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Investigation of the light-soaking effect in organic solar cells using dielectric permittivity and electric modulus approaches
Accepted Manuscript Investigation of the light-soaking effect in organic solar cells using dielectric permittivity and electric modulus approaches J. Symonowicz, M. Morawski, M. Dusza, P. Peksa, A. Sieradzki, F. Granek PII:
S1566-1199(17)30486-X
DOI:
10.1016/j.orgel.2017.09.048
Reference:
ORGELE 4329
To appear in:
Organic Electronics
Received Date: 24 October 2016 Revised Date:
30 July 2017
Accepted Date: 27 September 2017
Please cite this article as: J. Symonowicz, M. Morawski, M. Dusza, P. Peksa, A. Sieradzki, F. Granek, Investigation of the light-soaking effect in organic solar cells using dielectric permittivity and electric modulus approaches, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.09.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Investigation of the Light-Soaking Effect in Organic Solar Cells Using Dielectric Permittivity MANUSCRIPT andACCEPTED Electric Modulus Approaches J. Symonowicz a, M. Morawski b, M. Dusza b,c, P. Peksaa, A. Sieradzki a, F. Granek b,*
a
Division of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370, Wrocław, Poland b
Institute of Low Temperature and Structure Research, Polish Academy of Science, Okolna 2, 50-422 Wroclaw, Poland
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Wroclaw Research Centre EIT+, Stablowicka 147, 54-066 Wroclaw, Poland
ABSTRACT
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We demonstrate the influence of a light-soaking treatment on current-voltage characteristics, external quantum efficiency and dielectric response of an encapsulated P3HT:PCBM bulk heterojunction inverted organic solar cell. Furthermore, we apply dielectric permittivity and electric modulus approaches to identify relaxation peaks observed in this device. We discuss previously reported explanations of the light-soaking effect on the basis of our experimental results, showing that the light-soaking effect originates from dipole formation at the ZnO/P3HT:PCBM interface. In addition, we find that metal oxide hole and electron selective layers prevent the P3HT:PCBM molecules’ reorientations and, thus, enhance its thermal stability. This type of experimental observations are expected to provide new prospective on the fundamental aspect of elementary charge transfer modifications in organic solar cells. Key words: organic solar cell, light-soaking, P3HT:PCBM bulk heterojunction, ZnO, dielectric spectroscopy
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1. Introduction
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Rapid progress in the field of organic photovoltaics has been observed since polymer-fullerene diodes were first presented [1,2]. The first organic solar cells (OSCs) suffered from a very poor light to electricity power conversion efficiency (PCE) since, fifteen years ago, only OSCs with a breakthrough 2.5% efficiency were reported [3]. Today, multi-junction OSCs exceed 13% efficiency [4]. Despite the low-cost solution processability and the possibility to apply roll-to-roll manufacturing of OSCs on flexible substrates, this novel technology suffers from stability issues and high sensitivity to oxygen and moisture. It is of a great concern to improve shelf life and reproducibility of those devices. A commonly used organic bulk heterojunction (BHJ) consists of poly-3-hexylthiophene (P3HT) as a donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor material. Reproducible, air-processed solar cells incorporating such heterojunctions are reported [5]. The P3HT:PCBM BHJ solar cells in a so-called inverted structure are especially stable, incorporating metal oxides as hole and electron selective layers. However, the light-soaking effect is often observed in such architecture and it affects the device’s stability and alters its performance. Light-soaking effect is a phenomenon that manifests itself in the change of photovoltaic parameters of a solar cell due to irradiance provided in a given period of time. The effect is either reported to be permanent or reversible under dark storage or annealing [6–9]. This issue is observed in OSCs, as well as in other thin-film solar cells, based on perovskites [10,11], dyes [12,13], coper indium gallium di(selenide) (CIGS) [14,15] and amorphous silicon (a-Si) [16]. In the case of OSCs, the effect is very often interpreted as the initial kink or S-shape in the current-voltage characteristics of degraded solar cells that can be partially removed by UV illumination [5,17,18]. Authors suggest, that this issue is connected with electronic trap states present in metal oxides or at their interfaces with indium tin oxide (ITO) and the photoactive layer [8,17,19]. These states cause dipole formation and improve the energy level alignment between the metal oxide and the photoactive layer [8,17,19]. However, strong divergence in reports on the light-soaking effect indicates that it is not fully understood and that its origin can vary depending on materials and processes used in a given experiment. As it is going to be shown in the following article, in the solar cell under our investigation, we do not observe the S-shape in the I-V characteristics. The light-soaking effect manifests itself in the increased short circuit current and, as it is further revealed by the dielectric spectroscopy measurements, in the increased number of surface dipoles at the metal-oxide-BHJ interface.
In this paper, we apply dielectric spectroscopy technique to study the light-soaking effect in ITO/ZnO/P3HT:PCBM/MoO3/Ag invertedACCEPTED organic solar cells. Dielectric spectroscopy has been previously applied as a MANUSCRIPT complementary method for investigation of OSCs and other electronic devices [20–30]. This technique is mostly used for electronic characterization by an impedance approach where equivalent circuits are proposed to model electronic properties of solar cells [17,19-24,26]. However, dielectric spectroscopy also allows the possibility to analyse samples using dielectric permittivity ε*(ω) and electric modulus M*(ω) approaches [13-15,25]. This manner of data analysis allows a general characterization of the device with respect to the creation of internal electric fields, thus revealing polarization effects. The appearance of additional electric fields may significantly influence transport characteristics of photo-generated carriers, either enhancing or reducing the conductivity.
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2. Methods and experiment
2.1. Broadband dielectric spectroscopy – dielectric permittivity and electric modulus approaches
ε* (ω)= ε’(ω)+iε’’(ω)
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In this chapter, we define tools needed to apply dielectric permittivity and electric modulus approaches in dielectric spectroscopy analysis. In general, by applying to any sample an external oscillating electric field, we can measure its complex permittivity: (1)
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where the real and imaginary components are, respectively, the storage and loss of the energy of the electric field during each cycle [14]. The measured signal is a superposition of the dielectric relaxations, hopping mechanisms, and molecular structure deformations due to a diffusion of charge carriers [31]. In stable systems, ε’(ω) and ε’’(ω) contain the same information as they are interrelated by the Kramers-Kronigrelations. In general, a peak on the ε’’(ω) plot followed by the step-like behaviour of ε’(ω) indicates a dipolar relaxation process while the linear response of ε’(ω) and ε’’(ω) gives information about the diffusion of charge carriers [32]. The scheme of such behaviours is presented in fig. 1. The inverse of maximum value of a dipolar relaxation peak indicates a relaxation time of the process it represents. The relaxation time of charge translational movement cannot be determined [31]. In the case when the measured signal holds a significant dc-conductivity contribution that obscures relaxation processes, we may employ the well-known inversion method in order to obtain conductivity-free dielectric loss peaks [32,33]: .
ε’’ω = − ∙
(2)
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Due to the linear response theory, the dielectric data is equally described by the complex electric modulus M*(ω) [32], which relates to the complex permittivity in the following way:
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∗
= + , where:
=
=
(3) (4) (5)
From the physical point of view, the electric modulus M*(ω) corresponds to the relaxation of the electric field in the material when the electric displacement remains constant [34]. However, this formalism has been applied also to materials with non-zero conductivity [33]. Simultaneous peaks on both the M’’(ω) and the ε’’(ω) plots indicate dipolar relaxations. On the other hand, a peak on the M’’(ω) plot with simultaneous step-like behaviour on the M’(ω) plot and the linear ε’’(ω) response suggest a relaxation process originating from the dc-conductivity (see fig. 1) [32]. Therefore, in contrast to the permittivity approach, in the electric modulus approach both relaxation and translation mechanisms are expressed by peaks from which we may estimate relaxation times. Additionally, electric modulus representation allows us to verify the data obtained by the permittivity approach [33]. In order to obtain conduction-free dielectric response, M’’ω can be calculated in a way that is analogous to the inversion method for the permittivity representation:
.
M’’ω = ∙
(6)
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Fig. 1. Scheme of the real (solid line) and the imaginary (dashed line) part of the complex dielectric and modulus function for an ohmic conductivity (a) and a dipolar relaxation process (b). Equations (2) and (6) transform the dipolar relaxation peak obscured by the dcconductivity contribution (c) into a pure dipolar response (d).
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As shown above, the detailed electrical behaviour of a solar cell device can be studied by analysing its complex dielectric permittivity ε*(ω) and electric modulus M*(ω). However, such studies are very scarce due to the complexity of the investigated samples and its difficult electronic interpretation. The complex permittivity of P3HT:PCBM was obtained by Armbruster et al. [21,22]. However, their experiment was only performed for aged OSC previously illuminated with a halogen lamp, and they lacked the modulus approach. 2.2. Technological process and materials
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A P3HT:PCBM-based BHJ inverted organic solar cell on structured ITO glass substrate obtained from Ossila was prepared according to the following steps. The substrate was cleaned ultrasonically in distilled water, acetone, 2-propanol and subsequently dried with nitrogen. Zinc oxide nanoparticles (40 wt % dispersion in ethanol, Sigma-Aldrich) were spin-casted at 7000 rpm and annealed in 500 °C for 30 minutes in ambient conditions. The sample was then moved into a glovebox filled with nitrogen. A solution of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in 1,2-dichlorobenzene with a concentration of 30mg/ml was spin-casted on top of ZnO at 1000 rpm and pre-annealed at 150 °C for 30 minutes. Thermal evaporation of MoO3 (12 nm) and Ag (200 nm) was followed by glass encapsulation of the assembled device. Encapsulation was chosen in order to ease sample handling and to hinder the degradation caused by oxygen and moisture which could alter the results and, consequently, lead to misinterpretations of the light-soaking effect. The active area of the solar cell is 0.045 cm2 as is defined as an area overlap between the front and back contact (ITO and Ag respectively). In the case of P3HT:PCBM BHJ or ZnO placed between ITO and Ag contacts, the same procedure as described above was followed, with the exclusion of certain technological steps connected with material layers not present in those particular samples (see fig. 2).
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Fig. 2. Schemes of processed and studied solar cell and test structures: the inverted organic solar cell incorporating hole and electron transport layers (A:ETL/BHJ/HTL), sample prepared to investigate dielectric response of P3HT:PCBM BHJ without electron and hole transport layers (B:BHJ) and sample without MoO3 and P3HT:PCBM BHJ (C:ETL).
2.3. Experimental setup and conditions
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Each sample in this experiment was characterized before and after exposure to the light. In our experiment, we define the process of light-soaking as one hour of illumination under continuous solar simulator (AM1.5G solar spectrum, 1000 W/m2). The resulting change of photovoltaic parameters and/or dielectric properties of the samples due to light-soaking is understood as the light-soaking effect. The dielectric spectroscopy measurements of dark characteristics were performed by means of the Novocontrol Alpha impedance analyser in frequencies ranging from 100 Hz to 107 Hz. A very small voltage of 0.01 V was applied across the sample as for this value the relaxation peaks are most prominent. Measurements were performed before and right after illumination under continuous solar simulator for three different samples: ITO/ZnO/P3HT:PCBM/MoO3/Ag organic solar cell (sample A:ETL/BHJ/HTL), the P3HT:PCBM BHJ (sample B:BHJ) and a ZnO layer (sample C:ETL). The data was collected for various temperatures starting from -60 °C and increasing in steps of 5 °C up to 60 °C. The current-voltage characteristics of the A:ETL/BHJ/HTL before and after light-soaking were measured at room temperature (25 °C), using a Keithley 2401 source meter and under continuous class AM1.5G illumination by a solar simulator calibrated to 1000 W/m2 with monocrystalline silicon reference cell provided by the FraunhoferInstitute for Solar Energy Systems ISE. I-V characteristics were measured without a mask. External quantum efficiency (EQE) measurements were conducted using a Bentham PVE300 system with grating monochromator TMc300, 75W Xenon lamp and lock-in amplifier. EQE was measured without light bias.
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3. Results and discussion
In this chapter, the I-V characteristics and EQE of A:ETL/BHJ/HTL before and after light soaking are presented and discussed (fig. 2). The dielectric permittivity (ε*) and electric modulus (M*) data of the C:ETL, the B:BHJ, and the A:ETL/BHJ/HTL before and after light-soaking are depicted in fig. 4, fig. 5, and fig. 7, respectively. It is followed by the derivation of characteristic relaxation times and activation energies (fig. 10). The influence of the light-soaking treatment on the performance of the A:ETL/BHJ/HTL is presented in fig. 3. A nearly 20% increase in the short circuit current density JSC (from 10.9 mA/cm2 to 12.9 mA/cm2) is gained at the expense of increased reverse leakage current and lowered fill factor. Irradiance has no effect on the open circuit voltage which is found to be 0.61 V. The fill factor decreases from 42% to 38%. As a result, power conversion efficiency of the device increases from 2.8% to 3%. Therefore, the increased performance of the device is mostly correlated with the change in JSC. This correlation should also be seen in the EQE spectrum since JSC can be estimated from such measurement [35,36]. However, we do not observe a proportional increase in EQE response after lightsoaking (fig. 3). EQE is even hindered for long excitation wavelengths and increases only slightly for high-energy photons (400-450 nm). This divergence between JSC derived from I-V characteristics and EQE measurements could be caused by the existence of a barrier for photocurrent which is large under low-intensity monochromatic light (in the case of the EQE measurement) and becomes lower at AM1.5G illumination. A similar tendency was observed in the
UV-visible absorbance spectra of P3HT and P3HT:PCBM blend during photo-aging in an oxygen-free environment [37,38]. It is also argued that irradiation modifies the MANUSCRIPT chemical structure of P3HT, leading to a decrease of the ACCEPTED absorption in the 500-600 nm range.
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The large mismatch between the JSC and JSC calculated from EQE spectrum could also be caused by the lack of measurement mask during I-V scans [39,40]. Namely, the effective area of A:ETL/BHJ/HTL could be larger than the area of electrode overlap (0.045 cm2), leading to an overestimated JSC. It was previously shown, that JSC can be area dependent [41]. The smaller the area, the higher potential mismatch exists and masking should be applied. It is very likely, that we observe the discrepancy between JSC and JSC calculated from EQE due to incorrect active area assumption. However, this does not explain the increased JSC after light-soaking. In order to further reveal causes of such behaviour, a broadband dielectric spectroscopy study was conducted.
Fig. 3. Current-voltage characteristics (left) and external quantum efficiency (right) of the A:ETL/BHJ/HTL solar cell before and after 1h of lightsoaking under a solar simulator. Notice the discrepancies between JSC obtained from I-V characteristics and from integration of EQE with AM1.5G spectra.
3.1. Dielectric permittivity and electric modulus analysis of ZnO layer (sample C:ETL)
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According to the dielectric response depicted in fig. 4, the ZnO layer obtained from nanoparticles’ solution (for more details see section 2.2.) behaves like a perfect conductor in a wide range of input signal frequencies. Ambiguous lack of prominent relaxation effects and perfect conductivity of the C:ETL could be explained by the small thickness of the layer (50 nm) combined with possible discontinuities, creating local shunts between adjacent ITO and Ag electrodes. Those shunts could provide energetically favourable pathways for currents in the dielectric spectroscopy measurement, skipping ZnO and resulting in a conductor-like response of the sample under investigation. Moreover, there is no change in the dielectric spectrum after light-soaking. Regardless of whether or not local shunts exist, a light-soaking effect is observed when ZnO, deposited in the same process as described above, is used as an electron transfer layer in the A:ETL/BHJ/HTL. Therefore, it is rather that it is the ZnO/P3HT:PCBM interface that is responsible for the light-soaking effect, as will be discussed later. For further reference, the dielectric response of ZnO (synthesized with the use of different precursors) and the effect of doping on dielectric response are reported in several works [32–36]. It has also been proven that the P3HT:PCBM/MoO3 interface does not contribute to the light-soaking issue [8]. Additionally, a solar cell is illuminated from the ITO/ZnO side and light propagates only into the P3HT:PCBM absorber, not reaching the MoO3 layer. MoO3 absorption, and thus, its contribution to the light-soaking effect, is negligible. Therefore, we will take a deeper insight into the dielectric response of only samples A:ETL/BHJ/HTL and B:BHJ.
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Fig. 4. The real and imaginary parts of the complex permittivity ε* and electric modulus M* of the C:ETL structure before light-soaking (a,c) and just after light-soaking (b,d) in the temperature range of -60 °C to 60 °C (log-log scale representation). Light-soaking did not affect dielectric response of the C:ETL sample. The inserts calculated due to (2) and (6) indicate that there is no dielectric response and that ZnO is a perfect conductor.
3.2.Dielectric permittivity and electric modulus analysis of P3HT:PCBM BHJ (sample B:BHJ) Individual processes visible in the obtained data are marked in the figures 5-7 as follows: D1 – dipolar relaxation process for lower-frequency regime, D2 – dipolar relaxation process for higher-frequency regime, E1 – relaxation process originating from dc electric conductivity.
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The light-soaking effect is associated with the increased conductivity of OSC after illumination of a sample with light [6–9]. The unchanged linear response of imaginary dielectric permittivity ε’’ below the frequency of 103 Hz indicates that no light-soaking effect occurs (see fig. 5b).
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Fig. 5. The real and imaginary parts of the complex permittivity ε* and electric modulus M* of the B:BHJ structure before light-soaking (a,c) and just after light-soaking (b,d) in the temperature range of -60 °C to 60 °C (log-log scale representation). The inserts show the conductivityfree responses calculated due to (2) and (6) for the permittivity and modulus representations, respectively. Light-soaking enhances the number of dipoles throughout the BHJ but does not increase the sample’s conductivity.
The permittivity representation above the frequency of 103 Hz shows that in a P3HT:PCBM BHJ there are two relaxation processes (D1 and D2) which imply formation of dipole-like objects within the analysed material (see fig. 5a). Both of them are enhanced after light exposure (fig. 5b). Therefore, the lamp illumination results in the formation of a higher numbers of dipoles throughout the P3HT:PCBM BHJ.
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The D2 process is temperature-independent both before and after light illumination. The detailed behaviour of D1 process both before and after light illumination is presented in fig. 6. It initially shifts towards lower frequencies with increasing temperature becoming temperature-independent at about 0 °C. After light exposure the peak remains temperature-independent and then, within a week, it disappears completely without appearing ever again. Its behaviour, thus, is quite random and might be associated with temporary dipole formation at the reactive electrode-BHJ-film interface. What is more, in the B:BHJ complex permittivity plot obtained by Armbruster et al. only D2 peak is present [21,22].
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Fig. 6. The imaginary part of the complex permittivity ε’’ (top) and modulus M’’ of the B:BHJ structure before and after lamp illumination for T=300 K. The permanent disappearance of the D1 process is not associated with the light exposure.
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It has been previously described in the literature that P3HT:PCBM contains dipoles [37-39, 42-43]. Both PCBM and P3HT possess their own dipolar momenta and the P3HT:PCBM molecule is a dipole resulting from the dipole-dipole interaction [42,43]. Moreover, due to bulk interactions within P3HT:PCBM blend, different dipole momenta are present inside the bulk and on its surface [42]. Therefore, the dipoles between surface P3HT:PCBM molecules and molecules of a contact material might be formed and the second type of dipoles can be detected while measuring the dipolar response of B:BHJ [42,43]. We believe that this can be the origin of the instable D1 peak. The more pronounced D2 process must be due to the bulk dipoles [37-39, 42-43]. The analysis of the E1 peak in electric modulus representation allows us to estimate the relaxation times of the dc-conductivity process (fig. 5 c,d). The dc-conductivity increases with temperature which is a typical behaviour. Fig. 5 c,d also confirms the random behaviour of the D1 process. Dipolar relaxation peaks in modulus representation are always shifted towards higher frequencies as compared to the same analysis using the permittivity approach [15]. The spectrum of our measurements is not broad enough to detect the D2 peak in the modulus approach. 3.3. Dielectric permittivity and electric modulus analysis of P3HT:PCBM BHJ with ZnO and MoO3 transport layers (sample A:ETL/BHJ/HTL) The dielectric response of A:ETL/BHJ/HTL is very complex (see fig. 7). The light-soaking effect is apparent due to the increase in the dc-conductivity which is indicated in fig. 7 by the upwards shift of the low-frequency regime ε’’ω slope after the irradiation. Once again, two dipolar processes are present. The high-frequency peak accounts for the D2 relaxation as it possess the same magnitude and relaxation time as a process detected in B:BHJ. Since both MoO3 and ZnO layers are conductors, the lower-frequency regime peak can only be associated with dipoles at the BHJ/transportation layers interface. Moreover, its similar relaxation time to the D1 process indicates that similar parts of BHJ might be involved. Therefore, we will also call it D1. Once again, we observe the enhanced dipoles formation throughout the P3HT:PCBM BHJ after irradiation. In contrast to the B:BHJ case, the dc-conductivity is increased in the A:ETL/BHJ/HTL sample. We explain this phenomena by the fact that multiple stable interface dipoles form a stronger electric field inside the solar cell. This reduces the extraction barrier at the ZnO/P3HT:PCBM interface, allowing more electrons to be collected by the ITO cathode (see fig. 9). Such a barrier exists between ZnO and PCBM, as electrons in inverted structure are transported towards ZnO through the PCBM phase [8].The polymer/fullerene interface dipoles were previously proven to help
the exciton dissociation in the production of the photocurrent [37–40]. This can be caused either by the filling of deep interfacial traps during light exposure or by theMANUSCRIPT molecules’ reorientations [37-39]. However, a profound ACCEPTED understanding of the interface dipoles formation mechanisms in A:ETL/BHJ/HTL has not been described yet and requires further research.
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Since the increase of conductivity was not observed in the B:BHJ sample (see fig. 5), we conclude that lightsoaking is an interface effect between active and transport layers. Moreover, it should specifically originate from the ZnO/P3HT:PCBM interface rather than the P3HT:PCBM/MoO3 interface for two reasons. First, most of the incident photons should be absorbed in the solar cell by the time they reach the P3HT:PCBM/MoO3 interface. Thus, no lightinduced changes can occur. Secondly, it was previously examined that P3HT:PCBM/MoO3 is not modified by light irradiation [8]. Therefore, if the D1 process accounted for the dipoles at the BHJ/ MoO3 interface, we would not see the increase in D1 dipoles number after irradiation.
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The exact mechanism of absorption, charge recombination and charge extraction in OSCs is still under debate, however, the following picture can be outlined (see fig. 8). When photon of sufficiently high energy is absorbed in active material (most likely by polymer), the localized Frenkel exciton is formed on molecular site with relatively high binding energy (0.1 eV to 0.5 eV) [45–50]. Exciton has a limited lifetime and eventually it will recombine (geminate recombination with characteristic time ~100ps to 1 ns) to its ground state, unless it gets separated [46,48–50]. However, there is a probability, that the geminate pair will diffuse to the donor-acceptor interface, forming partially-delocalized, intermediate state with lower binding energy – the so-called charge transfer state (CT) with an electron in fullerene phase and a hole in polymer phase. Still, its energy is higher than thermal activation energy at RT (25 meV), therefore it is very unlikely to separate electron and hole in CT state by thermal excitations. Thus, separation of carriers forming CT state requires some additional driving force. This is realized by the existence of built-in-potential across the donor-acceptor BHJ. In inverted OSCs, the incorporation of electron and hole selective layers with different work functions provides required potential. By changing the abovementioned potential difference, e.g. by introducing dipoles and additional electric fields, the net conductivity can be either enhanced or suppressed.
Fig. 7. The real and imaginary parts of the complex permittivity ε* and modulus M* of the A:ETL/BHJ/HTL before light-soaking (a,c) and just after light-soaking (b,d) in the temperature range of -60 °C to 60 °C (log-log scale representation). The inserts show the conductivity-free
responses calculated due to (2) and (6) for the permittivity and modulus representations, respectively. After the light-soaking, the number of interface dipoles is strongly increased, which lowers the work functionMANUSCRIPT at the ZnO/P3HT:PCBM interface and, thus, improves carriers mobility. ACCEPTED
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Fig. 8. The schematic electron transport model in the A:ETL/BHJ/HTL: ① the formation of a Frenkel exciton inside P3HT phase; ② delocalization of an exciton at P3HT/PCBM junction (CT state); ③ electron transport through PCBM and extraction by ZnO. Some electrons are unable to overcome the potential barrier at ZnO/PCBM interface (③).
Fig. 9. The schematic picture of how light soaking effects the structure of A:ETL/BHJ/HTL (dark conditions). After Irradiation more dipoles are formed at the ZnO/P3HT:PCBM interface. This reduces the extraction barrier, allowing more electrons to be collected by the ITO cathode.
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It is worth noting that in the modulus representation, the E1 peak decays significantly after light-soaking. This means that many less carriers undergo this type of relaxation. Such a behaviour is also apparent in the B:BHJ sample. It is possible that the dc-conductivity relaxation is suppressed by the enhanced number of surface dipoles. In fig. 6 we see that once the D1 peak disappears in the B:BHJ, the E1 peak returns to its former magnitude. Once less charge carriers undergo relaxation, more of them can still diffuse through the P3HT:PCBM BHJ. This also slightly contributes to the enhanced dc-conductivity. In contrast to the B:BHJ sample, the dc-conductivity and surface dipolar relaxation processes are almost temperature-independent in the A:ETL/BHJ/HTL sample. Therefore, we conclude that the buffer layers suppress the relaxation of charge carriers and the reorientations of surface dipoles due to thermal excitations. 3.4. Analytical discussion of relaxation processes.
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The comparison of the characteristic relaxation times ( ) and activation energies ( !, ) of A:ETL/BHJ/HTL and B:BHJ reveals whether relaxation processes in both samples are indeed associated with the same segments of P3HT:PCBM BHJ. As a convenient measure of the relaxation process time (τ) for a particular temperature, one can choose the inverse of frequency of the maximum peak position [33], i.e.: τ = (2πfmax)−1
(7)
for both modulus and permittivity representations. The relaxation times gathered for D1, D2, and E1 processes in all temperatures for both samples all exhibit linear dependence satisfying the Arrhenius law (see fig. 10): '
(,) τ = exp & *+ ,
(8)
From the numerical fitting analysis based on (8), we find the !, and values for the D1, D2 and E1 processes in A:ETL/BHJ/HTL and the B:BHJ both before and after illumination by light. The results are shown in tab. 1. and tab. 2. Data for the E1(M*) process after the light-soaking could not be gathered in a A:ETL/BHJ/HTL sample due to its suppression by the D1 process. In the B:BHJ sample, the D1 behaviour below 0 °C cannot be approximated by any power function or, in particular, by the VRH model so it is not caused by an ion-hopping mechanism [33]. This behaviour changes during the first measurement above 0 °C to the Arrhenius one. It is not clear whether it is caused by some phase transition.
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EA [meV] τo [s]
D1(ε*)
D1(M*)
D2(ε*)
17.8
22.0
53.9
6.61·10-6 2.73·10-6
5.50·10-6
after light exposure
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A:ETL/BHJ/HTL before light exposure
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Fig. 10. The Arrhenius plot of the relaxation times obtained for the B:BHJ structure (left) and the A:ETL/BHJ/HTL (right) from dielectric permittivity (ε*) and modulus (M*) representations. D1(ε*) and D1(M*) depict the same dipolar relaxation process for 2 different representations. Some relaxation processes are not presented in a plot as they either vanished or were suppressed by other relaxation processes after the light-soaking.
E1(M*)
D1(ε*)
D1(M*)
D2(ε*)
E1(M*)
28.9
8.1
20.1
37.3
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3.35·10-4
7.99·10-5
6.46·10-5
1.92·10-6
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Tab. 1. Activation energies (EA) and characteristic relaxation times (τo) for the A:ETL/BHJ/HTL before and after light-soaking. D1(ε*) and D1(M*) are the same dipolar relaxation process obtained in the permittivity (ε*) and modulus (M*) representations. Data for the E1(M*) process after the light-soaking could not be gathered due to the suppression by the D1 process.
B:BHJ
τo [s]
D1(ε*) 44.7
after light exposure
D1(M*)
D2(ε*)
E1(M*)
D1(ε*)
D1(M*)
D2(ε*)
E1(M*)
122.1
31.0
522.3
52.6
70.6
34.6
567.6
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EA [meV]
before light exposure
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1.56·10-4 2.42·10-3
5.28·10-7
7.52·10-13 4.72·10-4 7.32·10-4 5.29·10-7
1.77·10-13
Tab. 2. Activation energies (EA) and characteristic relaxation times (τo) for the B:BHJ before and after light-soaking. D1(ε*) and D1(M*) are the same dipolar relaxation process obtained in the permittivity (ε*) and modulus (M*) representations. At the temperature below 0 °C the D1 process before light-soaking stops to obey Arrhenius behaviour.
For the A:ETL/BHJ/HTL sample, the characteristic dipolar relaxation times of about τ0≈10-6 s and !, <25 meV (25 meV is the room temperature activation energy) correspond to the data found in other experiments [20,42]. We are not able to verify the values obtained for the B:BHJ sample as we are the first ones to establish them. The τo value for the D1 process in the modulus representation (D1 (M*)) is only slightly smaller than the value in the permittivity representation (D1(ε*)) (see tab. 1). It indicates that both representations can equally describe the same relaxation processes. This is not completely true for the D1 process in B:BHJ (see tab. 2). In this case, the electric displacement is changing too quickly (see section 2.1) and the modulus representation is no longer perfectly exact. A very similar Arrhenius behaviour, characteristic relaxation times, and activation energies show that the D2 relaxation process is same for both B:BHJ and A:ETL/BHJ/HTL. On the other hand, due to the different vales of !,
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and , we assume that in the D1 process accounts for two different dipolar relaxation processes in B:BHJ and A:ETL/BHJ/HTL samples. ACCEPTED MANUSCRIPT The characteristic relaxation times of process E1 for the B:BHJ structure are τ0≈10-13 s and τ0≈10-4 s for the A:ETL/BHJ/HTL. The difference between their activation energies is 0.5 eV. This suggests that we are dealing with two different processes originating from dc electric conductivity. Indeed, the lack of electron and hole transport layers in the B:BHJ sample should result in an increased recombination rate. This is due to the absence of a built-in potential provided by the difference between the work functions of ZnO and MoO3 that sweeps out charges efficiently. When there is no such additional driving force, the energy required for a charge to be extracted from P3HT:PCBM BHJ should be much bigger. In this case, it exceeds the room temperature activation energy by the order of magnitude. The type dc-conductivity, therefore, depends on the surroundings of the P3HT:PCBM BHJ.
4. Conclusions and summary
Acknowledgements
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Light-soaking phenomena observed in inverted organic solar cells containing ZnO and MoO3 buffer layers were investigated using impedance spectroscopy, current-voltage and external quantum efficiency measurements. Dielectric permittivity (ε*) and electric modulus (M*) approaches were applied to the impedance spectroscopy of P3HT:PCBM BHJ, ZnO and a ITO/ZnO/P3HT:PCBM/MoO3/Ag solar cell device. We manifested that this approach is suitable for the detailed description of the charge transportation processes by investigating for the first time all dipolar processes occurring in P3HT:PCBM BHJ and establishing their activation energy and characteristic relaxation times. We have found that the light-soaking effect manifests itself mostly in the increased value of JSC, which is observed under AM1.5G solar spectrum illumination. However, JSC derived from EQE under low intensity, monochromatic illumination does not follow the same tendency. Most likely, the light-soaking effect occurs due to an increase in the number of dipoles at the ZnO/PCBM interface which induces stronger electric field and lowers the work function difference between ZnO and PCBM. This leads to an improved electron collection efficiency and, thus, enhancement of the extracted photo-current of the solar cell. A bigger number of dipoles after light exposure is also present in the P3HT:PCBM BHJ in the absence of the ZnO layer. However, this case does not result in better current extraction. In addition, we showed that buffer layers change the dielectric response of the P3HT:PCBM BHJ by preventing molecules’ reorientation and changing the types of dielectric and electric relaxations. Therefore, our theory supports the suggestions that light-soaking issues can be overcome by improving the energy level alignment between the metal oxide and the photoactive layer [8,17,19]. However, our description contains a straight-forward picture of the sample’s relaxation processes rather than a comparison of various materials which may improve cells’ efficiency. In order to fully understand a dipole formation mechanisms in P3HT:PCBM a further investigations are needed.
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This work was funded by the National Centre for Research and Development (NCBR) within the project POSCiS (Grant agreement no. LIDER/09/129/L-3/11/NCBR/2012).
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The mechanism of light-soaking effect is related to charge transport properties. Three peaks are distinguished from dielectric response of P3HT:PCBM BHJ. Light-soaking increases the number of dipoles at ZnO/P3HT:PCBM interface. Electron and hole selective layers enhance thermal stability of P3HT:PCBM.
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• • • •
S1566-1199(17)30486-X
DOI:
10.1016/j.orgel.2017.09.048
Reference:
ORGELE 4329
To appear in:
Organic Electronics
Received Date: 24 October 2016 Revised Date:
30 July 2017
Accepted Date: 27 September 2017
Please cite this article as: J. Symonowicz, M. Morawski, M. Dusza, P. Peksa, A. Sieradzki, F. Granek, Investigation of the light-soaking effect in organic solar cells using dielectric permittivity and electric modulus approaches, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.09.048. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Investigation of the Light-Soaking Effect in Organic Solar Cells Using Dielectric Permittivity MANUSCRIPT andACCEPTED Electric Modulus Approaches J. Symonowicz a, M. Morawski b, M. Dusza b,c, P. Peksaa, A. Sieradzki a, F. Granek b,*
a
Division of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrzeze Wyspianskiego 27, 50-370, Wrocław, Poland b
Institute of Low Temperature and Structure Research, Polish Academy of Science, Okolna 2, 50-422 Wroclaw, Poland
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c
Wroclaw Research Centre EIT+, Stablowicka 147, 54-066 Wroclaw, Poland
ABSTRACT
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We demonstrate the influence of a light-soaking treatment on current-voltage characteristics, external quantum efficiency and dielectric response of an encapsulated P3HT:PCBM bulk heterojunction inverted organic solar cell. Furthermore, we apply dielectric permittivity and electric modulus approaches to identify relaxation peaks observed in this device. We discuss previously reported explanations of the light-soaking effect on the basis of our experimental results, showing that the light-soaking effect originates from dipole formation at the ZnO/P3HT:PCBM interface. In addition, we find that metal oxide hole and electron selective layers prevent the P3HT:PCBM molecules’ reorientations and, thus, enhance its thermal stability. This type of experimental observations are expected to provide new prospective on the fundamental aspect of elementary charge transfer modifications in organic solar cells. Key words: organic solar cell, light-soaking, P3HT:PCBM bulk heterojunction, ZnO, dielectric spectroscopy
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1. Introduction
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Rapid progress in the field of organic photovoltaics has been observed since polymer-fullerene diodes were first presented [1,2]. The first organic solar cells (OSCs) suffered from a very poor light to electricity power conversion efficiency (PCE) since, fifteen years ago, only OSCs with a breakthrough 2.5% efficiency were reported [3]. Today, multi-junction OSCs exceed 13% efficiency [4]. Despite the low-cost solution processability and the possibility to apply roll-to-roll manufacturing of OSCs on flexible substrates, this novel technology suffers from stability issues and high sensitivity to oxygen and moisture. It is of a great concern to improve shelf life and reproducibility of those devices. A commonly used organic bulk heterojunction (BHJ) consists of poly-3-hexylthiophene (P3HT) as a donor and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor material. Reproducible, air-processed solar cells incorporating such heterojunctions are reported [5]. The P3HT:PCBM BHJ solar cells in a so-called inverted structure are especially stable, incorporating metal oxides as hole and electron selective layers. However, the light-soaking effect is often observed in such architecture and it affects the device’s stability and alters its performance. Light-soaking effect is a phenomenon that manifests itself in the change of photovoltaic parameters of a solar cell due to irradiance provided in a given period of time. The effect is either reported to be permanent or reversible under dark storage or annealing [6–9]. This issue is observed in OSCs, as well as in other thin-film solar cells, based on perovskites [10,11], dyes [12,13], coper indium gallium di(selenide) (CIGS) [14,15] and amorphous silicon (a-Si) [16]. In the case of OSCs, the effect is very often interpreted as the initial kink or S-shape in the current-voltage characteristics of degraded solar cells that can be partially removed by UV illumination [5,17,18]. Authors suggest, that this issue is connected with electronic trap states present in metal oxides or at their interfaces with indium tin oxide (ITO) and the photoactive layer [8,17,19]. These states cause dipole formation and improve the energy level alignment between the metal oxide and the photoactive layer [8,17,19]. However, strong divergence in reports on the light-soaking effect indicates that it is not fully understood and that its origin can vary depending on materials and processes used in a given experiment. As it is going to be shown in the following article, in the solar cell under our investigation, we do not observe the S-shape in the I-V characteristics. The light-soaking effect manifests itself in the increased short circuit current and, as it is further revealed by the dielectric spectroscopy measurements, in the increased number of surface dipoles at the metal-oxide-BHJ interface.
In this paper, we apply dielectric spectroscopy technique to study the light-soaking effect in ITO/ZnO/P3HT:PCBM/MoO3/Ag invertedACCEPTED organic solar cells. Dielectric spectroscopy has been previously applied as a MANUSCRIPT complementary method for investigation of OSCs and other electronic devices [20–30]. This technique is mostly used for electronic characterization by an impedance approach where equivalent circuits are proposed to model electronic properties of solar cells [17,19-24,26]. However, dielectric spectroscopy also allows the possibility to analyse samples using dielectric permittivity ε*(ω) and electric modulus M*(ω) approaches [13-15,25]. This manner of data analysis allows a general characterization of the device with respect to the creation of internal electric fields, thus revealing polarization effects. The appearance of additional electric fields may significantly influence transport characteristics of photo-generated carriers, either enhancing or reducing the conductivity.
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2. Methods and experiment
2.1. Broadband dielectric spectroscopy – dielectric permittivity and electric modulus approaches
ε* (ω)= ε’(ω)+iε’’(ω)
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In this chapter, we define tools needed to apply dielectric permittivity and electric modulus approaches in dielectric spectroscopy analysis. In general, by applying to any sample an external oscillating electric field, we can measure its complex permittivity: (1)
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where the real and imaginary components are, respectively, the storage and loss of the energy of the electric field during each cycle [14]. The measured signal is a superposition of the dielectric relaxations, hopping mechanisms, and molecular structure deformations due to a diffusion of charge carriers [31]. In stable systems, ε’(ω) and ε’’(ω) contain the same information as they are interrelated by the Kramers-Kronigrelations. In general, a peak on the ε’’(ω) plot followed by the step-like behaviour of ε’(ω) indicates a dipolar relaxation process while the linear response of ε’(ω) and ε’’(ω) gives information about the diffusion of charge carriers [32]. The scheme of such behaviours is presented in fig. 1. The inverse of maximum value of a dipolar relaxation peak indicates a relaxation time of the process it represents. The relaxation time of charge translational movement cannot be determined [31]. In the case when the measured signal holds a significant dc-conductivity contribution that obscures relaxation processes, we may employ the well-known inversion method in order to obtain conductivity-free dielectric loss peaks [32,33]: .
ε’’ω = − ∙
(2)
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Due to the linear response theory, the dielectric data is equally described by the complex electric modulus M*(ω) [32], which relates to the complex permittivity in the following way:
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∗ =
∗
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=
=
(3) (4) (5)
From the physical point of view, the electric modulus M*(ω) corresponds to the relaxation of the electric field in the material when the electric displacement remains constant [34]. However, this formalism has been applied also to materials with non-zero conductivity [33]. Simultaneous peaks on both the M’’(ω) and the ε’’(ω) plots indicate dipolar relaxations. On the other hand, a peak on the M’’(ω) plot with simultaneous step-like behaviour on the M’(ω) plot and the linear ε’’(ω) response suggest a relaxation process originating from the dc-conductivity (see fig. 1) [32]. Therefore, in contrast to the permittivity approach, in the electric modulus approach both relaxation and translation mechanisms are expressed by peaks from which we may estimate relaxation times. Additionally, electric modulus representation allows us to verify the data obtained by the permittivity approach [33]. In order to obtain conduction-free dielectric response, M’’ω can be calculated in a way that is analogous to the inversion method for the permittivity representation:
.
M’’ω = ∙
(6)
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Fig. 1. Scheme of the real (solid line) and the imaginary (dashed line) part of the complex dielectric and modulus function for an ohmic conductivity (a) and a dipolar relaxation process (b). Equations (2) and (6) transform the dipolar relaxation peak obscured by the dcconductivity contribution (c) into a pure dipolar response (d).
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As shown above, the detailed electrical behaviour of a solar cell device can be studied by analysing its complex dielectric permittivity ε*(ω) and electric modulus M*(ω). However, such studies are very scarce due to the complexity of the investigated samples and its difficult electronic interpretation. The complex permittivity of P3HT:PCBM was obtained by Armbruster et al. [21,22]. However, their experiment was only performed for aged OSC previously illuminated with a halogen lamp, and they lacked the modulus approach. 2.2. Technological process and materials
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A P3HT:PCBM-based BHJ inverted organic solar cell on structured ITO glass substrate obtained from Ossila was prepared according to the following steps. The substrate was cleaned ultrasonically in distilled water, acetone, 2-propanol and subsequently dried with nitrogen. Zinc oxide nanoparticles (40 wt % dispersion in ethanol, Sigma-Aldrich) were spin-casted at 7000 rpm and annealed in 500 °C for 30 minutes in ambient conditions. The sample was then moved into a glovebox filled with nitrogen. A solution of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in 1,2-dichlorobenzene with a concentration of 30mg/ml was spin-casted on top of ZnO at 1000 rpm and pre-annealed at 150 °C for 30 minutes. Thermal evaporation of MoO3 (12 nm) and Ag (200 nm) was followed by glass encapsulation of the assembled device. Encapsulation was chosen in order to ease sample handling and to hinder the degradation caused by oxygen and moisture which could alter the results and, consequently, lead to misinterpretations of the light-soaking effect. The active area of the solar cell is 0.045 cm2 as is defined as an area overlap between the front and back contact (ITO and Ag respectively). In the case of P3HT:PCBM BHJ or ZnO placed between ITO and Ag contacts, the same procedure as described above was followed, with the exclusion of certain technological steps connected with material layers not present in those particular samples (see fig. 2).
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Fig. 2. Schemes of processed and studied solar cell and test structures: the inverted organic solar cell incorporating hole and electron transport layers (A:ETL/BHJ/HTL), sample prepared to investigate dielectric response of P3HT:PCBM BHJ without electron and hole transport layers (B:BHJ) and sample without MoO3 and P3HT:PCBM BHJ (C:ETL).
2.3. Experimental setup and conditions
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Each sample in this experiment was characterized before and after exposure to the light. In our experiment, we define the process of light-soaking as one hour of illumination under continuous solar simulator (AM1.5G solar spectrum, 1000 W/m2). The resulting change of photovoltaic parameters and/or dielectric properties of the samples due to light-soaking is understood as the light-soaking effect. The dielectric spectroscopy measurements of dark characteristics were performed by means of the Novocontrol Alpha impedance analyser in frequencies ranging from 100 Hz to 107 Hz. A very small voltage of 0.01 V was applied across the sample as for this value the relaxation peaks are most prominent. Measurements were performed before and right after illumination under continuous solar simulator for three different samples: ITO/ZnO/P3HT:PCBM/MoO3/Ag organic solar cell (sample A:ETL/BHJ/HTL), the P3HT:PCBM BHJ (sample B:BHJ) and a ZnO layer (sample C:ETL). The data was collected for various temperatures starting from -60 °C and increasing in steps of 5 °C up to 60 °C. The current-voltage characteristics of the A:ETL/BHJ/HTL before and after light-soaking were measured at room temperature (25 °C), using a Keithley 2401 source meter and under continuous class AM1.5G illumination by a solar simulator calibrated to 1000 W/m2 with monocrystalline silicon reference cell provided by the FraunhoferInstitute for Solar Energy Systems ISE. I-V characteristics were measured without a mask. External quantum efficiency (EQE) measurements were conducted using a Bentham PVE300 system with grating monochromator TMc300, 75W Xenon lamp and lock-in amplifier. EQE was measured without light bias.
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3. Results and discussion
In this chapter, the I-V characteristics and EQE of A:ETL/BHJ/HTL before and after light soaking are presented and discussed (fig. 2). The dielectric permittivity (ε*) and electric modulus (M*) data of the C:ETL, the B:BHJ, and the A:ETL/BHJ/HTL before and after light-soaking are depicted in fig. 4, fig. 5, and fig. 7, respectively. It is followed by the derivation of characteristic relaxation times and activation energies (fig. 10). The influence of the light-soaking treatment on the performance of the A:ETL/BHJ/HTL is presented in fig. 3. A nearly 20% increase in the short circuit current density JSC (from 10.9 mA/cm2 to 12.9 mA/cm2) is gained at the expense of increased reverse leakage current and lowered fill factor. Irradiance has no effect on the open circuit voltage which is found to be 0.61 V. The fill factor decreases from 42% to 38%. As a result, power conversion efficiency of the device increases from 2.8% to 3%. Therefore, the increased performance of the device is mostly correlated with the change in JSC. This correlation should also be seen in the EQE spectrum since JSC can be estimated from such measurement [35,36]. However, we do not observe a proportional increase in EQE response after lightsoaking (fig. 3). EQE is even hindered for long excitation wavelengths and increases only slightly for high-energy photons (400-450 nm). This divergence between JSC derived from I-V characteristics and EQE measurements could be caused by the existence of a barrier for photocurrent which is large under low-intensity monochromatic light (in the case of the EQE measurement) and becomes lower at AM1.5G illumination. A similar tendency was observed in the
UV-visible absorbance spectra of P3HT and P3HT:PCBM blend during photo-aging in an oxygen-free environment [37,38]. It is also argued that irradiation modifies the MANUSCRIPT chemical structure of P3HT, leading to a decrease of the ACCEPTED absorption in the 500-600 nm range.
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The large mismatch between the JSC and JSC calculated from EQE spectrum could also be caused by the lack of measurement mask during I-V scans [39,40]. Namely, the effective area of A:ETL/BHJ/HTL could be larger than the area of electrode overlap (0.045 cm2), leading to an overestimated JSC. It was previously shown, that JSC can be area dependent [41]. The smaller the area, the higher potential mismatch exists and masking should be applied. It is very likely, that we observe the discrepancy between JSC and JSC calculated from EQE due to incorrect active area assumption. However, this does not explain the increased JSC after light-soaking. In order to further reveal causes of such behaviour, a broadband dielectric spectroscopy study was conducted.
Fig. 3. Current-voltage characteristics (left) and external quantum efficiency (right) of the A:ETL/BHJ/HTL solar cell before and after 1h of lightsoaking under a solar simulator. Notice the discrepancies between JSC obtained from I-V characteristics and from integration of EQE with AM1.5G spectra.
3.1. Dielectric permittivity and electric modulus analysis of ZnO layer (sample C:ETL)
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According to the dielectric response depicted in fig. 4, the ZnO layer obtained from nanoparticles’ solution (for more details see section 2.2.) behaves like a perfect conductor in a wide range of input signal frequencies. Ambiguous lack of prominent relaxation effects and perfect conductivity of the C:ETL could be explained by the small thickness of the layer (50 nm) combined with possible discontinuities, creating local shunts between adjacent ITO and Ag electrodes. Those shunts could provide energetically favourable pathways for currents in the dielectric spectroscopy measurement, skipping ZnO and resulting in a conductor-like response of the sample under investigation. Moreover, there is no change in the dielectric spectrum after light-soaking. Regardless of whether or not local shunts exist, a light-soaking effect is observed when ZnO, deposited in the same process as described above, is used as an electron transfer layer in the A:ETL/BHJ/HTL. Therefore, it is rather that it is the ZnO/P3HT:PCBM interface that is responsible for the light-soaking effect, as will be discussed later. For further reference, the dielectric response of ZnO (synthesized with the use of different precursors) and the effect of doping on dielectric response are reported in several works [32–36]. It has also been proven that the P3HT:PCBM/MoO3 interface does not contribute to the light-soaking issue [8]. Additionally, a solar cell is illuminated from the ITO/ZnO side and light propagates only into the P3HT:PCBM absorber, not reaching the MoO3 layer. MoO3 absorption, and thus, its contribution to the light-soaking effect, is negligible. Therefore, we will take a deeper insight into the dielectric response of only samples A:ETL/BHJ/HTL and B:BHJ.
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Fig. 4. The real and imaginary parts of the complex permittivity ε* and electric modulus M* of the C:ETL structure before light-soaking (a,c) and just after light-soaking (b,d) in the temperature range of -60 °C to 60 °C (log-log scale representation). Light-soaking did not affect dielectric response of the C:ETL sample. The inserts calculated due to (2) and (6) indicate that there is no dielectric response and that ZnO is a perfect conductor.
3.2.Dielectric permittivity and electric modulus analysis of P3HT:PCBM BHJ (sample B:BHJ) Individual processes visible in the obtained data are marked in the figures 5-7 as follows: D1 – dipolar relaxation process for lower-frequency regime, D2 – dipolar relaxation process for higher-frequency regime, E1 – relaxation process originating from dc electric conductivity.
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The light-soaking effect is associated with the increased conductivity of OSC after illumination of a sample with light [6–9]. The unchanged linear response of imaginary dielectric permittivity ε’’ below the frequency of 103 Hz indicates that no light-soaking effect occurs (see fig. 5b).
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Fig. 5. The real and imaginary parts of the complex permittivity ε* and electric modulus M* of the B:BHJ structure before light-soaking (a,c) and just after light-soaking (b,d) in the temperature range of -60 °C to 60 °C (log-log scale representation). The inserts show the conductivityfree responses calculated due to (2) and (6) for the permittivity and modulus representations, respectively. Light-soaking enhances the number of dipoles throughout the BHJ but does not increase the sample’s conductivity.
The permittivity representation above the frequency of 103 Hz shows that in a P3HT:PCBM BHJ there are two relaxation processes (D1 and D2) which imply formation of dipole-like objects within the analysed material (see fig. 5a). Both of them are enhanced after light exposure (fig. 5b). Therefore, the lamp illumination results in the formation of a higher numbers of dipoles throughout the P3HT:PCBM BHJ.
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The D2 process is temperature-independent both before and after light illumination. The detailed behaviour of D1 process both before and after light illumination is presented in fig. 6. It initially shifts towards lower frequencies with increasing temperature becoming temperature-independent at about 0 °C. After light exposure the peak remains temperature-independent and then, within a week, it disappears completely without appearing ever again. Its behaviour, thus, is quite random and might be associated with temporary dipole formation at the reactive electrode-BHJ-film interface. What is more, in the B:BHJ complex permittivity plot obtained by Armbruster et al. only D2 peak is present [21,22].
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Fig. 6. The imaginary part of the complex permittivity ε’’ (top) and modulus M’’ of the B:BHJ structure before and after lamp illumination for T=300 K. The permanent disappearance of the D1 process is not associated with the light exposure.
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It has been previously described in the literature that P3HT:PCBM contains dipoles [37-39, 42-43]. Both PCBM and P3HT possess their own dipolar momenta and the P3HT:PCBM molecule is a dipole resulting from the dipole-dipole interaction [42,43]. Moreover, due to bulk interactions within P3HT:PCBM blend, different dipole momenta are present inside the bulk and on its surface [42]. Therefore, the dipoles between surface P3HT:PCBM molecules and molecules of a contact material might be formed and the second type of dipoles can be detected while measuring the dipolar response of B:BHJ [42,43]. We believe that this can be the origin of the instable D1 peak. The more pronounced D2 process must be due to the bulk dipoles [37-39, 42-43]. The analysis of the E1 peak in electric modulus representation allows us to estimate the relaxation times of the dc-conductivity process (fig. 5 c,d). The dc-conductivity increases with temperature which is a typical behaviour. Fig. 5 c,d also confirms the random behaviour of the D1 process. Dipolar relaxation peaks in modulus representation are always shifted towards higher frequencies as compared to the same analysis using the permittivity approach [15]. The spectrum of our measurements is not broad enough to detect the D2 peak in the modulus approach. 3.3. Dielectric permittivity and electric modulus analysis of P3HT:PCBM BHJ with ZnO and MoO3 transport layers (sample A:ETL/BHJ/HTL) The dielectric response of A:ETL/BHJ/HTL is very complex (see fig. 7). The light-soaking effect is apparent due to the increase in the dc-conductivity which is indicated in fig. 7 by the upwards shift of the low-frequency regime ε’’ω slope after the irradiation. Once again, two dipolar processes are present. The high-frequency peak accounts for the D2 relaxation as it possess the same magnitude and relaxation time as a process detected in B:BHJ. Since both MoO3 and ZnO layers are conductors, the lower-frequency regime peak can only be associated with dipoles at the BHJ/transportation layers interface. Moreover, its similar relaxation time to the D1 process indicates that similar parts of BHJ might be involved. Therefore, we will also call it D1. Once again, we observe the enhanced dipoles formation throughout the P3HT:PCBM BHJ after irradiation. In contrast to the B:BHJ case, the dc-conductivity is increased in the A:ETL/BHJ/HTL sample. We explain this phenomena by the fact that multiple stable interface dipoles form a stronger electric field inside the solar cell. This reduces the extraction barrier at the ZnO/P3HT:PCBM interface, allowing more electrons to be collected by the ITO cathode (see fig. 9). Such a barrier exists between ZnO and PCBM, as electrons in inverted structure are transported towards ZnO through the PCBM phase [8].The polymer/fullerene interface dipoles were previously proven to help
the exciton dissociation in the production of the photocurrent [37–40]. This can be caused either by the filling of deep interfacial traps during light exposure or by theMANUSCRIPT molecules’ reorientations [37-39]. However, a profound ACCEPTED understanding of the interface dipoles formation mechanisms in A:ETL/BHJ/HTL has not been described yet and requires further research.
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Since the increase of conductivity was not observed in the B:BHJ sample (see fig. 5), we conclude that lightsoaking is an interface effect between active and transport layers. Moreover, it should specifically originate from the ZnO/P3HT:PCBM interface rather than the P3HT:PCBM/MoO3 interface for two reasons. First, most of the incident photons should be absorbed in the solar cell by the time they reach the P3HT:PCBM/MoO3 interface. Thus, no lightinduced changes can occur. Secondly, it was previously examined that P3HT:PCBM/MoO3 is not modified by light irradiation [8]. Therefore, if the D1 process accounted for the dipoles at the BHJ/ MoO3 interface, we would not see the increase in D1 dipoles number after irradiation.
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The exact mechanism of absorption, charge recombination and charge extraction in OSCs is still under debate, however, the following picture can be outlined (see fig. 8). When photon of sufficiently high energy is absorbed in active material (most likely by polymer), the localized Frenkel exciton is formed on molecular site with relatively high binding energy (0.1 eV to 0.5 eV) [45–50]. Exciton has a limited lifetime and eventually it will recombine (geminate recombination with characteristic time ~100ps to 1 ns) to its ground state, unless it gets separated [46,48–50]. However, there is a probability, that the geminate pair will diffuse to the donor-acceptor interface, forming partially-delocalized, intermediate state with lower binding energy – the so-called charge transfer state (CT) with an electron in fullerene phase and a hole in polymer phase. Still, its energy is higher than thermal activation energy at RT (25 meV), therefore it is very unlikely to separate electron and hole in CT state by thermal excitations. Thus, separation of carriers forming CT state requires some additional driving force. This is realized by the existence of built-in-potential across the donor-acceptor BHJ. In inverted OSCs, the incorporation of electron and hole selective layers with different work functions provides required potential. By changing the abovementioned potential difference, e.g. by introducing dipoles and additional electric fields, the net conductivity can be either enhanced or suppressed.
Fig. 7. The real and imaginary parts of the complex permittivity ε* and modulus M* of the A:ETL/BHJ/HTL before light-soaking (a,c) and just after light-soaking (b,d) in the temperature range of -60 °C to 60 °C (log-log scale representation). The inserts show the conductivity-free
responses calculated due to (2) and (6) for the permittivity and modulus representations, respectively. After the light-soaking, the number of interface dipoles is strongly increased, which lowers the work functionMANUSCRIPT at the ZnO/P3HT:PCBM interface and, thus, improves carriers mobility. ACCEPTED
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Fig. 8. The schematic electron transport model in the A:ETL/BHJ/HTL: ① the formation of a Frenkel exciton inside P3HT phase; ② delocalization of an exciton at P3HT/PCBM junction (CT state); ③ electron transport through PCBM and extraction by ZnO. Some electrons are unable to overcome the potential barrier at ZnO/PCBM interface (③).
Fig. 9. The schematic picture of how light soaking effects the structure of A:ETL/BHJ/HTL (dark conditions). After Irradiation more dipoles are formed at the ZnO/P3HT:PCBM interface. This reduces the extraction barrier, allowing more electrons to be collected by the ITO cathode.
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It is worth noting that in the modulus representation, the E1 peak decays significantly after light-soaking. This means that many less carriers undergo this type of relaxation. Such a behaviour is also apparent in the B:BHJ sample. It is possible that the dc-conductivity relaxation is suppressed by the enhanced number of surface dipoles. In fig. 6 we see that once the D1 peak disappears in the B:BHJ, the E1 peak returns to its former magnitude. Once less charge carriers undergo relaxation, more of them can still diffuse through the P3HT:PCBM BHJ. This also slightly contributes to the enhanced dc-conductivity. In contrast to the B:BHJ sample, the dc-conductivity and surface dipolar relaxation processes are almost temperature-independent in the A:ETL/BHJ/HTL sample. Therefore, we conclude that the buffer layers suppress the relaxation of charge carriers and the reorientations of surface dipoles due to thermal excitations. 3.4. Analytical discussion of relaxation processes.
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The comparison of the characteristic relaxation times ( ) and activation energies ( !, ) of A:ETL/BHJ/HTL and B:BHJ reveals whether relaxation processes in both samples are indeed associated with the same segments of P3HT:PCBM BHJ. As a convenient measure of the relaxation process time (τ) for a particular temperature, one can choose the inverse of frequency of the maximum peak position [33], i.e.: τ = (2πfmax)−1
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for both modulus and permittivity representations. The relaxation times gathered for D1, D2, and E1 processes in all temperatures for both samples all exhibit linear dependence satisfying the Arrhenius law (see fig. 10): '
(,) τ = exp & *+ ,
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From the numerical fitting analysis based on (8), we find the !, and values for the D1, D2 and E1 processes in A:ETL/BHJ/HTL and the B:BHJ both before and after illumination by light. The results are shown in tab. 1. and tab. 2. Data for the E1(M*) process after the light-soaking could not be gathered in a A:ETL/BHJ/HTL sample due to its suppression by the D1 process. In the B:BHJ sample, the D1 behaviour below 0 °C cannot be approximated by any power function or, in particular, by the VRH model so it is not caused by an ion-hopping mechanism [33]. This behaviour changes during the first measurement above 0 °C to the Arrhenius one. It is not clear whether it is caused by some phase transition.
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EA [meV] τo [s]
D1(ε*)
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D2(ε*)
17.8
22.0
53.9
6.61·10-6 2.73·10-6
5.50·10-6
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Fig. 10. The Arrhenius plot of the relaxation times obtained for the B:BHJ structure (left) and the A:ETL/BHJ/HTL (right) from dielectric permittivity (ε*) and modulus (M*) representations. D1(ε*) and D1(M*) depict the same dipolar relaxation process for 2 different representations. Some relaxation processes are not presented in a plot as they either vanished or were suppressed by other relaxation processes after the light-soaking.
E1(M*)
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Tab. 1. Activation energies (EA) and characteristic relaxation times (τo) for the A:ETL/BHJ/HTL before and after light-soaking. D1(ε*) and D1(M*) are the same dipolar relaxation process obtained in the permittivity (ε*) and modulus (M*) representations. Data for the E1(M*) process after the light-soaking could not be gathered due to the suppression by the D1 process.
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1.77·10-13
Tab. 2. Activation energies (EA) and characteristic relaxation times (τo) for the B:BHJ before and after light-soaking. D1(ε*) and D1(M*) are the same dipolar relaxation process obtained in the permittivity (ε*) and modulus (M*) representations. At the temperature below 0 °C the D1 process before light-soaking stops to obey Arrhenius behaviour.
For the A:ETL/BHJ/HTL sample, the characteristic dipolar relaxation times of about τ0≈10-6 s and !, <25 meV (25 meV is the room temperature activation energy) correspond to the data found in other experiments [20,42]. We are not able to verify the values obtained for the B:BHJ sample as we are the first ones to establish them. The τo value for the D1 process in the modulus representation (D1 (M*)) is only slightly smaller than the value in the permittivity representation (D1(ε*)) (see tab. 1). It indicates that both representations can equally describe the same relaxation processes. This is not completely true for the D1 process in B:BHJ (see tab. 2). In this case, the electric displacement is changing too quickly (see section 2.1) and the modulus representation is no longer perfectly exact. A very similar Arrhenius behaviour, characteristic relaxation times, and activation energies show that the D2 relaxation process is same for both B:BHJ and A:ETL/BHJ/HTL. On the other hand, due to the different vales of !,
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and , we assume that in the D1 process accounts for two different dipolar relaxation processes in B:BHJ and A:ETL/BHJ/HTL samples. ACCEPTED MANUSCRIPT The characteristic relaxation times of process E1 for the B:BHJ structure are τ0≈10-13 s and τ0≈10-4 s for the A:ETL/BHJ/HTL. The difference between their activation energies is 0.5 eV. This suggests that we are dealing with two different processes originating from dc electric conductivity. Indeed, the lack of electron and hole transport layers in the B:BHJ sample should result in an increased recombination rate. This is due to the absence of a built-in potential provided by the difference between the work functions of ZnO and MoO3 that sweeps out charges efficiently. When there is no such additional driving force, the energy required for a charge to be extracted from P3HT:PCBM BHJ should be much bigger. In this case, it exceeds the room temperature activation energy by the order of magnitude. The type dc-conductivity, therefore, depends on the surroundings of the P3HT:PCBM BHJ.
4. Conclusions and summary
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Light-soaking phenomena observed in inverted organic solar cells containing ZnO and MoO3 buffer layers were investigated using impedance spectroscopy, current-voltage and external quantum efficiency measurements. Dielectric permittivity (ε*) and electric modulus (M*) approaches were applied to the impedance spectroscopy of P3HT:PCBM BHJ, ZnO and a ITO/ZnO/P3HT:PCBM/MoO3/Ag solar cell device. We manifested that this approach is suitable for the detailed description of the charge transportation processes by investigating for the first time all dipolar processes occurring in P3HT:PCBM BHJ and establishing their activation energy and characteristic relaxation times. We have found that the light-soaking effect manifests itself mostly in the increased value of JSC, which is observed under AM1.5G solar spectrum illumination. However, JSC derived from EQE under low intensity, monochromatic illumination does not follow the same tendency. Most likely, the light-soaking effect occurs due to an increase in the number of dipoles at the ZnO/PCBM interface which induces stronger electric field and lowers the work function difference between ZnO and PCBM. This leads to an improved electron collection efficiency and, thus, enhancement of the extracted photo-current of the solar cell. A bigger number of dipoles after light exposure is also present in the P3HT:PCBM BHJ in the absence of the ZnO layer. However, this case does not result in better current extraction. In addition, we showed that buffer layers change the dielectric response of the P3HT:PCBM BHJ by preventing molecules’ reorientation and changing the types of dielectric and electric relaxations. Therefore, our theory supports the suggestions that light-soaking issues can be overcome by improving the energy level alignment between the metal oxide and the photoactive layer [8,17,19]. However, our description contains a straight-forward picture of the sample’s relaxation processes rather than a comparison of various materials which may improve cells’ efficiency. In order to fully understand a dipole formation mechanisms in P3HT:PCBM a further investigations are needed.
References
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This work was funded by the National Centre for Research and Development (NCBR) within the project POSCiS (Grant agreement no. LIDER/09/129/L-3/11/NCBR/2012).
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ACCEPTED MANUSCRIPT
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The mechanism of light-soaking effect is related to charge transport properties. Three peaks are distinguished from dielectric response of P3HT:PCBM BHJ. Light-soaking increases the number of dipoles at ZnO/P3HT:PCBM interface. Electron and hole selective layers enhance thermal stability of P3HT:PCBM.
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