A regioregular terpolymer comprising two electron-deficient and one electron-rich unit for ultra small band gap solar cells

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2][3] The efficiencies have been further increased to 11-12% via tandem and triple junction device architectures, 4,5 in which two or three photoactive layers with complementary absorption spectra are stacked and connected in series.By applying different conjugated polymers that absorb light from the visible light to the near-infrared region above 1000 nm, the conversion efficiencies of triple junction solar cells may achieve 15-20%. 6To reach this ambitious goal, it will be important to develop efficient conjugated polymers that absorb up to 1000 nm and beyond.
Conjugated polymers with tailored optical band gaps (E g ) can be obtained using electron donating and electron deficient units alternating along the main chain.In general stronger electron donors and stronger acceptors reduce the optical band gap.Using this design motif and by employing suitable side chains, the energy levels, solubility, aggregation and crystallinity of the polymers can be tuned.8][9] For E g below 1.25 eV the most efficient cells reach 5.3% PCE. 10 In many of the successful materials a significant photon energy loss between E g and open circuit voltage (V oc ) is incurred, E loss = E g À qV oc .E loss provides a driving force for photoinduced charge generation and often enhances the short circuit current density ( J sc ).When the optical band gap is reduced, charge generation is usually negligible when E loss becomes less than 0.6 eV. 11This problem becomes especially important when the absorption of conjugated polymers extends above 1000 nm where the optical band gap becomes less than 1.2 eV.To maintain a high quantum efficiency for charge generation, the V oc , which is controlled by the energy difference between the HOMO of the donor and the LUMO of the acceptor, must be designed to be less than 0.6 V.However, a too low V oc would quickly lead to an insignificant PCE.Hence, conjugated polymers with a very small band gap require a judicious design in terms of energy levels to be applicable in PSCs.
Several conjugated polymers with absorption beyond 1000 nm have been reported, often using strong electron deficient units such as thienoisoindigo, 12,13 thiadiazoloquinoxaline, 14,15 bisbenzo thiadiazole, 16 pyrazinoquinoxaline 17,18 or diketopyrrolopyrrole (DPP). 19hen applied in polymer solar cells PCEs up to 5.3% have been obtained. 10Especially, PDPP2T-DTP (Fig. 1a) with DPP as electron acceptor and dithienopyrrole (DTP) as strong electron donor affords E g = 1.23 eV 19 and provides J sc 4 20 mA cm À2 . 10The V oc of cells based on this polymer is, however, relatively low with 0.43 V, resulting in E loss = 0.80 eV.In order to reduce E loss and enhance V oc , we introduce the use of two electron deficient units, DPP and benzothiadiazole (BT), in combination with DTP as electron donor.The new ultra low band gap polymer, PDPP2T-BT-co-DTP (Fig. 1c) provides a significantly higher V oc of 0.52 eV and PCE of 3.7% with a reduced optical band gap of 1.19 eV in polymer solar cells.
PDPP2T-DTP 10 and PDPP2T-BT 20 were synthesized following previously reported procedures.The regioregular alternating The conditions applied in the polymerization reaction, toluene/DMF 10 : 1 (v/v) as solvent and Pd 2 (dba) 3 /PPh 3 1 : 4 (mol/mol) as catalyst, were chosen to reduce homo coupling defects and achieve a high molecular weight. 21The viscous gel formed during polymerization indicates a high molecular weight and the work-up required the use of 1,1,2,2-tetrachloroethane at 140 1C.It was not possible to measure the molecular weight by gel permeation chromatography (GPC) in o-dichlorobenzene (o-DCB) because the polymer seems to adhere to or decompose on the column.The same problem was previously encountered for PDPP2T-DTP. 10 The thermal stability of PDPP2T-BT-co-DTP is somewhat less than that of PDPP2T-BT and PDPP2T-DTP (Fig. S1, ESI †).The strong electron donating DTP unit makes polymers based on DTP susceptible to oxidation in air, and processing of devices was performed under inter atmosphere. 10ll three polymers absorb in the near-infrared region with onsets above 1000 nm (Fig. 2a).The absorption in thin films is slightly red-shifted compared to chloroform solution (Fig. S2, ESI †).PDPP2T-BT has a lower band gap (1.19 eV) than PDPP2T-DTP (E g = 1.23 eV).While we anticipated that the combination of two acceptors (DPP and BT) would increase the electron withdrawing character and could further reduce the band gap, the terpolymer PDPP2T-BT-co-DTP, that combines DPP, BT, and DTP also has E g = 1.19 eV.
Importantly, however, the introduction of two acceptors allows to effectively tune the frontier orbital energy levels. 22,23clic voltammetry (Fig. 2b and Table 1) reveals that as a consequence of incorporating two electron deficient units (DPP and BT), the PDPP2T-BT copolymer has deep HOMO and LUMO levels (À5.55 eV and À4.01 eV).Hence, the LUMO-LUMO offset (DE LL ) between PDPP2T-BT and [70]PCBM is only 0.15 eV and significantly less than the B0.3 eV threshold for charge separation.This problem often occurs in very small band gap conjugated polymers. 24,25With one electron deficient (DPP) and one electron rich unit (DTP), the PDPP2T-DTP copolymer exhibits significantly less negative HOMO and LUMO levels (À5.26 eV and À3.68 eV) to give DE LL = 0.48 eV.The new terpolymer PDPP2T-BT-co-DTP has the HOMO and LUMO in between those of the two copolymers PDPP2T-BT and PDPP2T-DTP, so that DE LL = 0.24 eV.The low lying HOMO level will help to achieve a higher V oc than that of PDPP2T-DTP, while the higher DE LL than that of PDPP2T-BT is beneficial for the efficiency of photoinduced charge separation.
The three polymers were applied in polymer solar cells with a transparent ITO/poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) (PEDOT:PSS) bottom contact for hole collection and LiF/Al top contact for electron collection.The photoactive layers, in which the polymers were blended with [70]PCBM, were carefully optimized for the donor to acceptor weight ratio, solvent, co-solvent, and thickness.In general, the optimized donoracceptor ratio is 1 : 2 (w/w) and the optimal thickness was around 110 nm, when spin coating from chloroform solution.For PDPP2T-BT 10% o-DCB as co-solvent gave the best performance.For PDPP2T-DTP and PDPP2T-BT-co-DTP the best co-solvents concentrations are 5% o-DCB and 3% 1-chloronaphthalene (1-CN), respectively.The J-V characteristics and external quantum efficiencies (EQEs) of the optimized devices are depicted in Fig. 3 and the device parameters are summarized in Table 2.
As expected from the low DE LL the PDPP2T-BT copolymer gives a low PCE of 1.2%, mainly due to a low short circuit   current density ( J sc = 2.8 mA cm À2 ).The cells have a high V oc of 0.66 V at this low band gap (1.19 eV).The photon energy loss E loss equals 0.53 eV and is below the threshold of 0.6 eV for efficient charge transfer, 11 rationalizing the low J sc .In contrast, photovoltaic devices based on the copolymer PDPP2T-DTP give a high J sc of 16.6 mA cm À2 and PCE of 3.9%, but a relatively low V oc of 0.43 V as consequence of the higher lying HOMO.The low V oc increases the photon energy loss to E loss = 0.80 eV.The new terpolymer PDPP2T-BT-co-DTP gives V oc = 0.52 V, i.e. in between the V oc 's obtained with the two copolymers, but a somewhat low J sc (12.2 mA cm À2 ) and PCE of 3.7%.Here E loss is 0.67 eV.The differences in photocurrent are also reflected in the EQE spectra (Fig. 3b).In PDPP2T-BT: As previously reported, a lower energy loss decreases the driving force for electron transfer and correlates with a lower EQE. 7Hence, the lower EQE of the terpolymer PDPP2T-BT-co-DTP is possibly related to the reduced DE LL and E loss .However, the energy loss of the terpolymer is 0.67 eV, i.e. above the threshold of 0.6 eV. 11Actually, another DPP-polymer, PDPP3T, where E loss = 0.65 eV gave a maximum EQE of 0.50, 7 which is significantly higher than the 0.32 of the terpolymer.Therefore other effects than E loss likely contribute to the low EQE.
First, TEM reveals that the morphology of the PDPP2T-BT-co-DTP:[70]PCBM blends consists of semicrystalline polymer fibrils (Fig. 4).The crystallinty can be readily seen from the lattice fringes in Fig. 4b.The width of these semicrystalline fibrils exceeds 10 nm, commonly considered as the maximum exciton diffusion length.We have shown recently for a range of DPP-polymers that the fibril width anti-correlates with the EQE and that exciton loss via intrinsic decay may contribute to a low EQE. 26econd, localization of excitons on the T-DPP-T-BT-T-DPP-T segments may occur.Because PDPP2T-BT has a smaller band gap than PDPP2T-DTP, excitons may prefer T-DPP-T-BT-T-DPP-T segments.Excitons on such segments may have a lower tendency to transfer an electron to the fullerene, as suggested by the fact that the PDPP2T-BT copolymer is unable to create charges efficiently.This explanation is supported by our recent work on the detrimental effect of low-LUMO chain segments on the charge generation efficiency. 21n conclusion, a new conjugated terpolymer with an ultra small band gap, E g = 1.19 eV, was designed and applied in polymer solar cells.By incorporating two electron-deficient and one electron-rich unit into the conjugated backbone the frontier energy levels were tailored to maximize V oc and J sc .The terpolymer provided a relatively high V oc (0.52 V) and PCE (3.7%) with a photoresponse up to 1050 nm, despite a moderate EQE.The study shows that by extending the push-pull approach and incorporating more units than single electron rich and electron poor units is viable way to create semiconducting polymers with finely tuned energy levels.
The work was performed in the framework of the Large cells and X10D projects that received funding from the European Commission's Seventh Framework Programme (Grant Agreement No. 261936 and No. 287818).The research forms part of the Solliance OPV programme and has received funding from the Ministry of Education, Culture and Science (Gravity program 024.001.035).

a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.E-mail: liweiwei@iccas.ac.cn b Molecular Materials and Nanosystems, Institute of Complex Molecular Systems, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands.E-mail: r.a.j.janssen@tue.nlc Dutch Institute for Fundamental Energy Research, De Zaale 20, copolymer PDPP2T-BT-co-DTP was synthesized using the dibromo-monomer of T-DPP-T-BT-T-DPP-T and the bisstannyl-DTP monomer via Stille polymerization (Scheme S1, ESI †).

Fig. 2
Fig. 2 (a) Optical absorption spectra of the polymers in solid state films and (b) cyclic voltammograms of the polymers thin films on ITO substrates.

Table 1 24 a
Fig. 3 (a) J-V characteristics in dark (dashed lines) and under white light illumination (solid lines) of optimized solar cells of the polymers with [70]PCBM.(b) EQE of the same devices.
[70]PCBM cells the contribution of light absorbed by the polymer in the 700-1050 nm region is negligible and photocurrent is only created via excitation of [70]PCBM.In contrast, a broad spectral response from 350 to 1050 nm, covering the absorption spectra of fullerene and polymer is found for PDPP2T-BT-co-DTP:[70]PCBM blends.Unfortunately, the PDPP2T-BT-co-DTP terpolymer provides a relatively low EQE (maximizing at 0.32) compared to that of the PDPP2T-DTP copolymer (maximizing at 0.53).

Table 2
Solar cell parameters of optimized solar cells of the DPP polymers with [70]PCBMPolymerJ sc a (mA cm À2 ) V oc (V) FF PCE a (%) E loss (eV) J sc and PCE were calculated by integrating the EQE spectrum with the AM1.5G spectrum. a