**6. Polymerizable methanofullerene as a buffer layer material for organic solar cells**

In recent years, new combinations of semiconductor materials based on fullerene derivatives (n-type materials) and electron-conjugated polymers (p-type materials) are being actively developed all over the world. It is believed that the high efficiency of conversion of light in organic solar cells can be achieved only by using charge-selective buffer layers [40]. Usual materials for producing such layers are PEDOT: PSS and a number of inorganic oxides. Since PEDOT: PSS exhibits acidic properties, its use adversely affects the duration of the operation of solar cells. At the same time, the metal oxides in high oxidation states (MoO3 , V<sup>2</sup> O5 and WO<sup>3</sup> ) show oxidizing properties on the materials of the photoactive layer facilitating their breakage. The problem is observed even with relatively unreactive titanium dioxide TiO<sup>2</sup> [41].

The photoactive layer of organic solar cells was created on the basis of the traditional composites: the acceptor component [60]PCBM or [70]PCBM and conjugated polymer Р3НТ or poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-

New Organic Polymers for Solar Cells http://dx.doi.org/10.5772/intechopen.74164 97

**Figure 8.** The molecular structures of the materials used to form ETL buffer layer of the devices.

In our study we propose usage of earlier synthesized {(1-methoxycarbonyl)-1-[2-(acryloyloxy) ethyloxycarbonyl]-1,2-methane}-1,2-dihydro-C60-fullerene **17** [42] and {(1-methoxycarbonyl)- 1-[2-(methacryloyloxy)ethyloxycarbonyl]-1,2-methane}-1,2-dihydro-C60-fullerene **18** [38], containing in their structure unsaturated acrylate and methacrylate fragments (**Figure 8a**), taking into account that buffer layer must comply with the number of requirements. First, the forming method of its film must be straightforward and reasonably technological. Covering the ITO surface with methanofullerene solution in chlorobenzene, as it turned out, was a pretty simple buffer layer-forming approach, which did not request such processes as vacuum thermal evaporation and high-temperature annealing. Second, the formed film should be resistant to the effect of other solvents. Therefore, after laying one on the ITO surface we have had before us challenge of FCM insolubilization. For that reason solid-state radical polymerization has been conducted, which resulted in the creation of fullerene-containing polyacrylates and

At the first stage, the influence of the temperature of the heating of the buffer layer on the efficiency of light conversion in solar batteries was studied on the example of photoactive

The current-voltage characteristics of organic solar cells (**Figure 9**) were measured under standard conditions using simulated solar light of AM 1.5 spectrum and intensity of 100 mW/cm<sup>2</sup> (calibrated Si diode used as reference) and a general-purpose source meter Keithley 400. The

The obtained data clearly demonstrate the positive effect on the characteristics of solar cell buffer layers produced by polymerization of fullerene derivatives **17** and **18**. Particularly exciting were high open-circuit voltages of 637–652 mV achieved by using polymerized **18** as a buffer layer. We would like to emphasize that such high voltages are very rare for the

4,7-diyl-2,5-thiophenediyl] (PCDTBT).

polymethacrylates.

materials [60]PCBM and Р3НТ [43].

P3HT-[60]PCBM solar cells.

resulting parameters of the solar cells are given in **Table 2**.

The greatest prospects in terms of practical implementation have inverted configuration organic solar cells that do not contain high active metals and have significantly increased operational stability. However, the creation of these devices requires development of selective electron-transport buffer layers (ETL) based on semiconductor materials of n-type. We have fabricated inverted solar cells which ITO cathode, fullerene-containing buffer layer or ETL, photoactive layer, hole-transporting layer MoO3 and Ag anode (**Figure 7**).

**Figure 7.** Schematic architecture of an inverted organic solar cell.

**Figure 8.** The molecular structures of the materials used to form ETL buffer layer of the devices.

The photoactive layer of organic solar cells was created on the basis of the traditional composites: the acceptor component [60]PCBM or [70]PCBM and conjugated polymer Р3НТ or poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT).

In our study we propose usage of earlier synthesized {(1-methoxycarbonyl)-1-[2-(acryloyloxy) ethyloxycarbonyl]-1,2-methane}-1,2-dihydro-C60-fullerene **17** [42] and {(1-methoxycarbonyl)- 1-[2-(methacryloyloxy)ethyloxycarbonyl]-1,2-methane}-1,2-dihydro-C60-fullerene **18** [38], containing in their structure unsaturated acrylate and methacrylate fragments (**Figure 8a**), taking into account that buffer layer must comply with the number of requirements. First, the forming method of its film must be straightforward and reasonably technological. Covering the ITO surface with methanofullerene solution in chlorobenzene, as it turned out, was a pretty simple buffer layer-forming approach, which did not request such processes as vacuum thermal evaporation and high-temperature annealing. Second, the formed film should be resistant to the effect of other solvents. Therefore, after laying one on the ITO surface we have had before us challenge of FCM insolubilization. For that reason solid-state radical polymerization has been conducted, which resulted in the creation of fullerene-containing polyacrylates and polymethacrylates.

At the first stage, the influence of the temperature of the heating of the buffer layer on the efficiency of light conversion in solar batteries was studied on the example of photoactive materials [60]PCBM and Р3НТ [43].

The current-voltage characteristics of organic solar cells (**Figure 9**) were measured under standard conditions using simulated solar light of AM 1.5 spectrum and intensity of 100 mW/cm<sup>2</sup> (calibrated Si diode used as reference) and a general-purpose source meter Keithley 400. The resulting parameters of the solar cells are given in **Table 2**.

The obtained data clearly demonstrate the positive effect on the characteristics of solar cell buffer layers produced by polymerization of fullerene derivatives **17** and **18**. Particularly exciting were high open-circuit voltages of 637–652 mV achieved by using polymerized **18** as a buffer layer. We would like to emphasize that such high voltages are very rare for the P3HT-[60]PCBM solar cells.

**Figure 7.** Schematic architecture of an inverted organic solar cell.

photoactive layer, hole-transporting layer MoO3

organic solar cells were about 2%. These values were obtained for the structures based on

Thus, it was demonstrated that a combination of PANI with fullerene-containing polymers is very important for formation of OSC on the basis of binary donor-acceptor systems. The solar cells investigated here differ from earlier ones [39] that they can be fabricated on the flexible

In recent years, new combinations of semiconductor materials based on fullerene derivatives (n-type materials) and electron-conjugated polymers (p-type materials) are being actively developed all over the world. It is believed that the high efficiency of conversion of light in organic solar cells can be achieved only by using charge-selective buffer layers [40]. Usual materials for producing such layers are PEDOT: PSS and a number of inorganic oxides. Since PEDOT: PSS exhibits acidic properties, its use adversely affects the duration of the operation

) show oxidizing properties on the materials of the photoactive layer facilitating their

and Ag anode (**Figure 7**).

, V<sup>2</sup> O5 and

[41].

**6. Polymerizable methanofullerene as a buffer layer material for** 

of solar cells. At the same time, the metal oxides in high oxidation states (MoO3

breakage. The problem is observed even with relatively unreactive titanium dioxide TiO<sup>2</sup>

The greatest prospects in terms of practical implementation have inverted configuration organic solar cells that do not contain high active metals and have significantly increased operational stability. However, the creation of these devices requires development of selective electron-transport buffer layers (ETL) based on semiconductor materials of n-type. We have fabricated inverted solar cells which ITO cathode, fullerene-containing buffer layer or ETL,

methanofullerene derivatives.

96 Emerging Solar Energy Materials

substrates.

WO<sup>3</sup>

**organic solar cells**

**Table 3** shows that PCEs of the devices with ETL are higher than PCE of the reference device. The data in **Table 3** also marks a strong increase in open-circuit voltage at the implementation of 17, which is also noticeable, while other characteristics differ. The most probable explanation is that an n-type semiconductor facilitates photoelectric work function increase, and in turn Voc depends on the work function. A low FF highlights the need to conduct an additional optimization for active-layer forming to improve photovoltaic cell morphology, since FF depends on photoactive film morphology. Authors reported that FF can achieve 60–70% for the PCDTBT:[70]PCBM system. With this value of FF, our devices could achieve PCE of 4.5–4.8%. Thus highest performance has been demonstrated by the device with minimal concentration of 1. It is obvious that more optimal PCEs are arranged in the low-value areas of concentration. Properly, the less the concentration of the compound, the less the thickness of the formed layer. Presumably, further studies on increasing solar cells' efficiency will be held

**Concentration 1, mg/ml Voc, mV Jsc, mA/cm2 FF, % PCE, %** — 446 8.7 36 1.4 0.625 618 11.1 39 2.7 1.250 587 9.1 39 2.1 2.500 620 8.6 36 1.8

**Table 3.** Current-voltage characteristics of inverted solar cells using different concentrations of **17**.

At the third stage, we investigated the effect of the concentration of the buffer layer on the efficiency of light conversion in solar batteries as the example of photoactive materials [60]PCBM and Р3НТ or PCDTBT [45]. For this we propose ETL in inverted organic solar cells using a polymerizable mixture of acrylate derivative of [60]fullerene **17** and pyrrolidinofullerene

P3HT/[60]PCBM — 409 6.9 46 1.3

PCDTBT/[60]PCBM — 585 6.6 42 1.6

\*note that concentration of FPI in the precursor solutions was always 25 mol % with respect to the amount of **17.**

**Table 4.** Parameters of inverted P3HT/[60]PCBM and PCDTBT/[60]PCBM organic solar cells comprising **17** + FPI buffer

1.25 582 7.4 42 1.8 2.50 591 6.5 43 1.7 5.00 486 7.0 43 1.5

0.625 618 11.1 39 2.7 1.25 677 8.3 54 3.0 2.50 707 9.1 46 2.9 5.00 712 7.5 41 2.2

**Voc, mV Jsc, mA/cm2 FF, % PCE, %**

New Organic Polymers for Solar Cells http://dx.doi.org/10.5772/intechopen.74164 99

using small thickness of the buffer layer

**Photoactive materials Concentration of 17 in the** 

layers as a function of **17** concentration in the precursor solution.

**precursor solution, mg/mL\***

**Figure 9.** Selected CV characteristics of the inverted P3HT/[60]PCBM solar cells prepared on bare ITO (reference) and using buffer layers formed from polymerized **17** or **18.**


**Table 2.** Parameters of the best inverted solar cells fabricated on bare ITO and using buffer layers formed from polymerized **17** and **18**.

At the second stage, we studied the impact of buffer layers on PCE and their forming methods on the substrate surface on the example of photoactive materials [70]PCBM and PCDTBT [44].

In recent years, a composite of PCDTBT: [70]PCBM was frequently used as an active layer in the standard organic solar cells OSC. This is based on the fact that the absorbance of [70] PCBM is much stronger than that of [60]PCBM and this property is very important for photovoltaic materials.

Four types of devices have been fabricated: without buffer layer (reference device) and with concentration of **17** in buffer layer 0.625, 1.25 and 2.5 mg/ml. Their current–voltage characteristic is given in **Table 3**.


**Table 3.** Current-voltage characteristics of inverted solar cells using different concentrations of **17**.

**Table 3** shows that PCEs of the devices with ETL are higher than PCE of the reference device. The data in **Table 3** also marks a strong increase in open-circuit voltage at the implementation of 17, which is also noticeable, while other characteristics differ. The most probable explanation is that an n-type semiconductor facilitates photoelectric work function increase, and in turn Voc depends on the work function. A low FF highlights the need to conduct an additional optimization for active-layer forming to improve photovoltaic cell morphology, since FF depends on photoactive film morphology. Authors reported that FF can achieve 60–70% for the PCDTBT:[70]PCBM system. With this value of FF, our devices could achieve PCE of 4.5–4.8%. Thus highest performance has been demonstrated by the device with minimal concentration of 1. It is obvious that more optimal PCEs are arranged in the low-value areas of concentration. Properly, the less the concentration of the compound, the less the thickness of the formed layer. Presumably, further studies on increasing solar cells' efficiency will be held using small thickness of the buffer layer

At the third stage, we investigated the effect of the concentration of the buffer layer on the efficiency of light conversion in solar batteries as the example of photoactive materials [60]PCBM and Р3НТ or PCDTBT [45]. For this we propose ETL in inverted organic solar cells using a polymerizable mixture of acrylate derivative of [60]fullerene **17** and pyrrolidinofullerene


At the second stage, we studied the impact of buffer layers on PCE and their forming methods on the substrate surface on the example of photoactive materials [70]PCBM and PCDTBT [44]. In recent years, a composite of PCDTBT: [70]PCBM was frequently used as an active layer in the standard organic solar cells OSC. This is based on the fact that the absorbance of [70] PCBM is much stronger than that of [60]PCBM and this property is very important for pho-

**Table 2.** Parameters of the best inverted solar cells fabricated on bare ITO and using buffer layers formed from

160 526 6.8 47 1.7

160 652 8.2 50 2.7 200 528 7.8 38 1.6

**Buffer layer Т(polymerization), °С\* Voc, mV Jsc, mA/cm2 FF, % PCE, %** — — 437 7.2 46 1.5 **17** 120 542 7.5 50 2.0

**Figure 9.** Selected CV characteristics of the inverted P3HT/[60]PCBM solar cells prepared on bare ITO (reference) and

**18** 120 608 7.5 55 2.5

\*annealing temperature of the buffer layer material **17** and **18** is provided.

using buffer layers formed from polymerized **17** or **18.**

Four types of devices have been fabricated: without buffer layer (reference device) and with concentration of **17** in buffer layer 0.625, 1.25 and 2.5 mg/ml. Their current–voltage character-

tovoltaic materials.

polymerized **17** and **18**.

98 Emerging Solar Energy Materials

istic is given in **Table 3**.

\*note that concentration of FPI in the precursor solutions was always 25 mol % with respect to the amount of **17.**

**Table 4.** Parameters of inverted P3HT/[60]PCBM and PCDTBT/[60]PCBM organic solar cells comprising **17** + FPI buffer layers as a function of **17** concentration in the precursor solution.

(FPI) (synthesized according to a published procedure [46]) (**Figure 8b**). The main parameters of the solar cells are given in **Table 4**.

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The obtained results suggest that the electron-selective buffer layers based on the blends of the fullerene derivatives FPI and polymerizable **17** can be successfully used for fabricating inverted organic solar cells. The power conversion efficiencies for the inverted devices presented in this chapter were only 25–30% lower than the parameters of the standardconfiguration organic solar cells. However, the latter contains reactive metal (calcium in our case) cathode that induces inherent instability leading to the rapid deterioration of the device parameters even under an inert atmosphere. Inverted devices showed lower opencircuit voltages (approximately by 100 mV) and fill factors as compared to the standard ones. Apparently, the electron work function of the fullerene-based buffer layer material is too high with respect to the conduction band (LUMO level) position of the n-type component of the photoactive layer ([60]PCBM).

Therefore, a Schottky-type barrier might be formed at the interface between the photoactive and the buffer layers. This might be a plausible reason for the observed reduction of the open-circuit voltages and fill factors of the inverted devices. To solve this problem, further research is needed with the aim to design some novel fullerene-based buffer-layer materials with lower-electron work functions.
