Applied Science and Convergence Technology 2021; 30(2): 41-44
Published online March 31, 2021
Copyright © The Korean Vacuum Society.
Ji Soo Gooa,∗
aAvionics R&D center, Hanwha Systems, Seongnam 13524, Republic of Korea
Correspondence to:E-mail: email@example.com
Indoor organic photovoltaics (OPVs) are drawing increasing attention due to their considerable potential for supplying energy to low-power devices under indoor lighting, and exhibit advantages such as changeable optical absorption and cost-effective fabrication. When designing indoor OPVs, the photovoltaic materials, electrodes, and interlayers should be developed considering the indoor lighting conditions, which differ from the outdoor conditions. In this review, design principles such as adoption of high shunt resistance and enhancement of the light absorption for the transparent conducting electrodes (TCEs) of indoor OPVs are suggested. Subsequently, an overview of the recent developments in the TCEs of OPVs under indoor lighting conditions is presented. Furthermore, the future implications of this research topic is discussed.
Keywords: Organic photovoltaic devices, Indoor lighting conditions, Transparent conducting electrode, Shunt resistance, Equivalent-circuit model
Recently, there has been an increasing interest in indoor energy harvesting systems with increasing use of small-scale electronic devices in residential applications, wearable electronic devices, and wireless sensor networks for Internet-of-Things (IoT) devices [1–3]. The replacement of chemical batteries and commercial electricity by the energy harvested from light, mechanical vibration, and heat have been extensively studied [4–6]. Among these energy sources, light and heat are preferred based of their performance in indoor applications. However, to obtain sufficient energy from a heat source, large thermal differences are necessary. Therefore, photovoltaic technology that converts light energy into electricity is more appropriate than others.
Organic photovoltaic (OPV) technology can be promising for in-door energy harvesting owing to its cost-effective fabrication, and optical properties such as high absorption coefficient and changeable optical bandgap. In addition, the advantages of OPVs including mechanical flexibility, light weight, environment friendly characteristics, and aesthetic advantages owing to the availability of various colors render them the preferred indoor energy harvester compared to the other sources [7–13]. In addition, recent reports indicate that OPVs have better photovoltaic performance compared to other photovoltaic technologies such as Si solar cells and copper-indium-gallium (di) selenide (CIGS) solar cells, as indoor light sources .
However, indoor and outdoor lighting conditions differ significantly in terms of the intensity and irradiation spectrum. The intensities of typical indoor lighting sources such as light emitting diode (LED), halogen light, and fluorescent light are only 0.05–1 % that of the out-door light. Moreover, the irradiation spectra of the indoor light differ from the solar spectrum, as depicted in Fig. 1. Therefore, different approaches are required while designing indoor OPVs .
In this study, we review the recent progress in the development of indoor OPVs, focusing on transparent conducting electrodes (TCEs) of the devices. Furthermore, after comparing the device performances under outdoor and indoor lighting conditions for different electrodes, key strategies for electrode selection to realize efficient indoor OPVs are discussed. Finally, the future implications on this research topic are discussed.
The typical OPV structures are displayed in Figs. 2(a) and 2(b). OPVs mainly comprise a donor:acceptor bulk-heterojunction (BHJ) sandwiched between two electrodes and two transport layers. When light passes through TCE, absorption occurs at BHJ and excitons are generated. The electrons and holes are separated from the excitons and transferred to the cathode and anode through the electron transport layer (ETL) and hole transport layer generating the required voltage and current [16–18].
To maximize OPV performance under indoor lighting conditions, approaches different from 1-sun illumination are required. For discussing the electrical properties, an organic solar cell can be approximated by an equivalent circuit [Fig. 2(c)] such as a current source (corresponding to the current density under illumination
The series resistance (
According to Eqs. (1) and (2), among the components of the circuit model,
As mentioned above, the electrode sheet resistances related to the
In 2019, Goo
The conductivity of PEDOT: PSS electrodes is insufficient for application in outdoor solar cells. However, Saeed
Interface engineering can be one of the strategies for indoor OPVs because the charge transport layer is directly related to the device
In addition, Shin
Furthermore, the series resistance effect for various ETLs was investigated by Shin
Another strategy to increase the efficiency of indoor OPVs is to enhance the light absorption. To maximize the absorption in the photoactive layer, the electrode must be designed considering the spectrum of indoor light. An oxide/metal/oxide structure can be utilized due to its electrical and optical advantages. ZnO/Ag/ZnO (ZAZ) electrodes are generally used as the conducting electrodes for various devices [27, 28]. Lee
Ultrathin ITO films can also be considered for application as TCEs of indoor OPVs . As the transmittance and electrical conductivity are inversely proportional, sufficient film thickness is required for use as electrodes, even at the expense of the transmittance. However, the filament-doping method permits ultrathin ITO films (10 nm) to have enhanced conductivity, while maintaining high transmittance (as shown in Fig. 5) and low surface roughness, resulting in high performance (11.1 %) under 500-lx LED conditions. Table 1 summarizes the overall photovoltaic properties of OPVs with various TCEs.
In this review, high-efficiency indoor OPVs with various types of TCEs were discussed. For low-light applications, the impact of the serial resistance is not drastic, and novel transparent electrode materials may be feasible. This property would not only contribute to the performance improvement of energy harvesting systems employing ambient energy to operate indoor electronic devices, but also lower the cost of OPVs by the usage of noble materials such as graphene or nanofibers. For outdoor applications, highly conductive electrode materials remain a requisite.
However, considerable progress needs to be made prior to large-scale manufacturing of indoor OPVs. Consequently, there is a need to increase the device efficiency, improve device life, and reduce the device cost. In addition, it is important to advance module efficiencies by utilizing roll-to-roll processing or employing new designs that can reduce electrical losses. Furthermore, the OPV stability needs to be improved; this will require deeper understanding of the interactions between all the interfaces of a given cell structure, as well as improvement of both the material and device structure for oxygen and water stability, for commercial use. In order to identify materials that can achieve the above goals, large-scale production of such materials with limited batch-to-batch variation is required, and these materials need to be utilized in high-throughput processes. Currently, OPVs are applied only in niche markets that require lightweight, flexibility, and variable angle performance. However, continuous development of indoor applications will lead to the extensive application of OPV technologies in next generation devices.