Efficiency Rise in PCDTBT:PC₇₀BM Organic Solar Cell Using Interface Additive

• Rashmi Swami, Rajesh Awasthi, Sanjay Tiwari

Abstract

Solar cell can be designed with photoactive layer of organic and inorganic materials. Organic materials, particularly polymers, are a promising alternative to traditional semiconductors as the active material for solar cell because of their low cost, low temperature & energy processing, low material requirement, can be used on flexible substrate, can be shaped to suit architectural application. Low efficiency is one of the biggest problem with organic solar cell. In order to increase the efficiency of bulk hetero-junction organic solar cell we are using interface surfactant additive poly(oxyethylene tridecyl ether) (PTE) with blend photoactive layer. Here we are reporting on the enhanced photovoltaic (PV) effects by means of a polymer bulk-hetero-junction (BHJ) layer having PCDTBT which is poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) as a low-band gap e’ donor/HTL polymer and PC₇₀BM which is [6,6]-phenyl C70 butyric acid methyl ester as an acceptor/ETL, doped with poly(oxyethylene tridecyl ether) (PTE) which is an interface surfactant additive. For PCDTBT:PC₇₀BM organic solar cell , we recorded 0.886 V open-circuit voltage (V_OC), 11.7 mA/cm² short-circuit current density (J_SC), and 47.3% fill factor (FF) and PCE of 4.9%. For PCDTBT:PCBM70:PTE organic solar cell, we recorded V_OC of 0.904 V, higher values of J_SC of 13.8 mA/cm², FF of 48.2% and improved PCE of 6.0% for a PTE concentration of ca. 0.164 wt%. Power conversion efficiency (PCE) reaches to 6.0%, by the addition of PTE to a PCDTBT:PC₇₀BM system which is much higher than a reference device not including the additive (4.9%). Increase in efficiency is because of the increase in lifetime of charge carrier, which is due to the existence of PTE molecules at the interfaces sandwiched between the BHJ photovoltaic active layer and the anode and cathode, in addition to the phase-separated BHJ domains interfaces.

Keywords – Organic Solar Cell, PCDTBT, PCBM, PTE, IPCE, Bulk hetero-junction.

1. Introduction

The global rising demand for low-priced electricity has triggered deep research on solar cells comprising organic semiconductors. Organic solar cell (OSC) technology has received significant attention over the past decade due to the simple, flexible nature of polymer photovoltaics and the potential to develop a clean, cost-efficient renewable energy source. The key development of organic solar cells has been made with the pioneering concept of ‘‘bulk hetero-junction (BHJ)’’ photoactive layers [1-2].The bulk hetero-junction (BHJ) PSC [1][3] is of particular interest, due to the efficient photo-induced generation of charge in its blended photovoltaic (PV) layer, that is consisted of interpenetrating, channel-like domains of separated fullerene and polymer. Following the annealing of the BHJ structure at elevated temperatures, PSCs with PV layers of P3HT which is poly(3-hexylthiophene) and PCBM60 which is phenyl C61-butyric acid methyl ester have shown high power conversion efficiencies (PCEs) of 3-5%. Efficiency of P3HT:PCBM organic solar cell is upto 5% because of the limitations of conventional P3HT, whose bandgap lies at around 1.9 eV, which limits absorbance to wavelengths below 650 nm [4]. To improve the efficiency of PSC we need new active materials having lower bandgap to harvest more solar photons. More recently, a PCE of 5-6% was reported for a BHJ PSC that used a blend of PCBM70 and PCDTBT having a bandgap of 1.88 eV [5,6]. Using ‘processing additives’ PCE of organic solar cell can be increased [7-9]. To increase carrier lifetimes (reduce recombination loss) we modify the BHJ interfaces between the phase-separated domains of the donor-conjugated polymer and the acceptor fullerene, and added a non-ionic surfactant poly(oxyethylene tridecyl ether) (PTE) as an additive to the PV layer. In this paper we investigated J-V characteristic and IPCE spectra of PCDTBT:PC₇₀BM organic solar cell with and without PTE.

1.1 Donor molecule

Next generation HTL/donor material for organic photovoltaics is Poly[[9-(1-octylnonyl)-9H-carbazole-2.7-diyl]-2.5-thiophenediyl-2.1.3 benzothiadiazole-4.7-diyl-2.5-thiophenediyl] (PCDTBT) shown in Fig. (1) which can produce better efficiencies and lifetimes. The main qualities of PCDTBT are –

• lower HOMO and LUMO levels
• narrow band gap
• Increased open circuit voltage
• Longer wavelength absorption
• Lower concentration and material usage
• Improved stability under ambient conditions
• High electron and hole generation rate and high mobility of electron and hole.

Fig. 1. Molecular structure of PCDTBT.

1.2 Acceptor molecule

Extremely symmetrical cage-shaped molecules of carbon atoms is Fullerenes as shown in Fig. (2). For the separation of photoexcited exciton into free charge carriers blending of conjugated polymers (electron donor) with fullerenes (electron acceptors), is extremely efficient way.

Fig. 2. Molecular structure of PC₇₀BM.

1.3 PTE additive

Poly(oxyethylene tridecyl ether) (PTE) shown in Fig. (3) as an additive have low (- 8.1 eV) highest- occupied-molecular-orbital (HOMO) and high (2.1 eV) lowest-unoccupied-molecular- orbital (LUMO) [10–12].

Fig. 3. Molecular structure of PTE.

2. Experimental Details

The sample BHJ PSCs were fabricated in a sandwich structure with an anode of indium tin oxide (ITO) and an Al:Li/Al cathode. Patterned 80-nm-thick ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, and then treated in an ultraviolet-ozone chamber for 15 min. Then, a ca. 40-nm-thick hole-collecting PEDOT:PSS buffer layer was spin-coated onto the ITO electrode. On the top of the PEDOT:PSS layer spin coat the blended solution of PCDTBT (0.456 wt%), PCBM70 (1.824 wt%), and PTE additive in dichlorobenzene. The PV layer was about 85 nm thick. Finally, for the cathode, a ca. 1-nmthick Al:Li alloy (Li: 0.1 wt%) layer and a pure Al (ca. 50-nm-thick) layer were created on the photovoliaic layer through thermal deposition (0.5 nm/s), at a foundation pressure below 2×10^-4 Pa. The sample device structure studied was therefore [ITO/PEDOT:PSS/PCDTBT:PC₇₀BM:PTE/Al:Li/Al] as shown in Fig. (4). The active area of the fabricated device was 3×3 mm². For comparison, a reference PSC was fabricated with the structure [ITO/PEDOT:PSS/PCDTBT:PC₇₀BM/Al:Li/Al] as shown in Fig. (5). In 100 mW/cm² illumination intensity produced by an AM 1.5G light resource, the performance of the PSCs was measured,. With the help of a source meter (Keithley 2400) the photocurrent-versus-voltage (J-V) characteristics were measured. The IPCE (incident photon-to-current collection efficiency) spectrum were measured for the PSCs studied using an IPCE measurement system.

Fig. 4. ITO/PEDOT:PSS/ PCDTBT:PC₇₀BM:PTE /Al:Li/Al Organic Solar Cell.

Fig. 5. ITO/PEDOT:PSS/ PCDTBT:PC₇₀BM /Al:Li/Al Organic Solar Cell.

3. Results And Discussion

As shown in Fig. (6) for PCDTBT:PC₇₀BM organic solar cell , under an illumination of AM 1.5G and 100 mW/cm2, we recorded 0.886 V open-circuit voltage (V_OC), 11.7 mA/cm² short-circuit current density (J_SC) and 47.3% of fill factor (FF) and PCE of 4.9% a value comparable with those reported by others [6]. For PCDTBT:PC₇₀BM:PTE organic solar cell, we recorded V_OC of 0.904 V, higher values of J_SC of 13.8 mA/cm², FF of 48.2% and improved PCE of 6.0% for a PTE concentration of ca. 0.164 wt%. These increased values resulted in an improved efficiency of 6.0%, which led to a PCE that was up to 22% higher than that of PCDTBT:PC₇₀BM based organic solar cell.

Fig. 6. The current-voltage characteristics of BHJ OSCs with and without the PTE additive.

We further investigated the PV performance of the OSCs that incorporated the PTE additive by studying the IPCE spectra. Fig. (7) shows the observed IPCE spectrum of the PSC devices. It can be seen that the IPCE values are consistent with the variations in JSC for the OSCs with and without the PTE additive. The maximum IPCE was 73.0% at 470 nm for the sample device with the PTE additive, which corresponded to the highest JSC (13.8 mA/cm² ), while the IPCE value was about 60.9% for the reference device without the additive, which had the lowest JSC (11.7 mA/cm² ).

Fig. 7. IPCE spectra of PCDTBT:PC₇₀ BM OSCs with and without the PTE additive.

4. Conclusions

In conclusion, we have reported on the use of a low-bandgap PCDTBT:PC₇₀BM-based PV layer that incorporates a PTE surfactant, which was used to the BHJ interfaces in OSCs. We have shown that BHJ OSCs that contain the interface PTE additive are more efficient than conventional OSCs. A high PCE (6.0%) was obtained for our PCDTBT:PC₇₀BM (1:4 w/w) OSC device using 0.164 wt% of the PTE additive, which yielded improvements in PCE of up to 22%. This improvement may be attributed to the increased selective flow of dissociated charge carriers, not only at the interfaces of the PV layer and the electrodes, but also at the BHJ interfaces between the PCDTBT and PC₇₀BM domains. Our findings show that a combination of PTE interface additives and high-performance low-band gap PV materials holds great potential for the development of a new generation of highly efficient OSCs.

References

[1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger. Polymer Photovoltaic Cells:Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science, New Series, 1995, 270(5243): 1789-1791.

[2] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes. Efficient photodiodes from interpenetrating polymer networks. Nature, 1995, 376: 498–500.

[3] C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen. Plastic solar cells. Adv. Funct. Mater. 2001, 11(1): 15–26.

[4] K. M. Coakley and M. D. McGehee. Conjugated polymer photovoltaic cells. Chem. Mater., 2004, 16(23): 4533–4542.

[5] S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, and A. J. Heeger. Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nat. Photonics, 2009, 3(5): 297–302.

[6] J. Zhou, X. Wan, Y. Liu, F. Wang, G. Long, C. Li, and Y. Chen. Synthesis and photovoltaic properties of a poly(2,7-carbazole) derivative based on dithienosilole and benzothiadiazole. Macromol. Chem. Phys., 2011, 212(11): 1109–1114.

[7] J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, and G. C. Bazan. Efficiency enhancement in low-bandgap polymer solar cells by processing with alkane dithiols. Nat. Mater., 2007, 6(7): 497–500.

[8] G. Garcia-Belmonte and J. Bisquert. Open-circuit voltage limit caused by recombination through tail states in bulk heterojuction polymer-fullerene solar cells. Appl. Phys. Lett., 2010, 96(11): 113301.

[9] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, and L. Yu. For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%. Adv. Mater. (Deerfield Beach Fla.), 2010, 22(20): E135–E138.

[10] Y. I. Lee, M. Kim, Y. Ho Huh, J. S. Lim, S. Cheol Yoon, and B. Park. Improved photovoltaic effect of polymer solar cells with nanoscale interfacial layers. Sol. Energy Mater. Sol. Cells, 2010, 94(6): 1152–1156.

[11] B. Park, Y. H. Huh, and M. Kim. Surfactant additives for improved photovoltaic effect of polymer solar cells. J. Mater. Chem., 2010, 20(48): 10862–10868.

[12] J. H. Park, S. S. Oh, S. W. Kim, E. H. Choi, B. H. Hong, Y. H. Seo, G. S. Cho, B. Park, J. Lim, S. C. Yoon, and C. Lee. Double interfacial layers for highly efficient organic light-emitting devices. Appl. Phys. Lett., 2007, 90(15): 153508.