Sarah E. Sofia

Techno-economic analysis of tandem photovoltaic systems

Tandem solar cells offer the potential of conversion efficiencies exceeding those of single-junction solar cells, but also incur higher fabrication costs. The question arises under which conditions a tandem solar cell becomes economically preferable to both of the single-junction sub-cells it comprises. We present an analysis based on cost and efficiency relations to answer this question for a double-junction tandem solar cell. We find that combining two ideally band-gap-matched single-junction solar cell technologies into a tandem should be a “marriage of equals”: the sub cells should be produced at similar

Light management in mechanically-stacked GaAs/Si tandem solar cells: Optical design of the Si bottom cell

per W costs, both sub cells should have similar efficiencies when operated independently, and the costs to turn both cells into a system should be similar. We discuss examples of different hypothetical and actual tandem solar cell technologies and show the intricacies of imbalances in the mentioned factors. We find that tandem-solar-cell-based PV power stations for existing solar-cell technologies offer the potential to reduce the levelized cost of electricity (LCOE), provided suitable top cells are developed.

Optical loss analysis of monolithic perovskite/Si tandem solar cell

Light management in the Si bottom cell of a GaAs/Si tandem system is crucial due to the bandgap mismatch of the two materials, which results in a low current of the bottom cell. To evaluate the light coupling and light trapping, we developed an optical model to simulate the light absorption in the silicon bottom cell. This optical model is an extension of Basores analytical model. By comparing the simulation with the measurement of the prototype tandem solar cell, we find that the Si bottom cell can gain up to 2.4 mA/cm2 photocurrent by improving light coupling. In addition, we study the impact of the rear surface reflectance on the photocurrent in the Si bottom cell. We observed that ~17% relative increase in current generated in this bottom cell can be achieve by changing the rear surface design from a full area metal contact to a local-back-surface-field configuration.

Tandem solar cell cost relations

Coupling perovskite and silicon solar cells in a tandem configuration is considered an attractive method to increase conversion efficiency beyond the single-junction Shockley-Queisser limit. While a mechanically-stacked perovskite/silicon tandem solar cell has been demonstrated, a method to electrically couple perovskite and silicon solar cell in a monolithic configuration has not been demonstrated. In this contribution, we design and demonstrate a working monolithic perovskite/silicon tandem solar cell, enabled by a silicon tunnel junction, with a VOC of 1.58 V. We further discuss possible efficiency loss mechanisms and mitigation strategies.

Metal grid contact design of four-terminal tandem solar cells

Tandem solar cells overcome conversion efficiency limitations of conventional single-junction solar cells by using multiple absorber materials with different band gap energies. The absorber materials are stacked in sequence with decreasing band gap energy, thus reducing thermalization losses in the conversion process. The improved conversion efficiency of a tandem solar cell comes at increased fabrication complexity and, hence, production cost. More fabrication steps and more materials are needed to produce a tandem solar cell than to produce a single-junction solar cell. Economically, a tandem solar cell is of interest if it generates electricity at a lower cost than either of its comprising single-junction counter parts. In this paper we present cost relations that link the costs and efficiencies of two single junction solar cells to those of a dual-junction tandem solar cell made from the combination of the two cells. In doing so, we establish criteria under which tandem solar cells are economically viable.

Device impact of photon recycling and luminescent coupling on InGaP/Si tandems

Flat-panel tandem solar cells have demonstrated the potential to exceed the efficiencies of their single-junction constituents. However, robust design rules for tandem solar cells are currently lacking, slowing the development of cost-effective implementations of this technology. A double-junction solar cell with four-terminal (4T) architecture stacks two electrically independent subcells and avoids current-matching losses, resulting in two main advantages over the conventional integrated two-terminal (2T) architecture: a higher energy yield and a loosened constraint on material bandgap combinations. Because both subcells are contacted independently in a 4T tandem, multiple stacked semitransparent contacts are needed, causing optical and series resistance losses. Moreover, for stationary flat-panel tandems, contacts need to be optimized for a varying direction of incident sunlight. In this study, we develop a framework for optimizing metal grid contacts for 4T tandem solar cells and quantify the electrical and optical loss associated with these contacts. We also explore the range of conditions for which it is beneficial to align metal grid contact fingers. We find that, for most applications, the front and back contacts of the top cell should be aligned, resulting in a decrease in energy yield loss between

23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability

\text{5}{{\% }}\;{\text{and}}\;15{{\% }}

The Bravyi-Kitaev transformation: Properties and applications

, while aligning the bottom cell contacts is not beneficial and may even reduce total energy yield. Finally, we show that for nonideally matched subcell pairings, the contacts loss in a 4T tandem is not enough to counter the yield benefit of 4T over 2T tandems, while the contact loss may make the yield for more ideally matched subcells be comparable for 2T and 4T devices.

Metal Grid Contact Design for Four-Terminal Tandem Solar Cells

We numerically evaluate the device impact of photon re-absorption on InGaP/Si tandem solar cells using a coupled optical-electronic device model. The presented simulation results provide guidelines for designing high performance InGaP/Si tandem device. We find that including the effects of photon recycling (PR) and luminescent coupling (LC) results in a 12.5% increase in optimum top-cell thickness for a two-terminal configuration. Furthermore, PR and LC affect the sensitivity of the tandems conversion efficiency to various device parameters. As the InGaP bulk lifetime increases, there is an absolute efficiency increase of up to 0.7% for the two-terminal as well as the four-terminal configuration. Considering PR and LC furthermore reduces the power generation sensitivity to shunting in the two-terminal configuration. For the four-terminal configuration, photon re-absorption has a less significant impact.