Photonic Management of Ultra-Thin Solar Cells
Thesis submitted by Blaise GODEFROID
in fulfilment of the requirements
of the PhD Degree in Physics
Academic year 2019-2020
Supervisor:
Prof. Gregory KOZYREFF
Thesis jury:
Thomas ERNEUX (ULB, Chair)
Pascal KOCKAERT (ULB, Secretary)
Jordi MARTORELL (ICFO)
Photonic Management of Ultra-Thin Solar Cells
Thesis submitted by Blaise GODEFROID
in fulfilment of the requirements of the PhD Degree in Physics (“Docteur en
physique”)
Academic year 2019-2020
Supervisor: Professor Gregory KOZYREFF
Optique Nonlinéraire Théorique
Thesis jury:
Thomas ERNEUX (Université libre de Bruxelles, Chair)
Pascal KOCKAERT (Université libre de Bruxelles, Secretary)
Jordi MARTORELL (ICFO - Institut de Ciencies Fotoniques)
Abstract
Despite the successive conferences of parties (COP) on climate change, our energy production remains dramatically dominated by fossil fuels and our energy consump-tion continues to increase. It is therefore crucial to develop alternative and renewable sources of energy. Solar energy is by far the largest accessible source of energy. Pho-tovoltaics therefore seems quite promising. However, to develop this technology on a large scale, we have to ful l certain challenges such as the high price of the commer-cialised solar panels, the scarcity of some of their raw materials, and the di culty of recycling them. In this context, organic solar cells are an interesting solution be-cause they are cheaper, do not use rare materials, consume less energy during their manufacture and are simpler to recycle.
Organic technology is, however, relatively new and researches are still needed to ensure simultaneously high e ciency, long lifetime and low-cost. On the side of the e ciency, the most limiting factor is the very short diusion length of the excitations produced by photon absorption. The cell is therefore forced to be very thin, which limits its absorption. To address this issue, materials scientists have developed many strategies to improve the quality of active materials. However, another, often less expensive, approach is to manage the distribution of light in the cell more e ciently. To exploit this, we have developed, in this thesis, new mathematical tools adapted to organic materials. We have generalised the theory developed by William Shockley and Hans Queisser in 1961 for crystalline semiconductor cells to new unconventional solar cells (e.g. organic). Moreover, we have proposed a new and more accurate way to approximate the optical response of these new materials. We then studied two promising, innovative and complementary photonic strategies:
• The rst one consists in managing the injection of light in a homo-tandem which is a stack of two sub-cells with identical absorbers. We show that absorption can be increased through interference management by inserting an ultra-thin metal lm between the two sub-cells. In parallel, we highlight the optical advantages of a new transparent electrode that is less expensive, less scarce and more environmentally friendly than conventional electrodes.
• The second one consists in managing the radiative losses during the recombina-tion of the excitarecombina-tions produced by the photon absorprecombina-tion. We show that with a judicious choice of geometrical parameters, we can reduce these radiative
losses - and thus increase the diusion length - thanks to microvavity eects. Moreover, we show that not considering the spatial dependence of radiative losses leads to a suboptimal conception of the cell, even if the radiative part of the recombination is very small.
Résumé
Malgré les conférences des parties (COP) successives sur les changements climatiques, notre production énergétique reste dramatiquement dominée par les énergies fossiles et notre consommation ne cesse d'augmenter. Il est donc crucial de développer des sources d'énergie alternatives et renouvelables. L'énergie solaire est de loin la plus grande source accessible d'énergie. Le photovoltaïque semble donc tout à fait prometteur. Cependant pour développer cette technologie à grande échelle, il nous faut relever certains dé s comme le prix élevé des panneaux solaires actuels, la rareté de certains de leurs matériaux, et la di culté de les recycler. Dans ce contexte, les cellules solaires organiques constituent une solution intéressante, car elles sont moins chères, n'utilisent pas de matériaux rares, consomment peu d'énergie lors de leur fabrication et sont simples à recycler.
La technologie organique est cependant relativement jeune et des recherches sont encore nécessaires a n que les panneaux solaires organiques présentent simultané-ment une grande e cacité, une longue durée de vie et un prix bas. Au niveau de l'e cacité, le facteur le plus limitant est la très courte longueur de diusion des excitations produites par l'absorption de photons. La cellule est donc contrainte à être très ne, ce qui limite son absorption. Pour remédier à ce problème, les chercheurs en sciences des matériaux ont développé de nombreuses stratégies a n d'améliorer la qualité des matériaux actifs. Cependant, une autre approche, souvent moins couteuse, consiste à gérer plus e cacement la répartition de la lumière dans la cellule. Pour exploiter cela, nous avons développé, dans cette thèse, de nouveaux outils mathématiques adaptés aux matériaux organiques. Nous avons généralisé la théorie élaborée par William Shockley et Hans Queisser en 1961 pour les cellules semi-conductrices cristallines, aux cellules solaires non conventionnelles (ex : or-ganiques), et nous avons proposé une nouvelle manière plus précise d'approximer la réponse optique de ce type de cellules solaires. Nous avons ensuite étudié deux stratégies photoniques prometteuses, innovantes et complémentaires:
• La première consiste à gérer e cacement l'injection de lumière dans une homo-tandem. Celle-ci est composée d'un empilement de deux sous-cellules avec des absorbeurs identiques. Nous montrons que l'absorption peut être augmentée grâce à une meilleure gestion des interférences via l'incorporation d'une couche métallique mince entre les deux sous-cellules. En parallèle, nous mettons en évidence les avantages optiques d'une nouvelle électrode transparente qui est
moins chère, moins rare et plus respectueuse de l'environnement que les élec-trodes conventionnelles.
Acknowledgements
Durant ma thèse de doctorat, un grand nombre de personnes m'ont, de près ou de loin, aidé et soutenu dans cette épreuve. Sans m'éppancher sur ma vie privée et les multiples autres facettes de mon investissement à l'ULB, je voudrais prendre ici l'opportunité de les remercier.
La thèse est d'abord une aventure intellectuelle et je ne pourrais commencer cette page sans adresser mes chaleureux remerciements à mon promoteur, Dr gory Kozyre, qui a accepté d'accompagner la réalisation de ce travail. Merci, Gre-gory, d'avoir encadré mon travail durant ces dernières années. Notre collaboration remonte au tout début de mon mémoire de master en 2014. Depuis, tes conseils avisés, ta nécessaire rigueur, nos discussions passionnées, tes relectures attentives et nos voyages scienti ques ont alimenté mes ré exions et ont rendu possible cet énorme travail. Merci aussi pour ta patience face à mon anglais maladroit et désolé pour les quelques sueurs froides que je t'ai données lors de la publication de notre dernier article. En n, ton attention toute particulière à relier nos travaux de physique fon-damentale à des problèmes concrets et palpables a donné plus de sens à mon travail, me permettant ainsi de garder le cap et de le mener à bien. Merci encore.
Rien n'aurait été possible non plus sans le soutien professionnel et amical de mon comité d'accompagnement. Je suis très reconnaissant au Dr Thomas Erneux et au Dr Pascal Kockaert d'avoir suivi mon travail de thèse depuis le début et d'avoir accepté de m'accompagner jusqu'à l'ultime étape que constitue ma défense publique. Vos nombreux conseils scienti ques sur le contenu d'une part, et concernant ma méthodologie d'autre part, m'ont été précieux et m'ont permis d'intégrer dans ma recherche des considérations auxquels je n'avais pas initialement pensées. Au-delà de ces aspects professionnels, j'ai pu également trouver auprès de vous des oreilles attentives qui ont grandement facilité le travail parfois péniblement solitaire que représente une thèse. En particulier, je voudrais vous remercier, Thomas, pour votre bienveillance et votre générosité qui ont garanti une ambiance au beau xe dans le service d'Optique Nonlinéaire Théorique durant toutes les années où vous l'avez dirigé. Je me souviendrai longtemps de nos discussions hétéroclites lors de nos déjeuners.
It is a great honour to have Dr. Jordi Martorell, Dr. Marina Mariano and Dr. Thomas Kirchartz, to review my research and I would like here take the opportunity
to thank them. I am very grateful to Dr. Martorell and his team at ICFO including Dr. Mariano and Dr. Quan Liu, for having given me the opportunity to link my theoretical work to experimental considerations. Our collaboration has been the basis of my research projects, particularly the one on TRTC. Finally, nothing would have been possible without the valuable information they provided us. Many thanks again!
I have read Dr. Kirchartz's work many times, which has allowed me to familiarise myself with the complex research eld of organic solar cells. That's why it's so a honour to have him in my jury.
Un tout grand merci aussi à Fabienne De Neym, Delphine Vantighem, Marie-France Rogge et Hélène Kajianis pour leur précieux support administratif qui m'ont permis de me concentrer sur ma recherche tout en sachant que le reste serait fait consciencieusement. Merci beaucoup!
La thèse est aussi une épreuve humaine et personnelle que je n'aurai pas surmonté sans mes camarades physiciens, Jehan, Sarah, Dr Gaetan, capt. Virginie, Dr Pierre-Brice, Dr Étienne et Nicolas. Je les remercie énormément pour nos bons moments et aussi pour leur relecture et leurs conseils. Merci également à Claire, à ma famille, à mes colocataires, aux copains de Payns, aux Découvreurs, aux Verts, au Cercle des Sciences, et à tant d'autres pour avoir toujours été là.
List of the Publications
Peer-Reviewed Publications
• B. Godefroid and G. Kozyre, Photonic eect of a central ultrathin metal lm on the performance of series and parallel homo-tandem solar cells, Solar energy materials and solar cells, 206 (2020)
• B. Godefroid and G. Kozyre, Photonic enhancement of parallel homo-tandem solar cells through the central electrode , Solar energy materials and solar cells, 193 (2019)
• B. Godefroid and G. Kozyre, Design of organic solar cells as a function of radiative quantum e ciency, Physical review applied, 8 (2017)
International Conferences
• B. Godefroid and G. Kozyre, Optimisation of metallic interconnecting layer in homo-tandem cells , International Conference on Hybrid and Organic Pho-tovoltaics 2019 (13/05/2019: Rome, Italy)
• B. Godefroid and G. Kozyre, Improvement of Light Harvesting with a Multi-Resonance Tandem Geometry in Thin-Film Solar Cells, 35th European Pho-tovoltaic Solar Energy Conference and Exhibition (25/09/2018: Brussels, Bel-gium)
• B. Godefroid and G. Kozyre, Organic solar cell design as a function of inter-nal luminescence quantum e ciency, Internatiointer-nal Conference on Hybrid and Organic Photovoltaics 2018 (28/05/2018: Benidorm, Spain)
• B. Godefroid and G. Kozyre, Multi-resonance tandem geometry for an im-proved light trapping at long-wavelength in thin- lm solar cells, International Conference on Hybrid and Organic Photovoltaics 2018 (28/05/2018: Benidorm, Spain)
• B. Godefroid and G. Kozyre, Solar cell e ciency as a function of blocking layer thicknesses and exciton uorescence quantum yield, 34th European
tovoltaic Solar Energy Conference and Exhibition (28/09/2017: Amsterdam, The Netherlands)
Other Communications
• B. Godefroid and G. Kozyre, Le photovoltaïque organique: Eet du con ne-ment sur les processus en jeu, Festival du Film Scienti que de Bruxelles 2018 (21/03/2018)
List of Acronyms
2T two-terminal (series connected) homo-tandem (Table 4.1)
2T+ 2T with an UTMF in the interconnecting layer (Table 4.1)
3T three-terminal (parallel connected) homo-tandem (Table 4.1)
AM air mass (p.50)
ARC anti-re ection coating (p.82)
BHJ bulk heterojunction (p.20)
BOS balance-of-system (p.15)
CED cumulative energy demand (p.38)
CPBT carbon payback time (p.37)
D/A donor/acceptor (p.18)
DCL dielectric cavity layer (p.82)
DSSC dye-sensitized solar cell (p.10)
e-h electron-hole (also called free charge carriers)
EPBT energy payback time (p.38)
EQE external quantum e ciency (number of collected
elec-trons per incoming photon) (p.56)
FF ll factor (p.50)
GFF geometrical ll factor (p.7)
GWP global warming potential (p.35)
HOMO highest occupied molecular orbital (p.18)
IQE internal quantum e ciency (number of collected
elec-trons per absorbed photon) (p.63)
LCA life cycle assessment (p.35)
LCOE levelized cost of energy (p.17)
LUMO lowest unoccupied molecular orbital (p.18)
OHJ ordered heterojunction (p.20)
OLED organic light emitting diode
OPV organic photovoltaic (PV)
OSC organic solar cell
PHJ planar heterojunction (p.20)
QD quantum dots (p.10)
SC single-cell (Table 4.1)
SQ Shockley-Queisser
TRTC two-resonance tapping cavity (front electrode) (p.10)
Table of Contents
Abstract i
Résumé iii
Acknowledgements v
List of the Publications vii
List of Acronyms ix
1 Introduction 1
2 A Renewable Source of Energy 5
2.1 Overview of Solar Cell Technologies . . . 5
2.1.1 Economic Considerations . . . 13
2.2 Challenges Facing Organic Solar Cells . . . 17
2.2.1 Active Materials . . . 19
2.2.2 Blocking Layers . . . 25
2.2.3 Electrodes . . . 26
2.2.4 Device Stability . . . 29
2.2.5 Roll-to-roll Printing Technique . . . 32
2.3 Environmental Impacts . . . 35
2.3.1 Impacts of the Electrodes . . . 38
2.3.2 Recyclable Solar Cells . . . 40
2.4 The TeraWatts Challenge by 2050 . . . 44
3 Theoretical Tools 49 3.1 E ciency and I(V ) Curve . . . 49
3.2 Shockley-Queisser Theory and its Generalization . . . 51
3.2.1 Detailed Balance Theory for Conventional Solar Cells . . . . 52
3.2.2 Detailed Balance Theory for Exciton-Regulated Solar Cells . 55 3.2.3 Complementary Considerations . . . 58
3.3 Computation of the External Quantum E ciency . . . 63
3.3.2 EQE for Organic Bilayer Solar Cells . . . 71
3.4 Conclusion . . . 75
4 Light Injection Management via Homo-Tandems 77 4.1 Introduction . . . 78
4.1.1 Advantages and Constraints of Parallel and Series Connection 78 4.1.2 ITO-free front electrode . . . 80
4.2 Materials and Method . . . 80
4.2.1 Studied Materials and Types of Cells . . . 80
4.2.2 Simulation Method . . . 82
4.2.3 Series vs Parallel Connection . . . 83
4.2.4 An Important Design Constraint : the Maximum Active Thick-ness, Lmax . . . 84
4.2.5 An Important Economic Constraint : the Total Active Thick-ness, h2+ h4 . . . 84
4.2.6 Special Emphasis on the 3T Architecture . . . 85
4.3 Numerical Results with the Theoretical Material . . . 86
4.3.1 Optimal Design as a Function of Lmax . . . 86
4.3.2 Comparison of TRTC and ITO Electrodes . . . 86
4.3.3 Signs of Microcavity Eects . . . 87
4.3.4 Saving Material Thanks to Interferences . . . 88
4.3.5 Eect of the UTMF Thickness in 2T . . . 88
4.3.6 Eect of the UTMF thickness in 3T . . . 90
4.4 Numerical Results with the Realistic Materials . . . 92
4.4.1 Additional Results about 3T . . . 96
4.5 Discussion . . . 99
4.5.1 Additional Discussion about 3T . . . 99
5 Fluorescence Loss Management 101 5.1 Introduction . . . 102
5.2 Numerical Results . . . 103
5.2.1 First Example: High-E ciency Cell . . . 105
5.2.2 Second Example: Low E ciency Cell . . . 106
5.3 Discussion . . . 108
6 Conclusions and Outlook 111
Bibliography 131
A Charge Carrier Diusion Derivation 135
A.1 PN-Junction . . . 137
A.2 Transport Equations . . . 137
A.3 Depletion Approximation and Boundary Conditions . . . 139
A.4 Current-Voltage Curve . . . 141
B How the Central UTMF In uence 2T Performances 143 C How the Central UTMF In uence 3T Performances 145 C.1 PTB7:PC71BM . . . 145
C.2 Perovskite . . . 150
C.3 P3HT:PC61BM . . . 153
Introduction
Despite four World Climate Conferences during the last 40 years, greenhouse gas emissions are still rapidly rising, with increasingly damaging eects on Earth's cli-mate [1]. The world must quickly implement massive energy e ciency and conser-vation practices and must replace fossil fuels with low-carbon renewables and other cleaner sources of energy for the safety of people and the environment. Unfortunately, the energy consumption curves do not seem to decline in the near future. The anual electricity demand, in particular, is expected to grow from 25 PetaWatt-hour (PWh) today [2] to 33 PWh [3] or even 42 PWh [4] in 2050. To ful l the climate goal of
staying below 2oC above pre-industrial global temperature, a large part (∼65% [3])
of the 2050 electrical production will have to be carried out by renewable sources of energy. Among these sources, solar have the biggest potential in terms of resources, see Figure 1.1. Moreover, photovoltaic technology is an ideal energy source as it is silent, has no moving parts and in principle requires little or no maintenance once installed [5]. It has demonstrated impressive developments in terms of the scale of deployment, cost reduction and performance enhancement, most visibly over the past decades. However, the photovoltaic technology is a new technology that is far from producing the 16% of the 2050 electricity production expected by the Inter-national Energy Agency in its high renewable scenario [3]. Moreover, eorts are still needed to further reduce the cost of the currently commercialised photovoltaic technologies (mainly silicon), their large thermal budgets, and their relatively slow manufacture [5]. In addition, we also need to address di culties in terms of raw material resources as well as recycling techniques on their way to ful l the TeraWatt challenge by 2050.
In this context, several new technologies have emerged using more complex ma-terials than silicon, which researchers have to improve in order to push them over the edge of conventional solar cells. In particular, organic solar cells have attracted considerable research interest since the 1990s as a low-cost, mechanically exible, using only earth-abundant raw material and low-temperature manufactured alter-native to inorganic solar cells. In Chapter 2, we highlight these various advantages of organic solar cells and detail their challenges. In addition, we emphasize their
capacity to ful l the TeraWatt objective by 2050 as a new source of solar energy respecting environmental constraints. One of the main problems we need to face with solar organic cells is to ensure broadband absorption by the sub-micron-thick layer of active material. Indeed, because of the poor transport of photo-generated electrical excitations, the active layers are restricted to thicknesses that are too thin to fully absorb the incident sunlight [7, 8, 9]. In order to overcome this di culty, the current research is led by material scientists who work to improve the quality of the employed materials and their longevity. On the other hand, light manage-ment researches conducted by optical scientists like us, is a complemanage-mentary way to address the absorption challenge. Indeed, several photonic strategies have been devised to e ciently trap light: randomly structured interface [10], hexagonal ar-rays of nanocolums [11, 12], nanoholes [13], nanospheres [14, 15] or photonic ber plates [16, 17]. Building better front electrodes also play an essential role in the light harvesting by solar cells. Transparent electrodes made of an Ultra Thin Metal Film (UTMF) [18, 19, 20, 21, 22] and in particular the Two-Resonance Tapping Cavity (TRTC) [23] have been demonstrated to produce constructive interference in the cell over broad spectral ranges.
The main contribution to this thesis is the study of two promising strategies to improve the solar cell performances via photonic management techniques. The rst one addresses light injection while the second one focus on uorescence management. They are strongly motivated by our collaboration with the experimental group of the Institut de Ciències Fotòniques (ICFO) who communicated to us their experimental data. Indeed, our goal is to oer answers to complex analytical problems keeping in mind - or taking into account - experimental constrains. In this sense, the collabo-ration with ICFO was greatly valuable. The two approaches detailed in this thesis for improving the solar cell performances are complementary, innovative and they do not require new complex techniques. To investigate these two approaches, we need to develop new mathematical tools appropriate for the speci c materials we are considering. Speci cally, we generalise the Shockley and Queisser theory (SQ) [24] elaborated in 1961 for crystalline semiconductor cells to non-conventional solar cells. The SQ theory is commonly used because the cell performance can be determined thanks to only two parameters, namely the bandgap energy and the cell tempera-ture. However, it has to be generalised when we model solar cells with more complex processes. The theory is detailed in Chapter 3 including the used transfer-matrix method [25, 26]. We perform all our simulations with python codes of our own. Moreover, the amorphous phase of the organic solar cells induces localised states in the bandgap. Consequently, the step-function used by SQ as ideal material re-sponse, does not hold up very well. Hence, we propose in Chapter 4, a new way to approximate organic electromagnetic behaviour. It consists of considering a step function plus an Urbach tail (decreasing exponential tail) as extinction coe cient. This particular extinction coe cient which stands just next in complexity to the SQ approximation, is found to capture all the essential features of the cell response.
the absorption of a cell using a single active material with a homo-tandem archi-tecture [27]. Homo-tandems are composed of a stack of two subcells with identical absorbers connected either in series or in parallel. We have the intuition, noting the favourable resonances coming with UTMF present in TRTC, that inserting another UTMF into the absorber can bring additional favourable resonances. It is our rst motivation to study the homo-tandem architecture. One of the goals of this reshearch is then to increase the light injection thanks to better interference management with the insertion of a thin metal layer between the two subcells. We emphasise sev-eral gensev-eral rules which can serve as guides for experimentalists in their quest of high performance solar cells. Secondly, we emphasise the eect of a TRTC front electrode comparing it with a conventional indium-thin oxide (ITO) layer [28, 29]. The advantage of TRTC, in addition to allowing better microcavity eect, is to be ITO-free. Indeed, as it will be discussed in Section 2.2.3 and 2.3.1, ITO includes some drawbacks such as high cost, indium scarcity, rigidity and high environmental impact.
Our strategy about uorescence management, presented in Chapter 5, is to ex-ploit the Purcell eect in order to reduce the spontaneous emission rate of the pho-togenerated particles with a judicious choice of geometrical parameters thanks to interferences. As a consequence, the diusion length of these particles is enhanced and so may be the thickness and the absorption of some organic active layers. One of our motivations is that authors sometime determine the exciton diusion length by resolving the exciton diusion equation without taking into account the spatial dependence of the recombination rate [30]. However, in doing so, they compute a value which still depends on the environment of the considered material layer. The aim is, on the one hand, to show that by properly managing uorescence losses, one could signi cantly increase the power conversion e ciency. On the other hand, we want to show that not to take into account the strong dependence of uorescence on the environment may lead to a suboptimal cell design and a degradation of cell performances because the presence of radiative losses, however small, signi cantly changes the optimal thicknesses [31, 32].
A Renewable Source of Energy
In this chapter, we discuss the potential of organic solar cells (OSCs). We brie y review the dierent kinds of photovoltaic (PV) technologies before reviewing their advantages and disadvantages with regard to their commercialisation, their impact on the environment and their capacity to reach the TeraWatts challenge by 2050 focussing on the main challenges facing organic solar cells.
2.1 Overview of Solar Cell Technologies
To be absorbed by a material, a photon must have an energy larger than a certain
threshold, namely the bandgap energy, EG. As this gap energy roughly corresponds
to the photogenerated electron energy eV , the excess photon energy prior to absorp-tion is lost through thermalisaabsorp-tion. As a result, there should be trade-o between absorbing a maximum of photons and generating electrons of a maximum nal en-ergy. In 1961, William Shockley and Hans Queisser found that the gap energy that maximises the product of the photogenerated electron and their nal energy, is equal to 1.1 eV, giving a 30% e ciency of conversion between the incoming solar power and the generated electrical power [24], see the description in section 3.2. Since then, researchers have focused on materials with a gap near that optimal value taking into account other practical constraints. As can be seen in Figure 2.1, the bandgap en-ergies of inorganic semiconductors are better matched to the SQ limit. However, as we will discuss later, they suer from a lower absorptivity than organic materials, requiring thicker absorbing layers, high purity and, hence, higher costs.
The most widely used classi cation of PV technologies is based on e ciency and area cost which leads to three dierent generations of solar cells [34, 35, 36]:
G1: Crystalline or wafer-based solar cells of silicon (c-Si), GaAs and III-IV multi-junctions.
G2: Thin- lm solar cells, including a-Si:H, CdTe, and CIGS.
G3: Emerging thin- lm devices, such as kesterite, dye-sensitized, organic, per-ovskite and quantum dot solar cells,
An overview of solar cell technologies is given in Figure 2.2 while the evolution of their record cell e ciencies is shown in Figure 2.3.
G1: First-generation solar cells use semiconducting wafers as substrates and are covered with glass for mechanical stability and protection.
The most common crystalline solar cells are made with single-crystalline (sc-Si) or multi-crystalline (mc-Si) silicon. They are commercialised since 1963 [36] and represented up to 95% of the global annual production in 2017 (33% for sc-Si and 62.4% for mc-Si) [37, 38]. Their record cell e ciencies stand at 26.7% and 22.3%
respectively while they achieve 24.4% and 19.9% in modulea con guration [39, 40].
The lower e ciency of mc-Si is explained by the grain boundaries which hinder the ow of electrons [35]. However, mc-Si is simpler to produce and require a lower energy input. Therefore, mc-Si is cheaper and has a lower energy payback time despite its lower e ciency [41]. Silicon PV also presents an impressively long module lifetime of 20-30 years. Modules are usually guaranteed for a lifetime of 25 years at a minimum 80% of their rated output and sometimes for 30 years at 70% [35]. Finally, despite 60 years of extensive research, challenges remain such as the restricted geometrical
ll factorb (GFF) and the poor absorption of silicium due to its indirect bandgap,
which requires the use of thick, rigid, impurity-free, and expensive wafers [34]. It is worth noting that even if c-Si present a relatively high e ciency, its performance has improved at a very low rate since 1995, see Figure 2.3, indicating the limit of its development.
Gallium arsenide (GaAs) is another active material of the rst generation. Its bene ts from a direct bandgap well matched with the SQ limit and from a very low non-radiative loss rate. Consequently, GaAs achieve the highest performances of any single material system with the e ciency of 29.1% for lab cells and 25.1% for modules [40, 39]. III-V multi-junction solar cells, using frequently GaAs, have also been developed. They consist of a complicated stack of two or more crystalline layers with dierent bandgaps in order to absorb most of the solar radiation and minimise thermalisation losses [34, 35]. With these technologies, researchers have been able to beat all records with e ciencies going up to 47% and 38.9% for 4-junction under concentrated illumination for cells and modules, respectively [40, 39]. However, the other side of the coin is the high cost of gallium arsenide solar cells due to the com-plex epitaxial technology involve in its production and the high cost of GaAs raw materials. Indeed, gallium is scarce and arsenic is toxic. One way to reduce the cost is to use concentration system. However, GaAs and multi-junction solar cells still are prohibitively expensive for large-area 1-sun terrestrial applications and are then reserved for aerospace applications [35, 34].
G2: Second generation solar cells consist of semiconducting thin lms de-posited onto a glass, plastic, or metal substrate. The few microns-thick active layers,
Figure 2.2: Typical solar PV device structures, divided into wafer-based and thin- lm technologies. Figure taken from [34].
compared to a few hundred microns for silicon, reduce the cost of these solar cells while the shorter light path is compensated by a 10-100 times higher light absorption. Typical 2G cells are made of hydrogenated amorphous silicon (a-Si:H), cadmium tel-luride (CdTe) and copper indium gallium diselenide (CIGS). In 2017, they constitute together less than 5% (0.3%, 2.4% and 1.9%, respectively) of the total annual pro-duction [37].
Amorphous silicon is cheap, abundant, and non-toxic and solar cells made of it can be lightweight and exible. Unfortunately, its e ciency decreases once initially illuminated due to the so-called Staebler-Wronski eect (i.e. ligh-induced increase of defect density) [35, 34]. Moreover, its large bandgap (1.7-1.8 eV) is not well matched with the solar spectrum, see Figure 2.1, leading to stabilised record e ciencies of only 14% for cell and 12.7% for two-junction modules with nanocrystalline silicon [40, 39]. This cell e ciency has remained almost constant since 1997, see Figure 2.3.
CdTe bene ts from a direct bandgap of 1.45 eV leading to record e ciencies of 22.1% for cells and 18.6% for modules [40, 39]. It has the easiest and fastest thin lm material deposition process leading to the cheapest module on the market. Neverthe-less, some CdTe drawbacks are remaining such as its high processing temperatures
(500oC), the cadmium toxicity and the scarcity of tellurium [34, 35].
CIGS is a compound semiconductor with a direct bandgap of 1.1-1.2 eV achiev-ing record e ciencies of 23.4% for cells and 19.2% for modules [40, 39]. The rela-tively low bandgap coupled with structural and electronic inhomogeneity leads to low open-circuit voltage. Moreover, this technology suers from high variability in lm stoichiometry and properties, limited understanding of the role of grain boundaries and scarcity of indium for large-scale deployment [34].
materials on the order of tens or hundreds nanometres. Their great advantages are to oer relatively easily tunable bandgaps, low material and manufacturing costs, high weight-speci c power (W/g), good electrical performance and potentially high exibility and visible transparency. However, these technologies are relatively recent and some of them still present relatively low e ciencies and short lifetimes [34, 43, 35]
Kesterite solar cells using copper zinc tin sul de (Cu2ZnSnS4, or CZTS) or its
variants, are a promising alternative to CIGS with a similar direct bandgap of 1.0-1.5 eV. Although CZTS is composed of more abundant and non-toxic elements, it has not been commercialised yet due to its still low e ciency of 12.6% [40, 39]. Indeed, kesterite still suers from some drawbacks as interface instabilities, cation disorders creating bulk defects and grain boundaries [44, 34].
Perovskite present an ABX3 crystal structure with a tunable optical bandgap
from 1.25 to 3.0 eV by cation (A, B) or anion (X) substitution. The most widely in-vestigated perovskite solar cells is the hybrid organic-inorganic lead halide perovskite:
CH3NH3Pb(I,Cl,Br). It is a very new technology which has presented a dramatic
increase in its e ciency, going from 15% to 25.2% in just over a decade [40]. This incredibly high e ciency for a 3G solar cell is thanks to long diusion lengths (up to 1 µm [45]) of both charge carriers (electrons and holes) and a high absorption
coe cientc, allowing a submicrometer thick layer to absorb all light in the visible
re-gion [48]. On the other hand, the module record is only of 11.6% [39]. The remaining challenges for perovkite solar cell improvement include re ned control of lm mor-phology and material properties, high sensitivity to moisture, unproven cell stability, and the use of toxic lead [34]. To avoid the use of Pb, some authors have suggested to rather use tin which also has the advantage of crystallise at room temperature. However, tin comes with other drawbacks such as shorter carrier lifetime due to the increase of the monomolecular recombination rate [49]. Another important issue of perovskite solar cells is the hysteresis in the current-voltage curve [50, 51, 52, 53] which is due, with a rather broad consensus, to low ions migration under illumina-tion [54, 55, 56].
Dye-sensitized solar cells (DSSCs) is the oldest 3G technology. It consists of
a transparent inorganic scaold anode (typically nanoporous TiO2) sensitized with
light absorbing dye molecules (usually ruthenium (Ru) complexes). DSSCs records e ciencies are 11.9% for cells (since 2012) and 8.8% for modules [40, 39]. A great advantage of DSSCs is their ability to make colourful modules while their key chal-lenges are long-term degradation under illumination and high temperatures, low absorption in the near-infrared, and low open-circuit voltages caused by interfacial recombination [34].
Colloidal quantum dot (QD) solar cells use solutions of absorbent nanocrystals. By changing the size of colloidal metal chalcogenide nanocrystals, researchers are cIt was also initialy attributed to an abnormally low bimolecular recombination rate (due to
able to easily tune the bandgap across a wide range of energy, making QD solar cells particularly suitable for multi-junction cells using a single material system. Since 2010, the cell record e ciency has quasi-linearly increase with an impressive 1.5%/year rate to achieve 16.6% today, see Figure 2.3. Moreover, QD solar cells are air-stable and fabricated at room temperature. In order to further increase these cell performances, better understanding of some processes is needed such as ligand exchange chemistries, electronic coupling in QD arrays, and the chemistry and physics of interfaces and bulk defects [57]. Moreover, the low open-circuit voltage is potentially limited fundamentally by mid-gap states or inherent disorder in QD lm [34].
Finally, organic solar cells (OSCs) use organic small molecules or polymers to absorb light. Despite their moderate record e ciencies of 16.6% for cells and 8.7% for modules [39, 40], organic photovoltaic (OPV) technologies present an important number of advantages in addition to already mentioned 3G assets:
Short energy payback times;
Low environmental impacts during manufacturing and operations; Mainly earth-abundant elements;
High defect tolerance facilitating multi-junction cells;
Continuous production of modules thanks to very cheap roll-to-roll printing technique;
Easy integration into buildings and other products as portable or wearable applications oering new market opportunities
Furthermore, there still are many ways to improve OSCs addressing the challenge facing these cells such as photo-, moisture-, temperature- and oxygen-sensibility, in-e cient exciton transport, low large-area deposition e ciency, etc [34, 58, 42]. These challenges will be further developed in Section 2.2.
To conclude this section, we show in Figure 2.4, the comparison between key metrics for the presented PV technologies. Three main observations can be done. Frist, c-Si and the three mentioned 2G technologies are the only commercialised solar cell at a large scale today and have been investigated for more than three decades. As indicated in Figure 2.3, the e ciency of these technologies appears, in general, to have come to a standstill, contrary to the emerging technologies which present encouraging increase. Secondly, 2G and 3G solar cells use 10 to 1000 times less ma-terial than crystalline silicon, thus increasing their speci c power. Finally, modules always exhibit dramatically lower e ciencies than lab cells [34]. Furthermore, noting
that material complexityd increases from one solar cell generation to the next, Jean
dComplexity is de ned here as the number of atoms in a unit cell, molecule, or other repeating
et al. have observed some general behaviours on this discriminant base [34]. Indeed, in most of the cases, increasing material complexity implies to:
increase absorption and then reduce the use of materials and the active layer thickness enabling potential high speci c power;
improve defect tolerance and then reduce manufacturing process complexity notably because the distance travelled by photogenerated carriers is reduced; enhance substrate exibility facilitating the transport of the solar cells and
opening new market opportunities;
allow visible transparency. For instance, thanks to the non-monotonic ab-sorption of their materials, OSCs are able to absorb infrared radiations while transmitting visible light.
Finally, several causes are identi ed to explain the e ciency drop when lab-cell are scaled up to module. First-of-all, the lowest-performing cell limits the module out-put current. As a consequence, even partial shadows by soiling or snow coverage can have disastrous consequences. Other issues are resistive losses due to longer wires, inverter and transformer losses, re ection losses at non-normal incidence or inaccu-rate tracking, e ciency losses due to non-constant irradiance, resistive losses due to larger travel distance of electrons when the cell size is increased while employing electrode grids [34]. Another critical issue is the aperture losses due to the separation space between cells. This reduces the active area of the module and then the GFF. In the case of OPV, the GFF can be as low as 45% [5]. Indeed, interconnecting subcells in OPV modules are patterned into stripes with narrow widths to enable the sheet resistance transparent electrodes to be neglected and to ensure contact areas for series connections between subcells [59]. Finally, sometimes manufacturers sacri ce e ciency to reduce cost by employing cheaper fabrication techniques than in laboratories and by avoiding scarce, expensive, or high-purity materials despite the fact that they induce better e ciencies [34]. We will try in the next sections to give a feeling about this competition between low cost and high performing consid-erations. First of all, we will brie y describe the energetic context, the PV market and what constitutes PV systems' costs.
2.1.1 Economic Considerations
The vertiginous increase of the world energy consumption over the past 50 years is plotted in Figure 2.5. Our energetic system is absolutely dominated by fossil fu-els. Renewable energies represent still less than 5% of global energy consumption and around 22% of the total electricity production (∼ 22 PW). The most developed renewable energy is the hydropower with a share of the renewable energy consump-tion of 56% while solar PV represents only 4.6%. Therefore, solar PV provides still less than 0.3% of the global energy consumption. However, between 2000 and 2018,
Figure 2.5: World primary energy consumption by source, measured in terawatt-hours (TWh). Note that this data does not include energy sourced from traditional biomass, which may form a signi cant component of primary energy consumption in low to middle-income countries. Figure taken from ourworldindata.org
it is illustrated in Figure 2.6-right. During the same period, the European installed
capacity grew from 0.13 GWp to 115 GWp (in 2017) with 36% of it in Germany
alone. Recently, Europe have been dethroned by China (with 133 GWp in 2017) as
the leading region in terms of cumulative PV installation while China leads the PV production since 2007, see Figure 2.6-left. Now, China provides more than 70% of
the annual PV production which has almost reached 100 GWp in 2017 [37]. The
Europe-China transfer may be partially attributed to the 2005 European policy de-cisions encouraging PV system. They have increased the PV demand too quickly, particularly in Spain and Germany, causing a shortage of materials and a lack in the production capacity. As a result, there have been a PV market slowdown which have forced several companies mainly European to close moving the production to China [35].
Figure 2.6: Left: Global annual production by region. Right: Global cumulative PV installations by region including o-grid systems. Figures taken from [37].
South America, Africa, Middle East,Central Asia and Australia [60]. Secondly, they are particularly suitable for non-conventional applications especially o-grid, such as building-integrated photovoltaic, portable devices, solar textiles, semi-transparent solar windows [34, 43]. In particular, according to the European Union (EU) direc-tive 2012/27/EU, PV technologies are expected to contribute to the energy e ciency targets of the EU by improving the energy performance of the building sector [43].
Figure 2.7: Evolution of the market share of solar panels by technology and expec-tation for 2030. Figure adapted from [37] and [38].
Figure 2.9: Levelized cost of energy (LCOE) of various energy sources in 2010 and in 2016. Figure taken from ourworldindata.org
sources even if it remains one of the most expensive. Figure 2.9 shows the levelized cost of energy (LCOE) of various energy sources. The LCOE includes the eect of actual system output, upfront capital costs, xed and variable operation and main-tenance costs, subsidies, nancing, cost of capital, degradation, and replacement. It can be seen as the average minimum price at which electricity must be sold in order to break-even over the lifetime of the project. Finally, despite its relatively high LCOE, solar energy, as well as wind energy, have been taking an increasing share of investment in renewable energy, especially over the last ve years, see Figure 2.10. This trend suggests that solar and wind energy are seen by the investors as the most promising renewable technologies for the future.
2.2 Challenges Facing Organic Solar Cells
In this section, we will focus on the challenges facing OSCs in terms of materials used for active, blocking and conducting layers as well as in terms of stability and roll-to-roll printing technique.
Figure 2.10: Global investment in renewable energy technologies, measured in USD per year. This gure excludes large-scale hydropower schemes. Figure taken from ourworldindata.org
separation of the photogenerated electron-hole (e-h) pair, two dierently doped semi-conductor layers are stacked together forming what is called a homojunction. That creates an electric eld able to guide the electrons toward the cathode and the hole toward the anode. More details are given on inorganic mechanisms in the appendix, Section A. In organic semiconductors, the situation is similar, the carbon atoms are covalently bound with alternating single and double bonds. During their formation, these molecular orbitals split into two kinds: the highest occupied molecular orbital (HOMO, similar to the valence band) and the lowest unoccupied molecular orbital (LUMO, similar to the conduction band). The organic gap energy is then the en-ergy dierence between the HOMO and the LUMO, see Figure 2.11. Contrary to inorganic solar cells, in OSCs, the absorption of a photon does not give rise to a free electron-hole pair but to an exciton, with a binding energy that is large compared to
kBT, typically on the order of 500 meV. This is a fondamental dierence between
or-ganic materials and inoror-ganic absorbers. Note that in perovskite, the exciton binding energy is typically between 20 and 60 meV, so that the majority of excitons disso-ciates at room temperature and we can neglect exciton population [47, 62, 48]. In order to dissociate these excitons, two dierent materials are brought into contact, namely a donor (D) and an acceptor (A), to form an heterojunction. If the energy
oset (∆ELU M O or ∆EHOM O) at the donor-acceptor (D/A) interface is larger than
Figure 2.11: Simpli ed energy level diagram for a donor-acceptor heterojunction.
The processes are: (i) absorption of a photon with energy larger than EG, generating
an exciton in the donor; (ii) transfer of the electron to the acceptor; (iii) diusion of electron and hole towards the electrodes. Figure adapted from [63]
the LUMO of the acceptor. The open-circuit voltage is then the dierence between the HOMO of the donor and the LUMO of the acceptor. These heterojunctions have the great advantage to potentially use materials with complementary absorption spectrum leading to great light harvesting.
2.2.1 Active Materials
Figure 2.12: Schemas of the three common interface geometries. Figure adapted from https://nanohub.org/courses/OED/01a/asset/5258 accessed 18 October 2019.
most used acceptors are small molecules of fullerene and of its derivatives [42, 64]. Nevertheless, we will brie y discuss the advantages of the nonfullerene alternatives. There exist thousands of organic materials, it is then important to choose the donor and acceptor materials wisely, considering the following intrinsic features: 1) complementary high absorption spectrum matched with solar spectrum; 2) suitable molecular energy levels alignment; 3) high crystallinity coming with high charge car-riers' mobility, and 4) good nanoscale phase separation [64]. Indeed, we need to control very precisely the nanomorphology of active lms because the optimal mor-phology requires the donor and the acceptor to be well-phase-separated. Idealy, the domain size have to be of the order of the short exciton diusion lengths (<20 nm) in order to achieve high performances. Authors distinguish three kinds of interface geometries, illustrated in Figure 2.12, namely, planar (PHJ [65]), bulk (BHJ [66]), and ordered (OHJ) heterojunctions. PHJ constrains the cell to be very thin which limits the absorption but also allows some improvement via light management ex-ploiting the Purcell eect, see Chapter 5. In BHJ, the random interface positions allow a bigger number of excitons to reach these interfaces than with a planar inter-face. Nevertheless, the non-radiative losses are also bigger because BHJ lms often exhibit large phase separation domains (>20 nm) or well-mixed compositions with homogeneous morphologies [42] causing dead-ends in the path of electrons and holes towards their respective electrodes [67, 68]. To limit these undesirable BHJ mor-phologies, one has to apply extra treatments, carefully choose the solvents and select the donors and acceptors as a function of their physical and chemical properties, such as the dierences or similarities in the surface energies, solubility parameters, and intermolecular interactions. Another way to address this issue is the OHJ. It consists of an heterojunction with controlled growth of the D-A materials which allows the structuring of one of the active components into vertically aligned structures with the size and periodicity of the order of the exciton diusion length. Hence, OHJs present better e ciencies than highly disoriented heterojunctions. However, they are more di cult to make [58, 69].
uc-tuations. Therefore, OSC e ciency has to be relatively insensitive to the active thickness. Strategies have been deployed to address this requirement such as [64]:
1. uorine substitution leading to high carriers' mobility thanks to a more face-on polymer crystallite orientation with respect to the substrate. This could sup-press charge recombination, facilitate the formation of larger purity domains, and exhibit strong inter chain aggregation.
2. enhancing the intermolecular π − π interactions via small molecules in order to exhibit highly ordered structure and excellent charge transport property. 3. extra treatments such as solvent additive processing, thermal annealing, and
solvent vapour annealing. It will be developed at the end of this section. Recently, relatively high e ciency maintained in a large range of thickness (10% with 660 nm and 9% with 1 µm) have been reported [70].
Donor Materials
As discussed in the beginning of Section 2.1, there is a dilemma between absorbing a lot of photons or absorbing high energy photons or equivalently, between favouring
large short-circuit current (Isc) or high open-circuit voltage (Voc).
On the side of polymer donors, researches have then oscillated between looking
at polymer donors with small bandgaps (EG < 1.5 eV) and high Isc or with deep
HOMO levels leading to high Voc. Starting with the high-EG(>2 eV) poly(phenylene
vinylene) (PPV) derivatives, researches have switched to poly(3-hexylthiophene) (P3HT), which has rapidly achieved an impressive e ciency for this material of
4.37% thanks to its highly crystalline structure and its lower EG of 1.9 eV [71].
However, its EG is mismatched with the sun spectrum which allows P3HT to only
harvest 22.4% of the available photons and limit further improvement of this ac-tive material [58]. The bandgap has then even decreased to 1.4 eV and 1.3 eV with poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H -cyclopenta[2,1-b;3,4-b']-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) or diketopyrrolopyrrole-based (DPP) so-lar cells. With these materials, authors have also focussed in increasing the crystalline structure in order to provide high charge carriers' mobility for both electrons and
holes (1.5 and 0.8 cm2V−1s−1 with DPP, respectively). These high mobilities allow
larger thicknesses up to 220 nm and then higher thickness insensitivity facilitating printing techniques. The performance of these cells have laboriously achieved 9.4%
because they are intrinsincly limited by their low Voc [72, 42]. Intermediate EG
ma-terials (1.5 eV<EG<1.8 eV) have then been investigated such as
Figure 2.13: Schema of the active molecules which are used in the simulations of this thesis. Left: a small-molecule donor. Centre: polymer donors. Right: acceptors.
for the lm formation. Finally, polymer donors also suer from non-constant molec-ular weight, solubility, polydispersity, and purity [42].
On the side of small molecule donors such as tetraphenyldibenzoperi anthene (DBP), their e ciency culminate to 10.29% [74] and they present several advantages such as reproducibility, high solubility, well-de ned structures, easy puri cation and higher molecular precision leading to a high charge carriers' mobility and making them suitable for printing technique [42]. On the other hand, they can suer from a deterioration of the device performance due to impurities, even at a very low concentration [75].
Acceptor Materials
PC60BM and PC70BM are the main acceptors used in high-performance devices [64].
Indeed, they are compatible with a lot of dierent donors, they present high electron mobility and ultrafast charge transfer at D/A interfaces and they have good solu-bility in common organic solvents. However, these fullerene acceptors suer from a
poor absorption in the visible region despite signi cant improvements with PC70BM.
Moreover, the LUMO level is too low even if attaching side chains can partially over-come this issue [42].
a wide range of new opportunities [79]. These results have opened the perspective of all-polymer OSCs in order to bene t from their following advantages: 1) high ab-sorption coe cients in the visible spectral region; 2) excellent mechanical exibility; 3) the excellent photochemical, thermal, and humidity stability; and 4) suitability for green solvent and extra-treatment-free fabrication technologie [42, 64]. However, all-polymer solar cells only achieve so far maximum 10.1% of e ciency [80] which is still behind those of small-molecule OSCs due to the large-scale phase separation [42, 64]. In our simulations, we have considered three dierent donor materials namely, P3HT, PTB7 and DBP, systematically blended with fullerene acceptors. Figure 2.13 illustrates the considered molecules. Our choice was constrained but also relevant. Constrained because the physical data required for our simulation, as the wavelength dependance of the refractive index, are not easy to get. For the majority of the considered molecules, they were provided to us by Prof. Jordi Martorell from ICFO, Barcelona. Relevant also because PTB7 is a high-performing polymer while P3HT, despite its relatively low performances, is one of the most extensively used and best understood polymers [77]. Moreover, P3HT are still in the center of researches about OSC fabricated via roll-to-roll printing techniques. Finally, DBP is a small-molecule donor, which is notably used in PHJ con guration.
Economical Considerations
The cost of OSC materials is, of course, an important factor in their commercialisa-tion potential especially since it represents a large part of the total module cost [64]. It is mainly determined by the achieved e ciency, the number of synthetic steps, the magnitude of reactions and the raw material costs [64]. In Table 2.1, we show the number of synthesis steps and market prices of the molecules which are used in
the simulations of this thesis from Ossilia supplier. For 1 m2 of PCBM:P3HT based
OSCs, one has to use typically a few more than 0.2 g of each active materials [81]. The best active materials are approaching the key target in order to be nancially Table 2.1: Price from Ossilia supplier (21/10/19) and number of synthesis steps [64] of the various active materials considered in this thesis.
Materials Synthesis steps Prices [e/g]
P3HT 3 392
PTB7 9 2170
DBP Not found 1030
PC60BM 3 414
PC70BM 3 1460
With a lower e ciency, the P3HT still exhibits a great commercialisation po-tential [64] because it is a low-cost material, relatively stable, earth-abundant and readily scalable due to its straightforward synthesis and its compatibility with high-throughput production techniques [77].
Processing and Extra-Treatments
Controlling the BHJ nanomorphology is crucial to ensure e cient exciton dissocia-tion. While many strategies have been developed to this end, in this section, we will focus on three of these namely, thermal/solvent annealing, choice of solvents and pro-cessing additive. However, these extra treatments have a cost and come with compli-cated fabrication process, therefore, we will also brie y discuss extra-treatment-free OSCs. Finally, note that this section in mainly inspired by the reviews [42, 64].
Thermal annealing consisting to apply a temperature above the glass-transition temperature of the polymer, has proven its e cient capability to induce polymer crystalline domains and then increase carriers' mobility. Moreover, this kind of post-treatment improves the thermal stability. Solvent annealing or solvent vapour an-nealing is another technique consisting to apply solvent vapour to the BHJ which will develop a well-aligned morphology. One of the great advantages of this technique is its ambient conditions processing. However, it is not compatible with the printing process due to its long processing time.
The organic solvents also play an important role in the nal lm morphology. Indeed, residual solvent molecules interact with active materials during the transition from solution to solid lms. The strength of these interactions depend on solvent properties such as the boiling point, solubility, vapour pressure, polarity, miscibility and rate of solvent evaporation. One of the most used solvent is chlorobenzene because it leads to smooth surface morphology, good interchain interactions and good solubility of fullerene. Unfortunately, these aromatic solvents and their non-aromatic counterpart (such as chloroform) have a negative impact on the environment and on human health. Therefore, environment-friendly green solvents have been investigated such as hydrocarbons, benzaldehyde derivatives, tetrahydrofuran, and its derivatives. Moreover, non-aromatic additives, aromatic additives or coblend additives can be added to the solvent host in order to tune some properties such as the matching with donor materials, the active material solubility, the solvent volatility (to reduce the drying time), the active material solvent interactions (to enlarge the D/A interface area and enhance the phase separation) and the molecular orientation relative to the donor/acceptor interfaces. However, residual additives in the BHJ lms can act as hole traps and reduce the device stability. Therefore, additional processes to remove residual additives are required.
en-vironmental burden. Finally, most solvents are not suitable for TeraWatt scales [83].
2.2.2 Blocking Layers
Once the free carriers are created at the donor-acceptor interface, they migrate to the electrodes through the selective layers called interfacial, buer, transporting or blocking layers (BL), see the cell diagram presented in Figure 2.14. These cost-eective, transparent, highly roll-to-roll compatible layers ful l several functions such as charge-selectors, surface modi ers, stabilisers and optical spacers [42]. The selec-tivity function allows changing the device polarity providing exibility in the cell architecture and then facilitating design such as inverted structures and tandem structures. BLs can modify the work function (WF) of the electrodes in order to match the energy-level of the target electrode and the LUMO or HOMO. This garan-tees a good ohmic contact between the photoactive layer and its electrode, which minimises the interfacial contact resistance. Moreover, these blocking layers ensure an encapsulation role preventing diusion of undesirable metal particles from the electrodes to the active layer. It also prevents water, oxygen and UV degradations during fabrication and operation, leading to more stable OSCs [42]. Finally, the BL optical function consists in maximising the distribution of sunlight intensity within the active layers by controlling the BL thicknesses and also in isolating the active region from the divergence of the exciton decay rate in the electrode vicinity due to dissipation [84, 31]. This will be detailed in Chapter 5. Finally, the cell e ciency has to be insensitive to the BL thicknesses to facilitate printing manufacture. Hole-Blocking Layers
A large number of materials have been used as hole-blocking layers (HBL) such as
transition-metal oxides (as TiOx and ZnO), water-soluble or alcohol-soluble polar
polymers (polyelectrolytes), and functional fullerene derivatives. Their thicknesses are often ultrathin (5-10 nm) in order to provide good ohmic contact for good elec-tron conductivity and collection. However, to be suitable with large-area printing technology, thicker HBL are needed without e ciency loss, ideally on a wide range of thicknesses. Researches have then been made about enhancing the conductivity via n-type self-doping or blend-doping strategy as well as self-assembly HBL [85] al-lowing these layers to be larger than 60 nm [86]. Note also that larger BL thicknesses have already been reported in organic light emitting diode (OLED) [87].
Figure 2.14: Schema of a simple organic solar cell. Depending on the fabrication the sun can be either to the right or to the left. HBL: hole-blocking layer, EBL: electron-blocking layer.
Electron-Blocking Layers
One of most widely used electron-blocking layer (EBL) material is the organic poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonicacid) (PEDOT:PSS). This one bene ts from its high work function, its good conductivity and high transmittance. However, its acidity and hygroscopicity lead to undesirable eects on OSC stability although this can be partially resolved with special polymer doping (such as na on doping) or with the addition of various components such as polymer additives, solvent additives, metal oxides and nanoparticles [64].
Metal oxides as EBL (such as MoO3 and V2O5) have also been developed (as
well as the associated solution processing) in order to increase the device stability and the HBL conductivity [42]. Graphene oxide is another candidate for more stable EBL [88].
2.2.3 Electrodes
Organic solar cells require low-cost electrodes which combine exibility (small bend-ing radius), high transparency (particularly in the visible region), good conductivity
(low sheet resistance, Rsh) and compatibility with roll-to-roll printing techniques.
However, e ciency may be sacri ced if the overall cost or energy input were re-duced. Indeed, electrodes typically represent 25% of the module cost and, as we will develop further in Section 2.3.1, 40 to 70% of the embedded energy [89]. Finally, note that this section is mainly inspired by the review [42].
Metal Based Electrodes
The current most commonly used electrode is indiun-tin oxide (ITO) despite other transparent conducting oxides exist such as aluminium-doped ZnO (AZO), or indium-doped ZnO (IZO). They have high transmittance and excellent conductivity. How-ever, they suer from several drawbacks such as 1) their fragility and rigidity which make them unsuitable for exible OSCs, 2) their low stability with organic mate-rials and 3) their high cost (raw material and manufacturing). Moreover, indium is also a scarce element and ITO require energy-intensive vacuum-based sputtering
high-temperature annealing process (>300oC) in order to garantee low sheet
and multi-layered hybrid electrodes as well as organic electrodes such as conduct-ing polymers, carbon nanotubes and graphene. Nevertheless, some metal oxides are
still in the race. For instance, SnO2 is an attractive candidate to replace In2O3 in
the indium-gallium-zinc-oxide (IGZO) system, and could thus lead to low-cost and earth-abundant transparent conductive oxide material [90].
Ultra Thin metal lms (UTMF) of Ag or Au present excellent conductivity, good stretchability and are more stable than ITO and those with Al [88]. They have been investigated in the past decade [18, 19, 20, 21, 22]. Moreover, it has been pointed out that today silver is the best material and the only metal suitable for roll-to-roll processing of electrodes [91]. However, they suer from 1) high-cost, 2) unstable metal migrations decreasing the cell stability and 3) poor transmittance coupled to high re ection leading to undesirable absorptions in the visible region. To bene t
from the excellent conductivity of the UTMF and then from a low Rsh(10-20 Ω/sq),
UTMF thicker than 8-10 nm are needed. However, this comes with the decrease of its transparency.
An answer of these weaknesses is the multi-layered hybrid electrode which consists of a stack of three-layer, dielectric/metal/dielectric (DMD) where each dielectric
layer can be made with either an oxide such as ZnO, ITO, TiOx, MoOxor polymers.
Thanks to the interferences caused by the dierent refractive index, the incident light is transmitted more easily through the DMD structure. Indeed, this dielectric layer, if it is well chosen, acts as an anti-re ection layer. We will investigate, in Chapter 4, the gain coming from the use of a multi-layered hybrid electrode, namely the Two-Resonance Tapping Cavity [23], compared to an ITO electrode. Furthermore, to address the conductivity issue, authors have presented several ways to overcome the dewetting problem of metal atoms on the oxide layer and found to improve the transmittance without conductivity loss. For example, they replace Ag by its
oxidised form, AgOx or they used MoOx, TeO2 or polymers such as PEDOT:PSS,
as dielectric layers.
Another way to avoid ITO is the recently investigated metal nanowires
(par-ticularly in silver) which present high transparency, low Rsh, good exibility and
good mechanical stability. By using a non-compatible roll-to-roll methods, OSCs with Ag-nanowire have obtained better performance than with ITO. However, fur-ther improvements are needed to obtain satisfactory metal nanowires with solution processability. Moreover, short-circuit losses are often observed with Ag-nanowires due to the penetration of wires through the solar cell to the opposite electrode.
Finally, metal grids present same advantage as those of metal nanowires and are printing-compatible. Moreover, high transparency can be ensured by the wide
spacing between metal lines but it comes automatically with an increase in Rsh.
Organic-Based Electrodes
On the side of organic electrodes, recent progress has been reported. In particular, highly doped conducting polymers present good charge-transport properties (high
conductivity and low Rsh). In this kind of electrodes, PEDOT:PSS is the common
employed material thanks to its high transmittance in the visible region contrary to other conducting polymers. The relatively low conductivity of PEDOT:PSS can be increased via strong acid treatment but then it cannot - or with di culty - be applied to most underlying plastic substrates. Another technique consists in making a thicker layer but the counterpart is to decrease the transparency. As a result, further improvements are still needed in order to maintain high conductivity and high transparency in roll-to-roll printing process. Finally, the undesirable hygroscopic eect, already mentioned in the case of HBL, is exacerbated when PEDOT:PSS act as an electrode.
Graphene electrodes and carbon nanotubes are other organic-based electrodes which bene ts from high conductivity and high transparency in lm form. In par-ticular, graphene presents extremely good optical properties such as extremely high transmittance (such as 97.7%) and bene ts from high chemical and thermal stabil-ities, high conductivity and good exibility combined with very low cost. However, the fabrication processing of graphene electrodes as well as carbon nanotubes, are not yet well compatible with roll-to-roll printing technique.
To summarise, Table 2.2 shows the best demonstrated properties of transparent electrodes and their share in the module cost. These lead to a module cost per
Watt-peak of around 4 e/Wp for PEDOT:PSS, metal grid and metal nanowires while it
is around 6 e/Wp for ITO. Despite the lower e ciency achieved with PEDOT:PSS,
it remains nancially competitive thanks to its extremely low cost. Finally, due to the very high cost of carbon nanotubes, the associated module cost stands at around
16-18 e/Wp.
Table 2.2: Best electrical and optical properties and the electrode share in the module cost for various exible transparent electrodes. Data extracted from [89].
Electrode Rsh (Ω/sq) Transparency (%) module cost share (%)
Sputtered ITO 60 79 24%
PEDOT:PSS 63.3 67.4 <1%
Metal grid 1 92 34%
Metal nanowires 8 80 10%
Carbon nanotubes 40 70 69%
Figure 2.15: Typical evolution of the device lifetime. Burn-in time being tipically from few hours [95] to hundreds [92]. Figure taken from [92].
2.2.4 Device Stability
One of the main limiting factors to commercialise OSCs is their short lifetime. The average cell lifetime varies typically between 2.5 and 6.2 years depending on active materials which are used (here, P3HT and PCDTBT respectively) [92]. Recently, an expected cell lifetime approaching 10 years has even been reported [93]. However, the module lifetime remains only around 2 years (versus 20-30 years for c-Si) which is already a promising result considering that 10 years ago, typical device lifetimes were in the range of a few days to weeks [94]. Nevertheless, we are still far from 5-10 years lifetime and 7% e ciency target [82]. Note that e ciency of 7% have already been achieved in large device areas, the limiting factor for future industrial production is then the remaining insu cient stability [88]. Furthermore, if we consider the total energy that a solar cell can potentially deliver during its operation life then a mediocre solar cell with a long operational lifetime easily outperforms over time a solar cell with high e ciency but short lifetime [5]. Encouragingly, OLEDs made with small molecular weight semiconductors have already presented near 5 years lifetimes [58]. Finally, note that this section in mainly inspired by the reviews [42, 88, 64].
Origins of Instabilities
There are many possible origins for OSC instabilities occurring in various cell layers and interfaces such as UV deterioration, mechanical stress, recrystallisation, ther-mal degradation as well as water, oxygen and air contamination causing chemical reaction. Moreover, there is an initial sharp e ciency loss (referred to as burn-in loss) which is a major contributor to the performance degradation and whose ori-gins are uncertain. This burn-in loss is then followed by a slowly and linearly decay regime. The device lifetime is usually de ned as the time after which the e ciency has decreased by 20% compared to its initial stabilised value, see Figure 2.15.
