Zhengshan J. Yu

PVMirror: A New Concept for Tandem Solar Cells and Hybrid Solar Converters

As the solar electricity market has matured, energy conversion efficiency and storage have joined installed system cost as significant market drivers. In response, manufacturers of flat-plate silicon photovoltaic (PV) cells have pushed cell efficiencies above 25%-nearing the 29.4% detailed-balance efficiency limit-and both solar thermal and battery storage technologies have been deployed at utility scale. This paper introduces a new tandem solar collector employing a “PVMirror” that has the potential to both increase energy conversion efficiency and provide thermal storage. A PVMirror is a concentrating mirror, spectrum splitter, and light-to-electricity converter all in one: It consists of a curved arrangement of PV cells that absorb part of the solar spectrum and reflect the remainder to their shared focus, at which a second solar converter is placed. A strength of the design is that the solar converter at the focus can be of a radically different technology than the PV cells in the PVMirror; another is that the PVMirror converts a portion of the diffuse light to electricity in addition to the direct light. We consider two case studies-a PV cell located at the focus of the PVMirror to form a four-terminal PV-PV tandem, and a thermal receiver located at the focus to form a PV-CSP (concentrating solar thermal power) tandem-and compare the outdoor energy outputs to those of competing technologies. PVMirrors can outperform (idealized) monolithic PV-PV tandems that are under concentration, and they can also generate nearly as much energy as silicon flat-plate PV while simultaneously providing the full energy storage benefit of CSP.

Silicon heterojunction solar cells with effectively transparent front contacts

We demonstrate silicon heterojunction solar cells with microscale effectively transparent front contacts (ETCs) that redirect incoming light to the active area of the solar cell. Replacing standard contact electrodes by ETCs leads to an enhancement in short circuit current density of 2.2 mA cm−2 through mitigation of 6% shading losses and improved antireflection layers. ETCs enable low loss lateral carrier transport, with cells achieving an 80.7% fill factor. Furthermore, dense spacing of the contact lines allows for a reduced indium tin oxide thickness and use of non-conductive, optically optimized antireflection coatings such as silicon nitride. We investigated the performance of ETCs under varying light incidence angles, and for angles parallel to the ETC lines find that there is no difference in photocurrent density with respect to bare indium tin oxide layers. For angles perpendicular to the ETC lines, we find that the external quantum efficiency (EQE) always outperforms cells with flat contact grids.

Evaluation of spectrum-splitting dichroic mirrors for PV mirror tandem solar cells

Single-junction solar cells are approaching their theoretical efficiency limit, and tandem or multi-junction architectures provide a route for further increasing efficiency. However, due to high material cost, these technologies have traditionally been restricted to high concentrations with the penalty of the loss of diffuse light in outdoor applications. We propose a new tandem concept called a “PVMirror” that makes use of the global spectrum. It utilizes PV cells as a three-in-one technology-they act as a concentrating mirror, spectrum splitter, and high-efficiency light-to-electricity converter. A key element of this technology is an effective spectrum-splitting dichroic mirror, and this paper evaluates three dichroic mirror designs. Prototype PVMirrors with these mirrors and silicon heterojunction solar cells were made, and their reflection and transmission spectra confirm the spectrum-splitting capability of the dichroic mirrors. Outdoor testing shows that reflected light is successfully concentrated to a focus at which the second sub-cell in the tandem is to be located.

Optimization of front TCO layer of silicon heterojunction solar cells for tandem applications

The front transparent conductive oxide (TCO) layers of silicon heterojunction solar cells need to be optimized electrically and optically to minimize losses due to sheet resistance and free carrier absorption. This optimization has already investigated for the wavelength range of 300-1100 nm, but not for the projected wavelength range of 700-1100 nm for a silicon cell that is applied in a tandem structure as the bottom cell. Here, we demonstrate a routine for determining the total loss associated with the front TCO layer and employ it to determine which carrier density, mobility, and finger pitch combinations minimize loss. For a representative ITO film with a mobility of approximately 20 cm2/Vs and a carrier density of approximately 2.5×1020 cm-3, the total loss over the wavelength range of 700-1100 nm is minimized by using a linger spacing of 3 mm and ITO thickness of 100-110 nm.

Modeling of GaAs/Silicon PVMirror tandem system: A case study

We propose a new, optically coupled tandem architecture with a silicon “PVMirror” as the bottom cell. A silicon PVMirror is a curved silicon module with an integrated dichroic mirror that transmits near-infrared light to the silicon cells and reflects and focuses visible light onto a GaAs receiver. Simulating such a system with record GaAs and silicon cells and an ideal dichroic mirror shows the tandem efficiency can reach over 36% with no diffuse light and over 30% with 20% diffuse light. A sensitivity analysis indicates that the PVMirror tandem is most sensitive to the GaAs cell performance and dichroic mirror cut-off wavelength but relatively insensitive to the silicon cell performance.