Effect of annealing treatment and ICL in the improvement of OSC properties based on MEH-PPV:PC70BM and P3HT:PC70BM
sub-cells
HAMZA BOUZID, HAMZA SAIDI* , NADIA CHEHATA and ABDELAZIZ BOUAZIZI
E´ quipe Dispositifs E´lectroniques Organiques et Photovoltaı¨que Mole´culaire, Laboratoire de la Matie`re Condense´e et des Nanosciences, Faculte´ des Sciences de Monastir, Universite´ de Monastir, Avenue de l’environnement, 5019 Monastir, Tunisie
*Author for correspondence (saidi-hamza@outlook.fr) MS received 26 January 2021; accepted 29 May 2021
Abstract. Tandem organic solar cells (OSCs), consisting of more than one (usually two) sub-cells, stacked and con- nected by a charge recombination layer (i.e., interconnecting layer (ICL)), are highly promising to boost OSC efficiency.
The ICL plays a critical role in regulating the performance of tandem device. Here, we report ZnO or C60as a simple ICL, modified then, by a PEDOT:PSS layer, can act as an efficient ICLs in tandem device to achieve a significant increase in performance compared to single sub-cell. After optimized optical properties of MEH-PPV:PC70BM and P3HT:PC70BM sub-cells by varying the annealing temperature, these ICLs are used to connect photoactive layers. For all tandem devices, we showed an improvement in optical properties, especially in the ground and excited states compared to optimized single sub-cells. The degree of improved performance in tandem devices is attributed to the connection which provides each ICL. For fully optimized tandem, based on C60ICL, reached*78% enhancement in charge transfer compared to the bottom sub-cell and 82% compared to the top sub-cell, obtained forkemission= 650 nm.
Keywords. Tandem structure; interconnecting layer (ICL); thermal annealing; PL quenching.
1. Introduction
Due to the promise of manufacturing, such as lightweight, low-cost and flexibility, organic solar cells (OSCs) have attracted the attention of the scientific community in recent years [1,2]. To date, different configurations have been adapted for the fabrication of OSCs. For that, the first configuration used is the single-layer structure cells, then, the double layer cells [3,4]. These structures demonstrated a low power conversation efficiency (PCE). To boost the PCE, a bulk heterojunction (BHJ) structure was invented, in which the donor and acceptor are blended in solution. To date, the BHJ structures have been widely studied and various informations have been collected [5–13]. The power PCE is enhanced as compared to single layer and bilayer structures. However, it is still limited due to transmission and thermalization losses. To overcome the limitation of the BHJ structure, the tandem architecture has been introduced.
Simply consisted by two sub-cells (front and back sub-cells) with complementary absorption spectra, stacked in series by charge recombination layer [14–18]. In fact, the front sub- cell absorbs lower wavelength light and the top one absorbs higher wavelength light [19,20]. As a result, the tandem structure covers the visible fraction of the solar spectrum.
The efficiency is maximized through the broad absorption spectrum and reduced carrier thermalization losses [21].
Recently, a high PCE of 17.3% was reached by Meng et al [22] in tandem structure based on PBDB-T:F-M and PTB7-Th:O6T-4F:PC71BM sub-cells. Subsequent resear- ches were mainly focussed on two aspects: (i) synthesis of new polymers to maximize light absorption, (ii) choosing appropriate interconnecting layer (ICL) to improve charges extraction from each sub-cell and decreased recombination process [23–25]. ICL is a criti- cal component for the high-performance tandem OSCs.
Generally, an ideal ICL needs to possess energy level matching with those of donor and acceptor materials in sub-cells. In addition, must have high optical transparency to minimize absorption losses and be able to efficiently collect electrons from one sub-cell and holes from the other sub-cell. It is required to be robust enough to prevent any damage by top sub-cell during the processing process [20,24, 26–32].
In this paper, we initially carried out the optimization of optical properties of the sub-cells by varying the thermal annealing from 80 to 150°C. The bottom sub-cell based on MEH-PPV:PC70BM and the top one based on P3HT:PC70BM. Then, to fabricate the tandem structure, the https://doi.org/10.1007/s12034-021-02525-z
sub-cells were connected by different ICL:ZnO (ICL1, tandem 1), C60 (ICL2, tandem 2), ZnO/PEDOT:PSS (ICL3, tandem 3) and C60/PEDOT:PSS (ICL4, tandem 4).
For all tandem structures, we showed an improvement on absorption and photoluminescence (PL) properties com- pared to single sub-cells.
2. Experimental
Donor materials (MEH-PPV and P3HT), acceptors (PC70BM) and materials for the ICLs (PEDOT:PSS and C60) were purchased from Sigma-Aldrich. ZnO sol–gel nanoparticles were synthesized as follows: 114 mg of zinc acetate dehydrate and 60 mg of ethanolamine were dis- solved in 7.7 ml of methanol. At 90°C for 45 min, the reaction mixture was then stirred. Finally, to eliminate any precipitates from the ZnO sol–gel, the resulting solution was filtered.
All structures were spin-coated on indium tin oxide (ITO) on a glass substrate, with a sheet resistance of 15 X per square. Prior to deposition, the ITO-coated glass substrates were sequentially washed in de-ionized water, acetone and 2-propanol by ultrasonic cleaning for 25 min, followed by drying with N2treatment. To fabricate the bottom sub-cell, a blend solution of MEH-PPV:PC70BM in chlorobenzene (1:1, W/W, 15 mg ml-1) was spin-coated at 800 rpm for 45 s. For the top sub-cell, a blend solution of P3HT:
PC70BM in chlorobenzene (1:1, W/W, 15 mg ml-1) was spin-coated at 900 rpm for 50 s. ZnO (ICL1) was spin- coated on top of bottom active layer at 2500 rpm for 1 min and annealed at 150°C for 10 min. A 60 nm thin layer C60= ICL2 was evaporated on the top of the bottom sub-cells. For the ICL3 and ICL4, a PEDOT:PSS layer was spin-coated at 4500 for 1 min and annealed at 150°C for 10 min. The elaborated structures and their corresponding energy dia- grams are shown in figure 1a and b. The absorption and transmittance ‘spectra were obtained using a ‘UV-1650PC’
spectrometer. A JOBIN YVON-SPEX Spectrum One’ CCD detector, cooled at liquid nitrogen temperature, has been used to conduct PL spectra. Atomic force microscopy (AFM) images were obtained in a tapping mode.
3. Results and discussion
3.1 Annealing effect on absorption and PL of single sub- cell
The absorption spectra of both, the bottom and top sub-cells with different annealing temperatures (80, 100, 120 and 150°C) are shown in figure 2a and b, respectively. The characteristic band of thep–p* transition around 500 nm is present in all absorption spectrums of MEH-PPV:PC70BM annealed at different temperatures. Similarly, the absor- bance spectrum of P3HT:PC70BM nanocomposites exhibits
three peaks characteristic at 518, 552 and 603 nm.
The peaks located at 552 and 603 nm were attributed to the crystallinep–p* stacking of P3HT [33,34]. Importantly, the annealing temperature has a remarkable effect on the absorption intensity of both the sub-cells. For the first sub-cell, we notice that when the annealing temperature increased from 80 to 120°C, the absorption intensity increases gradually, and less important for a higher tem- perature (150°C). However, in the case of second sub-cell, the absorption intensity begins to increase from 100°C to reach a maximum at 150°C [35].
The PL spectra of MEH-PPV:PC70BM and P3HT:PC70BM for different annealing temperatures are shown in figure 3a and b, respectively. The PL spectrum of the MEH-PPV:PC70BM with different annealing temperatures demonstrates the presence of two emission peaks. The first peak arises from the relaxation of excited p-electrons to the ground state and the second one is related to inter-chain interactions [36]. For the top sub- cells, we note the presence of prominent peak around 650 nm, corresponding to the intra-chain recombination of excitons in P3HT molecules [34]. It is worth noting that for both single sub-cells, the difference in positions of absorption and PL peaks are mainly due to energy loss through molecular collision and solvent relaxation, resulting in the red shift. Actually, PL spectrum demon- strates the effectiveness of charge transfer [37], shown by the decrease in PL intensity with annealing temperature.
In fact, annealing effect leads to improve the polymer/
PC70BM interfaces and provide a better separation. To confirm this we discuss the behaviour of space charge region SCR. The interfaces (MEH-PPV:PC70BM or P3HT:PC70BM) quality depends linearly with the SCR.
Thus, the photovoltaic applications require a significant depletion area. This zone is generated from an inner electric field which formed in the region near the inter- face between PC70BM and MEH-PPV or P3HT. This field is responsible for electrons and holes transportation in the opposite direction of the diffusion phenomena.
With increasing temperature, the space charge zone increases, leading to the reduction in the PL intensity.
Moreover, for both sub-cells, we showed a red shift between the as prepared and annealed films, which gen- erally attributed to the reorganization of the polymer chains and PC70BM acceptor during annealing. This facilitates exciton dissociation within the nanocomposite, stimulating additional quenching of the PL emission from the polymer chains.
The efficiency of charge transfer was evaluated by the parameterg [38]:
g¼1I=I0; ð1Þ
where I0 and I are the integrated PL intensities of the nanocomposite as cast and after annealing. From figure3c, we observe that the charge transfer in the bottom sub-cell is more efficient at 120°C. The charge transfer rate is
240 Page 2 of 9 Bull. Mater. Sci. (2021) 44:240
estimated at 55%. However, from figure 3d, it is clear that the charge transfer in the top sub-cell is more efficient at 150°C and it is estimated at 53%.
3.2 Absorption and PL study of tandem structure based on ZnO, C60ICLs
The transmittance spectrum of these ICLs is shown in figure4a. It can be seen that the transmittance of ICL1 and ICL3 is[85%, and of ICL2 and ICL4 is[75% in whole visible spectrum, ensuring enough light to be absorbed by
the top sub-cell. The optical bandgaps of individual layers consisting of different ICLs were calculated using the classical Tauc plot relation (ahm)2 vs. hm, where hm is the energy (eV), a is the absorption coefficient. The bandgaps of the electron transporting layers ZnO and C60 were measured and found to be 3.3 and 1.9 eV, respectively (figure4b). The calculated bandgap of the hole transporting layer (HTL) PEDOT:PSS is 1.9 eV.
The good connection between sub-cells through the interconnection layer, is crucial for the tandem operation.
Hence, we deeply investigated the electrical properties of different ICLs. Firstly, we measured the vertical Figure 1. (a) Schematic representation of the tandem with different interconnecting layers. (b) Energy levels of the
tandem.
Figure 2. UV–visible absorption spectra obtained before and after annealing at different temperatures for: (a) the bottom sub-cell based on MEH-PPV:PC70BM and (b) the top sub-cell based on P3HT:PC70BM.
Figure 3. Photoluminescence spectra obtained before and after annealing at different temperatures for: (a) the bottom sub-cell, (b) the top sub-cell. The wavelength excitation was selected at 488 nm. Variation of the quenching efficiency parameter as a function of the annealing temperature for: (c) the bottom-cell and (d) the top sub-cell.
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Figure 4. (a) Transmittance spectra of the interconnecting layers, (b) Tauc plot of ZnO, C60 and PEDOT:PSS layers, (c)J–Vcharacteristics of the hole only device (ITO/PEDOT:PSS/Au) and (d)J–V characteristics of the various interconnecting layers (ITO/ICL/Ag).
conductivity of individual film. For the HTL PED- OT:PSS, we measure the J–V characteristic of the hole only device ITO/PEDOT:PSS/Au. For the electron trans- porting layers (ETLs) ZnO or C60, we measure the J–V characteristic of the electron device ITO/ETLs/Ag.
The conductivity (r) is determined from the slope of (J–V) plot (figure4c and d), using the following equation [39]:
J¼rAd1V;
whereAis area of the sample (0.12 cm2) anddthe thickness of film. The conductivity of the HTL PEDOT:PSS is
1.7510-6S cm-1. For the ETLs, the conductivity is 2.510-4 S cm-1of the ZnO layer,and is higher (2.210–3S cm–1) for the C60 layer. To note, a high conductivity facilitates/en- hances the electron transfer at the interface BHJ/ETLs.
Then, we measured theJ–Vcharacteristic of various ICLs, in complete structure, sandwiched between the ITO and Ag, as shown in figure4d. The series resistance of the ZnO and ZnO/PEDOT:PSS ICLs is 7.2 and 6.3X.cm2. For the ICL3 = ZnO/PEDOT:PSS, the introduction of the PEDOT:PSS layer on the top of the ZnO reduce the mismatch energy, and thus, decrease the series resistance. The series resistance C60/
PEDOT:PSS ICL is higher than the C60 layer (1.4 vs. 1.7 X.cm2). Despite the reduction in mismatch energy level through the introduction of PEDOT:PSS on the top of C60, the series resistance increases. This may be explained by the non- compatibility in terms of processing between the evaporated C60 layer and the spin-coated PEDOT:PSS layer.
Hence, these ICLs with high transparency, appropriate energy levels with sub-cell, high conductivity and low series resistances satisfy the optical and electrical require- ment outlined in the beginning.
In figure5, the absorption spectra of the tandem structure 1 and 2 are shown. The absorption spectrum of these structures can be described as a superposition of the absorption spectra of each sub-cell. A range of wavelengths from 400 to 700 nm of the solar spectrum was covered.
Indeed, the first sub-cell based on MEH-PPV:PC70BM Figure 4. continued
Figure 5. Absorption spectra of the tandem cell using ZnO and C60 interconnecting layers.
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mainly absorbs the wavelength range between 400 and 550 nm, while the second sub-cell based on P3HT:PC70BM absorbs even in this region and in the range from 550 to 700 nm. The only difference among these spectra is at the absorption intensity. This can be explained by the trans- mittance, which provides each ICL to the top sub-cell. In the aim to study charge transfer, PL spectra of tandem 1 and 2, as well those of bottom and top sub-cells were displayed in figure6a and b. For all spectra, the excitation wavelength used was 488 nm. For this mode of excitation, light is absorbed by the second sub-cell passing through the first sub-cell and the ICL. Knowing that the absorption spectrum of ZnO, C60film presents no bands in the visible range. The PL spectrum of the tandem 1 exhibits an emission peak around 647 nm (figure 6a). This peak is linked to inter- chains interaction of MEH-PPV or P3HT. Whereas, the PL spectra of tandem 2 exhibits two emission peaks located at 650 and 700 nm (figure6b). We showed a decrease in PL intensity in tandem architectures (tandem 1 and tandem 3) as compared to single sub-cell taken separately. This proves that the dissociation of electron–hole pairs is more effective in the tandem cell combining two BHJ in one cell. Indeed, in addition to the dissociation produced at each BHJ sub- cell, the use of intermediate layer allows the creation of an interface with the bottom sub-cell (MEH-PPV:PC70BM/
ICLs) and an interface with the top sub-cell (ICLs/interface P3HT:PC70BM). For tandem 2, we notice a sharp decrease in PL intensity as compared to the single sub-cell. As well, a red shift was observed. This shift is not attributed to the reorganization of one of the polymers (MEH-PPV or P3HT), but rather to the presence of two interfaces disso- ciation between the ICL C60and sub-cells.
To highlight the charge transfer enhancement in tandem structure, we draw the diagram of tandem 1 and 2 as well as individual sub-cells, for an emission wavelength of 650 nm (figure6c). We chose the wavelength (k= 650 nm) because it represents the common emission peak for tandems and sub-cell structures. The charge transfer in tandem 1, with ICL1 = ZnO, is 28% more efficient as compared to the bottom sub-cell, and 41.5%, as compared to the top sub- cell.
The corresponding data for tandem devices changed significantly with the variation in the ICLs, indicating that the ICL play a critical role in controlling the device per- formance. Interestingly, for the tandem 2 (ICL = C60), an extinction of the PL signal is observed, as compared to single sub-cells. The charge transfer rate is estimated as 78 and 82% higher as compared to the first and second sub- cells, respectively. The efficient charge transfer is attributed to the high conductivity and low resistance of the C60 layer.
Figure 6. PL spectra of the single reference sub-cell and the tandem cell with: (a) ZnO and (b) C60
ICLs. (c) Diagram obtained forkemission= 650 nm.
Clearly, an effective connection between sub-cells can be realized by using a simple ICL as C60.
In tandem OSCs, the ICLs are generally consisting of a n-type electron layer (ETL) and p-type HTL. For that, we have introduced a PEDOT:PSS layer on top of C60 or ZnO layers, to obtain an appropriate HTLs/ETLs interconnecting structure.
To highlight the effect of incorporation of PEDOT:PSS on top of ZnO and C60, tandem devices with and without PEDOT:PSS were elaborated.
Figure 7a and b depicts the absorption and PL spectra of tandem structure with ZnO and ZnO/PEOT:PSS ICLs, respectively. It is clear that the insertion of the PED- OT:PSS layer on top of ZnO does not change the shape or the peak positions in the absorption and PL spectrum.
If we compare to the traditional tandem cell, insertion of
the PEDOT:PSS layer improves the absorption spectra, in particular, in the common absorption region of the two polymers (400–550 nm). In addition, even after the addition of PEDOT:PSS layer, the PL intensity decreases.
The load transfer is more efficient in tandem 3 than in tandem 1. The introduction of the PEDOT:PSS layer below the photoactive layer has generally shown the performance enhancement [40]. In our case, the deposi- tion of the PEDOT:PSS layer between the ZnO layer and the second sub-cell, results in the formation of defect states at the PEDOT:PSS/P3HT:PC70BM interface. The presence of defect states causes a further decline of PL of the second sub-cell [41].
In contrast, the incorporation of PEDOT:PSS on the top of C60layer leads to a decrease in the absorption intensity and the charge transfer efficiency (figure 8a and b), in Figure 7. Absorption spectra of tandem cell before and after incorporation of PEDOT:PSS on top of ICL: (a) ZnO
and (b) C60.
Figure 8. PL spectra of tandem cell before and after incorporation of PEDOT:PSS on top of ICL: (a) ZnO and (b) C60.
240 Page 8 of 9 Bull. Mater. Sci. (2021) 44:240
comparison with the traditional structure (with C60). The increase in PL signal may be attributed to (i) the unbalance in conductivity between the PEDOT:PSS and C60 layers (1.7510–6vs.2.210–3Scm–1) and (ii) the disappearance of the dissociating interface in the second sub-cell side (C60/P3HT:PC70BM interface) hidden following the incor- poration of PEDOT:PSS layer.
4. Conclusion
In summary, solution-processed tandem structures based on MEH-PPV:PC70BM and P3HT:PC70BM sub-cell have been successfully fabricated. For the first time, we optimized optical properties of each sub-cell by varying the annealing temperature. The optimum temperature for the absorption and the efficient charge transfer of bottom and top sub-cells are 120 and 150°C, respectively. The introduction of dif- ferent ICLs shows an improvement in optical properties as compared to single sub-cells. We also investigated the effect of incorporation of PEDOT:PSS layer on optical and morphological properties of tandem structure. The fully optimized tandem structure reached*78% enhancement in charge transfer as compared to the bottom sub-cell and 82%
as compared to the top sub-cell. We conclude that the introduction of a PEDOT:PSS layer within the ICL improves optical properties, particularly in the ground and excited states for tandem device based on ZnO ICLs, while it has the opposite effect for tandem device based on C60 ICLs.
References
[1] Kymakis E, Stratakis E, Stylianakis M M, Koudoumas E and Fotakis C 2011Thin Solid Films5201238
[2] Saidi H, Aloui W and Bouazizi A 2018J. Mater. Sci. Mater.
Electron.2918051
[3] Nelson J 2002Mater. Today520
[4] Rumer J W and Culloch I 2015Mater. Today18425 [5] Bijleveld J C, Zoombelt A P, Mathijssen S G, Wienk M M,
Turbiez M, Leeuw D Met al2009J. Am. Chem. Soc.131 16616
[6] Bouzid H, Prosa M, Bolognesi M, Chehata N, Gedefaw D, Albonetti Cet al 2019J. Polym. Sci. A: Polym. Chem.57 487
[7] Saidi H, Hidouri T, Fraj I, Saidi F and Bouazizi A 2015 Superlattices Microstruct.85925
[8] Beaujuge P M and Frechet J M J 2011J. Am. Chem. Soc.133 20009
[9] Zhong H, Li Z, Deledalle F, Fregoso E C, Shahid M, Fei Z et al2013J. Am. Chem. Soc.1352040
[10] Ma H, Yip H L, Huang F and Jen A K Y 2010Adv. Funct.
Mater.201371
[11] Seo J H, Gutacker A, Sun Y, Wu H, Huang F, Cao Yet al 2011J. Am. Chem. Soc.1338416
[12] Wang D H, Park J H, Seo J H, Seifter J, Jeon J H, Kim J K et al2011Adv. Energy Mater.1766
[13] Wang D H, Seifter J, Park J H, Choi D G and Heeger A J 2012Adv. Energy Mater.21319
[14] Liu Y P, Chang S Y, Li T F, Sun P, Wang R, Cheng H W et al2019Joule3432
[15] Min Nam Y, Huh J and Ho Jo W 2010Sol. Energy Mater.
Sol. Cells951095
[16] New E, Howells T, Sullivan P and Tim S J 2013 Org.
Electron.142353
[17] Mudrick J P, Cao W, Jian L and Jiangeng X 2014 Org.
Electron.153024
[18] Shockley W and Queisser H J 1961J. Appl. Phys.32510 [19] Aqoma H, Azmi R, Hwanh S and Sung-Yeon J 2017Nano
Energy31403
[20] Li W, Furlan A, Hendriks K H, Wienk M M and Janssen R A J 2013J. Am. Chem. Soc.1355529
[21] Jin F, Chu B, Li W, Su Z, Yan X, Wang Jet al2014Org.
Electron.153756
[22] Meng L, Zhang Y, Wan X, Li C, Zhang X, Wang Yet al 2018Science3611094
[23] Jo J, Pouliot R J, Wynands D, Collins S D, Kim J Y, Nguyen T Let al2013J. Adv. Mater.254783
[24] Zhou Y, Fuentes-Hernandez C, Shim J W, Khan M T and Kippelen B 2012Energy Environ. Sci.59827
[25] Li N, Stubhan T, Baran D, Min J, Wang H, Ameri Tet al 2013Adv. Energy Mater.3301
[26] You J B, Dou L T, Hong Z R, Li G and Yang Y 2013Prog.
Polym. Sci.381909
[27] Ameri T, Li N and Brabec C J 2013Energy Environ. Sci.6 2390
[28] Kouijzer S, Esiner S, Frijters C H, Turbiez M, Wienk M M and Janssen R A J 2012Adv. Energy Mater.2945 [29] Yang J, Zhu R, Hong Z, He Y, Kumar A, Li Yet al 2011
Adv. Mater.233465
[30] Chen Y, Lin W C, Liu J and Dai L 2015J. Am. Chem. Soc.
141467
[31] Tung V C, Kim J and Huang J 2012Adv. Energy. Mater.2 299
[32] You J, Dou L, Yoshimura K, Kato T, Ohya K, Moriarty T et al2013Nat. Commun.41446
[33] Huang Y C, Lia Y C, Li S S, Wu M C, Chen C W and Su W F 2009Sol. Energy Mater. Sol. Cells93888
[34] Chang S H, Chiang C H, Cheng H M, Tai C-Y and Wu C G 2016Opt. Lett.3824
[35] Sariciftci N S, Smilowitz L, Heeger A J and Wudl F 1992 Science2581474
[36] Traiphol R, Charoenthai N, Srikhirin T, Kerdeharoen T, Osotchan T and Maturos T 2007Polymer48813
[37] Chehata N, Ltaief A, Beyou E, Ilahi B, Salem B, Baron T et al2015J. Lumin.168315
[38] Al-Asbahi B A, Alsalhi M S, Al-Dwayyan A S and Jumali M H H 2012J. Lumin.90132
[39] Kakavelakis G, Maksudov T, Konios D, Paradisanos I, Kioseoglou G, Stratakis Eet al2017Adv. Energy Mater.7 1602120
[40] Peet J, Kim J Y, Coates N E, Ma W L, Moses D and Heeger A J 2007Nat. Mater.6497
[41] Van Dijken A, Perro A, Meulenkamp E A and Brunner K 2003Org. Electron.4131
