The application of luminescence imaging to silicon wafer solar cells has developed rapidly in recent years due to its convenience, speed, non-destructive nature, relatively low cost, and simple instrumentation requirements. This non-destructive optical characterization method has the potential to be used as a tool for development and processing of sili-
con wafer solar cells, and is able to yield information not obtained easily through other measurement methods [244]. It has been applied to acquire information such as carrier transport properties [184, 216, 245–250], the distribution of defects in silicon wafer solar cells [117, 136, 203, 251–257], for quantitative images of the lifetime of minority charge carriers in silicon wafer solar cells [166, 167, 171, 173, 174, 179, 258–261], and the mea- surement of local voltages of silicon wafer solar modules [182]. It has also been used to enhance fabrication parameters by non-destructive characterization implemented along with real time control of processing steps [116, 134, 262, 263]. This Section covers a brief overview of progress in luminescence-based characterization of silicon wafer based photovoltaic devices.
1.3.1 The spatial homogeneity of a silicon wafer solar cell
Figure 1.3.1 shows an electroluminescence image of a typical multicrystalline silicon wafer solar cell. Dark and bright regions of the image reveal the defect topography of the device, and the spatial inhomogeneity of the device’s performance characteristics.
In 1956, Chynoweth and McKay reported electroluminescence of a silicon pn junction [264]. This was applied to diagnostics of integrated circuits byKhurana and Chiang in 1986 [265]. Initially, the silicon wafer solar cell was thought to yield lumi- nescence like any light emitting diode under a bias voltage, and that the luminescence would be fairly uniform over the topography of the wafer, though weak since silicon has an indirect band-gap [266]. Luminescence of crystalline silicon was studied previously, mainly for the development of silicon emitters [243, 267–271] since this material may be processed easily on a chip. In 2001 Green et al. studied light emission from silicon, getting an efficiency of > 1% [272]. Such work has led to little practical application for silicon emitters [244, 273]. Luminescence was, however, used to characterize the ab- sorptivity of a silicon solar cell by Trupke et al. in 1998. Baeumler et al. discussed luminescence imaging for semiconductor homogeneity measurements in 1999 [157].
In 2005 Fuyuki et al. are credited with the discovery that an electroluminescence image of silicon wafer solar cells was not homogeneous [197]. Using a high-grade scien-
Figure 1.3.1: An electroluminescence image of a multicrystalline silicon wafer solar cell.
Dark regions indicate low injection and thus defects in the solar cell. The nature of these defects is not clear from such an image and further analysis must be performed to identify the problem. This same cell is characterized in Figures 1.3.2, 4.2.4, and 6.1.2.
tific silicon charge coupled device as an imaging camera, dark and light regions of the luminescence image of the cell were used to map defective regions of a silicon wafer solar cell in a direct measurement of the homogeneity of the device performance. The discov- ery showed it is possible to capture a large amount of information in a single, simple measurement, imaging the entire area of the solar cell using a camera array. Fuyukiat- tributed the measurement obtained to the diffusion length parameter [197]. In 2005 and 2006Trupke et al. developed non-contact photoluminescence imaging for application to silicon wafer and device inspection [162, 164, 274, 275]. These discoveries opened the door to a flood of research on the subject, which also reflects the enthusiasm of many
researchers for thehigh-growth solar energy industry7.
1.3.2 Measuring the electrical properties of a silicon wafer solar cell
Carrier transport of the solar cell, like that measured byFuyukiet al., is a focal point for luminescence-based characterization of solar cells; the lifetimeτ, and diffusion lengthL being important parameters governing photovoltaic device efficiency. In 2005 Trupke et al. reported photoluminescence as a contact-less replacement for suns-VOC mea- surements, where the intensity of photoluminescence was used to compute the carrier transport properties of the device [274]. In 2006 Abbott reported on photolumines- cence characterization for solar cell fabrication where the effective lifetime of carriers was determined [262].
Bardos et al. showed that quasi-steady-state photoluminescence is unaffected by the depletion region modulation effect which gives artificially high photo-conductance lifetime measurements [275]. Trupkeet al. studied photon reabsorption of luminescence showing the luminescence spectra change due to the absorption spectrum of crystalline silicon. Diffusion length imaging of silicon solar cells was improved from the method of Fuyuki et al. by W¨urfel et al. by using photon reabsorption in silicon to develop a ratio-based imaging technique [204] that may remove voltage dependences of an elec- troluminescence image. This study developed the generalized Planck law [276–280] to characterize silicon by its luminescence.
The determination of diffusion lengths using luminescence imaging was performed byGieseckeet al. using photoluminescence, as well asHinkenet al. who removed the dependence of the method to inhomogeneities of the optics, and proposed a measure- ment using both electro- and photoluminescence. Gieseckealso studied recombination issues in silicon solar cells by modeling electro- and photoluminescence, suggesting ways to separate bulk and surface recombination effects. In 2006 the advantages of photolu- minescence techniques to in-line production of solar cells was described by Trupke et
7In 1985, annual solar installation demand was only 21 MW, while in 2009 photovoltaic installations were 7.3 GW. It is estimated that solar energy demand has grown at about 30% per annum over the past 15 years. Source: 2010 cSolarbuzz
al. [162, 163]. This presented the use of luminescence imaging tools to in-line silicon wafer solar cell production, similarly proposed byAbbottet al. [262].
As well, the spatially resolved sheet resistance Rsh was measured by Isenberg et al. in 2006, where infrared imaging using luminescence replaced the four-point-probe method. Series resistance measurements were performed byPinket al., Hinkenet al.,
Figure 1.3.2: Quantitative series resistance image of a multicrystalline silicon wafer solar cell with the scale bar in Ohms per square centimeter. This is the same cell from Figure 1.3.1. The series resistance is generated using electroluminescence and photoluminescence to record open and closed circuit behavior of the cell, and so allow the determination of the series resistance at points on the cell. The same cell is characterized in Figures 1.3.1, 4.2.4, and 6.1.2.
and Trupke et al. [166, 167, 171] citing the advantages of luminescence imaging over other methods, such as Corescan [281], while developing separate quantitative methods.
In Figure 1.3.2 a series resistance image of a multicrystalline silicon wafer solar cell made at the Solar Energy Research Institute of Singapore is shown. Series resistance imaging was studied also byHaunschildet al.,Glaathaaret al., andBreitenstein
et al. proposed iterative methods for quantitative measurement of the series resistance [179, 261, 282, 283]. Series resistance imaging was again explored by bothGieseckeet al. and Kampwerth et al. who proposed iterative methods for Rs imaging [173, 174, 284]. Other studies on series resistances have developed the method based on differential voltages, or combined photo- and electroluminescence [166, 167, 179, 259, 260].
In 2007Rauet al. published a theoretical study focusing on connecting the electro- luminescence of solar cells to their quantum efficiencies through the reciprocity relations [285], a particular revision of theoretical models developed nearly a decade previously by the same author [286]. The path-length enhancement factor of a solar cell was measured using luminescence applying the reciprocity relationships by Kirchartz et al. [287].
Bruggemannet al. explored the theoretical limitations of the ideal diode model used for luminescence imaging [288].
Herlufsenet al. performed a photoluminescence-based lifetime imaging experiment on a solar cell calibrated with a photo-conductance signal to get absolute values of the excess carrier density in the device. Lifetime imaging was also revisited by R¨udiger et al. where the photon absorption of photoluminescence was used to obtain corrected lifetime values [248].
1.3.3 Identification of defects in solar cells
During 2007, studies of defects in solar cells were advanced using luminescence imaging by Sugimoto et al. who performed a detailed experiment of luminescence from grain boundaries interpreted alongside structural information, and separately, defects after a hydrofluoric acid etch probed by photoluminescence [169, 252]. As well, Stokkan et al. modeled the effect of dislocations and grain boundaries of multi-crystalline silicon on lifetime measurements [289]. Also, shunted regions of solar cells were detected using the luminescence imaging method byAbbott et al. [116].
Breitenstein et al. and others [258, 290, 291] studied shunted regions in solar cells combining lock-in thermography (LIT) [292] with photo- and electroluminescence, and showed that shunted regions of the cell can be weakly detected [117, 172, 253].
These shunts commonly occur close to screen printed contacts [293], and to laser scribed regions [294]. This allowed detection of pre-breakdown sites [264] in solar cells, which is important as these defects may cause photovoltaic module destruction and sometimes ignite fires, which was studied by Alonso-Garcia et al. [295–297]. Reverse-biasing of solar cells, initially performed by Dreckschmidtet al. in 2007, and observed to yield luminescence [298], was also an area of study for characterization of defects in the solar cells.
Reverse-bias luminescence is known to be correlated to defects in solar cells, and was studied by Usami et al. in 2008 [299], among others. Breitenstein et al. used three different types of lock-in thermography to understand pre-breakdown mechanism in solar cells [253]. Parameters were measured by these methods to generate a model that is useful in detecting a pre-breakdown region based on impact ionization processes.
Breitenstein et al. also studied electron transport and defects in solar cells using luminescence and electron beam induced current measurements [300].
Impurities of metals in silicon wafer solar cells were studied by MacDonald et al., where iron imaging was done using a strong laser to dissociate iron-boron pairs, while two images were then used to find the concentration of iron in the solar cell [176]. Sub- bandgap luminescence was imaged byDreckschmidtet al., showing defects occurring with luminescence energies within the bandgap of silicon [257].
Kasemann et al. performed luminescence imaging again for the detection of local shunts, where voltage drops over the surrounding series resistances were profiled through the luminescence intensity [175], and presented detailed summaries of infrared detection methods for solar cell characterization [301]. Trupke et al. performed shunt measure- ments as well, and discussed its use for in-line applications, where a laser may be used to neutralize defective regions of the solar cell.
The reverse-bias imaging technique was advanced by Kitiyanan et al., Bothe et al., andKwapilet al. [177, 255, 302] in a series of articles, illustrating the importance of the reverse-bias imaging for the detection of different kinds of defects in silicon wafer solar cells, and focusing on the impact ionization model. For example, Figure 4.2.4
shows defects in a multicrystalline silicon wafer solar cell obtained from a reverse-bias electroluminescence image.
Bauer et al. showed that etch pits in solar cells are typically located near regions where breakdown of the solar cell occurs [115], which may lead to the observation that silver paste can burn the pn junction at a defective region of a solar cell. In studies pre- sented in Sections 4.2 and 4.3, reverse-bias and sub-bandgap luminescence from defects in solar cells are compared, showing that both techniques may identify different kinds of defects [1, 303]. Fuyuki et al. applied electroluminescence to distinguish extrinsic defects (scratches or cracks) to intrinsic defects (intrinsic material properties like minor- ity carrier diffusion length, or lifetime), by applying luminescence imaging at different temperatures [304].