From Fluorine to Fluorene—A Route to Thermally Stable aza -BODIPYs for Organic Solar Cell Application
Melanie Lorenz-Rothe , Karl Sebastian Schellhammer , Till Jägeler-Hoheisel , Rico Meerheim , Stefan Kraner , Moritz P. Hein , Christoph Schünemann , Max L. Tietze , Markus Hummert , Frank Ortmann , * Gianaurelio Cuniberti , Christian Körner , and Karl Leo *
DOI: 10.1002/aelm.201600152
1. Introduction
Small-molecule organic solar cells (OSCs) have recently reached application-relevant power conversion effi ciencies (PCEs), offering the possibility of low-cost large-area photovoltaic applications since OSCs can be produced on fl exible substrates, with little material consumption, and at low process temperatures. [ 1 ] In particular, multijunction devices have attracted growing interest from both academia and industry since energy from a broad-band of the sun spectrum can be harvested with complementary absorber materials. [ 2 ] The usage of vacuum deposition tech-niques further enables very thin and clean absorber layers and versatile combination of different materials as well as non-toxic and cheap production processes as no solvents are necessary. Despite the advan-tages of vacuum-processed multijunction devices, their application is still limited by a lack of thermally stable and complemen-tary low-band gap absorbers with strong
absorption in the red and NIR region of the sun spectrum and still high open-circuit voltages ( V oc ). [ 3 ] Indeed, from the ther-mally stable NIR absorber materials absorbing close to 700 nm (or above) that have been employed in vacuum-processed OSCs, there are only very few with a PCE greater than 3%, and only three compounds have reached competitive values within a standard confi guration (using C 60 as acceptor). [ 4 ]
Table 1 compares the reported materials including their maximum achieved effi ciency, the maximum EQE and absorp-tion wavelength, and the sublimation yield. As sublimation yields are in most cases not reported, we add corresponding values from our own experiments. The table shows that high PCEs have only been reported for SubNc, Ph 2 -benz-aza-BODIPY, and DTDCTB. We believe that the reason is the strong constraint of thermal stability, limiting the substitution options for synthetic chemists. An important criterion in view of later mass production is the sublimation yield for material purifi cation, where DTDCTB has only a very low sublimation yield of around 30%, which appears insuffi cient for effi cient use in future mass production. This example underlines the
Despite favorable absorption characteristics, borondipyrromethenes (BODIPYs) often lack thermal stability preventing their application in vacuum-processed organic solar cells. In this paper, the replacement of the BF 2 unit by borafl uorene as a new functionalization strategy for this molecule class is explored. This approach is applied to a set of prototype molecules and demonstrates improved thermal stability, strong absorption in the red and near-infrared region of the sun spectrum, as well as excellent solar cell performance. Synthesis is realized from free ligands via complexation with 9-chloro-9-borafl uorene giving high yields up to 81%. Planar heterojunc-tion cells of these complexes exhibit high fi ll factors of more than 70%. Bulk heterojunction solar cells with C 60 are optimized yielding power conversion effi ciencies up to 4.5%, rendering the investigated prototype compounds highly competitive among other NIR-absorbing small-molecule donor mate-rials. Comprehensive experimental material characterization and solar cell analysis are carried out, and the results are discussed together with simula-tions of molecular properties. Based on this analysis, additional performance improvements are proposed by engineering the intramolecular steric interac-tions towards further red-shifted absorption.
M. Lorenz-Rothe, T. Jägeler-Hoheisel, Dr. R. Meerheim, Dr. S. Kraner, Dr. M. P. Hein, Dr. C. Schünemann, Dr. M. L. Tietze, Dr. M. Hummert, [+] Dr. C. Körner, Prof. K. Leo Institut für Angewandte Photophysik Technische Universität Dresden 01062 Dresden , Germany E-mail: [email protected] K. S. Schellhammer, Dr. F. Ortmann, Prof. G. Cuniberti Institute for Materials Science Dresden Center for Computational Materials Science and Max Bergmann Center of Biomaterials Technische Universität Dresden 01062 Dresden , Germany E-mail: [email protected] K. S. Schellhammer, Prof. G. Cuniberti, Prof. K. Leo Center for Advancing Electronics Dresden Technische Universität Dresden 01062 Dresden , Germany
necessity of thermally stable compounds and of high predict-ability of thermal stability before synthesis, which is also con-sidered in this paper.
Benzannulated aza -borondipyrromethenes (benz- aza -BODIPYs, 1 , Scheme 1 ) are a material class that shows high extinction coeffi cients, suitable energy levels for combination with C 60 as acceptor material, and suffi ciently good charge transport characteristics. [ 5 ] Consequently, such materials have already shown reasonable solar cell performances in single-junction devices as well as in semitransparent OSCs or solu-tion-processed ternary polymer solar cells. [ 4d,6 ] Furthermore, due to the complementary NIR absorption, this class of mol-ecules is of special interest for multijunction devices. For
instance, in combination with the green absorber material DCV5T-Me, we recently achieved an effi ciency of 10.4% in a triple-junction device using the phenyl-substituted benz-annulated aza -BODIPY (compound 1a in the following). [ 7 ]
In the last years, several functionaliza-tion strategies have been explored dem-onstrating various convenient synthesis routes of aza -BODIPYs with improved optical properties. [ 5,8 ] However, thermal stability, which is crucial for the produc-tion of vacuum-processed OSCs, remains
problematic. [ 8 ] For instance, methylthienyl-substituted benz- aza -BODIPY 1c exhibits good absorption characteristics out-performing similar molecules like 1a and 1b , but decomposes during purifi cation by thermal gradient sublimation or vacuum deposition. [ 9 ] For this compound as well as BODIPYs in general, the central borondifl uoride group is expected to be responsible for the reduced thermal stability (see Supporting Information).
In this article, we present a new functionalization strategy for benz- aza -BODIPYs with a fl uorene group at the boron atom ( 2 , Scheme 1 ), which leads to compounds with both a strong NIR absorption and a reliable thermal stability, thus combining the key properties which are crucial for the production of vacuum-deposited tandem OSCs. Our investigation covers all necessary
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Table 1. Performance of available thermally stable NIR absorber materials (absorption in the range of 700 nm or beyond) that have been employed in vacuum-processed single-junction OSCs with a PCE of at least 3%. Also given is the sublimation yield which was determined in our labs. Note that low sublimation yields prevent effi cient use in future mass production devices.
steps from synthesis via experimental and theoretical material characterization to solar cell analysis. We achieve an effi ciency of 4.5% for an optimized device using C 60 as acceptor (with an EQE of 62% at 690 nm) rendering this donor superior to the reference materials listed in Table 1 .
In particular, we fi nd high FFs of 74% in planar heterojunction (PHJ) devices, allowing for comparatively thick and, therefore, strongly absorbing blend layers in bulk heterojunction (BHJ) devices despite rather moderate hole mobilities. We propose fur-ther improvements (based on the control of intramolecular steric interactions) for the design of additional fl uorene-based BODIPY candidates with even more red-shifted absorption.
2. Results and Discussion
Experimental details as well as information on sample prepa-ration and characterization methods are presented in the Supporting Information.
2.1. Synthesis of BF 2 - and Borafl uorene Complexes
We fi rst present the synthesis of fl uorene-substituted benz- aza -BODIPYs in Scheme 1 , which—to our best knowledge—has not been reported previously. To reduce the steric interaction with the aryl groups in α-position of the BODIPY core, small phenyl rings were used for complexation. Surprisingly, fi rst coupling attempts between 1c and phenylmagnesiumbromide in different solvents (tetrahydrofuran and diethylether) led to only little reaction progress and many byproducts. Conse-quently, the single phenyl rings should be linked to fl uorene to give one defi ned, highly crystalline, and therefore easily extract-able complexation product at the end of the reaction. In this case, formation of the new complexes ( 2a , 2b , and 2c ) had to be started from the free ligands instead of the corresponding BF 2 -analogs which also avoided product losses during the complexa-tion step with BF 3 •OEt 2 and low-yielding fl uorine replacement.
The free ligands 7 were prepared according to the procedure of G resser et al. [ 9,10 ] We also synthesized the corresponding BF 2 -complexes ( 1a–c ) for comparison. As the reproduction of the literature procedure for the complexation reaction of 1a – c led to signifi cant amounts of unreacted starting material and
decomposition, this step was optimized by using higher excess of base (10 equivalents (eq.)) and boron species (15 eq.) as well as by carrying out the reaction in dichloromethane at room tem-perature overnight. [ 11 ] As the free ligands are supposed to be not fully stable under basic conditions and in solution exposed to sun light, the reaction was realized under light exclusion. [ 12 ] After further adjustment of the amount of solvent, the litera-ture yields could be signifi cantly increased to 80%–90% for all three BF 2 -complexes.
The adapted complexation reagent for the synthesis of the new borafl uorene complexes 2a – c , 9-chloro-9-borafl uorene ( 5 ), was prepared from 2,2-dibromobiphenyl ( 4 ) according to literature procedures. [ 13 ] The starting material could be easily received from 1,2-dibromobenzene ( 3 ) via bromine-lithium exchange. [ 14 ] To obtain the new aryl complexes 2a–c , the corre-sponding free ligands were reacted with 1.5 eq. of H ünig base and 2 eq. of 5 in dichloromethane. With such little excess of base and complexation reagent, 2a–c could already be isolated in high yields up to 81%. Notwithstanding, reaction times were relatively long for full composition of the starting mate-rial, because the reaction was less favored due to more steri-cally demanding 5 compared to BF 3 diethyletherate. Increasing the amount of base and boron compound would therefore be advantageous for a shorter reaction time, resulting in reduced decomposition of the starting material. This makes the syn-thesis of borafl uorene complexes from free BODIPY ligands as easy and effective as the reaction with BF 3 diethyletherate.
2.2. Material Characterization
2.2.1. Thermal Stability
We fi rst analyze the thermal stability and the sublimation behavior of the standard BF 2 -compounds 1a–c in comparison to the new compounds 2a–c . Thermogravimetric analysis (TGA) data are depicted in Figure 1 a. For compounds 1a and 1b , we fi nd fi rst mass losses at relatively high temperatures of about 350 °C. In contrast, material 1c appears less stable, showing weak mass losses already at 200 °C and a strong decrease at 325 °C. The borafl uorene complexes appear to be thermally equally stable ( 2a ) or more stable ( 2b , 2c ) than their BF 2 -analogs.
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Figure 1. a) TGA and b) DSC measurements (endo up) of BF 2 - and borafl uorene complexes. A vertical offset has been applied for clarity in (b).
To gain further insight, Figure 1 b presents differential scan-ning calorimetry (DSC) data for all six molecules. For molecule 1a , a sharp melting peak is observed at 270 °C. In contrast, 1b exhibits a melting peak at higher temperatures followed by an exothermic dip indicating thermal decomposition. 1c shows a similar behavior but shifted to smaller temperatures with a melting temperature around 250 °C. The melting temperatures of the new compounds 2a and 2c are shifted to higher tem-peratures supporting an improved sublimation behavior for the borafl uorene-functionalized molecules. In contrast, 2b melts already at slightly lower temperatures than 1b (not showing an exothermic dip behind the weak melting peak).
Finally, in order to ensure highest performance in devices all materials are purifi ed by thermal gradient sublimation in two consecutive steps. While the fi rst step is used to remove most of the impurities, the second step can be seen as a practical in-use measure for the thermal stability of the material. This number is furthermore important in view of later production because it directly infl uences the real price of a compound. From the compounds tested here, all but 1c can be sublimed with yields between 80% and 93% in the second step (see Table 2 ). As already indicated from TGA and DSC measurements, the replacement of F 2 by the fl uorene group enhances the thermal stability giving highest yields in the sublimation.
2.2.2. Crystal Structures
To analyze the impact of the borafl uorene group on the molecular packing, the crystal structures of molecules 1c , 7c , and 2c are presented in Figure 2 . While 1c reaches a similarly
dense packing as molecule 7c , although it exhibits torsionally twisted side groups, 2c is stacked more loosely due to sterical hindrance induced by the fl uorene-substitution, which suggests easier sublimation compared to 1c . This is confi rmed by a cohe-sive energy that is 400 meV less compared to compound 1a . In addition, the crystal structure of 2c exhibits only one nearest neighbor with suffi cient intermolecular interaction, while for 1c we observe a 3D network of CH⋅⋅⋅F hydrogen bonds.
The crystal structures of molecules 7c , 1c , and 2c differ also strongly in the orientation of the methylthienyl groups with respect to the molecular core (cf. Figure 2 ). These groups arrange parallel to the molecular core of 7c , but are slightly rotated out-of-plane for 1c and even more strongly for 2c , which results from a balance of intramolecular hydrogen bonds and steric hindrances. While for the free ligand 7c the absence of
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Table 2. Thermal characteristics of BF 2 compounds 1a–c and their fl u-orene analogs 2a–c . Thermal gradient sublimation was performed for input amounts of 1–2 g at temperatures between 230 °C and 250 °C.
Compound Melting point [°C]
Melting heat [kJ mol −1 ]
Total sublimation yield over two steps
[%]
Sublimation yield in second step
[%]
1a 270.6 36.79 68 84
1b 303.2 39.19 80 a) –
1c 252.8 13.81 0 –
2a 308.0 40.02 75 93
2b 290.4 48.15 85 93
2c 279.7 22.90 78 88
a) Only 1 sublimation step performed.
Figure 2. Molecular structures and crystal unit cells of molecules 2c, 7c , and 1c based on X-ray diffraction data. a) 2c , monoclinic unit cell containing four molecules: a = 16.068 Å, b = 8.675 Å, c = 21.631 Å, and β = 91.59°. b) 7c , monoclinic unit cell containing four molecules: a = 10.116 Å, b = 16.951 Å, c = 12.540 Å, and β = 95.98°. c) 1c , triclinic unit cell containing two molecules: a = 7.699 Å, b = 11.941 Å, c = 12.608 Å, α = 105.47°, β = 94.88°, and γ = 98.29°. [ 10 ]
hydrogen bonds to these groups results in a planar confi gura-tion, an interplay of the sp 3 hybrid orbital of the boron atom and the relatively strong intramolecular CH⋅⋅⋅F hydrogen bonds leads to a torsional angle of 34° for 1c . For 2c , due to intramo-lecular steric hindrances and the absence of hydrogen bonds, the methylthienyl groups prefer a more parallel orientation to the borafl uorene group and therefore a much higher torsional angle.
2.2.3. Optical and Electrochemical Properties
Motivated by the improved sublimation behavior of compounds 2a–c , we investigate the impact of the fl uorene-substitution on UV–vis–NIR absorption and electronic properties in Figure 3 and Table 3 . The borafl uorene complexation of 7a–c results in red-shifted absorption for 2a and 2b . This shift in solution is smaller than for 1a and 1b by about 40 nm. For 1c / 2c , the differ-ence is twice as high (79 nm) which means that absorption in 2c is even slightly blue shifted compared to the free ligand 7c . Extinction coeffi cients are slightly increased for 2a and 2b and almost unchanged for 2c compared to their BF 2 counterparts.
The absorption maxima of 2a–c are red shifted by about 30 to 40 nm from solution to thin fi lm, as depicted in Figure 3 b, hence 10 nm less than for 1a–c , which can be related to the
less densely packed crystal structures of the borafl uorene com-plexes resulting in a smaller orbital overlap. In addition, the absorption features are strongly broadened. We also observe that 2b and 2c show the same relative extinction as in solution, while the absorption strength of 2a is reduced. This reduction is attributed to the strong crystallinity of 2a , which is visible as well in atomic force microscopy and grazing incidence X-ray diffraction measurements (cf. Figures S7 and S8, Supporting Information). The absorption can be affected by the molec-ular orientation [ 15 ] or strong aggregation (reduced surface coverage). [ 16 ] Absorption measurements on full devices, where the donor is mixed with the acceptor C 60 and is therefore in a rather unaggregated state, show the same relation between the absorption strengths as observed in absorption measure-ments in solution (see Figure S14, Supporting Information).
The difference in the position of the absorption maximum between 1 and 2 is related to the electronic gap (Table 3 ). The BF 2 unit stabilizes the LUMO levels of 7a–c stronger than the HOMO levels leading to a reduction of the energy gap. This is similar though less pronounced for the borafl uorene unit causing larger energy gaps compared to compounds 1a–c .
Further insight into these trends and their correlation with the molecular structure is gained from density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations pre-sented in Figure 4 . The agreement between experimental and
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Figure 3. a) Absorption of BF 2 - and borafl uorene complexes in dichloromethane solution. b) Optical density (O.D.) of borafl uorene complexes in 50 nm thin fi lms prepared and measured on glass slides.
Table 3. Spectroscopic and electrochemical properties of 2a–c as well as reference data for 1a–c and 7a–c . [ 9 ]
Compound λ abs fi lm [nm] a)
λ abs sol. [nm] b)
ε [ M −1 cm −1 ]
λ em sol. [nm] b)
Stokes shift [nm]
HOMO [eV] c)
IP [eV] d)
LUMO [eV] c)
1a 756 (20) 712 106 000 746 34 −5.22 5.30 −3.65
1b 760 (20) 717 94 000 753 36 −5.16 5.27 −3.63
1c 842 (30) 793 95 000 840 47 −4.96 4.85 −3.70
2a 707 (50) 670 110 000 705 35 −5.13 5.27 −3.40
2b 705 (50) 673 113 000 706 33 −5.06 5.00 −3.43
2c 753 (50) 714 92 000 761 47 −4.99 4.96 −3.48
7a – 653 56 000 − − −4.97 − −3.38
7b – 657 55 000 − − −4.89 − −3.30
7c – 720 40 000 − − −4.78 − −3.35
a) Thin fi lm thickness is given in parentheses; b) Absorption and emission measured in CH 2 Cl 2 ; c) Determined via cyclovoltammetry (CV) measurements in CH 2 Cl 2 against ferrocene (Fc/Fc + ) as internal standard; d) From ultraviolet photoelectron spectroscopy (UPS, onset value).
theoretical trends is very good which enables the determination of polarization shifts for energy levels and absorption maxima (cf. Figures S1–S3, Supporting Information). The systematic blue shift of the absorption maximum of the borafl uorene com-pounds 2a–c with respect to their BF 2 -analogs is understood by analyzing the properties of the basic molecular cores 1* and 2* with H-substitution at the α-position. Surprisingly, in contrast to the undesired blue shift for 2a–c in Figure 4 , the excita-tion wavelength of both molecules 1* and 2* is mostly iden-tical demonstrating that the newly synthesized borafl uorene BODIPYs can have equally good absorption characteristics as their BF 2 -counterparts. Detailed analysis shows that intramo-lecular CH⋅⋅⋅F hydrogen bonds reduce the torsional angle of the
substituents at the α-position with respect to the molecular core for 1a–c leading to a red shift in the excitation spectrum com-pared to 1* . Such a strong red shift cannot be observed for mol-ecules 2a–c due to the sterically more demanding borafl uorene group causing increased torsional angles and, thus, reduced delocalization of the frontier MOs on the respective functional groups.
2.2.4. Charge Carrier Mobility
Reasonable hole transport is another criterion for donor materials in OSCs, although small-molecule-based devices
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Figure 4. Theoretical investigation of various BF 2 - and corresponding borafl uorene complexes. a) LUMO of BF 2 -complexes. b) HOMO of BF 2 -com-plexes and corresponding values of the torsional angle ϑ of the functional groups attached at the α-position with respect to the molecular core, the ionization potential (IP), and the electron affi nity (EA). c) LUMO of borafl uorene analogs. d) HOMO of borafl uorene analogs and obtained values of ϑ , IP, and EA. e) Oscillator strength ν versus absorption wavelength λ of the fi rst excited state, which corresponds to a transition from the HOMO to the LUMO. f) Internal reorganization energy for hole transfer Λ + versus absorption wavelength λ . Overall, excellent agreement with experimental trends is observed (cf. Figures S1–S3, Supporting Information).
have proven high effi ciencies even for moderate hole mobili-ties below 10 −4 cm 2 (V −1 s −1 ). [ 17 ] We measure hole mobilities µ h in organic fi eld-effect transistor geometry for molecules 1a–c and 2a–c giving values in the range between 10 −5 and 10 −4 cm 2 (V −1 s −1 ) (see Table 4 ). The borafl uorene compounds show smaller mobilities than their BF 2 -analogs except for molecule 2b . As transport simulations (cf. Figure S6, Sup-porting Information) indicate, most trends are consistent with the magnitude of the reorganization energy for hole transport Λ + , which is one key quantity infl uencing the hole mobility in organic materials. The increased reorganization energies of the new compounds can be assigned to relatively strong inter-actions between the substituents at the α-position and the borafl uorene group, which is also seen in the larger torsional angles.
As an exception, the higher hole mobility of material 1c cannot be related to the reorganization energy. Here, another key quantity, the orbital overlap described by the transfer inte-grals, is also of importance. More precisely, the tight packing of molecules in the crystalline phase results in a balanced dis-tribution of transfer integrals to neighboring molecules for 1c (cf. Figure S4, Supporting Information), which strongly pro-motes charge transport compared to results reported for 1a . [ 18 ] In less-ordered morphologies, one can expect that due to the
strong intermolecular interactions, its structure motifs may still appear, thus leading to the highest observed mobility.
2.3. Thin Film Characterization and Organic Solar Cells
The new compounds are tested in PHJ and BHJ solar cells as donor materials in combination with C 60 as acceptor, and the results are compared to their molecular properties. The device stacks are described in the Methods section in the Supporting Information and are depicted in Figure S13. Since the best per-formance was found for 2a , we present the results of this donor material in the following Section 2.3.1 in detail. In Section 2.3.2, we discuss differences found with respect to the devices based on 2b and 2c .
2.3.1. Organic Solar Cells with 2a
Solar cells based on 2a were built as PHJ devices with donor layer thickness of 10 nm and as BHJ devices with varying donor:acceptor volume mixing ratios ( V D : V A ), different BHJ layer thicknesses ( t B ), and under variation of the substrate temperature during deposition ( T s ). The results are summa-rized in Table 5 as well as Figures 5 and 6 . Further data are given in the Supporting Information.
Despite the moderate hole mobility, we achieve very high fi ll factors of 74.3% in PHJ devices, and their characteristics are competitive with respect to devices based on 1a . [ 9 ] In Figure 5 a, dark and illuminated current density–voltage ( J–V) characteris-tics are plotted for PHJ and BHJ devices under variation of the mixing ratio. Upon blending 2a with C 60 , we fi nd a signifi cant increase in V oc , which can be further promoted by increasing the volume fraction of C 60 in the BHJ. Changes in V oc for varying mixing ratios have been observed before in devices containing ZnPc:C 60 BHJs and for special devices containing low amounts of donor material diluted in C 60 (1–10 wt%). [ 19 ] In the fi rst case, the variation of V oc was attributed to morphological changes, while in the second reference, the authors demonstrated that
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Table 4. Mean values for the OFET-mobility µ h and reorganization energies for hole transfer Λ + for BF 2 - and borafl uorene complexes.
Compound µ h [cm 2 V −1 s −1 ]
Λ + [meV]
1a 6.9 × 10 −5 139
1b 1.6 × 10 −5 159
1c 2.0 × 10 −4a) 187
2a 2.7 × 10 −5 178
2b 2.7 × 10 −5 202
2c 1.1 × 10 −5 291
a) Material used without purifi cation by sublimation.
Table 5. Solar cell characteristics for 10 nm PHJ and BHJ devices produced with compounds 2a–c as donor and C 60 as acceptor material. Details on the measurement error can be found in the Methods section of the supporting information. Solar cell stacks are shown in Figure S13 in the Sup-porting Information. The mismatch corrected illumination intensity I L for the measurement was recalculated after the measurement using the respec-tive solar simulator spectrum for this particular measurement and the measured spectral response of the device.
Figure 5. Characteristics of PHJ and BHJ solar cells using compound 2a as donor material. a) J–V characteristics of the PHJ device and of 15 nm BHJ devices with different donor:acceptor mixing ratios at T s = 30 °C. b) EQE for PHJ and BHJ devices with different mixing ratios. On the right axis, the EQE (normalized to the maximum of the BODIPY absorption) is plotted on a logarithmic scale, revealing the contribution of direct CT state absorp-tion. c) J–V characteristics of BHJ devices with different active layer thicknesses and substrate temperatures T s for the active layer. d) Equivalent graph to (b) for the devices shown in (c).
Figure 6. Development of J sc and FF with increasing blend layer thickness t B for different substrate temperatures T S during layer deposition using a,d) 2a , b,e) 2b , and c,f) 2c as donor material, respectively.
the amount of interface area between donor and acceptor affects the voltage. Both effects may infl uence the trend in V oc observed here, but their respective contribution cannot be quantifi ed. In addition to increasing V oc s, we also observe higher short-circuit current densities ( J sc ) for the cells blended with increased C 60 content. Despite a weaker absorption, which is related to the reduced total amount of compound 2a in the BHJ, we observe a higher external quantum effi ciency (EQE; Figure 5 b) indicating more effi cient charge separation or transport processes and, therefore, higher internal quantum effi ciency (IQE). This obser-vation is supported by a reduced fi eld dependence of the photo-current for the 1:2 blend (Figure 5 a), which can be attributed to reduced charge carrier recombination.
Upon blending the BODIPY into C 60 , the EQE peak in the near-infrared broadens signifi cantly. While the short wave-length shoulder around 650 nm, which is present in the PHJ device, vanishes for the 1:2 mixing ratio (Figure 5 b), addi-tional photocurrent arises on the long-wavelength side (around 810 nm). Since this absorption occurs energetically below the optical gap of the pure material (727 nm), we attribute this fea-ture to direct absorption of the intermolecular charge transfer (CT) state between the donor molecule and C 60 . [ 20 ]
To further improve the device performance, the thickness of the BHJ is varied (blue data in Figure 6 a,d). Although J sc is increased for thicker layers due to stronger absorption, the effi -ciency steadily decreases owing to a steep decrease of the fi ll factor (FF), which is attributed to poor charge transport proper-ties of the BHJ.
Additional heating of the substrate during evaporation of the BHJ layer enhances the charge transport properties, which leads to an increased FF. This procedure allows for thicker photoactive layers, resulting in higher J sc . The evolution of J sc and FF with increasing BHJ thickness for higher substrate temperatures of 95 °C and 120 °C is included in Figure 6 as well. While the FF steadily increases with increasing substrate temperatures, even reaching FFs above 60% at t B = 50 nm, the photocurrent saturates at T S = 95 °C and drops at T S = 120 °C. The EQE in Figure 5 d shows a reduction only for the sensi-tivity region of the BODIPY, whereas the EQE of C 60 continues increasing from 95 °C to 120 °C. This reduction is known from literature [ 16 ] and is caused by the reduced absorption of the donor compound due to an increasing crystallinity (“bleaching effect”) as mentioned above for pure fi lms. Measurements of the absorption in full devices confi rm the reduced absorption in the BODIPY absorption range (see Figure S13, Supporting Information). The fact that both the decrease in photocurrent is lower than the decrease in absorption and that the C 60 -dom-inated part of the EQE is increased, indicate that charge extrac-tion is still improved and leads to higher FFs.
The effect of increasing FFs upon substrate heating is often attributed to an increased phase separation and/or crystalliza-tion in the BHJ. [ 21 ] Besides the observed indication of higher crystallinity obtained from the bleaching at T S = 120 °C, con-fi rmation for an increased phase separation is found in the normalized EQE of the devices depicted in Figure 5 d. We observe a lower contribution of the intermolecular CT states for λ > 750 nm with increasing substrate temperature, which corresponds to a reduced interface between donor and acceptor phases or a smaller fraction of a mixed phase. In addition, the
short wavelength feature below 700 nm is signifi cantly recov-ered upon heating, indicating the recovery and increased crys-tallinity of the pure donor phase.
Interestingly, compound 2a also exhibits a dependency of the V oc on the substrate temperature during deposition (see Table S2, Supporting Information). From 30 °C to 120 °C, V oc gradually decreases from 0.81 to 0.73 V at 120 °C. This decrease is attributed to an increased molecular aggregation and crystallization at higher substrate temperatures approaching a V oc of 0.69 V obtained for the PHJ device.
In the above measurement series, very good devices were obtained for a blend thickness of t B = 50 nm at T S = 120 °C. Due to the discussed bleaching at 120 °C, we performed another optimization cycle around this substrate temperature, including a wider blend-thickness variation as well as an optimization of the following layers, namely a pure donor layer and an addi-tional blocker layer. The best device is fi nally obtained for a blend layer thickness of 70 nm and a substrate temperature of 110 °C and features an excellent PCE of 4.5%. In comparison to the reference NIR absorber materials in Table 1 , these PCE values for 2a constitute a new record value for well-sublimable donor materials in combination with C 60 as acceptor. 2a outper-forms SubNc with a PCE of 4.4% and shows a very high EQE of 62% at 690 nm. Compared to DTDCTB, 2a has slightly lower effi ciency, but exhibits a much higher yield in the sublimation, reducing the price for potential production by a factor of 3–5.
2.3.2. Comparison with Materials 2b and 2c
Borafl uorene complexes 2b and 2c were tested in PHJ and BHJ devices with a volume mixing ratio of 1:2. Although we still reach FFs close to 70% in PHJ device using 2b and 2c , the slightly decreasing FF follows the trend of increasing reor-ganization energy and, therefore, a decreased hole transporting ability. The reduction in V oc follows the observed increase of the HOMO level, especially giving a signifi cantly lower V oc of 0.56 V for the red-shifted compound 2c . For increasing BHJ thicknesses and substrate temperatures (see Figure 6 ), 2b and 2c perform markedly different compared to 2a . Without substrate heating, the FF starts out much lower and steeply decreases for thicker active layers. Furthermore, J sc does not increase for thicker layers, pointing to strong recombination losses in the blend.
An increase of the substrate temperature from 95 °C to 120 °C still increases both J sc and FF for compounds 2b and 2c , which is attributed to an increased (but not yet negatively affecting) crystallinity. In contrast, molecule 2a , which showed higher crystallinity in pure fi lms, already demonstrated signifi -cant bleaching at this temperature, leading to reduced photo-current and lower device effi ciency. As a result, the photocur-rents achieved are similar for all three compounds, whereas the FFs still remain lower as discussed above (see Table 5 and Table S2, Supporting Information).
For the underlying optimization of substrate temperature and blend thickness, we fi nally obtain comparable effi ciencies of 3.9% and 3.7% for 2a and 2b , respectively, whereas 2c gives a reduced PCE of 2.6% due to the lowest FF and V oc in this series (see Table 5 ). Due to the overall lower effi ciencies and FFs for
compounds 2b and 2c , we refrain from additional optimization cycles here.
2.4. Discussion
Borafl uorene-based aza -BODIPYs provide highly competitive OSCs with respect to optimized BHJ OSCs of 1a and 1b and NIR absorbing donor materials in general, [ 4–6,22 ] and thus moti-vate further research of this functionalization strategy: DFT and TD-DFT simulations reveal that the strong interaction between the borafl uorene group and the substituents leads to large tor-sional angles of these groups with respect to the molecular core, which results in stronger localization of frontier molecular orbitals and, thus, a small blue shift and increased reorganiza-tion energies compared to their BF 2 -analogs. On the other hand, we found that the maximum absorption wavelength of the non-functionalized molecule 2* is nearly the same as for 1* (and also shows a reduced reorganization energy for holes). These prom-ising properties of the basic borafl uorene core can be better conserved either by attaching substituents at the α-position, which are more distant (as observed for compound 2d ), or by designing molecules with restricted planarity as found, for example, for 2e (cf. Figure 4 ). By applying a polarization shift of 160 nm to the gas-phase absorption spectrum (Figure S3, Supporting Information), we estimate an absorption maximum in thin fi lm at 840 nm for 2d and at 800 nm for 2e making these materials highly interesting for future studies on vacuum-processed OSCs.
Therefore, further structural optimization of the borafl u-orene complexes may enable PCEs above 4.5% or could further red-shift the absorption compared to the prototypical com-pounds 2a–2c studied here.
3. Conclusion
We propose a new functionalization strategy of aza -BODIPYs with improved thermal stability to be used as NIR absorber materials in vacuum processed OSCs. The borafl uorene com-pounds can be derived through an easy synthesis route with high yield and can be independently functionalized at the α-position. In this study, borafl uorene-BODIPYs with phenyl, tolyl, and methylthienyl substituents serve as prototypical exam-ples. They exhibit strong NIR absorption with maxima between 670 and 715 nm in solution (700–750 nm in thin fi lm).
Despite the increased reorganization energies, leading to a small reduction in hole mobility, the new materials 2a – c show high fi ll factors beyond 70% in PHJ OSC devices. Optimized BHJ solar cells with C 60 yield PCEs up to 4.5% for the phenyl substituent and an EQE as high as 62% around 700 nm. As we have summarized in the beginning (Table 1 ), this result is one of the highest effi ciencies achieved for vacuum-pro-cessed small-molecule OSCs using an NIR absorber as donor together with C 60 as acceptor. In addition, their strong and rela-tively narrow absorption band in the NIR region makes them very promising for use in tandem devices. Together with the
improved thermal stability and ease of further functionaliza-tion, these promising solar cell characteristics make borafl u-orene-functionalized aza -BODIPYs very attractive for further applications.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements M.L.-R. and K.S.S. contributed equally to this work. This work was fi nancially supported by the German Bundesministerium für Bildung und Forschung (BMBF) under the contract no. FKZ 03EK3505D (LOTsE project) and the DFG via priority program SPP1355 . The authors gratefully acknowledge support from the German Excellence Initiative via the Cluster of Excellence EXC 1056 “Center for Advancing Electronics Dresden” (cfAED). Additionally, this work was partly supported by the Heinrich Böll Stiftung e.V. and the DFG ( OR 349/1 ). Computational resources were provided by the Center for Information Services and High Performance Computing (ZIH) of Dresden University of Technology. The authors would like to thank A. Petrich for sublimation of the materials, A. Jäger for resolving the crystal structures, Dr. J. Widmer for discussion, Dr. L. Wilde (Fraunhofer CNT, Dresden) for performing the GIXRD measurements, and Dr. I. Kunert (TU Dresden) for the TGA/DSC-measurements. S. Furkert, T. Günther, and A. Wendel are also greatly acknowledged for their technical support.
Received: April 13, 2016 Revised: June 27, 2016
Published online: September 6, 2016
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