Effect of annealing treatment and ICL in the improvement of OSCproperties based on MEH-PPV:PC70BM and P3HT:PC70BMsub-cells

HAMZA BOUZID, HAMZA SAIDI* , NADIA CHEHATA and ABDELAZIZ BOUAZIZIEquipe Dispositifs Electroniques Organiques et Photovoltaıque Moleculaire, Laboratoire de la Matiere Condensee 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 C60 as 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 C60 ICL, reached *78% enhancement in charge transfer compared to the

bottom sub-cell and 82% compared to the top sub-cell, obtained for kemission = 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

Bull. Mater. Sci. (2021) 44:240 � Indian Academy of Scienceshttps://doi.org/10.1007/s12034-021-02525-z Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)

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 N2 treatment. 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 the p–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 crystalline p–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

parameter g [38]:

g ¼ 1�I=I0; ð1Þ

where I0 and I are the integrated PL intensities of the

nanocomposite as cast and after annealing. From figure 3c,

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 basedon ZnO, C60 ICLs

The transmittance spectrum of these ICLs is shown in

figure 4a. 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

(figure 4b). 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.

Bull. Mater. Sci. (2021) 44:240 Page 3 of 9 240

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.

240 Page 4 of 9 Bull. Mater. Sci. (2021) 44:240

Figure 4. (a) Transmittance spectra of the interconnecting layers, (b) Tauc plot of ZnO, C60 and

PEDOT:PSS layers, (c) J–V characteristics of the hole only device (ITO/PEDOT:PSS/Au) and (d) J–Vcharacteristics of the various interconnecting layers (ITO/ICL/Ag).

Bull. Mater. Sci. (2021) 44:240 Page 5 of 9 240

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 (figure 4c and d), using the following equation [39]:

J ¼ rAd�1�V ;

where A is area of the sample (0.12 cm2) and d the thickness

of film. The conductivity of the HTL PEDOT:PSS is

1.75�10-6 S cm-1. For the ETLs, the conductivity is 2.5�10-4

S cm-1 of the ZnO layer, and is higher (2.2�10–3 S 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 the J–V characteristic of various ICLs,

in complete structure, sandwiched between the ITO and Ag,

as shown in figure 4d. The series resistance of the ZnO and

ZnO/PEDOT:PSS ICLs is 7.2 and 6.3 X.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 figure 5, 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.

240 Page 6 of 9 Bull. Mater. Sci. (2021) 44:240

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 figure 6a 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, C60 film 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 (figure 6b). 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 C60 and 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

(figure 6c). 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 for kemission = 650 nm.

Bull. Mater. Sci. (2021) 44:240 Page 7 of 9 240

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 C60 layer 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

(1.75�10–6 vs. 2.2�10–3 Scm–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.

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