Elsevier

Organic Electronics

Volume 28, January 2016, Pages 275-280
Organic Electronics

Enhanced carrier dynamics of PTB7:PC71BM based bulk heterojunction organic solar cells by the incorporation of formic acid

https://doi.org/10.1016/j.orgel.2015.11.004Get rights and content

Highlights

  • •

    Formic acid (FA) was used as a novel additive in PTB7:PC71BM based organic solar cells.

  • •

    The best PCE of 9.04% with 24.6% improvement was obtained with 6 vol% FA doping.

  • •

    Enhanced PCE was attributed to the increase of exciton generation and enhanced charge-carrier mobility of the devices.

Abstract

Formic acid (FA) was used as a novel additive in bulk heterojunction (BHJ) solar cells, which contains blends of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The effect of FA on the performance of PTB7:PC71BM based BHJ solar cells is investigated. By the incorporation of FA, the device with the ratio of 6 vol % shows the best power conversion efficiency (PCE) of 9.04%, along with the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) being 24.11 mA/cm2, 0.72 V, and 52.11%, respectively. Experimental results suggest that FA has a strong influence on charge carrier dynamics with a significant increase in Jsc by ∼65% and the dramatically enhanced PCE is mainly due to the increase of absorption and exciton generation of the active layers and the improved charge-carrier mobility of the devices.

Introduction

Organic photovoltaics (OPVs) have been attracting impressive attention because of their advantages, including light weight, mechanical flexibility, low cost and easy fabrication [1], [2], [3], [4]. At the heart of the OPVs technology, the solution-processed bulk heterojunction (BHJ) solar cells is a simple, but successful technique, the active layer of BHJ solar cells consists an interpenetrating network of donor (p-type) and acceptor (n-type) domains, which is formed during the deposition/drying process [5], [6]. Currently, the favored materials for BHJ solar cells are composites of hole-conducting π-conjugated polymer as the donor material, with electron-conducting fullerene as the acceptor material. Among the large number of materials, poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl]] (PTB7) as π-conjugated polymer and [6], [6]-phenyl-C71-butyric acid methyl ester (PC71BM) (see Fig. 1a for material structure) have been widely used as active materials [5], [7], [8], [9], [10], yielding a power conversion efficiency (PCE) of 9.2% in single junction structure [11] and 10.7% in tandem cells [12].

The carrier dynamics which contain exciton generation, dissociation, and carrier transport are of paramount importance, because the performance of BHJ solar cells is strongly dependent on the ability of the cells to efficiently generate and dissociate excitons, and then transport charges to the electrodes [13], [14]. To achieve efficient exciton generation and dissociation, the ideal donor/acceptor domains scale should be smaller than 10 nm to allow generated excitons to diffuse to donor–acceptor interface [13], [15], [16], [17]. Moreover, a bicontinuous network of donor and acceptor phases must exist with sufficiently high mobility so that efficient charge transport to the electrodes can be allowed [14]. To date, the separation of phases has been optimized by lots of methods such as thermal annealing [18], [19], solvent annealing [20], solvent-fluxing treatment [21], [22], [23], ternary cells [9] and the introduction of processing additives [24], [25]. Among them, the solvent additive, especially 1,8-diiodooctane (DIO, as shown in Fig. 1a), has been widely adopted because it helps to form an ideal bulk heterojunction in active layers with a simpler inclusion in experimental operation [26].

Herein, formic acid (FA) was first employed as an additive to improve the performance of PTB7:PC71BM BHJ polymer solar cells (PSCs). To the best of our knowledge, FA has not previously been reported to improve BHJ solar cells performance as an additive. We controlled the amount of FA from 0 to 10 vol % in the BHJ solution. The Jsc of the cells was dramatically improved from 14.57 to 24.11 mA/cm2, demonstrating the best PCE of 9.04%. In order to identify more detailed effects of FA, we focused on the charge carrier dynamics of the PTB7:PC71BM BHJ cells. By comparing devices data obtained from control and FA doped devices, the improvement of devices performance was proven to be multifold: an increase in absorption and exciton generation of the active layers and enhanced charge-carrier mobility of the devices.

Section snippets

Materials and methods

Unless specified otherwise, all materials were used as received. PTB7 and PC71BM were purchased from 1-material. PTB7 and PC71BM were co-dissolved in chlorobenzene at 30 mg/ml with the weight ratio being 1:1.5. In order to form a homogeneous blend, the co-dissolved solution was stirred by a magnetic stirrer at 30 °C for 24 h. 3 vol % DIO and different volume ratios of FA were added into the solution. Then the mixed solution was stirred another 2 h before use.

All the solar cells were prepared on

Results and discussion

A series of films and devices with different volume ratios of FA were fabricated to investigate the effect of FA. The structure of the devices is shown in Fig. 1b. J-V characteristics of the devices with diverse doping ratios of FA are presented in Fig. 2a. The photovoltaic parameters of PSCs are summarized in Table 1. It is clear from Table 1 that the PCE of the BHJ cells is enhanced significantly after a small amount doping of FA into the PTB7:PC71BM blend and decreases as the FA becomes more

Conclusion

In summary, formic acid as a novel additive was employed to improve the devices performance of OPVs based on PTB7:PC71BM. By the incorporation of FA, the device with the ratio of 6 vol % shows the best PCE of 9.04% with a dramatic increase of Jsc from 14.57 to 24.11 mA/cm2 compared with the control device. The enhancement in Jsc originates from two important contributions: an increase in exciton generation of the active layers and enhanced charge-carrier mobility, which implies that FA improves

Acknowledgments

The authors express the thanks to the National Natural Science Foundation of China under Grant No. 61575019, the Research Fund for the Doctoral Program of Higher Education No. 20130009130001; the National Natural Science Foundation of China under Grant No. 11474018; the Fundamental Research Funds for the Central Universities with the Grant No. 2012JBZ001.

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