Reciprocity between electroluminescence and quantum efficiency used for the characterization of silicon solar cells

Spectrally and spatially resolved electroluminescence emission of crystalline silicon solar cells is interpreted in terms of two electro‐optical reciprocity relations. The first relation links the photovoltaic quantum efficiency to the electroluminescence spectrum. Both methods contain information on recombination and the optical pathlength of the incident light, simultaneously. From the electroluminescence spectrum, we derive the pathlength enhancement factor of textured and untextured crystalline silicon solar cells. Further, we use local quantum efficiency measurements to quantitatively explain light induced current as well as panchromatic electroluminescence images. A second reciprocity relation connects open circuit voltage of a solar cell with the light emitting diode quantum efficiency of the same device. For a given quality of light trapping and a given open circuit voltage, we predict the attainable LED quantum efficiency and verify our results experimentally. Copyright © 2009 John Wiley & Sons, Ltd.     

Electroluminescence as a spatial characterisation technique for dye‐sensitised solar cells

In this study, electroluminescence as a spatial characterisation technique is used to characterise a 6.9% efficient dye‐sensitised solar cell. The obtained image is compared with a light beam‐induced current scan image and a transmittance image. Results reveal the presence of inhomogeneities including those resulting from the topography of the cell and from defects, for example, presence of iodine crystals in the electrolyte, localised absence of dye in the active layer and poor adhesion of the active layer to the electrodes. The ability to identify such inhomogeneities within a relatively short acquisition time gives electroluminescence an advantage over the light beam‐induced current technique. Copyright © 2012 John Wiley & Sons, Ltd.     

Temperature‐dependent electroluminescence and voltages of multi‐junction solar cells

The spectral electroluminescence of a monolithic triple‐junction solar cell reveals the sub‐cell open‐circuit voltages under variation of temperature and carrier concentration. We present an electroluminescence set‐up that is able to acquire a full spectrum in less than 10 s and give insight into the voltage‐extraction process. The sum of the sub‐cell voltages is in excellent agreement to open‐circuit voltages acquired under concentrated illumination. The temperature‐induced voltage losses are investigated and explained by a bandgap dependent increase in intrinsic carrier concentration. Finally, the accurate extraction of the sub‐cell bandgaps helps to identify a temperature rise with increasing current density. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of cracks on the local current–voltage parameters of silicon solar cells

The existence of cracks in silicon solar cells can drastically reduce the electrical performance of an individual cell and even of an entire photovoltaic module. An in‐depth understanding of the influence of cracks on solar cells enables therefore calculations of the crack impact and other following effects on module level. This paper shows a detailed analysis of the electrical influence of cracks with two different spatially resolved methods including global and local current–voltage characteristics. The main influence of cracks is an increased recombination current density in the depletion region, which is clearly shown by spatially resolved dark lock‐in thermography measurements with local current–voltage investigation. This increased recombination current density affects further cell parameters such as the efficiency, which is confirmed also by the global current–voltage characteristics. The additionally used ratio image technique based on electroluminescence measurements is in comparison with the local current–voltage method, the more reliable and faster method for the crack detection itself, and allows on cell‐level and module‐level a continuous inspection of cracks. Copyright © 2013 John Wiley & Sons, Ltd.     

Detection of the voltage distribution in photovoltaic modules by electroluminescence imaging

We introduce an approach to determine the operating voltage of individual solar cells in photovoltaic (PV) modules by electroluminescence (EL) imaging. The highest EL signal of each solar cell is proportional to its operating voltage. Moreover the sum of all operating voltages equals the externally applied module voltage. Thus the operating voltage of individual solar cells is determined from the measured EL signal. The reliability of this relation is verified by measurements on specially prepared PV modules allowing us to measure the individual operating cell voltage. The experimentally measured cell voltages are deduced with an uncertainty of ±1% from an EL image. Moreover, the operating cell voltages determined from the EL image are used to calculate the module series resistance. Comparing experimentally determined values from the operating cell voltage and the total current flowing supplied to the module with calculated module series resistances using tabulated material and typical solar cell parameters, a very good correspondence is found. Copyright © 2010 John Wiley & Sons, Ltd.     

Spatial nonuniformity of current flow and its consideration in determination of characteristics of surface illuminated InAsSbP/InAs-based photodiodes

Current-voltage characteristics of surface-irradiated photodiodes based on the InAsSbP/InAs structures are analyzed using experimental data on the distribution of electroluminescence intensity over the diode surface and taking into account thickening the current streamlines near the contacts. The influence of the potential barrier associated with the N-InAsSbP/n-InAs junction in double heterostructures on the differential resistance of diodes under zero bias, the value of the reverse current, and spreading of the forward current is discussed.     

Influence of temperature on luminescent coupling and material quality evaluation in inverted lattice‐matched and metamorphic multi‐junction solar cells

Inverted metamorphic multi‐junction solar cells have reached efficiencies close to 46%. These solar cells contain very high‐quality materials that exhibit strong luminescent coupling between the junctions. The presence of luminescent coupling has a significant impact on the behavior of multi‐junction solar cells affecting the optimal design of these devices. Because of the importance of studying devices under real operating conditions, the temperature dependence of the luminescent coupling is analyzed over a range of 25–120°C. Luminescent coupling analysis results show a reduction of the luminescent coupling current as a function of temperature in two tandem components of an inverted metamorphic triple junction solar cell such as GaInP/GaAs and GaAs/GaInAs solar cells. This reduction is quantified and examined by means of luminescent coupling analysis and modeling, electroluminescence measurements and optical modeling at the device and subcell level. The results of the models are verified and discussed. Copyright © 2015 John Wiley & Sons, Ltd.     

On the detection of shunts in silicon solar cells by photo‐ and electroluminescence imaging

Recently electroluminescence (EL) and photoluminescence (PL) imaging were reported to allow detection of strong ohmic shunts in silicon solar cells. Comparing lock‐in thermography (LIT) images with luminescence images of various shunted cells, measured under different conditions, the ability of luminescence techniques for shunt detection is investigated. Luminescence imaging allows identifying ohmic shunts only if they reach a certain strength. The detection limit for PL measurements of linear shunts was estimated to be in the order of 15 mA at 0·5 V bias for a point‐like shunt in multicrystalline (mc) cells. Pre‐breakdown sites can also be detected by electroluminescence under reverse bias. Copyright © 2007 John Wiley & Sons, Ltd.     

Analysis of the fill factor for n-CdS/p-CdTe solar cells

The fill factor is calculated for a solar cell with series and parallel resistance, and with a voltage-dependent collection function for the light current that is important for solar cells using high optical absorption coefficient materials, or with significant junction interface recombination velocities. A parametric approach is used that is helpful and physically clarifying. Good agreement is demonstrated between the theoretical prediction and the experimental results for an 8% efficient n-CdS/p-CdTe heterojunction solar cell produced in our laboratory.     

The effects of interface states on the capacitance and electroluminescence efficiency of InGaN/GaN light-emitting diodes

The capacitance-voltage characteristics and external quantum efficiency of electroluminescence in blue GaN light-emitting diodes (LEDs) with an InGaN quantum well have been investigated in the temperature range 77–300 K. The results obtained are interpreted taking into account the effect of the InGaN/GaN interface states of structural defects and impurities on the capacitance of the GaN LEDs. The nonlinearity of the C^?2(U) characteristics observed at low forward bias is attributed to an increase in the interface charge resulting from tunneling of free electrons and their trapping at the interface states. According to estimates, states with a density of about 3 × 10¹² cm^?2 are present at the interface. A recombination current in the interface region suppresses the injection of charge carriers into the quantum well and decreases the electroluminescence efficiency at high forward bias. Degradation of the optical power of the LEDs, accompanied by an increase in the measured capacitance, is attributed to an increase in the density of charged interface states and changes in their distribution in the band gap.     

基于电致发光成像理论的硅太阳电池缺陷检测

基于半导体电致发光（EL）的基本理论,在理想P-N结模型条件下,定量计算正向偏压时硅太阳电池EL强度与少数载流子扩散长度的对应关系;分析了电池片中缺陷和扩散长度（EL强度）的关系,指出通过硅电池EL图像检测电池缺陷的可行性。搭建实验平台,分别拍摄单晶硅和多晶硅电池的EL图像,从中成功检测出各种缺陷;编写可视化裂纹自动检...     

多结太阳电池辐射损伤的电致发光研究

建立了电致发光测试方法,对一种国产GaInP/GaAs/Ge三结太阳电池1MeV电子辐照后各子电池的辐照特性进行了研究,并与光谱响应结果进行了比较。讨论了GaInP/GaAs/Ge三结太阳电池的辐射损伤机理。     

InGaN/GaN multi-quantum-well-based light-emitting and photodetective dual-functional devices

In this study, we fabricated and characterized an InGaN/GaN multi-quantum-well (MQW)-based p-n junction photodetector (PD) for voltage-selective light-emitting and photo-detective applications. The photode-tector exhibits a cutoff wavelength at around 460nm which is close to its electroluminescence (EL) peak position. The rejection ratio was determined to be more than three orders of magnitude. Under zero bias, the responsivity of the device peaks at 371 nm, with a value of 0.068 A/W, corresponding to a 23% quantum efficiency.The overall responsivity gradually rises as a function of reverse bias, which is explained by the enhanced photocarrier collection efficiency.     

The photoresponse of CdS/CuInSe2thin-film heterojunction solar cells

The effect of light bias on the spectral current response and spectral capacitance characteristics of CdS/CuInSe2thin-film heterojunction solar cells has been investigated. Monochromatic light bias has been used to identify specific wavelength regions responsible for the spectral behavior seen under white light bias. Variations with light or voltage bias are consistent with the effect of the field on interface recombination in both high and low CdS resistivity devices. Devices with high CdS resistivity show spectrally dependent enhancement and quenching effects very similar to those reported for CdS/Cu2S devices in which the space charge region was primarily in the CdS. It is concluded that in high CdS resistivity devices the junction behavior is controlled by the photoconductive CdS as has been established in CdS/Cu2S cells. Low CdS resistivity CdS/CuInSe2devices show none of these effects.     

三结太阳电池栅线对激光辐照中的传热影响研究

为了研究三结太阳电池表面的栅线在1 070 nm连续激光辐照过程中的传热影响机制,文中通过激光辐照过程中三结太阳电池实时的电致发光现象分析三结太阳电池的损伤情况,并建立三维锗基太阳电池模型,借助有限元分析软件COMSOL对连续激光辐照锗基太阳电池的温度分布进行仿真。结果表明:在连续激光功率密度为72.5 W/cm2、辐照时间为41 s时,三结太阳电池的顶电池出现轻微损伤,损伤区域首先沿着栅线分布。在锗基太阳电池的仿真模型中,电池的温度升高至1 318 K,栅线引起了三结太阳电池热量传递方向的各项异性,沿着栅线具有更高的热传导速率。仿真结果能够对实验现象给予合理的解释。     

Efficient 1.06-µm emission from InxGa1-xAs electroluminescent diodes

Vapor-grown p-n junctions of InxGa1-xAs have been prepared that emit near-bandgap infrared radiation at 1.06 µm with an external quantum efficiency in excess of 1 percent at room temperature. These diodes have an electroluminescence response time of 20 ns. In addition, InxGa1-xAs injection lasers have been fabricated with threshold current densities between 2000 and 3000 A/cm²at 80 K. The importance of internal absorption losses in determining the spectral distribution and the electroluminescence efficiency at room temperature is described.     

Improved Conversion Efficiency of GaN/InGaN Thin-Film Solar Cells

In this letter, we report on the fabrication and photovoltaic characteristics of p-i-n GaN/InGaN thin-film solar cells. The thin-film solar cells were fabricated by removing sapphire using a laser lift-off technique and, then, transferring the remaining p-i-n structure onto a Ti/Ag mirror-coated Si substrate via wafer bonding. The mirror structure is helpful to enhance light absorption for a solar cell with a thin absorption layer. After the thin-film process for a conventional sapphire-based p-i-n solar cell, the device exhibits an enhancement factor of 57.6% in current density and an increment in conversion efficiency from 0.55% to 0.80%. The physical origin for the photocurrent enhancement in the thin-film solar cell is related to multireflection of light by the mirror structure.      

Numerical modeling of photon recycling in solar cells

The incorporation of a detailed model for the photon recycling effect into an exact numerical solar cell simulation code is described. The commonly encountered radiative lifetime multiplication factor is shown to have both a spatial and bias point dependence. Example simulations show that photon recycling can significantly reduce the effective recombination current and thereby have a large effect on the open circuit voltage of a solar cell. Possible ways to capitalize upon this effect are discussed     

Electrical and photoelectrical properties of P3HT/n-Si hybrid organic–inorganic heterojunction solar cells

A detail analysis of electrical and photoelectrical properties of hybrid organic–inorganic heterojunction solar cells poly(3-hexylthiophene) (P3HT)/n-Si, fabricated by spin-coating of the polymeric thin film onto oxide passivated Si(1 0 0) surface, was carried out within the temperature ranging from 283 to 333 K. The dominating current transport mechanisms were established to be the multistep tunnel-recombination and space charge limited current at forward bias and leakage current through the shunt resistance at reverse bias. A simple approach was developed and successfully applied for the correct analysis of the high frequency C–V characteristics of hybrid heterojunction solar cells. The P3HT/n-Si solar cell under investigation possessed the following photoelectric parameters: J_sc = 16.25 mA/cm², V_oc = 0.456 V, FF = 0.45, η = 3.32% at 100 mW/cm² AM 1.5 illumination. The light dependence of the current transport mechanisms through the P3HT/n-Si hybrid solar cells is presented quantitatively and discussed in detail.