Planar Organic Photovoltaic Device. Saiful I. Khondaker

Transcription

1 Planar Organic Photovoltaic Device Saiful I. Khondaker Nanoscience Technology Center and Department of Physics University of Central Florida W Metal 1 L ch Metal 2 W SiO 2 Si NSF/ONR workshop on organic solar cell, September 21, 212

2 Outline Issues and challenges Planar OPV architecture Effect of different anode and cathode materials on the Voc and Isc Effect of independent control of channel lengths and film thickness Some control experiments to understand our results Summary and conclusion Perspective

3 Progress towards high efficiency OPV PCE (%) Brabec et al, Adv. Mater. 22, 3839 (21) 8 6 PBDTT-DPP:PCBM +P3HT:ICBA (Tandem) [ 8] PTB7:PCBM [7] PCPDTBT:PCBM+P3HT:PCBM (Tandem) [ 5] P3HT:PCBM [3] P3HT:PCBM [2] MDMO-PPV:PCBM [1] P3HT:PCBM [] PBDTTT:PCBM [6] Year 1. Shaheen et al, Appl. Phys. Lett. 78, 81 (21); 2. Padinger et al, Advanced Functional Materials, 13, 85 (23) 3. Ma et al, Advanced Functional Materials, 15, 1617, 25. Kim et al, Advanced Materials, 18, 572, 26 Improvements are due to: 1. Careful choice of active materials 2. Tuning of material morphology 3. Optimization of film thickness and absorption. Optimization of injection layers 5. Changing device design to tandem solar cell. 5. Kim et al, Science, 317, 222, Chen et al, Nature Photonics, 3, 69, Liang et al, Advanced Materials, 22,E135, Dou et al, Nature Photonics, 6, 18,212

4 Key issues and challenges.in BHJ cells, the photocurrent generation is governed by two main factors: (i) the fractional number of absorbed photons in the active layer (relative to the total flux of photons from the solar spectrum) and (ii) the IQE defined by the fraction of collected carriers per absorbed photon. In principle, one can simply increase the thickness of the active layer to absorb more light. However, because of the relatively low carrier mobility of the disordered materials (cast from solution with subsequent phase separation), increasing the thickness increases the internal resistance of the device. Consequently, the fill factor typically plummets as the thickness is increased..

5 Key issues and challenges: Materials 1. Light absorption depends on thickness: Thicker films gives more photon absorption 2. Charge transport and collection: Smaller separation between anode and cathode is preferable for efficient charge transport What will happen if we have independent control on film thickness and channel length? Brabec et al, Adv. Mater. 22, 3839 (21)

6 Key issues and challenges: Interface 1. Donor Acceptor Interface 2. Donor Donor and Acceptor-Acceptor Interface Charge transport is anisotropic in P3HT. The mobility could vary by almost two to three orders of magnitude. Does that matter? 3. Electrode Active material interface LUMO (A) Cathode Anode HOMO (D) What if we are able to change the electrode work function to whatever we need to without thinking of transparent electrode?

7 Planar Device Architecture Power input W Metal 1 W L ch SiO 2 Si Metal 2 Planar device may offer following advantages: (i) (ii) (iii) (iv) (v) Independent control of the active layer thickness and electrode separation, No need for transparent electrodes, A third gate electrode can be coupled to active layer, Electrode spacing can be reliably reduced below 1 nm without major potential for shorts while keeping the active layer thick for optimum absorption. This new device design will allow the study of material morphology, field effect mobility and charge transport properties in situ in the actual OPV device that is being tested.

8 Planar Device Architecture L= 1 um W=1 um PCE =.6%

9 Planar Device Architecture Power input Metal 1 W L ch t SiO 2 Si Metal 2 W A A illu min atred cross section JscVoc PCE(%) FF(%) P in A A illu min atred cross section PCE J A V P A sc cross section oc (%) FF(%) I P in sc in illu min atred Voc L W FF(%) A A illumin atred cross sec tion L ch W t W

10 Device fabrication Anode Anode Cathode Cathode L ch ~.2 nm to 3um The planner PV device was fabricated by evaporating two different metals with two different work functions on top of a 25nm of SiO 2 layer. P3HT /PCBM (1:2) ratio blend was spin coated on Si/SiO 2 substrate iside a nitrogen filled glovebox, then the device was thermally annealed at 1 o C for 1 minutes. I-V measurements were taken inside a glovebox under air mass AM 1.5 G illumination condition at 1 mw/cm2 calibrated with a Si reference solar cell.

11 -I DS (A) IDS(A) I (na) I ( A) Photovoltaic study of Au:P3HT/PCBM:Al solar cell 2-2 Au/(P3HT:PCBM)/Al Light Dark Al/P3HT/Al FET Voltage(V) Au/(P3HT:PCBM)/Ca-Al Voltage (V) Light Dark No diode characteristics in the dark nor any photovoltaic action was seen. This is due to rapid oxidation of Al which show MIS behavior with P3HT. 2.x x1-7 1.x1-7 5.x1-8 V th 2.x1-6 n type 1.5x1-6 1.x1-6 5.x VDS(V) P type V DS (V) High threshold voltage due to oxidation (MIS junction) p-type show high threshold voltage (V th ) oxides make large barrier for hole injection Al Al 2 O 3 ~ thickness -3.2 ev P3HT Al 2 O ev Al -.3eV

12 -I DS ( A) -I DS ( A) I DS ( A) I DS ( A) FET study of P3HT and PCBM with different metal electrodes Out put characteristics V DS (V) Transfer characteristics Mg In Cr Ti Au Pt Ag Al Pd 1 1 Mg 1 In Cr 1-1 Ti Au 1-2 Pt Ag 1-3 Al Pd V G (V) P3HT I-V with V G = -6V I-Vg with V ds = -6V V DS (V) Mg In Cr Ti Au Pt Au Al Pd 1 1 Mg 1 In Cr Ti Au Pt Ag Al Pd V G (V) PCBM The metals with low energy barrier with HOMO level of P3HT with relatively high hole mobility (Au, Pd) and also show good modulation in p and n channels are good candidates to be hole extractor electrode in the planar solar cell device. On the other hand metals such as Mg, Al, In, Cr and Ti have high electron mobility could be used as an electrons extractor electrodes. Al, Mg higher threshold voltage compared to all other metals due to the air sensitivity and because of oxidation problem.

13 FET study of P3HT and PCBM with different metal electrodes Mobility (cm 2 /Vs) Mobility (cm 2 /Vs) (Mg)=3.7 ev [Φe =.5 ev] (In)=.1 ev [Φe =.1eV] (Ag)=.26 ev [Φe =.ev] (Al)=.3 ev [Φe =.1eV]\ (Ti)=.3 ev [Φe =.1eV] (Cr)=.5 ev [Φe =.3 ev] (Cu)=.6 ev [Φe =.6eV] (Au)= 5.1 ev [Φe =.9 ev] (Pd)= 5.12 ev [Φe =.92eV] (Pt)= 5.6 ev [Φe =1. ev] Vacuum level LUMO 3.eV P3HT HOMO.9eV LUMO.2eV PCBM HOMO 6.1eV (Mg)=3.7 ev [Φe =.5 ev] (In)=.1 ev [Φe =.1eV] (Ag)=.26 ev [Φe =.ev] (Al)=.3 ev [Φe =.1eV]\ (Ti)=.3 ev [Φe =.1eV] (Cr)=.5 ev [Φe =.3 ev] (Cu)=.6 ev [Φe =.6eV] (Au)= 5.1 ev [Φe =.9 ev] (Pd)= 5.12 ev [Φe =.92eV] (Pt)= 5.6 ev [Φe =1. ev] Mg Hole mobility of P3HT Al Ag In Cr Au Pd Work function (ev) Ti Pt In Al 1-2 Cr Ag Pd Au 1-3 Mg Ti Electron mobility of PCBM Pt Workfunction (ev)

14 I (na) I (na) I (na) I (na) I (na) I (na) Photo voltaic study of P3HT/PCBM (1:2 ratio) L=2 nm, t=2 nm, w=1 um Au/In Au/Cr Au/Ti V (V) V (V) V (V) V (V) Isc = -.12 na Voc =.15 V PCE=.8 % Dark Light V (V) Isc = -7.1 na Voc =.25 V FF =.26 PCE= 2.25 % Dark Light Dark Light V (V) Isc = na Voc =. V FF =.3 PCE=.7 %

15 Diode characteristics and Circuit analysis Rectification ratio I I forward reverse Ideality factor e KT dv d ln I Series resistance (R s ), 1 di Rs [ ] I Shunt resistance (R p ), 1 [ ] dv di RP dv v Au/P3HT:PCBM/In Au/P3HT:PCBM/Cr Au/P3HT:PCBM/Ti Rectification ratio Ideality factor Series resistance(ωcm 2 ) Shunt resistance (Ωcm 2 ) Max PCE (%) Ideal Diode: Rectification is high and ideality factor is b/w 1 and 2 Ideal cell Series resistance (R s ) = and Shunt resistance (R p )= Oxidization of In and Cr: The presence of Schottky junctions leads to a counter-productive diode which injects a current opposite to the photovoltaic current. Such counter diodes can be identified by their production of a characteristic S-shape in their I/V-plots when subjected to illumination. Such an effect generally leads to low fill factors.

16 Why low Open Circuit Voltages (V OC )? Au/P3HT:PCBM/Cr Au/P3HT:PCBM/Ti Energy Lost LUMO (.2 ev, PCBM) E CT,MAX =.7 ev.3 ev W FT Cr =.5 ev.1 ev.2 ev W FT Ti =.3 ev IDEAL.15 ev. ev (E ACTUAL CT Au:P3HT/PBM:Cr ).25 ev (E CT Au:P3HT/PBM:Cr ) IDEAL.6 ev (E CT Au:P3HT/PBM:Ti ) ACTUAL. ev (E CT Au:P3HT/PBM:Ti ) HOMO (.9 ev, P3HT).2 ev W FT Au = 5.1 ev

17 Optimum P3HT:PCBM ratio for planar structure I (na) Abs (AU) Abs (AU) 1:1 1:2 1: :1 Ratio 1:2 Ratio 1:3 Ratio Wavelength (nm) Dark -6 1:1 Ratio -8 1:2 Ratio 1:3 Ratio Voltage (V)..3 (b) Both 1:1 and 1:2 ratio film can provide higher photon absorption on SiO 2 substrate compared to 1:3. 1:2 provides domain in range of 2 nm and a smoother surface. 1:1 Ratio 1:2 Ratio 1:3 Ratio 1:2 ratio provides highest PCE (~ 2% for Au and Cr electrodes).2.1

18 I (na) I SC (na) PCE (%) I (na) I sc (na) PCE (%) Channel length dependence solar cell performance Au/Cr 2-2 t=2nm 8 6 t = 5 nm t = 1 nm t = 15 nm t = 2 nm t = 5 nm t = 1 nm t = 15 nm t = 2 nm - Dark -6 L =.2 m L =.5 m -8 L =.8 m L = 1 m L = 3 m Au/Ti V (V) t = 2nm L ( m) L ( m) V (V) Dark L=.3 um L=.5 um L=.8 um L=1 um L=3 um t = 5 nm.5 t = 1 nm. t = 15 nm 3.5 t = 2 nm L ( m) L ( m) t = 5 nm t = 1 nm t = 15 nm t = 2 nm 3

19 I (na) I (na) I sc (na) PCE (%) I sc (na) PCE (%) Film thickness dependence solar cell performance Au/Cr L = 3nm (a) dark thckness ~ 5 nm thckness ~ 1 nm thckness ~ 15 nm thckness ~ 2 nm V (V) Au/Ti L = 3nm L =.3 m L =.5 m L =.8 m L = 1 m L = 3 m Thickness (nm) L =.3 m L =.5 m L =.8 m L = 1 m L = 3 m Thickness (nm) (c) V (V) 5 nm 1 nm 15 nm 2 nm L =.3 m L =.5 m L =.8 m L = 1 m L = 3 m Thickness (nm) L =.3 m L =.5 m L =.8 m L = 1 m L = 3 m Thickness (nm)

20 Comparison with vertical structure : ITO electrodes Metallic electrodes (e.g. Au) are more conductive than ITO. In planar structure the active layer is exposed directly to the light which makes it higher photo absorption (ratio of current to absorbed photons) compared to the vertical structure. In vertical solar cell structure photon absorption will never be 1% because the active layer is embedded in a multilayer's structure with different refractive indices (G. Dennler et al. Adv. Mater. 29, 21, ). J.Liu et al. Adv. Mater. 212, 2, 2228

21 Area calculation Top-Shining Au P3HT:PCBM SiO 2 (1 nm) ITO (1 nm) Ti Bottom-Shining

22 Area calculation Parallel Full Saw Half Saw (a) Metal 1 Metal 2 (b) All the Gaps are 3 nm

23 Area calculation I sc (na) Parallel Half Saw Full saw V (V) Full Half Parallel

24 I sc (na) PCE (%) T (%) I (na) I (na) Comparison with vertical structure : ITO electrodes Au 16 P3HT:PCBM SiO 2 (1 nm) ITO (1 nm) Top-Shining Ti Bottom-Shining Bottom-Shining - -8 Bottom shining Voltage (V) L=.3 m L=.5 m L=.8 m L= 1 m L= 3 m Top-Shining Up shining t = 2 nm L=.3 m L=.5 m L=.8 m L= 1 m L= 3 m Voltage (V) Bottom Shining.3 Top-Shining.5.8 L ( m) Bottom Shining.3 Top-Shining.5.8 L ( m) ITO ITO/SiO Wave Length (nm) PCE and Isc values are reduced by up to ~ % when the light is shined through the ITO and SiO2 layers

25 What is going on? Is the PCE equation we are using is valid for our device? 25 nm In electronics, if L < l (mean free path), we have ballistic transport In photonics, if L<λ, what happens?

26 Summary and conclusion We have studied planer solar cell using Au-Al, Au-In, and Au-Cr as contact and P3HT:PCBM (1:2) as active material. Au-Al showed MIS junction behavior and did not show any solar cell behavior due to rapid oxidation of Al. The Voc depends upon the choice of electrodes. For Au-Cr, the Voc is ~.26 V for Au-Ti it is ~.V. The short circuit current nd power conversion efficiency increases with increasing thickness and decreasing channel length. The maximum PCE was found to be ~%. This can be further improved by reducing the channel length and increasing thickness as well as choosing different sets of metal electrodes. The results indicate that control of the active layer thickness and electrode separation can improve the device performance by absorbing maximum photons as well as optimization of electrode separation for optimum charge carrier mobility which is not possible in present sandwich based design Question remains whether current PCE equations can be used when L<λ

27 Perspective PCE (%) Independent control of active layer thickness and channel length hold promise for fundamental understanding of solar cells and increasing OPV efficiency 1 L ( m) It is not clear, how do we calculate efficiency of nanoscale solar cell. Need to develop mathematical tools Interface engineering will be needed to reduce contact resistance and hence increase Voc and Isc Surface modification of SiO2 with HMDS or OTS may further improve device performance Small molecule devices can be tested using this geometry to gain insight t = 5 nm t = 1 nm t = 15 nm t = 2 nm 3