Single-junction Photovoltaic Devices Research Articles

Efficient wide-bandgap perovskite photovoltaics with homogeneous halogen-phase distribution

Wide-bandgap (WBG) perovskite solar cells (PSCs) are employed as top cells of tandem cells to break through the theoretical limits of single-junction photovoltaic devices. However, WBG PSCs exhibit severe open-circuit voltage (Voc) loss with increasing bromine content. Herein, inhomogeneous halogen-phase distribution is pointed out to be the reason, which hinders efficient extraction of carriers. We thus propose to form homogeneous halogen-phase distribution to address the issue. With the help of density functional theory, we construct a double-layer structure (D-2P) based on 2-(9H-Carbazol-9-yl)ethyl]phosphonic acid molecules to provide nucleation sites for perovskite crystallization. Homogeneous perovskite phase is achieved through bottom-up templated crystallization of halogen component. The efficient carrier extraction reduces the Shockley-Read-Hall recombination, resulting in a high Voc of 1.32 V. As a result, D-2P-treated device (1.75 eV) achieves a record power conversion efficiency of 20.80% (certified 20.70%), which is the highest value reported for WBG (more than 1.74 eV) PSCs.

Interstellar photovoltaics

The term 'Solar Cell’ is commonly used for Photovoltaics that convert light into electrical energy. However, light can be harvested from various sources not limited to the Sun. This work considers the possibility of harvesting photons from different star types, including our closest neighbor star Proxima Centauri. The theoretical efficiency limits of single junction photovoltaic devices are calculated for different star types at a normalized light intensity corresponding to the AM0 spectrum intensity with AM0 = 1361 W/m2. An optimal bandgap of > 12 eV for the hottest O5V star type leads to 47% Shockley-Queisser photoconversion efficiency (SQ PCE), whereas a narrower optimal bandgap of 0.7 eV leads to 23% SQ PCE for the coldest red dwarf M0, M5.5Ve, and M8V type stars. Organic Photovoltaics (OPVs) are the most lightweight solar technology and have the potential to be employed in weight-restricted space applications, including foreseeable interstellar missions. With that in mind, the Sun’s G2V spectrum and Proxima Centauri’s M5.5Ve spectrum are considered in further detail in combination with two extreme bandgap OPV systems: one narrow bandgap system (PM2:COTIC-4F, Eg = 1.14 eV) and one wide bandgap system (PM6:o-IDTBR, Eg = 1.62 eV). Semi-empirically modeled JV-curves reveal that the absorption characteristics of the PM2:COTIC-4F blend match well with both the G2V and the M5.5Ve spectrum, yielding theoretical PCEs of 22.6% and 12.6%, respectively. In contrast, the PM6:o-IDTBR device shows a theoretical PCE of 18.2% under G2V illumination that drops sharply to 0.9% under M5.5Ve illumination.

Toward Current Matching in Tandem Dye-Sensitized Solar Cells.

The tandem pn-type dye-sensitized solar cells (pn-DSCs) have received much attention in the field of photovoltaic technologies because of their great potential to overcome the Shockley-Queisser efficiency limitation that applies to single junction photovoltaic devices. However, factors governing the short-circuit current densities (Jsc) of pn-DSC remain unclear. It is typically believed that Jsc of the pn-DSC is limited to the highest one that the two independent photoelectrodes can achieve. In this paper, however, we found that the available Jsc of pn-DSC is always determined by the larger Jsc that the photoanode can achieve but not by the smaller one in the photocathode. Such experimental findings were verified by a simplified series circuit model, which shows that a breakdown will occur on the photocathode when the photocurrent goes considerably beyond its threshold voltage, thus leading to an abrupt increase in Jsc of the circuit. The simulation results also suggest that a higher photoconversion efficiency of the pn-DSCs can be only achieved when an almost equivalent photocurrent is achieved for the two photoelectrodes.

21.6%-Efficient Monolithic Perovskite/Cu(In,Ga)Se2 Tandem Solar Cells with Thin Conformal Hole Transport Layers for Integration on Rough Bottom Cell Surfaces

Perovskite-based tandem solar cells can increase the power conversion efficiency (PCE) of conventional single-junction photovoltaic devices. Here, we present monolithic perovskite/CIGSe tandem sola...

Towards Efficient Spectral Converters through Materials Design for Luminescent Solar Devices.

Single-junction photovoltaic devices exhibit a bottleneck in their efficiency due to incomplete or inefficient harvesting of photons in the low- or high-energy regions of the solar spectrum. Spectral converters can be used to convert solar photons into energies that are more effectively captured by the photovoltaic device through a photoluminescence process. Here, recent advances in the fields of luminescent solar concentration, luminescent downshifting, and upconversion are discussed. The focus is specifically on the role that materials science has to play in overcoming barriers in the optical performance in all spectral converters and on their successful integration with both established (e.g., c-Si, GaAs) and emerging (perovskite, organic, dye-sensitized) cell types. Current challenges and emerging research directions, which need to be addressed for the development of next-generation luminescent solar devices, are also discussed.

Energy transfer within small molecule/conjugated polymer blends enhances photovoltaic efficiency

We employed ternary blends capable of energy transfer—a synthesized small molecule (SM-4OMe) comprising benzodithiophene (BDT) units (a molecule designed for energy transfer), and a polymer (PTB7-TH) with BDT units with desired packing orientation, and a fullerene—as active layers for single junction photovoltaic devices.

Controlled exfoliation of (100) GaAs-based devices by spalling fracture

The importance of exfoliation techniques increases as the semiconductor industry progresses toward thinner devices as a way to reduce material costs and improve performance. The controlled spalling technique is a recently developed substrate removal process that utilizes the physics of fracture to create wafer cleavage parallel to the surface at a precise depth. In this letter, we apply principles of linear elastic fracture mechanics to predict the process conditions needed to exfoliate (100) GaAs of a desired thickness. Spalling can be initiated in a controllable manner, by depositing a stressor film of a residual stress value just below the threshold value to induce a spontaneous spall. Experimental data show process window requirements to controllably spall (100) GaAs. Additionally, experimental spall depth in (100) GaAs compares well to spalling mechanics predictions when the effects of wafer thickness and modulus are considered. To test spalled material quality, III-V single junction photovoltaic devices are lifted off of a (100)-GaAs substrate by spalling methods and electrical characteristics are recorded. No degradation is observed in the spalled device, illustrating the potential of this method to rapidly produce thin, high quality devices.

Development and characterisation of laser power converters for optical power transfer applications

Interest in single-junction photovoltaic devices has been renewed in recent years due to their potential to produce very high optical to electrical conversion efficiencies under monochromatic illumination. A single-junction photovoltaic converter was developed for monochromatic radiation (a laser power converter (LPC)) to specifically convert monochromatic light at a wavelength of 1.55 µm (which is eye and skin safe up to a power density of 1 kW/m2) into electrical energy. Such devices find use in space-based laser power transfer, as well as fibre optic power delivery and the remote powering of subcutaneous equipment and other electrical devices. The LPC design is based on lattice-matched InGaAsP/InP, which includes elements for photon recycling and efficient carrier extraction. Here, the design process is reported based on computer simulated characteristics, and compare them to experimental data from both laboratory and field tests. The LPC demonstrates a conversion efficiency of 45% ± 1% under illumination at 1.55 µm and 1 kW/m2, and a conversion efficiency of 13.3% under AM1.5 solar illumination for potential dual-use applications. With further design optimisation, it has been predicted that the conversion efficiency will exceed 50% at 1.55 µm and 1 kW/m2.

Plasmon-Enhanced Triplet–Triplet Annihilation Using Silver Nanoplates

Photon upconversion processes have attracted substantial interest as a means of circumventing the Shockley–Queisser limit for single-junction photovoltaic devices. Despite this promise, the quantum yield of most upconversion processes is very low at the light intensities typical of solar radiation (∼100 mW/cm2). Additionally, bimolecular upconversion processes that rely on molecular diffusion (e.g., triplet–triplet annihilation) typically see further reductions in quantum yield when the upconverting chromophores are confined to a solid state or thin film matrix. Here we report a plasmon-based enhancement of the triplet–triplet annihilation process when silver nanoplates are embedded in poly(methyl methacrylate) thin films containing the upconverting materials palladium(II) octaethylporphyrin and 9,10-diphenylanthracene. The silver nanoplates are synthesized with localized surface plasmon resonance bands tailored to overlap strongly with the Q-band of the porphyrin, leading to enhanced light absorption within the film and higher overall triplet concentrations. Optimization of the silver nanoplate loading leads to a nearly 10-fold increase in the upconverted light intensity compared with control samples containing no silver.

Synthesis and Characterization of Wurtzite Cu2ZnSnS4 Nanocrystals

Copper–zinc–tin–chalcogenide (CZTSSe) with earth abundant elements has attracted increasing attention due to large absorption coefficient and band gap of ∼1.5 eV which is near the optimum band gap of single-junction photovoltaic devices. In this study, we used commercially available precursors to produce wurtzite Cu2ZnSnS4 nanocrystals by simple solvothermal synthesis. Different from the typical kesterite or stannite phases of CZTS, the nanocrystals synthesized in this study are in wurtzite phase with hexagonal crystal cell. The n-dodecanethiol was used to control the reactivity of metal ions, leading to the controlled size of CZTS nanoparticle by simply varying the reaction time. Furthermore, the as synthesized CZTS nanocrystals have novel wurtzite crystal structure. As a result, a red shift of absorption band edge between the CZTS nanoparticles with different size was obtained. Our study provides an extending method of CZTS nanocrystal ink preparation awaiting for further photovoltaic device application.

Effective method to extract optical bandgaps in Si nanowire arrays

A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc's model. The optical bandgap of the SiNW arrays (112 nm in average diameter) was measured to be ~1.19 eV, which is larger than the ~1.08 eV bandgap of bulk Si. Further decreasing the wire diameter to 68 nm caused an increase of the bandgap to ~1.24 eV, which is closer to the optimal bandgap (~1.40 eV) required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays.