solar cell nanotechnology (tiwari/solar) || colloidal synthesis of cuins 2 and cuinse 2 nanocrystals...

97

Atul Tiwari, Rabah Boukherroub, and Maheshwar Sharon (eds.) Solar Cell Nanotechnology, (97–116) 2014 © Scrivener Publishing LLC

3

Colloidal Synthesis of CuInS2 and CuInSe2 Nanocrystals for Photovoltaic

Applications

Joanna Kolny-Olesiak

Energy and Semiconductor Research Laboratory, Department of Physics, Carl von Ossietzky University of Oldenburg, Oldenburg, Germany

Abstract Semiconductor nanocrystals attract scientifi c attention due to their size- and shape-dependent properties. They are interesting candidates for photovol-taic or light-emitting applications. However, the best studied nanomaterials contain toxic elements like cadmium or lead, which severely restricts their industrial applications. That is why I-III-VI semiconductor nanocrystals, which are much less toxic, but possess similar optical properties, are gaining scientifi c interest. The number of reports about colloidal synthesis of I-III-VI nanocrystals is steadily increasing. However, controlling the size, shape, composition and optical properties of these materials still remains a chal-lenge. This chapter summarizes the experimental methods of colloidal syn-thesis of CuInS2 and CuInSe2 nanocrystals, and the possibilities to control the properties of these nanomaterials by varying the conditions of their colloidal synthesis. In the last part of the chapter a short overview of the applications of CuInS2 and CuInSe2 nanocrystals in solar energy conversion is given.

Keywords: Colloidal synthesis, I-III-VI semiconductor nanocrystals, CuInS2, CuInSe2

3.1 Introduction

Optical and electrical properties of semiconductor nanocrystals depend not only on their composition and the crystallographic

*Corresponding author: [email protected]

98 Solar Cell Nanotechnology

structure , but also to a great extent on their morphological param-eters, such as size and shape [1]. This opens up new possibilities to control and design the properties of nanomaterials and pushes forward efforts to develop novel procedures to generate nanoscale particles. That is why colloidal synthesis of nanocrystalline materi-als, which allows for precise control of the morphological param-eters of nanocrystals, is a rapidly developing fi eld of research [2–4]. During the last two decades immense progress has been achieved in the synthesis of nanocrystals and, consequently, in controlling the properties of nanocrystalline materials. However, the main focus of the researchers laid for a long time on the II-VI or IV-VI semiconductor materials, which exhibit interesting optical proper-ties, but have a severe drawback of containing highly toxic heavy metals, such as cadmium or lead. Therefore, for large-scale applica-tions more environmentally friendly and less toxic alternatives are needed, which have similar properties. Appropriate candidates for applications in solar energy conversion were found among I-III-VI semiconductors. Especially the ternary copper-based materials, CuInS2 (CIS) and CuInSe2 (CISe), possess bandgaps, which match well the solar spectrum (1.4 eV and 1.04 eV respectively). Their opti-cal properties can be further adjusted by alloying them with ZnS or exchanging some fraction of indium ions with gallium. Because of their optical properties, exceptional radiation stability and high extinction coeffi cients in the visible range of the solar spectrum, alloys such as Cu(In,Ga)(S,Se)2 already fi nd application in thin-fi lm solar cells , where they exhibit power conversion effi ciencies up to 20% [5]. The fabrication of this kind of photovoltaic device requires, however, expensive production techniques (high investment costs, vacuum, high temperature ). Therefore, extensive research activities are focused on fi nding low-priced alternative manufacturing meth-ods of thin-fi lm solar cells. Here, the application of colloidal nano-crystals opens up new possibilities to generate thin layer devices using solution-based processes. Furthermore, new kinds of solar cells can be obtained from colloidal nanoparticles, which are not accessible with standard vacuum-based techniques, e.g., hybrid organic-inorganic solar cells (see Section 3.3.2).

Properties, which are important for successful application of inorganic nanocrystals in photovoltaic devices, can be adjusted by changing the parameters of the synthesis . The factors infl uencing the growth and shape control of binary materials have been exten-sively studied during the last years [2]. Much less is known, yet,

Colloidal Synthesis 99

about the growth of ternary and quaternary nanomaterials, such as I-III-VI semiconductor nanocrystals. The presence of two differ-ent cationic precursors with different reactivities makes planning the synthesis more complicated than in the case of binary materi-als. The formation of undesirable side products, which can have negative effects on the application of these materials, is frequently observed. On the other hand, in contrast to binary materials, the composition becomes another parameter, which can signifi cantly infl uence the properties of ternary or quaternary nanocrystals. This chapter summarizes the recent achievements in the fi eld of colloi-dal synthesis of CIS, CISe and their alloys. In particular, the possi-bilities of controlling parameters are discussed, which are relevant for the application of these materials in photovoltaics.

3.2 Synthesis of CuInS2 and CuInSe2 Nanocrystals

The synthesis of colloidal nanocrystals in organic high-boiling sol-vent turned out to be an effective way to produce a variety of highly crystalline semiconductor and metal nanostructures with control-lable size and shape [2–4]. In order to obtain uniform particles, it is important to separate the nucleation from the further growth step. An effective way to achieve this is the so called hot-injection- technique , which was developed in the early nineties for the syn-thesis of CdSe nanoparticles [6], and meanwhile, adapted for many other materials. For more details on this kind of synthesis the reader is referred to review articles [7–10]. Briefl y, in this method two solu-tions are prepared, which contain precursors for the reaction. One of them is heated to a high temperature (typically between 200°C and 300°C). Rapid oversaturation and, consequently, burst nucle-ation is achieved by the injection of the second solution into the fi rst one. The nucleation step leads to a fast consumption of the mono-mers; therefore, the oversaturation of the solution lasts only for a short time period and all the seeds are formed approximately at the same time. This reduces the differences between the particles; they all form and, then, grow under nearly identical conditions. This method, however, is not easy to scale up. Heating-up procedures , on the other hand, in which all the precursors are mixed together at room temperature and subsequently heated to the required reaction temperature, can be scaled up without problems. There drawback


is frequently the lack of precise control over the nucleation of the particles, and resulting broad size distribution of the nanoparticles. Both methods were already successfully applied for the synthesis of I-III-VI semiconductor materials.

Common starting materials for the formation of I-III-VI semi-conductor nanoparticles are metal salts (e.g. chlorides, iodides, acetates, acetylacetonates) and chalcogene compounds, such as thiols, thiourea, selenourea, trioctylalkyl selenide, or elemental sulfur and selenium. When brought together at high temperature , these chalcogene and metal precursors (also referred to as mono-mers) react rapidly with each other to form binary or ternary compounds. This reaction can be slowed down and controlled by the use of ligand molecules. Their role will be discussed in the following section.

3.2.1 Ligand Shell and Colloidal Stability

One of the biggest advantages of colloidal nanocrystals as alterna-tive materials for photovoltaic applications is their processability from solution. Because of their ability to form stable colloidal sus-pensions with common solvents, nanoparticles can be handled as “normal” chemicals: they can be dried to form powders and redis-persed again. This is achieved by covering the inorganic core of the nanoparticles with an organic ligand shell during the synthesis .

In the colloidal synthesis of inorganic materials the formation of the nanocrystals takes place in presence of organic stabilizer molecules. Compounds such as thiols, carboxylic acids, phospine oxides, phosphonic acids or amines, are the most common ligands . All of them have a functional group with an electron pair that can bind to the metal ions on the surface of the nanocrystals. They play an important role during the formation and the growth of the nanocrystals, because they not only stabilize the crystallites in solution during the synthesis and prevent them from fast, uncon-trolled growth, but also bind to the monomers and regulate their activity. Three kinds of effects infl uence the reactivity of the pre-cursors . High concentration of ligands reduces the activity of the monomers, as well as ligands, which form strong bonds with the precursors; fi nally, steric effects play a role: bulky ligands, which reduce the mobility of the monomers, also lower their activity. All this strongly affects the whole nucleation and growth process as will be discussed in more detail in the following sections.


Another important function of the ligands is providing chemical and colloidal stability of the particles. The ligand shell prevents the nanocrystals from aggregation . In unpolar solvents this is mainly due to steric effects, i.e., the repulsion of the carbon chains of the ligand molecules. Because the surfactant layer is strongly responsi-ble for the interaction of the nanocrystals in colloidal suspension, it also plays an important role in self-assembly processes of nanocrys-tals. From the point of view of colloidal stability, organic compounds with long carbon chains are especially suitable ligand molecules, because they effectively prevent aggregation of the nanoparticles. They also turn out to be the most suitable ligands for most syn-thetic procedures. However, a dense and thick ligand shell has disadvantages for the application of the particles in photovoltaics, where charge separation and charge transport are important steps of electrical power generation [11]. The organic ligand layer is an insulator, which severely interferes with these two elementary pro-cesses. Therefore, usually a ligand exchange is conducted after the synthesis , in which the original surfactant molecules are replaced with less bulky ones. Ideally, these ligands should still prevent the particles from aggregation, while enabling charge transport. In practice, small organic ligands frequently do not stabilize the par-ticles in a suffi cient manner, which leads to the formation of small aggregates in the colloidal suspension. This diminishes the process-ability of the particles. Another problem related to weak stabiliza-tion is the presence of defects on the surface, which act as traps for charge carriers and deteriorate the performance of the device [12]. The amount of surface defects, can, however, be reduced by cov-ering the surface with a layer of another inorganic material (see Section 3.2.5).

Even in cases, where the organic ligand shell is removed prior to application, such as in thin fi lms formed through thermal treatment of a layer of nanoparticles processed from colloidal suspensions, the presence of bulky ligands causes problems. Carbon usually can-not be removed completely from the inorganic fi lm; furthermore, cracks form because of the change in the volume resulting from the removal of the organic material.

During the last years, fully inorganic surfactants have also been developed, which can also provide colloidal stability of nanocrys-talline materials in polar solvents and have benefi cial effects on the charge transport in superlattices composed of these semiconduc-tor nanoparticles [13–16]. Even though the strategy of producing


nanomaterials with inorganic ligands has a lot of advantages, only a few examples can be found until now of the use of these kinds of ligands for I-II I-VI semiconductor nanocrystals [17].

3.2.2 Adjusting the Reactivity of the Precursors

The synthesis of ternary I-III-VI semiconductor nanocrystals requires a reaction between a chalcogen source and two differ-ent cations, e.g., Cu+ and In3+. A challenge for designing synthetic procedures for these particles arises from the fact that the two cations have different chemical properties: copper is a soft Lewis acid, while indium and gallium are hard Lewis acids. Selenium and sulfur are soft Lewis bases. Therefore, both chalcogens pref-erentially react with copper, which leads to the formation of cop-per chalcogenides. To avoid this, the reactivity of the cations has to be adjusted by using appropriate ligands . Among the common ligands used in the colloidal synthesis, thiols or phosphines are soft Lewis bases, while amines or carboxylic acids are hard ones. Thus, the reactivity of the cations cannot be controlled easily with one kind of organic stabilizer only, unless it is applied in a very high concentration [18]. A precise control of the activity of both cations can be achieved by using more than one kind of stabilizer, e.g., a thiol (a soft Lewis base) for copper and a carboxylic acid (a hard Lewis base) for indium [19]. Another possibility for overcoming the problems connected with the different reactivities of copper and indium is the use of single molecule precursors [20–23] containing both metals, and releasing them simultaneously during the reac-tion. In some reactions benefi cial effects on the stoichiometry and phase purity of the resulting nanoparticles were observed, if the indium and the sulfur source were mixed together prior to addition of copper precursor [24]. In this way an In-S bond could be formed before the reaction with copper. This shows that not only the right choice of the precursors, but also the way in which the synthesis is carried out, has a strong infl uence on the properties of the resulting particles.

Copper reacts faster with sulfur or selenium than indium or gallium, which frequently results in the formation of copper sul-fi de or copper-rich CIS nanocrystals in the beginning of the reac-tion [19, 24–28]. At later growth stages the element ratio in the nanoparticles can change towards higher indium contents, due to the copper depletion of the solution [25]. Thus, controlling


the size and the composition of the particles independently from each other is not always possible. Obtaining stoichiometries close to the ideal 1:1:2 atomic ratios requires a precise control of the activities of all the precursors . This was achieved by Li et al. [18] in a reaction taking place in dodecanethiol as solvent and sulfur source. The large excess of the anionic precursor prevented the formation of copper interstitials, which seem to be responsible for the changes in the stoichiometry during the growth process.

The formation of copper sulfi de particles in the beginning of the synthesis is usually considered a problem, and in most of the synthetic procedures the conditions are chosen so that this can be avoided. However, the formation of copper sulfi de particles may also have advantages and was successfully used for shape control of CIS nanorods (see next section) [26–29].

3.2.3 Shape Control

The shape of semiconductor nanocrystals depends on many dif-ferent factors. Particularly essential are the ligands used during the synthesis , because they can delay the growth of some facets of the crystals, and, thus, infl uence their shape. Furthermore, they infl uence the crystallographic structure , and consequently the symmetry of the crystallites (see Section 3.2.4), which strongly affects the fi nal shape. Also, the temperature plays an important role in the shape control because it can infl uence the crystallo-graphic structure of the nanocrystals. Furthermore, it depends on the temperature if the growth takes place in the thermodynamic or in the kinetic regime. Uniform facetted crystals formed under thermodynamic control (high temperature, low monomer fl ux towards the surface of growing nanocrystals) can serve as build-ing blocks for ordered superstructures of nanocrystals. They are, therefore, suitable for applications which require the formation of layers of one kind of material.

The fi rst report about successful size control of CIS nanocrystals was published in 2009 by Xie et al. [19]. Quasi-spherical particles with sizes between 2 nm and 16 nm were synthesized and the stoi-chiometry of the particles was controlled by adjusting the reactivities of the cationic precursors . In the same year, uniform CIS nanodiscs with wurtzite structure were synthesized by heating metal chlorides and thiourea in oleylamine [30]. Analog reaction with selenourea results in the formation of uniform CISe trigonal pyramides (Figure


3.1a) [31]. Uniform CISe nanorings (Figure 3.1b) were obtained in a reaction of CuCl, InCl3 and Se in oleylamine in the presence of trioctylphosphine [32]. Trioctylphosphine proved necessary for the shape control in this case; however the underlying mechanism is not clear. A non-injection method for the synthesis of CIS pyramids with sizes from 3.5 nm to 7.3 nm with chalcopyrite structure was developed by Zhong et al. [25]; two surfactants, dodecanethiol and oleic acid, were applied in this synthesis to control the activity of the monomers, however, together with the variation of the size, changes in the composition of the particles were observed. Another non-injection method, which applies dodecanethiol as solvent and sulfur source, yields particles with pyramidal shape, and without changes in the composition during the growth process [18].

Under kinetic control (low temperature , high monomer fl ux), shapes are accessible, which are thermodynamically less stable, such as elongated nanocrystals. These shapes are particularly suit-able for application in hybrid solar cells, which are composed of a conductive polymer and inorganic semiconductor nanocrystals. Conductive polymers have a good hole conductivity; blending them with inorganic nanocrystals increases the electron conduc-tivity of the active layer. This is especially effi cient if nanorods or nanowires are applied. However, nanorods tend to form aggre-gates, which, in turn, have a negative infl uence on the morphology of the polymer-nanorod-blend and impair the device performance. Branched nanocrystals (e.g. tetrapods) do not have this drawback; in general, they are better soluble than nanorods, while still offering pathways for effective electron transport through the active layer.

(a) (b)

50 nm 75 nm

Figure 3.1 Shape control of CISe nanocrystals: a) trigonal pyramids (adapted with permission from: J. Am. Chem. Soc., 2009, 131 (9), pp 3134–3135) Copyright (2009) American Chemical Society. and b) nanorings (adapted with permission from Nano Lett., 2008, 8 (9), pp 2982–2987) Copyright (2008) American Chemical Society.


In the synthesis of CIS nanoparticles a new strategy of shape con-trol has been developed in which the formation of Cu2-xS- CIS hybrid nanostructure plays a central role [26–28]. If the reaction conditions are chosen so that Cu2-xS nanocrystals are formed in the beginning of the reaction, these particles can serve as a starting point for the growth of CIS nanostructures (Figure 3.2). Hexagonal CIS and Cu2-xS have the same anion lattice; furthermore, the mobility of copper within the particles is relatively high at elevated temperatures. This facili-tates the formation of Cu2-xS-CIS hybrid nanostructures. During their growth the nucleation of CIS takes place within the Cu2-xS nanocrys-tals. When the CIS seeds reach a certain size, the Cu2-xS-CIS hybrid structure reorganizes to form two discs connected together. During the next growth stage both phases grow; however, the growth of the CIS phase takes place only at the interface between the two phases. Thus, the shape of the CIS particle can be controlled by changing the size and the position of the Cu2-xS part of the hybrid nanostructure. In the last stage of the reaction the size of the Cu2-xS particle decreases, until, fi nally, pure CIS nanocrystals are left (Figure 3.2d) [27].

(a)

(d) (e)

(b) (c)

20 nm

20 nm 20 nm 20 nm

20 nm

Figure 3.2 Growth process of CIS nanorods (adapted with permission from J. Am. Chem. Soc., 2010, 132 (45), pp 15976–15986. Copyright (2010) American Chemical Society).


Controlled aggregation resulting in the formation of multipods or nano-networks can be induced by destabilizing nanorods at later growth stages. This can be obtained if the ligands and the solvent react with each other to form a compound which cannot stabilize the particles. This reaction must be, however, slower than the forma-tion and the growth of the nanocrystals to avoid their uncontrolled growth and to maintain shape control . Also, other shapes such as fl ower-like particles can be obtained if the stabilization of the parti-cles is not suffi cient. To minimize the surface energy, thes e particles attach together to form small, fl ower-shaped aggregates [33].

3.2.4 Crystallographic Structure

Bulk CIS and CISe both crystallize in chalcopyrite structure at room temperature . Cation disordered polymorphs, wurtzite (WZ) and zincblende (ZB) are observed in bulk only at high temperatures. In contrast to this, nanocrystalline CIS and CISe are stable at room temperature in all three modifi cations. Structures with both cations sharing one lattice site allow a fl exibility of stoichiometry , which enables tuning the Fermi energy of the nanostructure in a wide range; this is benefi cial for device fabrication. Theoretical calcula-tion for wurtzite CISe particles show that this structure has advan-tages for being used as active layer of photovoltaic devices with improved absorption and electron transport, compared to chalco-pyrite structure [34].

In colloidal synthesis of I-III-VI semiconductor particles the crys-tallographic phase of the product can be controlled by the presence of specifi c ligand molecules and by the temperature of the reac-tion. Some ligands can stabilize structures, which are otherwise not thermodynamically stable. This has been observed, e.g., in II-VI semiconductor nanoparticles and can also be applied to generate I-III-VI semiconductor nanoparticles with the desired crystallo-graphic structure .

The fi rst synthesis of zincblende and wurtzite CIS nanoparticles was described by Pan et al. The crystallographic structure was controlled by the use of oleic acid (ZB) or dodecanethiol (WZ) as capping agent, under otherwise identical experimental conditions. Other examples of controlling the crystallographic structure fol-lowed in the next years, which not only describe the synthetic pro-cedures, but also attempt to give an explanation for the observed results [23, 28, 35]. The formation of thermodynamically more stable


chalcopyrite and zincblende particles was usually observed in slower reactions, taking place under thermodynamically controlled growth conditions. Fast reactions and the presence of thiols in the reaction solution favor the generation of wurtzite particles [28].

Also, in the synthesis of CISe nanoparticles the crystallographic structure could be controlled. In 2010 Norako and Brutchey reported the fi rst synthesis of wurtzite CISe nanoparticles [36], which had a rather broad size distribution and sizes around 30 nm. The presence of oleylamine and the use of diphenyl diselenide as selenium source turned out as critical for the formation of wurtzite NPs. Monodisperse CISe nanocrystals with hexagonal shape and a dia meter of about 21.3 nm were later obtained with a similar proce-dure by Wang et al. [37].

3.2.5 Composition

The possibility of adjusting the positions of the energy levels is one of the essential requirements for successful application of a mate-rial in photovoltaic devices. This can be achieved by reducing the size semiconductor nanocrystals below the value of the Bohr exci-ton radius (4.2 nm and 10.2 nm in CIS and CISe, respectively). Thus, this method can be applied effectively only in the size range of a few nanometers. However, small particles are less stable and have more defects than larger ones. An alternative approach for chang-ing the optical properties without changing the size of the particles uses alloying with another semiconductor material, e.g., CuInSe2 -CuGaSe2 [38, 39], CuInS2 -CuInSe2 [40, 41], or CuInS2-ZnS [42–45]. This is also the only method for adjusting the position of energy levels in materials, which are not going to maintain their nanocrys-talline character in the fi nal device (thin-fi lm solar cells processed from colloidal solution).

The bulk bandgap of CISe (1.04 eV) lies below the optimum value of a single junction solar cell (1.3 eV); therefore, Cu(In, Ga)Se2 with bandgap adjustable between 1.04 eV and 1.68 eV (bulk bandgap of CuGaSe2 ) is applied in thin-fi lm solar cells . Also, meth-ods to synthesize Cu(In, Ga)Se2 nanoparticles were studied; mainly with the goal of using the colloidal solution as ink for printable photovoltaic devices [39, 46, 47]. Size and composition control was achieved by adjusting the parameters of the synthesis , such as con-centrations, ligands and temperature . Unfortunately, gallium is a rare and expensive element; therefore, it would be benefi cial in


view of reducing the costs of photovoltaic devices to fi nd cheaper alternatives. Alloying of zincblende CISe with ZnSe offers the pos-sibility to vary the bandgap in the range between 1.04 eV and 2.7 eV. Because of the small lattice mismatch between the two materi-als, alloyed nanocrystals can be formed in the whole composition range [48].

Also, the crystallographic structures of CuInS2 and ZnS are simi-lar – the lattice mismatch between the two materials is only 2.2%; therefore copper and indium ions can be easily replaced by zinc ions and homogenous alloys can be produced within the whole composition range (between 0% and 100% ZnS). The bandgap of CuInS2-ZnS alloys can vary between 1.55 and 3.7 eV (bulk bandgap of ZnS), while the positions of the HOMO and the LUMO should lie between those of pure CuInS2 and ZnS. DFT calculations [49] show that the valence band of CuInS2-ZnS alloys is formed from Cu 3d and S 3p orbitals, while the In 5s5p and Zn 4s4p orbitals contrib-ute to the formation of the conduction band. Thus, only by varying the ZnS content in the alloy material can the position of the energy levels be infl uenced and nanocrystals with absorption covering the whole visible spectrum be obtained. Several examples of a direct synthesis of CuInS2-ZnS alloyed nanocrystals were described in the literature recently [43, 45, 50–52]; e.g., Pan et al. [51] applied ther-molysis of complexes of dithiocarbamate with Zn2+, Cu2+ and In3+ in the presence of oleylamine as activating agent, which facilitated the formation of homogenous nanocrystals. Another approach is a two-step synthesis, in which CuInS2 nanocrystals are synthesized fi rst and alloy particles are obtained by cation exchange [42, 45]. Some of these methods allow for a precise control of the composition, and, consequently, of the optical properties of quasi- spherical [43, 51] or cube-shaped [53] alloyed nanocrystals. Growth of elongated struc-tures can be obtained in a two-step method, using copper sulfi de particles as seeds [27, 54].

Combining two materials in a core-shell nanostructure can also be benefi cial for solar cell applications. Defects in CIS and CISe nanocrystals can trap the charge carriers in photovoltaic devices, and, thus, reduce the power conversion effi ciency . Surface defects can only partly be reduced by using strong capping agents. Another effective method for reducing the defects related to the surface is covering CIS or CISe with a shell of a material with larger bandgap, such as ZnS [18, 55, 56] or CdS [57–59]. The charge carriers are local-ized in the core of such a structure; the probability density of both


the electron and the hole on the surface of the shell material is rela-tively low, which reduces the impact of the surface defects on the properties of a core-shell structure. Furthermore, CIS-ZnS core shell nanocrystals exhibit longer lifetimes of the excited charge carriers, which is benefi cial for their application in solar cells [56].

3.3 Application of Colloidal CuInS2 and CuInSe2 Nanoparticles in Solar Energy Conversion

Several different solar cell geometries can be realized using nano-crystalline materials. Positive and negative charge carriers can be separated in a planar p-n junction, a Schottky barrier, an inter-penetrating network of a donor and an acceptor material, or at a semiconductor-liquid interface. The application of nanocrystals in all these kinds of devices offers a multitude of advantages (variable bandgaps, radiation stability, high extinction coeffi cients, and the possibility of multiple exciton generation); however, there are still many problems related mainly to the presence of defects and sta-bility issues which have to be solved before the potential of nano-crystaline building blocks for photovoltaic devices can be fully exploited.

3.3.1 All-Inorganic Solar Cells

Cu(In,Ga)Se2 solar cells reached 20% effi ciency , which is the current record value among the thin-fi lm devices [5]. However, the high production costs (investments and power consumption) hinder their widespread utilization; thus, less expensive alternative pro-duction methods are needed. Colloidal solutions of semiconduc-tor nanocrystals, which can be used as inks, are seen as one of the possible options to avoid the power consuming vacuum and high temperature processes and to reduce the investment costs [60]. As has been shown by Guo et al. [32, 47], Cu(In,Ga)Se2 thin fi lm com-posed of large crystalline domains can indeed be obtained by sin-tering colloidal nanocrystals. When thermal treatment is applied to Cu(In,Ga)Se2 nanoparticles, devices can be obtained exhibiting a power conversion effi ciency of 3.2% under AM1.5 illumination [32]. Because of the removal of the organic ligands during the sintering, the fi lm contains some void space inclusions. This can be avoided


when using Cu(In,Ga)S2 particles for the fi lm formation, which are heated in Se-atmosphere. The exchange of sulfur for selenium leads to an increase of the volume of the inorganic material, which com-pensates the loss of organic ligands. Therefore, fi lms with a better morphology are obtained, which yield higher power conversion effi ciencies (4.76%) [47].

The sintering step, applied to obtain a polycrystalline fi lm from nanoparticles, is, however, also power consuming; therefore, active layers, which do not require sintering, are also investigated. The highest effi ciency , 6% in nanocrystalline devices without thermal treatment, was obtained for PbS nanocrystals covered with inor-ganic ligands [61]. Devices produced with oleylamine-capped CISe nanocrystal inks reached 3% power conversion effi ciency under AM1.5 [62]. However, a Mott-Schottky analysis of the space-charge capacitance showed that the active region of these devices was only 50 nm; thus, only a fraction of the incident light absorbed in thicker active layers was effectively converted to electrical power. The effi ciency of these devices could be enhanced if the extrac-tion of photogenerated charge carriers from the CISe fi lm could be improved. CISe nanocrystals covered with inorganic ligands show lower power conversions effi ciencies: when a metal chalcogene-hydrazinium complex was applied as capping agent, effi ciencies up to 1.7% were obtained [17]. However, because of the need to use hydrazine as solvent, these kinds of nanocrystals cannot fi nd industrial application. Hydrazine-free ligands, such as S2- are also suitable capping agents for CISe nanocrystals, but the effi ciency of devices using them as the active layer are substantially lower [17].

3.3.2 Organic-Inorganic Hybrid Solar Cells

Solar cells composed of conducting polymers possess several advan-tages compared to thin-fi lm solar cells . They are low cost devices because they can be fabricated from solution, e.g., by printing, and if needed, on fl exible substrates. In a single-junction low-bandgap polymer cell an effi ciency of 8.4% has been reached [63]. A draw-back of organic polymers is their low chemical and photochemical stability, and relatively low electron mobility . Furthermore, most semiconducting polymers have wide energy gaps, and conse-quently, can utilize only a small fraction of the solar spectrum. The absorption of semiconductor nanocrystals can reach the near-IR spectral range; the mobility of electrons in inorganic semiconductor


materials is much higher than in polymers. Therefore, combining semiconducting polymers with inorganic nanomaterials seems to be a suitable way to overcome the limitations of the organic mate-rials. In spite of the theoretical advantages of organic-inorganic bulk heterjunction solar cells, and vivid research activity in this fi eld, these kinds of devices did not bring an increase of effi ciency, compared with purely organic ones. Hybrid solar cells suffer from problems related to insuffi cient passivation of the surface of the nanoparticles, trap states on the surface of the inorganic par-ticles and aggregation , which deteriorates the morphology of the active layer, as discussed in Section 3.2.1. With materials such as PbS , PbSe or CdSe , effi ciencies up to 5% could be achieved [64], so far, with optimized morphology and surface properties of the particles. Blends of CIS or CISe with conducting polymers such as poly hexylthiophene (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) or poly(3-(ethyl-4- butanoate)thiophene) (P3EBT) show photovoltaic response, however, hybrid solar cells based on these blends usually exhibit power conversion effi ciencies far below 1% [65–70].

3.3.3 Nanocrystal Sensitized Solar Cells

In dye-sensitized solar cells light is converted photoelectrochemi-cally into electrical power: a TiO2 electrode is covered with a dye, which absorbs light. The photogenerated electrons are transferred to the electrode, subsequently, the positively-charged dye reacts with a redox couple (typically I- and I-

3), which is dissolved in an organic solvent, and which transports the positive charge to the counter electrode. By using a co-sensitizer together with a black ruthenium dye an effi ciency of 11.4% was achieved in dye-sensitized solar cells [71]. Instead of the dye, nanoparticles can be applied as sensi-tizer. Nanocrystals have higher extinction coeffi cients and are more stable under illumination than conventional dye molecules. The relative positions of the energy levels of TiO2 and CIS nanoparticles allow for a charge separation, therefore combinations of this mate-rial show photocatalytic activity [59, 72–74] and can also be applied in nanocrystal-sensitized solar cells. Devices in which ZnSe- coated CIS particles were attached to TiO2 covered with a Cu2S buffer layer reach 2.52% effi ciency. Alloying CIS with selenium further extends the absorption into the near-IR spectral region, while the formation of a thin Cd(S, Se) shell can reduce surface trapping and increase


the stability of the particles. The resulting quantum-dot-sensitized solar cells exhibit up to 4.2% power conversion effi ciency [41, 58].

3.4 Conclusion and Outlook

Colloidal synthesis offers excellent possibilities for controlling the properties of CIS and CISe nanocrystals. Much progress has been achieved in the control of size, shape and composition of these nanomaterials in the past few years. This has provided an opportunity to apply nanocrystalline CIS and CISe in solar energy conversion, and to benefi t from the unique optical properties and stability of these materials together with the possibility of fabricat-ing photovoltaic devices from solution, which could signifi cantly reduce their production costs and enable large-scale application. Some of the studies devoted to the application of colloidal I-III-VI nanomaterials in photovoltaics show promising results, especially when the particles are used as inks for the generation of thin-layer devices or in nanocrystal-sensitized solar cells. However, in hybrid solar cells and in all-inorganic devices without a sintering step CIS and CISe do not reach the effi ciencies which have already been demonstrated for other materials, such as PbS or CdSe. Here, suc-cessful application of CIS and CISe requires further research; the surface properties and the stability of these materials have to be investigated in more detail and improved. Furthermore, synthetic procedures for particles with a smaller amount of internal defects have to be developed. To achieve this goal, the growth process and the underlying reaction mechanisms have to be extensively studied to enhance our understanding of the molecular processes taking place during the synthesis.

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solar cell nanotechnology (tiwari/solar) || colloidal synthesis of cuins 2 and cuinse 2 nanocrystals...

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