Advanced Concepts in Photovoltaics (Energy and Environment Series, Volume 11) - Hardcover

Hardcover

ISBN 10: 1849735913 ISBN 13: 9781849735919

Publisher: Royal Society of Chemistry, 2014

Synopsis

Photovoltaic systems enable the sun’s energy to be converted directly into electricity using semiconductor solar cells. The ultimate goal of photovoltaic research and development is to reduce the cost of solar power to reach or even become lower than the cost of electricity generated from fossil and nuclear fuels. The power conversion efficiency and the cost per unit area of the phototvoltaic system are critical factors that determine the cost of photovoltaic electricity. Until recently, the power conversion efficiency of single-junction photovoltaic cells has been limited to approximately 33% - the socalled Shockley-Queisser limit.

This book presents the latest developments in photovoltaics which seek to either reach or surpass the Shockley-Queisser limit, and to lower the cell cost per unit area. Progress toward this ultimate goal is presented for the three generations of photovoltaic cells: the 1st generation based on crystalline silicon semiconductors; the 2nd generation based on thin film silicon, compound semiconductors, amorphous silicon, and various mesoscopic structures; and the 3rd generation based on the unique properties of nanoscale materials, new inorganic and organic photoconversion materials, highly efficient multi-junction cells with low cost solar concentration, and novel photovoltaic processes.

The extent to which photovoltaic materials and processes can meet the expectations of efficient and cost effective solar energy conversion to electricity is discussed. Written by an international team of expert contributors, and with researchers in academia, national research laboratories, and industry in mind, this book is a comprehensive guide to recent progress in photovoltaics and essential for any library or laboratory in the field.

About this title

About the Author

Professor Nozik is a Professor Adjoint in the Department of Chemistry and Biochemistry and a Senior Research Fellow Emeritus at the National Renewable Energy Laboratory (NREL) in Golden, Colorado; NREL is one of the National Laboratories of the U.S. Department of Energy.   He maintains an active interaction and collaboration with his colleagues at NREL.

Professor Conibeer is deputy director of the School of Photovoltaics and Renewable Energy Engineering at the University of New South Wales, Australia. His research interests include third generation photovoltaics; hot carrier cooling in semiconductors and phonon dispersion modulation in nanostructures.

From the Back Cover

Photovoltaic systems enable the sun’s energy to be converted directly into electricity using semiconductor solar cells. The ultimate goal of photovoltaic research and development is to reduce the cost of solar power to reach or even become lower than the cost of electricity generated from fossil and nuclear fuels. The power conversion efficiency and the cost per unit area of the phototvoltaic system are critical factors that determine the cost of photovoltaic electricity. Until recently,the power conversion efficiency of single-junction photovoltaic cells has been limited to approximately 33% - the so-called Shockley-Queisser limit.

This book presents the latest developments in photovoltaics which seek to either reach or surpass the Shockley-Queisser limit, and to lower the cell cost per unit area. Progress toward this ultimate goal is presented for the three generations of photovoltaic cells: the 1st generation based on crystalline silicon semiconductors; the 2nd generation based on thin film silicon, compound semiconductors, amorphous silicon, and various mesoscopic structures; and the 3rd generation based on the unique properties of nanoscale materials, new inorganic and organic photoconversion materials, highly efficient multi-junction cells with low cost solar concentration, and novel photovoltaic processes.

The extent to which photovoltaic materials and processes can meet the expectations of efficient and cost effective solar energy conversion to electricity is discussed. Written by an international team of expert contributors, and with researchers in academia, national research laboratories, and industry in mind, this book is a comprehensive guide to recent progress in photovoltaics and essential for any library or laboratory in the field.

Excerpt. © Reprinted by permission. All rights reserved.

Advanced Concepts in Photovoltaics

By Arthur J Nozik, Gavin Conibeer, Matthew C Beard

The Royal Society of Chemistry

Copyright © 2014 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-591-9

Contents

Chapter 1 Crystalline Silicon Solar Cells with High Efficiency Stefan W. Glunz, 1, 
Chapter 2 Tandem and Multiple-junction Devices Based on Thin-film Silicon Technology Christophe Ballif, Mathieu Boccard, Karin Söderström, Grégory Bugnon, Fanny Meillaud and Nicolas Wyrsch, 30, 
Chapter 3 Thin-film CdTe Photovoltaic Solar Cell Devices Timothy Gessert, Brian McCandless and Chris Ferekides, 61, 
Chapter 4 III–V Multi-junction Solar Cells Simon P. Philipps and Andreas W. Bett, 87, 
Chapter 5 Thin-film Photovoltaics Based on Earth-abundant Materials Diego Colombara, Phillip Dale, Laurence Peter, Jonathan Scragg and Susanne Siebentritt, 118, 
Chapter 6 Chemistry of Sensitizers for Dye-sensitized Solar Cells Peng Gao, Michael Grätzel and M. D. K. Nazeeruddin, 186, 
Chapter 7 Perovskite Solar Cells Nam-Gyu Park, 242, 
Chapter 8 All-oxide Photovoltaics Sven Rühle and Arie Zaban, 258, 
Chapter 9 Active Layer Limitations and Non-geminate Recombination in Polymer–Fullerene Bulk Heterojunction Solar Cells Tracey M. Clarke, Guanran Zhang and Attila J. Mozer, 287, 
Chapter 10 Singlet Fission and 1,3-Diphenylisobenzofuran as a Model Chromophore Justin C. Johnson and Josef Michl, 324, 
Chapter 11 Quantum Confined Semiconductors for Enhancing Solar Photoconversion through Multiple Exciton Generation Matthew C. Beard, Alexander H. Ip, Joseph M. Luther, Edward H. Sargent and Arthur J. Nozik, 345, 
Chapter 12 Hot Carrier Solar Cells Gavin Conibeer, Jean-François Guillemoles, Feng Yu and Hugo Levard, 279, 
Chapter 13 Intermediate Band Solar Cells Yoshitaka Okada, Tomah Sogabe and Yasushi Shoji, 425, 
Chapter 14 Spectral Conversion for Thin Film Solar Cells and Luminescent Solar Concentrators Wilfried van Sark, Jessica de Wild, Zachar Krumer, Celso de Mello Donegá and Ruud Schropp, 455, 
Chapter 15 Triplet–triplet Annihilation Up-conversion Timothy W. Schmidt and Murad J. Y. Tayebjee, 489, 
Chapter 16 Quantum Rectennas for Photovoltaics Feng Yu, Garret Moddel and Richard Corkish, 506, 
Chapter 17 Real World Efficiency Limits: the Shockley–Queisser Model as a Starting Point Pabitra K. Nayak and David Cahen, 547, 
Chapter 18 Grid Parity and its Implications for Energy Policy and Regulation Muriel Watt and Iain MacGill, 567, 
Subject Index, 596, 

CHAPTER 1

Crystalline Silicon Solar Cells with High Efficiency

STEFAN W. GLUNZ

Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

Email: stefan.glunz@ise.fraunhofer.de

1.1 Introduction

Crystalline silicon photovoltaics is the dominant solar cell technology, with a market share of around 85% in 2012. Silicon has several advantages: It is non-toxic and abundantly available in the earth's crust. Crystalline silicon-based photovoltaic (PV) modules have proven their long-term stability over decades in the field and not only in accelerated module tests. The price reduction of silicon PV modules in the last 30 years can be described very well by a learning factor of 20%. Due to strong competition this price decline was even stronger in the last years, resulting in module prices well below $1/Wp. This is an excellent situation for customers and PV installers, but rather challenging for producers of silicon solar cells and modules. Thus cost reduction is still a major task.

The cost distribution of a crystalline silicon PV module is clearly dominated by material costs, especially by the cost of the silicon wafer and encapsulation materials (see Figure 1.1). Therefore, besides improved production technologies, the efficiency of the cells and modules is the main leverage to bring down the cost even more, especially when considering the full levelized cost of PV electricity.

The International Technology Roadmap for Photovoltaic recommends in their latest report:

(i) Continue the cost reduction per piece, along the whole value chain, but especially at the module level, by the efficient use of Si and non-Si materials, and (ii) Improve the module power/cell efficiency without significant increasing processing costs.

This chapter will mainly focus on the second route by using a detailed loss analysis to investigate the current developments and cell architectures in industry and research.

1.2 Efficiency Limitations

1.2.1 Theoretical Limitations: The Auger Limit

Based on a detailed balance calculation, Shockley and Queisser determined already as early as 1961 a maximum theoretical efficiency for solar cells. Using the AM1.5 spectrum and assuming no light concentration, the limit for a single junction solar cell is 33%. The main limitation of such cells is the thermalization of hot carriers generated by photons with energies greater than the bandgap energy and the non-absorption of photons with energy smaller than the energy gap (see Figure 1.2).

Thus, it is important to choose the right bandgap energy in order to balance and minimize these losses. Fortunately, semiconductors like silicon and GaAs are very close to the optimum bandgap energy (see Figure 1.3). However, comparing the best experimental values with the Shockley–Queisser limit (SQL), it is obvious that GaAs gets much closer to this limit.

This is actually not a consequence of a more advanced technological development, but due to the fact that the SQL is not the relevant limitation for solar cells fabricated from the indirect semiconductor crystalline silicon. In their groundbreaking article Shockley and Queisser write:

It is radiative recombination that determines the detailed balance limit for efficiency. If radiative recombination is only a fraction fc of all recombination, then the efficiency is substantially reduced below the detailed balance limit.

Other recombination channels can be caused by defect recombination, which could be controlled perfectly in theory. But there also other unavoidable intrinsic recombination channels such as non-radiative Auger recombination, which are given by the physical properties of the semiconductor. As silicon is an indirect semiconductor, radiative recombination also involves a phonon, thereby making the process quite unlikely (and silicon LEDs quite inefficient). Figure 1.4 shows the charge carrier lifetime limitation due to radiative and Auger recombination as a function of photogenerated excess carrier density. It is obvious that Auger recombination is the dominating intrinsic recombination channel for crystalline silicon in the ideal case of defect-free material.

Therefore it is very important to determine and parameterize the Auger recombination as a function of doping concentration and excess carrier density. The most recent parameterization of the intrinsic recombination (including Auger recombination) was published by Richter et al.

[MATHEMATICAL EXPRESSION OMITTED] (1.1)

(1:1) with n and p being the electron and hole density, respectively, n0 and p0 the electron and hole thermal equilibrium density, also respectively, Δn the excess carrier density, ni,eff the effective intrinsic carrier concentration, Blow the radiative recombination coefficient for lowly-doped and lowly-injected silicon (4.73 x 10-15 cm3 s-1 at 300 K; see Trupke et al.), Brel the relative radiative recombination coefficient and the enhancement factors:

[MATHEMATICAL EXPRESSION OMITTED] (1.2)

and

[MATHEMATICAL EXPRESSION OMITTED] (1.3)

with N0,eeh 1/4 3.3 x 1017 cm-3 and N0,ehh = 7.0 x 1017 cm-3.

Auger recombination included, it is possible to calculate the upper limit for crystalline silicon solar cells. With the most recent parameterization [see Richter et al. and Equation (1.1)] the maximum value was determined to be 29.4% (see Figure 1.5).

These maximum values are depicted as a dashed horizontal line in Figure 1.3. This makes it clear that the ratio of experimental record values and theoretical maximum values of GaAs and silicon are in both cases in the range of 85%.

1.2.2 Practical Limitations

Such idealized devices considered for the calculation of the efficiency limitations without additional recombination at contacts etc. are only of interest in theory and can not be realized. For a realistic, yet optimized silicon solar cell, an efficiency limit of 26% was predicted.

However, most of the cells in the industry and in development are far from this practical efficiency limit. They suffer from a variety of optical, recombination and resistive losses. In order to get a breakdown of the losses of different solar cell architectures, it is advantageous to analyse the loss currents at maximum power point (mpp). There are also very good calculations investigating the power losses at mpp or the free energy loss analysis, but for didactical reasons we have preferred to analyse the loss currents.

The starting point is jmax, i.e., the theoretical maximum photogeneration for a certain thickness. We have kept the cell thickness at 160 mm for all investigated cell structures to keep the different calculations comparable. For thickness of 160 μm and ideal Lambertian light-trapping this theoretical maximum is calculated to be 43.6 mA cm-2. The first loss is then the optical losses, Δjoptical, due to the primary surface reflection, parasitic absorption and escape light. Δjoptical is the difference between jmax and jgen, the latter referring to the photocurrent generated in the actual device. jgen was calculated for the individual device structures using the ray-tracing program in Sentaurus Device.

A part of jgen is lost by charge carrier recombination. This recombination loss current Δjrec is calculated with Senaturus Device at maximum power point (see Figure 1.6) In our simulation we have separated the recombin- ation in different parts of the cells by spatial integration of the two- or three- dimensional simulation output:

• Δjfront, the recombination loss current in the front emitter, at the front surface and at the front contacts (for interdigitated backjunction cells: front surface and in the front surface field).

• ΔjAuger,base, the intrinsic Auger recombination in the undiffused base region of the cell.

• ΔjSRH,base, the Shockley–Read–Hall recombination via defects and impurities.

• Δjrear, the recombination loss current in the back surface field and at the rear surface (for interdigitated back junction cells: rear emitter, back surface field and rear surface).

These losses are shown in Figure 1.7 for different cell structures on monocrystalline silicon. The black parts of the columns show the obtained current at mpp, jmpp, while the shaded sections denote the optical and recombination losses. jmpp and the loss currents add up to jmax. Additionally, the actual cell parameters VOC, Vmpp and efficiency are given above the figure. For our calculation, realistic resistive losses have been assumed. Since we have decided to discuss current losses and not power losses, it is not possible to show such power losses in the figure; however, they have been included in the calculation of the efficiency.

An important characteristic of our study is the fact that we used a consistent set of parameters, such as the bulk lifetime or surface recombination velocities, for example, to allow for a transparent comparison of the different cell structures. Details on the chosen parameters and the simulation procedures are given in Rüdiger et al. In the following we will discuss the cell structures shown in Figure 1.7 step by step.

1.3 Screen-printed Al-BSF Solar Cells on p-type Silicon

For decades the working horse of the PV industry has been the screen-printed aluminium back surface field (Al-BSF) cell on p-type silicon (Figure 1.8). Although some new process steps such as the fire-through process were introduced, most of the impressive increase in cell efficiency has been due to evolutionary improvements. Improved metal pastes and printing processes, increased emitter sheet resistance and better front surface passivation layers (SiNx by plasma-enhanced vapour deposition, PECVD) are a few examples of the changes which have been responsible for the increase in average monocrystalline silicon efficiency limits over the years, with values at around 14% in the 1990s to more than 18% today.

1.3.1 Standard Al-BSF Cell

Our calculation results in an efficiency of 18.4% (VOC 1/4 627 mV) for standard screen-printed Al-BSF cells. As can be seen in Figure 1.7, the biggest losses are the optical losses due to front reflectance, transmission losses and the recombination at the rear side of the cell. The Al-BSF at the rear side was already described in the 1970s. In today's standard process sequence, it is created by a rather elegant alloying process of screen-printed Al-paste with the base silicon. This alloying process takes place during a very short firing process in an inline belt furnace. During the cool-down phase the silicon that has been dissolved into the molten aluminium, recrystallizes and aluminium is incorporated in the silicon lattice according to the solubility at the actual temperature. Due to this process, the doping profile of Al-BSF cells shows a characteristic shape (black dots in Figure 1.9).

The main limitation of such profiles is the maximum Al doping concentration, which is in the range of 7x1018 cm-3, resulting from to the rather low solubility of Al in Si. Since the effectiveness of a BSF, i.e., its capability to reduce the surface recombination velocity, S, or the dark saturation current, J0, at the rear side, depends strongly on the doping step Nacc,BSF/Nacc,base, a higher doping concentration would be desirable.

1.3.2 Improved Al-BSF Formation by Boron Co-doping

The limitation of a maximum doping concentration in the Al-BSF can be overcome by co-doping with boron. This was suggested by Lölgen et al. already in 1994 and recently investigated in depth by Rauer et al. The pale grey squares in Figure 1.9 show the doping profile a BSF alloyed from an Al paste with B co-doping. Due to the higher solubility of boron, a significantly higher acceptor concentration is achieved. This makes the BSF more effective and the related dark saturation values j0 are reduced significantly (see Figure 1.10).

This reduction of j0 corresponds to the reduction of Δjrear (see second column in Figure 1.7) and leads to an improved VOC. The calculated efficiency of the cell is now 18.7%.

1.3.3 Improved Emitter

A second high-recombination region of a standard solar cell is the relatively highly phosphorous-doped emitter. Using a highly doped emitter has two advantages. Firstly, a low sheet resistance is achieved which allows one to increase the pitch between the front grid lines and thus reduces shadowing losses. Secondly, the high surface doping concentration of such emitters, Ndop,surface, makes it easy to create a metal contact with low ohmic losses, even when metals are used, which are not optimal for contacting n-type silicon, such as silver, for instance.

However, on the down side, such high doping concentrations Ndop will reduce the carrier lifetime due to Auger recombination (τAuger ~ 1/Ndop2 1/Ndop2), not to mention that the front surface recombination velocity Sfront increases with Ndop,surface as determined by Cuevas et al. Consequently, the dark saturation current increases and the current generated by high-energy photons with short absorption depths (blue response) is reduced. This dilemma can be solved in two ways. The first strategy is to improve the understanding of the contact formation and develop improved metallization pastes, which make it possible to contact lowly doped emitters. There has been a lot of progress in this field and pastes are now commercially available, which meet these demands. A second strategy is known as selective emitter or two-step emitter, whereby the region under the contact is highly doped to facilitate good contact properties and the region in-between the contacts is lowly doped ('shallow') to improve the blue response and reduce the dark saturation cur- rent (see Figure 1.11). The highly doped emitter under the contacts ('deep') emitter has a second advantage of 'shielding' the highly recombinative metal/ silicon contact and additionally reduces recombination. A very good overview on different technological approaches to fabricate such structures is given by Hahn. By using such a selective emitter structure, Δjfront can be reduced significantly with the conversion efficiency reaching a value of 19%.

1.4 Solar Cells with Dielectric Rear Passivation on p-type Silicon

Even with improved rear side Al pastes, as discussed in section 1.3.2, the Al-BSF structure is limited on account of the rear side recombination still being high and due to the fact that the internal reflection for deeply penetrating long-wavelength light reaching the rear side of the cell is only in the range of 65%. The most effective way to overcome this problem is through the introduction of a dielectric rear side passivation with local contact points or lines (partial rear contact, PRC). The PRC structure is the basis for a variety of successful cell architectures like the passivated emitter and rear cell (PERC), the passivated emitter, rear locally-diffused (PERL) or local back surface field structure (LBSF) and the passivated, rear totally diffused cells (PERT). PERL cells have set the actual world record of 25% for crystalline silicon-based solar cells (Figure 1.12).

With such a rear structure, low effective surface rear recombination velocities in the range of 60 to 200 cm s-1 and internal reflectance of 95% are achievable. Since the recombination at the rear side is very low resulting in a lower Δjrear and long-wavelength light is not lost due to transmission which reduces Δjoptical, efficiencies far above 20% are attainable. Although this cell structure achieved excellent efficiencies already as early as 1989, it took 20 years until the transfer into industrial production was performed. Efficiencies of 20.2% were recently shown using industrial production equipment.

The PRC structure can be easily combined with the so-called metal-wrap-through (MWT) cell architecture. When this is done, the shadowing loss due to the busbars can be strongly reduced. Efficiencies above 20% have been achieved with such MWT-PERC structures.

1.4.1 Rear Passivation Layers

Obviously the surface passivation layer plays a crucial role for this cell structure. Traditionally, the preferred layer type was silicon dioxide (SiO2), which was thermally grown. This is a technology well known by MOS technology, which was successfully introduced to photovoltaics in the 1980s. The reduction of surface recombination velocity in this case is mainly due to a reduction of interface defect density, Dit (Figure 1.13).

(Continues...)

Excerpted from Advanced Concepts in Photovoltaics by Arthur J Nozik, Gavin Conibeer, Matthew C Beard. Copyright © 2014 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2014
• Language: English
• ISBN 10: 1849735913
• ISBN 13: 9781849735919
• Binding: Hardcover
• Edition number: 1
• Number of pages: 630
• Editor: Nozik Arthur J., Conibeer Gavin, Beard Matthew C