Semiconductors for Bulkheterojunction OSC. Photonics Polymer Lab. MSE

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1 Semiconductors for Bulkheterojunction OSC

2 Strategy for high efficiency solar cells -Enhancement of light harvesting Application of low band gap conjugated polymer Towards 10 % energy conversion efficiency Move to Smaller band gap Developing light harvesting polymer Developing materials with very small exciton-binding energy Markus C. Scharber et al. Adv. Mater Improvement of carrier mobility Synthesis of new polymer Application of inorganic particle : organic/inorganic hybrid solar cells -Improvement of fill factor Decrease of series resistance Control of surface morphology

3 Design considerations for low band gap polymers There are several factors that influence the band gap of a conjugated polymer material. Among these are: (1) intra-chain charge transfer; (2) bond-length alternation; (3) aromaticity; (4) substituents effects; (5) intermolecular interactions; (6) p-conjugation length.

4 Electron acceptors used in OPV devices Fullerene derivatives were used electron acceptor in organic solar cell due to the strong electron withdrawing property of fullerene.

5 Low bandgap polymer - Polythiophene To improve the solubility of polythiophene many different R groups have been explored ranging from alkyl, alkoxy, acid, ester and phenyl groups etc. Since the thiophene ring is a 5-membered ring that is polymerized through the 2 and 5 position substitution introduces directionality in the polymer Regular material where all the molecules add in a head to tail (HT) fashion or it can be random with occasional head to tail (HT), head to head (HH) or tail to tail (TT) coupling.

6 Low bandgap polymer P3HT McCullough method Reike method Poly(3-hexylthiophene), P3HT, is one of the most studied conjugated polymers. The band gap of P3HT is 1.9 ev. The λ max of P3HT and for PAT in general varies with the percentage of head-to-tail coupling, i.e. the regularity of the side chains. Further, it has been shown that the absorption of P3HT depend on the molecular weight of the polymer. That is, λ max increases with an increase in molecular weight and absorption spectrum becomes broader up to 600nm. The regioregularity of P3HT is of great importance for the characteristics of the polymer such as conductivity and charge carrier mobility and transport. Several reports have investigated the regioregularity and its effect on packing of polymer. It has been shown that the regioregular P3HT with a head-to-tail coupling stack in lamella structure, which is separated by the side chains

7 Low bandgap polymer - Polyisothianaphthene (PITN) Resonance structures in PITN. The stars in the third resonance structure corresponds to radicals. Isothianaphthene (ITN) is a thiophene ring with a fused benzene ring in 3 and 4 positions. The band gap of PITN was determined by electronic spectroscopy to be 1 ev. Due to the many resonance structures of PITN and especially due to the increase in the quinoid structure in the backbone of PITN and the larger polarization causes the band gap of the polymer to decrease. Addition of further benzene rings resulted in a larger band gap due to the increase in electron delocalization over the aromatic system and a suppression of the quinoid structure. Derivatives of PITN where alkyl, alkoxy and halogen groups have been added to the backbone have been found to increase the solubility and hence the polymer is more processable.

8 Low bandgap polymer - Copolymers based on ITN There are a few examples of copolymers based on ITN that have been published. In these polymers ITN is the electron acceptor and the benzylic unit or the thiophene unit is the electron donor. The solubility of the copolymer was ensured by the alkoxy group on the benzene ring. The band gap of the polymer was found to be ev. However, the polymer product exhibited a very low conjugation length due to a nonplanar configuration. Hence, the charge carrier mobility was expected to be low.

9 Low bandgap polymer - Copolymers based on ITN P(T-ITNT) was first synthesized by electrochemical polymerization in Years later, in 2001, the copolymer was synthesized by oxidative ferric chloride polymerization from the monomer, which was obtained in 4 steps from 4,5- dichlorophthalic acid. The substituents on the benzene ring of ITN were either alkyl chains or chlorine. The band gap of the resulting polymers was found to be 1.5 ev and 1.76 ev. It is clear that the efficiency has increased tremendously when comparing the copolymers based on ITN with PITN. This is due to the addition of the thiophene units, i.e. the electron donating groups. Further, it is shown that the chlorine atoms on the benzene rings result in a higher efficiency. This is ascribed to the electron affinity of chlorine and hence, the ITN unit becomes a stronger electron acceptor when comparing with the copolymer having alkyl chains on the benzene ring.

10 Low bandgap polymer - Copolymers based on ITN The copolymer was synthesized by a Stille cross-coupling between a dibromoderivative of ITN-imide and a bis(trialkylstannyl) derivative of EDOT. The band gap was found to be 1.0 ev with a λmax at 807 nm and a tail that extended to 1240 nm. The low band gap of the copolymer was achieved, due to the low lying LUMO levels in ITN-imide and the high lying HOMO level of EDOT. The polymer was found to be highly stable towards n- and p-doping, and bulkhetero junctions of the copolymer and PCBM showed an IPCE of 3.5% at 700nm. While this is a low value compared to P3HT/PCBM bulk heterojunctions it shows that it is possible to achieve significant IPCE values with low band gap materials.

11 Copolymer of benzothiadiazole, pyrole and thiophene - PTPTB Poly-N-dodecyl-2,5-bis(2 -thienyl)pyrrole-2,1,3-benzothiadiazole: PTPTB the push-pull concept by altering electron rich N-dodecyl-2,5-bis(2 -thienyl)pyrrole and electron deficient 2,1,3-benzothiadiazole groups V oc = 720 mv I SC = 3 ma/cm 2 FF = 0.38 η = 1 % Enhancement of absorption area

12 Low bandgap polymer - PTPTB It has a maximum absorption around 650nm and a strong red shift ( nm) is observed when comparing the absorption spectra of solutions and films of the polymer. The band gap of the polymer varies with the length of the oligomers and for the longest oligomer synthesized the band gap was 1.6 ev. The position of the HOMO and LUMO was determined by electrochemical voltage spectroscopy (EVS) to be and ev, respectively, and the energy levels in OPVs shows good overlap between the polymer and the electrodes. The effect of the side chain on the nitrogen atom in the pyrrole ring caused an increase in the band gap from 1.2 to 2.0 ev. Addition of octyl side chains on thiophene, attached to achieve solubility, resulted in a larger torsion strain due to steric hindrance and thus caused the band gap to increase. PTPTB was also prepared with thermocleavable side chains on the thiophene units, which showed a significant decrease in band gap upon thermocleavage from 2.14 to 1.69 ev

13 Copolymers based on thiophene, benzothiadiazole or benzo-bis(thiadiazole) The copolymer based on thiophene, benzothiadiazole or benzo-bis(thiadiazole) is another example of an electron donor acceptor alternating polymer. Again the thiophene is the electron donor unit and here benzothiadiazole or benzobis(thiadiazole) is the electron acceptor unit. For the copolymer based on thiophene and benzothiadiazole results have shown that the number of thiophenes between the benzothiadiazole units affects the band gap of the polymer. Thus, it was possible to tune the band gap of the copolymer with the number of thiophene units between the benzothiadiazole units from 2.1 ev for one thiophene to 1.65 ev for four thiophenes.

14 Copolymers based on thiophene, benzothiadiazole or benzo-bis(thiadiazole) The synthesis of the copolymers has been carried out by three different methods as; (1) Stille cross-coupling polymerization of di-stannyl derivatives of thiophene or dithiophenes and di-bromo derivatives of benzothiadiazole or benzo-bis(thiadiazole) with a Pd catalyst. (2) Oxidative ferric chloride polymerization of the monomer with a FeCl3 catalyst. (3) Polymerization of the di-brominated monomers with Ni(COD)2 as a catalyst in a Yamamoto coupling.

15 Copolymers based on thienopyranozine (PTP) Structure of poly(thienopyrazine) Structure of thienopyrazine in a copolymer with thiophene. The copolymer based on thienopyranozine and thiophene is another example of the alternation between aromatic and quinoid units, which results in a low band gap polymer. Here thiophene is again used as the electron donor and thienopyranozine is used as the electron acceptor. The thienopyranozine is a thiophene with a fully fused aromatic pyrazine ring in position 3 and 4, and is thus similar in construction to ITN The synthesis of the homopolymer of thienopyrazine was first carried out by an oxidative ferric polymerization in chloroform. The band gap of the polymer was found to be 0.95 ev (solid state) and the conductivity was found to be 3.6 x 10-2 S cm -1. The polymer was also prepared with different alkyl side chains and polymerized electrochemically.

16 Copolymers based on thienopyranozine (PTP) The chemical synthesis of PTP has been carried out by two different methods as (1) Oxidative ferric chloride polymerization of TP with dodecyl side chains. (2) Polymerization of the dibrominated monomers with Ni(COD)2 as a catalyst in a Yamamoto coupling. The band gap of the polymers was found to be 1.3 ev for PB3OTP, 1.20 ev for PTBEHT and 1.28 ev for PBEHTT. The lower band gap of PTBEHT compared to PBEHTT is ascribed to the improved conjugation and more coplanar structure of PTBEHT caused by the absence of bulky substituents. PB3OTP showed a broad absorption range nm with a maximum at 730 nm [193].

17 Copolymers based on polyfluorene The homopolymer of polyfluorene have a large band gap (3.68 ev) and have been used as a blue light emitter in lightemitting diodes. However, the light emitted and hence the band gap of polyfluorene could be tuned with addition of different electron donating or electron accepting units in the back bone of the polymer. The copolymer based on thiophene, benzothiadiazole and fluorene is an example of the alternation of the electron donating unit, thiophene/fluorene, and the electron accepting unit, benzothiadiazole. The band gap of the copolymer was estimated from UV-vis to be 2.01 ev, found that due to exciton trapping in the benzothiadiazole unit efficient charge transfer was achieved.

18 Copolymers based on polyfluorene The reported band gaps for APFOGreen1, 2, 3 and 4 was 1.27 ev, 2eV, 1.4 and 1.3 ev, respectively. The reported λ max was at 400 and 780nm for APFOGreen1 and at 780 and 740 nm for APFOGreen3 and 4, respectively. Further, APFOGreen1 was found to have a high field effect mobility of 0.03 cm2/vs.

19 Low bandgap polymer - cyclopenta[2,1-b:3,4-b ]dithiophene PCPDTBT PCPDTBT The 4-carbon of the 4H-cyclopenta[2,1-b:3,4-b ]dithiophene can be readily functionalized by alkyl groups to increase solubility without causing additional twisting of the repeating units in the resulting polymers. Copolymer 7 was found to be only sparingly soluble in all solvents. To help increase the solubility, a 4H-cyclopenta[2,1-b:3,4-b ]-dithiophene monomer carrying two 2-ethylhexyl side chains was synthesized.

20 Low bandgap polymer - cyclopenta[2,1-b:3,4-b ]dithiophene Science, 317, 222 (2007) Tandem solar cell using the low band gap polymer, PCPDTBT and P3HT -> Power-conversion efficiencies of more than 6% were achieved

21 Metallated conjugated polymers Synthesized a soluble, intensely coloured platinum metallopolyyne with a low bandgap of 1.85 ev. The solar cells, containing metallopolyyne/fullerene derivative blends as the photoactive material, showed a power-conversion efficiency with an average of 4.1%, without annealing or the use of spacer layers needed to achieve comparable efficiency with P3HT.

22 High Voc Poly(ethynylene bithienylene) Because V OC originates from the energy difference b/w the HOMO level of the p-type pol ymer and the LUMO level of n-type fullerene, the open-circuit voltage and possibly the p ower conversion efficiency can be enhanced by lowering the HOMO energy of the polym er. In fact, for poly(p-phenylene vinylene)s and an alternating copolymer with fluorene (APFOs), the open circuit voltages are between 0.80 and 1.05 V as a result of their lower HOMO levels. Alternating ethynylene and 4,3 -dihexyl-2,2 -bithien-5,5 -diyl along the chain as in PEBT (Figure 1) lowers the HOMO energy level by 0.3 ev compared to P3HT and that when implemented in bulk heterojunction devices with PBCM, PEBT exhibits the expected increase in open-circuit voltage to values slightly over 1.0 V

23 Phenanthrenyl-imidazole-presenting rr-p3ot Copolymers Regioregular thiophene copolymers presenting phenanthrenyl-imidazole side chains in conjugation with the main polymeric chain, leading to lowered bandgaps.

24 Carbazole based copolymers Low MW due to the low solubility Introduction of 2,7-carbazole groups as a building block yields a linear structure wi th a high degree of conjugation along the backbone of the macromolecules, allowing easier charge transport. In addition, the introduction of the vinylene linkage in p-co njugated systems has been an effective strategy for modulating their electronic pro perties by lowering the bandgap.

25 Carbazole based copolymers bulky side chains -> increase solubility -> increase MW The combination of a low bandgap with good mechanical and transport properties has led to a PCE of 3.6%in bulk heterojunction solar cells New low-bandgap copolymers based on carbazoles should therefore lead to interesting features for photovoltaic applications. However, poly(n-alkyl-2,7-carbazole)s generally exhibit poor solubilities and low molecular weights. To solve these problems, bulky side chains are usually attached onto the conjugated backbone. -> Mn and Mw - 37 and 73 kda

26 Poly(thienylene vinylene) The soluble, conjugated regioregular polymer poly(3-dodecyl-2,5-thienylene vinylene), PDDTV, was synthesized and blended with 1-(3-methoxycarbonyl)propyl-1-phenyl- [6,6]-C61, PCBM, into bulk heterojunction photovoltaic devices, working toward the goal of being able to tune the spectral response. Alternatively, alkyl-substituted poly(2,5-thienylene vinylenes) with band gaps in the range Eg ) ev (absorption maxima ~ ev)12,13 allow for an improved overlap of the polymer absorption spectrum with the solar emission.

27 Poly(thienylene vinylene) Three bi(thienylenevinylene) substituted polythiophenes(bitv-pts), P1-3, were synthesized for obtaining the conjugated polymers with broad and strong visible absorption for the application in polymer solar cells.

28 Diblock-co-polymer with thiophene and PCBM