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Fast camera-based luminescence-imaging measurements on perovskite solar cells are presented. The fundamental correlation between the luminescence intensity and the open circuit voltage predicted by the generalised Planck law is confirmed, enabling various quantitative methods for the detection of efficiency-limiting defects to be applied to this new cell structure. Interstinegly, it is found that this fundamental correlation is valid only for light-soaked devices. Copyright © 2015 John Wiley & Sons, Ltd.

We present an improved method that uses photoluminescence images to calculate the spatially-resolved efficiency in addition to other performance parameters of silicon solar cells. This new method is simpler than our previously-presented two-diode method, using only one diode with a variable ideality factor. Experimental results show that the simplified method is more tolerant of very large variations in local series resistance, a characteristic commonly seen in silicon cells. Using dark lock-in thermography techniques, we quantitatively verify the efficiency images produced by our improved method.

We present a method to separate bulk lifetime and surface recombination velocity using transient photoluminescence (PL). With the use of the analytical expression of transient PL, the asymptotic separation of two transient PL decays excited by different wavelengths can be obtained. Combined with the use of the intensity ratio of two steady-state PL intensities excited by the two different wavelengths, the lifetime separation can be achieved. In addition, detailed methods to experimentally determine the two terms are presented.

Metals are an integral part of solar cell structures—as a metal contact and/or a back reflector. However it is a less known fact that there are plasmonic losses associated with these structures and it is important to minimise such losses especially when placed in proximity to scattering media like textures on the front of the cell that can introduce an angular dependence to the light incident on the rear. This study investigates the losses in a metal reflector when placed adjacent to a dielectric layer in a typical solar cell rear geometry. The experimental measurement was realised using a novel custom built optical setup for characterising intensity of the reflected light at various internal incident angles at the Si–dielectric interface. Our results show that the thickness of the dielectric layer, the refractive index of the dielectric layer and the type of the rear metal can all affect the degree of such losses and the distribution of the angular reflection. Both measured and simulation results indicate that a 250–320 nm SiO2 along with an Ag back reflector gives the best internal rear angular reflection in Si but is dependant on the wavelength of interest. It also shows that textured or scattering front interfaces with internal incident angles greater than 28° have the potential to provide light trapping through total internal reflection and at the same time minimise plasmonic losses at the metal reflector interface.

Reflected light from textured front surfaces of a solar cell contains useful information about the surface geometry as well as the optical properties of the cell. In this study, the 2-D reflected light distributions from front surfaces of silicon cells textured in various ways are characterised by an appropriate optical setup. The results are compared to those of conventional ray tracing. For surfaces with regular inverted pyramids, we identified reflected light diffraction patterns that can be explained by the principles of geometrical optics. These reflected light patterns can be used to extract details of surface morphologies and hence be used as a tool to fine tune and monitor fabrication processes. Surfaces with random pyramids reflect light without diffraction patterns but with other distinctive features. Their reflections lie primarily in the angular range between 0° and 50°, with a maximum intensity at about 25°. The impact of the encapsulant's refractive index on the amount of total internal reflection is discussed for various types of textured surface. For both random and regularly textured surfaces, it appears possible to achieve higher total internal reflection ratio when the encapsulant's index increases from conventional 1.5 to 1.6.

Silicon solar cell technology is evolving rapidly with recent improvements in both multicrystalline and monocrystalline silicon wafers and ingots prepared by high throughput directional solidification, using both semiconductor grade and upgraded metallurgical grade silicon. Photoconductance and photoluminescence measurements are being rapidly developed to guide the development of these materials, with increasing attention being paid to combining steady-state and transient photoluminescence measurements to obtain self-consistent results. Analytical transient solutions are presented for both photoconductance and photoluminescence from silicon bricks or thick wafers which will guide this development by highlighting important interdependencies.

We present a method to image the following parameters spatially resolved of a solar cell: voltage Vxy, current density Jxy, power density Pxy, efficiency ηxy, series resistance Rs,xy, fill factor FFxy and dark saturation current densities of a two diode model J01,xy and J02,xy. The algorithm determines this set of self-consistent parameters by using a minimum of 5 electrical biased photoluminescence images. A comparison with CELLO measurements supports the presented method.

In this article, a novel rear structure using Ag nanoparticles to create surface plasmons to enhance light trapping is applied on the rear of planar high efficiency PERT (Passivated Emitter and Rear Totally Diffused) silicon wafer cells, targeting the spectrum range from 1000 nm to 1200 nm. Variations of this rear structure that combine Ag nanoparticles, dielectric layers and back metal reflectors were studied and analysed. Thickness of the rear surface passivation SiO 2 spacer layer was optimised to achieve maximum optical enhancement using surface plasmons but with minimum electronic losses due to recombination effects. The effect of the precursor evaporated Ag film thickness was also studied as a means to vary the size/shape of the nanoparticles. The measured external quantum efficiency ( EQE ) of the best performing rear reflector shows a maximum enhancement of more than 4-fold at 1160 nm. This corresponds to a 16% photocurrent increase (calculated from 900 nm to 1200 nm) compared to the cell with conventional Al rear reflector. Moreover, from the measured spectral response and optical absorption data, we successfully separated and analysed the electrical and optical properties of the novel rear light trapping designs. Light trapping features were quantified using optical parameters characterised by an effective optical path length factor Z , while electrical parameters such as surface recombination velocity S (cm/s) and effective minority charge carrier lifetime τ bulk (μs) were also extracted. Relative errors for these parameters were also calculated. For the cell with the best performing rear structure, we report a maximum Z factor enhancement of around 6-fold using Ag nanoparticles in conjunction with a detached Ag reflector, in comparison to the reference at 1200 nm.

Only recently, the quality of liquid phase crystallized silicon directly on glass substrates made a huge leap towards the quality of multi-crystalline wafers with open circuit voltages well above 600 mV. In this paper, we investigate the material quality in order to identify the factors limiting further performance improvements. We employ photoluminescence imaging on a state of the art test structure with lifetime calibration by transient photoluminescence. The resulting lifetime map is converted into an effective diffusion length map and the origin of regions with short lifetimes is investigated with electron backscattering and transmission electron microscopy. High local dislocation densities in areas with dissociated coincidence site lattice boundaries were found to be responsible for the localised quenching of the photoluminescence signal.

We present a method to simultaneously determine bulk and surface recombination properties using time-resolved photoluminescence (PL) decay. The lifetime separation algorithm makes use of the analytical expression of the asymptotic separation of two time-resolved PL decays corresponding to different excitation wavelengths as well as that of the ratio of two steady-state PL intensities excited by the two different wavelengths. Detailed experimental methods of measuring these two terms are presented and the effect of signal-to-noise ratio is discussed to determine the applicability of this algorithm.

This paper presents an accurate and practical mathematical model of time-resolved photoluminescence (PL) response from silicon wafers generated by fast repetitive excitation pulses. The model is valid under low level injection condition and takes into account the depth dependence of carrier generation, diffusion, and surface recombination. Finite element analysis is employed for the carrier density and PL computations. By comparing computational results with results obtained from PC1D (a computer program solving fully coupled nonlinear equations for quasi-one-dimensional carrier transportation in crystalline semiconductor devices, especially focusing on photovoltaic devices), the validity of this method is confirmed. Early stage application and the limitations of this method have been studied, and future work has been proposed.

A technique for fast and spatially resolved measurement of the effective series resistance of silicon solar cells from luminescence images is introduced. Without compromising the speed of existing luminescence based series resistance imaging methods, this method offers significant advantages in that it is more robust against variations in local diode characteristics. Lateral variations in the series resistance of an industrial screen printed multicrystalline silicon solar cell obtained from this method show excellent correlation with a Corescan measurement and are also shown to be unaffected by lateral variations in the diode properties.

Both dark lock-in thermography (DLIT) and photoluminescence (PL) imaging deliver information about the local electronic properties of solar cells. Special methods have been proposed for evaluating DLIT or PL images taken under various conditions with the goal to extract the local two-diode parameters. Knowing these parameters, the locally contributing efficiency data in a cell or local expectation values of the efficiency parameters Voc, FF, or η may be calculated. In this contribution these DLIT- and PL-based approaches are applied to one and the same cell and the results are compared. It is found that both methods lead to similar but not to identical results. The physical reasons for the differences between DLIT and PL based local efficiency analysis are discussed. It is found that the PL-based J02 results are still corrupted by the local series resistance, probably because the simple two-diode model is not sufficient to describe the PL process under electric load with sufficient accuracy. For DLIT this approach is less disturbing.