Haejun Chung

Time domain simulation of tandem silicon solar cells with optimal textured light trapping enabled by the quadratic complex rational function.

Amorphous silicon/crystalline silicon (a-Si/c-Si) micromorph tandem cells, with best confirmed efficiency of 12.3%, have yet to fully approach their theoretical performance limits. In this work, we consider a strategy for improving the light trapping and charge collection of a-Si/c-Si micromorph tandem cells using random texturing with adjustable short-range correlations and long-range periodicity. In order to consider the full-spectrum absorption of a-Si and c-Si, a novel dispersion model known as a quadratic complex rational function (QCRF) is applied to photovoltaic materials (e.g., a-Si, c-Si and silver). It has the advantage of accurately modeling experimental semiconductor dielectric values over the entire relevant solar bandwidth from 300-1000 nm in a single simulation. This wide-band dispersion model is then used to model a silicon tandem cell stack (ITO/a-Si:H/c-Si:H/silver), as two parameters are varied: maximum texturing height h and correlation parameter f. Even without any other light trapping methods, our front texturing method demonstrates 12.37% stabilized cell efficiency and 12.79 mA/cm² in a 2 μm-thick active layer.

Flexible flux plane simulations of parasitic absorption in nanoplasmonic thin-film silicon solar cells

Photovoltaic light trapping theory and experiment do not always clearly demonstrate how much useful optical absorption is enhanced, as opposed to parasitic absorption that cannot improve efficiencies. In this work, we develop a flexible flux plane method for capturing these parasitic losses within finite-difference time-domain simulations, which was applied to three classical types of light trapping cells (e.g., periodic, random and plasmonic). Then, a 2 µm-thick c-Si cell with a correlated random front texturing and a plasmonic back reflector is optimized. In the best case, 36.60 mA/cm2 Jsc is achieved after subtracting 3.74 mA/cm2 of parasitic loss in a 2-µm-thick c-Si cell slightly above the Lambertian limit.

Design of CdZnTe and Crystalline Silicon Tandem Junction Solar Cells

A tandem photovoltaic system consisting of cadmium zinc telluride/crystalline silicon (CZT/c-Si) combines the successful technologies of cadmium telluride and silicon into a single platform and offers the potential efficiencies up to 46% in theory. However, the highest efficiency fabricated CZT/c-Si tandem cell is only 16.8% today. In this paper, we develop a detailed model for single-junction CZT and tandem CZT/c-Si PV cells, which is verified with experimental data. Based on this model, we propose three hypotheses to explain the anomalously low Voc observed in tandem cells: a lowquality tunnel junction, a Schottky barrier, and through-thickness shunting path. We then suggest a simple experiment to distinguish these hypotheses. After that, we provide a physics-based analysis of the magnitude of all the loss mechanisms present in the cell and an experimental strategy to mitigate each one. Ultimately, we predict that the ideal efficiency of CZT/c-Si tandem cell could reach 34.1%, if all these loss mechanisms were mitigated, and the CZT bandgap were adjusted to 1.8 eV, without requiring any improvement in bulk or surface recombination rates.

Hybrid dielectric light trapping designs for thin-film CdZnTe/Si tandem cells

Tandem solar cells consisting of high bandgap cadmium telluride alloys atop crystalline silicon have potential for high efficiencies exceeding the Shockley-Queisser limit. However, experimental results have fallen well below this goal significantly because of non-ideal current matching and light trapping. In this work, we simulate cadmium zinc telluride (CZT) and crystalline silicon (c-Si) tandems as an exemplary system to show the role that a hybrid light trapping and bandgap engineering approach can play in improving performance and lowering materials costs for tandem solar cells incorporating crystalline silicon. This work consists of two steps. First, we optimize absorption in the crystalline silicon layer with front pyramidal texturing and asymmetric dielectric back gratings, which results in 121% absorption enhancement from a planar structure. Then, using this pre-optimized light trapping scheme, we model the dispersion of the CdxZn1-xTe alloys, and then adjust the bandgap to realize the best current matching for a range of CZT thicknesses. Using experimental parameters, the corresponding maximum efficiency is predicted to be 16.08 % for a total tandem cell thickness of only 2.2 μm.

Collimated thermal radiation transfer via half Maxwell's fish-eye lens for thermophotovoltaics

Thermophotovoltaics (TPV) convert heat into electricity by capturing thermal radiation with a photovoltaic (PV) cell, ideally at efficiencies of 50% or more. However, excess heating of the PV cell from close proximity to the emitter substantially reduces the system efficiency. In this work, we theoretically develop and numerically demonstrate an approach to fundamentally improving TPV systems that allow for a much greater separation of an emitter and a receiver. Thus, we solve the excess heating dilemma, required for achieving theoretically high efficiencies. It consists of a spherically graded index lens known as Maxwells Fish-Eye (MFE) structure, capable of collimating hemispherical emission into a much narrower range of angles, close to the normal direction. To fully characterize the power radiation profile of the MFE, we perform finite-difference time-domain simulations for a quarter MFE and then map it onto a Gaussian beam approximation. The modeled beam properties are subsequently used to study a h...

Modeling and designing multilayer 2D perovskite / silicon bifacial tandem photovoltaics for high efficiencies and long-term stability

A key challenge in photovoltaics today is to develop cell technologies with both higher efficiencies and lower fabrication costs than incumbent crystalline silicon (c-Si) single-junction cells. While tandem cells have higher efficiencies than c-Si alone, it is generally challenging to find a low-cost, high-performance material to pair with c-Si. However, the recent emergence of 22% efficient perovskite photovoltaics has created a tremendous opportunity for high-performance, low-cost perovskite / crystalline silicon tandem photovoltaic cells. Nonetheless, two key challenges remain. First, integrating perovskites into tandem structures has not yet been demonstrated to yield performance exceeding commercially available crystalline silicon modules. Second, the stability of perovskites is inconsistent with the needs of most end-users, who install photovoltaic modules to produce power for 25 years or more. Making these cells viable thus requires innovation in materials processing, device design, fabrication, and yield. We will address these two gaps in the photovoltaic literature by investigating new types of 2D perovskite materials with n-butylammonium spacer layers, and integrating these materials into bifacial tandem solar cells providing at least 30% normalized power production. We find that an optimized 2D perovskite ((BA)2(MA)3(Sn0.6Pb0.4)4I13)/silicon bifacial tandem cell, given a globally average albedo of 30%, yields a normalized power production of 30.31%, which should be stable for extended time periods without further change in materials or encapsulation.

Characterization and redesign of perovskite/silicon tandem cells

Recently, metal-halide perovskites/crystalline silicon tandem cells have demonstrated extraordinarily rapid advances in efficiency, now exceeding 21%. However, the incomplete absorption of the bottom c-Si junction has still not been addressed. In this work, we build and characterize perovskite cells to obtain the losses of each layer. We then feed this data into an efficient simulation framework to analyze optical losses of both single and double junction world-record perovskite/silicon cells. Finally, asymmetric dielectric gratings, which are compatible with the fabrication constraints of perovskite/silicon tandem cells, are calculated to yield an increased Jsc of 19.17 mA/cm2.