Ultrathin and lightweight organic solar cells with high flexibility

Abstract

Application-specific requirements for future lighting, displays and photovoltaics will include large-area, low-weight and mechanical resilience for dual-purpose uses such as electronic skin, textiles and surface conforming foils. Here we demonstrate polymer-based photovoltaic devices on plastic foil substrates less than 2 μm thick, with equal power conversion efficiency to their glass-based counterparts. They can reversibly withstand extreme mechanical deformation and have unprecedented solar cell-specific weight. Instead of a single bend, we form a random network of folds within the device area. The processing methods are standard, so the same weight and flexibility should be achievable in light emitting diodes, capacitors and transistors to fully realize ultrathin organic electronics. These ultrathin organic solar cells are over ten times thinner, lighter and more flexible than any other solar cell of any technology to date.

Figure 1. Sub-2-μm-thick organic solar cells.

(a) Scheme of the ultra-light and flexible organic solar cell. The layer thicknesses are drawn to scale. (b) Extreme bending flexibility demonstrated by wrapping a solar cell around a 35-μm-radius human hair. Scale bar (also in c,d) 2 mm. (c) Stretchable solar cells made by attaching the ultrathin solar cell to a pre-stretched elastomer. They are shown flat (left) and at 30% (middle) and 50% (right) quasi-linear compression. (d) The device attached to the elastomeric support, under three-dimensional deformation by pressure from a 1.5 mm-diameter plastic tube. (e) SEM image of the PET surface of the solar cell in compressed state. The radius of curvature for the shown wrinkles is estimated to be on the order of 10 μm. Scale bar 500 μm.

Figure 2. AFM images of the individual device layers.

AFM topographical image of the 1.4 μm Mylar CW02 substrate foil on glass/PDMS support (left). The RMS roughness is ≈12 nm with a maximum peak-to-peak distance of 162 nm. Image of the PEDOT:PSS transparent electrode on PET (centre). RMS roughness is ≈12 nm and the maximum peak-to-peak distance is 116 nm. Image of the P3HT:PCBM active layer atop the transparent electrode (right). The RMS roughness is ≈16 nm and the maximum peak-to-peak distance is 130 nm. Scale bar 1 μm.

Figure 3. J–V characteristics.

(a) Linear–linear (top) and log-linear (bottom) J–V characteristics of an optimized ultrathin OPV device on 1.4 μm PET substrate. The device performance metrics are V_OC=580 mV, J_SC=11.9 mA cm⁻², FF=61% and η=4.2%. Solid lines are measured under one sun illumination and dashed lines in the dark (also in b,c). (b) Device performance of reference device, using the same materials but constructed directly on glass substrates without PET. The device performance metrics are V_OC=565 mV, J_SC=11.9 mA cm⁻², FF=61% and η=4.1%. Comparing the J–V curves of a and b, nearly identical behaviour is observed. The V_OC of this particular device is slightly lower, but overall the performance of the glass and PET-based solar cells is indistinguishable. (c) Ultrathin OPV devices on PET foil with underlying silver grid can attain FF up to 71% (top) and dark diode rectification of up to 6 orders of magnitude at ±1 V (bottom). However, they typically show lower J_SC due to shading from the silver grid.

Figure 4. Extreme mechanical deformation.

(a) I-V curves during compression on pre-stretched elastomer. The solid black line represents 0% and the purple line represents 80% compression with the intermediate steps of 10% distributed between accordingly. The dashed black line represents the device after re-stretching from 80% compression to its initial flat state. (b) Measured device parameters 0–80% compression, which correspond to 400–0% tensile strain or five times change in size. Green downward triangles represent FF, blue upward triangles V_OC, red circles I_SC and black diamonds power. All parameters are normalized to their pre-compression value. The I_SC and correspondingly the total maximum power of the solar cell decrease, but maintain higher values than would be expected with the corresponding decrease of device area, represented by the black line. Upon re-stretching the device to its original size, the values are nearly identical to the pre-compression state. The symbol size is chosen to represent the reproducibility of the experiment, as indicated by error bars for selected data points for the power metric.

Figure 5. Biaxial compression.

(a) The device performance under reversible biaxial compression of 0% (black solid line), roughly 10% (green solid line) and roughly 20% (red solid line). Upon re-stretching to near flat conditions, the device shows comparable performance (black dashed line). (b) Device performance under very high biaxial compression of more than 50% (red solid line) compared with 0% flat state (black solid line).

Figure 6. Resilience to cyclic compression–stretching.

Device performance during 22 consecutive compression–stretch cycles to 50% compression. (a) The I-V characteristics are shown at 1 (black), 11 (red) and 22 (blue) cycles for both the fully extended and 50% (quasi-linear) compressed states. (b) Device performance metrics plotted at each of the 22 cycles for the fully extended state. Green downward triangles represent FF, blue upward triangles V_OC, red circles I_SC and black diamonds power. All parameters are normalized to their initial value. Upon cycling, a reduction of the FF and I_SC is observed. The origin of these losses is unknown, but is certainly in part owing to the experimental apparatus and loss of contact. The overall efficiency is shown to be 73% of its original value after 22 repeated cycles. Multiple separate devices were subjected to cyclic testing. For the sake of clarity, error bars are indicated only for the power metric for selected data points, as this is the primary metric for device characterization.

Figure 7. Comparing solar cell technologies.

Specific weight of various photovoltaic technologies for complete modules (left) and individual cells (right). Module data are taken from manufacturer-specified efficiency and weight of commercially available panels and should be representative: Kaneka 12V 55W PV Module, First Solar FS272, Sunforce 70W Pro Series, Bosch Solar Module c-Si M60 and Kyocera KD245Gx-LPB. Cell data were taken from published academic results. In many cases, the cell weight was not published and so was estimated from published layer thicknesses and known material densities, assuming 2% areal coverage for metal grid contacts. For a direct comparison of cell efficiency, please see Green et al.