Jinhyung
Park
abc,
Muhammad T.
Sajjad
d,
Pierre-Henri
Jouneau
e,
Arvydas
Ruseckas
d,
Jérôme
Faure-Vincent
abc,
Ifor D. W.
Samuel
*d,
Peter
Reiss
abc and
Dmitry
Aldakov
*abc
aUniv. Grenoble Alpes, INAC-SPRAM, F-38000 Grenoble, France. E-mail: dmitry.aldakov@cea.fr
bCNRS, INAC-SPRAM, F-38000 Grenoble, France
cCEA, INAC-SPRAM, F-38000 Grenoble, France
dOrganic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, UK. E-mail: idws@st-andrews.ac.uk
eCEA, INAC-SP2M, LEMMA, F-38000 Grenoble, France
First published on 1st December 2015
Recent progress in quantum dot (QD) sensitized solar cells has demonstrated the possibility of low-cost and efficient photovoltaics. However, the standard device structure based on n-type materials often suffers from slow hole injection rate, which may lead to unbalanced charge transport. We have fabricated efficient p-type (inverted) QD sensitized cells, which combine the advantages of conventional QD cells with p-type dye sensitized configurations. Moreover, p-type QD sensitized cells can be used in highly promising tandem configurations with n-type ones. QDs without toxic Cd and Pb elements and with improved absorption and stability were successfully deposited onto mesoporous NiO electrode showing good coverage and penetration according to morphological analysis. Detailed photophysical charge transfer studies showed that high hole injection rates (108 s−1) observed in such systems are comparable with electron injection in conventional n-type QD assemblies. Inverted solar cells fabricated with various QDs demonstrate excellent power conversion efficiencies of up to 1.25%, which is 4 times higher than the best values for previous inverted QD sensitized cells. Attempts to passivate the surface of the QDs show that traditional methods of reduction of recombination in the QD sensitized cells are not applicable to the inverted architectures.
Similar shortcomings observed in DSSCs have led to the emergence of the cells of second generation based on the principle of hole injection from excited organic dyes to nanostructured p-type semiconductors, such as NiO.12,13 While being much less investigated than conventional Grätzel cells, p-DSSCs attract increasing attention over last years and due to a series of recent developments a record efficiency of 2.51% has been achieved.14 Moreover, probably the most important and promising application of p-type sensitized solar cells is their combination with n-type ones in a tandem configuration.13 Indeed, combining two absorbers in the photoanode and the photocathode can allow overcoming the Shockley–Queisser limit. Recently, very promising first examples of the use of such tandem architectures have been demonstrated for dye sensitized solar cells15–17 and water splitting.18 Among the reasons for the generally lower efficiency of these cells compared to n-DSSCs the most critical point is fast recombination of holes injected into the p-type semiconductor with the reduced dye.13,19,20 It is also worth mentioning in this context the progress obtained last year in the field of hybrid perovskite solar cells using NiO as p-type electrode in an inverted cell structure.21–26 An impressive efficiency value of 17.3% was reached very recently for nanostructured NiO obtained via pulsed laser deposition.27 At the same time, the field of hybrid perovskite photovoltaics has its own problems to be solved such as for example the presence of toxic lead in soluble form and the low operational stability and it is therefore important to continue to develop alternative solutions.
By combining the advantages of n-QDSSCs and p-DSSCs for the design of an inverted QDSSC (p-QDSSC) one can overcome their inherent weak points. Such a cell can benefit from the easy tuning of optoelectronic properties of the QDs in order to adjust the energy levels in the system and optimize the hole injection while the whole arsenal of QD surface chemistry methods can be used to reduce the recombination of the separated charges. In such cell after the light absorption the QDs inject a hole into a nanostructured p-type wide band gap semiconductor, while the electron is regenerated by the redox electrolyte and recovered on a counter-electrode (Fig. 1). Even though the concept of inverted cells sensitized by inorganic nanomaterials is very appealing, most attempts to fabricate them made in recent years showed very low efficiencies. Typically, because of problems with energy alignment and/or charge recombination, previous research efforts were generally limited to charge transfer28–30 and photoelectrochemical studies31–33 while the photovoltaic efficiency was very low or not reported34–36 despite relatively decent incident-photon-to-electron conversion efficiency (IPCE) values reaching in some cases 30%.33 To the best of our knowledge, for solar cells using p-QDSSCs architecture the highest efficiency reported so far is 0.35%.37 Generally, the layer of inorganic sensitizer is deposited on the p-type semiconductor by in situ fabrication using chemical bath deposition (CBD),28 successive ionic layer adsorption and deposition (SILAR),33,37 electrodeposition34 or spray pyrolysis.35 None of these methods gives control over the surface states or over the size distribution (and thus energy levels) of the nanometric coatings, both of which are extremely important for efficient cell functioning. Barcelo et al. compared in detail NiO/CdS assemblies obtained by different QD deposition techniques and concluded that the charge injection efficiency follows the order: directly adsorbed colloidal QDs > colloidal QDs deposited via linker > SILAR.31 This study demonstrates that ex situ synthesized colloidal QDs are better adapted for p-QDSSCs because of the higher degree of control of their physical properties.
In most previous studies on QDSSCs the inorganic sensitizers were composed of cadmium chalcogenides (CdS or CdSe). An obvious disadvantage of this type of sensitizers is the toxicity of cadmium compounds whose use should be avoided. Recently, a series of important research works have reported the successful use of colloidal ternary or quaternary QDs for solar cells,6,38–42 such as CuInS2, CuInSe2, AgInS2 which are non-toxic as they do not contain heavy metals, have high absorption coefficients, long photoluminescence lifetimes and band gaps (1–1.5 eV) adapted for efficient sunlight harvesting.43 Moreover, due to crystalline flexibility and defect tolerance such QDs offer alternative band gap tuning strategies in addition to the classical size control: a mixture of the chalcogenide atoms (S2−, Se2−) and introduction of new cations (Zn2+, Ga3+) into the ternary crystalline structure allows varying the band gap in a wide range.44–46 For example, upon introduction of selenium into CuInS2 QDs the band gap decreases thus extending the absorption spectrum and allowing to harvest more infrared photons of the sunlight.6 Another technique that has proved highly beneficial for photovoltaic applications is QD surface passivation by inorganic materials. To give an example, reduced recombination losses were observed in QDSSCs sensitized with CuInS2 QDs when using surface cation exchange with Zn2+.47
In the present paper we report on the development of p-type QDSSCs based on mesoporous NiO sensitized with non-toxic ternary quantum dots of CuInS2 and CuInSxSe2−x with various ligands and surface passivation. The morphology of the obtained assemblies of NiO/QDs, studied in detail by scanning transmission electron spectroscopy (STEM) with High Angle Annular Dark Field (HAADF) detector and Energy Dispersive X-ray spectrometer (EDX), shows excellent penetration of the QDs into the mesoporous NiO layer, which can enhance the QD loading and lead to better light harvesting and charge transfer properties. Transient photoluminescence spectroscopy was used to study hole transfer processes in the cells and high hole injection rates from QDs to NiO of 108 s−1 were observed, which is comparable to the electron injection rates in analogous n-type TiO2/CuInS2 QDs solar cells. Inverted cells fabricated using such assemblies yielded high photoconversion efficiencies of up to 1.25%, i.e. around 4 times more than the highest value reported in this field. Our approach offers a pathway to efficient inverted QD solar cells and paves the way to tandem sensitized cells.
Transmission electron microscopy (TEM) allows determining the shape and average size of the CuInSxSe2−x QDs as tetrahedrons with a height of 4.9 ± 0.3 nm and an edge length of 6.4 ± 0.2 nm (see Fig. S1†). Selected area electron diffraction (SAED) has been used for the determination of the crystal structure lattice parameters of the obtained QDs. While it is generally difficult to differentiate between the cubic zinc blende and the tetragonal chalcopyrite phase in these systems, the observed 2a/c ratio of 1.016 indicates that the latter is predominant in the present case (Fig. S1†). The band gap of CuInSxSe2−x:Zn2+ QDs calculated from electrochemical measurements was about 1.9 eV, which means that the QDs are in the quantum confinement regime (the band gap values for bulk CuInS2 and CuInSe2 are 1.5 and 1.15 eV, respectively). It is corroborated by the fact that the size of such alloyed QDs is above the Bohr radius of CuInS2 but below the one for CuInSe2 (4.1 and 10.6 nm, respectively).
After the deposition of the QDs on the electrode XPS analysis shows unambiguous presence of all the elements of CuInSxSe2−x:Zn2+, moreover the initial peaks of NiO are intact confirming that the deposition process does not alter the chemical composition of NiO (Fig. S11†).
To investigate the penetration of the QDs into the mesoporous layer of NiO after the solution deposition, we have performed FIB-assisted cross-section of the sensitized films followed by HAADF-STEM studies coupled with EDX analysis. On the image of NiO/QD layer it is problematic to distinguish individual QDs because the size is comparable to the morphological features of the NiO itself (see Fig. S5†). At the same time, an image containing chemical information allows to highlight QDs. In the case of CuInSxSe2−x, we have chosen indium and selenium as EDX elemental markers because their peaks are well resolved and can be easily separated from the contributions of nickel and oxygen originating from NiO, whereas copper and sulfur can be present in the sample support (copper TEM grid) and/or top conducting coating. STEM EDX microscopy reveals that the QDs fill homogeneously all the depth of the NiO layer down to the fluorine-doped tin oxide (FTO) layer (Fig. 3A–C and S10†). Higher resolution microscopy allows obtaining a detailed image of the QDs deposited on NiO showing that the former are indeed attached to the walls leaving the interior volume of the pores void, which is beneficial for the subsequent complete solvent pore filling, and its good contact with the excited QDs for charge extraction (Fig. 3D). Moreover, we can conclude that the QD loading on mesoporous NiO is high judging from the respective Se intensity superimposed on Ni one on high resolution images.
![]() | ||
Fig. 3 STEM EDX cartography of a slice of NiO/CuInSxSe2−x:Zn2+ film fabricated by FIB. (A–C): Full substrate thickness slices; (D): high resolution image with Ni and Se elements. |
QDs | Substrate | PL QY [%] | k HT [s−1] |
---|---|---|---|
CuInS2:Zn2+ | Glass | 2.0 | 5.4 × 107 |
NiO | 0.2 | ||
CuInS2:Cd2+ | Glass | 1.1 | 3.9 × 107 |
NiO | 0.2 | ||
CuInS2Se2−x:Zn2+ | Glass | 0.9 | 8.2 × 108 |
NiO | 0.1 | ||
CuInS2Se2−x:Cd2+ | Glass | 1.0 | 5.1 × 108 |
NiO | 0.1 |
To unravel the interplay of all the processes emerging after the excitation of QDs in contact with NiO from a timescale viewpoint, time-resolved photoluminescence studies of NiO/QD films have been performed. First, QD films on glass were excited and their PL decay measured. In agreement with previous studies, a triexponential model appears to be optimal to fit the decay, each exponential corresponding to one of three components: radiative decay involving surface states (shortest decay time on the order of 1 ns), closest and next-to-closest donor acceptor pair recombination (typical decay times of 10 ns and 100 ns, respectively).50,51 Therefore, unlike classical CdS or CdSe QD systems, the fitting of the CuInS2 and CuInSxSe2−x PL decays results in three distinct lifetimes. These could in principle be averaged but taking into account the different underlying de-excitation processes it appears judicious to avoid the use of average lifetimes (details about fitting parameters are given in Table S2, ESI†). Upon the deposition of QDs on NiO, significant shortening of all three components of the PL decay was observed compared to those of QD films on glass as well as a dependence on the QDs composition. For the QDs in this study, the long-lived component decreased by a factor of 5 to 12. Similar substantial reduction of the lifetime has been previously demonstrated in the case of individual CdSe QDs deposited on NiO.30 The interpretation of this observation is based on the same grounds as the decrease of the QY, i.e. appearance of an alternative fast non-radiative decay pathway related to the hole transfer (HT) from the excited QD to NiO due to the favorable position of the energy levels of both components.
One of the indicators of the efficiency of this hole transfer is an apparent HT constant, kHT, which is typically calculated using:
![]() | (1) |
![]() | (2) |
The resulting hole transfer rates for the four materials studied are shown in Fig. 5. The key point is that quenching rate (hole transfer rate) is very strongly time-dependent.
![]() | ||
Fig. 5 Hole transfer rate determined by taking the derivative of PL ratios for CuInS2:Zn2+ and CuInS2:Cd2+ (left panel), and CuInSxSe2−x:Zn2+ and CuInSxSe2−x:Cd2+ (right panel). |
For comparison with other studies, we estimated the average hole transfer rate by integrating the time dependent rate shown in Fig. 5 over time period equivalent to 1/e of fluorescence decay, i.e.
![]() | (3) |
The observed hole transfer is about 15 times faster in the case of mixed selenide-sulfide QDs (Table 1). One of the possible reasons explaining this important difference is better energy alignment at the interface NiO/QD allowing for higher driving force for the hole transfer: introduction of selenium atoms raises the valence band of the CuInS2 QDs approaching it to the VB of NiO.
It is known that under the conditions of high excitation fluence and slow regeneration of the oxidized QDs upon electron transfer in n-type QDSSCs, formation of positive excitons (trions) is possible. Such trions decay subsequently by Auger recombination with a similar rate to that determined for electron transfer, which may lead to confusion.55 In the case of CuInSxSe2−x QDs the decay lifetime of negative trions has been recently determined to be 230 ps resulting in an Auger recombination rate of 4.3 × 109 s−1.55 This means that under the experimental conditions used here hole transfer appears on a longer timescale than negative exciton decay.
QDs | Surface treatment | NiO thickness [μm] | J sc [mA cm−2] | V oc [V] | FF | Efficiency [%] |
---|---|---|---|---|---|---|
a Results for the best cell in series. b Ex situ stands for the ligand exchange in the solution of the QDs prior to deposition; in situ indicates ligand exchange on the QDs deposited on NiO and “NiO-MPA” indicates treatment of NiO by MPA prior to the deposition of QDs with pristine ligands. | ||||||
CuInS2:Zn2+ | — | 3.5 | 1.54 | 0.33 | 0.28 | 0.14 |
CuInSxSe2−x:Cd2+ | — | 3.5 | 0.66 | 0.34 | 0.32 | 0.07 |
CuInSxSe2−x:Zn2+ | — | 3.5 | 5.72 | 0.34 | 0.41 | 0.80 |
5.05 | 0.35 | 0.53 | 0.95a | |||
CuInSxSe2−x:Zn2+ | — | 5.0 | 7.50 | 0.35 | 0.35 | 0.91 |
9.13 | 0.35 | 0.39 | 1.25a | |||
CuInSxSe2−x:Zn2+ | MPA ex situb | 3.5 | 0.87 | 0.46 | 0.25 | 0.10 |
CuInSxSe2−x:Zn2+ | tBA ex situ | 3.5 | 1.64 | 0.38 | 0.34 | 0.21 |
CuInSxSe2−x:Zn2+ | NiO-MPA | 3.5 | 0.96 | 0.45 | 0.25 | 0.11 |
CuInSxSe2−x:Zn2+ | S2−in situ | 3.5 | 1.03 | 0.35 | 0.33 | 0.12 |
CuInSxSe2−x:Zn2+ | ZnS | 3.5 | 0.36 | 0.38 | 0.28 | 0.04 |
In general, the cells demonstrate high short circuit currents of 2–9 mA cm−2, strongly dependent on the QD surface modifications, with relatively stable Voc of 0.35–0.4 V and moderate fill factors of 0.35 (Table 2). The latter is related to charge recombination at interfaces and appears to be the inherent problem of all nanostructured semiconductor NiO films. They exhibit a high density of states in the band gap above the valence band, which act as traps for photogenerated carriers.56,57 The cells obtained with 3.5 μm thick NiO and CuInSxSe2−x:Zn2+ develop a high Jsc of 5 mA cm−2 together with a Voc of 0.35 V and a fill factor of 0.53 resulting in an efficiency of 0.80% in average (with the best cell showing 0.95%) (Fig. 6). Cation exchange of the QDs with Zn2+ and Cd2+ plays an important role for the surface passivation and consequently conversion efficiency as non-exchanged QDs result in poor performance of the cells. By optimizing different parameters of the cells' components we attempted to improve this performance. Not surprisingly, we were not able to modify substantially the open circuit voltage, which is generally defined by the energy difference of the valence band of the photocathode (NiO) and the redox level of the electrolyte (polysulfide). At the same time, the effect of different treatments of the QDs on the obtained photocurrents is significant. Generally, the cells based on QDs containing selenium result in higher Jsc and constant Voc and fill factor, and thus higher photovoltaic efficiencies compared to pure CuInS2 QDs. This is in agreement with photophysical studies showing much slower hole injection of the latter compared to CuInSxSe2−x. Better-aligned energy levels in the case of mixed sulfide-selenide QDs could be at the origin of this behavior. Other possible reasons contributing to the better photovoltaic performance is the larger absorption range of mixed sulfide-selenide QDs. Zn2+ cation exchanged QDs equally result in higher obtained photocurrent compared to Cd2+ exchanged ones due to better surface passivation and lower charge recombination.
The homogeneous coating of the pores of a 3.5 μm thick NiO mesoporous layer by QDs seen on STEM EDX images (see Fig. 3) allows assuming that QD penetration is not limited by the thickness of NiO. In order to increase the QD loading on the NiO scaffold, we have decided to increase its thickness. For cells with a 5 μm thick NiO mesoporous layer the efficiency further augmented reaching the value of 1.25% for the champion cell, which is to the best of our knowledge the highest efficiency for any inorganically sensitized p-type solar cells. This comparably high efficiency is primarily due to the unprecedented Jsc increased from 5.05 (for thinner cells) up to 9.13 mA cm−2. The higher Jsc observed for thicker electrodes allows also understanding the influence of light penetration into the cells: it is known that one of the major drawbacks of mesoporous NiO electrodes is their opacity because of various impurities, such as Ni0.48 This opacity can lead to a decreased light-harvesting efficiency (LHE) and the capacity of light absorption by the sensitizers. The fact that the short circuit current significantly grows upon the increase in photocathode thickness reveals that the light generated by AM1.5G solar simulator penetrates at least up to 5 μm into the optimized NiO scaffold confirming that the cell performance is not limited by its thickness. It is worth mentioning that NiO electrodes with thickness above 5 μm are increasingly hard to fabricate because of the mechanical constraints.
Surface ligands play an important role in the charge transfer both to the wide gap electrode and to the redox electrolyte.58,59 Long organic chains passivating the QD surface after the synthesis prevent an efficient transfer in QD sensitized solar cells because they create a physical and energy barrier for the charge carriers. A typical strategy is thus to replace them with shorter ligands using either an ex or in situ approach.8 Ligand exchange strategies were applied to replace original long dodecanethiol and oleylamine ligands passivating the surface of the QDs used in this work. Ex situ stands for the ligand exchange in the solution of the QDs prior to deposition; in situ indicates ligand exchange on the QDs deposited on NiO and “NiO-MPA” indicates treatment of NiO by mercaptopropionic acid (MPA) prior to the deposition of QDs with pristine ligands. Contrary to conventional n-type cells, where they were shown to work efficiently,6,60 short capping ligands (MPA; tert-butylamine, tBA; sulfide anions) when applied to inverted systems lead to a decreased photovoltaic performance, especially due to a significantly worse Jsc after the exchange although the Voc can be sometimes higher (in the case of MPA treatment). Several reasons can be proposed to explain this decrease of photovoltaic performance as a consequence of surface modifications: less efficient surface passivation, perturbation of energy levels, and change in QD loading. Less efficient QD surface passivation by the new ligands can indeed result in higher losses related to unpassivated surface traps. In the case of “classical” QDSSCs this effect is largely compensated by a gain in charge (hole) transfer to the electrolyte, however probably it is not the case for the electron transfer for the inverted cells. A shift of QD energy levels as a function of passivating ligands can be an additional reason for the efficiency loss. It is known that in some cases ligand exchange can lead to a shift of valence and/or conduction bands of the QDs because of the strong coupling between the ligand and QD surface atoms with consequent electronic perturbation.61,62 As a result, electronic levels modification upon ligand exchange might lead to less favorable energy alignment for the charge transfer. In order to check this hypothesis and estimate the energy levels of colloidal QDs before and after the ligand exchange, electrochemical studies have been performed. Differential pulsed voltammetry (DPV) reveals that the shift of energy levels as a result of surface modification is not very pronounced (less than 0.1 eV) (Table 3), therefore, it is not expected that the ligand exchange would have a significant effect on the band alignment and charge injection at the interfaces with NiO and electrolyte. At the same time, slightly higher increase of the conduction band energy of tBA-coated QDs can cause less efficient charge transfer to the electrolyte compared to native dodecanethiol (DDT)-coated QDs as a result of the increased energy difference with the polysulfide couple redox potential (4.1 eV).
QD/ligands | Native (DDT) | MPA | tBA |
---|---|---|---|
E VB, eV | 5.43 | 5.44 | 5.46 |
E CB, eV | 3.54 | 3.51 | 3.42 |
It is worth noting that electronic level alignment at the interface with NiO and electrolyte in real devices may be different because of band bending and levels pinning. Additional studies to probe such interfaces by UPS are currently underway to give a more detailed answer based on which alternative strategies of ligand exchange could be developed. In addition, the method of QD surface treatment can also play a role in the performance in the cells: QDs processed using ex situ exchange possess the new ligands on the entire surface, while for the in situ exchanged ones only the surface not in contact with the substrate is modified.8 The studied systems, however, do not seem to depend on the order of ligand exchange: the negative factors described above probably dominate the overall performance.
Finally, the role of QD loading in the devices as a function of the surface treatment used was determined. From the EDX studies it was possible to find the ratio of the mass of CuInSxSe2−x QDs to the overall mass of sensitized mesoporous NiO scaffold. The highest loading (15–16 mass%) was achieved using unmodified QDs with native DDT and oleylamine ligands, and tBA. Contrary to the case of CdSe sensitization of NiO,31 MPA pre-treatment of NiO and ex situ MPA ligand exchange was found to lead to much lower QD loading in the film (3–4 mass%) (see details in ESI†), which corresponds very well to much lower photocurrent measured in the corresponding cells. Lower loading achieved using NiO functionalization by MPA can be due to the lower penetration of the QDs to the pores of mesoporous scaffold, while ex situ MPA ligand exchange probably lowers the attainable concentration of the QD solution because of the decreased solubility of MPA coated QDs. Taken together, surface modification of QDs strongly influences the photovoltaic performance of the inverted QDSSCs essentially because of the changed QD loading with some potential contribution of the energy band modification.
Another approach developed to optimize the charge transfer for the conventional QDSSCs and thus improve their photovoltaic performance consists in inorganic coating of the sensitized electrodes by SILAR. While the surface of QDs already contains a passivation layer due to the presence of Zn2+ cations, it probably represents only a thin (sub)monolayer and a thicker coating could be necessary for more efficient passivation. The ZnS layer deposited by SILAR is known to decrease the rate of recombination in n-type QDSSCs.1,63 The same strategy adapted in the case of p-type cells studied in this work turned out to be counterproductive: even though the Voc slightly increased from 0.34 to 0.38 V, the almost 4-fold drop of the Jsc has wiped out any positive effects of the ZnS coating leading to a more than triple decrease in the cell efficiency. The major role of the wide band gap ZnS coating in n-QDSSCs is to suppress undesired charge recombination between the electron on the conduction band of excited QD and the redox level of the polysulfide electrolyte (Fig. 7). However, in the case of p-type QDSSCs, electron transfer from the QD to the electrolyte is an integral part of the working principle of the photovoltaic process. Therefore, by introducing a high-lying conduction band of ZnS we render the electron transfer less efficient, which is manifested by a decreased photovoltaic efficiency. While a thin layer of Zn2+ has a positive effect, a SILAR-grown layer is probably too thick to allow for tunneling. Similar dependence of the solar cell efficiency on the thickness of the passivation layer on CuInS2 QDs (in the case of n-type cells) has been observed by other groups.6,64,65 By consequence, other strategies are needed to suppress undesired recombination pathways originating here from hole transfer from the QD valence band to the electrolyte. A more n-type material showing type II band alignment with respect to the CuInS2Se2−x could be an option.
![]() | ||
Fig. 7 Scheme of energy levels alignment after inorganic ZnS passivation of QDs on standard (left) and inverted (right) configurations. |
IPCE spectra of the cells generally followed the shape of the absorption spectra of CuInS2Se2−x QDs confirming that the photogenerated current is indeed due to the QDs (Fig. S6 and S12†). The cells demonstrate efficient charge generation (up to 72% efficiency at 390 nm) over the broad absorption range up to 900 nm. By integrating the IPCE spectra over the full wavelength range short-circuit photocurrents can be estimated. For the CuInS2Se2−x sensitized NiO cell integrated IPCE values yield the current of 5.30 mA cm−2, which is lower than the real Jsc value measured in the cell (7.50 mA cm−2). Similar phenomena between the estimated and measured photocurrents have been already observed in the case of both n-66 and p-type QDSSCs.33 To understand the origin of this discrepancy the techniques of measurement under IPCE and simulated sunlight conditions need to be compared. The IPCE for the sensitized cells is defined67 by the product of three parameters: LHE of the sensitizer (QDs), charge injection yield (hole injection from the excited QDs to the NiO) and charge collection efficiency by the back contact. The LHE of a QD sensitized cell according to Lambert–Beer law is determined by the QD loading, their extinction coefficient, and the optical absorption depth in the NiO film. While most of the parameters listed above basically stay constant for the two techniques, the absorption depth can vary considerably as a function of the light source used. Indeed, as mentioned before, the mesoporous NiO electrode is much less transparent compared to TiO2, therefore the intensity of incident light could play an important role on its penetration depth and the intensity of the simulated sunlight (100 mW cm−2) is a thousand times stronger compared to the monochromatic light used for IPCE measurements (env. 100 μW cm−2). After the noise correction, the real IPCE spectrum for an inverted QDSSC exhibits the expected shape (Fig. S12†). Moreover, upon the integration the spectrum yields a Jsc of 7.22 mA cm−2, which is very close the current measured for the cell (7.50 mA cm−2), which confirms our hypothesis about the origin of the initial short circuit current discrepancy.
Focused ion beam (FIB) tomography has been realized in a Zeiss NVision 40 dual-beam instrument. In this technique, the NiO layer is cut in cross-section, slice by slice, with a Ga+ ion beam (with a 700 nA current at 30 kV), and each slice is imaged in scanning electron microscopy (SEM) at 5 kV using the in-chamber secondary electron detector. To minimize curtaining effects, a 1 μm thick carbon layer is used prior to the etching in order to smooth the surface of the sample. 400 slices of 4k × 4k pixels have been acquired, with a slice thickness of 3 nm and a pixel size of 3 nm for SEM images.
Electrochemical DPV measurements were performed inside a glove box using an Autolab3 potentiostat/galvanostat using Ag wire pseudo-reference electrode and Pt counter electrode and working electrode (diameter: 4 mm). The samples were prepared by drop-casting 10 μL of a 20 mg mL−1 QD colloidal solution in chloroform on the working electrode and subsequent immersion of the electrodes in the solution of electrolyte (0.1 M tetrabutylammonium hexafluoride in acetonitrile).
Footnote |
† Electronic supplementary information (ESI) available: TEM images of QDs, XPS spectra, UV-vis and PL spectra of the sensitized electrodes, details about photophysical characterization and IPCE spectra interpretation. See DOI: 10.1039/c5ta06769c |
This journal is © The Royal Society of Chemistry 2016 |