A method for measuring light absorption using photothermal deflection spectroscopy (PDS) is investigated. Typical reflectance and transmittance (R & T) absorption measurements have low spectral resolution (SR) with standard setups. It is believed that absorption measurements using PDS solve this problem because of its high sensitivity, even at a high SR. However, accurate absorption measurements have not been obtained using conventional PDS. The reflectance of the sample must be considered in the absorptance calculations. Various absorption measurement methods are compared and a novel calculation method for accurate PDS absorption measurements is proposed. A sample with quantum structures is fabricated and PDS measurements are performed. Exciton absorption peaks are also observed. The high SR of PDS is demonstrated and the measured exciton absorption peaks are confirmed using piezoelectric photothermal measurements. Furthermore, it is confirmed that the absolute values of absorptance derived from the R & T and PDS measurements agree well. The new PDS absorption measurement technique enables to obtain accurate absorption characteristics, which are indispensable for designing and optimizing optoelectronic devices.

A novel, camera‐based method for direct implied open‐circuit voltage (i V OC ) imaging via the use of a single bandpass filter (s‐BPF) is developed for large‐area photovoltaic solar cells and precursors. The photoluminescence (PL) emission is imaged using a narrow BPF with centre energy inside the high‐energy tail of the PL emission, utilising the close‐to‐unity and nearly constant absorptivity of typical photovoltaic devices in this energy range. As a result, the exact value of the sample's absorptivity within the BPF transmission band is not required. The use of an s‐BPF enables a fully contactless approach to calibrate the absolute PL photon flux for spectrally integrated detectors, including cameras. The method eliminates the need for knowledge of the imaging system spectral response. Through an appropriate choice of the BPF centre energy, a range of absorber compositions or a single absorber with different surface morphologies, such as planar and textured, can be imaged, all without the need for additional detection optics. The feasibility of this s‐BPF method is first validated. The relative error in i V OC is determined to be ≤1.5%. The method is then demonstrated on device stacks with two different perovskite compositions commonly used in single‐junction and monolithic tandem solar cells.

Crystalline Si solar cells based on thin wafers, with thicknesses in the range of 5–50 μm, can find applications in a wide range of markets where flexibility and bendability are important. For these cells, avoiding standard macroscopic texture is desirable to increase structural integrity. Herein, a nanopatterned SiN x antireflection (AR) coating that consists of 174 nm‐radius and 118 nm‐high SiN x nanodisks arranged in a square lattice on a thin (59 nm) SiN x layer is introduced. This geometry combines Fabry–Pérot AR and forward scattering by a resonant Mie mode to achieve high transmission into the Si absorber over a broad spectral band. The nanostructured coating is patterned on a commercial interdigitated‐back‐contact (IBC) Si solar cell, experimentally demonstrating a short‐circuit current density (J sc) of 36.9 mA cm−2, 2.3 mA cm−2 higher than for a single‐layer AR coated cell, and an efficiency of 16.3% at a thickness of around 100 μm. It is shown that light incoupling efficiency is comparable to that of pyramidal texturing, while the absorption in the infrared is lower, due to less‐effective light trapping. Overall, nanopatterned SiN x broadband AR coatings are an appealing option for improving light management in ultrathin solar cells and other optoelectronic devices.

Antimony-based chalcogenides have emerged as promising candidates for next-generation thin film photovoltaics. Particularly, binary Sb2S3 thin films have exhibited great potential for optoelectronic applications, due to the facile and low-cost fabrication, simple composition, decent charge transport and superior stability. However, most of the reported efficient Sb2S3 solar cells are realized based on chemical bath deposition and hydrothermal methods, which require large amount of solution and are normally very time-consuming. In this work, Ag ions are introduced within the Sb2S3 sol-gel precursors, and effectively modulated the crystallization and charge transport properties of Sb2S3. The crystallinity of the Sb2S3 crystal grains are enhanced and the charge carrier mobility is increased, which resulted improved charge collection efficiency and reduced charge recombination losses, reflected by the greatly improved fill factor and open-circuit voltage of the Ag incorporated Sb2S3 solar cells. The champion devices reached a record high power conversion efficiency of 7.73% (with antireflection coating), which is comparable with the best photovoltaic performance of Sb(2)S(3 )solar cells achieved based on chemical bath deposition and hydrothermal techniques, and pave the great avenue for next-generation solution-processed photovoltaics.

Greater stability of low-dimensional halide perovskites as opposed to their three-dimensional counterparts, alongside their high extinction coefficient and thus excellent emission properties, have made them popular candidates for optoelectronic applications. Topological edges are found in two-dimensional perovskites that show distinct electronic properties. In this work, using Kelvin Probe Force Microscopy, performed on butylammonium lead bromide (BA2PbBr4) single crystals with optical bandgap of ~413 nm, we elucidate the electronic response of the edges and their potential impact on photodetector devices. We show that the charge-carriers are accumulated at the edges, increasing with the edge height. Wavelength-dependent surface photovoltage (SPV) measurements reveal that multiple sub-bandgap states exist in BA2PbBr4. As the edge height increases, the SPV amplitude at the edges reduces slightly more as compared to the adjacent regions, known as terraces, indicating relatively less reduction in band-bending at the surface due possibly to increased de-population of electrons from sub-bandgap states in the upper bandgap half. The existence of sub-bandgap states is further confirmed by the observation of below-bandgap emission (absorption) peaks characterised by spectral photoluminescence and photothermal deflection spectroscopy measurements. Finally, we fabricated a photodetector using a millimetre size BA2PbBr4 single crystal. Noticeable broadband photodetection response was observed in the sub-bandgap regions under green and red illumination, which is attributed to the existence of sub-bandgap states. Our observations suggest edge-height dependence of charge-carrier behaviour in BA2PbBr4 single crystals, a potential pathway that can be exploited for efficient broadband photodetector fabrication.

P-type multi-crystalline PERC silicon solar cells are susceptible to strong light and elevated temperature-induced degradation (LeTID). Various approaches, such as illuminated annealing and modified thermal processes, have been investigated to suppress LeTID. An undesirable side effect of these processes is an increased contact resistance at the front screen printed imgers, and therefore a lower fill factor. Previous work has indicated that this may be caused by the transport and accumulation of hydrogen at the contacts and that this behaviour can be limited by applying a reverse bias across the cell during a dark anneal. In this paper, we have conducted a similar biased annealing process to investigate the effect of different cooling conditions on PERC cell performance, and developed a biased annealing treatment that not only improves the efficiency but also maintains the stability during the light soak test. Preliminary results of transferring this process into 6-inch PERC cells using a full-scale industrial tool are also presented.

The sub-bandgap photoluminescence (PL) arising from dislocations in crystalline silicon (known as “D-lines”) has been studied for over half a century. However, many properties of the D-lines such as the defect parameters and the underlying recombination mechanism are poorly understood. In this study, we perform both temperature-dependent and injection-dependent hyperspectral mapping and apply this to a cast-mono silicon sample held at room-temperature and above. We parameterize the energy levels and defect densities of the D-lines in this sample. We also demonstrate for the first time that the D1 line in silicon wafers originates from the donor–acceptor pair recombination mechanism.

Reduction of surface reflection loss is crucial for high efficiency next generation Si solar cells. Surface texturing provides a viable method to reduce loss over the full solar bandwidth. Previous studies have concentrated on simple moth-eye silicon pillar arrays protruding from the surface. Using FDTD simulation methods, we undertake a systematic investigation into performance benefits provided by complex semi-random photonic quasi-crystal surface patterning methodologies whereby arrays of air holes are etched deep into the solar cell surface. In contrast to other studies we carefully investigate the effect of lattice symmetry, systematically comparing performance of simple 6-fold symmetric triangular photonic crystal patterning to 12 fold symmetry photonic quasicrystal patterning and infinitely symmetric 2D Fibonacci patterning. We optimize key geometric parameters such as lattice pitch, hole size and etch depth to maximize optical performance for each lattice type. 12 fold photonic quasi crystal lattice is found to provide best overall anti-reflectance performance providing a solarcorrected average reflectance of 8.3% for a hole depth of 1.5 mu m and 300 nm diameter, in comparison to 36.4% for a bare silicon solar cell surface. Practical feasibility of the optimal designs is demonstrated by fabrication of physical prototypes consisting of arrays of nm scale air-holes etched into the surface of a silicon slab fabricated Using e-beam lithography and ICP/RIE etching. FDTD Simulation methodology is validated by convergence studies as well as comparison to optical measurements on these fabricated devices. Furthermore, in contrast to previous studies we provide an in depth analysis of the physical mechanisms responsible for reduction in surface reflection, determining the parameter space where conventional Gaussian optical processes such as effective refractive index, refraction and Fresnel reflection dominate, vs parameter space where sub wavelength photonic crystal scattering effects play the main role. We finish up with an analysis of electrical performance for the optimal designs to further validate real world performance. Taking electrical performance into account we determine that infinite-symmetry 2D Fibonacci patterning far outperforms lower symmetry 12 fold and triangular arrangement. We believe that this is the first in depth investigation into 2D Fibonacci patterning in silicon solar cells.

We present a "post-mortem" procedure for analyzing degradation of fielded photovoltaic modules. We use this procedure to investigate degradation mechanisms in silicon heterojunction (SHJ) modules. We study cell fragments cored from two modules, one operated in the field for a decade and one stored in the dark as a control. We find that both the front surface passivation and bulk have likely been degraded. Through comparisons of spectral photoluminescence emissions, we conclude that although an increase in density of the pre-existing types of radiative defects is possible, it seems that any new defects types are purely non-radiative.

Sub-bandgap luminescence is characteristic of radiative transitions from defects in semiconductors. However, methods to extract defect-identifying parameters from this luminescence are lacking. Here, we present a method to extract these parameters from temperature- and intensity-dependent micro-photoluminescence (μPL) spectra. The initial “coarse” analysis determines the relevant radiative recombination mechanism by fitting the integrated defect PL spectra with phenomenological models for the temperature- and intensity-dependence. The subsequent “detailed” analysis fits the integrated defect PL spectra using rigorous physical models for the defect radiative recombination and spectral line-shape. Finally, defect parameters are extracted, including the defect energy level(s). As we obtain these values directly from the defect luminescence, our method provides higher confidence than more traditional indirect methods, such as those involving band-to-band PL and photoconductance. We demonstrate our method on spatially non-uniform defects with radiative transitions in n-type monocrystalline silicon samples. It is shown that the defect PL originates from the donor-acceptor pair recombination mechanism, involving a shallow acceptor and deeper donor energy level. The acceptor level is extracted from the temperature-dependent spectra, whilst the intensity-dependent spectra give the sum of acceptor and donor energies.

Light trapping in thin silicon solar cells demands radically different fabrication approaches to standard commercial cells. Weaker optical absorption and increased sensitivity to surface recombination requires light trapping to be achieved over a broader spectral range and, ideally, without texturing the absorber itself. Nano-scale light trapping structures allow the strongest scattering to be tuned to wavelengths, where oblique scattering into the absorber is needed most. Furthermore, applying these structures "externally," i.e., on a well-passivated planar silicon surface, reduces the surface area and permits optimal electronic conditions to be maintained. Despite these advantages, the challenges of balancing efficiency gain, cost, and lithographic fidelity have prevented the commercial use of nano-scale light trapping schemes. Here, we demonstrate the use of nanosphere lithography for producing high-quality and cast-effective nano-scale light trapping structures suitable for incorporation in thin solar cells. We have successfully fabricated large-area and uniform metal nanosphere grating structures, with embedded dielectric nanospheres, on 30 mu m thick c-Si pseudo cells and measured their effectiveness for light trapping. Comparison between simulations and the fabricated pseudo cells' characteristics highlighted key challenges in fabricating uniform structures, including the impact of air gaps within non-conformal coatings and minor changes in the geometry. Optical characterization via absorption spectroscopy and both spectral and spatially resolved photoluminescence showed a clear enhancement in the short-circuit current density of up to 4.33 mA/cm(2) in comparison with a planar 30 mu m thick device and a 3.7 times absorptance enhancement close to the bandgap of Si.

In this paper, we propose a hybrid quantum dot (QD)/solar cell configuration to improve performance of interdigitated back contact (IBC) silicon solar cells, resulting in 39.5% relative boost in the short-circuit current (JSC) through efficient utilisation of resonant energy transfer (RET) and luminescent downshifting (LDS). A uniform layer of CdSe1-xSx/ZnS quantum dots is deposited onto the AlOx surface passivation layer of the IBC solar cell. QD hybridization is found to cause a broadband improvement in the solar cell external quantum efficiency. Enhancement over the QD absorption wavelength range is shown to result from LDS. This is confirmed by significant boosts in the solar cell internal quantum efficiency (IQE) due to the presence of QDs. Enhancement over the red and near-infrared spectral range is shown to result from the anti-reflection properties of the QD layer coating. A study on the effect of QD layer thickness on solar cell performance was performed and an optimised QD layer thickness was determined. Time-resolved photoluminescence (TRPL) spectroscopy was used to investigate the photoluminescence dynamics of the QD layer as a function of AlOx spacer layer thickness. RET can be evoked between the QD and Si layers for very thin AlOx spacer layers, with RET efficiencies of up to 15%. In the conventional LDS architecture, down-converters are deposited on the surface of an optimised anti-reflection layer, providing relatively narrowband enhancement, whereas the QDs in our hybrid architecture provide optical enhancement over the broadband wavelength range, by simultaneously utilising LDS, RET-mediated carrier injection, and antireflection effects, resulting in up to 40% improvement in the power conversion efficiency (PCE). Low-cost synthesis of QDs and simple device integration provide a cost-effective solution for boosting solar cell performance.

We investigate the limitations of two-photon absorption time-resolved photoluminescence to measure the low-injection bulk lifetime of different semiconductor materials under varying surface recombination. The excitation source is assumed to be a sub-bandgap pulsed laser and the localized absorption and carrier generation was modeled using a focused TEM00 Gaussian beam under the assumption of diffraction-limited performance. The subsequent carrier kinetics were simulated by applying the finite-difference time-domain method to the continuity equation. Three typical semiconductor materials were modeled: direct bandgap low-mobility material (such as CZTS), direct bandgap high mobility (such as GaAs), and indirect bandgap high mobility (such as float-zone silicon). The extracted effective lifetime as a function of surface recombination velocity was compared to the bulk lifetime and the effective lifetime calculated using an analytical 1D approximation. For the direct bandgap materials, focusing inside the material yields an effective lifetime within a few percent of the bulk lifetime, regardless of the surface recombination velocity, while for excitation close to the surface it is up to 30% lower than the bulk lifetime at high surface recombination velocities (>104 cm/s). For the indirect bandgap material, the effective lifetime is dominated by the surface, making the bulk lifetime inaccessible, even at surface recombination velocities of 100 cm/s. Finally, we use the 1D approximation to find under what conditions the bulk lifetime can be extracted by this method and determine that both the bulk diffusion length and the product of the bulk lifetime and surface recombination velocity must be much less than twice the device thickness.

The development of new photovoltaic (PV) materials and cell structures requires detailed optical and electrical characterisation. Although a multitude of characterisation methods are presently used, data are often inconclusive and some properties remain difficult to measure. This is particularly true for materials and cells that are based on thin films: thinner layers interact weakly with light, making optical measurements challenging. Ultra-sensitive characterisation methods that are especially suited to thin films have long been described in the literature. Unfortunately, most of these techniques are not commercially available and thus are not widely used. One example is photothermal deflection spectroscopy (PDS), one of the few methods available to directly measure the nonradiative recombination of excess carriers. In this paper we report on our effort to develop a commercially-available PDS instrument and propose extensions of the technique to make it more useful for researchers. In particular, optical or electrical biasing during the measurement allows the excess carrier concentration in the material to be controlled, indirectly influencing the Fermi level and occupation of defects within the material. The effect on the absorption spectrum can then be observed with high sensitivity. Biased PDS has previously been investigated for PV devices [1–3] and photoelectrochemical cells [4] but here we demonstrate new directions for PV characterisation with the technique.

We investigate the limitations of two-photon time resolved photoluminescence to measure the bulk lifetime of different semiconductor materials used for photovoltaic applicationsThe alternating difference implicit finite-difference time-domain method was employed to simulate the carrier kinetics, following the localized generation of carriers by an ultra-fast laser pulse. Three hypothetical materials were modelled: direct bandgap material for thin-film applications (such as CZTS), direct band-gap for thick--film applications (such as GaAs), and indirect band-gap thick material (such as silicon). It was found that the effective lifetime of the direct band-gap materials remains within an order of magnitude of the bulk lifetime, even for surface recombination velocities up to 10 7 cm/s. For the indirect band-gap material, the bulk lifetime is inaccessible, at even moderate surface recombination velocities, due to the combination of high bulk lifetime and high diffusivity. This is the firststudy that highlights the limit of the two-photon time-resolved method for silicon applications.

Mie resonances of high-refractive index nanostructures provide strong and spectrally broadband scattering. In this numerical work, we investigate the feasibility of amourphous silicon nanodisks at the planar rear side of crystalline silicon solar cells for light trapping.

In this study, we use temperature-dependent microphotoluminescence spectroscopy to investigate non-uniform bulk defects in -type Czochralski and float-zone silicon wafers. For both wafer types, we observe defect photoluminescence (PL) with peak energy between 0.78 eV and 0.85 eV emerge as the temperature is decreased from 300 K to 80 K, along with an associated 10-100 times reduction in the band-to-band (BB) PL. This similarity in the shape of the spectral PL between the two wafer types suggests that the origin of the defects may be related. For the float-zone wafers, the defect PL is present even in regions appearing bright in conventional band-to-band PL imaging, suggesting that the bulk defect exists across the entire wafer, but is spatially inhomogeneous. Defect parameters are extracted by fitting the ratio of the integrated defect to band-to-band PL ratio using a modified Arrhenius equation derived from the ShockleyRead-Hall defect theory. We determine similar defect parameters for the FZ wafer within the bright and dark regions: a single defect species located at 150 to 160 meV from the valence band-edge and a capture cross-section temperature-dependence following an inverse power law with exponent of approximately 2. For the Cz wafer the defect is located 325 meV from the valence band-edge, as expected from the peak PL position, and a capture cross-section temperature exponent of approximately 2.5 was determined. This suggests that a different recombination process is responsible for the same defect PL band in each wafer type.

In this work, we develop a fabrication process for an interdigitated back contact solar cell using BBr3 diffusion to form the p+ region and POCl3 diffusion to form the n+ regions. We use the industry standard technology computer‐aided design modelling package, Synopsys Sentaurus, to optimize the geometry of the device using doping profiles derived from electrochemical capacitance voltage measurements. Cells are fabricated using n‐type float‐zone silicon substrates with an emitter fraction of 60%, with localized back surface field and contact holes. Key factors affecting cell performance are identified including the impact of e‐beam evaporation, dry etch damage, and bulk defects in the float zone silicon substrate. It is shown that a preoxidation treatment of the wafer can lead to a 2 ms improvement in bulk minority carrier lifetime at the cell level, resulting in a 4% absolute efficiency boost.

A quasi-crystal structure yields coupling to 12 modes from single chromatic color laser source. The device was fabricated in thin film of SiON on glass substrate, and simultaneously acts as both coupler and beam splitter.