Highly Efficient Hybrid Polymer and Amorphous Silicon Multijunction Solar Cells with Effective Optical Management

Theoretical design of low bandgap donor-acceptor (D-A) monomers for polymer solar cells: DFT and TD-DFT study

Endeavors have been made to construct new donor-acceptor (D-A) monomers utilizing 9 H-carbazole (CB) as electron donors and different electron acceptors. All estimations were finished using DFT and TD-DFT, and B3LYP level with a 6-311 G basis set in the gas and chloroform solvent. The impacts of the distinctive acceptors on the geometry of molecules and optoelectronic properties of these D-A monomers were discussed to dissect the connection connecting the molecular structures and the optoelectronic properties. Likewise, the HOMO - LUMO energies, atomic orbital densities are calculated theoretically. Notwithstanding the charge transfer measure between the carbazole electron donor unit and the electron acceptor one is upheld by breaking down the optical spectra of the acquired monomers and the restriction of involved HOMO and LUMO. The outcomes show that the D-A monomers, CB-ODP, CB-TDP, and CB-SDP, are acceptable for optoelectronic applications in organic solar cells like BHJ.

Recent Progress in Interconnection Layer for Hybrid Photovoltaic Tandems

Hybrid tandem solar cells offer the benefits of low cost and full solar spectrum utilization. Among the hybrid tandem structures explored to date, the most popular ones have four (simple stacking design) or two (terminal/tunneling layer addition design) terminal electrodes. Although the latter design is more cost-effective than the former, its widespread application is hindered by the difficulty of preparing an interface between two solar cell materials. The oldest approach to the in-series bonding of two or more bandgap solar cells relies on the introduction of a tunneling layer in multijunction III-V solar cells, but it has some limitations, e.g., the related materials/technologies are applicable only to III-V and certain other solar cells. Thus, alternative methods of realizing junction contacts based on the use of novel materials are highly sought after. Here, the strategies used to realize high-performance tandem cells are described, focusing on interface control in terms of bonding two or more solar cells for tandem approaches. The presented information is expected to aid the establishment of ideal methods of connecting two or more solar cells to obtain the highest performance for different solar cell choices with minimized energy loss through the interface.

Multifunctional Bilayer Template for Near-Infrared-Sensitive Organic Solar Cells

For organic solar cells (OSCs) based on nonplanar phthalocyanines, it has previously been reported that a thin film composed of triclinic crystals with face-on (or flat-lying)-oriented molecules, typically obtained with a CuI template layer, is desired for optical absorption in the near-infrared (NIR) spectral region. However, this work demonstrates that for a PbPc-C₆₀ donor-acceptor pair, less face-on orientation with a broader orientation distribution obtained with a new template layer consisting of a ZnPc/CuI bilayer is more desirable in terms of solar cell efficiency than the face-on orientation. A NIR-sensitive PbPc-C₆₀ OSC employing this bilayer-templated PbPc film is found to increase the internal quantum efficiency (IQE) by 36% on average in the NIR spectral region compared to a device using a CuI-templated PbPc film. Analyses of the change in IQE using the exciton diffusion model and the entropy- and disorder-driven charge-separation model suggest that the improved IQE is attributed to the facilitated dissociation of charge-transfer excitons as well as the reduction in exciton quenching near the indium tin oxide surface.

Flexible Semiconductor Technologies with Nanoholes-Provided High Areal Coverages and Their Application in Plasmonic-Enhanced Thin Film Photovoltaics

Mechanical flexibility and advanced light management have gained great attentions in designing high performance, flexible thin film photovoltaics for the realization of building-integrated optoelectronic devices and portable energy sources. This study develops a soft thermal nanoimprint process for fabricating nanostructure decorated substrates integrated with amorphous silicon solar cells. Amorphous silicon (a-Si:H) solar cells have been constructed on nanoholes array textured polyimide (PI) substrates. It has been demonstrated that the nanostructures not only are beneficial to the mechanical flexibility improvement but also contribute to sunlight harvesting enhancement. The a-Si:H solar cells constructed on such nanopatterned substrates possess broadband-enhanced light absorption, high quantum efficiency and desirable power conversion efficiency (PCE) and still experience minimal PCE loss even bending around 180°. The PCE performance without antireflection coatings increases to 7.70% and it improves 40% compared with the planar devices. Although the advantages and feasibility of the schemes are demonstrated only in the application of a-Si:H solar cells, the ideas are able to extend to applications of other thin film photovoltaics and semiconductor devices.

Scalable Production of Mechanically Robust Antireflection Film for Omnidirectional Enhanced Flexible Thin Film Solar Cells

Antireflection (AR) at the interface between the air and incident window material is paramount to boost the performance of photovoltaic devices. 3D nanostructures have attracted tremendous interest to reduce reflection, while the structure is vulnerable to the harsh outdoor environment. Thus the AR film with improved mechanical property is desirable in an industrial application. Herein, a scalable production of flexible AR films is proposed with microsized structures by roll-to-roll imprinting process, which possesses hydrophobic property and much improved robustness. The AR films can be potentially used for a wide range of photovoltaic devices whether based on rigid or flexible substrates. As a demonstration, the AR films are integrated with commercial Si-based triple-junction thin film solar cells. The AR film works as an effective tool to control the light travel path and utilize the light inward more efficiently by exciting hybrid optical modes, which results in a broadband and omnidirectional enhanced performance.

Influence of surface relaxation of strained layers on atomic resolution ADF imaging

Surface relaxation of thin transmission electron microscopy (TEM) specimens of strained layers results in a severe bending of lattice planes. This bending significantly displaces atoms from their ideal channeling positions which has a strong impact on the measured annular dark field (ADF) intensity. With the example of GaAs quantum wells (QW) embedded in a GaP barrier, we model the resulting displacements by elastic theory using the finite element (FE) formalism. Relaxed and unrelaxed super cells served as input for state of the art frozen phonon simulation of atomic resolution ADF images. We systematically investigate the dependencies on the sample´s geometric parameters, i.e. QW width and TEM sample thickness, by evaluating the simulated intensities at the atomic column´s positions as well as at the background positions in between. Depending on the geometry the ADF intensity can be affected in a range several nm from the actual interface. Moreover, we investigate the influence of the surface relaxation on the angular distribution of the scattered intensity. At high scattering angles we observe an intensity reduction at the interface as well as in the GaP barrier due to de-channeling. The amount of intensity reduction at an atomic column is directly proportional to its mean square displacement. On the contrary we find a clearly increased intensity at low angles caused by additional diffuse scattering. We discuss the implications for quantitative evaluations as well as strategies to compensate for the reduced intensities.

Nanoimprint-Transfer-Patterned Solids Enhance Light Absorption in Colloidal Quantum Dot Solar Cells

Colloidal quantum dot (CQD) materials are of interest in thin-film solar cells due to their size-tunable bandgap and low-cost solution-processing. However, CQD solar cells suffer from inefficient charge extraction over the film thicknesses required for complete absorption of solar light. Here we show a new strategy to enhance light absorption in CQD solar cells by nanostructuring the CQD film itself at the back interface. We use two-dimensional finite-difference time-domain (FDTD) simulations to study quantitatively the light absorption enhancement in nanostructured back interfaces in CQD solar cells. We implement this experimentally by demonstrating a nanoimprint-transfer-patterning (NTP) process for the fabrication of nanostructured CQD solids with highly ordered patterns. We show that this approach enables a boost in the power conversion efficiency in CQD solar cells primarily due to an increase in short-circuit current density as a result of enhanced absorption through light-trapping.

Open-Source Automated Mapping Four-Point Probe

Scientists have begun using self-replicating rapid prototyper (RepRap) 3-D printers to manufacture open source digital designs of scientific equipment. This approach is refined here to develop a novel instrument capable of performing automated large-area four-point probe measurements. The designs for conversion of a RepRap 3-D printer to a 2-D open source four-point probe (OS4PP) measurement device are detailed for the mechanical and electrical systems. Free and open source software and firmware are developed to operate the tool. The OS4PP was validated against a wide range of discrete resistors and indium tin oxide (ITO) samples of different thicknesses both pre- and post-annealing. The OS4PP was then compared to two commercial proprietary systems. Results of resistors from 10 to 1 MΩ show errors of less than 1% for the OS4PP. The 3-D mapping of sheet resistance of ITO samples successfully demonstrated the automated capability to measure non-uniformities in large-area samples. The results indicate that all measured values are within the same order of magnitude when compared to two proprietary measurement systems. In conclusion, the OS4PP system, which costs less than 70% of manual proprietary systems, is comparable electrically while offering automated 100 micron positional accuracy for measuring sheet resistance over larger areas.