solar cells is suggested and optimized for best implied photocurrent density.@en

A 3-D optical modelling was calibrated to calculate the light absorption and the total reflection of fabricated CIGS solar cells. Absorption losses at molybdenum (Mo) / CIGS interface were explained in terms of plasmonic waves. To quench these losses, we assumed the insertion of a lossless dielectric spacer between Mo and CIGS, whose optical properties were varied. We show that such a spacer with low refractive index and proper thickness can significantly reduce absorption in Mo in the long wavelength regime and improve the device’s rear reflectance, thus leading to enhanced light absorption in the CIGS layer. Therefore, we optimized a realistic two-layer MgF2 / Al2O3 dielectric spacer to exploit (i) the passivation properties of ultra-thin Al2O3 on the CIGS side for potential high open-circuit voltage and (ii) the low refractive index of MgF2 on the Mo side to reduce its optical losses. Combining our realistic spacer with optically-optimized point contacts increases the implied photocurrent density of a 750 nm-thick CIGS layer by 10% for the wavelengths between 700 and 1150 nm with respect to the reference cell. The elimination of plasmonic resonances in the new structure leads to a higher electric field magnitude at the bottom of CIGS layer and justifies the improved optical performance.@en

The optical analysis of optically-textured and electrically-flat ultra-thin crystalline silicon (c-Si) slabs is presented. These slabs were endowed with decoupled front titanium-dioxide (TiO2) / back silicon-dioxide (SiO2) dielectric textures and were studied as function of two types of back reflectors: standard silver (Ag) and dielectric modulated distributed Bragg reflector (MDBR). The optical performance of such systems was compared to that of state-of-the-art flat c-Si slabs endowed with so-called front Mie resonators and to those of similar optical systems still endowed with the same back reflectors and decoupled front/back texturing but based on textured c-Si and dielectric coatings (front TiO2 and back SiO2). Our optimized front dielectric textured design on 2-µm thick flat c-Si slab with MDBR resulted in more photo-generated current density in c-Si with respect to the same optical system but featuring state-of-the-art Mie resonators ( + 6.4%), mainly due to an improved light in-coupling between 400 and 700 nm and light scattering between 700 and 1050 nm. On the other hand, the adoption of textured dielectric layers resulted in less photo-generated current density in c-Si up to −20.6% with respect to textured c-Si, depending on the type of back reflector taken into account.@en

The influence of a dielectric layer between Molybdenum and CIGS on the performance of CIGS solar cells is investigated. Using optical simulations, thickness and dielectric constant of such dielectric spacer are evaluated.@en