for industrial application
Dissertation
zur Erlangung des akademischen Grades des Doktors der Naturwissenschaften (Dr. rer. nat.)
an der Universität Konstanz
Mathematisch-Naturwissenschaftliche Sektion Fachbereich Physik
vorgelegt von Alexander Edler
Tag der mündlichen Prüfung: 12.03.2014 1. Referent: Prof. Dr. E. Bucher
2. Referent: Prof. Dr. M. Fonin
Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-275017
“There are no passengers on spaceship earth. We are all crew.”
Richard Buckminster Fuller (1895-1983), American inventor and architect, one of the first strong supporters of renewable energy
“We’re killing people in foreign lands in order to extract 200-million-year-old sunlight.
Then we burn it in order to boil water to create steam to drive a turbine to generate electricity. We frack our own backyards and pollute our rivers, or we blow up our mountaintops just miles from our nations capital for an hour of electricity, when we could just take what is falling free from the sky.”
Danny Kennedy (1971-), clean-technology entrepreneur and environmental activist
1 Introduction 1
1.1 Background and Motivation . . . 1
1.2 The Zoo of n-type cell concepts . . . 3
1.2.1 Aluminium rear emitter cells . . . 4
1.2.2 PERC, PERL and PERT cell architectures . . . 4
1.2.3 Interdigitated back-contact cells . . . 5
1.2.4 Heterojunction device concepts . . . 6
1.3 Outline of the thesis . . . 6
2 Manufacturing technology and relevant concepts 9 2.1 Cell processes and Cell Manufacturing . . . 9
2.1.1 Material . . . 9
2.1.2 Wet Chemical Processing . . . 10
2.1.3 High Temperature Processes . . . 10
2.1.4 Passivation . . . 11
2.1.5 Metallization . . . 12
2.2 Solar Cell Characterization and Definitions . . . 13
2.2.1 Cell parameters and IV characterization . . . 13
2.2.2 Optical losses . . . 16
2.2.3 Recombination losses and lifetime measurements . . . 17
2.2.4 Resistive losses and resistance measurements . . . 22
2.2.5 Wafer characterization . . . 23
3 Boron emitter diffusion and passivation 25 3.1 Diffusion of boron in silicon . . . 25
3.2 Bulk lifetimes of silicon wafers after high temperature processing . . . 27
3.2.1 Experiment . . . 28
3.2.2 Results and Discussion . . . 29
3.2.3 Conclusion . . . 32
3.3 Homogeneity of emitter diffusion and BSG layer . . . 33
3.3.1 Conclusion . . . 36
3.4 Passivation of highly boron doped surfaces . . . 36
3.4.1 Introduction . . . 36
3.4.2 Overview of passivation methods . . . 37
3.4.3 Experimental results . . . 40 iii
3.4.4 Conclusion . . . 46
4 Cell processes 47 4.1 Cell processes with homogeneous diffusions . . . 47
4.1.1 Potential of cell precursors in PC1D device simulations . . . 48
4.1.2 Cell processing of full ingot range . . . 52
4.1.3 Cell process with lowly doped emitters . . . 53
4.1.4 Cell process with optimized BSF diffusions . . . 54
4.2 Concepts for selective cell doping . . . 58
4.2.1 Motivation . . . 58
4.2.2 Laser doping for selective emitter and BSF formation . . . 58
4.2.3 Screen-printable doping pastes . . . 69
4.2.4 Chemical etch-back for selectively doped structures . . . 70
4.2.5 Conclusion . . . 74
4.3 Rear emitter cell concept . . . 75
4.3.1 Motivation . . . 75
4.3.2 Experiment . . . 76
4.3.3 Conclusion . . . 78
5 Metallization of bifacial solar cells 81 5.1 Introduction . . . 81
5.2 Experimental Details . . . 82
5.3 Results and Discussion . . . 83
5.3.1 Identification of main losses . . . 83
5.3.2 Losses in open-circuit voltage . . . 85
5.3.3 Quantifying the metallization losses . . . 87
5.3.4 Modelling the metallization losses . . . 88
5.4 Conclusion . . . 92
6 Measurement of high efficiency solar cells 93 6.1 Motivation . . . 93
6.2 Measurement uncertainties of bifacial solar cells . . . 94
6.3 Flasher measurements of high efficiency n-type cells . . . 96
6.3.1 Introduction . . . 96
6.3.2 Observation . . . 96
6.3.3 Explanation . . . 97
6.3.4 Maximum power point scan . . . 99
6.3.5 Conclusion . . . 100
6.4 Bifacial measurements of solar cells . . . 102
6.4.1 Motivation . . . 102
6.4.2 Experimental setup . . . 102
6.4.3 Experimental evaluation of bifacial properties . . . 104
6.4.4 Conclusion . . . 106
7 Summary and Outlook 107
Zusammenfassung 111
List of Acronyms, Symbols and Constants 115
References 130
List of Publications 131
Acknowledgement 133
1
Introduction
1.1 Background and Motivation
The first practical solar cell was invented in 1954 at the Bell Laboratories and was a rear contact technology and based on arsenic doped silicon [1]. With an efficiency of 6% it presented an efficiency leap from former device architectures. Their invention was praised by the New York Times to mark “the beginning of a new era, leading even- tually to the realization of one of mankind‘s most cherished dreams-the harnessing of the almost limitless energy of the sun for the uses of civilization”. For the following two decades energy generation from solar cells was extremely expensive. Consequen- tially solar power was applied mainly in space applications, where the higher power-to- weight ratio over depleting energy sources mattered. These early applications drove the inventions in the solar field. It was then demonstrated that p-type based cells showed higher radiation hardness than n-type based solar cells in the space radiation environ- ment. Accordingly the research focus shifted towards cells based on boron or gallium doped p-type silicon. It is this early focus on manufacturing techniques for p-type cells that is at least partly responsible for the dominating position of p-type solar cells in the industry today. At the current stage of development it seems that p-type efficiencies have saturated. Over the years many researchers have acknowledged the higher suitability of n-type Cz-material for solar cell processing [2–4]. A shift from p-type to n-type based cell concepts could consequentially be the next evolutionary step. However, this requires the adoption of new fabrication methods and sequences. Although many of those have been successfully demonstrated in lab scale, manufacturers are only reluctantly moving in that direction. It can be assumed that the booming expansion in the first decade of the century and the preoccupation with p-type manufacturing, has lead to an inherent inertia of the industry. Self-explaining, any new emerging technology has to prove its competitiveness compared to current technology. If a technology has evolved over many decades, like the p-type cell technology, it is possible that adapting a new technology at one point becomes prohibitively costly. This can be the case, even if the inherent properties argue for that new technology in the long run. In this case economists speak about “path-dependency” or “lock-in” phenomena to describe this barrier. Several hints indicate that such a scenario exists in the photovoltaic industry today. Examples can be found all along the standard silicon photovoltaic production sequence. State-of-the-art phosphorous diffusion in tube furnaces is far more advanced than boron tube diffu- sions, n+ surface passivation using PECVD SiNx is well established, whereas suitable boron emitter passivation techniques have accumulated only little production experi- ence. The metallization of n-type cells, and boron emitters in particular, requires new paste formulations and state-of-the-art products are not nearly as advanced as their p-
1
type counterparts. Nevertheless, even with current production methods, the attainable efficiencies rival the best p-type production results, indicating that n-type cell technology is ready for the transition to the industry. Due to the novelties and innovations involved, the easiest way to introduce new n-type based cells will be to maintain as far as possible production methods well known from p-type manufacturing. This approach comprises using tube diffusions for the formation of emitter and/or back surface field, PECVD deposition of dielectrics for passivation and screen-printing and firing for the formation of the contacts. The reluctance of the industry, due to decades of p-type manufacturing experience, must be overcome today to enable the continuation of the development of PV production efficiencies. It is the scope of this work to understand the limitations and improve the efficiency of one of the closest neighbours of the p-type PERC cell, the n-PERT concept.
Figure 1.1: Learning curve for the price of c-Si module as a funtion of cumu- lative installed capacity, from Ref. [5]
Solar cell market situation
Although the solar cell market has been growing rapidly over the last 10 years, the mar- ket is in turmoil. In a market that was largely driven by government incentives and is therefore highly fluctuating it is very difficult to anticipate market development in the long term. For several years in a row the market growth exceeded all expectations. The massive ramp up of production capacity, especially in China, was the result. As a conse- quence the industry has built up a massive oversupply of production capacity. In 2012 solar installations with a rated power output of 31.1 GW have been installed, this was met by global PV production capacity of around 57 GW [6]. As a result the price for solar modules has plummeted below the manufacturing costs for many suppliers. The price decline can be represented by the famous learning curve for PV (Fig. 1.1), which is the result of thorough research and development work that transitioned from laboratories to production lines while perfecting the manufacturing techniques and enabling laws of scale. The oversupply has lead to an accelerated price reduction, which can be seen from the kinks in the learning curve. While this makes solar modules currently as cheap
as 0.69USD/Wpeak for the end user, it is at the same time a dangerous situation for an emerging industry. The low prices impede the ability of manufacturers to invest in R&D or new production equipment. Many large manufacturers have already been driven into insolvency. In order to reduce manufacturing costs to progress along the learning curve it is therefore most likely to rely on industry proven processing that enables new cell concepts.
1.2 The Zoo of n-type cell concepts
It has been demonstrated that current p-type solar cell architectures are increasingly lim- ited by inherent bulk recombination from the metastable boron oxygen defect [7]. For advanced cell concepts the bulk limitation has to be overcome. The substitution of boron by gallium is discussed, but leads to a very large resistivity spread over the ingot and interstitial iron is still an issue causing degradation. The magnetic Cz (MCz) approach is another option to reduce the influence of this defect by reducing the oxygen concen- tration in the crystal. It is however very expensive and therefore not industrially viable today. Even if the boron oxygen defect could be suppressed, the sensitivity of p-type for common metal impurities is higher than for n-type material due to asymmetric cap- ture cross sections of their defects [2, 4, 8]. This could still be the limiting factor for the bulk lifetime. However this highly quoted assumption should be taken with care. On n- type cells with boron emitter even higher cleanliness requirements might apply since the boron diffusion is extremely sensitive towards emitter bulk contamination. Even though the bulk might enable high efficiency limits, the recombination within a contaminated p+ emitter can easily destroy the cell performance. In case of high impurity concentra- tions in the bulk also the limited gettering efficiency of boron diffusions [9] can become a drawback for example for cell processes on multi-crystalline n-type material [10]. Ev- ery p-type module installed today suffers from light induced degradation (LID), which reduces power output substantially over the working life of a module. This effect is fully negligible in cells made from non-compensated n-type silicon. Today it is taken as nor- mal by an installer, that the energy output of a Si-module diminishes over its working life, simply because p-type modules have been the only type available. The stable per- formance will certainly be a major advantage for all kinds of n-type devices, but it will require marketing efforts. Many n-type cell concepts moreover are inherently bifacial.
The module assembly can then be adjusted to make use of the bifacial properties, en- abling new ways of installation in the field but also for building integrated photovoltaic.
Last but not least there is evidence that n-type devices are more sensitive to low light intensity as well as having a lower temperature coefficient mostly due to the higherVOC. All these effects can lead to a remarkably higher kWh energy harvesting and reduction of BOS (balance of system) costs due to lower area needs for the same installation capac- ity.
The following list gives a non-exhaustive overview of existing n-type cell concepts.
1.2.1 Aluminium rear emitter cells
The closest relative of the aluminium BSF p-type cell employing n-type material, is the aluminium rear emitter cell. Basically it can be fabricated using the process sequence of standard p-type Al-BSF cells simply by starting with n-type Cz material. This makes the process appealing at first, since existing production lines can be re-fitted to produce this cell. The potential of the cell concept has been evaluated by Rüdiger et al. [11] and effi- ciencies of 19.4% on 6 inch wafers have been demonstrated by Booket al. [12]. Ultimately, the benefits over its p-type equivalent are limited. The poor electrical properties of a full area aluminium emitter limit the cellVOC and the internal optical properties put an up- per bound on the achievable cell current density JSC. Also the module interconnection imposes challenges, due to the aluminium alloyed rear emitter.
1.2.2 PERC, PERL and PERT cell architectures
An advanced line of cell concepts deals with improving the rear surface compared to alu- minium BSF cells. As a hint about the time of their conception their acronyms indicate what these concepts have in common, namely a passivated emitter. Today, a modern cell cannot be imagined without emitter passivation, the rear surface, however, much looks like it did since the aluminium BSF was invented. The PERC (passivated emitter and rear cell) concept, which is gradually being adopted by the industry, adds rear side passiva- tion to the structure. As rear side dielectric commonly Al2O3is employed. The rear side is opened locally, usually by laser ablation, to allow for local alloying of aluminium. This forms the BSF and contacts, thus improving the recombination and optical properties.
Accordingly the PERC concept has been promoted for many years as the next candi- date for widespread industrial implementation. By now initial efficiencies exceeding 20% have been demonstrated by many manufacturers. This approach can not easily be transferred to n-type cells. In case of a PERL (passivated emitter, rear locally diffused) cell this is refined even further by diffusing a local BSF, which is contacted by evapo- rated and plated metal contacts. The long-standing efficiency world record for silicon devices was established using such a structure. Zhaoet al.[13] reported 24.7% on a des- ignated area of 4cm2 on FZ p-type wafers. This is achieved by combining all cell design ingredients necessary for achieving highest efficiencies. Those include amongst others selective front doping, ideal inverted pyramid texture with double layer anti-reflection coating, an evaporation coated rear side for improved light trapping and front contacts structured by photolithography. Using a process of similar complexity 23.4% efficiency has been shown for 4cm2 n-type PERL devices by Benicket al.[14, 15]. Needless to say that these processes are far too complicated and costly for a production environment.
The direct contender of the p-type PERC cell in the race for industrial implementation can be found in the n-type PERT (passivated emitter, rear totally diffused) cell discussed in this work. These can be fabricated with front emitter or as back junction cells with an emitter on the rear side. The best lab results for front junction n-PERT devices have been reported by Zhaoet al.[16] with 21.1% on small area Cz and 21.9% on FZ material.
Newer experimental results using Al2O3-based passivation were reported by Richter et al.[17], who could demonstrate 20.5% on 4cm2 FZ cells with a p+nn+ structure using advanced metallization techniques. On 5 inch Cz-material still 19.6% has been shown by
the same group using industrial diffusions but still evaporated contacts. Developments on back junction devices with rear boron emitter have been pioneered byQ-Cells/Hanhwa as reported from Bordihnet al.[18]. In a recent update by Mertenset al.[19] efficiencies up to 21.3% on large area Cz based solar cells have been shown, using PVD aluminium metallization on the rear side. Several research groups with a focus on industrial imple- mentation are currently working on the front emitter version of this cell concept. ECN (Energieonderzoek Centrum Nederland) is promoting this concept under the name n- Pasha(towards 20% [20]) and attempts to merge the concepts with metal wrap through (MWT) technology have been successfully demonstrated [21]. Also in France, at CEA- INES (Institut national de l’énergy solaire) these cell types are being developed, recently the efficiency of 20.1% [22] was reported. Both share the formation of doped layers with conventional tube diffusions, passivation by aluminium oxide or nitric acid oxide based passivation stacks and industrial screen-printing. The advent of ion implantation for solar cell manufacturing also has implications for the process sequences of n-PERT cells.
At CEA-INES phosphorous implanted or fully implanted n-PERT cells achieving 19.9%
and 19.5% maximum efficiency respectively have been fabricated. Recently alsoBOSCH Solar Energy AG reported n-PERT results from pilot line production of up to 20.6% us- ing implanted phosphorous BSF structures [23]. These results have been achieved in a collaborative project betweenISC-Konstanzand BOSCH Solar Energy AG also including work presented in this thesis. P-type PERT cells with full area boron BSF have also been demonstrated [24].
1.2.3 Interdigitated back-contact cells
The interdigitated back contact (IBC) cell concept is characterized in that all metal con- tacts can be found on the rear side of the cell. This enables a front side optimized entirely for higher light absorption without the requirements of contact formation. Since the con- tacts are all on the rear side, the contact layout can be optimized for transport properties irrespective of shading considerations. These advantages stand in opposition to higher process complexity due the requirement of patterned diffusions and the risk of fatal cell shunting due to the proximity of n+ and p+ doped regions. At ISC-Konstanz a pro- cess for IBC cells based on 6 inch Cz n-type wafers has been developed. By using only industrial processing techniques an efficiency of 21% could already be demonstrated.
For back-contact back junction designs high bulk lifetime is further required to achieve highest efficiencies as analysed by Granek [25]. This structure can also be fabricated using the aforementioned aluminium emitter, but will then suffer from the same limita- tions. A report on the status of development for this cell is given in Ref. [26]. The most prominent example for IBC cells comes from the companySunpower Corp. With a rated maximum efficiency of 24.2% and a reported average production efficiency of 23.6% their implementation of the IBC concept stands out from the range of commercially available silicon solar cells [27]. These results are achieved on 5 inch wafers and employing a very complex production sequence.
1.2.4 Heterojunction device concepts
A new category of devices that combines the advantages of n-type bulk material and semiconductor grade passivation are heterojunction structures. Here a high quality n- type substrate is coated with stacks of intrinsic and doped amorphous silicon to give a p+i n i n+ structure. The wafer is coated on either side with indium tin oxide (ITO) to enhance current collection from the contacts. The a-Si enables superb passivation of the bulk silicon and enables a tunneling junction to the doped emitter layer. This both results in extremely high voltage potential and low temperature coefficients (≤0.23%/◦C). The cost benefit of low temperature deposition for the amorphous layers leads to the require- ment of exceptionally high bulk lifetime. Additionally the low temperature processing forbids the use of firing through metallization. The commonly employed low tempera- ture contact pastes show significantly larger resistances compared to their sintered coun- terparts. Despite these drawbacks very high efficiencies have been demonstrated by sev- eral companies. The most famous example certainly is the concept of heterojunction cells with intrinsic thin layer (HIT) fromPanasonic, earlierSanyo. The certified peak efficiency was just raised by one percent absolute to 24.7% for a 100µmthick cell on 101.8cm2. One competitor,Kaneka, reported a certified efficiency of 24.2% on large area cells, using cop- per plated contacts at the same time. These results demonstrate the very high efficiency potential of this concept. It is expected that average production efficiencies range 1-2%
lower than the best cell efficiencies in the research environment. Other companies, like Silevo, a California based start-up, and Tetrasun, a subsidiary ofFirst Solar, are commer- cializing slightly different proprietary cell technologies. Their cell concepts combine an n-type silicon bulk, with an amorphous silicon emitter. This heterojunction architecture enables over 730 mV in VOC. A semiconductor grade tunneling oxide is deposited on top, forming a tunneling contact to the copper plated metallization. Peak efficiencies of 22.1% have been demonstrated on 239cm2 wafer size. An average production efficiency of 21.3% was also reported recently [28].
1.3 Outline of the thesis
This introductoryChapter 1gives a summary of the general motivation for this research.
In the previous section many cell concepts have been introduced. Some of them are being applied commercially, but generally they are adopted only by few and very specialized companies (e.g. Panasonic, Sunpower, Silevo). The largest group of cell or module manu- facturers is still producing mainly p-type aluminium BSF cells. It is assumed that several technological challenges have so far prevented a widespread adoption of next generation n-type cells and it is the scope of this work to understand and improve these processing and measurement steps.
InChapter 2we introduce the employed manufacturing tools and concepts of solar cell characterization. The facilities at ISC Konstanzembrace a pilot scale solar cell man- ufacturing line which is shown here. It can be employed to process various different cell types and wafer sizes. The extensive characterization equipment is needed to allow comprehensive loss analysis of solar cells and wafers.
The analyses inChapter 3are intended to tune the properties of a boron emitter and its interface. Tube diffusion of boron is commonly perceived to be more complicated than phosphorous diffusion. We have reviewed the properties of boron diffusions and focused on the formation of homogeneous and stable boron emitters of different sheet resistance. The concerns about associated lifetime degradation could be invalidated. A different important aspect about n-type cell technology is the passivation forp+surfaces.
Over a long period only thermal oxide was suited to reliably passivate boron emitters.
In the course of growing research interest in n-type technologyAl2O3-based passivation has received much attention. Here we have investigated the properties of an alternative passivation stack based on boron silicate glass (BSG) and compared it to state-of-the-art methods.
Chapter 4deals with the cell processes for n-PERT cells. The tolerance of the cell pro- cess towards utilization of base material with different resistivity is an important aspect in view of the large resistivity spread found for n-type ingot. The impact of doping pro- files of emitter and BSF has been evaluated in experiment and simulation. Especially the BSF doping can be adjusted to benefit from new developments of Ag contacting pastes.
Also the implications of two consecutive high temperature steps for the cell potential have been tested. The high doping concentration and especially metal contact recombi- nation has been demonstrated to constitute main loss mechanisms. Therefore processes for the selective doping of emitter or BSF have been evaluated.
In Chapter 5 we analyse the metal related recombination losses in-depth. A new technique using printing layouts with different metallization fractions is employed to distinguish between the contributions from front and rear side contacts. The losses are quantified in terms of J0e(met), to demonstrate that metal induced recombination is in- deed the primary loss mechanism for this concept. It is found that the damage at p+ contacts is far more detrimental for minority carriers than then+BSF contact. Moreover the effect is also stronger than forn+emitters of conventional p-type cells. A simulation model was built to explain the effects underneath the contacts in detail.
Chapter 6deals with the adequate IV characterization of bifacial high-efficiency cells.
The high VOC of advanced cell concepts brings up the topic of hysteresis errors in fast IV measurements. A strategy to cope with this problem with limited flash durations is suggested. The open rear architecture, found in many n-type cells, leads to uncertainties in theJSC generation depending on the measurement background. The standard testing conditions (STC), so successful in allowing comparability for all kinds of devices, do not encompass bifaciality. We have constructed a sample holder to measure the cell perfor- mance under varying bifacial illumination conditions. It is suggested that STC should be reviewed to enable a fair assessment of this property, in order to allow marketing of the higher energy yield of such modules.
2
Manufacturing technology and relevant concepts
In this chapter we will introduce the manufacturing steps that are involved in the ad- vanced solar cell process, basic solar cell parameters, as well as the characterization methods employed throughout this work.
2.1 Cell processes and Cell Manufacturing
2.1.1 Material
The starting material are phosphorous doped wafers from Czochralski grown ingots (Cz), that were slurry sawn to a thickness of 180µm. In an n-type ingot of typical length the resistivity range is larger compared to a p-type crystal doped with boron. This is due to the lower segregation coefficient of phosphorous (0.35) compared to boron (0.8).
The doping concentration along the crystal length for perfectly mixed melts is described by the Scheil equation 2.1, wherein k is the segregation coefficient, C0 the total doping concentration in the melt and fsthe fraction solidified.
c(fs) =k C0(1− fs)k−1 (2.1) Fig. 2.1 shows a typical resistivity distribution, which was measured in an n-type production ingot as an example. We have mainly employed wafers from the middle regions of the ingots, showing a resistivity from 3−5Ωcm. The doping concentration in
Figure 2.1:Exemplary resistivity distribution along a Cz n-type crystal.
9
the wafer is an important quantity in solar cell design and the tolerance of a cell process towards a variation in resistivity needs to be tested. For Cz ingots the usable fraction solidified for a given cell process is a key parameter which determines the wafer cost. An interesting approach to get around this limitation is the “continuous feeding” method.
Here the dopant concentration in the melt is held at a constant level by continuously adding doped material during crystal growth. Ingots with very uniform resistivity and high bulk lifetimes over the length of the whole crystal have already been demonstrated.
2.1.2 Wet Chemical Processing
At the beginning of the cell process the saw damage is removed with alkaline (NaOH) etching of 5−7µmof silicon on each side. After saw damage removal and before every high temperature or passivation step either one of two cleaning sequences is required.
The cleaning procedures employed at ISC-Konstanz are listed below, whereas the first one being called standard cleaning and the second one IMEC cleaning hereafter.
• Standard cleaningBatch process, max. 50 Wafers/carrier, DI H2O rinse, 3% HCl (5min) bath, DIH2Orinse, 2% HF (2min) bath, DI H2Orinse, drying at 110◦C
• IMEC or Piranha cleaning Batch process, max. 50 Wafers/carrier, comprises the standard cleaning followed byH2SO4(conc)/H2O2(80◦C, 10min) bath, DIH2Orinse, 2% HF (2min) bath, DI H2Orinse
For the laboratory process we perform a full IMEC cleaning sequence before each high temperature or passivation step. This sequence is too complicated to be imple- mented in a production environment and the cleaning efficiency of different cleaning steps has also been tested and is presented by Buchholz et al.[29]. It could be demon- strated that also less sophisticated cleaning sequences can reduce the surface contami- nation sufficiently to achieve high lifetimes in the solar cell process. In order to reduce the reflectivity surface texturing in a potassium hydroxide (KOH) and isopropyl alco- hol (IPA) bath is employed. For monocrystalline wafers with (100) surface orientation the anisotropic etching of (111) over (100) surfaces results in a random pyramid tex- ture. This reduces the reflection of the surface to about 11%. The reflection is further minimized by the deposition of an anti-reflection coating (ARC), such as silicon nitride (SiNx). In the standard solar cell process 70 nm of SiNx are deposited, for passivation stacks comprising different dielectrics the thickness of depositedSiNxhas to be adjusted.
2.1.3 High Temperature Processes
For the bifacial n-type solar cell two high-temperature steps are required to form the emitter and BSF regions by diffusion of dopant atoms. At ISC-Konstanzthe high tem- perature diffusion steps are carried out in an industrial quartz tube furnace from Cen- trotherm. The furnace features four separate tubes, whereas wet oxidation (O2−DCE), POCl3-based phosphorous diffusion andBBr3-based boron diffusion can be carried out.
The wafers are loaded into quartz holders which are driven into the tube. At 4 mm spac- ing up to 200 wafers can be diffused simultaneously. The gas inlet for the precursor gases is situated on the left side of the tube, while the exhaust outlet is close to the loading
door. The wafers are always positioned standing upright with the surface to be diffused facing the gas inlet. Since the boron silicate glass deposition is more homogeneous on the side facing the gas inlet, lower variation in sheet resistance is measured on that sur- face. Throughout this work diffusions are performed with a capacity of 50-150 wafers.
The homogeneity of the sheet resistance over the wafer and over the tube position may vary, depending on the amount of wafers. The second high temperature step also in- fluences the dopant distribution of the initial diffusion step. The doping distribution is hence defined by the combined thermal budget as well as the surface capping layer.
Throughout this work the formation of the boron emitter has been performed as the second high temperature step. There are several reasons for that. First, the risk of cross contamination of the boron emitter with phosphorous atoms during diffusion makes the sequence where boron is diffused second favourable. It has been found that minimal contamination of phosphorous, as might leak through pinholes or other imperfections in the protection layer, can degrade the cells through shunting effects. In any process sequence with an initial boron emitter diffusion a diffusion barrier would be required.
The protection against cross contamination with amorphous dielectric layers is difficult especially for textured surfaces. Such layers would have to be easy to remove even af- ter high temperature densification and at the same time dense enough to prevent any phosphorous penetration. PECVD deposition of SiO/SiNx stacks is currently the only viable method for this application. The second motivation to start with the phosphorous diffusion is the potential utilization of BSG as a passivation layer. Since it was shown that the passivation quality degrades during further high temperature diffusion steps, the only option is to employ the emitter forming step at the end of the process.
2.1.4 Passivation
At the surfaces of a semiconductor wafer the crystal structure is abruptly interrupted.
This discontinuity in the crystal order can be imagined as a big crystal defect. The forma- tion of discrete energy bands is hence disturbed and multiple defect states exist within the bandgap. These defect states allow for multistep recombination of minority carriers, which drastically limits the lifetime. Therefore all surfaces need to be passivated in order to reduce the recombination activity. This can be done in two ways. Chemical passivation reduces the density of defect states in the bandgap, i.e. by saturating dangling bonds at the surface with hydrogen. In the case of field effect passivation electric fields keep the region close to the surface depleted of one type of carriers to avoid their recombination.
Generally different dielectrics are required to passivate surfaces with different dopant type and surface concentration. Especially the industrial passivation of the p+ surfaces required extensive development as is reported in Section 3.3. Available methods atISC- Konstanzfor the passivation of boron emitters are thermal oxidation followed by PECVD deposition ofSiNxand a stack system ofBSG/SiNx. Phosphorous surfaces are best pas- sivated by thermally grownSiO2 andSiNx, but pureSiNx can also effectively passivate the surface. As for all components of solar cells it is required to demonstrate not only high initial passivation quality but also to confirm its stability in the interplay with other manufacturing steps and the durability in the long operating life of the cell. Special requirements in this respect would be firing stability during contact formation, the long
term stability, e.g. against UV radiation and compatibility with common anti-reflection layers.
2.1.5 Metallization
Metal contacts are required on surfaces of either polarity to extract the generated current from a solar cell. These contacts are widely formed using screen-printing of metal pastes.
This technique has a long history in solar cell manufacturing due to its favourable prop- erties, such as high throughput capability and reliability. In this step the grid design is defined by printing a mesh of thin metal lines (fingers) interconnected with broader conductive paths (busbars) on the light sensitive side. While the rear side of a p-type cell is fully covered with aluminium paste, the diffused and passivated rear surface of the n- type cell also allows for the application of an open grid on the rear side. The metal pastes required for solar cell contact formation differ widely depending on the type of cell and contacting purpose. For bulk silicon solar cells in general silver based pastes are used to form the front grid pattern due to the high conductivity of the sintered silver. The basic ingredients of contacting pastes, beside the metal powder, are glass frits, binders and an organic vehicle. The contact formation to p+doped layers further requires the addition of aluminium in the percentage range to the silver paste. All commercial products con- tain aluminium since it is currently the only option to achieve low contact resistivities.
At the same time, however, the aluminium reduces the line conductivity and leads to serious surface damage, representing a major drawback for n-type based cells. After the printing step the paste is dried in a drying furnace at 200◦C where the solvents evap- orate. The cells are then transferred to an IR furnace, where the contact to the silicon is actually formed. In the IR furnace the solvents and binders are further burned off at temperatures up to 550◦C. Above that temperature the glass frit liquefies and etches the passivation layer while the silver powder starts to dissolve. The glass frit oxidizes the silicon surface and the liquid glass lead phase is saturated by silver. Upon rapid cooling from the peak temperature around 800◦C, silver crystallites grow epitaxially into etch pits at the silicon surface. The sintered silver bulk is separated from the silicon by a thin SiO2 layer, with embedded silver precipitates. This glass layer plays a vital role in establishing mechanical adhesion of the contacts. It is assumed, that especially direct current paths via silver imprints connecting the silicon and silver bulk are responsible for a low contact resistivity. Growth of such silver crystallites is promoted by sharp sur- face features, as well as excess surface doping concentration. During this work different generations of commercially available contacting paste have been employed. Each was fired at the best known firing settings, which were individually determined during firing optimization test runs. The firing step moreover fulfils the function of enabling hydrogen passivation, which diffuses from theSiNx to the interfaces during the temperature step.
The metallization scheme has a great influence on the cell performance as will be shown in Chapter 5.
2.2 Solar Cell Characterization and Definitions
The following section explains influences that limit the conversion efficiency of solar cells and methods that have been employed to characterize wafers and solar cell performance during this work. Like any other solar cell, devices made from silicon in principal have to fulfil two very basic tasks. The first is light absorption in order to generate electron hole pairs and the second is the separation of these generated electron hole pairs. Silicon is an indirect semiconductor with a bandgap energy of 1.12 eV. In the presence of lattice phonons, photons of energy portions larger than the bandgap energy can be absorbed and thus contribute to the photo-generated current. Excess energy of photons is lost due to rapid thermalization of charge carriers to their respective band edges. Photons of lower energy than the bandgap are not absorbed. These two effects fundamentally limit the conversion efficiency of single-junction devices made from silicon to around 44% [30]. Further influences that limit the efficiency of solar cells will be discussed in this chapter and can be sorted in either one of the following categories:
• Optical losses
• Recombinative losses
• Resistive losses
2.2.1 Cell parameters and IV characterization
For the characterization of the finished device the cell parameters are extracted from current-voltage (IV) curves, measured under dark or illuminated conditions. During an illuminated IV measurement a voltage ramp is applied in forward bias direction to change the voltage at the cell terminals stepwise, while the generated current is measured for each step. More details on IV measurements are reported in Chapter 6. From this IV curve the cell parameters that constitute the power conversion efficiency (η) are extracted.
The most important parameters are the fill factor (FF), the open circuit voltage (VOC) and the short circuit current density (JSC). The power conversion efficiencyηis defined as the ratio between the maximum electrical power densitypel and the intensity of the incident light IL. It is measured according to Standard Testing Conditions which are described in DIN EN 60981. Herein the spectrum of incident light (AM 1.5G) as well as an incident intensity of 1000W/m2 at 25◦Care specified.
η= pel,max
IL = VOC·jsc·FF
IL (2.2)
Fig. 2.2 gives a graphical representation of the main cell parameters. Instead of abso- lute current and incident power usually the current density and illumination intensity is stated to yield comparable numbers independent of the cell geometry. The short circuit current density is the maximum current density that can be extracted from the cell under 1 sun illumination. The shape of that curve is established by Shockley’s ideal diode law, wherein Jph is the photo-generated current density, J01 is the saturation current density, kB is the Boltzmann constant and T the temperature in Kelvin. By convention the signs are chosen so that the power generating points of the curve lie in the first quadrant.
0 . 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 . 6 0 . 7
02468
1 0
P M P P
F F =
V o l t a g e [ V ] V O C
I l l u m i n a t e d I - V c u r v e P o w e r c u r v e
Current [A] and Power [W]IS C
M P P
IS C V O C
Figure 2.2:Typical IV characteristic and power curve with indication of main cell parameters
J(V) = Jph−J01h
eqV/kBT−1i
(2.3) The efficiency is determined at the point of maximum power generation, indicated by the maximum of the product of current and voltage along this curve. The ratio of this maximum power point (MPP) and the product of JSC andVOCis called the fill factor:
FF= JMPPVMPP
JSCVOC (2.4)
For an ideal diode the maximum FF can be approximated according to Green [31]
from the normalized voltage voc=VOC/(kbT/q)by:
FF0= voc−ln(voc+0.72)
voc+1 (2.5)
The light sensitive diode is in reality afflicted with leakage currents through the junc- tion, represented in the equivalent circuit by a parallel or shunt resistance Rp. This is accounted for by the third term in Eq. 2.6 and thus reduces the externally measured cur- rent. The voltage is diminished by the influence of series resistances Rs within the cell, which distort the shape of the curve. The series resistance, encountered by the current along its path, is constituted of the bulk resistanceRbulk, the lateral resistance in the emit- ter or BSF layer Rsheet, the contact resistance Rcontact and the resistance of the metal grid Rline. The influence of typical series resistances (fewmΩ) and shunt resistances (several kΩ) distorts the shape even more and reduces the FF to around 79% down from its ideal value of around FF0of 84%:
J(V) = JL−J01
h
eq(V+JRs)/kT−1i
− V+JRs
Rp (2.6)
The dark saturation current density J01 is generally the sum of the recombination losses in the emitter and base regions according to:
J01= J0e+J0b (2.7) For the metallized cellJ0eandJ0bcan be subdivided into contributions from the met- allized (J0e(met)) and passivated (J0e(pass)) areas, weighted according to their area fractions Fmetas:
J0e = (1−Fmet)J0e(pass)+FmetJ0e(met) (2.8) The contribution of an n-type baseJ0bcan be expressed after [31] by the hole diffusion coefficientDp, the intrinsic doping concentrationn2i and the diffusion length Lpas:
J0b= qDpn
2i
LpND (2.9)
The influence of finite cell dimensions modifies this term by a geometry factor FN which relates the bulk diffusion length Lp to the dimensions of the cell, represented by the base thickness W and depends on the rear surface recombination velocitySrear. It is given by:
FN = Srearcosh (WL
p) + DLp
psinh(WL
p)
D
Lpcosh(LW
p) +Srearsinh(WL
p) (2.10)
When no current is extracted from the illuminated cell, the open circuit voltage is measured between its terminals. In this case the external current is zero and the entire light generated current is consumed by recombination within the cell. For ideal diodes theVOCis found to be:
VOC = kBT q ln
Jph J01 +1
(2.11) This J01 takes into account the radiative, Auger and SRH recombination. It was found, however, that deviations from this shape can apply whenever recombination in the space charge region (SCR) is present. Also a large variation in series resistance over the cell area can distort the IV curve. These influences are best described by an extended double exponential model analogue to a second diode with ideality factor 2 connected in parallel. According to Mcintosh [32], a dark saturation current density J02 in the range of nA/cm2 yields the best description of the IV characteristic in this case. This J02 determines the shape of the IV curve at low voltages and up to the maximum power point, while at open circuit voltage the influence of J01 still dominates. Alternatively a parametrization with a single exponential function with variable local ideality factor can be used. In the thesis of McIntosh it is shown how the value of the local ideality in the low or high voltage regions of the IV curve can be used to distinguish between dominant non-SRH recombination mechanisms.
J(V) = JL−J01h
eq(V+JRs)/m1kT−1i
−J02h
eq(V+JRs)/m2kT−1i
− V+JRs
Rp (2.12)
Aside from current-carrying IV measurements the SunsVoc technique can be used to measure a VOC over intensity curve of the finished device [33]. The cell voltage is
measured between a conductive chuck and contact pins on the front side, while the cell is kept at open circuit condition at all times. A slowly decaying light pulse from a photo flash is monitored by a reference cell close-by. When the actual JSC under 1 sun illumination is known, the ISC−VOC curve can be constructed from theSunsVoc curve.
Since no current is extracted from the cell during the measurement, the resulting I-V diagram is free of series resistance contributions and therefore easy to fit. The evaluation of the FF of this curve shows the influence of the shunt resistance RP and the junction quality in terms of J02contribution. It is hence called the pseudo fill factor (pFF).
Spectral response and quantum efficiency
The spectral response of a solar cell can be defined as the ratio between short circuit current density JSC(λ)and the monochromatic light intensityI(λ)of the illumination.
SR(λ) = JSC(λ)
I(λ) (2.13)
It is a wavelength-dependant quantity in units of A/W. A more comprehensive figure can be calculated when the current is expressed in units of elementary charges and the light intensity in number of incident photons. This quantity is called the external quantum efficiency (EQE) and represents the conversion efficiency of incident photons of a specific wavelength into externally detected electrons.
EQE(λ) = JSC q
hc
λI(λ) = hc
qλSR(λ) (2.14)
From this expression the internal quantum efficiency (IQE) can be calculated by tak- ing into account the reflectivity of the front surfaceR(λ)and the metal covered areaFmet as:
IQE(λ) = EQE(λ)
(1−R(λ))(1−Fmet) (2.15) This term therefore represents the conversion efficiency of photons inside the device.
Measurement of the cell IQE is a vastly useful, non-destructive analysis tool since the dif- ferent absorption lengths of light of different wavelengths allow to discriminate between various recombination effects. The cell IQE in the blue and green wavelength range car- ries information about emitter recombination and surface passivation. The IQE towards near infrared radiation instead carries information about bulk lifetime and rear surface recombination, as well as free-carrier absorption. From the IQE the attainable JSC of the cell can be calculated by integration over the spectral intensity of the AM1.5G spectrum up to the cut-off wavelength, which is around 1107 nm for silicon:
Jsc= q hc
λ2
Z
λ1
EQE(λ)I(λ)λdλ (2.16)
2.2.2 Optical losses
The maximum current generated in a silicon wafer of 160µmthickness illuminated under the AM1.5G spectrum at1 sunintensity is around 43.6mA/cm2. In reality this theoretical
maximum value is reduced by optical losses which can have several origins. The biggest factor for double side contacted cells is the shading due to the front metal contacts.
In the case of 6.5% metal coverage, this reduces the maximum attainable cell current to 40.8mA/cm2. Another influence is the non-ideal reflectivity of the surface. Bare silicon reflects more than 30% of the light, and therefore surface texturing is required to reduce this figure. In case of monocrystalline silicon a random pyramid texture is created in a KOH/IPA bath, which reduces the reflectivity to around 11%. On top of that an antireflection coating (ARC) is deposited to enable even lower reflectivity due to destructive interference of waves, which are either reflected at the ARC surface or the wafer surface, respectively. By tuning the refractive index and thickness of the layer, the reflectivity is minimized for a specific wavelength. Typically it is adjusted for minimal reflection around 600 nm, where the solar photon flux peaks. Further optical losses occur due to parasitic absorption in the dielectric layers, free-carrier absorption in highly doped silicon and escape light due to insufficient light trapping. All of these effects limit the current generation currently to around 39.0mA/cm2 for a bifacial n-PERT device.
2.2.3 Recombination losses and lifetime measurements
Recombination refers to the annihilation of electron hole pairs, the reverse process of carrier generation. The average time for this reverse process in a population of excess carriers is called carrier lifetime. In solar cells high lifetime is required to fulfil the second task of each cell, which is separation of charges. In order to contribute to an external current, electron hole pairs need to be separated at the p-n junction. The minority charge carrier needs to reach the junction by diffusion. The mean distance that is covered in the silicon bulk is called the diffusion length, which is related to the lifetime viaLp =pDpτ (for holes). As a rule of thumb, the diffusion length needs to be at least 3 times the wafer thickness to allow for high collection probability. In the following we present several processes that limit carrier lifetime. A general relation between the carrier lifetimeτand the recombination rate U for a specific recombination process is:
τ= ∆n
U (2.17)
Two mechanisms are inherent to semiconductors and can not be suppressed tech- nically. The first is radiative recombination, the directly opposite process of carrier generation which is therefore proportional to the carrier concentration product. The total recombination rate can be written with the coefficient for radiative recombination B(300K) =4.7·10−15cm3/s[34] as:
Urad= Bnp (2.18)
Since silicon is an indirect bandgap semiconductor, radiative recombination plays a minor role in the balance of charge carriers. The second mechanism is the Auger recombination which is the reversed process of impact ionization. The recombination rate of this three particle process can be expressed by:
Uaug =Cnn2p+Cpnp2 (2.19)
The associated Auger lifetime (for n-type silicon) in low injection (∆n≈ ND) or high injection conditions (∆nND) is given by:
τaug,low = 1
CnN2D and τaug,high = 1
(Cn+Cp)∆p2 (2.20) The auger coefficients take values ofCn =2.8·10−31cm6/sandCp =0.99·10−31cm6/s as determined by Dziewioret al. [35]. A revised model has been suggested by Kerret al.
[36]
The presence of imperfections in the crystal matrix gives rise to a different category of recombination. Impurities or dislocations result in discrete energy levels within the en- ergy bandgap. The recombination of carriers due to these trap levels is greatly enhanced because they allow for recombination via a two-step process. The following description of defect related recombination is based on the description in the thesis of Kerr [36].
The first formal description was given by Shockley, Read and Hall [37, 38] after whom this mechanism is hence called (SRH). They described the recombination rateUSHRfor a single defect level by:
USRH = np−n2i
τp0(n+n1) +τn0(p+p1) (2.21) Herein τp0 and τn0 are the fundamental lifetimes related to the probability to en- counter a defect of the density Nt and capture cross sections for holesσp and electrons σn, when moving at the thermal velocity vth.
τp0 = 1 σpvthNt
and τn0 = 1 σnvthNt
(2.22) n1 and p1 express the contribution to the carrier densities due to the occupation of the trap level.
n1= NCexp(Et−EC
kT ) and p1 = NVexp(EC−EG−Et
kT ) (2.23)
HereNC and NV are the state densities at the band edges andEC andEV the energy levels of the band edges of conduction and valence band. Et is the defect energy level.
Accordingly the SRH lifetime can be expressed by:
τSRH = τn0(p0+p1+∆n) +τp0(n0+n1+∆n)
n0+p0+∆n (2.24)
From this term it can be shown that defect levels close to the center of the bandgap are the most recombination active. It should be noted that a variety of interstitial defects due to common metal impurities (Fe, Ti, Mo) shows higher capture cross sections for electrons than for holes [4]. This leads to lower impact of SRH recombination for a common degree of contamination in Cz n-type wafers and accordingly high diffusion length of holes in such wafers. For silicon it is usually the dominating recombination for low injection conditions, eventually overtaken by Auger recombination in the higher injection regimes. Analogous to the formalism for bulk defects the lifetime due to defects at the wafer surface can be described. The variables however, need to be adjusted to
express defect densities and recombination events averaged over surface area instead of volume. The surface recombination rate thus becomes analogue to 2.21:
US= nsps−n2i
ns+n1
Sp0 + psS+p1
n0
(2.25) Nowns and ps are the densities of carriers at the surface while, Sn0 and Sp0 are the surface recombination velocities for electrons and holes according to:
Sp0= σpvthNtS and Sn0 =σnvthNtS (2.26) In reality the surface produces a whole distribution of defect levels within the bandgap Dit(E) so the actual US follows from integration over the whole bandgap. It can be concluded that two fundamental approaches exist for reducing the surface recom- bination rate. On the one hand the density of interface statesDit(E)should be reduced (principle of chemical passivation), while on the other hand the surface concentration of either type of charge carrier should be reduced (field effect passivation) (refer 2.1.4).
Lifetime measurement techniques
In the cell or wafer the lifetime is influenced by all of the mechanisms presented above.
The resulting lifetime is called effective lifetime. Since the recombination mechanisms occur independently the effective recombination rate is formed as the sum of the indi- vidual rates.
Ue f f ective =Urad+UAug+USRH+Uemitter+US (2.27) The effective lifetime then calculates as:
1 τe f f ective
= 1 τrad
+ 1 τAug
+ 1 τSRH
+ 1
τemitter + 1
τS (2.28)
= 1
τbulk + 1
τemitter + 1
τS (2.29)
Effective lifetime is also the only quantity that is accessible experimentally. Most often we are also interested in the contributions of individual recombination processes.
A key technique to distinguish between those, is to measure the effective lifetime while knowing as much as possible about the other recombination mechanisms. This can be done either by suppressing a recombination path, e.g. by taking high bulk lifetime mate- rial or by calculating and subtracting known contributions, e.g. the Auger recombination.
Since the efficiency of the solar cell is determined by different limiting influences, lifetime measurements are an important tool in solar cell characterization. In order to evaluate the efficiency potential of the finished device we measure the effective lifetime on cell precursors, which are fully processed wafers without any metallization. Also dedicated symmetric lifetime structures (p+np+ or n+nn+) are produced to separate out the con- tributions from the different surfaces. Since the recombination properties at the surface or in the bulk generally depend on the injection level a technique should be able to de- termine the effective lifetime over a wide range of injection levels. This is accomplished
in the quasi-steady state photoconductance (QSSPC) method. Also the spatial resolution of the effective lifetime can reveal information about the origin of the recombination. We therefore employ the microwave detected photoconductance decay (µWPCD) technique which has a high spatial resolution. This method however works with pulsed laser ra- diation resulting in arbitrary intensity levels, leaving the exact injection level unknown.
The techniques are therefore complementing each other. Both are applied throughout this work and will be briefly discussed in the following section, while more extensive explanation can be found in the literature [39–41].
Quasi-steady state photoconductance
The most widely used instrument for lifetime testing in research and development is the quasi-steady state photoconductance (QSSPC) tool fromSinton Instruments. A photo flash is employed to generate carriers in a semiconductor sample, which is inductively coupled to an RC circuit. A sketch of the setup is seen in Fig. 2.3. The photo flash exhibits an exponential decay and its intensity is constantly monitored by a reference cell. Typi- cally the flash decay time is much longer than the measured lifetimes, so that the excess carrier distribution can be considered in steady-state condition at all times. The excess conductance is measured by a coil underneath the sample. The carriers in the sample couple to the electromagnetic field in the vicinity of the coil, leading to eddy currents which induce a current of opposing direction in the coil. The voltage in the RC circuit is then directly linearly related to the conductance, which makes the calibration of the de- vice using reference wafers of known conductivity straight forward. The measurement of the conductance is averaged over the area of the coil, which results in a measurement spot of 2 cm in diameter. This measurement of the excess photoconductanceσLis related to the average excess carrier density via:
σL=q ZW
0
(∆nµn+∆pµp)dx (2.30)
≈q∆nav(µn+µp)W (2.31)
This term is valid for homogeneous carrier distributions throughout the sample and equal generation of holes and electrons (∆n = ∆p) which requires that this balance is not disturbed by trap states. The evaluation of the excess carrier density therefore requires knowledge of the sample thickness W, which can be easily measured and the carrier mobilities which are well known from literature [42]. In steady state condition the photo-generated current Jph is always balanced by the recombination current Jrec. The latter can be calculated as:
Jrec= q RW
0 ∆n dx
τe f f = q∆navW
τe f f (2.32)
Combining 2.30 and 2.32 returns:
τe f f = σL
Jph(µn+µp) (2.33)
Figure 2.3:Drawing of QSSPC measurement setup from [40]
The measurement of excess photoconductance is therefore a direct way to measure τe f f, given the photo-generated current in the sample is known. Therefore the current of the calibrated reference cell is measured and the result is converted into the average photo-generated current Jph in the test sample by applying an optical factor. This factor relates the optical properties of the reference cell to the measured sample in order to correct for different surfaces. From the excess carrier density an implied voltage can be calculated according to:
VOC= kT
q ln[(∆p(ND+∆n)/n2i) +1] (2.34) This implied VOC can be evaluated at all intensity levels. When evaluated at the intensity equivalent of 1 sun illumination, it can be a meaningful figure of merit. Implied VOC takes into account all the recombination effects in the bulk and at the surfaces as well as base doping, which is why it is more suitable to compare the performance of cell precursors than the effective lifetime as such. Since the implied VOC is determined on cell precursors without any metal it represents an upper limit to the cellVOC, without metallization induced losses. Another application of photoconductance measurements is the determination of emitter saturation currents J0E for the characterization of highly doped surfaces. Kane and Swanson have suggested the method to be presented here [43].
The measured effective lifetime can be separated into the contributions coming from the bulk and the two surfaces via:
1
τe f f = 1
τbulk +2 J0
qn2iW∆n (2.35)
This is valid for two identically processed surfaces as in symmetrically diffused and passivated test structures. For this method homogeneous carrier generation and distri- bution throughout the wafer is required. This can be attained by near infrared excitation which shows sufficiently high absorption length. For samples with passivated surfaces
