Scaling of inverted PTB7-Th: PC₇₁BM organic solar cell for large area organic photovoltaic modules

Abstract

Performance studies of large area inverted organic photovoltaic (OPV) modules of configuration ITO/ZnO/PTB7-Th: PC₇₁BM/MoO₃/Ag are performed. At a laboratory of scale 0.06 cm², this device configuration repeatedly demonstrates the power conversion efficiency (PCE) of ∼9%, which is within the range of PCE normally achieved for this configuration. The OPV modules with active area of 9.25 cm² and 63 cm² are fabricated employing spin coating techniques comprising a total area 25 cm² (5 cm × 5 cm) and 144 cm² (11 cm × 11 cm), respectively. The 25 cm² module, composed of five cells connected in series show PCE of 3.256%, with short-circuit current (J_sc), open circuit voltage (V_oc), and fill factor (FF), 3.210 mAcm⁻², 3.20 V, and 31.719%. However, The 144 cm² modules, composed of 10 cells connected in series show PCE 1.019, J_sc, V_oc, and FF, 0.87 mAcm⁻², 4.20 V, and 27.877%. The PCE dropped by 63.89% for modules of active area of 9.25 cm² and 88.68% of modules of active area of 63 cm². The PCE of the modules is decreased sharply due to loss in FF, and J_sc of the modules. These losses are exhibits due to quality of layer morphology, layer interfaces, and design of module. The PCE could be potentially improved up to the desired value by the further optimization of layer morphology, layer interfaces, design of module geometry, and film deposition/printing methods. The results showed that PTB7-Th: PC₇₁BM is a splendid structure for future organic solar modules due to its high performance and compatibility with large area coatings.

Introduction

The fast development, industrial growth, and extensive usage of electricity consuming devices across the globe demand energy reservoirs. However, extensive energy utilization causes the depletion of natural resources and increases global warming [1–4]. Solar energy is one of the most potential renewable energy resources, which can be utilized by directly converting it into electricity by employing photovoltaic technology [5–8]. Several photovoltaic technologies such as based on Silicon (Si), Cadmium Telluride (CdTe), and Copper Indium Gallium Selenide (CIGS) are competent to fulfill a significant fraction of this demand. However, these technologies showed hurdles due to high manufacturing process cost, manufacturing obstacles because of complex chemistry, and environmental concerns related to Cd in CdTe [9–13]. Organic solar cells (OSCs) technologies comprising donor and acceptor type of organic semiconductors intermixed to form bulk-heterojunction (BHJ) have potential to become an alternative photovoltaic technology because of cost-effectiveness, light weight, mechanical flexibility, and have the competence to manufacture on the large area [14–20]. There has been immense progress in BHJ organic solar cells. The PCE of BHJ organic solar cells has reached up to 18% [21] because of remarkable progress in donor/acceptor blend materials. However, these tremendous PCE of OSC devices would be noted with laboratory scale area (> 1 mm²). The transformation of laboratory scale area OSC devices to large area modules with stability and without significant loss in PCE exhibits several challenges. Particularly, the deposition of films accompanied by required morphologies over a large area, optimum designs of device structure, and appropriate processing techniques [22–24].

In the company of various blends employed in BHJ organic solar cells, those consist of Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b; 3,3-b]dithiophene]{3-fluoro-2[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th) donor materials blended along with [6, 6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) acceptor have demonstrated magnificent PCE > 10% [25–27]. However, the PCE efficiency of these devices is reported for small scale device areas. On the other hand, this blend (PTB7-Th: PC₇₁BM) of BHJ OSCs is not more investigated for large area modules. Hong et al. demonstrated a new module architecture for making large area modules of blend PTB7-Th: PC₇₁BM BHJ OSCs and reported a high module efficiency of 7.5% with an area of 4.15 cm² [28]. Barreiro-Arguelles et al. investigated the OSCs modules performance and stability with blend PTB7: PC₇₁BM as a photoactive layer [29]. In this work, we extended our study of lab scale optimized OSC devices (0.06 cm² and PCE of 9.0%) with an inverted structure of ITO/ZnO/PTB7-Th: PC₇₁BM/MoO₃/Ag to large area modules [27]. The organic solar modules based on the blend PTB7-Th: PC₇₁BM are fabricated employing spin coating techniques. The OPV modules comprising 5 cells and 10 cells connected in series and possessing total active areas of 9.25 cm², and 63 cm² showed a PCE of 3.256%, and 1.019%, which in the future could be further improved by optimizing layer morphology and interface, and processing techniques. The study stimulated that the OPV modules based on PTB7-Th: PC₇₁BM blend may be a promising candidate for the large area OSCs applications.

Experimental section

Materials

ITO coated glass substrates with a sheet resistance of 14 Ω/sq were purchased from Lumtec, Taiwan. Zinc acetate (99.99%), 2-methoxyethanol (99.8%), and ethanolamine (99.0%) were purchased from Sigma Aldrich. Poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b; 3,3-b]dithiophene]{3-fluoro-2[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7-Th) and [6, 6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) (PC₇₁BM) were purchased from 1-Materials, Canada. 1,8-Diiodooctane was purchased from Sigma-Aldrich. Chlorobenzene used as a solvent in the device fabrication process was purchased from Alfa Aesar. All these commercially available materials were used as received without further purification [27].

Modules design, fabrication, and characterization

Schematic of inverted ITO/ZnO/PTB7-Th: BC₇₁BM/MoO₃/Ag OSC device structure and energy band diagram shown in Fig. 1a and b, which is followed for the fabrication of OPV modules. Inverted structure of OPV modules are fabricated on 5 cm × 5 cm and 11 cm × 11 cm ITO coated glass substrate. Schematic representation of cross-sectional view as well as design parameters of the 5 cm × 5 cm OPV module shown in Fig. 2a and b, it comprises 5 cells connected in series. Each cell has included the size of 5 cm in width and 3.7 cm in length and each cell is parted by 0.1 cm space. The total active area is 9.25 cm². The OPV module is designed in series to obtain the output voltage. In a series of circuitry, each cell should be worked properly to complete the circuit of the module. Hereof, to get the maximum performance of the module film and interface uniformity of each cell is very important. Figure 2c shows the photograph of a 5 cm × 5 cm OPV module. Schematic representation of cross-sectional view as well as designs parameters of the 11 cm × 11 cm OPV module shown in Fig. 3a and b, it comprises 10 cells connected in series. One cell of the 10 cells have size of 0.9 cm in width and 10 cm in length and the other nine cells are comprised of a size of 0.6 cm in width and 10 cm in length and each cell is parted by 0.1 cm space. The total active area is 63 cm². Figure 3c and d shows the photograph of 11 cm × 11 cm OPV module and metal mask for making interconnection, respectively. These designs are selected to perform the study of the given device structure (Fig. 1a) with active layer PTB7-Th: PC₇₁BM. However, for commercial application purpose need to rethink for the design to get the maximum active area and performance of the module.

5 cm × 5 cm and 10 cm × 10 cm pieces of ITO coated glass substrate were cut from a large piece of substrate. First, ITO coated glass substrates were patterned using the photolithography technique. The patterned substrates were ultrasonically cleaned in a boiled soap solution for 10 min, followed by rinsing and ultrasonication in DI water, acetone, and isopropyl alcohol (IPA) for 10 min, respectively, followed by drying with nitrogen. Patterned substrates were UV Ozone (UVO) treated for ∼ 20 min. For the preparation of ZnO films appropriate solutions were spin coated onto patterned ITO coated UVO treated glass substrates for 60 s at 2000 rpm followed by thermal annealing at 250°C for 10 min. Organic photoactive layer solution was prepared by dissolving PTB7-Th (10 mg) and PC71BM (15 mg) in chlorobenzene (970 μl) and 1,8-Diiodooctance (30 μl). The solution was stirred at room temperature for 8 h and then filtered using of 0.22 μm polytetrafluoroethylene (PTFE) filter [27]. The deposition of active layer onto ZnO/ITO/glass substrate was performed by spin coating the solution at 2000 rpm for 15 sec (for the module size: 5 cm × 5 cm) and 800 rpm for 15 sec (for the module size: 11 cm × 11 cm) followed by thermal treatment on a hot plate at 40C for 2 h the in nitrogen glove box [27]. Finally, MoO₃ film of 10 nm thickness to act as hole transporting layer (HTL) and Ag layer of 100 nm thickness as top metal electrode were thermally evaporated under high vacuum, respectively. The estimated active area of devices as defined by the overlap of the anode and cathode was 9.25 cm² and 63 cm². The current (I) – Voltage (V) characteristics of the OPV modules were measured in air using TriSol^TM concentrated photovoltaic (CPV) solar simulator in the dark as well as under one sun illumination (simulated solar radiation of AM 1.5G spectrum) [27].

Results and discussion

The performance of OPV modules is affected by several factors such as high series resistance, effect of defective cells, and aperture ratio [30]. To examine the performance of organic photovoltaic (OPV) modules, we fabricated inverted structured modules with configuration Glass/ITO/ZnO/PTB7-Th: PC₇₁BM/MoO₃/Ag with device structure and energy band diagram [27] shown in Fig. 1a and b. The photovoltaic performance of OPV modules with two different active areas 9.25 cm² and 63 cm² comprising a total area of 25 cm² and 144 cm², under illumination of AM 1.5G, 100 mW cm⁻² and under dark is shown in Fig. 4a and b. The photovoltaic performance of OPV modules with varying active areas is summarized in Table 1. OPV modules with an active area of 9.25 cm² are characterized by the following average photovoltaic performance parameters (from three samples): short-circuit current density (J_sc) of 2.88 mA cm⁻², open circuit voltage (V_oc) of 2.85 V, fill factor (FF) of 0.30 and power conversion efficiency (PCE) of 2.53%. The OPV module with active area 63 cm² shows short-circuit current density (J_sc) of 0.87 mA cm⁻², an open-circuit voltage (V_oc) of 4.2 V, and a fill factor (FF) of 0.27, leading to PCE of 1.01%. As we observed (Table 1) the average photovoltaic performance of small active area (0.06 cm²) devices is comparable to the PTB7-Th: PC₇₁BM blend based organic photovoltaic devices [27]. However, when the active area scaled up ∼154 times (to 9.25 cm²) and 1050 times (to 63 cm²), there is notable decrease in the PCE of OPV modules leading to an average value of 2.53% and 1.01% (Table 1). These values are approximately 27% (for 9.25 cm²) and 10% (for 9.25 cm²) of PCE value corresponding to devices with 0.06 cm² active area (Fig. 5b). In the same way, the values of J_sc were reduced by approximately 15% and 4% (Fig. 5a), and FF values by approximately 49%, and 45%, respectively (Fig. 5b). The deficiency in PCE of OPV module with large active area is mainly concerned with the decrease in FF and J_sc. The main reason for the reduction of FF with increasing active area of the OPV modules is effective contribution of series resistance from ITO [31, 32]. However, the losses in current density (J_sc) are attributed to non-uniformity in morphology and thickness of the OPV modules with large area [32, 33]. V_oc is not the function of the active area of the devices. The V_oc of the OPV module is the addition of the subcells V_oc. On the contrary, the V_oc values of the increased number of subcells showed less advancement (Table 1 and Fig. 5a). The values of V_oc are related to the junction properties of the donor and acceptor layers, and the work function of cathode and anodes [34–36]. The present study noticed that controlling the series resistance, uniformity in morphology and thickness across the entire active area is crucial to retain the higher efficiency demonstrated by small area devices.

Conclusions

In conclusion, the photovoltaic performance of PTB7-Th: PC₇₁BM based large area OPV modules was studied when the active area was scaled from 0.06 cm² to 9.25 cm² and 63 cm². PCE of the 5-cell and 10-cell modules were 2.53% and 1.01% with active area 9.25 cm² and 63 cm². These values were 27% and 10% of PCE value corresponding to devices with 0.06 cm² active area. The present study noticed that controlling the series resistance, uniformity in the film thicknesses and morphology throughout the entire active area is crucial to retaining the higher efficiency demonstrated by small area devices. However, it is possible to reform preparation conditions and reduce series resistance from ITO to get better performance from PTB7-Th: PC₇₁BM based OPV modules.

Data availability

The data underlying this article are available in the article.

Authors’ contributions

Belal Usmani (Conceptualization [lead], Data curation [lead], Formal analysis [lead], Investigation [lead], Methodology [lead], Visualization [lead], Writing—original draft [lead], Writing—review and editing [lead]), Rahul Ranjan (Conceptualization [Supporting], Data curation [supporting], Formal analysis [supporting]), Raju Kumar Gupta (Resources [supporting], Supervision [Supporting]), and Ashish Garg (Conceptualization [lead], Resources [lead], Supervision [lead], Validation [lead], Writing—review and editing [supporting]).

Conflict of interest statement: the authors declare no conflict of interest.

Acknowledgements

Authors thank Department of Science and Technology for funding through India-UK APEX-II project, Newton Prize Funds, and EPSRC (UK) for funding through SUNRISE under GCRF call.

References