Fabrication and Characterization of Hybrid organic-inorganic electron

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Fabrication and Characterization of Hybrid organicinorganic electron extraction layers for polymer solar cells towards improved processing robustness and air stability Donia Fredj, Florent Pourcin, Riva Alkarsifi, Volkan Kilinc, Xianjie Liu, Sadok Ben Dkhil, Nassira Chniba Boudjada, Mats Fahlman, Christine VidelotAckermann, Olivier Margeat, Jörg Ackermann, and Mohamed Boujelbene ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Applied Materials & Interfaces

Fabrication and Characterization of Hybrid organicinorganic electron extraction layers for polymer solar cells towards improved processing robustness and air stability Donia Fredj,1 Florent Pourcin,2 Riva Alkarsifi,2 Volkan Kilinc,2 Xianjie Liu,3 Sadok Ben Dkhil,2,# Nassira Chniba Boudjada,4 Mats Fahlman,3 Christine Videlot-Ackermann,2 Olivier Margeat,2,* Jörg Ackermann,2,* Mohamed Boujelbene.1

Laboratoire Physico-Chimie de l’Etat Solide, LR11 ES51, Faculté des Sciences de Sfax, Université de Sfax, BP 3071 Sfax, Tunisie. 1

2

Aix Marseille Univ, CNRS, CINaM, Marseille, France.

3

Department of Physics, Chemistry and Biology Linkoping University, S8183, Linkoping,

Sweden. 4

Laboratoire de Cristallographie, CNRS, 25 avenue des Martyrs, BP 166, 380, France.

[*] Corresponding Authors: E-mail address: [email protected] ; [email protected] [#] Present address: Dracula Technologies, 4 Rue Georges Auric, 26000 Valence, France.

KEYWORDS: hybrid material; interfacial layer; nanocrystals; morphology; electron extraction; solar cell.

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Abstract Organic-inorganic hybrid materials composed of bismuth and diaminopyridine are studied as novel materials for electron extraction layers in polymer solar cells using regular device structures. The hybrid materials are solution processed on top of two different low band gap polymers (PTB7 or PTB7-Th) as donor materials mixed with fullerene PC70BM as acceptor. The intercalation of the hybrid layer between the photo-active layer and the Aluminum cathode leads to solar cells with power conversion efficiency of 7.8% due to significantly improvements in all photovoltaic parameters, i.e. short-circuit current density, fill factor and open-circuit voltage, similar to the reference devices using ZnO as interfacial layer. However when using thick layers of such hybrid materials for electron extraction, only small losses in photocurrent density are observed in contrast to the reference material ZnO of pronounced losses due to optical spacer effects. Importantly, these hybrid electron extraction layers also strongly improve the device stability in air compared to solar cells processed with ZnO interlayers. Both results underline the high potential of this new class of hybrid materials as electron extraction materials towards robust processing of air stable organic solar cells.

1. Introduction To date, the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) could be increased beyond 13 % for both single and tandem heterojunctions due to intense researches on improved device structures, low band gap donor polymers absorbing more light in the visible,13

and recently new non-fullerene acceptors. 4-6 The latter is particularly interesting due to its

complementary absorption in the visible-near infrared region, providing an effective approach for getting a high short-circuit current density (Jsc). While the optoelectronic properties of the photoactive layer determine theoretically the maximum photocurrent generation and thus the


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efficiency of the solar cell, interfacial layers (ILs) sandwiching the photoactive layer are of equal importance as they provide efficient charge carrier extraction towards the electrodes avoiding hereby losses such as non-ohmic contact, charge carrier recombination and exciton quenching at the interfaces. 7,8 In order to further improve the device performance of PSCs, increasing effort is needed to optimize device structures, processing techniques and the development of new material systems for both active layer and ILs. It is important to mention that interfacial materials in organic solar cells used in both regular and inverted device structures are either pure organic9-11 or inorganic materials.12-14 In contrast, hybrid organicinorganic materials (HMs) has only used in form of nano-composites as interfacial layers15-17 despite the unique properties of HMs such as tunable band gap and energy levels, or more suitable optical properties thanks to the infinite combination of organic-inorganic composition. These novel features of HMs may have the potential to overcome the drawbacks of classical ILs. For instance, metal oxides such as ZnO or TiOx are often used in regular device structures as electron extraction layers (EELs). It is well known that the use of such metal oxide based EELs in regular device structures introduce so called optical spacer (OSP) effects that modify the light distribution inside solar cells.18-19 The choice of spacer thickness and material index of refraction modifies the position of the maximum intensity inside the device stuck and allows to shift it within the photoactive layer and consequently, to increase the overall light absorption. However, we have demonstrated in our previous work7,20 that, in optimized solar cells, ZnO based optical spacers give high efficiency only when used as very thin layer of about 20 -25 nm. By increasing the thickness of the OPS from 20 to 75 nm, which would make their processing at large scale easier and thus more robust, there is a strong loss in photo current density over 33%. Thus, only thin layers lead to high efficiency, which makes the processing of ZnO based EELs at large scale highly challenging.



In this work, we investigate organic-inorganic materials composed of bismuth and diaminopyridine as novel materials for EELs, that aim to combine the advantages of organic and inorganic components in term of electronic and optical properties in order to overcome OSP limitation of metal oxide based EELs in normal device structures. Two novel HMs using bismuth as metal are developed, in which simple change of the organic-inorganic molar ratio leads to two different materials. Moreover, these new materials ([C5H8N3]2[BiCl5] referred as H1 and [C5H8N3]12[BiCl6]4 referred as H2) are found compatible with low temperature solution processing and are less toxic compared to other HMs. Indeed, among the heavy metals, bismuth is outstanding as its toxicity is much lower than its neighbors in the periodic table such as lead, thallium and antimony. In the following sections, we first describe the synthesis of the two HMs followed by detailed characterizations using UV–Vis absorption spectroscopy, Fourier transform infrared (FT-IR), current density–voltage (J–V) analysis, Ultraviolet Photoelectron Spectroscopy (UPS), transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction analysis (XRD). Furthermore, we develop solution of welldispersed HM nanocrystals to solution process polymer solar cells using HM layers as electron extraction layers. We selected PTB7 and PTB7-Th as donor materials in combination with PC70BM to produce high performance polymer solar cells as demonstrated by several groups recently.20-22 Furthermore we have studied intensively electronic and OSP properties of ZnO EELs in solar cells using these two donor materials.7,20 The chemical structures of the used donor and acceptor materials are shown in Figure 1. Under optimized conditions, high efficient PTB7-Th-based solar cells in regular device structure with PCE of 7.8% are obtained demonstrating the potential of HMs approach as ILs. The use of 80 nm thick and thus easier processable EELs, we observe clearly smaller losses in photocurrent density in solar cells using H1 and H2, respectively, compared to those using ZnO EELs due to less pronounced optical spacer effects for the hybrid materials. We further show that these novel hybrid electron 4 ACS Paragon Plus Environment

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extraction layers strongly improve air stability of the solar cells under dark storage compared to device using ZnO.

Fig. 1. Schematic representation of the regular solar cell structure and the corresponding energy level diagram. Molecular structure of donor (PTB7 and PTB7-Th) and acceptor (PC70BM) materials.

2. Experimental part

2.1 Materials

For the synthesis of the HMs, all materials and solvents were purchased from Sigma–Aldrich and used without further purification. PTB7 and PTB7-Th were purchased from 1-Materials. PEDOT:PSS (CLEVIOSTM AI 4083) was purchased from Heraeus CLEVIOSTM. The acceptor



material used in this study is PC70BM from nano-C (purity 99%). The solvent additives 1, 8diodoctane (DIO) and ethylene glycol (EG) were also purchased from Sigma-Aldrich.

2.2 Instruments and procedures Single crystal X-Ray analysis: The intensity data collections were performed using a Brüker APEXII CCD diffractometer with Ag Kα radiation (λ=0.5608 Å). The final cycle of refinement lead to the final discrepancy factors R1 =0.05 and wR2 = 0.16. All the crystallographic details and refinement parameters are summarized in Table S1 and the final atomic coordinates are reported in Table S2. Ultraviolet Photoelectron Spectroscopy: UPS measurements were performed in an ultrahigh vacuum (UHV) surface analysis system with a Scienta-200 hemispherical analyzer and HeI photons (hυ = 21.22 eV). Samples of the two hybrid materials were prepared by spin-coating onto ITO substrates with a resulting film thickness of 40 nm. The work function (WF) of the samples was derived from the secondary electron cut-off. The WF and the valence band edge of the samples were obtained with an error margin of ±0.03 eV. Infrared measurements: The Fourier transform infrared (FT-IR) powder measurements of the hybrid materials were conducted at room temperature, with a Perkin–Elmer FT-IR Paragon 1000 PC spectrometer over the 4000–400cm-1 region. UV-Vis measurements: The UV–vis spectra of the hybrid materials were recorded in solution and thin films spin-coated on glass substrates over the 200–900 nm range using a Cary 5000 UV-Vis-NIR spectrophotometer. Thicknesses measurements: Film thicknesses were measured by a contact profilometer Dektak XTS (Bruker, Germany) equipped with a stylus of 2 µm radius. Transmission electron microscopy measurements: TEM measurements of hybrid materials drop casted on a carbon grid were performed on a JEOL 3010 operating at 300kV. 6 ACS Paragon Plus Environment

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Solar Cell Fabrication and Characterization: The solar cells were fabricated and characterized in regular device structure of (ITO)/PEDOT:PSS/active layer/ZnO/Al as reported in our previous work.20 The hybrid materials in isopropanol (IPA) with 2% v/v EG as additive were filtrated with 4.5 micron pore size, spin-coated on the top of active layers at different speeds (ranging from 1000 to 2500 rpm) and then dried on a hot plate at 80°C for 10 min. J-V curves of solar cells and photovoltaic parameters are presented for best devices, while average PCEs obtained with standard deviation analysis were calculated using nine devices.

3. Results and discussion

3.1 Synthesis of hybrid materials The hybrid compound [C5H8N3]12[BiCl6]4 referred as H2, was synthesized by slow evaporation which is the same method used to prepare the first material [C 5H8N3]2[BiCl5], (referred as H1) as already reported in our previous work.[27] This method consists in dissolving separately organic and inorganic part of 2,6 diaminopyridine and bismuth trichloride in the appropriate solvent which is water in our case. Then, we mixed the two aqueous solutions and we gradually added concentrated hydrochloric acid solution until complete dissolution of the reactants. The solution was stirred and kept at room temperature for 3 hours. All reactions were carried out at room temperature.

For H1, we employed a 1:2 ratio (0.001 mole for BiCl3 to 0.002 mole for 2,6 diaminopyridine) as described in our previous work,27 while the H2 hybrid material [C5H8N3]12[BiCl6]4, is a new compound that has different molar ratio between organic chain and metal part compared to H1 (see hybrid complexes on Figure 2). To synthesize H2, we selected a bismuth trichloride and 2,6 diaminopyridine in order to study the impact of the 7 ACS Paragon Plus Environment


organic part on the electronic properties of HM since these properties are closely related to structural order and organization between organic and inorganic network. H2 was obtained by a 1:1 ratio of bismuth trichloride (0.001 mole) and 2,6 diaminopyridine (0.001 mole). After few days under ambient conditions, the growth of crystals was observed in the solution. In order to separate the crystals from the solutions, filtration was used with adequate pore size. Larger crystals needed for X-Ray diffraction were obtained by growth over several days.

3.2 Detailed analysis of the hybrid materials H1 and H2

The new H2 HM were characterized by X-ray diffraction analyses of single crystals and compared to the already published structure of H1.23 The crystal structure of H2 adopts a triclinic system with space group P1 and lattice parameters a =13.115 (5) Å, b = 14.229 (5) Å, c = 28.969 (5) Å, α = 90.199 (5)°, β = 92.527 (5)°, γ = 90.004 (5)°, which are slightly larger than those of H1.23 In contrast to H1, the independent part of the unit cell consists of an octahedral chloride bismuthate and three 2,6 diamino-pyridinium cations (see Figure 2).


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Fig. 2 Hybrid complexes H1 (left) and H2 (right)

The material H2 is formed by isolated anions of [BiCl6]3- leading to (0D) network 24,25 and independent protonates 2,6-diamino-pyridinium (see Figures S1 and S2). The anions of the inorganic part sit in distorted octahedral coordination due to the stereochemical activity of the Bi(III) lone pair electrons26,27 and the deformations caused by hydrogen bond interactions,28 as observed for H1.23 Thus, Bi_Cl distances vary from 2.603(12) to 2.808(14) Å and angles ClBi-Cl vary from 85.5(5) to 178.1(5)° (Table S3-a) which are similar to those found in H1. Organic cation organization can be described as parallel layers, with face-to-face distances of 3.746 Å to form π-π interactions (< 3.8 Å).29,30 These interactions play a major role in the connection of each two organic groups. The 2,6 diaminopyridinium cations involve distances C-C and C-N ranging from 1.22(9) to 1.62(8) Å, the C-C-C, C-N-C, N-C-C and C-C-N angles vary from 101 (7) to 136 (10)°(see Table S3-b). Organic cations occupy cavities between inorganic entities and these two parts are related via hydrogen bonds, which make the stability of the compound with N-Cl length in the range of 3.135-3.632 Å (see Figure S1 and Table S4).



FTIR spectroscopy is used to detect the functional groups related to the crystal structure. Figure 3 and Table S5 present the FTIR spectra and characteristic vibrational bands of the [C5H8N3]12[BiCl6]4 H2 compound. Referring to some previous works of similar materials, and especially of the H1,23 we propose an assignment of the observed bands in the domain of high frequencies. Peaks associated with stretching NH2 vibration are detected between 3392 and 3265 cm-1. The peaks appearing between 3265 and 2921 cm-1 are assigned to stretching modes of C-H and N − H … Cl. The bands observed in the range 1647-1488 cm-1 can be accredited to scissoring mode of N-H2. The peaks appearing in the two domains 1588-1233 cm-1 and 1092983 cm-1 are related to the stretching modes of C = N and C = C. Additionally, the infrared spectra exhibit bands between 1233 and 983 cm-1 resulting from bending C-H. Both mode of rocking NH2 and stretching C-N are manifested by peaks located at 1092, 1055 and 983 cm-1. The last bands observed at 839, 767 and 689 cm-1 are characteristic of bending modes of C = C − C, scissoring modes of C=C-C, scissoring modes of C − C = N and wagging NH2. These different bands are similar to those found in the case of H1.

Fig. 3. FTIR spectra of H2 material.


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In order to implement the new HMs as ILs into organic solar cells, thin film processing and optimization of the morphology of the layers are essential. In general, high performance in solar cells is obtained by processing closely packed films quality of low roughness.7,20 Thus, we studied the influence of surfactant, solvent and ultrasonic treatment to convert H1 and H2 crystals into solution containing finely dispersed nanocrystals of HMs, which is important to produce high quality films with controlled morphology, crystallinity and corresponding optoelectronic properties. As isopropanol (IPA) is a suitable solvent for ZnO nanocrystals to deposit this material as EELs onto f PTB7-Th:PC70BM blends,20 we selected this solvent to prepare hybrid nanocrystal solution. After intense ultrasonic treatment of the large hybrid crystals to form nanocrystals clusters, the dispersion in IPA were improved by adding ethylene glycol (EG) as dispersing agent leading to highly dispersed HMs solutions as shown on Figure 1. Importantly, ultrasonic treatment was needed to gradually reduce the crystal size after EG treatment and must be performed at least during 5 min to obtain good dispersion of HMs in IPA. The obtained nanocrystals were visualized by TEM as represented on Figure S3. We see that the ultrasonic treatment has produced hybrid nanocrystals with broad size distribution from few nanometers to hundreds of nanometers. In order to solution process interfacial layers of high quality, the dispersion of HMs was filtered by PTFE syringe filters (0.45 μm) to remove the large nanocrystals from the solution. Remarkably, these filtered HMs in IPA form highly stable solution up to a concentration of 25 mg/mL over several days in both air and glove box. The morphology and surface roughness of the solution-processed hybrid films on top of the polymer blend were studied by AFM. As shown on Figure 4, more closely packed layers were observed in the case of H1 with low root-mean square (RMS) roughness of 1.3 nm compared to 6.4 nm in the case of H2 films and better film quality bearing less cracks and pinholes. These optimized



conditions for layer processing make H1 the most promising candidate to behave as efficient IL in corresponding devices.

Fig. 4. AFM images of layers obtained with H1 (left) and H2 (right).

In order to evaluate the opto-electronic properties of these new HMs, we also determined absorption spectra of crystals in solution as well as in thin films processed by spin-coating the solution on glass substrates. Figure 5 shows the UV–Vis absorption spectra of both solution and thin films deposited for the two compounds H1 and H2. Both HMs exhibit two distinct absorption bands in solution centered at 273 and 333 nm. The highest absorption peak at 333 nm is attributed to the metal centered (MC) 6s to 6p transition from the bismuth atoms.31 The band at 273 nm are attributed to ligand to metal charge transfer (LMCT) transition from nonbonding or weakly π bonding chloride 3p orbital to the Bi(III) 6p orbital as demonstrated in some previous work. The hybrid films of H1 and H2 exhibit an additional shoulder at higher wavelength when compared to solution spectra, suggesting specific interaction between nanocrystals during film formation.


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Fig. 5. UV-vis absorption spectra of H1 (a) and H2 (b) for both solution and thin films.

The key parameters to apply the novel HMs as IL in organic solar cells are the optical bandgap together with the energy levels of the material. Optical band gap was determined for H1 and H2 to 3.38 eV and 3.37 eV, respectively, by using the absorption band cutoff on the absorption spectra of the hybrid materials. Assuming a direct band gap and negligible exciton binding energy for the HMs, we can estimate the position of the conduction band edge versus the vacuum level based on the position of the valence band edge (ionization potential, IP) derived from the ultraviolet photoelectron spectroscopy spectra and the onset of the optical absorption. Figure S4 shows UPS data of the HMs, from which we obtain WF and IP values of respectively 4.28 eV and 7.43 eV for H1 and 4.2 eV and 7.55 eV for H2, yielding conduction band edges of 4.13 eV (H1) and 4.25 eV (H2). Both WF values of the HMs are low enough to form pinned contacts with the fullerene domains, but not with the donor polymers.32,33 The shallower positioned conduction band edge of H1 hence will provide a larger donor-acceptor gap than H2, and is then expected to produce a larger open circuit voltage in the devices.



3.3 Polymer solar cells using the hybrid materials H1 and H2 as electron extraction layer In order to evaluate the potential of HMs as EELs, we processed the hybrid nanocrystal solutions on top of polymer blends as described before and implemented these layers into polymer solar cells using the device structure ITO/PEDOT:PSS/ (PTB7 or PTB7-Th) :PC70BM/EEL/Al. While Figure 6 shows the J–V curves of best performing devices for the two organic polymers and the two HMs, key photovoltaic parameters along with standard deviations associated with nine cells are summarized in Table 1. In this comparison of the HMs, the best performance in solar cells using PTB7-Th as donor was observed for H1 as it can be expected from better energy level matching and nanoscale morphology. The best solar cells based on H1 EELS produced a short-circuit current density (Jsc) of 16.06 mA.cm-2, an open-circuit voltage (Voc) of 786 mV and a fill factor (FF) of 62% leading to a PCE of 7.82 % (see Table 1). H2 shows lower but still reasonable performance of 6.52 %. Compared to the reference device without any EEL showing an efficiency of 6.4 %, we clearly see an improvement in Voc and photocurrent density in the case of H1 indicating efficient EEL properties of this material. The improvement of the device performance can be attributed to improved hole blocking (H1 and H2) and improved electron extraction (H1) properties of the new materials, leading to a reduction of charge recombination at the polymer blend/Aluminum interface.7 H1 was also applied as EEL in PTB7 based solar cells using a layer thickness of 40 nm. This leads to efficiency for PTB7:PC70BM solar cells of 6.64%, demonstrating the general use of the H1 material as layer to produce efficient polymer solar cell.


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Fig. 6. J-V curves for ITO/PEDOT:PSS/(PTB7-Th or PTB7):PC70BM/(H1 or H2)/Al devices structures.

Table 1. Photovoltaic parameters (PCE, Voc, Jsc and FF) of ITO/PEDOT:PSS/(PTB7-Th or PTB7):PC70BM/(H1 or H2)/Al devices structures.

In order to further optimize the performance of H1 based PTB7-Th solar cells and to study potential optical spacer effect introduced by the hybrid materials in the normal device structure, we varied the layer of the hybrid EELs from 20 nm to 80 nm. Solar cells using ZnO EELs of 20nm and 80 nm were processed for comparison. Figure 7a shows the performance of the solar 15 ACS Paragon Plus Environment


cells for hybrid EELs with thicknesses of 20 nm, 40 nm, 60 nm and 80. The photovoltaic parameters along with standard deviations associated with nine cells are summarized in Table 2.

Figure 7: J(V) curves and reported solar cell performances for a) ITO/PEDOT:PSS/PTB7Th:PC71BM/H1/Al devices structures for various H1 thicknesses, b) ITO/PEDOT:PSS/PTB7Th:PC71BM/EEL/Al devices structures with EEL based either on 30 and 80 nm of ZnO or 40 nm and 80 nm of H1 .

Table 2. Photovoltaic parameters (PCE, Voc, Jsc and FF) of ITO/PEDOT:PSS/PTB7-Th:PC70BM/H1/Al devices structures for various H1 thicknesses.


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The highest PCE was obtained with film thickness of 40 nm and the standard deviations in PCE were less than 1% for all devices, independently of the thickness. It has to be emphasized that deviations were calculated using three substrates bearing each three solar cells with an active area of 0.27 cm2. This clearly implies a very high homogeneity in layer quality and high reproducibility of solar cells performance. Furthermore, looking at the variation of the photocurrent density with EEL thickness, there are only 16% photocurrent losses for the 80 nm EELs compared to optimal layer thickness of 40 nm. In Figure 7b we compare the J-V curve of solar cells using of H1 and ZnO, respectively, as EELs with an optimal thickness (40 nm for H1, 30 nm for ZnO) and a thickness of 80 nm. It can be clearly seen that the photocurrent density of the both solar cells using thin EELs layers is almost equal to 16.19 mA/cm2 and 16.06 mA/cm2 for ZnO and H1, respectively. The overall efficiency of ZnO based solar cells is 8.93 % and thus clearly higher than the H1 based devices, which is due to a better FF and slightly higher Voc. However, when one compares the device using a 80 nm thick EELs, ZnO leads to a much stronger loss in photocurrent density of 33% compared to 17 % in the case of H1. The strong loss in Jsc for device using ZnO is identical to our recent work and addressed to a negative optical spacer effect in the regular device structure for solar cells with optimized device structures.7,20 In order to understand in how far the photocurrent density losses are also related to electrical limitation of the materials used, we determine the serial resistance of the diodes as a function of the EELs thickness. From Table S6, we see that, in general, the serial resistances of ZnO based solar cells are clearly smaller than those for devices using H1 EELs. While, in the case of ZnO, RS increases from 4.7 Ωcm2 to 5.1 Ωcm2 with layer thickness, the RS of the H1 materials changes from 8.1 Ωcm2 to 9.9 Ωcm2. The value RS and its variation with layer thickness are reflected by only small FF and Voc losses in the case of ZnO, while the larger RS values of H1 based devices leads to stronger losses in both parameters, as observed already before in PTB7 based solar cells.7 Theses results reveal on one hand a less efficient 17 ACS Paragon Plus Environment


electron transport inside of the H1 layer compared to ZnO as all other components of the solar cells are identical. The lower electrical conductivity of H1 can potentially reduce photocurrent density as charge recombination at the interface between the blend and the EEL is increased due to the slower charge extraction by the H1 layers. On the other hand, the photocurrent density of solar cells using H1 as EELs is less reduced by increase in layer thickness and stay as high as almost 14mA/cm2 for a 80 nm EEL. This clearly demonstrates that the OSP effects are reduced in polymer solar cells using theses hybrid EEL materials, making them promising candidates towards thick layer robust processing of EELs. As spacer thickness and material index of refraction impact on the OSP effect, i.e the position of the maximum intensity inside the device stuck, our results suggest that the more suitable refraction index of the hybrid materials is at the origin of the OSP reduction. Our future work will focus on deeper understanding of the optical and electrical properties of these hybrid interfacial materials to improve their electron extraction properties, while further reducing OSP effect in the corresponding solar cells. In order to investigate another potential of these new hybrid interfacial materials, we study the stability of PTB7:Th solar cells in air under dark storage using open circuit conditions. The performance of the solar cells using either the hybrid materials H1 and H2 or ZnO nanoparticles as EELs were measured during one week and the performance losses compared. Figure 8 shows the evolution of the J-V curves of three different device types under the same storage conditions over 7 days, while Table S7 presents the time evolution of the photovoltaic parameters (fill factor FF, short-circuit density Jsc, open-circuit voltage Voc and power conversion efficiency PCE).


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Figure 8: J(V) curves for ITO/PEDOT:PSS/PTB7-Th:PC70BM/(H1 or H2 or ZnO)/Al devices structures under the same storage conditions in air over 7 days.

In the case of ZnO based EELs, a continuous degradation over time is found, and after 7 days only around 20 % of their initial value of PCE is preserved. The losses in ZnO devices are due to strong losses in all parameters, i.e. Jsc, Voc and FF. More importantly an S-shape quickly appears in the J–V curves and the series resistance increases dramatically, leading to a sharp decline of the solar conversion efficiency. The formation of such an S-shape and similar device degradation in polymer solar cells using ZnO EELs in normal device structures have been observed and studied in details recently by Lechene et al.34 The authors demonstrated that there is a chemical reaction at the interface between ZnO and the Aluminum electrode, although these devices were stored under inert atmosphere, fast degradation over days with similar losses 19 ACS Paragon Plus Environment


and S-shape formation as in our work was found. This degradation was addressed to the formation of a ultrathin Aluminum oxide layer at the interface between Zno and the Al cathode. In our stability tests, we can expect additional degradation mechanism due to the presence of water and oxygen. The air may further accelerate the formation of such an isolating aluminum oxide layer as well as decrease of the ZnO conductivity. However, the work of Lechene et al.34 suggest that the main degradation mechanism is related to the reaction of the ZnO/Al interface. In contrast to the fast degradation of the ZnO based device, solar cells using H1/Al or H2/Al contacts are much more stable in air over the whole period of observation. Indeed, the solar cells based on H1 and H2 retain 70 % and 60 % of their initial value of PCE, respectively. Furthermore, there is no appearance of an S-shape indicating the Al and H1 do not form an isolating Aluminum oxide layer as found for ZnO. This clearly indicates that these materials have a general tendency to be more stable than metal oxide materials like ZnO and improve strongly the device stability in air.

4. Conclusion In summary, we demonstrate a novel family of hybrid high band gap material composed of bismuth and diaminopyridine that is highly suitable for electron extraction layers in polymer solar cells. PTB7-Th:PC70BM solar cells were realized in regular device structures with allsolution-processed hybrid electron extraction layers leading to PCEs exceeding 7.8 %. We could show that the increase of the hybrid extraction layer thickness only slightly reduces the photocurrent density of the solar cell indicating small optical spacer effect an thus improved compatibility with robust layer processing compared to standard materials for electron extraction layers such as ZnO. Additionally, we could evidence the higher air stability of solar


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cells using these hybrid interfacial layer compared to interlayer based on ZnO. We believe that, together with the compatibility of facile low temperature solution processing, these novel hybrid high band gap material opens interesting opportunity towards use in low cost high efficiency solar cells. Especially the synthesis of bismuth and other metal based organicinorganic materials may be a promising strategy to further improve their electronic properties, stability as well as process robustness.

Supporting Information. Unit cell of the H2 material. Projection of the atomic arrangement. TEM images of the two hybrid materials. UPS spectra of the two hybrid materials. Solar cell characterizations for various H1 thicknesses. Crystallographic data for the H2 material. Assignments of IR wavenumbers.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements DF acknowledges the Ministry of Higher Education, Scientific Research, and Technology in Tunisia for their financial support. JA acknowledges financial support by the French Fond Unique Interministériel (FUI) under the project “SFUMATO” (Grant number: F1110019V/201308815) as well as by the European Commission under the Project “SUNFLOWER” (FP7-ICT-2011-7, Grant number: 287594). MF and XL acknowledge support from the Swedish Research Council project grant 2016-05498 and the Swedish Government 21 ACS Paragon Plus Environment


Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO Mat LiU No 2009 00971). The authors thank D. Chaudanson from CINaM for assistance in the use of the TEM facility; and I. Ozerov & F. Bedu for profilometer measurements performed at PLANETE CT PACA cleanroom facility (CINaM, Marseille) and Dr. N. C. Boudjada from NEEL institute (Grenoble) for the crystallographic measurement.

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Fabrication and Characterization of Hybrid organic-inorganic electron

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