Showing papers in "IEEE Journal of Photovoltaics in 2014"

Achievement of More Than 25% Conversion Efficiency With Crystalline Silicon Heterojunction Solar Cell

Abstract: The crystalline silicon heterojunction structure adopted in photovoltaic modules commercialized as Panasonic's HIT has significantly reduced recombination loss, resulting in greater conversion efficiency. The structure of an interdigitated back contact was adopted with our crystalline silicon heterojunction solar cells to reduce optical loss from a front grid electrode, a transparent conducting oxide (TCO) layer, and a-Si:H layers as an approach for exceeding the conversion efficiency of 25%. As a result of the improved short-circuit current (J sc ), we achieved the world's highest efficiency of 25.6% for crystalline silicon-based solar cells under 1-sun illumination (designated area: 143.7 cm 2 ).

A Maximum Power Point Tracking Method Based on Perturb-and-Observe Combined With Particle Swarm Optimization

Abstract: Conventional maximum power point tracking (MPPT) methods such as perturb-and-observe (P&O) method can only track the first local maximum point and stop progressing to the next maximum point. MPPT methods based on particle swarm optimization (PSO) have been proposed to track the global maximum point (GMP). However, the problem with the PSO method is that the time required for convergence may be long if the range of the search space is large. This paper proposes a hybrid method, which combines P&O and PSO methods. Initially, the P&O method is employed to allocate the nearest local maximum. Then, starting from that point on, the PSO method is employed to search for the GMP. The advantage of using the proposed hybrid method is that the search space for the PSO is reduced, and hence, the time that is required for convergence can be greatly improved. The excellent performance of the proposed hybrid method is verified by comparing it against the PSO method using an experimental setup.

On the Parameter Extraction of a Five-Parameter Double-Diode Model of Photovoltaic Cells and Modules

Abstract: The main contribution of this paper is to present a new set of approximate analytical solutions for the parameters of a photovoltaic (PV) five-parameter double-diode model that can be used as initial values for the numerical solutions based on the Newton-Raphson method. The proposed formulations are developed based on only the limited information given by the PV manufacturers, i.e., the open-circuit voltage ( Voc), the short circuit current ( Isc), and the current and voltage at the maximum power point (Im and Vm). Compared with the existing techniques that require the entire experimental I-V curve or additional information such as the slope of the I-V curves of the open circuit and the short circuit points, the proposed technique is quite independent of these additional data, and, it is therefore, a low cost and fast parameter extraction method. The accuracy of the theoretical I-V curves is evaluated through the comparison of the simulation results and experimental data. The results of the application of the proposed technique to different PV modules show the accuracy and validity of the proposed analytical-numerical method.

Toward the Practical Limits of Silicon Solar Cells

Abstract: This paper reports recent efficiency results achieved by SunPower (SPWR) using industrially relevant manufacturing processes and then reexamines the following age-old question: “What efficiency can a single-junction Silicon solar cell suited to large-scale manufacturing achieve?” Through examination of a gap analysis of the SPWR cell and use of other relevant proof-points, insights are provided into the remaining challenges to achieving the practical limits.

Light Unmanned Aerial Vehicles (UAVs) for Cooperative Inspection of PV Plants

Abstract: After a fast photovoltaic (PV) expansion in the past decade supported by many governments in Europe, in this postincentive era, one of the most significant open issues in the PV sector is to find appropriate inspection methods to evaluate real PV plant performance and failures. In this context, PV modules are surely the key components affecting the overall system performance; therefore, there is a main concern about the occurrence of any kind of failure in PV modules. This paper aims to propose a novel concept for monitoring PV plants by using light unmanned aerial vehicles (UAVs) or systems (UASs) during their operation and maintenance. The main objectives of this study are to explore and evaluate the use of different UAV technologies and to propose a reliable, cost-effective, and time-saving method for the inspection of PV plants. In this research, different UAVs were employed to inspect a PV array field. For this purpose, some thermal imaging cameras and a visual camera were chosen as monitoring tools to suitably scan PV modules. The first results show that the procedure of utilizing UAV was effective in the detection of different failures of PV modules. Moreover, such a process was much faster and cost effective than traditional methods.

Tandem Solar Cells Based on High-Efficiency c-Si Bottom Cells: Top Cell Requirements for >30% Efficiency

Abstract: Tandem solar cells based on crystalline silicon present a practical route toward low-cost cells with efficiencies above 30%. Here, we evaluate a dual-junction tandem configuration consisting of a high-efficiency c-Si bottom cell and a thin-film top cell based on low-cost materials. We show that the minimum top cell efficiency required to reach 30% tandem efficiency ranges from 22% for a bandgap of 1.5 eV to 14% for a bandgap of 2 eV. We investigate these limits using a simple model for a four-terminal tandem to identify the material requirements for the top cell in terms of optical absorption, electronic bandgap, carrier transport, and luminescence efficiency. In particular, we show that even relatively low-quality earth-abundant semiconductor materials with luminescence efficiencies of 10-5 and diffusion lengths below 100 nm are compatible with tandem cell efficiencies above 30%. Introducing light trapping in the top cell can increase the efficiency beyond 32% and reduce the required diffusion length below 50 nm. This analysis establishes clear research targets for high-bandgap semiconductor materials and novel thin-film solar cell concepts that can be combined with existing c-Si technology. Such tandem approaches could enable the rapid development of a new generation of low-cost high-efficiency cells.

A Novel Simplified Two-Diode Model of Photovoltaic (PV) Module

Abstract: This paper proposes a novel simplified two-diode model of a photovoltaic (PV) module. The main aim of this study is to represent a PV module as an ideal two-diode model. In order to reduce computational time, the proposed model has a photocurrent source, i.e., two ideal diodes, neglecting the series and shunt resistances. Only four unknown parameters from the datasheet are required in order to analyze the proposed model. The simulation results that are obtained by MATLAB/Simulink are validated with experimental data of a commercial PV module, using different PV technologies such as multicrystalline and monocrystalline, supplied by the manufacturer. It is envisaged that this work can be useful for professionals who require a simple and accurate PV simulator for their design.

Maximum Power from PV Arrays Using a Fixed Configuration Under Different Shading Conditions

Abstract: A major challenge in photovoltaic (PV) systems is making them energy efficient. One of the major factors that contribute to the reduction of PV power is partial shading. The reduction in power depends on module interconnection scheme and shading pattern. Different interconnection schemes are used to reduce the losses caused by partial shading. This paper presents a fixed interconnection scheme for PV arrays that enhances the PV power under different shading conditions. The proposed scheme facilitates distribution of the effect of shading over the entire array thereby reducing the mismatch losses caused by partial shading. The performance of the system is investigated for different shading conditions and the MATLAB/SIMULINK results are presented to show that the power extracted from the PV arrays under partial shading conditions is improved. Experimental results are provided to validate the proposed approach using a laboratory experimental setup. A comparison is also made between the electrical array reconfiguration scheme and the proposed scheme for a 5 × 5 PV array.

Development of Heterojunction Back Contact Si Solar Cells

Abstract: An energy conversion efficiency of 25.1% was achieved in heterojunction back contact (HBC) structure Si solar cell utilizing back contact technology and an amorphous silicon thinfilm technology. A new patterning process was established, and it was applied to the fabrication process of HBC cells. In addition, the unique technology of the surface mount technology concept contributed to the superior performance of HBC cell. A short circuit current density (J sc ) and an open-circuit voltage (V oc ) were 41.7 mA/cm2 and 736 mV, respectively. The high J sc as well as the high Voc indicates the strength of HBC structure cell. Besides, a high fill factor of 0.82 was obtained, which shows that HBC structure cell does not have any fundamental critical losses caused from series resistance or shunt resistance. Such high values of I-V parameter means that the patterning process was properly performed.

Organic–Inorganic Halide Perovskites: Perspectives for Silicon-Based Tandem Solar Cells

Abstract: We investigate the efficiency potential of organic-inorganic halide perovskite/crystalline silicon tandem solar cells, a new class of photovoltaic devices targeting long-term cost reductions by ultrahigh conversion efficiencies. Methyl ammonium lead triiodide perovskite solar cells are particularly interesting as the top cell in Si-based tandem devices due to their suitable band gap, high photovoltage, and low sub-bandgap absorption. We derive optical models for a perovskite/Si tandem cell with Lambertian light trapping in the perovskite top cell, as well as for a top cell in the single pass limit. We find that unlike for other thin-film device architectures, light trapping is not required for the triiodide perovskite/Si tandem to reach matched top and bottom cell currents. While a Lambertian top cell could be employed in a four-terminal tandem, a top cell in the single pass limit enables a current-matched monolithic device with realistic top cell thicknesses. We calculate a limiting efficiency of 35.67% for an ideal (no parasitic absorption, ideal contacts) monolithic tandem, assuming a top cell open-circuit voltage of 1100 mV.

Optics and Light Trapping for Tandem Solar Cells on Silicon

Abstract: The rapid advancement of thin-film photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% efficiency, although light in these devices must be managed carefully. We identify the optical requirements to reach high efficiencies. Strict conditions are placed on material parasitic absorption and transmission of contacts: Absorption of 20% of sub-bandgap light leads to the required top-cell efficiencies of 18% at a bandgap of 1.5 eV to break even and 23% to reach tandem efficiencies of 30%. Perovskite-silicon tandem cells present the first low-cost devices capable of improving standalone 25% efficiencies and we quantify the efficiency gains and reduced thickness afforded by wavelength-selective light trapping. An analytical formalism for Lambertian tandem light trapping is introduced, yielding stringent requirements for wavelength selectivity. Applying these principles to a perovskite-based top cell characterized by strong absorption and high luminescence efficiency we show that tandem efficiencies greater than 30% are possible with a bandgap of E g = 1.55 eV and carrier diffusion lengths less than 100 nm. At an optimal top-cell bandgap of 1.7 eV, with diffusion lengths of current vapor-deposited CH 3 NH 3 PbI x Cl 1-x perovskites, we show that tandem efficiencies beyond 35% are achievable with careful light management.

A 12% Efficient Silicon/PEDOT:PSS Heterojunction Solar Cell Fabricated at < 100 °C

Abstract: Solar cells based on a heterojunction between crystalline silicon and the organic polymer PEDOT:PSS were fabricated at temperatures <;100 °C by spin coating. The Si/PEDOT interface blocks electrons in n-type silicon from moving to the anode and functions as a low-temperature alternative to diffused p- n junctions. The device takes advantage of the light absorption and transport properties of silicon and combines it with the simplicity of fabrication afforded by organics. Reverse recovery measurements were used to analyze the electron-blocking effectiveness of the heterojunction. The data show that current in the device is primarily due to holes injected from the anode into the silicon. At AM1.5, Si/PEDOT heterojunction solar cells achieve power conversion efficiency of 11.7%, which is among the highest reported values for this class of devices.

Progress in Laser-Crystallized Thin-Film Polycrystalline Silicon Solar Cells: Intermediate Layers, Light Trapping, and Metallization

Abstract: Diode laser crystallization of thin silicon films on the glass has been used to form polycrystalline silicon layers for solar cells. Properties of an intermediate layer stack of sputtered SiOx/SiNx/SiOx between the glass and the silicon have been improved by reactively sputtering the SiNx layer, which result in enhanced optical and electrical performance. Light trapping is further enhanced by texturing the rear surface of the silicon prior to metallization. An initial efficiency of 11.7% with VOC of 585 mV has been achieved using this technique, which are the highest values reported for poly-Si solar cells on glass substrates. Cells suffer a short term, recoverable degradation of VOC, and fill factor. The magnitude of the degradation is reduced via the repeated thermal treatment. A selective p+ metallization scheme has been developed which eliminates the degradation altogether.

Potential-Induced Degradation (PID): Introduction of a Novel Test Approach and Explanation of Increased Depletion Region Recombination

Abstract: In recent years, a detrimental degradation mechanism of solar cells in large photovoltaic fields called potential-induced degradation (PID) has been intensively investigated and discussed. Here, the module efficiency is decreasing down to a fractional part of their original efficiency. In this study, we introduce a PID test at a solar-cell level and for individual module components applicable as a tool for process control in industries and root cause analyses in science departments. Using the proposed method, one example analysis of a solar cell that is degraded by the PID tester is presented. It is shown that PID of the shunting type influences both the parallel resistance (Rp) and the depletion region recombination behavior (J02) of the solar cell. Increased recombination in the depletion region is caused by Na decorated stacking faults crossing the depletion region. This strongly influences recombination behavior in the depletion region, leading to an increased J02 and an ideality factor n2 > 2. However, the defects leave the base of the solar cell primarily unaffected, and hence, J01 recombination remains rather low. Based on these findings, a model for the shunting and the increased depletion region recombination behavior is discussed.

Direct Semiconductor Bonded 5J Cell for Space and Terrestrial Applications

Abstract: Spectrolab has demonstrated a 2.2/1.7/1.4/1.05/0.73 eV 5J cell with an efficiency of 37.8% under 1 sun AM1.5G spectrum and 35.1% efficiency for 1 sun AM0. The top three junctions and bottom two junctions were grown on GaAs and InP substrates, respectively, by metal organic vapor phase epitaxy. The GaAs- and InP-based cells were then direct bonded to create a low-resistance, high-transmissive interface. Both the space and terrestrial cells have high 1 sun Voc between 4.75 and 4.78 V. Initial tests of the terrestrial cells at concentration are promising with efficiencies increasing up to 10× concentration to a maximum value close to 41%.

Comparison of Direct Growth and Wafer Bonding for the Fabrication of GaInP/GaAs Dual-Junction Solar Cells on Silicon

Abstract: Two different process technologies were investigated for the fabrication of high-efficiency GaInP/GaAs dual-junction solar cells on silicon: direct epitaxial growth and layer transfer combined with semiconductor wafer bonding. The intention of this research is to combine the advantages of high efficiencies in III-V tandem solar cells with the low cost of silicon. Direct epitaxial growth of a GaInP/GaAs dual-junction solar cell on a GaAsyP1-y buffer on silicon yielded a 1-sun efficiency of 16.4% (AM1.5g). Threading dislocations that result from the 4% lattice grading are still the main limitation to the device performance. In contrast, similar devices fabricated by semiconductor wafer bonding on n-type inactive Si reached efficiencies of 26.0% (AM1.5g) for a 4-cm2 solar cell device.

Impact of the Band Offset for n-Zn(O,S)/p-Cu(In,Ga)Se $_{2}$ Solar Cells

Abstract: The conduction-band offset (CBO) of the Zn(O, S)/Cu(In,Ga)Se2 heterojunction can play a significant role in the performance of solar cells. The individual electron affinities and bandgaps are controlled by the oxygen-to-sulfur and gallium-to-indium ratios, and the resulting offsets can range from +1.3 eV in the “spike” direction to -0.7 eV in the “cliff” direction if the full range of the two ratios is considered. The optimal CBO of near +0.3 eV can be achieved with various combinations of the two ratios. The traditional CdS emitter is near optimal for the commonly used 1.15-eV Cu(In,Ga)Se 2 (CIGS) but less optimal for higher Ga. The flexibility with Zn(O,S) emitters ranging from above 90% oxygen for CIS down to 50% oxygen for CGS allows an optimal CBO over the full gallium range. Assuming that other factors remain constant, the optimal offset should also be able to reduce the loss in cell efficiency between room temperature and temperatures more typical of field conditions by about 1% absolute.

Oscillator-Based Inverter Control for Islanded Three-Phase Microgrids

Abstract: A control scheme is proposed for an islanded low-inertia three-phase inverter-based microgrid with a high penetration of photovoltaic (PV) generation resources. The output of each inverter is programmed to emulate the dynamics of a nonlinear oscillator. The virtual oscillators within each controller are implicitly coupled through the physical electrical network. The asymptotic synchronization of the oscillators can be guaranteed by design, and as a result, a stable power system emerges innately with no communication between the inverters. Time-domain switching-level simulation results for a 45-kW microgrid with 33% PV penetration demonstrate the merits of the proposed technique; in particular they show that the load voltage can be maintained between prescribed bounds in spite of variations in incident irradiance and step changes in the load.

Advanced Bulk Defect Passivation for Silicon Solar Cells

Abstract: Through an advanced hydrogenation process that involves controlling and manipulating the hydrogen charge state, substantial increases in the bulk minority carrier lifetime are observed for standard commercial grade boron-doped Czochralski grown silicon wafers from 250-500 μs to 1.3-1.4 ms and from 8 to 550 μs on p-type Czochralski wafers grown from upgraded metallurgical grade silicon. However, the passivation is reversible, whereby the passivated defects can be reactivated during subsequent processes. With appropriate processing that involves controlling the charge state of hydrogen, the passivation can be retained on finished devices yielding independently confirmed voltages on cells fabricated using standard commercial grade boron-doped Czochralski grown silicon over 680 mV. Hence, it appears that the charge state of hydrogen plays an important role in determining the reactivity of the atomic hydrogen and, therefore, ability to passivate defects.

Silicon Heterojunction Solar Cells With Copper-Plated Grid Electrodes: Status and Comparison With Silver Thick-Film Techniques

Abstract: Copper electroplating is investigated and compared with common silver printing techniques for the front metallization of silicon heterojunction solar cells. We achieve smaller feature sizes by electroplating, significantly reducing optical shadowing losses and improving cell efficiency by 0.4% absolute. A detailed investigation of series resistance contributions reveals that, at maximum power point, a significant part of the lateral charge-carrier transport occurs inside the crystalline bulk, rather than exclusively in the front transparent conductive oxide. This impacts optimization for the front-grid design. Using advanced electron microscopy, we study the inner structure of copper-plated fingers and their interfaces. Finally, a cell efficiency of 22.4% is demonstrated with copper-plated front metallization.

Review of Microcrack Detection Techniques for Silicon Solar Cells

Abstract: Microcracks at the device level in bulk solar cells are the current subject of substantial research by the photovoltaic (PV) industry. This review paper addresses nondestructive testing techniques that are used to detect microfacial and subfacial cracks. In this paper, we mainly focused on mono- and polycrystalline silicon PV devices and the root causes of the cracks in solar cells are described. We have categorized these cracks based on size and location in the wafer. The impact of the microcracks on electrical and mechanical performance of silicon solar cells is reviewed. For the first time, we have used the multi-attribute decision-making method to evaluate the different inspection tools that are available on the market. The decision-making tool is based on the analytical hierarchy process and our approach enables the ranking of the inspection tools for PV production stages, which have conflicting objectives and multi-attribute constraints.

Light Trapping Textures Designed by Electromagnetic Optimization for Subwavelength Thick Solar Cells

Abstract: Light trapping in solar cells allows for increased current and voltage, as well as reduced materials cost. It is known that in geometrical optics, a maximum 4 n2 absorption enhancement factor can be achieved by randomly texturing the surface of the solar cell, where n is the material refractive index. This ray-optics absorption enhancement (AE) limit only holds when the thickness of the solar cell is much greater than the optical wavelength. In subwavelength thin films, the fundamental questions remain unanswered: 1) what is the subwavelength AE limit and 2) what surface texture realizes this optimal AE? We turn to computational electromagnetic optimization in order to design nanoscale textures for light trapping in subwavelength thin films. For high-index thin films, in the weakly absorbing limit, our optimized surface textures yield an angle- and frequency-averaged enhancement factor ~39. They perform roughly 30% better than randomly textured structures, but they fall short of the ray optics enhancement limit of 4 n2 ~ 50.

Improved Rear Surface Passivation of Cu(In,Ga)Se $_{\bf 2}$ Solar Cells: A Combination of an Al $_{\bf 2}$ O $_{\bf 3}$ Rear Surface Passivation Layer and Nanosized Local Rear Point Contacts

Abstract: An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (VOC) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al2O3 is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al2O3 rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in VOC compared with unpassivated reference cells.

Optimal Orientation and Tilt Angle for Maximizing in-Plane Solar Irradiation for PV Applications in Singapore

Abstract: The performance of photovoltaic (PV) modules and systems is affected by the orientation and tilt angle, as these parameters determine the amount of solar radiation received by the surface of a PV module in a specific region. In this study, three sky models (Liu and Jordan, Klucher, and Perez et al .) are used to estimate the tilted irradiance, which would be received by a PV module at different orientations and tilt angles from the measured global horizontal irradiance (GHI) and diffuse horizontal irradiance (DHI) in Singapore (1.37°N, 103.75°E). Modeled results are compared with measured values from irradiance sensors facing 60° NE, tilted at 10°, 20°, 30°, 40°, and vertically tilted irradiance sensors facing north, south, east, and west in Singapore. Using the Perez model, it is found that a module facing east gives the maximum annual tilted irradiation for Singapore's climatic conditions. These findings are further validated by one-year comprehensive monitoring of four PV systems (tilted at 10° facing north, south, east, and west) deployed in Singapore. The PV system tilted 10° facing east demonstrated the highest specific yield, with the performance ratio close to those of other orientations.

A Multilevel Medium-Voltage Inverter for Step-Up-Transformer-Less Grid Connection of Photovoltaic Power Plants

Abstract: Recently, medium (0.1-5 MW) and large (>5 MW) scale photovoltaic (PV) power plants have attracted great attention, where medium-voltage grid connection (typically 6-36 kV) is essential for efficient power transmission and distribution. A power frequency transformer operated at 50 or 60 Hz is generally used to step up the traditional inverter's low output voltage (usually ≤400 V) to the medium-voltage level. Because of the heavy weight and large size of the power frequency transformer, the PV inverter system can be expensive and complex for installation and maintenance. As an alternative approach to achieve a compact and lightweight direct grid connection, this paper proposes a three-phase medium-voltage PV inverter system. The 11-kV and 33-kV PV inverter systems are designed. A scaled down three-phase 1.2-kV test rig has been constructed to validate the proposed PV inverter. The experimental results are analyzed and discussed, taking into account the switching schemes and filter circuits. The experimental results demonstrate the excellent feature of the proposed PV inverter system.

Performance of Mismatched PV Systems With Submodule Integrated Converters

Abstract: Mismatch power losses in photovoltaic (PV) systems can be reduced by the use of distributed power electronics at the module or submodule level. This paper presents an experimentally validated numerical model that can be used to predict power production with distributed maximum power point tracking (DMPPT) down to the cell level. The model allows the investigations of different DMPPT architectures, as well as the impact of conversion efficiencies and power constraints. Results are presented for annual simulations of three representative partial shading scenarios and two scenarios where mismatches are due to aging over a period of 25 years. It is shown that DMPPT solutions that are based on submodule integrated converters offer 6.9-11.1% improvements in annual energy yield relative to a baseline centralized MPPT scenario.

Review of Experimental Results Related to the Operation of Intermediate Band Solar Cells

Abstract: The intermediate band solar cell (IBSC) has drawn the attention of the scientific community as a means to achieve high-efficiency solar cells. Complete IBSC devices have been manufactured using quantum dots, highly mismatched alloys, or bulk materials with deep-level impurities. Characterization of these devices has led, among other experimental results, to the demonstration of the two operating principles of an IBSC: the production of the photocurrent from the absorption of two below bandgap energy photons and the preservation of the output voltage of the solar cell. This study offers a thorough compilation of the most relevant reported results for the variety of technologies investigated and provides the reader with an updated record of IBSC experimental achievements. A table condensing the reported experimental results is presented, which provides information at a glance about achievements, as well as pending results, for every studied technology.

Flexible Thin-Film Tandem Solar Cells With >30% Efficiency

Abstract: Alta Devices, Inc. has previously reported on single-junction thin-film GaAs photovoltaic devices on flexible substrates with efficiencies up to 28.8% under AM1.5G solar illumination at 1-sun intensity. Here, we show that the same technology platform can be extended to tandem devices that are capable of even higher efficiencies: so far up to 30.8%. Specifically, here, we report on a lattice-matched, series-connected, two-junction device with InGaP as the light-absorbing material of the top cell and GaAs as the absorber in the bottom cell. The material is grown by metallorganic chemical vapor deposition, and then, the device is lifted off by the epitaxial liftoff (ELO) process, as previously reported. This demonstrates that ELO is not only capable of record-setting single-junction performance but capable of achieving world-class efficiency with a multijunction architecture as well.

Hot Carriers in Quantum Wells for Photovoltaic Efficiency Enhancement

Abstract: In a hot carrier solar cell, the steady-state carrier population is hot relative to the surrounding lattice. This requires an absorber material which restricts carrier-phonon interaction and, therefore, reduces entropic loss during thermalization. The limiting efficiency of these devices approaches 85%: the Carnot limit for a solar energy collector. A spectroscopic analysis of GaAsP/InGaAs quantum well structures shows that carrier cooling in single quantum well samples is dominated by the rate of radiative recombination, leading to unprecedented carrier cooling lifetime (τ = 5.8 ±0.1 ns). This exceptional lifetime arises due to state saturation, frustrating the carrier scattering processes. A steady-state carrier population temperature >100 K above the lattice temperature is measured under illumination equivalent to 10 000 Suns. We calculate the projected efficiency >40% for a device with these characteristics, amounting to a 3% efficiency enhancement over equivalent single-junction devices.

Performance Degradation of Various PV Module Technologies in Tropical Singapore

Abstract: The performance of ten photovoltaic (PV) modules with nine different solar cell technologies (and one different module construction) is monitored in the tropical climate of Singapore. The types of modules included in this study are monocrystalline Si (glass-backsheet with frame and glass-glass without frame), heterojunction crystalline Si, monocrystalline Si back-contact, multicrystalline Si, double-junction “micromorph” Si, single-junction/double-junction amorphous Si, CdTe, and CIGS. Three years of outdoor monitoring data are used to extract degradation trends of the performance of the various modules. Statistical decomposition methods are used to extract trends for performance ratio (PR), short-circuit current (I SC ), open-circuit voltage (V OC ), and fill factor (FF). The degradation rates of the monocrystalline Si modules are found to be equal to or less than -0.8% per year, mainly contributed by the decrease in I SC . The multicrystalline Si module shows a slightly higher degradation rate of -1.0% per year. The amorphous Si, micromorph Si, and CdTe modules show degradation rates of around -2% per year. The CIGS module showed an exceptionally high degradation rate of -6% per year. The decrease in FF and VOC is found to be significant for all the thin-film modules but not for the crystalline silicon modules.