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Energy losses of photovoltaic (PV) plants because of soiling are a problem in all regions, including Germany. Soft soiling, caused by a uniform dust film, is shading a PV module and is reducing yield depending on the thickness of the debris layer. Hard soiling, caused by agglomerations of dust or dirt, only covers parts of a PV module, and it causes local shading. A spot check of modules from different PV power plants in Germany revealed a power reduction because of soft- and hard soiling of up to 6%. Determination of soiling type and amount is a prerequisite for decisions about cleaning and for optimizing yield. In this article, we show that luminescence imaging can be used to detect and characterize soiling and quantify losses. For this purpose, we compare luminescence images before and after cleaning and we show that soiling becomes detectable and power losses quantifiable from difference images. We also show that the impact of hard soiling can be quantified from a single photoluminescence (PL) image. This technique may enable a fully automated quantification of power losses because of hard soiling in PV- modules and strings in the future.

We address the challenge in depositing ultra-thin GaAs cells (

Although two-terminal perovskite-silicon tandem solar cells have rapidly increased in efficiency, they have only been demonstrated with n-type silicon, which currently constitutes less than 5% of the global photovoltaics market. In this paper, we realize the first two-terminal perovskite tandem with p-type silicon by developing a recombination contact that enables voltage addition without damaging either subcell. We find that silicon interband recombination contacts are limited by a SiO x charge-extraction barrier, which forms during oxidative top-cell fabrication. A sputtered 30-nm indium tin oxide layer is found to protect the silicon cell surface from oxidation, while forming a recombination contact with the p-type nickel oxide hole transport layer for the perovskite top cell. Using this recombination contact we achieve voltage addition between the perovskite top cell and aluminum back-surface field p-type silicon bottom cell. We also find that minimizing moisture on the nickel oxide surface is important for achieving a stable open-circuit voltage under illumination. The recombination contact developed herein could play an important role in near-future developments.

Efficient optimization of the boron-doped region of silicon solar cells requires reliable process simulation of boron tube diffusion. Established simulation models and parameters are mostly calibrated for complementary metal oxide semiconductor device fabrication, where the doping processes are significantly different from those used in solar cell fabrication. In this paper, we present models and a set of corresponding parameters that are suitable for process simulation of BBr3 tube diffusion for solar cell applications with Sentaurus TCAD. Experimental doping profiles obtained with a wide range of diffusion recipes are compared with simulation results. Additionally, with the process parameter sensitivity analysis, we demonstrate the dominant process parameters that alter the boron distribution profile and its effect on the electrical performance.

Flat-panel tandem solar cells have demonstrated the potential to exceed the efficiencies of their single-junction constituents. However, robust design rules for tandem solar cells are currently lacking, slowing the development of cost-effective implementations of this technology. A double-junction solar cell with four-terminal (4T) architecture stacks two electrically independent subcells and avoids current-matching losses, resulting in two main advantages over the conventional integrated two-terminal (2T) architecture: a higher energy yield and a loosened constraint on material bandgap combinations. Because both subcells are contacted independently in a 4T tandem, multiple stacked semitransparent contacts are needed, causing optical and series resistance losses. Moreover, for stationary flat-panel tandems, contacts need to be optimized for a varying direction of incident sunlight. In this study, we develop a framework for optimizing metal grid contacts for 4T tandem solar cells and quantify the electrical and optical loss associated with these contacts. We also explore the range of conditions for which it is beneficial to align metal grid contact fingers. We find that, for most applications, the front and back contacts of the top cell should be aligned, resulting in a decrease in energy yield loss between $\text{5}{{\% }}\;{\text{and}}\;15{{\% }}$ , while aligning the bottom cell contacts is not beneficial and may even reduce total energy yield. Finally, we show that for nonideally matched subcell pairings, the contacts loss in a 4T tandem is not enough to counter the yield benefit of 4T over 2T tandems, while the contact loss may make the yield for more ideally matched subcells be comparable for 2T and 4T devices.

Motivated by the primary benefit of reduced material cost, the thickness of crystalline silicon solar cells has been continuously reduced. Laboratory and industrial studies have explored ultrathin crystalline silicon solar cells below 50 μ m with ambitious endeavors toward thicknesses of only a few micrometers. Ultrathin crystalline silicon solar cells require compatible small-scale surface textures to enhance the optical absorption. For this purpose, a novel submicron periodic nanostructure—periodic upright nanopyramids (PuNPs)—is fabricated by an integrated process of laser interference lithography and anisotropic etching of silicon in an alkaline solution. By simulation and measurements, we demonstrate that PuNPs are able to reduce front surface reflectance more effectively than conventional micron-scale pyramid textures and previously investigated periodic inverted nanopyramids (PiNPs). With a silicon nitride antireflection coating, we predict that PuNPs reduce the front surface reflectance to below 1% at an angle of incidence of 8°, which is comparable to black silicon. The superior antireflective property of PuNPs contributes to an absorbed photocurrent density of 40.8 mA/cm2 for a 40 μ m silicon absorber layer, which is 0.7 mA/cm2 higher than PiNPs, 0.8 mA/cm2 higher than inverted pyramids and 1 mA/cm2 higher than upright pyramids.

Honeycomb textures provide excellent reflectance reduction for multicrystalline silicon solar cells. Achieved reflectance levels are comparable or even superior to those of pyramidal textures for monocrystalline silicon. Honeycombs were used to achieve record efficiencies for multicrystalline silicon solar cells. In this paper, we present an analytical optical model for the calculation of the front surface reflectance of honeycomb-textured silicon wafer solar cells in air environment and in a module. Reflectance is calculated using an analytical path tracer for a geometric representation of the texture's symmetry. Light trapping in the module is calculated using a retro-reflection model. We evaluate the approach for a selected set bare and antireflection-coated samples against air and for encapsulated samples. For the used samples, we show that as little as 1% of the net incoming photons are reflected at the solar cell–air interface. Compared with state-of-the-art isotextures, the presented honeycomb textures reduce the net reflectance loss in a photovoltaic (PV) module from 0.8 to 0.3 mA/cm2 .

We demonstrate a numerical analysis of the device impact of photon reabsorption on single-junction GaAs and tandem GaAs/Si solar cells. A self-consistent optical–electrical model that considers nonideal losses within the devices is developed. For single-junction devices, we find that the impact of photon recycling on the voltage increases monotonically with the injection level. For record-level GaAs solar cells, the voltage boost is 33 mV under open-circuit conditions and 13 mV at the maximum power point. For tandem GaAs/Si solar cells, photon reabsorption moderates the sensitivity of tandem efficiency to both obvious parameters like absorber thickness and implicit parameters like shunt resistance ( $R_{{\rm sh}}$ ) and bulk lifetime. Considering luminescent coupling results in a GaAs top cell that is 9.5% thicker than without luminescent coupling. The tandem device is 50% more sensitive to $R_{{\rm sh}}$ changes in the GaAs cell than $R_{{\rm sh}}$ changes in the Si cell. The impact of the GaAs top-cell bulk lifetime on tandem efficiency is reduced by 61% if photon reabsorption is not considered. This integrated optoelectronic device model allows one quantification of the implicit effects of photon recycling and luminescent coupling on device parameters for GaAs/Si tandem, providing a valuable tool for high-performance device optimization.

Bifacial solar cells can be encapsulated in modules with either a glass/glass or a glass/backsheet structure. A glass/backsheet structure provides additional module current under standard test conditions (STC), due to the backsheet scattering effects, whereas a glass/glass structure has the potential to generate additional energy under outdoor conditions. In this study, we quantify the current contributions due to various mechanisms in both module structures under STC. The current contributions due to different mechanisms are calculated by measuring the reflectance and transmittance of mini-modules with both structures, together with a MATLAB-based simulation. Our results show that under STC, glass/backsheet modules provide approximately 2.2% more power, as compared with glass/glass modules using the same bifacial solar cells with a standard cell gap of 2.0 mm. Using module optimization, we demonstrate that the maximum possible cost reduction benefit in $/WP of glass/backsheet modules over glass/glass modules under STC is limited to 3.3%. Due to the potential outdoor energy yield advantages of glass/glass modules reported in the literature, we recommend a glass/glass module structure for bifacial solar cells. Furthermore, in order to compensate for the lower performance of glass/glass modules under STC, we propose a methodology to measure and fairly rate bifacial glass/glass photovoltaic (PV) modules.

Short-term power output forecasting of photovoltaic (PV) systems has become increasingly important to balance the supply and demand of grid electricity in an efficient manner. It is well known that the operating temperature has a strong influence on the power output of photovoltaic devices; therefore, improvements in simulating the short-term PV module temperature add to the accuracy of PV power output forecasting. In this study, a new empirical model is proposed, which is suitable for transient weather conditions. This model uses four variables to predict the temperature of PV modules, namely ambient temperature, irradiance, wind speed, and relative humidity. The model is applied and evaluated for various silicon-based PV technologies at two Southeast Asian locations (Singapore and Jayapura) on a 1-min basis. Results show root-mean-squared errors (RMSE) of 1.5–3.2 K, which marks an average improvement in RMSE of 46%, compared with similar existing empirical models.

In this study, we evaluate the impact of interdigitated metallization patterns on the 1-sun performance of poly-Si thin-film solar cells on glass. We implement a model of the solar cell with an interdigitated metallization pattern in the simulation software Silvaco Atlas. Simulation and experimental results show consistently that the dominant factor for the performance of the metallization pattern is its impact on the current generation capability of the solar cell. For this reason, we study the effect of a variation of the emitter finger pitch, rear contact size, and carrier lifetime on current generation. For a pitch between 500 and 600 μm, the solar cell efficiency varies by less than 0.2% absolute. We also find that the optimum emitter finger pitch does not depend strongly on the charge carrier lifetime in the absorber layer.

The spectral content of sunlight directly affects the power output of solar photovoltaic (PV) devices. The extent of the effect of seasonal and weather-related spectral variations on the power output will depend largely on the semiconductor bandgap. In this study, haze, which is a common weather condi- tion in many parts of the world, is found to affect the power output of PV systems. An analysis of a recent haze event in Singapore in mid-June 2013 reveals that haze has an impact on the performance ratios and short-circuit currents of PV systems. The performance ratio of amorphous silicon thin-film PV systems dropped during the haze event, while that of crystalline silicon wafer-based systems ex- hibited a slight increase. A detailed analysis showed that the main cause of the observed performance ratio variations was changes in the generated short-circuit currents, which were due to a red shift of the solar spectrum arriving in the module plane during the haze period. Therefore, PV systems with different semiconductor bandgaps were affected differently. Index Terms—Average photon energy (APE), haze, performance ratio (PR), photovoltaic (PV) module, spectral analysis.

In this study, we propose a geometric optical model to represent alkaline saw-damage-etched (SDE) surfaces of monocrystalline silicon wafers. An experimental study is carried out to characterize the optical properties of alkaline SDE surfaces on monocrystalline silicon wafers. Based on the surface characteristics measured by goniometry and height profiling, a geometric optical model is developed to describe the SDE surface with two parameters: characteristic angle and planar fraction. Using the path-tracing method, spectral reflectance simulations are carried out for four different types of samples. With the measured characteristic angle of 22° and planar fraction of 0.25 or 0.36, we find that this representation of SDE surface can predict the reflection and transmission with a root-mean-square error (RMSE) of the equivalent current density from 0.19 to 0.57 mA/cm 2 . The developed model is also applied to the optical loss analysis of aluminum local back surface field (Al-LBSF) solar cells with an SDE rear surface. We find that SDE rear surfaces provide better light trapping than planar surfaces. As a consequence, Al-LBSF solar cells with pyramids on the front and an SDE rear are predicted to produce 0.6 mA/cm 2 more photocurrent than similar cells with a planar rear surface.

We present a top-down method to quantify optical losses due to encapsulation of textured multicrystalline silicon wafer solar cells in a photovoltaic module. The approach is based on a combination of measurements and mathematical procedures. Seven different loss mechanisms are considered: 1) reflection at the glass front surface, 2) reflection at the metal fingers, 3) reflection at the textured solar cell surface, 4) absorption in the antireflection coating, 5) absorption in the glass pane and the encapsulation layer, 6) front surface escape, and 7) losses due to a non-perfect solar cell internal quantum efficiency. Losses for each of these mechanisms are obtained as a function of wavelength, and the corresponding current loss for each loss mechanism is calculated. Comparing simulated and measured results, the method predicts the module quantum efficiency with an error of less than 2% and the collected current with an error of less than 1%. In the presented example, the biggest loss (7.4 mA/cm 2) is due to the nonperfect quantum efficiency, followed by reflection losses at the glass front (2.2 mA/cm 2) and absorption in the glass and encapsulation layer (1.1 mA/cm 2).