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# Intrinsically stable organic solar cells under high-intensity illumination

## Abstract

Organic photovoltaic cells are now approaching commercially viable efficiencies, particularly for applications that make use of their unique potential for flexibility and semitransparency1,2,3. However, their reliability remains a major concern, as even the most stable devices reported so far degrade within only a few years4,5,6,7,8. This has led to the belief that short operational lifetimes are an intrinsic disadvantage of devices that are fabricated using weakly bonded organic materials—an idea that persists despite the rapid growth and acceptance of organic light-emitting devices, which can achieve lifetimes of several million hours9. Here we study an extremely stable class of thermally evaporated single-junction organic photovoltaic cells. We accelerated the ageing process by exposing the packaged cells to white-light illumination intensities of up to 37 Suns. The cells maintained more than 87 per cent of their starting efficiency after exposure for more than 68 days. The degradation rate increases superlinearly with intensity, leading to an extrapolated intrinsic lifetime, T80, of more than 4.9 × 107 hours, where T80 is the time taken for the power conversion efficiency to decrease to 80 per cent of its initial value. This is equivalent to 27,000 years outdoors. Additionally, we subjected a second group of organic photovoltaic cells to 20 Suns of ultraviolet illumination (centred at 365 nanometres) for 848 hours, a dose that would take 1.7 × 104 hours (9.3 years) to accumulate outdoors. No efficiency loss was observed over the duration of the test. Overall, we find that organic solar cells packaged in an inert atmosphere can be extremely stable, which is promising for their future use as a practical energy-generation technology.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

This work was supported by the United States Department of Energy SunShot Program under awards DE-EE0006708 and DE-EE0005310 and the Department of the Navy, Office of Naval Research under award N00014-17-1-2211.

## Author information

Authors

### Contributions

Q.B. constructed the high-intensity LED ageing setup and designed experimental protocols. Q.B. and X.H. fabricated all samples, performed all measurements, and analysed and fitted the data. X.L. assisted with electroluminescence measurements and performed the transient photoluminescence spectroscopy. C.J. collected the mass spectra on organic films. C.C. and X.H. constructed the ultraviolet ageing system and C.C. performed the error analysis of the acceleration factor. S.R.F. supervised the project and analysed the data. The manuscript was written by Q.B., X.H. and S.R.F., and edited by all co-authors.

### Corresponding author

Correspondence to Stephen R. Forrest.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data figures and tables

### Extended Data Fig. 1 Spectrum of an Xe-arc solar simulator lamp over time.

Spectrum (left axis) of the Xe-arc lamp used for 1-Sun ageing of the OPVs, both before (black) and after (red) operating continuously for 2,000 h. EQEs (right axis) of the Si reference cell (blue) used for calibrating the intensity of the Xe-arc lamp intensity, and the EQE of a DBP:C70 OPV cell (pink). Previously, we reported the operational stability of DBP:C70 OPV cells with an identical structure to those studied here, and found that JSC degraded linearly with time when measured in situ under continuous illumination from a Xe-arc lamp7. The spectrum of the Xe-arc lamp red-shifts with time as shown, leading to an underestimation of the lifetime of the OPV cells. In this study, the EQE and current–voltage characteristics were measured between solar-ageing periods to avoid this error.

### Extended Data Fig. 2 Charge-extraction efficiency and wavelength-dependent EQE spectra.

a, Charge-extraction efficiency at V = 0 (extracted from the reverse-bias photocurrent data, as described in Methods) plotted against time. b, Wavelength-dependent EQE spectra after 47 days of ageing at 9.5, 20 and 37 Suns. The inset shows the change in EQE after 47 days; that is, ΔEQE = [EQE(t = 47 days) − EQEfresh]/EQEfresh.

### Extended Data Fig. 3 High-intensity LED ageing system and performance characterization.

a, b, Side (a) and top (b) views of the water-cooled white-LED setup used to supply high-intensity illumination to the OPVs. Each of the four stations consists of a cool-white LED (colour temperature = 5,000 K) with a heat sink and active water cooling of an Al block attached to the substrate. Cylindrical Ag-coated light pipes are used to collimate the LED light onto the OPV, shown detail in c. Despite the reflectivity of the Ag coating being greater than 99%, the OPV and its reflections are visible through the pipe owing to the extreme light intensity within. d, The spectrum of the white LEDs, ultraviolet LED and Xe-arc lamp measured at the surface of the OPV, compared with the AM1.5G spectrum. e, Surface temperature of the OPV cells plotted against intensity, across the range of white LED output intensities. The three ageing conditions used for OPV cells and thin films are indicated on the plot.

### Extended Data Fig. 4 Current–voltage characteristics of aged OPVs.

Dark current (Jdark, left axis) and current under 1-Sun illumination (J1-Sun, right axis) as a function of applied voltage for an OPV before and after ageing for 47 days under illumination at 37 Suns.

### Extended Data Fig. 5 Correlation between degradation of VOC and decrease in electroluminescence intensity.

To quantify the effect of recombination on the performance of the device, we express the change in VOC as: $$\Delta {V}_{{\rm{OC}}}={V}_{{\rm{OC}}}\left(0\right)-{V}_{{\rm{OC}}}\left(t\right)=\frac{{k}_{{\rm{B}}}T}{q}{\rm{ln}}\left(\frac{{J}_{{\rm{SC}}}\left(0\right)}{{J}_{{\rm{SC}}}\left(t\right)}\frac{{{\rm{EQE}}}_{{\rm{EL}}}\left(t\right)}{{{\rm{EQE}}}_{{\rm{EL}}}\left(0\right)}\frac{\int {{\rm{EQE}}}_{{\rm{PV}}}\left(t\right){\Phi }_{{\rm{BB}}}{\rm{d}}E}{\int {{\rm{EQE}}}_{{\rm{PV}}}\left(0\right){\Phi }_{{\rm{BB}}}{\rm{d}}E}\right)$$ where kB is Boltzmann’s constant, T is the cell temperature, q is the charge of an electron, EQEEL is the electroluminescence quantum efficiency of the charge-transfer states, EQEPV is the photovoltaic quantum efficiency, ΦBB is the black-body flux incident on the device, and E is the photon energy22. Predicted values of ΔVOC calculated using the above equation are plotted as a function of time along with the measured change in VOC. For the above equation to accurately predict the change in VOC, each of the parameters should be measured under an illumination intensity of 1 Sun. Because EQEEL was measured in the dark at a constant current density of 4 mA cm−2 rather than at JSC (that is, under 1-Sun illumination), the absolute value of the prediction from the above equation differs from the measured ΔVOC by approximately a factor of 2. Nevertheless, there is a qualitative match between the degradation time constants of the predicted and measured ΔVOC, which suggests that EQEEL measurements are a useful test of recombination and voltage loss in photovoltaic-degradation studies.

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Burlingame, Q., Huang, X., Liu, X. et al. Intrinsically stable organic solar cells under high-intensity illumination. Nature 573, 394–397 (2019). https://doi.org/10.1038/s41586-019-1544-1

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