Journal of Materials Science: Materials in Electronics https://doi.org/10.1007/s10854-018-9265-z

The effect of alcohol solvent treatment on the performance of inverted polymer solar cells Qi Li1 · Yuan‑Cong Zhong1 · Yong Zhang1,2  Received: 27 March 2018 / Accepted: 8 May 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract Significant performance change has been observed for inverted polymer solar cells (PSCs) with the structure of ITO/ZnO/ P3HT:PC61BM/MoO3/Ag when the photoactive layer was rinsed by spin-coating alcohol solvent before the deposition of ­MoO3 and Ag anode. As a result, isopropanol (IPA) treatment can dramatically enhance the device performance of inverted PSCs while methanol, ethanol, and 1-butanol treatment led to worse photovoltaic performance. The enhancing device performance should be attributed to the remove of ­PC61BM near the top of the P3HT:PC61BM active layer by spin-coating IPA due to better wetting with the photoactive layer, resulting in the power conversion efficiency (PCE) and short-circuit current ­(Jsc) increased from 3.96% and 8.97 mA/cm2 to 4.49% and 9.92 mA/cm2 for IPA treatment, respectively. This facile alcoholtreated route provides a promising method to enhance the device performance of inverted PSCs.

1 Introduction Polymer solar cells (PSCs) have been extensively studied as a promising photovoltaic technology due to the unique advantages including low-cost, light weight, and great potential for large area flexible fabrication, and high-throughput roll-to-roll manufacture [1–5]. To achieve high power conversion efficiency, many efforts have been designed to the development of new materials with suitable energy alignment [6–9], the optimization of device structures [10–12] and processing methods [13, 14]. A variety of materials for the interface engineering of PSCs have been utilized, such as metal oxides [15], self-assembled monolayer [16, 17] and water/alcohol soluble conjugated polymers [18]. Recently, several groups have found that the power conversion efficiency (PCE) of PSCs can be significantly improved by polar solvent treatment on the photoactive layer before deposition of metal electrodes [19–23]. Zhou et al. showed the improvement performance originated from an increase in built-in * Yong Zhang [email protected] 1



Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, People’s Republic of China



Guangdong Engineering Technology Research Center of Low Carbon and Advanced Energy Materials, Guangzhou 510631, People’s Republic of China

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voltage across the device due to the passivation of surface traps and a correspondingly increase of surface charge density after methanol treatment [19]. Zhang et al. demonstrated methanol treatment can induce the modification of the interface between the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and the photoactive layer [20]. Similar effects of solvent treatment were found to remove the residual additives after spin-coating methanol, thus improving the morphological stability [21–23]. Moreover, these research were mostly focused on the effect of solvent treatment on a conventional PSC structure, in which the photoactive layer is sandwiched between the PEDOT:PSScoated ITO anode and low work function metal cathode. Investigations on the impacts of inverted polymer solar cells (IPSCs) are still very limited [24]. The inverted device configuration can improve the environmental stability due to replacing the acidic PEDOT:PSS with metal oxide cathodes [25, 26]. Moreover, several findings indicate the polymer donor tends to cluster at the upper of the photoactive layer due to lower surface energy while PCBM acceptor prefers to gather at the bottom with higher surface energy during the spin-coating [27–29]. Obviously, this kind of donor–acceptor distribution should facilitate carrier transportation and reduce their recombination, resulting in having a positive effect on an inverted device structure. In this study, we investigate the effect of the different alcohol treatment on the device performance of inverted PSCs. UV–Vis measurements show that

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alcohol treatment can remove partial P ­ C61BM while the thickness of P3HT remains unchanged. The contact angle tests show IPA has better wetting with the P3HT:PC61BM active layer. As a consequence, IPA treatment can induce the vertical phase separation and the morphology change of P3HT:PC61BM photoactive layer for the inverted structure, where a P3HT pure film resides on the top of the photoactive layer and ­PC61BM gathers underneath the photoactive layer, resulting in the enhanced device performance.

2 Experimental P3HT and ­P C 61 BM were purchased from Luminescence Technology Corporation. Methanol, ethanol, IPA, 1-butanol, and ZnO nanoparticles were purchased from Sigma-Aldrich. Prior to the device fabrication, patterned indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 Ω were ultrasonically cleaned with detergent, deionized water, acetone, and IPA sequentially for 15 min and then baked in an oven for 12 h. The precleaned ITO substrates were moved into a nitrogen-filled glovebox to deposit subsequent multilayer films. ZnO nanocrystals (in ethanol, 30  mg/ml) were spin-coated on the precleaned ITO substrates at 2000 rpm for 50 s and then baked at 150 °C for 10 min. Then, blend films of P3HT:PC61BM were casted on the top of ZnO from a solution with P3HT:PC61BM ratio of 1:0.8 (P3HT concentration of 12 mg/mL) in chlorobenzene/1,8-diiodoctane (97:3 vol%) mixed solvent at 800 rpm for 45 s. The fresh P3HT:PC61BM blend films were put into a glass Petri dish to undergo solvent annealing for 7 h. After annealed at 150 °C for 15 min to remove the residual solvent, alcohol solvent treatments were carried out by spin-coating methanol, ethanol, IPA, and 1-butannol on the top of the P3HT:PC61BM at 2000 rpm for 60 s, respectively. Finally, the samples were moved into an evaporation chamber under vacuum (6 × 10−4 Pa) to deposit 5 nm ­MoO3/100 nm Ag as anode. Current density–voltage (J–V) characteristics of inverted PSCs were measured with a Keithley 2400 voltage and current source meter in the dark and under the illumination of 100 mW cm −2 AM 1.5 G using a solar simulator (Oriel model 91160). EQE measurements were performed by Solar Cell Spectral Response Measurement System QE-R3011 (Enlitechnology Co. Ltd, Taiwan). UV–Vis absorption spectra were recorded with a HP 8453A UV–Vis spectrophotometer (Agilent, USA). SEM and AFM images were obtained using Zeiss Ultra 55 (Carl Zeiss, Germany) and SOLVER PRO-M (NT-MDT, Russia), respectively. The X-ray photoelectron spectroscopy (XPS) of blend films was recorded by a K-ALPHA+ X-ray

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photoelectron spectroscopy (Thermo Fisher Scientific, USA).

3 Results and discussion The device structure of the fabricated inverted PSCs is shown in Fig. 1. The current density–voltage (J–V) characteristics are shown in Fig. 2a and the corresponding photovoltaic parameters open-circuit voltage ­(Voc), short-current density ­(Jsc), fill factor (FF), power-conversion efficiency (PCE) of inverted PSCs are summarized in Table 1. It can be observed that the different alcohol treatments have an important effect on the device properties of inverted PSCs. Methanol treatment led to a decline device performance relative to the control device without alcohol treatment, which show a PCE of 3.22 ± 0.3% with a ­Jsc of 7.74 ± 0.2 mA/ cm2, a ­Voc of 0.64 ± 0.01 V and a FF of 64.68 ± 0.6%. The ethanol-treated device showed a similar PCE to the control device. However, it was found that the IPA-treated device exhibited a distinctly improved PCE, particularly in terms of ­Jsc and FF, which exhibited a Jsc of 9.92 ± 0.3 mA/cm2, a FF of 70.32 ± 0.9%, and a relatively high PCE of 4.49 ± 0.2% whereas the 1-butanol treatment presented the degraded device performance with the number increasing of carbon atoms in the used alcohol. Different alcohol-treated inverted PSCs all showed a similar reduce V ­ oc from 0.65 to 0.64 V. Figure 2b shows the dark J–V characteristics of the corresponding inverted PSCs with the different alcohol treatments. Methanol and ethanol treated devices showed larger dark leakage current than that of the control device whereas IPA treatment led to much lower dark leakage current relative to the control device, which suggests the IPA treatment can induce good hole-transporting and the electron-blocking properties at the same cathodes for the different alcoholtreated inverted PSCs. Table 1 also gives the series resistance ­(Rs) values. It can be found that methanol and ethanol treatment gave rise to an increase of the contact resistance from 10.7 ± 0.3 Ω cm2 to 11.4 ± 0.4 and 12.1 ± 0.3  Ω  cm 2, respectively while for IPA-treated inverted PSCs, the R ­ s value reduced to

Fig. 1  Device structure of inverted PSCs

Journal of Materials Science: Materials in Electronics

7.0 ± 0.2 Ω cm2. The 1-butanol treatment led to the increase of the contact resistance than that of IPA-treated device. IPA treatment showed much lower contact resistance than the control device, indicating the formation of better Ohmic contact between the P3HT:PC61BM active layer and the metal anode, which is consistent with the higher ­Jsc compared to the control device. For conventional PSCs, alcohol-treatment can generally lead to a simultaneous improvement of ­Jsc, ­Voc, and FF [19–23]. The improvement device performance is usually attribute to reduce the function of metal cathodes and passive

the surface traps due to alcohol treatment, thus enhancing ­Voc and ­Jsc. Our results show that alcohol-treated inverted PSCs gave rise to a different trend from the conventional PSCs. To study the role of alcohol treatment, the UV–Vis absorption spectra of P3HT, ­PC61BM, and P3HT:PC61BM films before and after the different alcohol treatments were measured. As shown in Fig. 3a, no obvious change was observed in the absorption spectrum of P3HT before and after methanol, ethanol, IPA, and 1-butanol rinsing by spincoating, respectively. But the absorption of P ­ C61BM film significantly reduced after alcohol-treatment (as shown in

Fig. 2  J–V characteristics of inverted PSCs with the different alcohol treatments a under illumination of 100 mW cm−2 AM 1.5 G solar simulator and b in the dark Table 1  The statistic photovoltaic parameters of inverted PSCs associated with ten devices based on alcohol treatments

Alcohol treatments

Voc [V]

Jsc [mA/cm2]

FF [%]

PCE [%]

Rs [Ω ­cm2]

Control Methanol Ethanol IPA 1-Butanol

0.65 ± 0.01 0.64 ± 0.01 0.64 ± 0.01 0.64 ± 0.01 0.64 ± 0.01

8.97 ± 0.4 7.74 ± 0.2 9.24 ± 0.5 9.92 ± 0.3 8.13 ± 0.2

67.91 ± 0.8 64.68 ± 0.6 65.85 ± 1.2 70.32 ± 0.9 68.93 ± 0.8

3.96 ± 0.1 3.22 ± 0.3 3.90 ± 0.3 4.49 ± 0.2 3.63 ± 0.3

10.7 ± 0.3 11.4 ± 0.4 12.1 ± 0.3 7.0 ± 0.2 8.5 ± 0.4

Fig. 3  Absorption spectra of a P3HT, b ­PC61BM, and c P3HT:PC61BM films before and after solvent rinsing by spin-coating from methanol, ethanol, IPA and 1-butanol, respectively

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Fig. 3b). 1-Butanol-treatment led to the largest reduction of ­ C61BM absorption intensity, which implied that much more P ­PC61BM were dissolved and removed during the 1-butanolrinsing by spin-coating. The thickness of ­PC61BM film reduced from 65.0 to 57.9 nm for methanol, 55.8 nm for ethanol, 53.0 nm for IPA, and 40.6 nm for 1-butanol, respectively. In addition, alcohol-treatment induced the red-shift of ­PC61BM absorption edge, indicating that a more orderedpacking structure of P ­ C61BM and an enhanced conjugation length occurred due to the hydrogen-bond interactions between the carboxyl groups of ­PC61BM and –OH groups in alcohol molecules. Figure 3c shows that absorption spectra of P3HT:PC61BM blend film before and after alcoholrinsing. After alcohol-rinsing, the P3HT:PC61BM blend film exhibit an increase absorption in the wavelength region of 400–650 nm, indicating the enhanced π-stacking of P3HT chains. However, there is a bit of reduce absorption in the ultraviolent region for methanol, ethanol, and IPA treatment, suggesting that P ­ C61BM near the top of P3HT:PC61BM active layer was removed during the alcohol-rinsing. However, for the 1-butanol-treated, the absorption intensity reduced by about 30%, indicating that the 1-butanol treatment led to much more ­PC61BM removed than methanol,

Journal of Materials Science: Materials in Electronics

ethanol or IPA. In general, for a P3HT:PC61BM photoactive layer, ­PC61BM tends to gather at the bottom and P3HT prefers to aggregate at the upper of the photoactive layer. Alcohol-rinsing can further reduce P ­ C61BM near the top of the photoactive layer, resulting in a P3HT pure film residing on the top of the photoactive layer and P ­ C61BM gathering underneath the photoactive layer. This concentration distribution will be benefit to the transport of charge carrier to their corresponding electrodes and reduction charge recombination near the electrodes. Compared with the different alcohol solvent-treated P3HT:PC61BM film, the 1-butanoltreatment maybe induce internal PC61BM removed of the photoactive layer while the IPA-treatment showed a moderate absorption reduce in the ultraviolent region. As a result, IPA-treated inverted PSC exhibited a significantly enhanced short-circuit current, which is consistent with Fig. 2a. To investigate effect of alcohol solvent on the photoactive layers, the contact angle of the different alcohol solvent on the surface of P3HT:PC61BM active layer was measured. Figure 4 displays the contact angle images of methanol, ethanol, IPA, and 1-butanol dropped on the surface of P3HT:PC61BM blend film. As shown in Fig. 4a–d, the contact angles of methanol, ethanol, IPA, and 1-butanol on

Fig. 4  Contact angle images of methanol (a), ethanol (b), IPA (c), and 1-butanol (d) dropped on the surface of P3HT:PC61BM blend film

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the surface of P3HT:PC61BM are 51.27°, 39.67°, 31.81°, and 11.01°, respectively, indicating that 1-butanol has a better wetting capability with P3HT:PC 61BM active layer. For 1-butanol dropped on the top of P3HT:PC61BM film, 1-butanol can quickly spread out on the surface of P3HT:PC61BM film due to a smaller contact angle. As a result, much more distribution of 1-butanol molecules on the surface of the blend film leads to easier dissolving and removing ­PC61BM during the spin-coating than methanol, ethanol or IPA, which is in accordance with the largest remove of PC61BM for 1-butanol treatment. Moreover, water contact angle measurements were used to gain further insights into the effect of IPA-treatment on the P3HT:PC61BM blend film. As shown in Fig. 5a, b, the contact angles of pure P3HT and P ­ C61BM are 103.94° and 81.56°, respectively. After IPA-rinsing, the contact angle of P3HT:PC61BM blend film increased from 91.73° to 100.88°, which is near to that of the pure P3HT, indicating that the P3HT is enrich on the top surface of the blend film after IPA-treating. To quantify the content of ­PC61BM at the top surface, the fractions of the blend film before and after IPAtreating were calculated by fitting the contact angle results to the Cassie–Baxter equation [30]. The surface coverage of

P3HT was calculated to increase from 45.8 to 97.6% after IPA treating, resulting in most ­PC61BM on the top surface of the P3HT:PC61BM photoactive layer removed during the IPA-rinsing by spin-coating. To further testify the remove of ­PC61BM near the surface of P3HT:PC61BM during the IPA-rinsing, X-ray photoelectron spectroscopy (XPS) was used to probe the changes of the ­PC61BM at the surface of P3HT:PC61BM blend film before and after IPA-rinsing by spin-coating. XPS is an useful tool to determine the composition of surface (0–10 nm) in the P3HT:PC 61BM blend photoactive layers can be calculated directly from the peak intensities of individual elements [31, 32]. According to the molecular structures of P3HT and P ­ C61BM, the sulfur 2p (S 2p) spectral line (~ 163 eV) and the oxygen 1s (O 1s) spectral line (532 eV) are attributed to P3HT and P ­ C61BM, respectively. After IPArinsing, the atomic ratio of O/S decreases from 3.65 to 2.87 (as shown in Fig. 6), further indicating there are more P3HT aggregation on the top surface of the blend photoactive layer by IPA-rinsing. The IPA-treating can remove ­PC61BM near the surface of P3HT:PC61BM photoactive layer and induce the reconsitution of the blend photoactive layer. SEM and AFM

Fig. 5  Water contact angle images of the top surface of pure P3HT film (a), pure PC61BM film (b) and P3HT:PC61BM blend film c before and d after IPA-treating

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4 Conclusions

Fig. 6  XPS of P3HT:PC61BM blend films before and after IPA-rinsing

were used to investigate the surface morphology change of the P3HT:PC61BM blend films before and after IPArinsing. Figure 7 shows the SEM images of P3HT:PC61BM photoactive layers before and after IPA-rinsing. The surface of P3HT:PC61BM film is smooth before IPA-rinsing. However, a large number of pits occurred on the surface of the photoactive layers after IPA-rinsing. Figure  8 shows the AFM images of P3HT:PC 61BM photoactive layers before and after IPA-rinsing. The roughness of P3HT:PC61BM photoactive layer increased from 8.35 to 12.85 nm after IPA-rinsing, which can increase the contact area between top metal anode and the photoactive layer, resulting in the enhanced hole-collecting ability of anode [13].

The effect of different alcohol treatment on inverted PSCs has been examined with alcohol-rinsing by spin-coating on the surface of P3HT:PC61BM photoactive layer before metal electrode deposition. Alcohol treatments on inverted PSCs have significantly different function from conventional PSCs. As a consequent, IPA treatment dramatically improved the device performance of inverted PSCs while methanol, ethanol or 1-butanol treatment had a negative role on the photovoltaic performance. The PCE and ­Jsc of inverted PSC increased from 3.96% and 8.97 mA/cm2 to 4.49% and 9.92 mA/cm2 after IPA-treating, respectively. The improved photovoltaic performance should be attributed into IPA having a suitable wetting ability with the P3HT:PC61BM photoactive layer, which ­PC61BM near the surface of the photoactive layer was disolved and removed during the IPArinsing, resulting in P3HT enriching on the top surface of the photoactive layer and inducing the π-stacking of P3HT chains. Our results show the well-chosed alcohol-treatment can improve the performance of inverted PSCs.

Fig. 7  SEM images of P3HT:PC61BM photoactive layers a before and b after IPA-rinsing

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Fig. 8  AFM images of P3HT:PC61BM photoactive layers a before and b after IPA-rinsing Acknowledgements  This work was supported by the Nature Science Foundation of China (61377065, 61574064), and the Science and Technology Planning Project of Guangdong Province (2013B040402009, 2014B090915004, 2015B010132009). The authors thank Shaohu Han at the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology) for providing a contact angle test.

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