A new NIR absorbing DPP-based polymer for thick organic solar cells

Gulce Oklem a, Xin Song *b, Levent Toppare acd, Derya Baran b and Gorkem Gunbas *acd
aDepartment of Polymer Science & Technology, Middle East Technical University, Ankara, Turkey. E-mail: ggunbas@metu.edu.tr
bPhysical Sciences and Engineering Division, KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), KSC Thuwal 23955-6900, Saudi Arabia. E-mail: xin.song@kaust.edu.sa
cDepartment of Chemistry, Middle East Technical University (METU), Ankara, Turkey
dThe Center for Solar Energy Research and Applications, METU, Ankara, Turkey

Received 8th January 2018 , Accepted 4th February 2018

First published on 5th February 2018

Sunlight covers a broad spectrum from ultra-violet to infrared, and low band gap materials are required to utilize the near infrared region (NIR) for better photon harvesting in organic solar cells. It has been shown that copolymers comprising diketopyrrolopyrrole-based acceptors and simple donors (thiophene or furan) achieve an absorption maximum at around 800 nm. In this study, selenophene was coupled with a diketopyrrolopyrrole based acceptor to yield a polymer (PFDPPSe) with an absorption maximum at 830 nm and an absorption onset at 930 nm. The optimized organic solar cells with PFDDPSe:PC71BM active layer blends at 210 nm showed a maximum PCE of 6.16% (average 6.02%) via solvent additive engineering with an inverted device structure. Their charge transport, recombination loss mechanism, and morphology were systematically studied. The results demonstrate that a highly efficient NIR-absorbing polymer can be achieved by the introduction of selenophene and a suitable solvent additive process for NIR organic solar cells. PFDPPSe is also one of the rare examples of a polymer with a PCE of over 6% that does not contain any thiophene-based unit in its backbone.


Interest in organic solar cells (OSCs) has grown steadily and the 10% power conversion efficiency (PCE) barrier has recently been surpassed.1–10 Additionally, continuous progress in the development of non-fullerene acceptor (NFA)-based OSCs has brought a new level of excitement and opportunities to this lively research area,11–13 which has pushed the efficiency over 13% with structural design and device optimization.14–17 However, the absorption range of the active layer in fullerene and non-fullerene based solar cells is mainly between 300 nm and 800 nm.18 The utilization of the near infrared (NIR) region of the solar spectrum is limited. Towards achieving this aim, a number of NIR-absorbing polymers were designed, synthesized and characterized.19–22 Additionally, utilization of NIR absorbing NFAs for the realization of high performance solar cells has recently gained significant attention.23,24 Among donor polymers that have been evaluated, thiophene based diketopyrrolopyrrole (DPP) acceptor containing polymers showed promising results.22,25 In addition to thiophene based DPP acceptors, their furan containing counterparts are also successfully utilized in organic solar cells. Frechet and co-workers showed that when a furan based DPP acceptor (FDPP) copolymerized with a simple donor, thiophene, the resulting polymer (PDPP2FT) has an absorption maximum at 789 nm and a PCE of 5% can be achieved utilizing this material in OSCs.26 To further push the absorption towards the NIR region, replacing thiophene with selenophene is a valid strategy. The larger size of selenium compared to sulfur results in lower aromaticity of selenophene due to the poor overlapping of selenium orbitals with the π-system of carbon framework.27–29 This ultimately results in an increased quinoidal contribution to the molecular energy levels and hence a red-shift in the absorption profile.30 There are a number of examples in the literature where this approach results in a red-shift in absorption and in some cases a better photovoltaic performance was observed.31–33

In this work, we designed a NIR-absorbing polymer, PFDPPSe, where furan-based DPP was used as the acceptor and selenophene was used as the donor, which is one of the rare examples of DPP-based polymers where the polymer backbone does not include any thiophene or thiophene-based unit (BDT, thienothiophene, etc.). With the construction of an inverted device structure, a high performance of over 6% was obtained upon the introduction of diphenyl ether (DPE), a high boiling point solvent additive, which surpasses the as-cast devices, with an improvement of over 10-fold. Deeper morphological and electrical analysis demonstrated the beneficial effects on the morphology, charge transport and recombination, which resulted in a much higher performance of the organic solar cells.

Results and discussion


First, we concentrated our efforts towards the synthesis of the FDPP unit (1) (Fig. 1). There are a number of approaches reported in the literature for the synthesis of this unit. Condensation of 2-furonitrile and diethyl succinate is the most common method due its convenience.26 In this method the FDPP unit is obtained as a salt which can be directly utilized in the subsequent alkylation reaction. However when this approach is used, the reported yields are generally low.26 Alternatively, at the end of the same reaction, the solution can be acidified and FDPP can be obtained in the neutral, protonated form. In the literature, acetic acid is commonly used for this purpose and the precipitated FDPP was collected by filtration and washed with methanol. However, in our study, very low yields were obtained after successive washing.34 On the other hand, when HCl was used instead of acetic acid, the target product was obtained in good yield.35 Next alkylation reactions were carried out. Here two different alkyl chains were utilized: a linear C12 chain and a linear C18 chain. The alkylation reactions were carried out successfully (utilizing both the approaches mentioned above) to obtain compounds 235 and 3 albeit in relatively low yields. Then the obtained alkylated FDPP units were brominated with NBS to give literature compound 435 and compound 5. Selenophene was stannylated from both 2 and 5 positions36 and Stille coupling polymerizations were performed to obtain target polymers (PFDPPSe-12, PFDPPSe-18). As mentioned above it was stated in the literature that FDPP-based polymers tend to have better solubilities compared to their thiophene analogues, and C12 linear chain or C8 branched chain substituted FDPP-thiophene copolymers were shown to be soluble in common organic solvents. A significant difference was observed when a thiophene donor unit in the polymer backbone was replaced with selenophene; the resulting material, PFDPPSe-12, was completely insoluble, even in o-DCB. On the other hand, PFDPPSe-18 was highly soluble in CHCl3, chlorobenzene and o-DCB. GPC anaylses showed that PFDPPSe-18 has an Mn of 14.6 kDa and an Mw of 50.3 kDa with a PDI of 3.44. PFDPPSe-18 will be referred as PFDPPSe for simplicity.
image file: c8tc00113h-f1.tif
Fig. 1 Synthetic route to the polymers.

Optical and electrochemical characterization of FDPPSe

A thin film of PFDPPSe was spray coated onto ITO glass slides and the UV-Vis spectrum was recorded. Two absorption maxima were observed similar to that of PDPP2FT (Fig. 2a). The lower energy absorption maximum of PDPP2FT was reported to be 789 nm, and as expected, PFDPPSe showed a significant red-shift and the absorption maximum was determined to be 832 nm. The onset of absorption also shifted substantially (880 nm vs. 927 nm). The optical band gap was calculated from the onset and was found to be 1.34 eV. The cyclic voltammogram of PFDPPSe was also recorded and from the onset of oxidation and reduction potentials the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PFDPPSe were observed at 5.46 eV and 3.81 eV respectively (Fig. 2b). These values are comparable with those obtained for PDPP2FT.
image file: c8tc00113h-f2.tif
Fig. 2 (a) Thin film UV-Vis spectrum and (b) cyclic voltammogram of FDDPPSe, and (c) band alignment of PFDPPSe in the proposed solar cell device structure.

Solar cell fabrication and optimization

Solar cells were fabricated using PFDPPSe as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor with an inverted device structure, ITO/ZnO(∼40 nm)//PFDPPSe:PC71BM(∼210 nm)/MoOx((∼7 nm)/Ag(∼100 nm)). The band alignment for the proposed device structure is given in Fig. 2c. We first optimized the donor[thin space (1/6-em)]:[thin space (1/6-em)]acceptor blend ratio. In detail, the polymer[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM ratio was changed from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]5 and the device performances were evaluated (Table 1). While the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio gave very poor results (PCE of 0.15%), increasing the PC71BM loading resulted in a significant enhancement (PCE of 0.93%) in device performance, due to the increase of the fill factor (FF). Notably, the increase in the PC71BM ratio did not have a strong effect on the short circuit current (Jsc). It is possible that due to the long alkyl chains of the donor unit, large gaps are present in the morphology. The increase in the PC71BM ratio helps in the filling of these gaps which results in a higher FF.37 However, the morphology still does not provide a pathway for efficient charge extraction, hence the low Jsc. The effect of additives was then examined to see if the morphology can be improved for better charge extraction.38
Table 1 Results of the polymer[thin space (1/6-em)]:[thin space (1/6-em)]PCBM ratio optimization (film thickness of the active layer is 95 nm)
Ratio J sc V oc FF PCE
1[thin space (1/6-em)]:[thin space (1/6-em)]1 0.78 0.56 33.2 0.15
1[thin space (1/6-em)]:[thin space (1/6-em)]2 1.89 0.63 54.6 0.65
1[thin space (1/6-em)]:[thin space (1/6-em)]3 2.22 0.63 62.8 0.93
1[thin space (1/6-em)]:[thin space (1/6-em)]4 2.05 0.64 58.6 0.77
1[thin space (1/6-em)]:[thin space (1/6-em)]5 1.85 0.61 52.1 0.59

Diphenyl ether (DPE) was chosen as the additive.25,39 A strong additive effect was clearly observed in solar cell devices. With the optimization of film thickness via changing the concentration of the polymer[thin space (1/6-em)]:[thin space (1/6-em)]PC71BM solution and the rotation speed for spin coating, a high efficiency of 6.16% (average 6.02% over six devices) was achieved. The optimum film thickness was found to be 210 nm. It is important to note here that a decent FF was achieved with a thick active layer which is rare for NIR organic solar cells. For a clear comparison, devices under optimized conditions were prepared without additive and the results are given below (Table 2). The devices processed with solvent additive resulted in almost a 13-fold increase in the PCE. This is one of the highest enhancements using DPE as an additive for NIR organic solar cells. External quantum efficiency (EQE) data for the devices were also recorded and the integrated Jsc data (1.50 mA cm−2 and 15.4 mA cm−2) were consistent with the actual Jsc values (1.66 mA cm−2 and 16.0 mA cm−2) from the devices, which are in good agreement with the Jsc values from JV curves with errors smaller than 5% (Fig. 3a and b). A significant difference in the photoresponses was reflected in the EQE curves after the introduction of DPE. The devices revealed a broad and high incident light response with the maximum EQE approaching 60% with the help of DPE. In contrast, the device without DPE shows a low photoresponse (<10%) at all wavelengths ranging from 300 nm to 930 nm (Fig. 3b). According to a previous report, PDPP2FT derivatives (thiophene analogues of PFDPPSe with a variety of alkyl chains) showed EQE values below 40% in the 700–850 nm region because of insignificant charge contribution. However, in our blend system, a stronger contribution was achieved from the NIR region where EQE values were roughly higher than 40% between 700 nm and 850 nm (Fig. 3b). These data clearly show that PFDPPSe provides a stronger contribution in the NIR region than the polymer with thiophene units.

Table 2 Comparison of device performance without additive (CB) and DPE additive (CB + 3% DPE) under optimized conditions. Active layer film thickness is 210 nm
J sc V oc FF PCE Average PCE
CB 1.66 0.63 50.0 0.52 0.47
CB + 3% DPE 16 0.64 60.4 6.16 6.02

image file: c8tc00113h-f3.tif
Fig. 3 (a) The JV curves of the device with/without DPE, (b) EQE curve of device with/without DPE, (c) and (e) the topographical AFM and TEM images of the blend film without additive, (d) and (f) the topographical AFM and TEM images of the blend film with additive.

Morphological and electronic characterization of FDPPSe:PC71BM blends

In general, the additive can efficiently help to obtain better micromorphology and phase separation, which is beneficial for charge transport. The surface and inner morphology of blend films with/without DPE were studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in the AFM image in Fig. 3c, the blend film without DPE had a rough surface with a root-mean-square (RMS) over 10 nm, whereas the blend film after the solvent additive treatment became smooth with a reduced RMS of 2.3 nm (Fig. 3d).40 The flat blend film is beneficial for the contact between the active layer and top electrode. More importantly, the blend films with DPE indicated an interpenetrating nanoscale structure, which indicates an obvious phase separation has appeared. We utilized the TEM technique to further study the inner morphology on the nanoscale. Similar to the AFM images, strong aggregation of PC71BM was observed in the blend film without additive treatment (Fig. 3e). Conversely, the incorporation of 3% DPE in solvent results in significant changes in the morphology of the film. The interpenetrating network with optimal phase separation size is shown in Fig. 3f, showing an important sign of efficient charge generation and transport.41 Clearly, AFM and TEM images explain the order of magnitude increase in the Jsc and the improvements observed in the FF.

The charge carrier mobility in the blend film is critical, especially for carrier sweeping-out. In order to determine the electron and hole mobilities in the blend film with/without additive treatment, we measured the dark current density–voltage (JV) characteristics of the single carrier device and then analysed the data using a space-charge-limited current (SCLC) method. The detailed equation and device fabrication are presented in the ESI. The fitting results shown in Fig. 4a and Table 3 revealed that the increase in hole mobility with the introduction of DPE is in line with the conclusion of morphological analysis, in which the interpenetrating network was formed when DPE was added to the solvent. Conversely, there is a decrease in electron mobility of the blend film when the amount of additive changed from 0% to 3% (Fig. 4b). We speculate that the distribution of the over-aggregated fullerene domain may be the main reason for the decrease in electron mobility. More importantly, the electron and hole mobilities are more balanced in the film processed with 3% DPE. The balanced carrier transport may be one reason that contributes to the higher device performance.42

image file: c8tc00113h-f4.tif
Fig. 4 (a) Hole mobilities of FDPPSe spin coated from CB and from CB + 3% DPE, (b) electron mobilities of PFDPPSe spin coated from CB and from CB + 3% DPE, (c) Jscversus light intensity and (d) Vocversus light intensity measurements of solar cells with/without additive.
Table 3 Comparison of hole and electron mobilities for the films coated from CB and CB with 3% DPE additive
Hole mobility Electron mobility Ratio
CB 3.52 × 10−5 3.21 × 10−4 9.12
CB + 3% DPE 3.44 × 10−4 2.68 × 10−4 0.78

To further characterize the significant changes in current density and fill factor, we focused on the recombination loss in the blend films with/without DPE by collecting the JV characteristics at different light levels to show the Voc evolution as a function of light intensity. Particularly, Jsc follows a power-law dependence on light intensity (Pin) as JscPαin, where α represents the power-law exponent.43 Normally, α = 1 indicates that bimolecular recombination becomes negligible, whereas α < 1 suggests that bimolecular recombination is a limiting factor for device performance. As shown in Fig. 4c, it is apparent that bimolecular recombination is one of the limiting factors that significantly surpassed charge transportation and/or extraction due to the moderate morphology of the film without additive because α = 0.83. Interestingly, the α value is approaching unity (α = 0.98) after the introduction of additive, demonstrating that the bimolecular recombination was one of the factors that hinder the device performance. On the other hand, the relationship between Voc and light intensity can give information about trap-assisted recombination by the following theoretical consideration. In detail, trap-assisted recombination is associated with 2kT/q in a semi-logarithmic plot of Vocversus Pin, while the slope of the curves indicates a pure bimolecular recombination. Fig. 4d demonstrates that trap-assisted recombination has a significant decrease after the addition of additive to the blend film (1.56kT/q in CB, 1.23kT/q in CB + 3% DPE).44


A new NIR-absorbing copolymer, PFDPPSe, one of the rare examples of a polymer that does not contain any thiophene-based unit in its backbone, was successfully synthesized and used as a donor in organic solar cells with inverted device configuration. Compared to its thiophene containing analogues, PFDPPSe showed a broader absorption in the NIR region which translated to a stronger response in the 700–900 nm region by the corresponding EQE studies. Detailed device optimizations indicate a strong additive influence on the blend morphology, whose PCE as high as 6.16% was recorded for PFDDSe (a 13-fold increase in PCE compared to the device without additive), which is favourably comparable to PCEs reported for a number of DPP-based polymers. More fundamentally, the NIR absorption ability of PFDPPSe shows broad application prospects in outdoor windows and semitransparent device applications. Additionally PFDPPSe is a good candidate for high band gap perovskite/NIR bulk heterojunction bilayer devices. Studies on these applications are currently underway in our laboratories.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK), Grant No: 115M036. D. B. acknowledges the KAUST Solar Center (KSC) Competitive Fund.

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c8tc00113h

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