A. Motivation

Research students: Jingchuan Wang

S-shape properties in the J-V curves of bilayer heterojunction organic solar cells

Acknowledgements

This work is supported by research grants (Grant No. A-PJ28 and A-PJ73) from Hong Kong Polytechnic University. Funding from HKSAR through UGC grant (Grant No. PolyU 5112/08E) is also acknowledged.

E. Conclusion

References [1] AMPS-1D approach is developed by Prof. S. J. Fonash and his students at The Pennsylvania State University. [2] A. Wagenpfahl, et, al, Phys. Rev. B 82, 115306 (2010) [3] P. Peumans et al, Appl. Phys. Lett. 79, 126 (2001) [4]C. Alexander, et al, Appl. Phys. Lett. 89, 203506 (2006) [5]H. Jin, et al J. Phys. Chem. C 2009, 113, 16807–16810 [6]R. Prakash, et al, Appl. Phys. Lett. 93, 033315 (2008) [7]B. Fan et al, Org. Electron. 11 (2010) 583–588 [8]B. Anthony, et al, Nano. Let. 8 (2008) [9]I. A.Levitsky, et al, Appl. Phys. Lett. 85 (2004)

The origin of S-shape in J-V curves was investigated by inducing it experimentally and theoretically. The charge accumulation induced S-shape J-V can be viewed as a interfacial recombination modulation’s result.

Device structure:

Devices were fabricated by thermal evaporation under < torr6101 −×

(i)  Charge accumulation at the C60 and BCP interface

(ii)  Gaussian distributed gap states in the BCP, àpinning of Fermi level.

D. Simulation of S-shape effect

1φ

EgA

EgD

Eg

2φ

light anode HOMOD

LUMOD

LUMOA

HOMOA

cathode

Gap states

e

h

(i) Band diagram of the simulation model

(ii) Parameters used in simulation

S-shape appearance Dipole Electrode

degradation Injection barrier

Charge accumulation

Polymer BHJ √ √ √ √ √

Small molecule

PHJ √ √ √ ?

DSSC √ √

The reduction of FF will significantly drop the device efficiency ! ! !

(ii) Appearance of S-shape in OPVs and possible mechanisms

The exact mechanism in PHJ device is still unclear ! ! !

B. Device structure

Devices were fabricated by thermal evaporation under < 1×10-6 torr

(i) S-shape effect on fill factor (FF)

“S-shape” effect frequently appears in the J-V curves, which reduces the fill factor (FF) significantly, specially in tandem structure due to incomplete recombination of charge accumulation.

Investigate the S-shape effect in the J-V curve due to BCP layer

Bathocuproine (BCP)

Exciton blocking layer

FF will be remarkably reduced by the S-shape effect

C. Open circuit voltage and fill factor varies with BPC thickness

Voc increases, WHY???

Band diagram

Voc increases----reasons

Buffer layer thickness à trap state recombination

Buffer layer

By considering the direct and indirect recombination, the S-shape J-V curves can be simulated by increasing the buffer layer thickness.

Buffer layer thickness

Research student: Jingchuan Wang

Short Circuit Current Improvement in Planar Heterojunction Organic Solar Cells by Multijunction Charge Transfer

Acknowledgements

This work is supported by research grants (Grant No. A-PJ28 and A-PJ73) from Hong Kong Polytechnic University. Funding from HKSAR through UGC grant (Grant No. PolyU 5112/08E) is also acknowledged.

E. Conclusion

References

D. Mechanism of the efficiency enhancement

B. Schematic of device structure, energy diagram and performance

A. Motivation

C. Absorption and EQE measurement

Extra absorption in the 340-440nm contributes to EQE enhancement à charge generation !!!

Different from the previous findings related to Förster resonant energy transfer due to strong overlapping of emission and absorption, the enhancement in our multijunction devices is correlated to extra free charges generation and transfer. Our findings show that an alternative method to enhance solar spectra absorption by carefully designed band alignment for efficient charge transport throughout device.

1. J. Y. Kim; et.al Science 2007, 317, 222. 2. N. Lopez; et.al Phys. Rev. Lett. 2000,85, 1552 3.Liang, Y; et.al, Advanced Materials 2010, 22,135. 4.Hardin, B. E.; et.al, Nat Photon 2009, 3, (7), 406 5.Driscoll, K; et.al,Nano Letters 2010 10, (12), 4981 6. Mor, G. K ; et.al. Nano Letters 2010 10, (7), 2387 7.Jadhav, P; et.al. Nano Letters, 2011, (11), 1495

Enhance the light absorption, other than using the tandem and multiband structures. (i) Tandem (ii) Multiband

SubPc can quench the excitons generated by TTPA efficiently

Both enhancements are attributed to the Voc improvement. In our case, we design a multijunction for the the short circuit current density(Jsc) improvement in planar organic solar cells.

The bilayer OPV formed by TTPA/SubPc only!!!

The low overlap between the emission of TTPA and absorption of SubPc limits Förster energy transfer (FRET)

The multijunction (TTPA/SubPc) and (SPbPc/C60), both generate free charges à JSC increases

Enhanced optical absorption from 340nm to 440nm, contributed by the absorption of TTPA