Showing papers in "IEEE Journal of Photovoltaics in 2016"

A GaAs Nanowire Array Solar Cell With 15.3% Efficiency at 1 Sun

Abstract: A GaAs nanowire array solar cell with an independently verified solar energy conversion efficiency of 15.3% and open-circuit voltage of 0.906 V under AM1.5g solar illumination at 1-sun intensity has been fabricated. This is the highest published efficiency for nanowire array solar cells and is twice the prior record for GaAs nanowire array solar cells. The solar cell has been fabricated by substrate-based epitaxy but is structurally compatible with substrate-less aerotaxy fabrication, providing a path to high-volume manufacturing. The short-circuit current of 21.3 mA/cm2 was generated with axial p-n junction GaAs cores covering 13% of the surface area, which is a volume of GaAs equivalent to a 370-nm-thick planar layer.

Four-Junction Wafer-Bonded Concentrator Solar Cells

Abstract: The highest solar cell conversion efficiencies are achieved with four-junction devices under concentrated sunlight illumination. Different cell architectures are under development, all targeting an ideal bandgap combination close to 1.9, 1.4, 1.0, and 0.7 eV. Wafer bonding is used in this work to combine materials with a significant lattice mismatch. Three cell architectures are presented using the same two top junctions of GaInP/GaAs but different infrared absorbers based on Germanium, GaSb, or GaInAs on InP. The modeled efficiency potential at 500 suns is in the range of 49–54% for all three devices, but the highest efficiency is expected for the InP-based cell. An efficiency of 46% at 508 suns was already measured by AIST in Japan for a GaInP/GaAs//GaInAsP/GaInAs solar cell and represents the highest independently confirmed efficiency today. Solar cells on Ge and GaSb are in the development phase at Fraunhofer ISE, and the first demonstration of functional devices is presented in this paper.

Industrial Silicon Solar Cells Applying the Passivated Emitter and Rear Cell (PERC) Concept—A Review

Abstract: Even though the passivated emitter and rear cell (PERC) concept was introduced as a laboratory-type solar cell in 1989, it took 25 years to transfer this concept into industrial mass production. Today, PERC-type solar cells account for 10% of the worldwide produced solar cells, and their share is expected to rapidly increase up to 35% within the next few years. Record efficiencies up to 22.1% of industrial PERC cells approach an efficiency of 22.8% of the lab-type PERC cell in 1989. This paper reviews the most important research results and technological developments of the past 25 years, which enabled the successful transfer of the lab-type PERC concept into industrial mass production. Particular attention is paid to the development of AlO x /SiN y layer stacks with excellent rear surface passivation properties and low production costs. In addition, we summarize the most important research results and technological improvements of industrially processed local aluminum rear contacts. Furthermore, we describe the most relevant process flows to manufacture industrial PERC cells and address silicon wafer material requirements regarding high and stable charge carrier lifetimes. An outlook is provided on future development opportunities, which may further increase the conversion efficiency and the energy yield of industrial PERC solar cells.

Contact Selectivity and Efficiency in Crystalline Silicon Photovoltaics

Abstract: Highly doped junctions of a Si solar cell function as membranes that block minority carriers and, at the same time, provide a high conductivity for transporting majority carriers to the contacts. They are thus said to be selective contacts. We propose a quantitative definition for the selectivity of contacts. The selectivity provides a figure of merit for electron and hole contacts that is helpful in designing solar cells and in identifying the efficiency limiting components. Applying the definition of selectivity to poly-Si junctions reveals that the root cause of their high selectivity is the highly asymmetric carrier concentration rather than a specific contact geometry.

A Method to Detect Photovoltaic Array Faults and Partial Shading in PV Systems

Abstract: Abnormal conditions such as faults and partial shading lead to a reduction in the maximum available power from a photovoltaic (PV) array. Thus, it is necessary to detect partial shading and faults in a PV array for improved system efficiency and reliability. Conventional protection devices fail to detect faults under cloudy and low irradiance conditions, leading to safety issues and fire hazards in the PV field. This paper proposes a method to detect faults and partial shading under all irradiation conditions using the measured values of array voltage, array current, and irradiance. The proposed method enables classification of the status of the PV array into three possible scenarios, viz., normal operating condition, partial shading, and fault. The proposed method is tested experimentally to verify its effectiveness under different irradiation conditions.

Engineering Solutions and Root-Cause Analysis for Light-Induced Degradation in p -Type Multicrystalline Silicon PERC Modules

Abstract: We identify two engineering solutions to mitigate light-induced degradation (LID) in p -type multicrystalline silicon passivated emitter and rear cells, including modification of metallization firing temperature and wafer quality. Lifetime measurements on etched-back samples confirm that LID has a strong bulk component. Spatially resolved lifetime maps indicate that the defects responsible for LID are dispersed ubiquitously across the wafer. Reversibility of LID upon low-temperature annealing suggests a low-activation-energy barrier inconsistent with precipitated impurity dissolution. Lifetime spectroscopy of the LID-affected state reveals an asymmetry of electron and hole capture cross sections of $\sim \text{28.5}$ , consistent with a deep-level donor point defect (e.g., interstitial Ti, interstitial Mo, substitutional W), charged nanoprecipitate, or charged structural defect, such as a dislocation. Finally, we explain two possible root causes of this LID, including 1) a point-defect complex involving a hydrogen atom and a deep-level donor and 2) configurational change of a point-defect complex involving fast-diffusing impurities.

Performance of III–V Solar Cells as Indoor Light Energy Harvesters

Abstract: Energy-harvesting systems provide power autonomy to individual wireless sensor nodes, allowing simpler deployment, as compared with wired nodes and longer lifetimes than battery-powered ones. Indoor light energy is an abundant source in many applications where solar cells are used to convert the radiant energy to electricity. To allow the greatest functionality in nodes powered by light energy-harvesting systems, the highest power output indoor photovoltaic (PV) devices are required. Amorphous-silicon and dye-sensitized solar cells have been tested extensively as indoor light harvesters due to their wide bandgaps that are optimum for absorption of the spectra from compact fluorescent lamp (CFL) and light-emitting diode (LED) bulbs. III–V solar cells are the highest performing PV devices under 1-sun conditions but have not been thoroughly investigated under indoor illumination conditions. Here, we show experimentally that III–V single-junction solar cells made from GaAs and GaInP significantly outperform amorphous-silicon and dye-sensitized solar cells at low illumination intensities with 2× greater measured power densities. Our measured data show that credit-card-sized GaAs and GaInP solar cells will provide ∼ 4 mW of power under 1000 lx (i.e., in a bright office space). This is the first thorough presentation of GaInP and GaAs solar cells under CFL and LED illumination and provides a key insight for indoor PV as we move toward wider deployment of these energy efficient bulbs.

Rapid Stabilization of High-Performance Multicrystalline P-type Silicon PERC Cells

Abstract: Light-induced or, more broadly, carrier-induced degradation (CID) in high-performance multicrystalline silicon (HP mc-Si) solar cells remains a serious issue for many manufacturers, and the root cause of the degradation is still unknown. In this paper, the impact of firing temperature on the stability of lifetime test structures is investigated, and it is found that substantial CID can be triggered if peak temperatures exceed approximately 700 °C. We then investigate two pathways to stabilize the performance of industrially produced HP mc-Si passivated emitter rear contact cells which have been fired at CID-activating temperatures (∼740 °C–800 °C) currently required for silver contact formation. The first is a fast-firing approach, whereby it is demonstrated that an additional firing step at a reduced temperature after cell metallization can suppress the extent of ${V_{{\rm{oc}}}}$ degradation by up to 80%. The second approach is the accelerated degradation and subsequent recovery of carrier lifetime through the use of high-intensity illumination during annealing at elevated temperatures. A 30 s process is found to suppress the maximum extent of degradation in ${V_{{\rm{oc}}}}$ by up to 60% and up to 80% for longer processes. Ultimately, the results suggest that a combined approach of fast-firing and a high-intensity-illuminated anneal could achieve the best results in terms of ${V_{{\rm{oc}}}}$ stability.

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Abstract: Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

Photovoltaic (PV) Impact Assessment for Very High Penetration Levels

Abstract: This paper describes a granular approach for investigating the impacts of very high photovoltaic (PV) generation penetration. Studies on two real-world distribution feeders connected to PV plants are presented. The studies include both steady-state and time-series power flow analyses, which include the effects of solar variability. The goal of the study is to predict the effects of increasing levels of PV generation as it reaches very high penetration levels. The loss and return of generation with and without regulation is simulated to capture short-term problems such as voltage fluctuations. Impact results from the analyses are described along with potential mitigations.

Optimized Cleaning Cost and Schedule Based on Observed Soiling Conditions for Photovoltaic Plants in Central Saudi Arabia

Abstract: For photovoltaic (PV) systems suffering energy loss due to an accumulation of soil, there is an optimum cleaning interval that balances the economic cost of lost revenues due to soiling loss with the cost of cleaning operations and, thereby, minimizes the total cost per unit energy delivered by the system and, thus, the levelized cost of electricity. This optimum interval may vary, depending on cleaning costs, as well as seasonal variations in soiling loss and energy production. In this work, a general expression has been derived for the optimal cleaning interval and then applied to losses actually observed over a one-year interval at a test site in central Saudi Arabia and fitted to a simple exponential loss model. The results show that costs attributable to cleaning to an optimum schedule are a small fraction of typical operation and maintenance costs even for completely manual cleaning, and even more so with machine-assisted cleaning.

GaAs $_{0.75}$ P $_{0.25}$ /Si Dual-Junction Solar Cells Grown by MBE and MOCVD

Abstract: Monolithic, epitaxial, series-connected GaAs0.75P0.25/Si dual-junction solar cells, grown via both molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD), are reported for the first time. Fabricated test devices for both cases show working tandem behavior, with clear voltage addition and spectral partitioning. However, due to thermal budget limitations in the MBE growth needed to prevent tunnel junction failure, the MBE-grown GaAs0.75P0.25 top cell was found to be lower quality than the equivalent MOCVD-grown device. Additionally, despite the reduced thermal budget, the MBE-grown tunnel junction exhibited degraded behavior, further reducing the overall performance of the MBE/MOCVD combination cell. The all-MOCVD-grown structure displayed no such issues and yielded significantly higher overall performance. These initial prototype cells show promising performance and indicate several important pathways for further device refinement.

Parameter Estimation Method to Improve the Accuracy of Photovoltaic Electrical Model

Abstract: This paper proposes an estimation method to identify the electrical model parameters of photovoltaic (PV) modules and makes a comparison with other methods already popular in the technical literature. Based on the full single-diode model, the mathematical description of the I–V characteristic of modules is generally represented by a coupled nonlinear equation with five unknown parameters, which is difficult to solve by an analytical approach. The aim of the proposed method is to find the five unknown parameters that guarantee the minimum absolute error between the P–V curves generated by the electrical model and the P–V curves provided by the manufacturers' datasheets for different external conditions such as temperature and irradiance. The first advantage of the proposed method is that the parameters are estimated using the P–V curves instead of I–V curves, since most of the applications that use the electrical model want to accurately estimate the extracted power. The second advantage is that the value ranges of each unknown parameter respect their physical meaning. In order to prove the effectiveness of the proposition, a comparison among methods is carried out using both types of P–V and I–V curves: those obtained by manufacturers' datasheets and those extracted experimentally in the laboratory.

Semitransparent Perovskite Solar Cell With Sputtered Front and Rear Electrodes for a Four-Terminal Tandem

Abstract: A tandem configuration of perovskite and silicon solar cells is a promising way to achieve high-efficiency solar energy conversion at low cost. Four-terminal tandems, in which each cell is connected independently, avoid the need for current matching between the top and bottom cells, giving greater design flexibility. In a four-terminal tandem, the perovskite top cell requires two transparent contacts. Through detailed analysis of electrical and optical power losses, we identify optimum contact parameters and outline directions for the development of future transparent contacts for tandem cells. A semitransparent perovskite cell is fabricated with steady-state efficiency exceeding 12% and broadband near infrared transmittance of >80% using optimized sputtered indium tin oxide front and rear contacts. Our semitransparent cell exhibits much less hysteresis than opaque reference cells. A four-terminal perovskite on silicon tandem efficiency of more than 20% is achieved, and we identify clear pathways to exceed the current single silicon cell record of 25.6%.

New World-Record Efficiency for Pure-Sulfide Cu(In,Ga)S 2 Thin-Film Solar Cell With Cd-Free Buffer Layer via KCN-Free Process

Abstract: World-record conversion efficiency of 15.5%, which is a number independently certified by an external accredited laboratory, was achieved on a pure-sulfide Cu(In,Ga)S2 (CIGS) thin-film solar cell fabricated by metal sputtering and H2S gas sulfurization. This is the highest conversion efficiency reported for pure-sulfide CIGS thin-film solar cells. We exceeded the limit of 15% efficiency thanks to the buffer layer optimization. The buffer layer consists of Cd-free materials, and the absorber was fabricated via a KCN-free process.

Design Flexibility of Ultrahigh Efficiency Four-Junction Inverted Metamorphic Solar Cells

Abstract: The inverted metamorphic solar cell has highly tunable bandgaps, in part due to the metamorphic subcells. Using phosphide-based compositionally graded buffers, we show a wide variety of GaInAs solar cells, ranging in bandgap from 1.2 to 0.7 eV. These metamorphic subcells are all high quality and can be used for a wide variety of multijunction designs. GaInAs solar cells with 0.70 eV bandgaps are developed using an InAsP buffer that extends beyond the InP lattice constant, allowing access to an additional 2 mA/cm2 of photocurrent at AM1.5D and 25 °C. This subcell is implemented into a four-junction inverted metamorphic solar cell combined with an appropriate antireflective coating, which increases the series-connected multijunction current by 0.5 mA/cm2 with respect to designs using 0.74-eV GaInAs. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. One-sun devices achieve 35.3 ± 1.2% efficiency under the AM0 spectra and 37.8 ± 1.2% efficiency under the global spectra at 25 °C. Concentrator devices designed for elevated operating temperature achieve 45.6 ± 2.3% peak efficiency under 690× the direct spectrum and 45.2 ± 2.3% efficiency at 1000× and 25 °C. Device optimization is performed for the direct spectrum on 1-sun devices with 2% shadowing, which achieve 39.8 ± 1.2% efficiency under the direct spectrum at 1 sun, highlighting the excellent performance and bandgap tunability of the four-junction inverted metamorphic solar cell.

Monitoring and Diagnostics of PV Plants by a Wireless Self-Powered Sensor for Individual Panels

Abstract: In this paper, an innovative sensor suited to perform real-time measurements of operating voltage and current, open-circuit voltage, and short-circuit current of string-connected photovoltaic (PV) panels is presented. An effective disconnection system ensures that the sensor does not affect the behavior of the string during the measurement phase and offers many benefits like the automatic detection of bypass events; moreover, the sensor does not require additional cables thanks to a wireless communication and a power supply section based on energy harvesting. An extensive experimental campaign is performed to prove the reliability and usefulness of the sensor for continuous monitoring of PV plants. The capability to detect faults and accurately localize malfunctioning panels in a PV string is highlighted.

Nanocrystalline Silicon Carrier Collectors for Silicon Heterojunction Solar Cells and Impact on Low-Temperature Device Characteristics

Abstract: Silicon heterojunction solar cells typically use stacks of hydrogenated intrinsic/doped amorphous silicon layers as carrier selective contacts. However, the use of these layers may cause parasitic optical absorption losses and moderate fill factor ( FF ) values due to a high contact resistivity. In this study, we show that the replacement of doped amorphous silicon with nanocrystalline silicon is beneficial for device performance. Optically, we observe an improved short-circuit current density when these layers are applied to the front side of the device. Electrically, we observe a lower contact resistivity, as well as higher FF . Importantly, our cell parameter analysis, performed in a temperature range from −100 to +80 °C, reveals that the use of hole-collecting p-type nanocrystalline layer suppresses the carrier transport barrier, maintaining FF s in the range of 70% at −100 °C, whereas it drops to 40% for standard amorphous doped layers. The same analysis also reveals a saturation onset of the open-circuit voltage at −100 °C using doped nanocrystalline layers, compared with saturation onset at −60 °C for doped amorphous layers. These findings hint at a reduced importance of the parasitic Schottky barrier at the interface between the transparent electrodes and the selective contact in the case of nanocrystalline layer implementation.

Study of Soiling Loss on Photovoltaic Modules With Artificially Deposited Dust of Different Gravimetric Densities and Compositions Collected From Different Locations in India

Abstract: Evaluation of soiling loss on photovoltaic (PV) modules in a geographical location involves collecting data from a fielded PV system of that location. This is usually a time-consuming and expensive undertaking. Hence, we propose collecting dust samples from various location of interest, preferably from the module surface, and use them as dust samples so that the soiling experiments can be conducted in the laboratory. In this work, a low-cost artificial dust deposition technique is utilized that could be used to deposit dust on a module surface in a controlled manner, which helps in predicting soiling loss associated with various dust properties, including densities, chemical compositions, and particle sizes. The soil samples covering diverse climatic conditions and six different geographic locations covering all of India were collected and investigated. Soiling loss on a silicon solar cell with Mumbai dust (17.1%) is about two times that of Jodhpur dust (9.8%) for the same soil gravimetric density of 3 g/m2. The dust collected from Mumbai showed the highest spectral loss, followed by Pondicherry, Agra, Hanle, Jodhpur, and Gurgaon. The worst affected module technology was amorphous silicon (17.7%), followed by cadmium telluride (15.7%), crystalline silicon (15.4%), and CIGS (14.5%) for the same density (1.8 g/m2) of dust from Mumbai.

Fundamental Studies of Adhesion of Dust to PV Module Surfaces: Chemical and Physical Relationships at the Microscale

Abstract: Photovoltaic (PV) module soiling is a growing area of concern for performance and reliability. This paper provides evaluations of the fundamental interactions of dust/soiling particles with several PV module surfaces. The purpose is to investigate the basic mechanisms involving the chemistry, morphology, and resulting particle adhesion to the first photon-incident surface. The evaluation and mapping of the chemistry and composition of single dust particles collected from operating PV module surfaces are presented. The first correlated direct measurements of the adhesive force of individual grains from field-operating collectors on identical PV module glass are reported, including correlations with specific compositions. Special microscale atomic force microscopy techniques are adapted to determine the force between the particle and the module glass surface. Results are presented for samples under dry and moisture-exposed conditions, confirming the effects of cementation for surfaces having soluble mineral and/or organic concentrations. Additionally, the effects of hydrocarbon fuels on the enhanced bonding of soiling particles to surfaces are determined for samples from urban and highly trafficked regions. Comparisons between glass and dust-mitigating superhydrophobic and superhydrophilic coatings are presented. Potential limitations of this proximal probe technique are discussed in terms of results and initial proof-of-concept experiments.

43% Sunlight to Electricity Conversion Efficiency Using CPV

Abstract: A concentrator standard test condition (CSTC) efficiency of 43.4% for the direct conversion of sunlight into electricity is demonstrated with a subunit of a concentrator photovoltaics (CPV) module. This is the highest reported CSTC efficiency for CPV applications thus far. This test device consists of a four-junction solar cell and uses an achromatic full-glass lens for concentrating the sunlight. The high efficiency is made possible by the combination of a high-efficiency solar cell (46%) and a high-efficiency lens (95%). The I–V curve of the solar cell has been measured for varying distances to the glass lens and compared with the results for a silicone-on-glass Fresnel lens. In this manner, chromatic aberration effects on solar cell performance are discussed in this study.

Snail Trails and Cell Microcrack Impact on PV Module Maximum Power and Energy Production

Abstract: This paper analyzes the impact of the snail trail phenomena on photovoltaic (PV) module performances and energy production. Several tests (visual inspection, maximum power determination, dielectric withstand, wet leakage current, and electroluminescence test) were carried out on 31 PV modules located in a PV plant in Italy. The electroluminescence test highlighted the strong correlation between the appearance of snail trails and presence of damaged cells in PV modules. The daily energy produced by four PV modules affected by snail trails ranged between 68% and 88% of the energy produced by a damage free commercial PV module over the same period.

Toward Totally Flexible Dye-Sensitized Solar Cells Based on Titanium Grids and Polymeric Electrolyte

Abstract: In this work, we present a novelty in the dye-sensitized solar cell scenario: a quasi-solid and completely flexible configuration based on plastic substrates and metallic meshes as support. The aim is to obtain a portable efficient device that can be competitive in the solar market due to the low cost and easy-to-prepare materials used for its fabrication. To fulfill this purpose, three different typologies of devices are proposed and tested in order to move from a rigid to a completely flexible setup in a gradual way. Materials and cells have been thoroughly characterized and tested by means of physicochemical, electrical, and electrochemical measurements to investigate the observed performances and the results that are reported in this paper.

Reliability and Cost Analysis of Multistage Boost Converters Connected to PV Panels

Abstract: In photovoltaic (PV) generation systems, dc–dc boost converters are responsible for maximum power point tracking and voltage regulation. The intent of this paper is to determine the number of the stages in an interleaved boost converter interfacing PV panels for achieving a reliable and costly optimized structure. A comparative study has been done on different modes of operation. including redundant operation or parallel operation in a two-stage interleaved converter. The comparison indicates that working in simultaneous mode would be more reliable. Contemplating this fact, reliability equation of a three-stage interleaved converter is calculated for simultaneous mode of operation. Considering dark hours of the day, a k -means clustering technique has been utilized in the reliability calculations based on the output data of a PV generation system installed at the campus. Besides, mean time to failure criterion is considered in the reliability analysis. Simulation and experimental results are analyzed, considering the costs. The results determine the optimum structure for interfacing the PV panels to the grid.

A Robust Continuous-Time MPC of a DC–DC Boost Converter Interfaced With a Grid-Connected Photovoltaic System

Abstract: The main function of the dc–dc converter in a grid-connected photovoltaic (PV) system is to regulate the terminal voltage of the PV arrays to ensure delivering the maximum power to the grid. The purpose of this paper is to design and practically implement a robust continuous-time model predictive control (CTMPC) for a dc–dc boost converter, feeding a three-phase inverter of a grid-connected PV system to regulate the PV output voltage. In CTMPC, the system behavior is predicted based on Taylor series expansion, raising concerns about the prediction accuracy in the presence of parametric uncertainty and unknown external disturbances. To overcome this drawback, a disturbance observer is designed and combined with CTMPC to enhance the steady-state performance in the presence of model uncertainty and unknown disturbance such as the PV current, which varies nonlinearly with the operating point. An interesting feature is that the composite controller reduces to a conventional PI controller plus a predictive term that allows further improvement of the dynamic performance over the whole operating range. The effectiveness of the proposed controller was tested numerically and validated experimentally with the consideration of the grid-connected PV inverter system and its controller.

Lifetime Spectroscopy Investigation of Light-Induced Degradation in p-type Multicrystalline Silicon PERC

Abstract: When untreated, light-induced degradation (LID) of p -type multicrystalline silicon (mc-Si)-based passivated emitter and rear cell (PERC) modules can reduce power output by up to 10% relative during sun-soaking under open-circuit conditions. Identifying the root cause of this form of LID has been the subject of several recent investigations. Lifetime spectroscopy analysis, including both injection and temperature dependencies (IDLS and TIDLS), may offer insight into the root-cause defect(s). In this paper, to illustrate the root-case defect identification method, we apply room-temperature IDLS to intentionally Cr-contaminated mc-Si. Then, we apply this technique to the p -type mc-Si that exhibits LID in PERC devices, and we provide further insights by analyzing qualitatively the injection-dependent lifetime as a function of temperature. We quantify the sensitivity of the capture cross-section ratio to variations in the measured lifetime curve and in the surface recombination. We find that the responsible defect most likely has an energy level between 0.3 and 0.7 eV above the valence band and a capture cross-section ratio between 26 and 36. Additionally, we calculate the concentrations of several candidate impurities that may cause the degradation.

Rear Surface Optimization of CZTS Solar Cells by Use of a Passivation Layer With Nanosized Point Openings

Abstract: Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2(Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage ( $V_{{\rm OC}}$ ; +17%rel.), short-circuit current ( $J_{{\rm SC}}$ ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.

Gallium Phosphide Window Layer for Silicon Solar Cells

Abstract: The integration of III–V compound semiconductors on a silicon bottom cell offers the opportunity to form two- and three-junction solar cells with a conversion efficiency exceeding 30%. This paper reports on the progress in the heteroepitaxial nucleation of gallium phosphide (GaP) on silicon, which allows the fabrication of a silicon bottom cell with front-surface passivation by a thin single-crystalline GaP window layer. GaP has a low lattice-mismatch to Si and an indirect bandgap energy of 2.26 eV, which leads to low absorption. At the same time, GaP can be doped with silicon to form an n-type contact layer. In this publication, we investigate n-Si/p-Si homojunction solar cells with a GaP window and contact layer. Metal–organic vapor phase epitaxy was used to deposit the 60-nm GaP window layer with a low density of antiphase boundaries at the heterointerface and without misfit dislocations. Open-circuit voltages of up to 634 mV have been obtained under 1-sun AM1.5g conditions for devices without antireflective coatings.

Strategies for Doped Nanocrystalline Silicon Integration in Silicon Heterojunction Solar Cells

Abstract: Carrier collection in silicon heterojunction (SHJ) solar cells is usually achieved by doped amorphous silicon layers of a few nanometers, deposited at opposite sides of the crystalline silicon wafer. These layers are often defect-rich, resulting in modest doping efficiencies, parasitic optical absorption when applied at the front of solar cells, and high contact resistivities with the adjacent transparent electrodes. Their substitution by equally thin doped nanocrystalline silicon layers has often been argued to resolve these drawbacks. However, low-temperature deposition of highly crystalline doped layers of such thickness on amorphous surfaces demands sophisticated deposition engineering. In this paper, we review and discuss different strategies to facilitate the nucleation of nanocrystalline silicon layers and assess their compatibility with SHJ solar cell fabrication. We also implement the obtained layers into devices, yielding solar cells with fill factor values of over 79% and efficiencies of over 21.1%, clearly underlining the promise this material holds for SHJ solar cell applications.

Measurement of the Optical Constants of Soda-Lime Glasses in Dependence of Iron Content and Modeling of Iron-Related Power Losses in Crystalline Si Solar Cell Modules

Abstract: It is well known that the absorbance of soda-lime glass is very sensitive to the amount of iron contamination; therefore, it strongly affects the power output of mass-produced crystalline silicon solar cell modules. We use a combination of ellipsometry and transmission measurements to determine the optical constants, at wavelengths between 300 and 1690 nm, of soda-lime-silica glasses containing an iron content between 1‰ and 0.01‰, measured with inductive coupled plasma optical emission spectroscopy. We derive two different semiempirical models for the extinction coefficient of soda-lime-silica glass as a function of its iron content: one model for iron alone and the other model for iron including other typical remaining coloring agents. Furthermore, we use ray tracing and spice simulations to predict the power losses in standard modules as a function of iron content in their cover glass sheet. Considering a module with 3.2-mm glass thickness, our results predict a decline in module output power due to iron content in the glass of 1.1% (3 W) for Fe $_2$ O $_3$ = 0.1‰ and 9.8% (28 W) for Fe $_2$ O $_3$ = 1‰.