High efficiency polymer photodiodes

ELSEVIER

Synthetic

Metals

102 (1999)

957-958

High Efficiency Polymer Photodiodes Magnus Granstrom, Klaus Petritsch, Ana Claudia Arias, Richard H. Friend Cavendish Laboratory,

Dept. of Physics, University of Cambridge, Madingley

Road, Cambridge

CB3 OHE, U.K.

Abstract Organic polymer photodiodes have been fabricated using different poly(thiophene) and poly(phenylene vinylene), a broad wavelength quantum yield of 4.5% is achieved under short-circuit conditions, found for double layer devices, having a spectral response covering quantum yield at short circuit and reaching almost 60% at -2V bias.

device designs, and by the use of blends between derivatives of response is accomplished. For single layer devices, an external increasing to 28% at 2V negative bias. Even better results are the range from 3.50 nm to 750 nm, and exhibiting 28% external

Keywords: Poly(phenylene

and derivatives, Solar cells, Interface preparation.

vinylene) and derivatives, Polythiophene

1. Introduction Polymer based photodiodes use the photovoltaic effect, by which electrons and holes are produced and subsequently collected at electrodes when a semiconductor device is illuminated. With organic materials the absorption creates bound electron-hole pairs (excitons). These can be dissociated by using a combinations of materials, one having a small ionisation potential and good hole transport properties and the other having a large electron affinity acts as an electron transport material. This route has proven successful for organic photovoltaic devices, both as multilayer devices [l] and single-layer blend devices [2, 31. If this combination of materials can be done so that a percolation path for the charge carrier is created, the efficiency of the devices is increased dramatically [4].

MEH-CN-PPV

(MCP)

POPT

Fig. 1. Chemical structures of the polymers. Glass substrate /I

2. Experimental The polymers used are shown in Figure 1. The cyan0 derivative of poly(phenylene vinylene), MEH-CN-PPV [5], is used as electron acceptor, the octyl-phenyl polythiophene derivative, POPT [6], as hole acceptor and poly(ethylene dioxythiophene), PEDOT, as electrode material. MEH-CNPPV and POPT are both soluble in chloroform and toluene, and can therefore be mixed easily in solution in order to make blends. The diodes were made in a laminated structure. One layer comprised an MEH-CN-PPV-rich film spin-coated onto the calcium-coated glass and the other layer consisted of a POPT-rich film on a glass substrate covered with a PEDOT film on top of a thin (< 10 nm) gold layer. The two polymer covered substrates were laminated together at an elevated temperature (approx 200 “C). This temperature is also needed to obtain the desired, red-absorbing, form of POPT 171.

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S.A.

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PEDOT

Electron acceptor layer

PEDOTIPSS on Au

Glass substrate

Fig. 2. Device structure reserved.

/l

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M. Granstrtim

et al. I Synthetic

Metals

102 (1999)

957-958

3. Results and discussion The laminated double layer arrangement shown in Figure 2, using this specific combination of polymers makes it possible to reach the desired coverage of the visible spectrum. Also, it allows us to ensure simultaneously that the electronacceptor material is in contact with the low work-function contact and the hole-acceptor is in contact with the high workfunction contact. Laminated devices made from homolayers showed good spectral response, but not particularly high efficiencies. However, the addition of a small amount of ‘the opposite’ polymer (POPT into the MEH-CN-PPV layer and vice versa) resulted in a drastic increase in the efficiency. The external quantum efficiency (EQE) as a function of wavelength for such a device is shown in Figure 3, together with the absorption spectrum. That the curves follow each other rather well suggests that there is no inactive layer acting as an efficiency decreasing optical filter. 30g 6 .-5 .o

zo-

Ti E 2

15-

-1.6

25-

P 2 -,g 2

-1.4 -1.2

=

z 2. -* c. m 2 z 0 m

-1 -0.6

2 g m E 0 z w

D

-1.6

IO-

-0.6 -0.4

5

5 -0.2 0

I 400

I 450

I 500

I 550

Wavelength

I 600

I 650

I 700

I 750

2

800

(nm)

Fig. 3. Optical absorption of a POPT/MEH-CN-PPV double layer (- - -) and external quantum efficiency, EQE, of an Au/PEDOT/POPT:MEH-CN-PPV (19: l)/ MEH-CN-PPV:POPT (19: l)/Ca device (-). The devices clearly show a diode behaviour with a rectification ratio (in darkness) of more than lo3 at +1.5 V and a strong photocurrent response. The external quantum efficiency has a peak value of 29% at 480 nm excitation with no applied bias, increasing to more than 60% at -2 V bias. The fill factor is in the range between 0.3 and 0.35 and the power conversion efficiency at the optimum wavelength (480 nm) is 4.8%. The dependence of the open-circuit voltage and shortcircuit current on the intensity of incident light (L) is shown in Figure 4. The short-circuit current scales almost linearly with intensity (I,, = L’.‘*) up to the highest value measured here (100 mA/cm2), whereas the open-circuit voltage increases sublinearly with intensity, saturating when the value approaches 2.2 V, which is close to the estimated difference in the electrode work functions. From the results outlined above, a power conversion efficiency for a simulated solar spectrum AM 1.5 can be calculated, giving a value of 1.9%.

Incident light intensity (mW/cm’) Fig.4. Dependence of open-circuit voltage (open circles) and short-circuit current density (tilled circles) on incident light intensity (488 nm). We have used phase-detection tapping-mode scanning force microscopy to investigate the structure of the devices. Such investigations show that there is interpenetration between the two layers following the lamination and annealing procedure, with a length scale of 20-30 nm. Ongoing investigations also indicate that there is interpenetration between the two polymers on an even smaller scale, well within the exciton diffusion range [8]. We believe that this, together with the fact that this device design used here ensures that the each polymer is in contact with the desired electrode explains why these devices show such high efficiencies compared to previous polymer based photovoltaic cells. 4. Acknowledgements Financial support from the Engineering and Physical Sciences Research Council, the European Commission (TMR Marie Curie Fellowship and TMR Network SELOA), and from CNPq, Brazilian Government is gratefully acknowledged. 5. References

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