Improved efficiency of single-component active layer photovoltaics by optimizing conjugated diblock copolymers

Dengke Shi ab, Huabin Wang b, Hua Sun a, Wenbo Yuan b, Shifan Wang *a and Wei Huang *b
aSchool of Material and Chemistry Engineering, Xuzhou University of Technology, Xuzhou, 221018, China. E-mail: shifanwang@xzit.edu.cn
bKey Laboratory of Flexible Electronics & Institute of Advanced Materials, Jiangsu National Synergistic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China. E-mail: iamwhuang@njtech.edu.cn

Received 25th November 2019 , Accepted 17th January 2020

First published on 20th January 2020


The synthesis of single-component, all-conjugated D–A diblock copolymers that exhibit high photovoltaic activity remains a significant challenge. To date, the synthesis of all-conjugated block copolymers typically produces a mixture of donor and acceptor homopolymers, diblock copolymers, ternary block copolymers or multi-block copolymers. In this work, we report the use of an A–B type monomer to synthesise a diblock copolymer. Importantly, by avoiding AA + BB polymerization, we have eliminated the production of ternary block and multi-block copolymers. In addition, the donor and acceptor blocks had orthogonal solubility in order to increase the purity of the block copolymer and give significant nanoscale phase separation. Two single-component diblock copolymers (P3HT-b-TBTF8 and P3HT-b-F8TBT) were synthesized, of which different molecular weights were obtained using Soxhlet extraction and preparative GPC. Both copolymers exhibited good thermal stability and effective self-aggregation. Thermal annealing was found to reduce the self-aggregation time and enhance the crystallization of the P3HT block, which effectively reduced the self-aggregation of the polymers. The power conversion efficiency of a single component active layer based P3HT-b-F8TBT photovoltaic device reached 1.22%. The higher efficiency of this device was thought to be caused by the linkage between the donor and acceptor blocks, whose P3HT and PF8TBT chains are separated by the wider bandgap F8 unit. Moreover, the unique molecular structure and self-assembly properties of these diblock copolymers will allow them to be used as model materials for further studies on charge and energy transfer in photovoltaic devices.


Introduction

At present, bulk heterojunction (BHJ) solar cells are the most widely used and efficient organic solar cells. These devices contain physically mixed donor/acceptor materials to form an interpenetrating network structure.1–9 Post-treatment of these networks using thermal annealing or solvent annealing can effectively control the phase separation of the active layer. However, significant challenges remain with regards to morphology control and thermal stability.10,11 Since the active layer prepared using this process is metastable, it is highly susceptible to temperature, humidity, and pH. In addition, the size of phase separation and the domains also change with time, which results in decreased photovoltaic performance.12,13 Phase separation in BHJ solar cells can be further complicated because they are typically prepared by blending multiple active layer materials, and therefore slight changes in the device structure or processing conditions can result in changes in the film morphology, which impacts reproducibility.14,15 To overcome these problems, single component active layers that can avoid complicated phase separation processes have attracted significant attention.16–20

Fully conjugated D–A block copolymers (BCP) consist of donor and acceptor polymers that are covalently linked. Compared with BHJ structures, single-component, fully conjugated D–A block copolymers simplify device preparation and stabilize the phase separation of the D–A component. Moreover, fully conjugated block copolymers typically exhibit strong self-assembly, which results in more regular and ordered structures that are more favorable for exciton diffusion and charge transfer.19–25 Therefore, the synthesis and examination of new, fully conjugated D–A diblock copolymers is very important for advancing all-polymer solar cell materials. The synthesis method of a fully conjugated D–A type diblock copolymer is classified into two types based on the monomer type. As shown in Fig. 1, polycondensation of AA + BB or A–B functional groups is the most common method for synthesizing diblock copolymers, where A represents a bromine atom and B represents a group such as a boronic ester or an alkyl tin. The product of polycondensation of an AA + BB type functional group is a mixture of a diblock copolymer, a ternary block copolymer and a homopolymer, and it is difficult to separate a single component diblock copolymer by a subsequent process. The product obtained by polycondensation of an A–B type functional group is only a mixture of a diblock copolymer and a homopolymer, and a single component of a fully conjugated D–A type diblock copolymer can be obtained by post-treatment to remove the homopolymer. However, the preparation of an A–B type monomer is more challenging than the preparation of an AA + BB type monomer. The preparation of the block copolymer P3HT-b-PFTBT using a Suzuki–Miyaura polycondensation reaction was reported, which was incorporated into a solar cell that achieved an energy conversion efficiency of 3.1%, however, the block copolymer still contained 14% P3HT homopolymer.26 A single-component solar cell is prepared by using a double cable conjugated polymer containing a conjugated main chain as the donor and a perylene bisimide side unit as the acceptor. It was found that both the donor and acceptor segments tended to form ordered nanostructures, which resulted in effective charge transport, reduced charge recombination and a record efficiency of 6.3% for single-component organic solar cells.27,28 A significant amount of work has been undertaken to improve this particular block copolymer, including the introduction of different alkyl side chains on the TBT unit, using solvent additives during the preparation of the active layer, and examining the effects of thermal annealing on device performance.29–36 However, the interference of ternary block copolymers and homopolymers has resulted in a number of challenges, including batch to batch variation of the polymer materials.37–40


image file: c9nj05869a-f1.tif
Fig. 1 Schematic of the synthesis of all-conjugated D–A type diblock copolymers by AA + BB type and A–B type polycondensation reactions.

Herein, we describe the use of a donor (P3HT) and an acceptor block (PF8TBT), which have orthogonal solubility, to synthesize the fully conjugated D–A diblock copolymers P3HT-b-TBTF8 and P3HT-b-F8TBT using monofunctionalized polymer P3HT-Br and an A–B monomer. This method effectively avoided the formation of ternary and multi-block copolymers. The products of these polycondensation reactions contained a mixture of the desired diblock copolymers and both donor and acceptor homopolymers. High molecular weight PF8TBT is insoluble in chloroform, and therefore this could be removed by extraction with chloroform. The chloroform solution was collected to obtain the diblock copolymer, which also contained small amounts of low molecular weight P3HT and PF8TBT homopolymers. These homopolymers were removed using preparative GPC with a chlorobenzene mobile phase at 80 °C to obtain the pure, fully conjugated D–A diblock copolymers P3HT-b-TBTF8 (BCP1-1 and BCP1-2) and P3HT-b-F8TBT (BCP2-1 and BCP2-2). The photophysical and electrical properties, and film morphology of the diblock copolymers were examined, and single-component organic solar cells that contained these diblock copolymers were prepared, which resulted in a photoelectric conversion efficiency of 1.25%.

Results and discussion

Synthesis and characterization

As shown in Scheme 1, P3HT-b-F8TBT (BCP1) was synthesized by Suzuki–Miyaura cross-coupling reactions between BF8TBT and P3HT-Br (Mw ∼ 10 kg mol−1, PDI ∼ 1.11), and P3HT-b-TBTF8 (BCP2) was synthesized by Stille cross-coupling reactions between F8TBTSn and the same P3HT-Br. The use of an A–B type monomer instead of AA and BB type monomers avoided the production of ternary block and multi-block copolymers. After the reaction, the crude product (Fig. 2, crude GPC trace) was purified using Soxhlet extraction, which effectively removed the majority of the P3HT and PF8TBT homopolymers and the low molecular weight block copolymers.41 Importantly, the orthogonal solubility of the donor and acceptor regions of these diblock copolymers should promote effective phase-separation and self-assembly. Following Soxhlet extraction, unreacted P3HT was removed using preparative GPC (five to six fractions were collected). Among them, BCP1-1, BCP1-2, BCP2-1 and BCP2-2 had lower PDI values and higher molecular weights than those of the homopolymers, as shown in Table 1. As shown in Fig. 2, all fraction traces with sharp and monomodal profiles are different from P3HT, and were clearly shifted to shorter elution times compared to P3HT. No shoulder was observed within the overlap between the BCP and P3HT curves, indicating efficient purification of the block copolymer by preparation of GPC. The diblock copolymers were further characterized using 1H-NMR spectroscopy (Fig. S1–S3, ESI). The triplets at 2.57 ppm from the P3HT-Br homopolymer were not observed in the spectra from the block copolymers, which was consistent with the successful polymerization of F8TBT and P3HT in the diblock copolymers.42,43
image file: c9nj05869a-s1.tif
Scheme 1 Synthesis of the diblock copolymers P3HT-b-F8TBT (BCP1) and P3HT-b-TBTF8 (BCP2).

image file: c9nj05869a-f2.tif
Fig. 2 GPC analysis of various (a) P3HT-b-F8TBT and (b) P3HT-b-TBTF8 fractions purified by preparative GPC.
Table 1 Molecular weights and polydispersity indices of P3HT, PF8TBT and the D–A diblock copolymers, as determined using GPC
Polymer M n (kg mol−1) M w (kg mol−1) PDI
P3HT 9.4 10.5 1.11
BCP1-1 49.3 57.2 1.19
BCP1-2 24.7 32.9 1.41
BCP2-1 64.6 84.5 1.30
BCP2-2 32.9 43.9 1.33
PF8TBT 5.1 9.6 1.87


Thermal properties

The thermal stability of the diblock copolymers, P3HT and PF8TBT was examined using DSC from −50 up to 350 °C. All samples exhibited good reproducibility after two heating cycles, which indicated that the diblock copolymers were thermally stable and could withstand the temperatures required for device preparation (Fig. 3). The melting temperature of the diblock copolymers did not change significantly when compared with P3HT. P3HT exhibited good crystallinity, with distinct melting and crystallization peaks at 234 °C and 190 °C, respectively. The PF8TBT homopolymer was amorphous and did not have any melting or crystallization peaks, as shown in Fig. S4 (ESI). The BCP consists of a P3HT block and a PF8TBT block linkage, which may play a major role in the melting temperature of the BCP. However, the crystallization temperature decreased and its peak intensity also decreased, which indicated a significant decrease in crystallinity. Interestingly, the crystallinity and crystallization temperature of BCP1-1 were significantly lower than those of BCP1-2, which may be because BCP1-1 contains more PF8TBT blocks than BCP1-2.
image file: c9nj05869a-f3.tif
Fig. 3 DSC curves from the D–A diblock copolymers.

Optical and electrochemical properties

UV-visible absorption spectra of P3HT, PF8TBT and the BCP in chloroform and as films are shown in Fig. 4 (related data are shown in Table 2). All diblock copolymers had similar absorption profiles and contained absorption bands that were characteristic of P3HT and PF8TBT. Each diblock copolymer exhibited broadened and red-shifted absorption spectra in films when compared to those recorded in chloroform. The optical band gaps of the diblock copolymers were calculated from the onset of the film absorption spectra (∼1.89 eV), and were similar to those of the PF8TBT and P3HT homopolymers. The absorption behavior was strongly dependent on the ratio of P3HT to PF8TBT. The proportion of P3HT in the diblock copolymers was estimated by examining the spectra of different P3HT and PF8TBT blends in chloroform (Fig. S4, ESI). The P3HT content in BCP1-1, BCP1-2, BCP2-1 and BCP2-2 was estimated to be 32%, 46%, 28% and 43%, respectively.
image file: c9nj05869a-f4.tif
Fig. 4 UV-vis absorption spectra of P3HT, PF8TBT and the diblock copolymers (a) in chloroform and (b) as films.
Table 2 Optical properties and energy levels of P3HT, PF8TBT and the BCP
Polymer λ max (nm) λ max (nm) λ onset (nm) E g (eV) HOMOd (eV) LUMOe (eV)
a Absorption peaks in chloroform. b Absorption peaks in thin films. c E g estimated from the absorption onset of the thin films (Eg = 1240/λonset). d HOMOs estimated using cyclic voltammetry. e LUMOs calculated using the HOMO and Eoptg (LUMO = HOMO + Eoptg).
P3HT 457 521/559/597 649 1.91 −5.13 −3.22
BCP1-1 398/548 400/553 656 1.89 −5.25 −3.36
BCP1-2 404/474/550 405/540 656 1.89 −5.22 −3.33
BCP2-1 401/544 402/553 656 1.89 −5.28 −3.39
BCP2-2 404/475/550 407/540 656 1.89 −5.25 −3.36
PF8TBT 390/554 395/570 659 1.88 −5.35 −3.47


Cyclic voltammetry was used to estimate the HOMO levels of the diblock copolymers (the onset of the oxidation curves), as shown in Fig. 5. All of the diblock copolymers exhibited quasi-reversible oxidation potentials (Fig. 5a and Fig. S6, ESI). Both the HOMO and LUMO levels of the diblock copolymers were located between PF8TBT and P3HT (Fig. 5b). As the P3HT content increased, the HOMO and LUMO levels of BCP1-1 and 1-2 (or 2-1 and 2-2) reduced, which was likely influenced by changes in the conjugation length and interactions between the P3HT and PF8TBT chains.


image file: c9nj05869a-f5.tif
Fig. 5 (a) Cyclic voltammograms of the BCP. (b) Energy levels of P3HT, PF8TBT and the BCP.

Polymer solar cells

Polymer solar cells were prepared using the diblock copolymers (P3HT-b-F8TBT and P3HT-b-TBTF8) as single-component active layers. Detailed device data are shown in Fig. 6 and Table 3. The devices that contained BCP1-1 had low efficiencies (0.07–0.12%), even when the active layer was subjected to annealing treatment. This was likely because of the imbalanced donor/acceptor ratio in this diblock copolymer. The efficiency of the device that contained BCP1-2 was only 0.09%, and this was primarily caused by the low short-circuit current (0.33 mA cm−2). When the active layer was annealed at 150 °C for 120 min, the short-circuit current, open circuit voltage and fill factor all improved significantly (2.74 mA cm−2, 1.14 V and 39.06%, respectively), which caused the efficiency to increase to 1.22%. This was likely because the morphology of the active layer film was greatly improved after annealing. When the annealing temperature was increased to 200 °C, the open circuit voltage remained basically unchanged, and the fill factor increased to 43.53%, but the short circuit current was reduced, which resulted in a slight reduction in efficiency to 1.04%. BCP1-2 shows a significantly high Voc compared to BCP1-1, which is due to the leakage current (Fig. S7, ESI). However, the efficiency of the device that contained P3HT-b-TBTF8, which was fabricated using optimal conditions, was only 0.39%. This was likely because of the difference in the structure of the bridging unit between the donor and the acceptor blocks.21,29
image file: c9nj05869a-f6.tif
Fig. 6 JV characteristics of photovoltaic devices that contained the D–A diblock copolymers.
Table 3 Photovoltaic properties of photovoltaic devices that used the diblock copolymers as single-component active layers
V oc (V) J sc (mA cm−2) FF (%) PCEmax (%) PCEavr[thin space (1/6-em)]a (%)
a Average PCE was obtained from more than 12 devices.
BCP1-1 As-cast 0.85 0.26 27.06 0.07
150 °C 0.75 0.58 28.01 0.13 0.12
BCP1-2 As-cast 0.95 0.33 25.50 0.09
150 °C 1.14 2.74 39.06 1.22 1.11
200 °C 1.15 2.06 43.53 1.04 0.93
BCP2-1 As-cast 1.07 0.28 23.11 0.07
150 °C 0.70 0.85 27.21 0.17 0.16
BCP2-2 As-cast 0.83 0.54 25.50 0.12
150 °C 0.70 1.65 33.35 0.39 0.34
200 °C 0.35 0.48 29.62 0.05


Film morphology

To understand the differences in device efficiencies after annealing, the morphology of the active layers was examined using atomic force microscopy (AFM) before and after annealing, as shown in Fig. 7 and Fig. S8 (ESI). The diblock copolymer films showed fine structure and distinct domain images both at room temperature and after annealing. The root mean square roughness of BCP1-1 and BCP1-2 was 0.71 and 0.95 nm, respectively, which is slightly lower than the results after annealing, at 0.74 and 1.04 nm, respectively. In contrast, annealing can promote P3HT crystallization, which greatly reduces intermolecular aggregation and increases the surface roughness. When an annealing temperature of 200 °C was used, the surface roughness (1.27 nm) of the active layer increased and the P3HT domains were more pronounced. This increased surface roughness is detrimental to effective carrier transport and results in decreased device performance.
image file: c9nj05869a-f7.tif
Fig. 7 AFM height images (top) and phase images (bottom) over a 3 μm × 3 μm area. (a and f) BCP1-1 as-cast, (b and g) BCP1-1 after annealing at 150 °C, (c and h) BCP1-2 as-cast, (d and i) BCP1-2 after annealing at 150 °C, and (e and j) BCP1-2 after annealing at 200 °C.

The orientation and aggregation of the diblock copolymers in the active layer were investigated further using grazing incidence wide-angle X-ray scattering (GIWAXS), as shown in Fig. 8 and Fig. S9 (ESI). Prior to annealing, the block copolymers exhibited strong crystallization. Both BCP1-1 and BCP2-1 had larger molecular weights, and therefore they both built up more along the face-on direction, but excessive molecular packing is not conducive to efficient carrier transport. After annealing at 150 °C, P3HT crystallization reduced the aggregation of the diblock copolymers (Fig. S10, ESI), and the film formation time of the active layer was shortened, which caused the molecular aggregation strength of the block copolymer to decrease. The stacking of BCP1-1 and BCP2-1 along the face-on direction was greatly reduced, and the molecular packing of BCP1-2 and BCP2-2 was also slightly reduced. After annealing, BCP1-2 and BCP2-2 have suitable crystallization, which facilitates carrier transport and ultimately leads to improved device performance.


image file: c9nj05869a-f8.tif
Fig. 8 GIWAXS images of (a) BCP1-1 as-cast, (b) BCP1-1 after annealing at 150 °C, (c) BCP1-2 as-cast, and (d) BCP1-2 after annealing at 150 °C.

Conclusion

In conclusion, two fully conjugated diblock copolymers, P3HT-b-TBTF8 and P3HT-b-F8TBT, were synthesized using A–B type monomers. These polymers contained different linkages between the donor and acceptor blocks. The different polymers did not exhibit any major differences in their optoelectronic properties, size of their molecular packing, or nanoscale ordering. However, a photovoltaic device that contained the P3HT-b-F8TBT polymer as a single-component active layer exhibited a power conversion efficiency of 1.22%, which was significantly higher than that of the devices that contained a P3HT-b-TBTF8 active layer. Moreover, the unique molecular structure and good self-assembly properties of these all-conjugated D–A diblock copolymers may serve as a useful model for understanding and examining the donor–acceptor interface in photovoltaic devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Jiangsu Province (No. BK20180178) and Xuzhou Science and Technology Plan Project (No. KC18010). We would like to thank Dr Y. Tao (Nanjing Tech University) for support during the experiment.

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Footnotes

Electronic supplementary information (ESI) available: See DOI: 10.1039/c9nj05869a
These authors contributed equally to this work.

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