1. Geometrical Structure of Fabricated Inverted OPVs

Fig. 1 shows the (a) geometrical structure, (b) field emission scanning electron microscopy (FE-SEM, Zeiss Ultra Plus), and (c) developed solution-processed large-area inverted OPVs on a flexible substrate (PEN). The inverted structure has an anode electrode on top and a cathode electrode on the bottom. To determine the feasibility of low-cost mass production, the reason for the adoption of the inverted structure was to fabricate OPVs under ambient air conditions, not inside a N$_{2}$ filled glove box. This is because inverted OPVs have been reported for high stability under air ambient than conventional OPVs ⁽¹²⁾. Moreover, as HTL and anode electrode are intimate in inverted OPV, so realization of all-solution process could be possible by replacing HTL and anode electrode to aqueous materials such as conductive polymer ^(7,8).

The thicknesses of the cathode and anode were 150 and 228 nm, respectively, with semiconducting materials having a thickness of 358 nm. It should be noted that all processes except the anode electrode were applied from a solution with a low temperature of below 160 $^{\circ}$C, which also opens the possibility of a low thermal budget production, such as a roll-to-roll process, for this type of OPV fabrication. Note that the sample for the FE-SEM image was fabricated onto a rigid glass substrate because of the clear FE-SEM image.

Fig. 2 shows an atomic force microscopy (AFM, Bruker Dimension Icon) image of the final OPVs fabricated on glass and PEN substrates. The average roughness (Ra) of the OPV with a PEN substrate was 13.8 nm, whereas that with a glass substrate was 6.3 nm. As expected, the Ra of the OPV with the PEN substrate was higher because the rigid substrate induced an even spread of the liquid solution and easy handling.

2. Characteristic Parameters of Scalable Flexible Large-area Inverted Organic Photovoltaic Cells

This section evaluates the scalability of the developed solution-process flexible inverted OPVs. In addition to the PCE, the area of the solar cell is critical for its power production. One technique that cannot scale-up up to a PV module of a certain area cannot be chosen by the customer. Therefore, in this section, OPVs with different areas (0.04–1.6 cm$^{2}$) were fabricated on glass and PEN substrates to account for the impact of the area of the OPV and the rigidity of the substrate. Fig. 3 shows the normalized (a) open-circuit voltage (V$_{\mathrm{oc}}$), (b) short-circuit current density (J$_{\mathrm{sc}}$), (c) fill factor (FF), and (d) photoconversion efficiency (PCE) of the OPV with a glass or PEN substrate as a function of its area. It was found that V$_{\mathrm{oc}}$ has no significant difference for a different solar cell area. A slight degradation of the J$_{\mathrm{sc}}$ may come from the less even spread of the solution materials for larger areas owing to the limitations of the spin-coating.

A significant degradation for higher area solar cells was mainly due to FF degradation. The FF of the solar cell can be defined as Eq.(1). Where FF$_{o}$ is the ideal fill factors which does not account for parasitic resistance, v$_{oc}$ is a normalized voltage calculated from the measured V$_{\mathrm{oc}}$, and r$_{s}$ is a normalized resistance calculated from the measured R$_{\mathrm{se}}$ (series resistance), V$_{\mathrm{oc}}$, and J$_{\mathrm{sc}}$ ⁽¹³⁾. The R$_{\mathrm{se}}$ is mainly dependent on the sheet resistance (R$_{\mathrm{sh}}$) of the anode and cathode electrodes and the area of the solar cell, as shown in Eq.(2)⁽¹⁴⁾. As the thickness of the organic layer (t$_{org}$) was few hundred nanometers, so other series resistance components $\left(\rho _{org}\cdot t_{org}+r_{int}\right)$ can be negligible. Therefore, R$_{\mathrm{se}}$ will be almost double when the area is double and will degrade the corresponding FF of the solar cells. It should be noted that $\rho _{org}$ and $t_{org}$ are the resistivity and thickness of the organic bulk materials, respectively, $r_{int}$ is the interface resistance. From the numerical calculations, more 1cm$^{2}$ area degrades absolute FF of ~11% and ~15% for OPV with glass and PEN substrates. As determined experimentally, an absolute FF of 10.1% occurred as the area of the solar cell increased from 0.04 to 1.0 cm$^{2}$.

Fig. 4 shows the calculated FF as a function of the solar cell area with experimental data. Numerical calculations were conducted using Eq.(1) and (2) with input from the experimental data. It can be seen clearly that the numerical calculations matched the experimental data for both the OPV with glass and PEN substrates. Fig. 4 also provides a road map for the solar cell performance for various areas. The performance of a solar cell when scaling up from its initial performance can be anticipated. For example, if OPV area is increased from 1.0 to 3.0cm$^{2}$, FF will degrade from 52% to 30% and corresponding PCE will degrade from 2.0% to 1.2%.

Table 1 summarizes the characteristic parameters of a solar cell, and Fig. 5 shows the I-V curve of the best OPV with different areas and substrates. As the area of the OPV increased from 1.0 to 1.6 cm$^{2}$, the FF decreased from 43.2% to 38.9%, and the corresponding PCE decreased from 2.0% to 1.8%, for the PEN substrate. For reference, OPV on a glass substrate with an area of 1.6 cm$^{2}$ provided an FF of 44.0% and a PCE of 2.0%. From this result, we can conclude that a PCE degradation for higher areas is mostly from an FF degradation. Therefore, from the numerical calculations in Fig. 4, we can achieve an FF of 31.3% and PCE of 1.4% when the area increased to 2.0 cm$^{2}$. In addition, as shown in Fig. 5, the generated power of the higher area produced a higher energy proportional to its area.