Munira Raja. Organic Electronics Group Department of Electrical Engineering & Electronics.

Transcription

1 MOS-AK/GSA ESSDERC/ESSCIRC Workshop, Bordeaux, 21 st Sept 2012 Munira Raja Organic Electronics Group Department of Electrical Engineering & Electronics

2 Motivation Organic Electronics is rapidly emerging technology for flexible, high volume and low-cost systems i.e. RFID tags, Displays, Smart sensors (e.g. SIMS) SIMS is focused in detection of cholesterol levels in human blood however will provide platform in industrial and environmental testing SIMS integrates nanosensor, organic circuitry, printed battery and printed display on a same flexible substrate Biosensor Battery Display Circuit

3 Polycrystalline - Background theory Polycrystalline consists of arrays of ordered grains separated by disordered grain boundaries Grain (of width 2a) is treated as a single crystal i.e. E F moves freely compared to boundaries Grain boundaries comprise of large density of traps which results in pinning of the E F thereby limiting the conduction The traps are assumed to comprise of density of states (DOS) similar to that of disordered material, which follows an exponential distribution The edge of E LUMO (assuming n-type) dips at centre and extent of the dip depends on relative size of effective Debye length 3 (intrinsic material) or depletion region (doped material). At minimal point, the concentration of electron is at the highest 3 M. Raja and W. Eccleston, J. Appl. Phys. 110, , (2011)

4 Diffusion mechanism Diffusion mechanism is dominant at low voltages There is no potential drop between adjacent grain boundaries and/or across the channel i.e. between the source and drain contacts The energy difference between E LUMO minimum and E F changes between adjacent grains Carrier density decreases at grain centres down the channel n 1 > n 2 > n 3 S O U R C E n 1 n 2 n 3 D R A I N 2a 2a 2a

5 Drift mechanism Drift mechanism is dominant at higher voltages There is a net potential drop between adjacent grains (i.e. V = 2aF xmean ) however no change in the position of E F with respect to E LUMO minimum Carrier density is constant (at grain centre) Conduction down the channel is enhanced due to lowering of the barriers as a result of an applied drain voltage S O U R C E n n n D R A I N (x-2a) (x) (x+2a)

6 Diffusion and Drift mechanisms Upon application of gate and drain biases, the various fluxes F X flowing across the grain boundaries are determined, x S O U R C E z D R A I N (x-2a) (x) (x+2a) The resultant total fluxes at equilibrium is found to follow: diffusion (F diffusion ) drift (F drift )

7 Drain Current models Currents expressions are obtained by integrating individual fluxes over the depth of channel (in z-direction) so as to include total accumulated charge: The expression for field strength F Z is obtained by solving Poisson s equation as: Assuming the density of traps at the grain boundary to be given as: Where T C is characteristic temperature associated with degree of disorder

8 Drain Current models Consequently, the drain current expressions 4 in terms of applied gate and drain voltages under diffusion and drift are given as: K diffusion K drift Where C ox is gate capacitance, W and L are channel width and length, a is half grain size, ν is frequency of attempted jumps, N o is density of traps at boundary, ε b is relative permittivity of OSC, T c characteristics temperature and c = 4T c /T 1 (in diffusion) and 2T c /T 1 (in drift) 4 M. Raja et. al, in Print JAP

9 Parameter Extraction (I) For validation of the model, respective parameters i.e. K drift, K diffusion, c drift and c diffusion need to be extracted from experimental data i.e. sub-threshold plots of TIPS OTFT. This initially requires to extract threshold voltages V T Respective values of V T are extracted in: i. Diffusion Region Requires multiple transfer plots at low V D, with no dependency on V G ii. Drift Region In this region, drift current dominates. More dependency on V G i.e. transfer plots begin to separate Drain Current, I D (A) 10-5 Drift W = 2 mm, L = 100 µm V D = - 1V V D = - 5V V D = - 40V Diffusion Gate Voltage, V G (V) Bulk leakage Sub-threshold plots of TIPS OTFT

10 Parameter Extraction (II) Using respective values of V T, values of K and c (or T C ) were extracted from derivatives of current expressions as below: log [di D /d(v GS - V Tdiff )] (AV -1 ) Experimental data Linear Fit log (V GS - V Tdiff ) (V) Diffusion Drift V T 11 V - 4 V K (SI unit) (SI unit) c Tc 777 K 566 K MNE (kt C / q) mev 49 mev log [di D /d(v GS - V Tdrift )] (AV -1 ) -6 Experimental data Linear fit log (V GS - V Tdrift ) (V)

11 Validation of the Polycrystalline model Drain Current, I D (µa) Experimental data Polcrystalline model a) V DS = -5 V Drain Current, I D (A) Drain Current, I D (µa) Experimental data Polycrystalline model b) V DS = -40 V Drain current, I D (A) Gate Voltage, V GS (V) Gate Voltage, V GS (V) Drain Current, I D (µa) 0.5 Experimental data Polycrystalline model 0.4 V GS = -20 V (a) V GS = -10 V Drain Voltage, V DS (V) Drain Current, I D (µa) V GS = -40 V V GS = -30 V V GS = -20 V V GS = -10 V Experimental data Polycrystalline model Drain Voltage, V DS (V) (b)

12 Significance of Mobility in Polycrystalline devices Assume a crystalline and disordered materials to be in direct contact (ignoring work function difference and trapping effects) such as: Subsequently, at thermal equilibrium the carrier fluxes between the two materials are equal and thus:

13 Significance of Mobility in Polycrystalline device Substitute for carrier concentration, Then, The measured effective mobility contains additional pre-factor but with carrier dependency similar to the Universal Mobility Law A. R. Brown, D. M. de Leeuw, E. E. Havinga and A. Pomp, Synth. Met. 68 (1), 65 (1994). 6 C. P. Jarret, R. H. Friend, A. R. Brown and D. M. de Leeuw, J. Appl. Phys. 77 (12), 6289 (1995).

14 Universal Mobility Law Carrier concentration increases due to doping or field-effects effects Doping of disordered semiconductor with DDQ 7 in solution resulted in increase in mobility for small changes in dopant (or carrier) concentration 10-7 Bulk mobility, µ (m 2 V -1 s -1 ) P3HT PTAA m = 2, T C = 900 K and MNE = 78 mev Conductivity, σ (Sm -1 ) 7 M. Raja et. al. J. Appl. Phys. 92 (3), 1441, 2002

15 Effective Mobility Vs. Gate voltage In terms of applied gate voltage, the effective mobility increases with increase in V G, to a power exponent dependent on T C (i.e. degree of disorder) Effective drift mobility, µ eff (cm 2 V -1 s -1 ) Gate Voltage, V GS (V) T C (drift) = 566 K

16 Disordered Vs. Polycrystalline Comparing drift currents for Disordered 8 and Polycrystalline OTFTs below : and where and The equations are similar (i.e. power exponent of T C on applied voltages) possibly because the surface potential due to gate bias is affected by E F pinning in the grain boundaries which are disordered in nature 8 M. Raja and W. Eccleston, IET Circ. Dev. Syst. 6 (2), 122, (2012)

17 Disordered Vs. Polycrystalline For TIPS OTFT data, the polycrystalline model fits better than disordered model particularly at low voltages where diffusive component is dominant Disordered model assumes a drift mechanism only thus a power exponent of 2T c /T across the whole range of the applied voltage. Note for the diffusive component in polycrystalline the power exponent is 4T C /T - 1 Drain Current, I D (µa) (a) Experimental data Polycrystaline model Disordered model Gate Voltage, V GS (V) Drain Current, I D (A) Drain Current, I D (µa) (b) Experimental data Polycrystaline model Disordered model Gate Voltage, V GS (V) Drain Current, I D (A)

18 Conclusions Polycrystalline OTFT model was developed, taking into account various modes of conduction i.e. diffusion and drift, and expressed in terms of essential parameters i.e. grain sizes, characteristic temperature Polycrystalline OTFT model showed good agreement with experimental data of TIPS Pentacene OTFT Similar dependency of the effective mobility on carrier concentration was observed in polycrystalline to disordered materials. Further studies show similar power dependencies of the applied voltages on both OTFT models However better fits to the experimental data are attained with the Polycrystalline rather than Disordered model due to the presences of the diffusive component at low voltages For complete compact models, other effects such as contact resistance need to be included, and also transient models developed

19 Acknowledgments Bill Eccleston, David Donaghy, Robert Myers, Sidra Afzal, Grace Carradice, Lin Sheng and Paul Rimmer

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