Progression of metalorganic chemical vapour‐deposited CdTe thin‐film PV devices towards modules

This paper reports important developments achieved with CdTe thin‐film photovoltaic devices produced using metalorganic chemical vapour deposition at atmospheric pressure. In particular, attention was paid to understand the enhancements in solar cell conversion efficiency, to develop the cell design, and assess scalability towards modules. Improvements in the device performance were achieved by optimising the high‐transparency window layer (Cd_0.3Zn_0.7S) and a device‐activation anneal. These increased the fill factor and open‐circuit voltage to 77 ± 1% and 785 ± 7 mV, respectively, compared with 69 ± 3% and 710 ± 10 mV for previous baseline devices with no anneal and thicker Cd_0.3Zn_0.7S. The enhancement in these parameters is associated with the two fold to three fold increase in the net acceptor density of CdTe upon air annealing and a decrease in the back contact barrier height from 0.24 ± 0.01 to 0.16 ± 0.02 eV. The optimum thickness of the window layer for maximum photocurrent was 150 nm. The cell size was scaled from 0.25 to 2 cm² in order to assess its impact on the device series resistance and fill factor. Finally, micro‐module devices utilising series‐connected 2‐cm² sub‐cells were fabricated using a combination of laser and mechanical scribing techniques. An initial module‐to‐cell efficiency ratio of 0.9 was demonstrated for a six‐cell module with the use of the improved device structure and processing. Prospects for CdTe photovoltaic modules grown by metalorganic chemical vapour deposition are commented on. Copyright © 2015 John Wiley & Sons, Ltd.     

Multijunction solar cell model for translating I–V characteristics as a function of irradiance,spectrum, and cell temperature

We describe here a lumped diode model for concentrator multijunction solar cells, in which the temperature, irradiance and spectrum dependences are explicitly included. Moreover an experimental method based on it for the prediction of the I‐V curve under any irradiance‐spectrum‐temperature conditions from a single input measurement is proposed and applied to a set of commercial triple‐junction solar cells in order to demonstrate its validity. Component ‘isotype’ cells are used as reference cells for intensity and spectrum, sparing the measurement of light spectrum and cell spectral response. Finally, a mean RMS prediction error of 0.85% over a range of 100X‐25°C to 700X‐75°C is reported for the whole set when the model parameters inherent in the cell are assumed to be the same for every sample. If optimum parameters are extracted for every cell, the RMS error is reduced to 0.53%. Copyright © 2010 John Wiley & Sons, Ltd.     

Validation of energy prediction method for a concentrator photovoltaic module in Toyohashi Japan

III–V concentrator photovoltaic systems attain high efficiency through the use of series connected multi‐junction solar cells. As these solar cells absorb over distinct bands over the solar spectrum, they have a more complex response to real illumination conditions than conventional silicon solar cells. Estimates for annual energy yield made assuming fixed reference spectra can vary by up to 15% depending on the assumptions made. Using a detailed computer simulation, the behaviour of a 20‐cell InGaP/In_0.01GaAs/Ge multi‐junction concentrator system was simulated in 5‐min intervals over an entire year, accounting for changes in direct normal irradiance, humidity, temperature and aerosol optical depth. The simulation was compared with concentrator system monitoring data taken over the same period and excellent agreement (within 2%) in the annual energy yield was obtained. Air mass, aerosol optical depth and precipitable water have been identified as atmospheric parameters with the largest impact on system efficiency. Copyright © 2012 John Wiley & Sons, Ltd.     

Determination of built-in-potential in N-I-P a-Si:H solar cells

This paper describes a simple method to evaluate built-in-potential of P-N junction a-Si:H solar cells. Built-in-potential (Vbi) was calculated from photovoltage-current characteristics at various temperature and illumination levels. Results from two independent, experimental techniques agree extremely well. Vbi= 1.02 eV ± 0.02 eV was obtained for a N-I-P cell having Voc= 650 mV under AM1 illumination.     

BICON: high concentration PV using one‐axis tracking and silicon concentrator cells

BICON is a two‐stage concentrator system developed at Fraunhofer ISE which is one‐axis tracked. The innovation of this one‐axis tracked system is that it enables a high geometrical concentration of 300 × in combination with a high optical efficiency (upto 78%) and a large acceptance angle of ± 23·5° all year through. For this, the system uses a parabolic mirror (40·4 ×) and a three dimensional second stage consisting of compound parabolic concentrators (CPCs, 7·7 ×). For the concentrator concept and particularly for an easy cell integration, rear‐line‐contacted concentrator (RLCC) cells with a maximum efficiency of 25% were developed and a hybrid mounting concept for the RLCC cells is presented. The optical performance of different CPC materials was tested and is analysed in this paper. Finally, small modules consisting of six series interconnected RLCC cells and six CPCs were integrated into the concentrator system and tested outdoor. A BICON system efficiency of 16·2% was reached at around 800 W/m² direct irradiance under realistic outdoor conditions. Copyright © 2006 John Wiley & Sons, Ltd.     

Self‐Powered Sensors Enabled by Wide‐Bandgap Perovskite Indoor Photovoltaic Cells

A new approach to ubiquitous sensing for indoor applications is presented, using low‐cost indoor perovskite photovoltaic cells as external power sources for backscatter sensors. Wide‐bandgap perovskite photovoltaic cells for indoor light energy harvesting are presented with the 1.63 and 1.84 eV devices that demonstrate efficiencies of 21% and 18.5%, respectively, under indoor compact fluorescent lighting, with a champion open‐circuit voltage of 0.95 V in a 1.84 eV cell under a light intensity of 0.16 mW cm^?2. Subsequently, a wireless temperature sensor self‐powered by a perovskite indoor light‐harvesting module is demonstrated. Three perovskite photovoltaic cells are connected in series to create a module that produces 14.5 µW output power under 0.16 mW cm^?2 of compact fluorescent illumination with an efficiency of 13.2%. This module is used as an external power source for a battery‐assisted radio‐frequency identification temperature sensor and demonstrates a read range by of 5.1 m while maintaining very high frequency measurements every 1.24 s. The combined indoor perovskite photovoltaic modules and backscatter radio‐frequency sensors are further discussed as a route to ubiquitous sensing in buildings given their potential to be manufactured in an integrated manner at very low cost, their lack of a need for battery replacement, and the high frequency data collection possible.     

Development of concentrator modules with dome‐shaped Fresnel lenses and triple‐junction concentrator cells

The status of the development of a new concentrator module in Japan is discussed based on three arguments, performance, reliability and cost. We have achieved a 26·6% peak uncorrected efficiency from a 7056 cm² 400 × module with 36 solar cells connected in series, measured in house. The peak uncorrected efficiencies of the same type of the module with 6 solar cells connected in series and 1176 cm² area measured by Fraunhofer ISE and NREL are reported as 27·4% and 24·8% respectively. The peak uncorrected efficiency for a 550× and 5445 cm² module with 20 solar cells connected in series was 28·9% in house. The temperature‐corrected efficiency of the 550 × module under optimal solar irradiation condition was 31·5 ± 1·7%. In terms of performance, the annual power generation is discussed based on a side‐by‐side evaluation against a 14% commercial multicrystalline silicon module. For reliability, some new degradation modes inherent to high concentration III‐V solar cell system are discussed and a 20‐year lifetime under concentrated flux exposure proven. The fail‐safe issues concerning the concentrated sunlight are also discussed. Moreover, the overall scenario for the reduction of material cost is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four‐Terminal Tandems with Silicon and CIGS

Wide‐bandgap perovskite solar cells (PSCs) with optimal bandgap (E_g) and high power conversion efficiency (PCE) are key to high‐performance perovskite‐based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double‐cation wide‐bandgap PSCs with engineered bandgap (1.65 eV ≤ E_g ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open‐circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in four‐terminal perovskite/c‐Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four‐terminal tandem configuration with respect to variations in the perovskite bandgap for two state‐of‐the‐art bottom solar cells is experimentally validated.     

Ions Matter: Description of the Anomalous Electronic Behavior in Methylammonium Lead Halide Perovskite Devices

Carrier transport in methylammonium lead iodide (MAPbI₃)‐based hybrid organic–inorganic perovskites (HOIPs) is obscured by vacancy‐mediated ion migration. Thus, the nature of migrating species (cation/anion) and their effect on electronic transport in MAPbI₃ has remained controversial. Temperature‐dependent pulsed voltage–current measurements of MAPbI₃ thin films are performed under dark conditions, designed to decouple ion‐migration/accumulation and electronic transport. Measurement conditions (electric‐field history and scan rate) are shown to affect the electronic transport in MAPbI₃ thin films, through a mechanism involving ion migration and accumulation at the electrode interfaces. The presence of thermally activated processes with distinct activation energies (E_a) of 0.1 ± 0.001 and 0.41 ± 0.02 eV is established, and are assigned to electromigration of iodine vacancies and methylammonium vacancies, respectively. Analysis of activation energies obtained from electronic conduction versus capacitive discharge shows that the electromigration of these ionic species is responsible for the modification of interfacial electronic properties of MAPbI₃, and elaborates previously unaddressed issues of “fast” and “slow” ion migration. The results demonstrate that the intrinsic behavior of MAPbI₃ material is responsible for the hysteresis of the solar cells, but also have implications for other HOIP‐based devices, such as memristors, detectors, and energy storage devices.     

Wide bandgap Cu(In,Ga)Se2 solar cells with improved energy conversion efficiency

We report on improvements to the energy conversion efficiency of wide bandgap (E_g > 1.2 eV) solar cells on the basis of CuIn_1−xGa_xSe₂. Historically, attaining high efficiency (>16%) from these types of compound semiconductor thin films has been difficult. Nevertheless, by using (a) the alkaline‐containing high‐temperature EtaMax glass substrates from Schott AG, (b) elevated substrate temperatures of 600–650 °C, and (c) high vacuum evaporation from elemental sources following National Renewable Energy Laboratory's three‐stage process, we have been able to improve the performance of wider bandgap solar cells with 1.2 < E_g < 1.45 eV. The current density–voltage (J–V) data we present includes efficiencies >18% for absorber bandgaps of ~1.30 eV and efficiencies of ~16% for bandgaps up to ~1.45 eV. In comparing J–V parameters in similar materials, we establish gains in the open‐circuit voltage and, to a lesser degree, the fill factor value, as the reason for the improved performance. The higher voltages seen in these wide gap materials grown at high substrate temperatures are due to reduced recombination. We establish the existence of random and discrete grains within the CIGS absorbers that yield limited or no generation/collection of minority carriers. We also show that interfacial recombination is the main mechanism limiting additional enhancements to open‐circuit voltage and therefore performance. Solar cell results, absorber materials characterization, and experimental details and discussion are presented. Copyright © 2012 John Wiley & Sons, Ltd.     

Operation of series‐connected silicon‐based photovoltaic modules under partial shading conditions

Partial shading has been recognized as a major cause of energy losses in photovoltaic (PV) power generators. Partial shading has severe effects on the electrical characteristics of the PV power generator, because it causes multiple maximum power points (MPPs) to the power‐voltage curve. Multiple maxima complicate MPP tracking, and the tracking algorithms are often unable to detect the global maximum. Considerable amount of available electrical energy may be lost, when a local MPP with low power is tracked instead of the global MPP. In this paper, the electrical characteristics of series‐connected silicon‐based PV modules under various partial shading conditions are studied by using a Matlab/Simulink simulation model. The simulation model consists of 18 series‐connected PV modules, corresponding to a single‐phase grid‐connected PV power generator. The validity of the simulation model has been verified by experimental measurements. The voltage and power characteristics of the PV power generator have been investigated under various system shading and shading strength conditions. The results can be utilized to develop new MPP tracking algorithms and in designing, for example, building integrated PV power generators. Copyright © 2011 John Wiley & Sons, Ltd.     

Critical issues in the design of polycrystalline,thin‐film tandem solar cells

We use an empirical technique for modeling the efficiency of thin‐film tandem solar cells and calculate an approximate upper limit on the range of performance of these hypothetical devices. This is shown to be approximately 28.2%, without losses due to inactive layers at the front of the device, or other parasitic sources. Reduction of the value of the reverse saturation current density by a factor of ten, increases the lossless efficiency by approximately 4% absolute. This change also greatly broadens the range of top and bottom cell bandgaps that would lead to efficiencies greater than 25%, the project goal. These observations emphasize the critical importance of focusing future research on gaining a thorough understanding of recombination losses. We then calculate daily energy density outputs for various direct spectra, computed from meteorological data, and show that the optimum bandgap pairs are relatively insensitive to the detail of the spectral irradiance. We also show that the use of daily energy density output may be a more useful criterion than efficiency in designing tandem thin‐film solar cells. We compute contours of equal daily energy density output and show that the range of potentially suitable bandgap pairs is much larger than simple maximization of efficiency implies. The simple parametric approach enables us to investigate the effect of partial loss of photons with energies less than that of the bandgap of the top cell, but greater than that of the bottom cell. These photons are essential to the project goal of 25% efficiency, which emphasizes the need to evaluate the optical properties in this wavelength range very carefully. We also discuss the reduction of the thickness, or the area, of the top cell. When the top subcell generates a greater current than the bottom subcell, either of these parameters may be reduced to enable current‐matching, and increased efficiencies, to be achieved. Again, this approach greatly extends the range of bandgaps that could lead to a 25% tandem thin‐film cell. Next, we consider the case of concentrated sunlight and show that the optimum bandgap pairs decrease with concentration ratio. This is due to the atmospheric absorption bands. The efficiency increases by approximately 4% absolute per decade increase in concentration ratio. Finally, we comment on some of the practical difficulties that can already be anticipated in constructing these devices. Published in 2003 by John Wiley & Sons, Ltd.     

Hybrid Cd‐free CIGS solar cell/TEG device with ZnO nanowires

A green energy device with a CuInGaSe₂ (CIGS) photovoltaic (PV) cell covered with a passive light‐trapping structure (ZnO nanowires (NWs)) and connected to an active energy‐harvesting device (thermoelectric generator (TEG)) is presented. The efficiency of the ZnO NWs/CIGS PV device obtained using a deposition temperature of 550 °C and Cd‐free processes reaches 16.5%. The series‐connected CIGS PV cell with a TEG had a record‐high efficiency of 22% at a cool‐side temperature (T_c) below 5 °C. The open‐circuit voltage (V_oc) of the hybrid CIGS PV/TEG device was increased from 0.64 to 0.85 V. This technology has potential for high‐efficiency energy‐harvesting applications. Copyright © 2014 John Wiley & Sons, Ltd.     

A method for modeling the current–voltage curve of a PV module for outdoor conditions

A method has been developed for modeling the current–voltage curve of a photovoltaic (PV) module for outdoor conditions. An indoor characterization procedure determines a PV module's temperature and irradiance correction factors, which are used in conjunction with equations to translate a reference curve to outdoor conditions of PV module temperature and irradiance. A PV technology's spectral response characteristics are accommodated in the equation for irradiance. The modeled and measured energy is compared for a one‐year period for seven PV modules of different technologies. The results validate the method's use for modeling the hourly performance of PV modules, and for modeling daily energy production for PV module energy rating purposes. Published in 2002 by John Wiley & Sons, Ltd.     

A Study of Deep Defect Levels in Semi-Insulating SiC Using Optical Admittance Spectroscopy

Optical admittance spectroscopy (OAS), supported by electron paramagnetic resonance (EPR) measurements, is used to identify the controversial vanadium acceptor levels in vanadium-doped semi-insulating (SI) 4H-SiC and 6H-SiC. The V^3+/4+ levels for the cubic site are likely located at E _c − 0.67 ± 0.02 eV and E _c − 0.70 ± 0.02 eV in 6H-SiC and E _c − 0.75 ± 0.02 eV in 4H-SiC. A peak at 0.87 ± 0.02 eV in the 6H-SiC is tentatively assigned to the same transition at the hexagonal site and the associated transition in 4H-SiC is thought to occur near 0.94 eV. All assignments are supported by the observation of V³⁺ in the EPR spectrum.     

Current–voltage curve translation by bilinear interpolation

By means of bilinear interpolation and four reference current–voltage (I–V) curves, an I–V curve of a photovoltaic (PV) module is translated to desired conditions of irradiance and PV module temperature. The four reference I–V curves are measured at two irradiance and two PV module temperature levels and contain all the essential PV module characteristic information for performing the bilinear interpolation. The interpolation is performed first with respect to open‐circuit voltage to account for PV module temperature, and second with respect to short‐circuit current to account for irradiance. The translation results over a wide range of irradiances and PV module temperatures agree closely with measured values for a group of PV modules representing seven different technologies. Root‐mean‐square errors were 1·5% or less for the I–V curve parameters of maximum power, voltage at maximum power, current at maximum power, short‐circuit current, and open‐circuit voltage. The translation is applicable for determining the performance of a PV module for a specified test condition, or for PV system performance modeling. Copyright © 2004 John Wiley & Sons, Ltd.     

Practical method for estimating the power and energy delivered by photovoltaic modules operating under non‐standard conditions

This work describes a method developed for estimating the energy delivered by building integrated photovoltaics systems operating under non‐standard conditions of irradiance and temperature. The method is based on calculation of the maximum power (P_Gmax) supplied by the modules array as a function of irradiance and ambient temperature, achieved by simulating its I–V and P–V curves using an algorithm which needs only the performance parameters supplied by the manufacturers. The energy generated by the PV system is estimated from monthly average values of P_Gmax calculated for using monthly average values of ambient temperature and irradiance obtained from data measured during 2 years. The method is applied to crystalline Si modules and tested by comparing the simulated I–V and P–V curves with those obtained by outdoor measurements as well as for comparing the energy produced during the years 2009 and 2010 with a 3.6 kWp building integrated photovoltaics system installed at the Universidad Nacional located in the city of Bogotá, Colombia, at 4°35′ latitude and 2.580 m altitude. The contrast of the simulated I–V and P–V curves for two different types of commercial Si‐modules with those experimentally obtained under real conditions indicated that the simulation method is reliably. Copyright © 2012 John Wiley & Sons, Ltd.     

Semi‐quantitative temperature accelerated life test (ALT) for the reliability qualification of concentrator solar cells and cell on carriers

An adequate qualification of concentrator photovoltaic solar cells and cell‐on‐carriers is essential to increase their industrial development. The lack of qualification tests for measuring their reliability together with the fact that conventional accelerated life tests are laborious and time consuming are open issues. Accordingly, in this paper, we propose a semi‐quantitative temperature‐accelerated life test to qualify solar cells and cell‐on‐carriers that can assure a minimum life when failure mechanisms are accelerated by temperature under emulated nominal working conditions with an activation energy >0.9 eV. A properly designed semi‐quantitative accelerated life test should be able to determine if the device under test will satisfy its reliability requirements with an acceptable uncertainty level. The applicability, procedure, and design of the proposed test are detailed in the paper. Copyright © 2015 John Wiley & Sons, Ltd.     

Optimum design of integrated tandem-type a-Si solar cell submodule

A new optimum design, in which the actual daily spectral data under outdoor conditions over a year were considered, was developed for the integrated tandem-type a-Si solar cell submodule with a structure of glass/TCO/a-Si: H(pinpin)/metal. It was found by this design that the optimum cell number connected in series, at which maximum total annual output power was obtained, was small compared with that obtained by the conventional optimum design under standard conditions (AM1.5, 100 mW cm⁻²). It was also found that when light-induced degradation was considered, the thickness of the i-layer for the second pin cell was thinner than that by the conventional optimum design.     

Energy‐Modulated Heterostructures Made with Conjugated Polymers for Directional Energy Transfer and Carrier Confinement

In this paper we demonstrate that multilayer structures with modulated bandgaps can be used for efficient energy transfer and carrier confinement inside a nanostructured film of a light‐emitting polymer. The films were produced with the layer‐by‐layer technique (LbL) with a poly(p‐phenylene vinylene) (PPV) precursor and a long chain dodecylbenzenesulfonate ion (DBS). DBS is incorporated selectively into the precursor chain, and with a rapid, low temperature conversion process (100 °C) superstructures with variable HOMO–LUMO gap could be formed along the deposition direction by changing the DBS concentration. Structures with different stair‐type energy modulations were produced, which are thermally stable and reproducible, as demonstrated by UV‐VIS. absorption measurements. Energy differences of up to 0.5 eV between the lowest and highest conjugated layers inside the stair structure could be achieved, which was sufficient to guide the excitation over long distances to the lower bandgap layer.