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Thermal Stabilisation of Polymer–Fullerene Bulk
Heterojunction Morphology for Effi cient Photovoltaic Solar Cells
Lionel Derue , Olivier Dautel , Aurélien Tournebize , Martin Drees , Hualong Pan , Sébastien Berthumeyrie , Bertrand Pavageau , Eric Cloutet , Sylvain Chambon , Lionel Hirsch , Agnès Rivaton , Piétrick Hudhomme , Antonio Facchetti , and Guillaume Wantz *
Dr. L. Derue, Dr. S. Chambon, Dr. L. Hirsch, Dr. G. Wantz
IMS Laboratory University of Bordeaux
16 avenue Pey Berland F-33607 , Pessac , France E-mail: guillaume.wantz@ims-bordeaux.fr Dr. O. Dautel
AM2N Laboratory Charles Gerhardt Institute
8 rue de l’Ecole Normale 34296 , Montpellier , France A. Tournebize, Dr. S. Berthumeyrie, Dr. A. Rivaton ICCF Laboratory
University of Blaise Pascal F-63000 Clermont-Ferrand France
Dr. E. Cloutet LCPO Laboratory University of Bordeaux
16 avenue Pey Berland F-33607 , Pessac , France Prof. P. Hudhomme
MOLTECH-Anjou University of Angers
2 boulevard Lavoisier 49045 , Angers , France B. Pavageau
LoF Laboratory, Solvay Pessac , France
Dr. M. Drees, Dr. H. Pan, Prof. A. Facchetti POLYERA Corporation, 8045 Lamon Avenue Skokie, IL 60077
USA and Center of Excellence for Advanced Materials Research (CEAMR) King Abdulaziz University
Jeddah , Saudi Arabia
DOI: 10.1002/adma.201401062
level, and operational stability. [ 3 ] Most of BHJ photoactive blends are composed of a mixture of an electron-donor polymer and an electron-accepor fullerene derivative, where the latter mate- rial is typically a soluble C 60 -fullerene (PC 61 BM) or C 70 -fullerene (PC 71 BM) derivative ( Figure 1 ). The BHJ layer is sandwiched between charge carrier selective interlayers and the electrodes.
The bottom electrode is typically indium tin oxide (ITO) or other transparent conductors. Interlayers choice governs the polarity of the photovoltaic cells. Metal oxides such as TiO x or ZnO are commonly used as electron selective layer whereas MoO x or con- ducting polymers (PEDOT:PSS) are used as hole transporting layers. An optimised BHJ layer requires specifi c phase segrega- tion of the BHJ donor-acceptor components to allow optimum charge carrier photogeneration in the blend and charge perco- lation pathways for effi cient electron and hole collection to the respective electrodes. An important morphological parameter of the BHJ blend to achieve large PCEs is that nano-sized fullerene crystallites are necessary within the polymer matrix to prevent electron-hole recombination mechanisms. [ 4–7 ] Thus, the domain size must be in the order of the excitons diffusion length, which typically ranks from 3 to 30 nm. [ 8 ] Such optimal polymer- fullerene blend morphology is achieved with a different effi - ciency depending on the material combinations. [ 9 ] Optimized phase segregation can be promoted using appropriate solvent(s) and/or specifi c solvent additives during blend deposition as well as post-deposition fi lm processing such as thermal or solvent annealing. [ 10 ] Semicrystalline polymers such as poly(3-hexylth- iophene) (P3HT, Figure 1 ) tend to expel fullerenes during their crystallization into nano-objects upon drying of the solvent or during post-fi lm deposition thermal annealing. This property enabled to fi nely tune P3HT:PCBM blend morphology and led to a tremendous amount of data concerning OPV cells based on this specifi c polymer. [ 11 ] However, P3HT cells are severely lim- ited in terms of the maximum achievable PCEs. Therefore, low band gap polymers, which can harvest a larger portion of the solar spectrum, were developed to reach greater performances.
Unfortunately, several of these high-potential polymers are less crystalline and do not have such a strong tendency for molec- ular organization. As a consequence, manipulating BHJ mor- phology of less crystalline/amorphous polymers is not trivial.
Solvent additives, such as 1,8-diiodooctane or 1,8-octanedithiol for example, enable to preferentially solvate fullerene derivatives rather than the polymer, were chosen to tune BHJ morphology and achieve effi ciencies >9%. Thus, the major problem that the The use of a bulk heterojunction (BHJ) blend of an electron-
donor and an electron-acceptor organic semiconductors to fab- ricate photovoltaic solar cells and to understand fundamental light-to-charge phenomena in organic solids has attracted the interest of the international scientifi c community for the last 20 years. These efforts recently led to the demonstration of lab-scale organic photovoltaic (OPV) cells with power conver- sion effi ciencies (PCE) of 9.2% and 10.6% for single cells [ 1 ] and tandem cells [ 2 ] confi gurations, respectively. OPV cells are becoming a credible revolutionary thin-fi lm photovoltaic tech- nology with advantages such as lightweight, mechanical fl exi- bility, roll-to-roll large area and low-cost solar module production.
The three major challenges to OPV module realization are the cost of the active/encapsulation layers, effi ciency at module www.MaterialsViews.com
OPV fi eld is now facing is whether high-effi ciency devices can be stabilized at a level larger than that of P3HT and whether the formulation enabling this morphological stabilization can be easily manipulated and can be stable themselves. If these goals can be achieved, then transition into signifi cant module tests can begin.
It is now recognized that OPV cell stability correlates to inter- face material issues, morphological stability of the OPV blends, and the kinetic of photodegradation of organic materials. [ 12 ] These three aspects require careful investigation in order to obtain stable polymeric solar cells. Here, the morphological stability of the BHJ blend is the subject of study. Bulk hetero- junctions that embed fullerene derivatives are not stable since nanometer-sized fullerene domains always tend to enlarge, resulting in PCE losses because of exciton recombination. [ 13 ] For semicrystalline polymer-fullerene blends, crystal growth is dramatically increased upon heating. For amorphous polymer- based blends, even without the strong driving force to polymer reorganization, fullerene crystals will inevitably start growing with time due to Ostwald ripening. [ 14 ] The effect of continuous thermal annealing at 85 °C on the power conversion effi ciency of a device based on P3HT:PC 61 BM has been studied (Figure S1, see Supporting Information). This mild temperature has been defi ned as a reference temperature for OPV cell stability evaluation. [ 15 ] From these data it is clear that after 19 days of continuous heating, microscopic crystals of fullerene are vis- ible under a optical microscope. After 120 days, the effi ciency has decreased to 50% of the initial value (Figure S1, Supporting Information). It is therefore crucial to stabilize bulk heterojunc- tions which we will address via crosslinking of the acceptor as described in the following sections.
a bromide atom at the end of the lateral chain for cross-linking upon UV irradiation with effi cient stabilisation of the BHJ mor- phology. The use of alkylazide functionalized thiophene-based copolymers has enabled the bonding to fullerenes on the polymer chains resulting in the stabilisation of the BHJ. [ 19 ] More recently, acrylate-functionalised polymers have been demonstrated to slightly enhance the thermal stability of the BHJ fi lm. [ 20 ] These approaches are important but require careful design and time-consuming synthesis of customized polymers. On the other hand, there have been only few attempts to cross-link the acceptor fullerene rather than the polymer. The fi rst approaches were developed by Drees et al. using a cross-linkable epoxide-based fullerene [ 21 ] and Cheng et al. with a fullerene bearing a thermally cross- linkable styryl moiety (PCBSD). [ 22 ] The latter cross-linkable molecules were used as additives with PC 61 BM and P3HT to enhance the stability of the BHJ morphology. The crystallization of the fullerene acceptor was reduced thus restricting PC 61 BM from fast migration upon thermal annealing. However, with the rapid progress in polymer-fullerene OPV development, a more versatile approach is now necessary. To minimize the cost, the development of easy attainable third component cross-linkers to be introduced into any kind of a BHJ remains of essential importance. Ho et al. [ 23 ] fi rst demonstrated that π-conjugated polymers can be cross-linked using an aromatic bis-azide without affecting its semiconducting properties. Recently, the same team used the crosslinker named sFPA (Figure 1 ) in OPV cells made of back-fi lling PC 61 BM in cross-linked P3HT thin fi lms under various conditions to achieve high internal quantum effi ciencies. [ 24 ] Meredith et al. simultaneously devel- oped the same approach using sFPA on P3HT and a top layer of PC 61 BM. [ 25 ] Nevertheless, these concepts have never been utilized to crosslink the fullerene phase, which would be a far more general approach as the best solar cells are based on polymer donor-fullerene blends. Here we introduce a specifi - cally-designed bis-azide cross-linker: 4,4′-bis(azidomethyl)-1,1′- biphenyl (BABP, Figure 1 ) in order to stabilize thermally dif- ferent polymer:fullerene BHJ. This new compound was easily synthesized by reacting NaN 3 with the bis-chloromethyl biaryl precursor (see Supporting Information). Compared to liquid Figure 1. Molecular structures of the materials described in this study: BABP, sFPA, P3HT,
PTB7, PDPPTBT, PC 61 BM, PC 71 BM and bis-PC 61 BM.
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sFPA, [ 23 ] BABP is far easier to synthesize and, most impor- tantly, this powder is much less reactive enabling storage at room temperature. BABP is used as a crosslinker here in a mix- ture with the semiconducting polymer and the fullerene deriva- tive dissolved in an organic solvent. Thus, our strategy enables the chemistry to occur only in the solid state after photoactive fi lm deposition upon mild thermal treatment, and only once the BHJ thin fi lm with optimised morphology is formed.
Our azide derivative can react according to two major path- ways ( Figure 2 ). Photochemically ( λ = 254 nm), it decomposes into dinitrogen and a highly reactive nitrene function (pathway A). In this case, the nitrene (in either singlet or triplet state) can react randomly with both the donor polymer and the fullerene acceptor. Ho et al. used these chemical pathways and demonstrated that cross-linking occurs between semicon- ducting polymer backbones on their alkyl side chains. [ 23 ] On the other hand, upon mild thermal activation (Δ1 ) the azide func- tion does not react with the polymer. Under such conditions, it is thus able to react selectively with fullerene derivatives through a 1,3-dipolar cycloaddition to give triazolino(4′,5′:1,2) fullerenes. Eventually, subsequent thermal annealing at higher temperatures (Δ2 ) affords open [5,6] azafulleroid and/or closed
[6,6]aziridinofullerene regioisomers through dinitrogen elimi- nation, resulting in a three-membered ring-functionalized fullerene (pathway B). [ 26 ] The cross-linking reactions were investigated here using a combination of thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared spectroscopy (IR) as well as nanoindentation via atomic force microscopy (AFM). From the TGA thermogram ( Figure 3 a), one can clearly observe that BABP is stable up to 180 °C. In a fi rst approach, heating the BABP below to 180 °C does not generate nitrene. At 180°C, BABP starts to decompose to fi nally lose 70% of the initial mass at 210 °C. Since nitrene generation from BABP with N 2 elimination should result in a mass loss of only ∼21%, this means that the generated nitrenes alone reacts forming thermally labile species which under- goes further thermal decomposition. To further prove that the B pathway is undoubtedly followed in the present case, we studied the morphological behaviour of the BABP material as a function of the temperature by DSC experiments (Figure 3 b).
Heating of BABP powder from 20 to 100 °C shows two phase transitions at 57 and 72 °C, which correspond to a crystal-liquid transition at 57 °C and melting at 72 °C. Remarkably, these two transitions are reversible with an isotropic-liquid crystal www.MaterialsViews.com
Figure 2. Possible reaction pathways for BABP blended with P3HT:PC 61 BM. Pathway A is promoted photochemically by deep UV (254 nm) or thermally at high temperatures (>200 °C). In this scenario, BABP can react nonselectively with several organic compounds. This scheme represents the case where BABP has reacted with PC 61 BM and to the alkyl side chain of P3HT. Pathway B illustrates the cross-linking reaction of BABP with PC 61 BM upon mild thermal annealing (∼80 °C) forming a triazolino ring. Further heating of this compound at high temperatures (>200 °C) can lead to N 2 elimina- tion and the formation of an aziridine ring.
Figure 3. a) TGA of BABP realized at 10°C/min under nitrogen. b) DSC analysis of BABP as a powder upon thermal cycle between 25–100 °C (black line), 25–160 °C (blue line), and 25–250 °C (red line). c) DSC of various blends : P3HT:BABP (1:1wt), PC 61 BM:BABP (1:1 wt) and P3HT:PC 61 BM:BABP(1:1:1 wt).
presence of PC 61 BM (blue curve), the BABP starts to melt at 58 °C, however, the latter reacts with PC 61 BM before complete melting. Indeed, an exothermic reaction is clearly observed at
∼100 °C (? 1 ), suggesting a 1,3-dipolar cycloaddition of BABP to the fullerene to afford the corresponding triazolino(4’,5’:1,2) fullerene. Moreover, in a second step, one can also observe the exclusion of N 2 , associated with an exotherm at ? 2 = 145 °C, to afford the respective azafulleroids and/or aziridinofuller- enes according to the pathway B. The reactivity of BABP with PC 61 BM is not altered by the presence of P3HT as it can be seen from the thermogram (red curve) of the ternary mixture.
Additionally, infrared spectra of thermally annealed fi lm made of P3HT:PC 61 BM:BABP during 10 minutes at 150 °C clearly show that the bands attributed to azide functions of BABP (526 cm −1 ) are drastically reduced without formation of new bands (Figure S2, Supporting Information). Furthermore, additional evidence of the cross-linking reactions promoted by BABP are seen by analyzing the mechanical properties of these blends by nano-indentation measurements. Thus, cross-linking PC 61 BM induces a signifi cant increase of the fi lm nanohard- ness from 110 MPa (pure PC 61 BM) to 520 MPa (after thermal annealing at 150 °C for 10 min). Similarly, cross-linking the P3HT:PC 61 BM in the BHJ leads to an increase of the nanohard- ness from 50 MPa to 90 MPa (see Table S1, Supporting Infor- mation). These results further confi rms the effi cient chemistry occurring between BABP and fullerene derivatives.
In this work, we investigated several donor polymer-fullerene blend with BABP combinations, fabricated the corresponding solar cells, and tested them under several conditions. A sum- mary of these results are presented in Table S2 (see Supporting Information). However, we initiated this study by fi rst inves- tigating the P3HT-PC 61BM-based devices in more details.
Thus, BABP was incorporated into P3HT: PC 61 BM solutions at various concentrations. Figure 4 a shows that the PCE of the devices based on P3HT:PC 61 BM depends on the BABP weight content. Below 20 wt% versus fullerene, the presence of BABP does not affect the initial PCE, that remains at ∼3.3%, similar to the reference cells without BABP. Increasing the amount of BABP reduces dramatically the initial PCE of solar cells mainly because BABP does not participate in the photon harvesting nor in the transport of charge carriers. As seen from Figure 4 b, thermal annealing of a P3HT: PC 61 BM fi lm at 150 °C reduces performance and shows fullerene crystals growth as
AFM further demonstrates that the typical fi brillar structure of the pristine P3HT:PC 61 BM layer (Figure 4 i) completely change after 24h at 150 °C (Figure 4 j). However, the addition of an optimised content of BABP in P3HT:PC 61 BM (5 wt% versus fullerene) enables to stabilize the morphology. From the optical microscopic images, no fullerene crystal appears even after 7 days of continuous thermal annealing at 150 °C (Figure 4 h). The optical absorption spectra of Figure 4 d demonstrate this micro- scopic-scale stabilization with similar contributions of PC 61 BM and P3HT before and after annealing at 150 °C during 15 h, thanks to the cross-linked fullerenes. For the device annealed at lower temperatures, similar decrease of PCE is observed for P3HT:PC 61 BM cells and similar stabilization is achieved when BABP is used as the cross-linker (Figure S1, Supporting Infor- mation) over more than 4 months of continuous treatment of layers at 85 °C. The power conversion effi ciency of the cor- responding devices drop by one half of its initial value after 120 days of continuous heating at 85 °C. Using BABP under these thermal stress, the effi ciency is stabilized at ∼90% of its initial value. Since 85 °C has been defi ned as a standard tem- perature for solar cells ageing, [ 15 ] our results clearly demon- strate that our cross-linking enables BHJ stabilization.
An optimal BABP weight ratio versus fullerene must be used in the blend. Higher BABP amounts do freeze effi ciently the bulk heterojunction morphology with no crystal growth, but the OPV performances drop after thermal annealing. This result could be explained by the fact that high BABP concentrations result in multiple functionalization of PC 61 BM with formation of multi-adducts. If bis-adduct fullerenes such as bis-PC 61 BM [ 27 ] or bis-indene derivatives such as ICBA [ 28 ] are well known to favour P3HT-based OPV performances by increasing the open- circuit voltage (V oc ), multi-adducts having three or more groups on the fullerene are known to reduce fullerene acceptor capa- bilities. [ 29 ] Indeed, P3HT-PC 61BM cells having 5 wt% BABP exhibit no signifi cant increase of V oc , indicating only partial cross-linking (Figure S3). Under these conditions, there is only one molecule of BABP for about fi ve molecules of PC 61 BM. As a consequence, not all the fullerene derivatives are cross-linked, which explains the invariance of V oc. These results demon- strate that a frozen P3HT:PC 61 BM bulk heterojunction can be achieved by cross-linking only a fraction of the PC 61 BM mol- ecules. Finally, our morphological stabilization strategy is also effective for PC 71 BM when blended with P3HT corroborating
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that the cycloaddition chemistry is effective for both C 60 and C 70 derivatives. Thus, by BABP (5 wt%) addition to a P3HT:
PC 71 BM blend enables to stabilize the corresponding solar cells PCE also after 20 h of thermal annealing at 150 °C, whereas the control cell without BABP shows complete performance degra- dation (Figure S4).
P3HT is a particular semicrystalline polymer often utilized as a donor model for OPV devices and it enables to achieve devices with effi ciencies of ∼3–5%. 3 rd generation semicon- ducting polymers, as named by Heeger, [ 30 ] offer the possibility to dramatically improve effi ciencies up to 7–9%. For this reason, state-of-the-art polymers, i.e. low-bandgap materials, have also been investigated (Figure 1 and Table S2, Supporting Informa- tion). The low-bandgap semiconducting polymer thieno[3,4-b]
thiophene/benzodithiophene (PTB7, Figure 1 ) introduced by Yu et al. , [ 31 ] is a known donor enabling outstanding PCEs of up to 9.2%. [ 32 ] PTB7 is a less crystalline polymer than P3HT, leading to smaller BHJ morphological features where the fullerene nano-domains are better interpenetrated within the polymer. [ 33 ] The PTB7:PC 61 BM fi lms do not show the longer range order observed for the P3HT-based blends because of the strong interactions between PTB7 and PC 61 BM. Despite these
features, PTB7 based BHJ are thermally unstable as fullerene crystals appear after thermal annealing ( Figure 5 b). For our devices, the initial phase segregation of this BHJ layer change completely after 16 h at 150 °C, resulting in a signifi cant drop of the PCE from ∼5.4% to ∼0.9%. The addition of an optimized 2 wt% ratio of BABP avoids fullerene crystallization and stabi- lizes the devices at a PCE of 4.6% (Figure 5 c) under the same stress conditions. However, there is a loss of effi ciency here due to a reduced short-circuit current (J sc ) as can be seen on the cur- rent-voltage curves on Figure 5 d. This loss is discussed below.
In order to extend our approach, we investigated mor- phology stabilization upon cross-linking for a DPP-based donor (PDPPTBT, see chemical structure on Figure 1 ) con- sidering the importance of this class of polymers. [ 34–36 ] Our PDPPTBT:PC 71 BM-based solar cells exhibit an initial PCE of
∼5%. As displayed in Figure 5 , after 15 h of thermal annealing of the PDPPTBT-fullerene blends at 150 °C, optical microscopy images showed micro-crystallization of PC 71 BM resulting in a drop of power conversion effi ciency from 5% to 1.5%. The addition of an optimal content of BABP (2 wt%) does not affect initial performances, as indicated by the J–V curves (Figure 5 h), and the PCE stabilizes at ∼3% mostly due to a drop of the www.MaterialsViews.com
Figure 4. a) Effect of BABP concentration (wt% vs PC 61 BM) on the initial power conversion effi ciency (blue symbols) of P3HT:PC 61 BM:BABP- based cells. The effect of BABP content for layers on P3HT:PC 61 BM:BABP that have been aged for 122 h at 150 °C is also shown (red symbols).
b) Power conversion effi ciency of P3HT:PC 61 BM:BABP-based devices as a function of annealing time at 150°C with 5wt% BABP or without cross-linker.
c) UV-Vis spectra of P3HT:PC 61 BM as a function of time at 150 °C without cross-linker and d) with 5wt% BABP. e) Optical microscopy of a pristine P3HT: PC 61 BM BHJ solar cell without cross-linker f) After 5 h at 150°C without cross-linker g) After 24 h at 150 °C without cross-linker. Large crystals of PC 61 BM are visible and responsible of the PV performance drop. h) P3HT:PC 61 BM:BABP (5 wt%) image of the fi lm after 122 h at 150 °C. No large crystals of PC 61 BM are formed after long annealing thanks to the use of BABP as cross-linker in the bulk heterojunction. i) AFM phase images of a P3HT:PC 61 BM before thermal annealing and j) After thermal annealing at 150 °C for 24 h. k) AFM phase images of a P3HT:PC 61 BM:BABP fi lm before thermal annealing and l) After thermal annealing at 150 °C for 122 h. Nano-domains of PC 61 BM are preserved thanks to the cross-linker. All AFM images size is 2 × 2 µm 2 .
short-circuit current. As in the case of PTB7:PC 61 BM devices, PDPPTBT-based solar cells with cross-linked fullerenes exhibit stabilized performances but with reduced effi ciencies mostly originating from decreased short-circuit currents (Table S2, Supporting Information). The J sc reduction in these devices upon thermal stress of the crosslinked blends is a behaviour specifi c of BHJs comprising a low-bandgap polymer. In fact, ageing of these cross-linked blends having optimized BABP concentrations (2wt%) invariably reduces the short circuit cur- rents, does not affect signifi cantly the open-circuit voltages, nor the fi ll factors. We suspect the J sc drop is due to the presence of fullerene multi-adducts in the blend after cross-linking which is consistent with recent studies by two independent groups ana- lyzing the effect of bis-adduct fullerenes on the performance of solar cells based on low-bandgap polymers. [ 37,38 ] Since in our experiments only a fraction of fullerenes are cross-linked, future optimizations will involve the test of additional high-per- formance donor polymers, optimal amounts of cross-linkers, and a compromize between champion device performance for a given blend, initial performance upon cross-linking and after morphology stabilization.
In conclusion, we developed a new specifi cally designed cross-linker for the morphological stabilization of donor- acceptor bulk heterojunctions. Our approach is demonstrated by using three donor polymers (P3HT, PTB7 and PDPPTBT) and two fullerene derivatives (PC 61 BM and PC 71 BM). In con- trast to other bis-azide reagents, our BABP allows the prepa- ration of stable BHJ formulations and enables specifi c cross- linking with fullerenes only after fi lm deposition and under mild conditions (∼80 °C). This type of cross-linking chem- istry is effi cient in stabilizing the bulk heterojunction mor- phology by preventing the fullerenes to diffuse and form large
crystallites. Depending on the donor polymer semiconductor and the initial amount of cross-linker, different stabilization of the PCEs were achieved, which could be further optimized by additional studies. Thus, for cells based on P3HT with PC 61 BM the relative effi ciency stabilizes to 3.4% after thermal annealing at 150°C for 45 h. For lower bandgap polymers, such as PTB7 and PDPPTBT blends with PC 61BM, the PCEs stabilize at 4.6% and 3% (150 °C for ∼15 h). Our study demonstrates that freezing optimized bulk heterojunction morphologies with a stable reagent is now possible and, consequently, this approach combined with other interfacial stabilization methodologies may fi nd application for the future development of commercial organic photovoltaic modules.
Experimental Section
OPV Devices fabrication and characterizations : Inverted and direct solar cells have been fabricated with the structure glass/ITO/TiO x / P3HT:PC 61 BM; PDPPTBT:PC 71 BM/MoO 3 /Ag or glass/ITO/PEDOT-PSS/
PTB7:PC 71BM/Ca/Al, respectively, using standard procedures.
15 mm × 15 mm ITO-coated glass sheets (10 Ohm square, Kintec) were successively cleaned in acetone, ethanol, and isopropanol in an ultrasonic bath and exposed to UV-ozone for 20 min. 15 nm-thick titanium oxide was prepared as previously described by Chambon et al. [ 39 ] P3HT (Plexcore OS2100, Plextronics), PTB7 (1-materials), PDPPTBT (Polyera Corp.), (PC 61BM (99.5%, Solaris-Chem Inc) and PC 71 BM (99.5%, Solaris-Chem Inc) were used as received. Solutions were prepared in o-dichlorobenzene for P3HT or chlorobenzene for PTB7 and chloroform for PDPPTBT, for the following optimized weight ratio and concentrations: P3HT:PC 61 BM ratio 1:1 at 40 mg/mL; PTB7:PC 61 BM ratio 1:1.5 at 25 mg/mL; PDPPTBT:PC 71 BM ratio 1:1.5 at 15 mg/mL.
Solutions were fi rst stirred at 90°C for 10 min and, subsequently, at 50°C for 24 h. BABP was introduced in the polymer:fullerene solutions Figure 5. Optical microscopy images (scale bar is 50 µm) of a PTB7:PC 61 BM BHJ solar cell a) Before thermal annealing and b) After thermal annealing for 16 h at 150 °C without cross-linker. Numerous micrometer-sized PC 61 BM crystals are observed. c) Image of the same blend with the addition of BABP after thermal annealing for 16 h at 150 °C, indicating the absence of microcrystals. d) Current density–voltage curves measured under 1 sun of PTB7:PC 61 BM (green symbols) and PTB7:PC 61 BM:BABP (blue symbols) cells before thermal annealing and after thermal annealing for 16 h at 150 °C without cross-linker (black symbols) and with BABP (red symbols). A similar set of images and current–voltage characteristics are shown for solar cells based on PDPPTBT:PC 71 BM (panels e, f, g, and h).
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at room temperature from diluted solutions in the respective solvent at various concentrations and stirred for at least 5 min. BHJ layers were spin-coated in a glove-box. Optimized phase segregation between the polymer and the fullerene derivative has always been achieved here with no thermal stress to avoid undesired cross-linking. Solvent annealing in closed Petri dishes was used on P3HT:PC 61BM. The resulting P3HT:PC 61 BM thickness was 241 ± 7 nm. 1,8-Diiodooctane (3% v/v) was used as processing additive in the case of PTB7 and PDPPTBT (2%v).
PTB7 and PDPPTBT fi lm thickness is at 145 ± 5 nm and at 115 ± 6 nm.
Thermal ageing was performed on a temperature-controlled hot plate in the glove-box. This thermal treatment is mostly performed before top electrode deposition to avoid thermal diffusion of atoms such as Mo or Ag in the active layer. For inverted architectures, MoO 3 (Serac) followed by 60 nm-thick silver electrodes were successively thermally-evaporated at deposition rates of 0.1 nm·s −1 and 0.2–0.3 nm·s −1 respectively under secondary vacuum (10 −6 mbar) through a shadow mask to defi ne a 8.6 mm 2 active area. For direct architectures, Ca (20 nm) / Al (80 nm) top electrode was evaporated under the same conditions (0.5 nm·s −1 ).
Experiments were repeated on multiple individual cells and the overall experiments were also independently repeated multiple times in different laboratories to evaluate standard deviations. The devices were characterized using a K.H.S. SolarCelltest-575 solar simulator with AM1.5G fi lters set at 100 mW/cm 2 with a calibrated radiometer (IL 1400BL). Labview controlled Keithley 2400 SMU enabled the current density-voltage ( J – V ) curves measurements. Devices were fabricated, thermally aged and characterized under nitrogen in a set of glove-boxes (O 2 and H 2 O < 0.1 ppm). External quantum effi ciency was measured using Newport’s QE setup. Incident light from a xenon lamp (300 W) passing through a monochromator (Newport, Cornerstone 260) was focused on the active area of the cell. The output current was measured using a current pre-amplifi er (Newport, 70710QE) and a lock-in amplifi er (Newport, 70105 Dual channel Merlin). A calibrated silicon diode (Newport 70356) was used as a reference.
Spectroscopic analysis : UV-Visible spectra were recorded on a Safas Monaco spectrometer or a Shimadzu UV-2101PC and UV-2550 equipped with an integration sphere (transmission mode). The spectra were recorded between 200 and 800 nm with a resolution of 0.5 nm. Infrared (IR) spectra were recorded in transmission mode with a Nicolet 7600- FTIR spectrophotometer (nominal resolution of 4 cm −1 ; summation of 32 scans).
Atomic Force Microscopy : AFM imaging was carried out at room temperature using an AFM Nanoman from Bruker Instrument with Nanoscope 5 controller. Images were obtained in tapping mode using silicon tips (PointProbe Plus AFM-probe, Nanosensors, Switzerland) with a spring constant of 50 N·m −1 and a resonance frequency of approximately 160 kHz. (512 × 512 data points were acquired.)
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
Authors are grateful to the Agence Nationale de la Recherche (ANR) for funding the HABISOL 2010 (PROGELEC) program CEPHORCAS.
RHODIA, member of the SOLVAY group, is acknowledged for its funding contribution. Mylène Leborgne, Clémence Lecourtier, Aude Salamero and Lise Malassenet are acknowledged for the synthesis of BABP as part of a graduate lab project in 2010. Authors are thankful to Maxime Lebail and Qiana Carswell for their participation in photovoltaic device optimizations, to Michel Ramonda for his help with AFM, to Pascale Guiffrey for her help with DSC, to Sokha Khiev for his technical contribution. AF thanks KAU for support. Finally the Cluster of Excellence “Advanced Materials by Design” LabEx AMADEus directed
by Prof. E. Duguet is acknowledged for funding the research period of Lionel Derue at POLYERA Corporation.
Received: March 7, 2014 Revised: May 28, 2014 Published online: July 17, 2014
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