3.4 Measurement Modi
3.4.1 Illuminated Lock-In Thermography (iLIT) under different Conditions Depending on the electrical load applied to the solar cell, different
measurement conditions have to be distinguished for iLIT.
Voc-iLIT
Without any contacting of the solar cell and illumination by pulsed light (IR LEDs), the the cell is in (pulsed) Voc-condition. This is the most simple and practical measurement mode. The main issues are described in paragraph 3.3.
In this case, all Joule-like losses are minimal, since no net current is flowing.
However, lateral current flow occurs, since the generation of the photocurrent is homogeneous, but its back-injection as a diffusion current or at certain shunt positions is inhomogeneous.
If the measurement is performed under 1 sun illumination intensity26, Voc-iLIT basically images the local diffusion current, which is proportional to the inverse of the local lifetime [97], [98]. Hence, the iLIT image anti-correlates with a LBIC image, as Figure 103 and Figure 104 shows27.
Figure 104: LBIC mapping (left) and associated Voc-iLIT (right) measurement of the same RGS solar cell. In areas of high IQE (high Jsc) a lowered thermography signal is measured due to lower local diffusion current densities.
If Voc-iLIT is performed under reduced intensity, with Voc close to mpp at 1 sun, the measurement is equivalent to a dLIT measurement performed at mpp, except that there is the inevitable homogeneous thermalisation signal across the whole cell.
Jsc-iLIT, mpp-iLIT and Rs-iLIT
For a measurement performed with pulsed light under short circuit conditions of the cell (Jsc-iLIT), linear and non-linear shunts and also recombination in the bulk show a minor influence, since the bias is very small and the dominating local heat sources are Joule heat of lateral current flows as well as thermalisation heat. In Jsc-iLIT, inhomogeneities of the lateral current flow in the emitter are demonstrated more effectively by the 0°-image according to eqn. (6) instead of the amplitude image [98]
because in the 0°-image the homogeneous thermalisation heat is suppressed.
By applying an electrical load (resistor) to a solar cell, to obtain mpp conditions, the resulting mpp-iLIT image reflects all energy losses appearing in real solar cell operation [98], [99].
26 IIlumination and measurement are performed at the same (short) time. Thus, the light intensity of 1 sun (100 mW/cm2) has to be provided only for a short time, allowing the usage of LEDs as a light source.
27 Shown are the same measurements as presented in Figure 40, but with the iLIT measurement
For a solar cell with a homogeneous contacting28, the bias in all regions is close to zero, when the cell is kept short circuited under illumination. However, for regions of increased contact resistance between the emitter and the front side metallisation or non-contacted regions a considerable forward bias develops.
At such positions, the thermalisation heat of photo-induced electrons crossing the junction is reduced and thus a lower thermal signal is expected. Hence, Jsc-iLIT is also able to detect non-contacted areas in solar cells as dark regions. However, these regions are surrounded by a bright halo, which is due to Joule heat dissipated in the emitter by the lateral current flowing out of the non-contacted region. For mpp-iLIT in non-contacted regions the signal is locally increased, since in these regions the local diffusion current is enlarged due to the higher forward bias. Also in this case, the Joule heat dissipated in the emitter at the edge for non-contacted regions appears. This Joule effect is widely compensated if the difference between the mpp-iLIT and Jsc-iLIT images is displayed. This so-called Rs-iLIT technique may be realised either by subtracting these separately measured iLIT images or by permanent irradiation of light and pulsing the bias from zero to 0.5 V, which is close to mpp. In this case the -90° image has to be displayed, since this signal may be positive or negative. In Rs-iLIT well-contacted regions appear dark (negative signal) and non-contacted regions bright (less negative or positive signal), and the influence of Joule heating is considerably reduced [97].
3.5 Summary
Lock-In Thermography (LIT) uses the averaging nature of the Lock-In technique, which improves the thermal resolution of an infrared camera system significantly. Already after seconds of measurement time most of heat dissipating loss mechanisms in a solar cell are spatially resolved detected. In addition, the lateral resolution of the thermographic image benefits from the phase sensitive Lock-In method compared to a steady state technique.
LIT provides a whole class of investigation techniques, which allows to image not only heat dissipating areas (shunts) in solar cells but also inhomogeneities of the lifetime (Voc-iLIT), the series resistance (Rs-iLIT), and the distribution of Joule losses (Jsc-iLIT) under realistic operation conditions (mpp-iLIT) of a solar cell. The ohmic or rectifying nature of occurring shunts can be investigated in detail using dark Lock-In Thermography (dLIT) under forward and reverse bias. This makes LIT to an important tool for spatially resolved solar cell characterisation.
The illuminated Lock-In Thermography (iLIT) is a newly established measurement technique. Hereby the signal necessary for the Lock-In calculation is generated by the sample itself using photovoltaic conversion. As a result, the method is contactless and thus contamination is negligible.
In contrast to conventional dLIT this allows even the measurement on p/n-structures without a metallisation and thus enables a monitoring of emerging shunts during solar cell processing. Within this work shunting occurring during solar cell processing of RGS silicon could be clarified and thus solar cell efficiency could be enhanced.
28 i.e. with no inhomogeneously distributed lateral contact resistance of the front side metallisation to the emitter
The possibility to measure relevant shunts fast and contactless gives iLIT the potential to be applied to inline process control.
The thermography setup built up during this work served as a prototype for the system “LimoLIT Test Bench” distributed by the Infratec GmbH Dresden.
3.6 Outlook
The used thermography setup can easily be expanded allowing the spatially resolved measurement of minority charge carrier lifetimes. This CDI (Carrier Density Imaging) method [101] is much faster compared to scanning techniques as e.g. the μPCD technique and allows the mapping of the actual local lifetimes within seconds.
The measurement technique is based on the free carrier absorption (or emission) of infrared light in silicon29 combined with the fast noise reducing Lock-In principle. For CDI measurements only a hot plate has to be attached to the iLIT measurement setup shown in Figure 98.
Figure 105: Schematic setup of the CDI measurement assembly. Only a hotplate and a modified sample holder has to be attached to the iLIT setup besides software adaptions.
The hotplate (black body) acts as a source for infrared light. The thermo-camera measures the infrared light transmitted through the silicon wafer located above. This is done for a simultaneously pulsed illumination with the IR LED panels. For one period (i.e. duration of the LED IR light pulse and time to the next LED IR light pulse), only for one half of the time (during illumination) excess charge carriers are generated. The difference between the images gathered during one complete period is proportional to the IR absorption of the excess free-carriers and thus to the local excess free-carrier density. With the known generation G(x,y) actual lifetimes τeff = Δn(x,y)/G(x,y) are obtained [101].
The expansion of the thermography setup is currently performed in the context of a diploma thesis.
Summary
Within this thesis two silicon ribbon materials were analysed regarding their utilisation in photovoltaics. Both materials were produced by industry partners for R&D purposes and are close to commercialisation.
Further on, a new spatially resolved measurement technique was developed, which allows the contactless imaging of heat dissipating loss mechanisms in pn-structures and solar cells.
The first ribbon material, RGS (Ribbon Growth on Substrate), is grown directly as a wafer from the silicon melt using a supporting substrate. The very fast producing technique (one wafer per second), with no kerf loss incurred allows for a significant cost reduction, sufficiently high cell efficiencies provided.
Crystallographic investigations revealed, that the columnar grain structure of the material shows a defect-rich crystal layer on the wafer front and substrate side. To apply a screen-printing based solar cell process, the uneven wafer front side was mechanically planarised. As a result of the mechanical load, cracks can be generated. Experiments revealed that the generation and the impact of such cracks on the parallel resistance of solar cells can be reduced by an etching step prior to planarisation.
RGS material with a high > 1018 cm-3 and a low < 1018 cm-3 interstitial oxygen concentration was analysed. The diffusivity of hydrogen in RGS material with a high oxygen content is slowed down due to an additional trapping based diffusion mechanism. Thus, hydrogenation during an industrial-type cell process based only on PECVD-SiN30 is unsufficient. Hence, an additional hydrogenation step using the MIRHP-technique31 was applied. Cell results showed, that the effectiveness of the MIRHP hydogenation was higher when performed after and not before the PECVD-SiN deposition. Thereby the duration for a complete passivation was determined by means of lifetime measurements. To substitute the fraction of hydrogen effusing during the deposition of the PECVD-SiN antireflection coating, which follows in the cell process, a thermal annealing step was introduced. During this annealing step, hydrogen is released which can passivate bulk defects. This was confirmed by solar cell parameters as well as spatially resolved LBIC32 measurements.
For material with a low oxygen content and thus accelerated hydrogen kinetics it was shown in terms of solar cell parameters that a thermal annealing step results in no improvement of the material quality and further on, that an additional MIRHP hydrogenation improves cell efficiencies only marginally.
Besides enhanced efficiencies of solar cells processed from low oxygen RGS material, areal and point-like shunts were sometimes observed in this material.
Thermographic investigations revealed an ohmic behavior of the areal shunts which was attributed to carbon-containing defects. As a cause of the point-like shunts, aluminium was identified, which was drawn from the rear side to the front side of the cell via extended defects during alloying.
Substituting the fully covering rear side metallisation with a grid structure leads to enhanced fill factors due to a reduced probability of contacting shunts. A solar cell concept with an open rear side metallisation, which includes only one additional
30 Plasma Enhanced Chemical Vapor Deposition – silicon nitride (hydrogen-rich)
31 Microwave Induced Remote Hydrogen Plasma
32 Laser Beam Induced Current
processing step compared to an industrial-type screen-printing process, showed due to phosphorous and aluminium gettering as well as hydrogenation of bulk defects an improvement in lifetime by a factor of 10 from 0.5 μs to 5 μs for the low oxygen material. With 13% the highest measured efficiency for a screen-printed RGS solar cell with a single anti-reflective layer was reached so far. With this result, the silicon consumption per WattPeak is reduced by 50% compared to a multicrystalline industrial-type solar cell.
100 μm thin solar cells processed from unplanarised RGS wafers reached efficiencies up to 10.6% using a cell process not yet optimised to the significantly reduced wafer thickness. The silicon consumption per WattPeak compared to multicrystalline industrial-type solar cells (10.5 g/WattPeak) is further reduced above 70% (2.9 g/WattPeak).
To lower the reflection and thus to enhance the short circuit current density, different textures were investigated. Acidic texture solutions, however, showed due to a not sufficiently isotropic etching characteristic a pronounced etching of grain boundaries.
This resulted in enhanced diode saturation currents J02 and compensated a possible gain in efficiency as a result of texure based enhanced short circuit current densities.
The scalability of the material quality and the cell process was demonstrated by large 10x10 cm2 RGS solar cells, which showed approximately the same efficiency compared to smaller 5x5 cm2 solar cells processed from material with a comparable oxygen content.
Gallium doped RGS solar cells were investigated with the intention to enhance short circuit current densities of solar cells by a drift field as a result of segregation in combination with the specific crystallisation process and further on, to avoid degrading boron-oxygen complexes. A depth dependent dopant concentration was measured using ECV33 and a model was developed, which describes the characteristics of the charge carrier concentration. A drift effect correlated to the base doping could be measured in terms of enhanced or reduced Jsc-values, respectively, depending on the wafer side the emitter was applied to.
Phosphorous doped n-type RGS wafers showed compared to p-type wafers similar as-grown lifetimes, however, the potential of material improvement due to processing related gettering and hydrogenation turned out to be lower.
Finally, a mini-module produced from four 5x5 cm2 RGS wafers with efficiencies of 12% was tested and showed no degradation effects after one year of outdoor exposure.
The second silicon ribbon material analysed within this work, MW (Molded Wafer), is recrystallised from silicon powder on a supporting substrate. The established screen-printing process was adapted to the high wafer thickness of 500 - 800 μm and the relatively uneven wafer surfaces.
MW material, which was gettered during production, showed a more than doubled diffusion length compared to ungettered material, as calculated from IQE34 -measurements of solar cells.
For this material, different isotexture solutions were tested in order to enhance short circuit current densities. One isotexture resulted in a preferred etching of grain boundaries and thus lowered Voc-values for this material as well. Solar cells textured with another acidic etching solution, however, showed no limitation in the open circuit
33 Electrochemical Capacitance Voltage
voltage. Up to 1.3 mA/cm2 enhanced Jsc-values enabled the highest efficiency of 11.9% for a screen-printed solar cell published for this material.
A Lock-In Thermography measurement setup was built up within this work and was advanced with a new measurement mode, the so called “illuminated Lock-In Thermography”. The measurement system as well as the used Lock-In principle were presented. The improvement of the thermal resolution as a result of the Lock-In calculation was demonstrated in terms of measurements. It could be shown, that already after a measurement time of 3.3 s a thermal resolution of 2 mK is reached, corresponding to an improvement of one order of magnitude.
Measurements of industrial-type solar cells revealed that all efficiency limiting, heat dissipating shunts can be detected already after measurement times of a few seconds.
iLIT35 uses no external signal for the generation of the Lock-In reference signal but generates the signal by photovoltaic conversion within the sample. For the sample no metallisation is needed besides a pn-structure, which enables a contactless and thus contamination free monitoring of all processing steps for solar cells already after emitter formation.
LIT36 and iLIT was compared in terms of measurements. It was shown, that iLIT can describe loss mechanisms occurring during real solar cell operation more precisely due to a homogeneous injection and altered current paths compared to LIT.
The Lock-In Thermography setup served as a prototype for the already commercialised system “LimoLIT Test Bench” of Infratec GmbH Dresden.
35 illuminated Lock-In Thermography
36 (conventional) Lock-In Thermography
Zusammenfassung
In der vorliegenden Arbeit wurden zwei Silizium-Folien Materialien im Hinblick auf ihre Verwendung in der Photovoltaik analysiert. Beide Materialien wurden von Industriepartnern zu Forschungszwecken hergestellt und stehen vor der Kommerzialisierung.
Weiterhin wurde eine neue ortsaufgelöste Charakterisierungsmethode entwickelt, die es erlaubt, wärmeerzeugende Verlustmechanismen in pn-Strukturen und Solarzellen kontaktlos bildhaft darzustellen.
Das Folienmaterial RGS (Ribbon Growth on Substrate) wird auf einem Substrat direkt als Wafer aus der Silizium-Schmelze gezogen. Das mit einem Wafer pro Sekunde sehr schnelle Herstellungsverfahren, bei dem keine Sägeverluste anfallen, verspricht bei ausreichend hohen Wirkungsgraden eine deutliche Kostenreduktion.
Kristallographische Untersuchungen zeigten, dass die kolumnare Kornstruktur des Materials an der Vorder- und Substratseite des Wafers jeweils eine defektreiche Kristallschicht aufweist. Um einen auf Siebdruck basierenden Solarzellenprozess anwenden zu können, wurde die unebene Vorderseite des Wafers mechanisch planarisiert. Durch die dabei wirkende mechanische Last können Risse im Material entstehen. Experimente zeigten, dass die Entstehung bzw. die Auswirkungen solcher Risse auf den Parallelwiderstand von Solarzellen durch einen Ätzschritt vor der Planarisierung vermindert werden können.
RGS Material mit einer hohen > 1018 cm-3 sowie einer niedrigen < 1018 cm-3 interstitiellen Sauerstoffkonzentration wurde untersucht. Die Diffusivität von Wasserstoff ist in RGS Material mit hohem Sauerstoffgehalt durch einen zusätzlichen, auf Trapping basierenden Diffusionsmechanismus verlangsamt. Eine nur auf PECVD-SiN37 basierende Wasserstoff-Passivierung während eines industrie-typischen Zellprozesses ist damit nicht ausreichend. Daher wurde zusätzlich mit Hilfe der MIRHP-Technik38 passiviert. Anhand von Zellergebnissen konnte gezeigt werden, dass die MIRHP-Passivierung effektiver ist, wenn sie im Zellprozess nach und nicht vor der PECVD-SiN Abscheidung durchgeführt wird. Dabei wurde durch Lebensdauermessungen die Dauer bis zu einer vollständigen Passivierung bestimmt.
Um den Anteil an Wasserstoff zu ersetzen, der während der im Zellprozess folgenden Abscheidung der PECVD-SiN Antireflektionsschicht effundiert, wurde ein thermischer Ausheilschritt direkt nach der Abscheidung durchgeführt, während dem Wasserstoff freigesetzt wird und somit Volumendefekte passiviert werden können, was anhand von Zelldaten und ortsaufgelösten LBIC-Messungen39 bestätigt wurde.
Für Material mit geringerem Sauerstoffgehalt und damit beschleunigter Wasserstoffkinetik wurde anhand von Zellergebnissen gezeigt, dass ein thermischer Ausheilschritt keine Verbesserung der Materialqualität mit sich bringt und dass weiterhin eine zusätzliche MIRHP-Passivierung die Zellwirkungsgrade nur unwesentlich erhöht.
Neben erhöhten Wirkungsgraden wurden bei Solarzellen aus RGS mit geringer Sauerstoffkonzentration jedoch mitunter flächige wie auch punktförmige Kurzschlüsse beobachtet. Thermographische Untersuchungen zeigten ein ohmsches Verhalten der flächenartigen Kurzschlüsse, die kohlenstoffhaltigen Defekten zugeschrieben wurden. Als Quelle punktförmiger Shunts konnte durch eine
37 Plasma Enhanced Chemical Vapor Deposition – Silizium-Nitrid (wasserstoffreich)
38 Microwave Induced Remote Hydrogen Plasma
thermographische Prozessüberwachung Aluminium ausgemacht werden, das während des Legierungsvorganges durch ausgedehnte Defekte von der Zellrückseite auf die Zellvorderseite gesogen wurde.
Das Ersetzen der flächigen Rückseitenmetallisierung durch eine Gridstruktur führt wegen der reduzierten Wahrscheinlichkeit, Kurzschluss-Pfade zu kontaktieren, zu einer Erhöhung des Füllfaktors. Ein Zellkonzept mit offener Rückseitenmetallisierung, das nur einen Prozessschritt mehr als ein industrienaher Siebdruckprozess beinhaltet, zeigte als Folge von Phosphor- und Aluminium-Gettern sowie der Wasserstoff-Passivierung von Volumendefekten eine Erhöhung der Lebensdauer um den Faktor 10 von 0.5 μs auf 5 μs in sauerstoffarmem Material. Mit 13% wurde damit der bisher höchste gemessene Wirkungsgrad für eine siebgedruckte RGS Solarzelle mit einfacher Antireflexionsschicht erreicht. Mit diesem Ergebnis reduziert sich der Siliziumverbrauch pro WattPeak im Vergleich zu einer multikristallinen Industriesolarzelle um knapp 50%.
100 μm dünne Solarzellen, prozessiert aus unplanarisierten RGS Wafern, erreichten Effizienzen von bis zu 10.6% für einen noch nicht auf die stark reduzierte Waferdicke optimierten Zellprozess. Der Siliziumverbrauch pro WattPeak im Vergleich zu multikristallinen Industriesolarzellen (10.5 g/WattPeak) reduziert sich mit diesem Ergebnis weiter auf über 70% (2.9 g/WattPeak).
Zur Reflexionsminderung und damit zur Steigerung der Kurzschlussstromdichte wurden verschiedene Texturen untersucht. Saure Textur-Lösungen zeigten jedoch aufgrund nicht ausreichend isotroper Ätzcharakteristik ein zu schnelles Ätzen von Korngrenzen, was erhöhte Diodensättigungsströme I02 zur Folge hatte und die durch die Textur erhöhte Kurzschlussstromdichte im Hinblick auf den Zellwirkungsgrad kompensierte.
Die Skalierbarkeit sowohl der Materialqualität als auch des Zellprozesses wurde durch große 10x10 cm2 RGS Solarzellen demonstriert, die annähernd gleiche Wirkungsgrade erreichten wie kleinere 5x5 cm2 Zellen, die aus Material mit vergleichbarer Sauerstoffkonzentration prozessiert wurden.
Mit dem Ziel, die Kurzschlussstromdichte von Solarzellen durch ein Drift-Feld als Folge von Segregation und der speziellen Art des Kristallisationsprozesses zu erhöhen und um weiterhin degradierende Bor-Sauerstoff-Komplexe zu vermeiden,
Mit dem Ziel, die Kurzschlussstromdichte von Solarzellen durch ein Drift-Feld als Folge von Segregation und der speziellen Art des Kristallisationsprozesses zu erhöhen und um weiterhin degradierende Bor-Sauerstoff-Komplexe zu vermeiden,