The power conversion efficiency of a solar cell corresponds directly to the power per unit area, and is the primary metric for describing solar cell performance. However, area is not always the limiting parameter. When considering solar power for certain applications, the power output per weight (specific weight in Watts per gram) may be a more appropriate metric. This is particularly true for situations that require portable power, where payload is a premium such as aircraft, spacecraft or personal pack load. The specific weight of various solar technologies (panels and cells) for terrestrial and space applications is compared in . Commercially available space-rated high-efficiency Si and triple junction cells have 0.82 and 0.39 W g−1
. A recent report showed that CIGS cells were constructed on thin (25 μm) polyimide films with 18.7% efficiency18
. These devices have an extremely high specific weight of roughly 3 W g−1
. The OPV devices constructed on PET presented here have a per-area mass of 4 g m−2
, and 4% efficiency, giving 10 W g−1
Comparing solar cell technologies.
This extreme specific weight may be advantageous for certain applications. Weather balloons, unmanned aircraft or any other remote sensing systems may prioritize specific weight and have known project duration. The same is true for pack-weight for remote wilderness use, where there is often a need for lightweight power sources. Modern robotics applications, such as electroadhesive wall climbing and at-scale robotic insects, have very specific needs for low-weight power19
. Ultrathin OPV may be ideal for such applications. These particular materials are not comparable to the most efficient or air-stable organic semiconductors, inter-layers or electrodes used in state-of-the-art OPV devices6
; so there is a large potential for optimization by enhancing both efficiency and lifetime of such ultrathin OPV devices while maintaining low weight and high flexibility.
In conclusion, polymer-based solar cells have been constructed on 1.4-μm-thick PET substrate. The devices use standard OPV materials and obtain nearly identical efficiency to those constructed on ITO-coated glass substrates with over 4% power conversion efficiency. Here, the high conductivity PEDOT:PSS electrode replaces the ITO, which is considered a cost- and production-volume-limiting material13
. The polymer electrode is highly flexible compared with the ternary oxide22
. The device layers consist of roughly 450 nm total thickness, so the total device including substrate is <2 μm.
These ultrathin, flexible solar cells show unprecedented mechanical resilience. By adhering the flexible device to a pre-stretched elastomer, we have shown that the devices survive quasi-linear compression to below 70% of their original area. Furthermore, we have shown cyclic compression and stretching to 50%, over more than 20 full cycles with marginal loss in device performance and no visible defect formation beyond the external contact points.
Though the electrically active components of this device are in principle no different than other standard OPV devices, the solar cell now constitutes roughly 25% of the total thickness and 45% of the total weight. OPV is emerging as a viable technology where temporary, high-volume and low-weight power sources are required. The extreme flexibility and specific weight demonstrated here realize the unique potential for OPV as an ultrathin-film power source.