Dang-Trung
Nguyen
a,
Sunil
Sharma
a,
Show-An
Chen
*a,
Pavel V.
Komarov
bc,
Viktor A.
Ivanov
de and
Alexei R.
Khokhlov
bd
aChemical Engineering Department, National Tsing-Hua University, Hsinchu 30013, Taiwan. E-mail: sachen@che.nthu.edu.tw
bLaboratory of Physical Chemistry of Polymers, A.N. Nesmeyanov Institute of Organoelement Compounds RAS, Vavilova St. 28, Moscow 119991, Russia
cGeneral Physics Department, Tver State University, Sadovii per. 35, Tver 170002, Russia
dFaculty of Physics, Moscow State University, Leninskie Gory 1, Moscow 119991, Russia
eInstitut für Physik, Martin-Luther-Universität, Halle 06120, Germany
First published on 29th December 2020
Hybrid quantum dot solar cell (HQDSC) based on solution-processed blends of poly(3-hexylthiophene) (P3HT) with PbS quantum dots (QDs) is a potential candidate toward practical use for its low material cost and simple fabrication process. However, P3HT is highly incompatible with oleic acid (OA)-capped PbS QDs (OA-PbS QDs) due to strong phase separation, giving poor quality in the desired bi-continuous networks morphology and thus leading to inefficient charge collection. Here, for the first time, a block copolymer of P3HT with polystyrene (P3HT-b-PS) was confirmed to improve the miscibility between the polymers and OA-PbS QDs, leading to the formation of a desirable bi-continuous network morphology, as predicted by us via dissipative dynamic simulations previously. The bi-continuous network morphology for charge transport is an ideal morphology in bulk heterojunction solar cells. For the active layer, using the block copolymer P3HT-b-PS as the donor and PbS QDs as the acceptor at the weight ratio of 1:20, the power conversion efficiency (PCE) of HQDSC was found to be 4.18%, which is higher than P3HT and PbS QDs (3.66%) having the same weight ratio even though the content of the P3HT component in P3HT-b-PS was 28% less than that of homo-polymer of P3HT. The formation of the desired morphology for electron and hole collections of the device with the block copolymer was confirmed via scanning electron microscopy. Further, the addition of P3HT into the blend of the block copolymer with OA-PbS QDs still retains the desired morphology. Therefore, further improvement of PCE was made by taking the blend of P3HT and P3HT-b-PS at the weight ratio of 0.7:0.3 as the donor, thus achieving the PCE of 4.91%, which is better than that of P3HT alone by 1.25% and P3HT-b-PS alone by 0.73%. Thus, this methodology could be applicable for hybrid solar cells with a low bandgap molecular or polymeric material as the donor.
An alternative simpler device fabrication method (yet allowing an extension of the light absorption spectrum) of blending a conjugated polymer with QDs as the active layer was proposed.18–21 The obtained product was designated as hybrid quantum dot solar cell (HQDSC) and is one of the most potential candidates for replacing silicon-based solar cells due to its advantages such as low-cost, simple fabrication process, high efficiency, and device stability. A critical review on the progress and prospect of solar cells based on PbX QDs has been given by W. Ma et al.22 By this blending method, the absorption range of solar cells can be extended due to the absorption of different parts of the incident light spectrum by the addition of a conjugated polymer. In addition, tuning HOMO and LUMO levels of the conjugated polymer is required to provide energy level offset for facilitating the dissociation of exciton and charge transport in the active layer of the electrodes. Poly(3-hexylthiophene) (P3HT) is a model polymer in organic electronics due to its low cost, simplicity of its chemical structure, and promising optoelectronic properties, and hence has been studied extensively. However, a large surface energy difference exists between quantum dots and the polymer, and it could lead to strong phase separation giving less efficient charge extraction.23,24 To date, the highest reported PCE of the P3HT:QD hybrid system is 4.32%,25 for which Tian et al. fabricated HQDSC based on the aqueous-solution-processed poly(3-hexylthiophene) (P3HT) dots and CdTe nanocrystals (NCs); however, studies on their morphologies were not reported. In addition, HQDSC based on a low band-gap polymer, namely poly(2,6-(N-(1-octylnonyl) dithieno[3,2-b:20,30-d]pyrrole)-alt-4,7-(2,1,3-benzothiadiazole)) (PDTPBT), was reported with a PCE of only up to 5.5%, for which PbSxSe1−x alloyed NCs was used as an electron acceptor.26 However, there is no experimental report on the blend of a block copolymer with QDs as the active layer for HQDSC. Here, we propose to study the blend of the block copolymer poly(3-hexylthiophene)-b-polystyrene (P3HT-b-PS) with PbS QDs to see if the inclusion of the nonpolar polymer block (PS), which is more compatible with the non-polar ligand of QDs, could lead to the formation of a bi-continuous network morphology for the effective extraction of electrons and holes towards the electrodes, as we predicted previously.27 In our previous study, we used mesoscopic simulation to study the effect of di-block copolymers with different solubility parameters (δ) between the two blocks27 and found that if the δ of one block is close to that of the ligand of QDs, a bi-continuous gyroid morphology of the polymer and QD phases could form.
In this study, we synthesized two single block polymers, P3HT with bromo terminal groups and polystyrene with pinacol boronic ester as terminal end functional groups. Then, these two blocks were coupled via the Suzuki coupling reaction to form the block copolymer P3HT-b-PS, which was confirmed by gel permeation chromatography (GPC) and FTIR. The solar cell with the block copolymer (the weight ratio of the P3HT block to the PS block is 72:28) and PbS QDs at the weight ratio of 1:20 delivers the optimal PCE of 4.18%, which is better than that of P3HT alone (3.66%). The improved PCE was due to the formation of a better quality bi-continuous network morphology, as confirmed by scanning electron microscopy (SEM), which allows more efficient charge transport and collection even though the content of the P3HT component was 28% less. Partial replacements of the block copolymer with P3HT were carried out in order to improve the extent of light absorption, and it was found that such replacements do not lead to any significant disturbance in the bi-continuous network morphology. When the weight ratio of P3HT and P3HT-b-PS in the blend was 0.7:0.3, an improved PCE of 4.91% was observed. Thus, for the first time, the present study demonstrates experimentally that the inclusion of a compatible block to the QDs in the block copolymer donor could lead to the formation of a bi-continuous network morphology, which conforms with our previous simulation study.27
The polystyrene block with pinacol boronic ester (PS-B(OR)2) as the end functional group was synthesized by the living polymerization method, in which polymer termination was carried out by the addition of isopropyl pinacolyl borate in the final stage. Subsequently, the formation of the pinacol boronic ester group on the PS chain ends was confirmed via1H NMR spectra, in which the signals for the end group (–C(CH3)2) resonated at 0.92 and 1.2 ppm in CDCl3. The number average molecular weight (Mn) and polydispersity (PDI) were determined as 7340 Dalton and 1.23, respectively, by gel permeation chromatography (GPC).
P3HT with the bromo terminal group (P3HT-Br) was synthesized by the Grignard metathesis method. Mn and PDI were also determined by GPC as 9,700 Dalton and 1.09, respectively. The end group-Br in P3HT-Br was identified by 1H NMR spectra with two small triplets appearing at δ = ∼2.6 ppm, as also reported by McCullough et al.28,29 Then, P3HT-Br was coupled with PS-B(OR)2via the Suzuki coupling reaction, with the equivalent mole ratio being 1:5 for the reaction to obtain the di-block copolymer. The formation of the block copolymer from two different blocks was also indicated by the GPC results (Fig. 1). The increase of Mn of the resulting block copolymer relative to its two components is shown in the GPC chromatogram (Mn = 11520 Dalton, PDI = 1.20). The GPC result indicates that P3HT-Br prefers to couple with the short-chain of PS-B(OR)2 due to the higher probability of coupling reaction between shorter chains. The excess PS was removed in the purification step using acetone, and the block copolymer in the GPC chromatogram shows the absence of the polystyrene homopolymer peak from 15 to 16 min in the elution time. The Fourier-transform infrared spectroscopy (FTIR) of the P3HT-Br shows the presence of C–Br stretching vibration at 690 (cm−1). However, in the present block copolymer sample, this stretching vibration is absent, indicating the absence of unreacted P3HT-Br in the sample as shown in Fig. S2 (ESI†). The composition of the copolymer was determined from the UV-Vis spectra via calculating the spectral area (from 300 to 565 nm) at an equal concentration of polymers in chloroform (1 mg mL−1) (Fig. S3a, ESI†). The area ratio of P3HT-b-PS to P3HT-Br was 72:100, thus reflecting that the weight ratio of P3HT:PS in the copolymer was 72:28. Thus, this information regarding the weight ratio of P3HT:PS was confirmed with our inference about the reaction priority of P3HT-Br with the short chains of PS. In addition, the π–π conjugation in the P3HT block in the copolymer was not affected by the presence of the PS block, as shown by the overlapping of the normalized spectra of the two blocks (P3HT and PS) at their absorption maxima (Fig. S3b, ESI†).
Fig. 2 The SEM image of the polymer:PbS QD film at the weight ratio of 1:20 with various weight ratios of P3HT and P3HT-b-PS in polymer phases. |
To further confirm the formation of a bi-continuous network morphology, we performed the hole mobility measurement for the blend of P3HT:QD and P3HT-b-PS:QD at the weight ratio of 1:20 using the hole-only device having the structure ITO/PEDOT:PSS/polymer:OA-PbS QD/MoO3/Al, in which the active layer was a thin film of P3HT:PbS QD or P3HT-b-PS:PbS QD of 120 nm thickness. The hole currents versus the applied voltage plots of the blend are shown in Fig. 3. The hole-current profile of P3HT-b-PS:PbS QD was observed to be higher than that of P3HT:PbS QD by the factor of 2.4 at 1 × 105 Vcm−1, indicating the formation of a more efficient hole transport channel by using P3HT-b-PS though the content of the conductive component P3HT was 28% less than that of the device having only homopolymer P3HT. Thus, this result further supports that the P3HT-b-PS system gives better quality of the bi-continuous network morphology than the P3HT system.
Fig. 3 Hole current density versus electrical field of the device having different active layers in the hole-only device: ITO/PEDOT:PSS/polymer:OA-PbS QD/MoO3/Al. |
To investigate the effect of the block copolymer, P3HT-b-PS, on the device performance, we fabricated the device with the structure ITO/PEDOT:PSS(40 nm)/polymer:BDT-PbS (120 nm)/BDT-PbS (30 nm) LiF (0.6 nm)/Al (100 nm), as illustrated in Scheme 2. Since device performance of HQDSC usually depends on the weight ratio of the polymer and QDs,25,26 we first investigated the weight ratio dependency of P3HT:PbS QD-based solar cells, and the results are presented in Fig. S4 and Table S1 (ESI†). The values of all the parameters, VOC, JSC, and FF, initially increased monotonically when the content of P3HT in P3HT:PbS QDs decreased from the ratio 1:5 to 1:20, but then dropped at 1:25. Thus, the optimal weight ratio was 1:20, at which the values of VOC, JSC, and FF were 0.50 V, 13.68 mA cm−2, and 38.0%, respectively, thus obtaining the optimal PCE of 2.58%. The thermal annealing of the active layer has been proven to be an effective strategy to increase the electrical conductivity of the active layer.32,33 Here, we investigated HQDSC with the weight ratio of 1:20 at different annealing temperatures from room temperature to 225 °C to search for the optimal annealing condition. The result of device performances (J–V curves) are shown in Fig. S5 (ESI†), and the J–V characteristic parameters are listed in Table S2 (ESI†). Both the VOC and JSC values were found to slightly increase with the increase in annealing temperature up to 175 °C, and the PCE reached 3.52%, while when the annealing temperature was increased further to 225 °C, both the VOC and JSC values slightly decreased, leading to a lower PCE of 3.45%. In the following studies on device performances of the devices with various compositions of the active layer, the optimal polymer to the QD weight ratio of 1:20 and annealing temperature of 175 °C was used.
Scheme 2 (a) Chemical structures of polymers and ligands, (b) the schematic of the hybrid photovoltaic device, and (c) ligands and energy diagram of the quantum dot hybrid solar cell. |
The J–V curves of the devices with various ratios of P3HT to P3HT-b-PS, as a donor, are shown in Fig. 4, and the characteristic device parameters are listed in Table 1. The P3HT-b-PS device gives the improved current density of 16.58 mA cm−2 relative to that of the P3HT device (15.81 mA cm−2) due to better quality of the bi-continuous network morphology for more efficient charge transport and collection. In addition, VOC and FF also slightly improved to 0.53 V and 47.7% from 0.52 V and 44.6%, respectively. Thus, the PCE was improved to 4.18% from 3.66% even though the content of the photo-active component P3HT in the block copolymer was less. The parameters RSH and RS, referring to the leakage current and bulk resistance, respectively, were calculated from the current density–voltage curves.34 The corresponding RSH of the P3HT-b-PS device increased to 191.2 Ω·cm2 relative to that of the P3HT device (140.0 Ω·cm2), which indicates that the leakage current in the former was higher than that in the latter. In contrast, the RS of the P3HT-b-PS device (12.2 Ω·cm2) was slightly lower relative to that of the P3HT device (12.7 Ω·cm2). Thus, the device performance results are in good agreement with the morphology of the active layers and hole current profile measurement.
Fig. 4 Current density–voltage (J–V) characteristics of the devices with Polymer:BDT-PbS as the active layer at the weight ratio of 1:20 and various contents of the block copolymer. |
Polymer ratio (P3HT:P3HT-b-PS)b | V OC (V) | J SC (mA cm−2) | FF (%) | PCE (%) | R SH (Ω·cm2) | R S (Ω·cm2) | Width of polymer domains (nm) |
---|---|---|---|---|---|---|---|
a The device structure is ITO/PEDOT: PSS(40 nm)/polymer: BDT-PbS(120 nm)/BDT-PbS(30 nm)LiF(0.6 nm)/Al(100 nm). b The polymer ratio is referred to the weight ratio of P3HT to P3HT-b-PS. | |||||||
1:0 | 0.52 | 15.81 | 44.6 | 3.66 | 140.0 | 12.7 | 120–390 |
0.9:0.1 | 0.53 | 16.75 | 47.7 | 4.24 | 194.6 | 11.6 | 75–390 |
0.7:0.3 | 0.57 | 16.21 | 53.2 | 4.91 | 256.6 | 10.4 | 95–341 |
0.5:0.5 | 0.54 | 17.29 | 50.7 | 4.70 | 183.5 | 9.2 | 87–245 |
0.3:0.7 | 0.53 | 17.61 | 49.0 | 4.60 | 212.5 | 10.7 | 64–256 |
0:1 | 0.53 | 16.58 | 47.7 | 4.18 | 191.2 | 12.2 | 68–220 |
Further, we fabricated HQDSCs at the weight ratio of 1:20 of the polymer to QDs and various weight ratios of P3HT to P3HT-b-PS with the device configuration: ITO/PEDOT:PSS (40 nm)/polymer:PbS QD (120 nm)/PbS QD (30 nm)/LiF (0.6 nm)/Al (100 nm). The device performances (current density versus voltage) are shown in Fig. 4, and the characteristic parameters are listed in Table 1. VOC exhibited slight composition dependency, with the optimal value being 0.57 V at the ratio of 0.7:0.3 of P3HT:P3HT-b-PS. JSC and FF both exhibited an improvement with the increase in the block copolymer content. Consequently, an optimal PCE value was obtained at the ratio of 0.7:0.3. Thus, these observations indicate that the addition of P3HT into the block copolymer could lead to the generation of more excitons but might cause a drop in the quality of the bi-continuous network morphology. Further, the RSH and RS dependency on the polymer composition shows higher and lower values at the optimal ratio of 0.7:0.3, respectively, which are beneficial to the device performance due to the presence of lower series resistance and high shunt resistance at this composition.
The device performance results indicate that the formation of bi-continuous charge transport channels in the active layer was still retained even when P3HT was added into the block copolymer. At the optimal ratio of 0.7:0.3 of P3HT to P3HT-b-PS, PCE significantly improved from 3.66% to 4.91% because the high quality bi-continuous network morphology in the active layer was still retained, and the content of the photoactive P3HT was increased. To the best of our knowledge, this performance is the best among the reported state-of-the-art P3HT/QD systems, as summarized in Table 2.
HTLa | Active layer | Ligand | V OC (V) | J SC (mA cm−2) | FF (%) | PCE (%) | Ref. |
---|---|---|---|---|---|---|---|
a HTL: hole transport layer. | |||||||
PEDOT:PSS | P3HT:P3HT-b-PS:PbS QDs (0.7:0.3:20) | BDT | 0.57 | 16.21 | 53.2 | 4.91 | This work |
PEDOT:PSS | P3HT:PbS QD:MWCNTs (20:4:1) | OLA (oleylamine) | 0.54 | 10.81 | 54.0 | 3.03 | 35 |
PEDOT:PSS | P3HT:PbS QD (1:9) | MPA (3-mercaptopropionic acid) | 0.56 | 10.8 | 50 | 3.0 | 36 |
MoO3 | P3HT:CdTe QD (1:24) | MA (2-mercaptoethylamine hydrochloride) | 0.54 | 16.95 | 47.2 | 4.32 | 25 |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ma00770f |
This journal is © The Royal Society of Chemistry 2021 |