Prospects of Chalcopyrite-Type Nanocrystals for Energy Applications

Prospects of Chalcopyrite-Type Nanocrystals for Energy Applications Martina Sandroni,†,‡,§,∥,⊥ K. David Wegner,†,‡,§,⊥ Dmitry Aldakov,†,‡,§ and Peter Reiss*,†,‡,§ †

Université Grenoble Alpes, INAC-SyMMES, F-38054 Grenoble Cedex 9, France CEA, INAC-SyMMES, Laboratoire STEP, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France § CNRS, UMR 5819 SyMMES, F-38054 Grenoble Cedex 9, France ∥ Département de Chimie Moléculaire, UMR CNRS-UGA 5250, Laboratoire CIRE, 301 rue de la chimie, Université Grenoble Alpes, CS 40700, 38058 Grenoble Cedex 9, France ‡

ABSTRACT: Colloidal chalcopyrite-type nanocrystals M + M 3+ E 2 (M+ = Cu, Ag; M3+ = In, Fe; E = S, Se) are an emerging class of materials used in energy conversion systems, owing to their high absorption coefficients, nontoxicity, and appropriate band gaps. Their properties can be tuned by varying their size and composition, and they are characterized by peculiar photophysical properties due to a high density of intra band gap electronic states, which arise from defects in the crystal structure and from trap states at the surface. In this Perspective we first discuss optical and electrochemical studies, which give insight into the electronic structure of chalcopyrite nanocrystals. In the second part, their exploitation for energy conversion is highlighted, in particular their use in photovoltaics (with power conversion efficiencies already exceeding 10%) as well as in luminescent solar concentrators, photocatalytic systems, and thermoelectrics. Finally, we discuss current challenges in the chemistry and applications of the title compounds. challenges and future directions in this field. Quaternary compounds such as kesterites (Cu2ZnSnE4, E = S, Se) are beyond the scope of this article; their properties and energy applications have been recently reviewed by others.1,7−9

T

ernary I−III−VI2 chalcopyrite-type nanocrystals (NCs) M+M3+E2 (mainly M+ = Cu, Ag; M3+ = In, Fe; E = S, Se) emerged around 10 years ago and have drawn tremendous research interest for energy applications, in particular in the fields of solar energy, photocatalysis, and thermoelectrics.1 These materials are among the most promising alternatives to established Cd- or Pb-containing binary NCs (e.g., CdSe, CdTe, PbS) and to more recently developed perovskite NCs such as CsPbX3 (X = Cl, Br, I) or MAPbX3 (MA = methylammonium), all of which are of limited interest for real-life applications because of their toxicity.2 Replacing formally the +2 element in metal chalcogenides by two elements in the +1 and +3 state gives access to a new class of NCs showing a diversity of structural and optoelectronic properties, which largely surpasses that of the corresponding bulk materials.3 Ternary chalcopyrite NCs tolerate various off-stoichiometries from the nominal 1:1:2 composition, which has a profound impact on their properties.4 Similar to their binary counterparts, they exhibit quantum confinement effects when their size becomes smaller than the Bohr exciton radius. By consequence, both size- and composition-dependent effects are observed in electron-transfer reactions, which are the basis of photovoltaics5 and photocatalysis.6 The goal of this Perspective is to summarize the present state of knowledge of the structure and optoelectronic properties of the ternary metal chalcogenide NCs, their applications in energy conversion, as well as current © XXXX American Chemical Society

Ternary chalcopyrite NCs show a diversity of structural and optoelectronic properties which largely surpasses that of the corresponding bulk materials. Among the most-studied chalcopyrite type NCs are CuInS2 (CIS) and related compounds containing additionally Zn and/ or Se. In the bulk, the band gap of CIS is around 1.5 eV, and the photoluminescence (PL) is characterized by a sharp band at 1.53 eV [full width at half-maximum (fwhm) ≈ 2 meV] as well as broader deep trap features at 1.3 and 1.4 eV (fwhm ≈ 20 meV).10 The transitions are attributed to exciton emission and donor−acceptor pair (DAP) recombinations, implying a deep acceptor level due to vacancies (V) or antisite defects (e.g., VCu, VIn, CuIn) and a shallow donor level (e.g., VS, InCu, Ininterstitial). Received: January 2, 2017 Accepted: April 7, 2017

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DOI: 10.1021/acsenergylett.7b00003 ACS Energy Lett. 2017, 2, 1076−1088

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http://pubs.acs.org/journal/aelccp

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in the conduction band and holes being localized at Cu+ ions or at point defects generally assigned to copper vacancies VCu (Figure 1).18

In contrast, CIS NCs have a broad PL band with a line width of more than 100 nm (fwhm, 200−500 meV), large effective Stokes shifts of 180−500 meV, and PL decay times of several hundred nanoseconds.11 These features provide strong evidence that, in contrast to classical binary QDs, direct exciton emission is not the major radiative pathway and that the PL emission involves a range of intra band gap states. Therefore, for the determination of the optical band gap in general the absorbance spectrum is used. However, because of the pronounced red-tail and the absence of clear excitonic features, the use of the so-called Tauc plot becomes necessary for analyzing the absorption spectra of chalcopyrite NCs. It is based on eq 1: αhν = C(hν − Eg )n

(1)

with α the absorption coefficient (α = A/d; A, absorbance; d, optical path length of the cuvette), hν the photon energy, and C a constant; n is 1/2 in the case of a direct and 2 in the case of an indirect transition. The extrapolation of the linear region in the (αhν)2 versus hν plot to its intersection with the energy axis can thus be used to determine the direct band gap of chalcopyrite NCs. By means of steady-state and time-resolved analysis of the PL emission, a broad range of defect states similar to those in the bulk have been evidenced for CIS NCs. However, the nature of these defects also depends on the crystalline phase, as CIS and other types of ternary I−III−VI2 NCs can exist in the cationordered chalcopyrite (CH) structure, the cation-disordered zinc blende (ZB) structure, and the hexagonal wurtzite (WZ) structure.3 The ZB and WZ phases are observed only at high temperatures in the bulk. In the ground-state CH structure, according to the Grimm−Sommerfeld rule (the average number of valence electrons per atom must be four), the two cations are ordered alternately on the (201) planes of the metal sublattice. This leads to M+2M3+2 tetrahedra with the chalcogenide atom in the center. However, other ordered structures exist with only slightly higher formation energies, such as the CuAu-like structure in CISe, which consists of alternating M+ and M3+ (100) metal planes.12 Recent Z-contrast STEM studies on rather large (20 nm) CIS NCs indicate that also in the WZ phase the Cu and In ions are not randomly distributed in the cation sublattice, but exhibit a range of ordered interlaced domains and phases.13 In these “interlaced crystals” the nonunique ordering patterns form an uninterrupted, global Bravais lattice free of defects and strain. WZ CIS NCs exhibit another unique feature compared to the CH and ZB structures, namely, a broad, weak PL emission in the near-infrared region centered around 950 nm, attributed to an Ininterstitial point defect in the crystal structure.14 For CH CIS NCs the main contributions to the optical transitions have been assigned to sulfur vacancies (VS) and indium on copper sites (InCu) as well as holes localized on copper sites.15 Because of the slightly weaker Cu−S bond in comparison with In−S, VCu are preferably generated, which can also induce antisite defects.16 Initial timeresolved PL studies showed that the peak energies of the different spectral components shifted to lower energies with increasing delay time from the excitation moment.17 This behavior is characteristic for the DAP mechanism. On the other hand, the DAP mechanism does not explain the observed pronounced size dependence of the PL spectra of CIS NCs. Recent excitation power-dependent, ultrafast transient absorption, and time-resolved PL studies on CH CIS NCs converge toward a “free-to-bound” mechanism,11 with electrons being delocalized

Figure 1. “Free-to-bound” mechanism describing the PL emission in CH CIS NCs. It involves the transition from an electron delocalized in the conduction band (CB) to a hole localized at a copper vacancy, a lattice Cu+ ion or a surface trap state. The higher energy−shorter lifetime recombination involving surface states can be suppressed by passivation with a shell (e.g., CdS, ZnS).

The spectra of CIS NCs are generally composed of several components, which comprise in addition to the discussed recombinations higher-energy transitions involving surface trap states.17,19,20 Time-resolved ensemble measurements reveal at least two very different decay times in the range of tens and hundreds of nanoseconds; the shorter one is attributed to surface defects. Similar surface passivation techniques, growth of a larger band gap semiconductor shell, were used as in binary QD systems to suppress surface defects and increase the quantum yield (QY). As expected, in core/shell structures, the amplitude of the decay component with the longer lifetime increases while the shorter decay component decreases.20 The comparison of CdS and ZnS as shell material revealed a better passivation with higher QY values for CdS along with a nearly monoexponential decay, while for ZnS still a biexponential behavior was observed.20 The ionic diameter of Zn is similar to that of Cu, and cation exchange can compete with ZnS shell growth. An increase of the Zn:Cu ratio leads to a significant blue shift of the absorbance band edge and PL emission.16 De Trizio et al. studied the influence of Zn cation exchange on CIS NCs resulting in alloyed Cu−In−Zn−S (CIZS) NCs. QYs up to 80% and nearly monoexponential decay kinetics were observed for a composition of Cu0.13In0.74Zn0.59S2.21 This evidences that the alloying process eliminates internal trap states, e.g., via filling of vacancies or expulsion of interstitial atoms. Another possible explanation is the rigidification of the crystal lattice and reduced interatomic diffusion. The lack of size increase during the exposure to Zn ions and the shifted Cu 2p3/2 peak measured with high-resolution X-ray photoelectron spectroscopy (XPS) confirmed the formation of an alloyed structure. In comparison with the parent CIS NCs, the blue-shifted absorption and emission spectra indicate that the introduction of Zn ions leads to a widening of the band gap. By additionally passivating surface traps with a ZnS shell, Zhang et al. have shown strongly reduced blinking of the resulting CIZS/ZnS NCs.22 For a certain composition, i.e., Cu:In:Zn = 1:4:3 and a shell growth time of 15 h, more than 98% of the investigated 1077


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individual NCs did not exhibit fluorescence intermittency. For other compositions and thinner or thicker ZnS shells, this fraction was lower. Because of the analogous optical properties observed in CIS and in copper-doped II−VI and III−V NCs, Gamelin and co-workers suggested that the microscopic nature of the electronic transitions is similar.23 In copper-doped NCs, the PL originates from transitions between partially localized electrons in the conduction band and holes strongly localized at deep acceptor levels associated with the copper dopant. These transitions are characterized by a long decay time (dependent on the spatial separation of electron and hole) and a large effective Stokes shift depending on the energy difference between the copper 3d orbitals and the valence band edge. In these systems, the observed large PL line width has its origin in the distribution of copper energy levels originating from excited-state local structural distortion (Jahn−Teller effect). The localization of holes at Cu+ sites of the lattice rather than at vacancies is also discussed in the case of CIS NCs. This phenomenon is called exciton self-trapping and occurs because of pronounced vibronic coupling, which induces local lattice distortion. It leads to the modulation of the hole potential and hole localization at a lattice Cu+ ion.23,24 This effect is supposed to contribute to the broad PL emission of CIS NCs, which is only partially related to the size inhomogeneity as shown by singleparticle PL spectroscopy. First single-particle measurements on CIS NCs capped with a thin CdS shell reported by Whitham et al. revealed a narrower PL band with respect to the ensemble, but the line widths were still 190−270 meV, i.e., much broader than typical values reported for II−VI NCs (≈ 50 meV).24 Furthermore, there was no correlation between the PL peak energy and its line width. However, the samples exhibited pronounced emission intermittency (blinking), and the singleparticle nature of the measured objects was not demonstrated unambiguously. This has been achieved by means of photon correlation (antibunching) experiments in a very recent PL study by Zang et al.25 Single CIS NCs capped with a thick ZnS

CIS NCs were calculated, which are consistent with experimental results, underpinning the involvement of intra band gap states and the hole as localized carrier in the emission process.17,19,20,26 Another highly interesting material of the ternary chalcopyrite family is AgInS2 (AIS). The PL band of AIS NCs is characterized by a Stokes shift that is even larger (800− 1000 meV) than that of CIS NCs and a broad line width of ca. 400 meV. It shifts to higher energy with increasing excitation power, which is a characteristic sign for DAP recombination.27 Intra band gap defects arise in the bulk from vacancies and interstitials of sulfur and silver, which have donor and acceptor character, respectively. Similar defects are assumed to be present in AIS NCs. Time-resolved PL analyses revealed three contributions to the radiative decay: a short component originating from the recombination of surface states and two slower components of DAP recombination involving surface and core states. In general, the probability for recombination is higher for close-distance DAPs, resulting in an exponential behavior of the decay time. Furthermore, the DAP recombination involving surface states is more efficient than that from the core, as the decay time exponentially increases with the emission wavelength. Using ZnS as a shelling material results in an increase of the decay times for all components. The strong contribution from DAP recombination involving surface states leads to a decreased PL QY with increasing size of the particles.28 Single-particle measurements revealed the existence of several spectral components with different transition energies leading to a broadening of the line width of AIS NCs.29 It varies around 230 meV and shows a correlation with the PL maxima. The coexistence of multiple donor levels within one particle has been proposed to be one reason for the large homogeneous line width in AIS NCs. As a last type of ternary NCs we discuss CuFeS2 whose mineral name gives the designation to the chalcopyrite family. This earth-abundant material offers unique properties as an antiferromagnetic semiconductor with a small band gap of 0.5−0.6 eV. This feature, combined with a high absorption coefficient make CuFeS2 in principle a good candidate for photovoltaic applications. Much less well-known is the fact that this material can also exhibit appealing PL properties, as shown by Bhattacharyya et al., who grew a CdS shell on CuFeS2 NCs.30 By using XPS and inductively coupled plasma optical emission spectroscopy, they detected a composition gradient with a copper-rich core and a Cd-rich surface. The PL peak position could be tuned from 0.7 to 2.5 eV, with a maximum PL QY of 87% and an average fwhm of 480 meV. Similar to CIS NCs, the PL is based on a defect state emission with a delocalized electron in the conduction band and a localized hole in an internal defect state above the valence band. This assumption could be confirmed by transient absorption spectroscopy, which showed a bleach feature with significantly smaller width than the emission feature, indicating strong phonon coupling. Additionally, there is a linear relationship between the Stokes shift and the band gap energy, which can be explained by the reduced energy difference between the valence band edge and the acceptor state with increasing NC size. The emission decay kinetics follows the general behavior of a defective system: the lifetime increases with decreasing band edge energy because the overlap of a confined quantum level with a defect scales inversely with the NC volume. While optical studies on NCs give access to the band gap or ELUMO − EHOMO energy difference, they do not

Single CIS NCs capped with a thick ZnS shell (≈6 monolayers) showed surprisingly narrow PL line widths with values down to 60 meV. shell (≈6 monolayers) showed surprisingly narrow PL line widths with values down to 60 meV. The authors explain the broad ensemble line widths (>300 meV) with the random positioning of the Cu-related emission centers, leading to variations in the contribution from the electron−hole Coulomb coupling to the PL energy. A monoexponential decay behavior with a time constant of ≈180 ns has been determined on a single CIS/ZnS NC. This value is close to that determined by Whitham et al.24 who observed two different time regimes in CIS/CdS NC ensembles with a “prompt” PL decay showing a time constant of ≈140 ns and a delayed emission, attributed to a long-lived metastable state. Because of the long lifetime, the possibility for multiple excitations is enhanced, inducing an offstate (blinking) by nonradiative deactivation processes due to Auger recombination or Shockley−Read−Hall type recombination. Theoretical calculations revealed that the tetragonal symmetry of the crystal lattice results in a weakly allowed recombination between the electron and hole ground state.26 Large Stokes shifts of the PL up to 300 meV in the smallest CH 1078


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Table 1. Electrochemical and Optical Properties of Ternary I−III−VI2 NCs Having Different Sizes and Compositionsa material CuInS2 CuInS2

CuInS2

shape − plate

pyramid

size (nm)

[I]:[III]

ligand

bulk 29 ± 6 × 1.5 ± 0.5

1 2.9

− DDT

2.68

DDT

1.23

DDT

1.02

DDT

0.72

DDT

7.3

0.92

DDT

6.1

1.03

DDT

5.6

1.21

DDT

5.2

1.36

DDT

3.5

1.71

DDT

CuInS2

sphere

2−4

1.1

PhSH

CuInS2

sphere

5

1

t

CuInS2

sphere

7

0.72

EHT

CuInS2

sphere

≈5

1

OLA/DDT

CuInSxSe2−x:Zn

pyramid

5

0.45

wz-CuInS2

sphere

6.9

4.6

DDT MPA tBA DDT, OLA

wz-CuInS2

pyramid

10

n.r.

Py

bullet

50*18

n.r.

HA

spheres, necklace

16

n.r.

DDT, OA

3.4

n.r.

DDT, OA

2.7

1.62

OLA, ODE

9.3

1.64

OLA, ODE

CuInSe2

CuFeS2

pyramid

BuSH

Eox (V)

Ered (V)

EHOMO (eV)

ELUMO (eV)

Eec g (eV)

Eopt g (eV)

ref.

−5.6 0.92 −5.63b 1.02 −5.73b 1.04 −5.75b 1.06 −5.77b 1.04 −5.75b 1.026 −5.74b 1.05 −5.76b 1.11 −5.82b 1.13 −5.84b 1.18 −5.89b 1.08 −5.72d 1.16 −6.26c 1.09 −5.90c 0.9 −5.54d −5.73c −5.74c −5.76c 1.13 −5.84b −0.1 −5.0c −0.1 −5.0c 0.36 −5.07b 0.63 −5.34b 0.72 −5.43b 0.57 −5.28b

−4.1 −0.27 −4.44b −0.32 −4.39b −0.31 −4.40b −0.36 −4.35b −0.41 −4.30b −0.68 −4.03b −0.73 −3.98b −0.73 −3.98b −0.75 −3.9b −0.85 −3.86b −0.72 −3.92d −1.48 −3.62c −0.69 −4.40c −0.76 −3.88d −3.84c −3.81c −3.72c −0.71 −4.00b −1.5 −3.6c −1.5 −3.6c −0.67 −4.04b −1.08 −3.63b −0.82 −3.89b −0.17 −4.54b

1.5 1.19

1.5 n.r.

42 34

1.34

n.r.

1.35

n.r.

1.42

n.r.

1.45

n.r.

1.70

1.67

1.78

1.72

1.84

1.77

1.88

1.80

2.04

1.95

1.80

1.74

43

2.64

2.76

44

1.5

n.r.

45

1.66

n.r.

46

1.89 1.93 2.04 1.84

n.r. n.r. n.r. 1.47

41

47

1.4

1.62

48

1.4

1.58

1.03

n.r.

1.71

1.6

1.54

1.39

0.74

0.69

39

35

36

a

DDT, dodecanethiol; OLA, oleylamine; EHT, 2-ethylhexanethiol; MPA, mercaptopropionic acid; ODE, 1-octadecene; tBA, tert-butylamine; OA, oleic acid; Py, pyridine; HA, hexylamine. bUsing Ag/AgNO3 = −4.71 eV. cUsing Fc/Fc+ = −4.8 eV. dUsing Fc/Fc+ = −5.1 eV; n.r., not reported.

to probe the reduction and oxidation potentials. They provide complementary information to spectroscopic methods, in particular about surface states, involved in electron transfer with the electrodes.31 However, various experimental configurations and reference systems have been used in the literature; therefore, we will briefly recapitulate some basic considerations concerning electrochemical measurements on NCs.

provide information about the absolute positions of these levels. This knowledge is, however, of capital importance for designing efficient NC-based systems for energy conversion. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) level positions can be determined using X-ray and ultraviolet photoelectron spectroscopy as well as the latter. Electrochemical measurements are a very convenient and accessible technique 1079


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Figure 2. EHOMO and ELUMO for ternary I−III−VI2 NCs having different sizes and compositions, determined by electrochemical measurements.

band gap Eec g by the interaction energy between electron and hole, i.e., the exciton binding energy Eb:

The ionization potential, EHOMO, and the electron affinity, ELUMO, can be deduced from the onset potentials of the oxidation and reduction waves in the case of cyclic voltammetry or from the oxidation and reduction peak positions in the case of differential pulse voltammetry, respectively.31 Electrochemical measurements on NCs are generally carried out by depositing a thin layer of NCs on the working electrode surface and controlling the potential versus a reference electrode. When the work is conducted in organic solvents, Ag/AgNO3 in acetonitrile can be used, or simply a silver wire acting as a pseudoreference electrode. In this case, calibration has to be carried out with a reference redox system, e.g., the ferrocene/ ferrocenium redox couple (Fc/Fc+), Ag/AgCl, or a saturated calomel electrode (SCE, Hg/Hg2Cl2 in saturated KCl). The Fc/Fc+ potential accounts for +380 mV vs SCE in acetonitrile containing 0.1 M Bu4N+PF6−.32 Redox potentials can also be indicated versus the standard hydrogen electrode (SHE), which serves as the universal reference electrode in protic solvents. The standard potential of the SCE is +244 mV vs the SHE; hence, the redox potential of Fc/Fc+ is +624 mV vs SHE. [In numerous papers potentials are reported versus the “normal hydrogen electrode” (NHE); the approximate difference between NHE and SHE is 6 mV.] For comparison, the potential of the Ag/AgCl reference electrode is +197 mV vs SHE. Using the absolute electrode potential of the SHE (4.44 V, IUPAC), electrochemical measurements can be correlated to orbital energies, and the HOMO and LUMO levels of NCs can be calculated according to eq 2 and 3 when using Fc/Fc+ as the internal standard (5.1 eV is generally used instead of 5.064 eV): Ionization potential: E HOMO = − e(Vox + 5.1) eV

(2)

Electron affinity: E LUMO = −e(Vred + 5.1) eV

(3)

Egec = E LUMO − E HOMO = Egopt + E b

(4)

The electrochemical and optical data available for ternary I−III−VI2 NCs having different sizes and compositions are summarized in Table 1 and graphically represented in Figure 2. Some important trends can be deduced. For CIS, copperdeficient NCs show an increased optical and electrochemical band gap, with a more anodic oxidation potential and a more cathodic reduction potential (blue series in Figure 2). This is in accordance with the blue-shift of the luminescence peak when the Cu content decreases.37 Due to quantum confinement effects, NCs with decreased size show a wider electrochemical and optical band gap (red and orange series in Figure 2). The LUMO (conduction band, CB) level is more affected than the HOMO (valence band, VB) level, as expected from the lower effective mass of the electron compared to the hole (0.16 vs 1.3) in CIS.16,38 However, the two effects, size and composition, can hardly be studied separately because of the evolution of the composition of the NCs during their growth. For many syntheses it has been reported that CIS NCs start from a copper-rich to a quasi stoichiometric or slightly copperdeficient composition with increasing reaction times.39 Importantly, surface ligands have a dramatic influence on the energy level positions, which can overcompensate size and composition effects (green and violet series in Figure 2). There is a lack of detailed electrochemical data for alloyed or core/ shell CuInS2/ZnS NCs, but it is known that the optical band gap increases with an increasing fraction of ZnS.40 This is also confirmed by the available data for CuInSxSe2−x:Zn2+ NCs.41 This compound shows an electrochemical band gap similar to that of CIS NCs: while the incorporation of selenium tends to shrink Eg, the addition of Zn cations compensates this effect. The rare data available for CuFeSe2 NCs indicates that their energy levels are situated in the same region as those of CISe. Both materials exhibit strong quantum confinement effects for small particle sizes, with the LUMO level being once again more strongly affected than the HOMO level position. In terms of energy applications, the interest of chalcopyrite NCs is manifold. First, they have a high potential for photovoltaic (PV) applications, as has mainly been shown for CIS,

It should be noted that in a number of reports the value of 4.8 eV is used instead of 5.1 eV to calculate the energy levels versus vacuum, based on an estimation published by Pommerehne et al.33 However, this conversion has been invalidated by the more recent data reported by Pavlishchuk and Addison.32 When the potentials are reported directly versus Ag/AgNO3 (0.01 M),34−36 4.71 eV is used for calculating the energy levels instead of 5.1 eV. The band gap extracted from optical measurements Eopt differs from the electrochemical g 1080


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optimal synthesis parameters is more delicate, because the reactivity of the cationic precursors must be carefully balanced to obtain the desired stoichiometry for a given NC size. Furthermore, the high density of surface and volume trap states leads to a high charge recombination rate. Another concern could be related to a higher exciton binding energy (Eb) of ternary chalcopyrites as compared to PbS, as suggested by the bulk values of 18−20 meV for CIS, 7−8 meV for CISe, and 4 meV for PbS. However, when going to the small dimensions of NCs, the electron−hole Coulomb interaction becomes substantial (it scales approximately as 1/radius), leading to values of 200−50 meV for NCs of 1−2 nm radii.50 More precisely, for 3.5−7.3 nm CIS NCs, Eb values of 30−100 meV have been determined experimentally by using eq 4, which are similar to those calculated for 3−8 nm PbS NCs (102−38 meV).39,51 Finally, depending on the synthesis method, ternary chalcopyrite NCs can possess tightly bound surface ligands, which are hard to remove or replace. To give an example, one of the most widely applied synthesis methods is based on the use of 1-dodecanethiol (DDT) as the sulfur source and surface ligand. While being an excellent compound for the high-yield production of ternary chalcopyrite NCs of small size (2−7 nm),19,20,39 DDT has been shown to be tightly bound by the integration of thiolate groups into the surface layer of the crystalline core (crystal-bound sulfur atoms).52 The strategies to circumvent or alleviate the above-mentioned drawbacks of ternary chalcopyrite NCs for PV applications will be discussed below. Three main types of solar cells exploiting semiconductor NCs exist: thin film (TFSCs), hybrid organic−inorganic, and QD sensitized solar cells (QDSSCs).53 Figure 3 shows schematically their function principles, and Table 2 summarizes the characteristics of the best-performing solar cells reported using these different approaches. The TFSC operation principle is based on efficient charge generation and transport within the nanocrystalline film; consequently, the interparticle coupling should be maximum and the exciton binding energy minimum. In the simplest configuration, a film of NCs is illuminated through a transparent conductive oxide (TCO) and forms on the back side of the cell a Schottky junction with a shallow work function metal electrode. While with lead chalcogenide NCs maximum PCEs around

Surface ligands have a dramatic influence on the energy level positions, which can overcompensate size and composition effects. CISe, and derived structures containing Zn. Their popularity in this field is based on their band gap values, situated in an ideal range for single-junction solar cells (1.5 and 1.04 eV for bulk CIS and CISe, respectively) and tunable by changing size or composition; their high absorption coefficients (≈105 cm−1); and the absence of toxic elements. Bulk chalcopyrite semiconductors are very well-known as active materials in photovoltaics: CuIn1−xGaxSe2 (CIGS) modules range among the top three thin-film technologies (together with CdTe and amorphous Si), accounting for around 2% of the global solar cell production and showing a record efficiency of 22%. At the same time, alternative technologies for the fabrication of solar cells based on chalcopyrites are needed, because the relative scarcity of indium would become an issue if CIGS thin-film solar cells (TFSCs) were produced on a massive scale. In addition, coevaporation or cosputtering manufacturing technologies are not compatible with cost-efficient deposition techniques, such as, for example, roll-to-roll processing. This point has been addressed by the development of solution-processed TFSCs using chalcopyrite absorbers generated from molecular precursors, with power conversion efficiencies (PCEs) reaching almost 15%.49 However, the fabrication of high-efficiency cells requires toxic and hazardous compounds such as hydrazine for the precursor preparation and H2Se for the selenization process. Chemically synthesized colloidal NCs can provide a viable alternative to these techniques as the band gap and composition can be optimized independently from the solar cell fabrication process. Additionally, in the so-called quantum-dot sensitized solar cell (QDSSC) configuration, the amount of active material is much lower than in traditional TFSCs, which is a critical advantage when elements of lower Earth abundance are considered, such as indium. Nonetheless, compared to other types of NCs such as PbS, the progress in the field of chalcopyrite NCs for photovoltaic applications has been slower for several reasons. Determining

Figure 3. Comparison of three device architectures of QD solar cells under operation close to maximum Voc. (a) The Schottky solar cell is characterized by lower Voc and FF for a given Jsc because of the low barrier for hole injection into the electron-extracting contact and Fermi level pinning at the NC−metal interface. (b) The depleted heterojunction cell shows improved characteristics with maximized Voc, Jsc, and FF but still relies on efficient charge transport in the NC phase. (c) The QDSSC design gives access to good Voc and FF values but requires a sufficient thickness of the mesoporous oxide scaffold to provide sufficient light absorption and hence elevated Jsc values. (EF,n and EF,p are electron and hole quasi-Fermi levels, respectively; EV and EC are valence and conduction band edges; Jn,PV and Jp,PV are electron and hole photocurrents, respectively; Jp,PV is hole current in forward bias direction.) Adapted from ref 54. Copyright 2010 American Chemical Society. 1081


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Table 2. Best-Performing Solar Cells Using Ternary Metal Chalcogenide NCs as Absorbers cell configuration a

Schottky depleted heterojunctiona hybrid organic−inorganic QDSSC (n-type) QDSSC (p-type) a

type of NCs

Voc (mV)

Jsc (mA/cm2)

FF

PCE (%)

ref.

CIS CIS-Zn oleate AgBiS2 CIS/PSiF-DBTb CIS-ZnS aqueous Zn−Cu−In−Se Zn−Cu−In−S,Se

410 600 450 540 600 739 350

1.40 1.67 22.1 10.3 9.6 25.25 9.13

0.38 0.33 0.63 0.50 0.42 0.62 0.39

0.2 0.33 6.8 2.8 8.15 11.6 1.25

55 63 59 60 61 62 41

Without the use of sintering and/or selenization processes. bPoly[(2,7-silafluorene)-alt-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)].

5% have been reported, the values obtained with ternary chalcopyrite NCs are much lower. In the former case, the efficiencies are mainly restricted by the open-circuit voltage (Voc), which is intrinsically limited in Schottky-type solar cells because of Fermi level pinning at the NC−metal interface. In the second case, an additional limiting factor is the comparable low short-circuit current density (Jsc). As an example, Borchert et al. measured a PCE of 0.2% for ≈200 nm CIS films sandwiched between ITO/PEDOT:PSS and Mg/Al contacts, with a maximum Jsc of 1.4 mA/cm2,55 i.e., more than 10 times lower than in PbS- or PbSe-based cells.56 In contrast to chalcopyrite NCs, significant research efforts have been dedicated to the optimization of the surface passivation of lead chalcogenide NCs, using various organic and inorganic surface ligands. These strategies have two important consequences: (i) Because of the substitution of long, insulating surface ligands on the pristine NCs by shorter molecules and/or inorganic ions, the interparticle electronic coupling is strongly enhanced. This leads to much higher charge carrier mobilities in the NC solids, reaching over 10−1 cm2/(V s) in PbS NC films. (ii) Through the combination of organic and inorganic surface passivation, the density of trap states is significantly reduced. Defects in the band gap act as recombination centers for photogenerated carriers and also reduce the Voc. In the case of lead chalcogenide NCs, the best PCEs in TFSC configuration, exceeding 10%, have been obtained using heterojunction approaches.57 There, the limitations of Schottky solar cells are overcome as charge separation takes place at a p−n junction formed between a highly doped n-type oxide (e.g., TiO2, ZnO) and the NC film, leading to a depletion layer localized in the latter. Still, efficient charge transport across the NC layer is required, which leads to the so-called absorption− extraction compromise: the thickness of the NC film has to be kept low enough to prevent transport losses from becoming the limiting factor of the solar cell; consequently, the thickness does not reach the value theoretically required for optimum solar light absorption. Only very recently have promising results appeared for TFSCs using ternary chalcopyrite NCs. As mentioned before, one of the main problems is related to the removal of tightly bound surface ligands, such as DDT, which impede efficient charge transport in the NC film. Therefore, alternative methods have been developed including, for example, a DDT-free synthesis of CIS and CIS-Zn alloyed NCs using oleylamine and thiol-free sulfur precursors,63 which facilitate subsequent ligand exchange. Another possibility for eliminating insulating surface ligands is their thermal decomposition or ligand thermolysis. Using 1-ethyl-5-thiotetrazol as a thermally degradable ligand, Lauth et al. obtained “virtually bare” CISe and CIGS NCs, which allowed obtaining a significantly improved interparticle coupling and conductivity in the corresponding thin films.64

Ligand exchange or removal can be completely avoided when ternary nanoparticles are directly synthesized with the desired short-chain ligands. Lefrançois et al. proposed the use of tertbutylthiol in the synthesis of CIS NCs in organic solvent, which led to 5 nm particles with a 0.3−0.4 nm organic surface layer.44 The obtained NCs showed a 400 times increased conductivity as compared to DDT-capped CIS NCs of the same size. By refluxing with pyridine to remove native ligands from CIS NCs followed by the introduction of short allylamine ligands, Niyamakom et al. reported TFSCs with 6.5% efficiency, but still selenization at 550 °C was required.58 Halpert et al. fabricated thin films of CIS NCs using 1,3-benzenedithiol to replace oleylamine ligands and to enhance the interparticle coupling.65 By means of layer-by-layer deposition, thin films have been obtained, which yielded up to 1.5% efficiency after integration into solar cells without an additional annealing step. In parallel, films of CIS grown in situ from the same precursors have led to an efficiency of 3.2%. The difference between the two results has been explained by the higher trap density in the ex situ synthesized colloidal NCs. Using In-rich alloyed Cu− Zn−In−S NCs in the TFSC architecture, So et al. obtained efficiencies of 0.1−0.3%.63 The authors concluded that a high density of traps in the NC film led to significant recombination losses that critically limit the photovoltaic performance. Deposition of the CIS film on a mesoporous TiO2 scaffold reduced these recombination processes through the better quenching of deep tail states in CIS and resulted in efficiencies reaching 1.16%. Finally, Bernechea et al. developed TFSCs based on AgBiS2, a novel type of ternary metal chalcogenide NCs crystallizing in the cubic rock salt structure.59 Taking advantage of the high absorption coefficient, suitable band gap (1.3 eV), air stability, and facilitated ligand exchange of AgBiS2 NCs, solar cells with a PCE of 6.3% were prepared at low temperature (200 meV). Recently, several important efforts have been made to implement them into LSCs. By engineering alloyed Zn−Cu−In−S,Se NCs Meinardi et al. increased the Stokes shift to 0.53 eV and obtained LSCs with an optical power efficiency of 3.2%.73 In an alternative approach, Wang et al. applied highly luminescent Zn−Cu−In−S NCs (QY: 81%), which allowed reaching an optical efficiency of over 26%.74 As in PV, chalcopyrite type NCs also hold great promises in the field of photocatalysis, due to their efficient absorption of the solar spectrum and tunable position of their valence and conduction bands (cf. Figure 5a). Schematically, in

Figure 5. (a) Solar light-driven hydrogen generation using chalcopyrite NCs. At high pH, water molecules are reduced by photogenerated electrons under formation of hydroxide ions, at low pH, protons are reduced. Sacrificial reducing agents (red, e.g. mixtures of sulfide and sulfite ions) are oxidized by photogenerated holes. (b) Thermoelectric generator. The p- and n-type NCs, processed to form dense, monolithic pillars (“legs”), are sandwiched between the hot and cold faces of the device, which leads to the generation of an electrical current due to the Seebeck effect. The figure of merit (ZT), of each leg depends on the Seebeck coefficient (S), the electrical (σ) and thermal (κ) conductivity, as well as the temperature.

photocatalysis the photogenerated charge carriers are used to induce a chemical reaction, instead of generating an electrical current used in an external circuit as in the case of photovoltaics. In particular, NCs can be used in the field of solar fuel production to convert solar energy into chemical energy (for example hydrogen from water splitting), or in the field of environmental remediation to degrade organic pollutants. The use of ternary and quaternary NCs in photocatalysis has recently 1084


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mediators. They combine several appealing features like tunable redox potentials, noncorrosiveness, and low visible light absorption; however, their comparably low long-term stability remains an issue to be solved.78 In the field of photocatalysis, the combination of QD light absorbers with molecular catalysts is a promising avenue for improved performance.79 Initially metallic cocatalysts in the form of nanoparticles (e.g., platinum or rhodium), crystalline domains, or dopants of semiconductor QDs have been applied. Meanwhile, molecular coordination complexes of transition metals (e.g., cobalt, nickel) have been shown to provide much higher versatility and tunability of the electronic and photocatalytic properties. To improve the efficiency and the rational design of such new catalytic systems based on chalcopyrite NCs, more systematic studies of their electrochemical behavior are needed. As highlighted throughout this Perspective, for all cited applications of chalcopyrite-type NCs, the precise control of their size, composition, crystalline phase, and surface state is of prime importance. Therefore, substantial efforts should be devoted to the development of their chemical synthesis and to the detailed studies of their structural and optoelectronic properties in order to be able to exploit the full potential of this exciting class of materials.

existence of the claimed chalcopyrite structure in the obtained NCs. Comparing both structures in the bulk, X-ray diffraction analysis reveals a slight tetragonal lattice distortion leading to a lattice parameters ratio of c/2a ≈ 1.004 in the chalcopyrite phase, and additional peaks corresponding to the (101), (103), and (211) reflections are visible. However, these features are generally very difficult to detect in the case of small NCs ( 10% have already been reported, many aspects still leave large room for improvement. In addition to the aforementioned optimization of chalcopyrite NC synthesis, surface engineering has turned out to be the key for achieving high PCEs by decreasing losses through nonradiative recombination processes in the particles and at the interfaces. Furthermore, the polysulfide electrolyte is clearly a bottleneck, limiting in particular the open-circuit voltage of the devices when combined with standard mesoporous oxides such as TiO2 and ZnO. As in the case of DSSCs, cobalt complexes could be promising alternative redox

M.S. and K.D.W. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Martina Sandroni studied chemistry at University of Torino (Italy) and earned her Ph.D. degree from University of Nantes (France) in 2012. After postdoctoral stays at University of Sherbrooke (Canada) and University of Brest (France) she is currently working in a joint collaboration between University Grenoble-Alpes/DCM and CEAGrenoble/INAC on nanocrystal-based systems for photocatalytic hydrogen generation. Karl David Wegner studied chemistry at University of Potsdam (Germany) and obtained his Ph.D. in Physics from University ParisSud in 2015. He is now a postdoctoral associate under the supervision of Dr. Peter Reiss at CEA-Grenoble/INAC. His research focuses on the synthesis of III−V and I−III−VI2 colloidal semiconductor nanocrystals and their application in optoelectronics and as biosensors. Dmitry Aldakov graduated from Mendeleev University, Moscow, Russia and earned his Ph.D. from Bowling Green State University, United States (2004). After postdoctoral research at CEA Grenoble and Ecole Polytechnique Paris (2005-2011) he joined CEA-Grenoble/ INAC as a CNRS researcher. His current research interests include third-generation PV, perovskite materials, self-assembly, quantum dots, interface characterization, and surface chemistry. Peter Reiss is a researcher at CEA Grenoble/INAC and Head of the STEP laboratory. He graduated from University of Karlsruhe (Germany) and earned his Ph.D. in inorganic chemistry in 2000. His current research focuses on the chemistry of semiconductor nanocrystals, nanowires, and hybrid perovskites in view of their applications in energy conversion and storage as well as in life sciences. 1085


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ACKNOWLEDGMENTS The authors acknowledge the French National Research Agency ANR for financial support (Grants NIRA ANR-13BS08-011-03, QuePhelec ANR-13-BS10-0011-01, SuperSansPlomb ANR-15-CE05-0023-01, and PERSIL ANR-16-CE050019-02). They further gratefully acknowledge the Labex SERENADE (Grant Saquado), the Labex ARCANE (Grant QDPhotocat), as well as CAPES/COFECUB (Grant 858/15).

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