Solar energy offers a viable solution to our growing energy need. While adoption of conventional photovoltaics on rooftops and in solar farms has grown rapidly in the last decade, there is still plenty of opportunity for expansion. See-through solar technologies with partial light transmission developed over the past 30 years have initiated methods of integration not possible with conventional modules. The large-scale deployment necessary to offset global energy consumption could be further accelerated by developing fully invisible solar cells that selectively absorb ultraviolet and near-infrared light, allowing many of the surfaces of our built environment to be turned into solar harvesting arrays without impacting the function or aesthetics. Here, we review recent advances in photovoltaics with varying degrees of visible light transparency. We discuss the figures of merit necessary to characterize transparent photovoltaics, and outline the requirements to enable their widespread adoption in buildings, windows, electronic device displays, and automobiles. Transparency offers integration routes unavailable to opaque photovoltaics. Here, Lunt and co-workers review recent progress in transparent solar technologies, highlight technical challenges and measurement considerations, and review performance requirements for various applications.

Emergence of highly transparent photovoltaics for distributed applications

Highly sensitive photodetection even approaching the single-photon level is critical to many important applications. Graphene-based hybrid phototransistors are particularly promising for high-sensitivity photodetection because they have high photoconductive gain due to the high mobility of graphene. Given their remarkable optoelectronic properties and solution-based processing, colloidal quantum dots (QDs) have been preferentially used to fabricate graphene-based hybrid phototransistors. However, the resulting QD/graphene hybrid phototransistors face the challenge of extending the photodetection into the technologically important mid-infrared (MIR) region. Here, we demonstrate the highly sensitive MIR photodetection of QD/graphene hybrid phototransistors by using plasmonic silicon (Si) QDs doped with boron (B). The localized surface plasmon resonance (LSPR) of B-doped Si QDs enhances the MIR absorption of graphene. The electron-transition-based optical absorption of B-doped Si QDs in the ultraviolet (UV) ...

Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors

The transition to fully energetically sustainable architecture through the realization of so-called net zero-energy buildings is currently in progress in areas with low population density. However, this is not yet true in cities, where the cost of land for the installation of ground photovoltaic (PV) is prohibitively high and the rooftop space is too scarce to accommodate the PV modules necessary for sustaining the electrical requirements of tall buildings. Thus, new technologies are being investigated to integrate solar-harvesting devices into building facades in the form of PV windows or envelope elements. Luminescent solar concentrators (LSCs) are the most promising technology for semi-transparent, electrodeless PV glazing systems that can be integrated ‘invisibly’ into the built environment without detrimental effects to the aesthetics of the building or the quality of life of the inhabitants. After 40 years of research, recent breakthroughs in the realization of reabsorption-free emitters with broadband absorption have boosted the performance of LSCs to such a degree that they might be commercialized in the near future. In this Perspective, we explore the successful strategies that have allowed this change of pace, examining and comparing the different types of chromophores and waveguide materials, and discuss the issues that remain to be investigated for further progress. Luminescent solar concentrators (LSCs) offer a unique opportunity to ‘invisibly’ integrate semi-transparent photovoltaic architectural elements, such as electrodeless glazing units, into the building envelope. This Review highlights the advancements making LSCs a realistic technology for near zero-energy buildings, along with the remaining challenges and strategies for further device optimization.

Luminescent solar concentrators for building-integrated photovoltaics

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for terrestrial and space-based photovoltaics. Due to their high emission efficiencies and readily tunable emission and absorption spectra, colloidal quantum dots have emerged as a new and promising type of LSC fluorophore. Spectral tunability of the quantum dots also facilitates the realization of stacked multilayered LSCs, where enhanced performance is obtained through spectral splitting of incident sunlight, as in multijunction photovoltaics. Here, we demonstrate a large-area (>230 cm2) tandem LSC based on two types of nearly reabsorption-free quantum dots spectrally tuned for optimal solar-spectrum splitting. This prototype device exhibits a high optical quantum efficiency of 6.4% for sunlight illumination and solar-to-electrical power conversion efficiency of 3.1%. The efficiency gains due to the tandem architecture over single-layer devices quickly increase with increasing LSC size and can reach more than 100% in structures with window sizes of more than 2,500 cm2. Multijunctions have long been used to enhance photovoltaic solar cell efficiency. Here, a large-area tandem luminescent solar concentrator is demonstrated using two types of quantum dot with low reabsorption.

Tandem luminescent solar concentrators based on engineered quantum dots

Halide perovskite nanocrystals (NCs) are promising solution-processed emitters for low-cost optoelectronics and photonics. Doping adds a degree of freedom for their design and enables us to fully decouple their absorption and emission functions. This is paramount for luminescent solar concentrators (LSCs) that enable fabrication of electrode-less solar windows for building-integrated photovoltaic applications. Here, we demonstrate the suitability of manganese-doped CsPbCl3 NCs as reabsorption-free emitters for large-area LSCs. Light propagation measurements and Monte Carlo simulations indicate that the dopant emission is unaffected by reabsorption. Nanocomposite LSCs were fabricated via mass copolymerization of acrylate monomers, ensuring thermal and mechanical stability and optimal compatibility of the NCs, with fully preserved emission efficiency. As a result, perovskite LSCs behave closely to ideal devices, in which all portions of the illuminated area contribute equally to the total optical power. The...

Doped Halide Perovskite Nanocrystals for Reabsorption-Free Luminescent Solar Concentrators

Building-integrated photovoltaics is gaining consensus as a renewable energy technology for producing electricity at the point of use. Luminescent solar concentrators (LSCs) could extend architectural integration to the urban environment by realizing electrode-less photovoltaic windows. Crucial for large-area LSCs is the suppression of reabsorption losses, which requires emitters with negligible overlap between their absorption and emission spectra. Here, we demonstrate the use of indirect-bandgap semiconductor nanostructures such as highly emissive silicon quantum dots. Silicon is non-toxic, low-cost and ultra-earth-abundant, which avoids the limitations to the industrial scaling of quantum dots composed of low-abundance elements. Suppressed reabsorption and scattering losses lead to nearly ideal LSCs with an optical efficiency of η = 2.85%, matching state-of-the-art semi-transparent LSCs. Monte Carlo simulations indicate that optimized silicon quantum dot LSCs have a clear path to η > 5% for 1 m2 devices. We are finally able to realize flexible LSCs with performances comparable to those of flat concentrators, which opens the way to a new design freedom for building-integrated photovoltaics elements. Reabsorption losses in luminescent solar concentrators can be avoided by the use of indirect-bandgap semiconductor nanostructures. The technology has been used to demonstrate flexible luminescent solar concentrators with performance comparable to flat concentrators.

Samantha K. Ehrenberg

Papers

Highly efficient luminescent solar concentrators based on earth-abundant indirect-bandgap silicon quantum dots

Silicon nanocrystals from plasma synthesis