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High performance low band gap polymer solar cells with a non-conventional acceptorw

Downloaded by University of California - Los Angeles on 26 June 2012 Published on 13 June 2012 on http://pubs.rsc.org | doi:10.1039/C2CC33282E

Youjun He,a Jingbi You,a Letian Dou,a Chun-Chao Chen,a Eric Richard,a Kitty C. Cha,a Yue Wu,b Gang Lia and Yang Yang*a Received 8th May 2012, Accepted 11th June 2012 DOI: 10.1039/c2cc33282e A novel C70 fullerene derivative was designed and synthesized by [4+2] cyclic addition reaction between indene derivative (methyl 1H-indene-3-carboxylate) and C70. The absorption and photoluminescence of H120 and its mixed films with different polymer donor materials were investigated, as well as its electrochemical property and electron mobility. It was found that H120 has 0.05 eV higher LUMO level than that of PC70BM. Its electron mobility reached 6.32 104 cm2 V1 s1, which is slightly lower than 9.55 104 cm2 V1 s1 of PC70BM. The photovoltaic devices based on P3HT, and two high efficiency low band gap polymers, PBDTTT-C and PBDTTDPP as donors, with H120 as an acceptor gave power conversion efficiencies of 4.2%, 6.0% and 6.2%, respectively. Due to the diversity of organic materials, easy device fabrication and low cost, polymer solar cells (PSCs) have been one of the hottest research topics in the organic electronics field.1 Most PSCs are based on the bulk-heterojunction (BHJ) concept with an active layer of a conjugated polymer donor and soluble fullerene derivative acceptor blend, an anode of transparent conducting oxide (e.g. ITO), and a cathode with a low work function.2a In most situations, some buffer layers between electrodes and the active layer are also widely used2b–e in polymer electronics (for improving charge collection in PSCs). The power conversion efficiency (PCE) of PSCs is decided by the short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor (FF) of the device. Much effort has focused on the development of donor materials with low band gaps for harvesting more sunlight and thus increasing the Jsc of PSCs,3 and choosing materials with suitable energy levels for maintaining enough exciton separation driving force and increasing Voc. Development of device optimization methods has been important for improving exciton diffusion in the donor phase, exciton separation at the interface between the donor phase and the acceptor phase, charge transport in the active layer a

Department of Materials Science and Engineering & California Nanosystems Institute, University of California, Los Angeles, California 90095, USA. E-mail: [email protected] b Solarmer Energy Inc, El Monte, California 91731, USA w Electronic supplementary information (ESI) available: Experimental details including polymer synthesis, fabrication and characterization of the polymer solar cells, measurements, and instruments. See DOI: 10.1039/c2cc33282e

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The Royal Society of Chemistry 2012

and charge collection by the electrodes.4 As a result of these efforts, the PCE of PSCs has improved to over 8%5 from initially less than 1%.2 Almost all of the novel low band gap donor materials were evaluated with PC60BM and PC70BM, which are still the best and most used acceptor materials due to their good solubility, high electron mobility and good miscibility with many donor materials.6 Brabec et al. showed that for obtaining 10% PCE, the ideal band gaps for donor materials should be in the range of 1.2 eV to 1.8 eV, the LUMO levels offset between donor materials and acceptor materials should be ca. 0.3 eV, and the device external quantum efficiency (EQE) should be over 65%.7 There have been many excellent donor materials such as (poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b 0 ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), poly[2,6-(4,4 0 -bis(2-ethylhexyl) dithieno[3,2-b:2 0 ,3 0 d]silole)-alt-4,7(2,1,3-benzothiadiazole)] (PSBTBT), poly[benzo[1,2-b;4,5-b 0 ]dithiophene-co-thieno[3,4-b]thiophene] (PBDTTT) series etc. with band gaps in those ideal regions, but compared with the energy levels of PCBM, the HOMO and LUMO energy levels of these donor materials are too high, which induces a big energy loss, as the Voc is mainly decided by the difference between the HOMO energy level of polymer donor material and the LUMO energy level of an acceptor. It is highly necessary to develop novel fullerene acceptor materials with LUMO levels that are higher than that of PCBM to further increase the photovoltaic performance of the above mentioned polymers. Many novel fullerene acceptors with better performance than PCBM in the P3HT system have been reported.8 Due to their higher LUMO levels than that of PCBM, the PSCs based on BisPCBM,9 Lu3N@C80–PCBH,10 ICBA,11 IPC60BM,12 NCBA13 have higher Voc than the counterpart PCBM devices, high Jsc and FF, and thus have higher PCE than devices based on P3HT and PCBM. Although these new acceptors have better performance than PCBM in the P3HT system, their photovoltaic performance in low band gap polymer systems is not very good. The reasons are mainly their lower electron mobility, weaker exciton separation and charge transportation when applied to these polymer systems. Here, we report one novel fullerene acceptor which not only has a higher LUMO level (and thus larger Voc) than PCBM, but also has high photovoltaic performance in low band gap polymer systems. The structure and synthetic route of the H120 acceptor are Chem. Commun.

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Fig. 3 Electrochemical properties of H120 and PC70BM.


Scheme 1

Chemical structure and synthesis route for H120.

shown in Scheme 1. It was synthesized by a [4+2] cyclic addition reaction between methyl 1H-indene-3-carboxylate and C70. In the refluxing solution of dichlorobenzene, methyl 1H-indene-3-carboxylate will first become its isomer, and then this isomer will react with C70. After reaction completion, the mixed solution was cooled to room temperature, and dropwise added into a stirred methanol. The solid was collected, absorbed by silica gel, and purified by a silica gel column using toluene as the eluent. H120 was obtained as a brown solid. The structure of H120 was characterized by MALDI-TOF, 1 H-NMR and 13C-NMR spectra. H120 has good solubility in common organic solvents such as chlorobenzene and dichlorobenzene. The photovoltaic devices with H120 and PC70BM as acceptors, P3HT, PBDTTT-C and PBDTTDPP as donors were fabricated. The structures of these materials are shown in Scheme S1 (ESIw). PBDTTT-C and PBDTTDPP were synthesized according to ref. 3c and g. The absorption of H120 and PC70BM in diluted THF solution at a concentration of 105 mol L1 is shown in Fig. 1. In the range from 450 nm to 800 nm, the two fullerene derivatives have similar absorptions. The absorption of H120 is weaker than that of PC70BM in the range from 350 nm to 450 nm. The absorption of H120 and PC70BM films mixed with P3HT, PBDTTT-C and PBDTTDPP was also tested, as shown in Fig. 2. In the P3HT system, the absorption of the H120 film is stronger than that of the PC70BM film in the range from 500 nm to 650 nm. In the PBDTTT-C system, the H120 film has weaker absorption than that of the PC70BM film in the whole UV-vis region. In the PBDTTDPP system, the absorption of the H120 film in the range from 400 nm to 600 nm is weaker than that of the PC70BM film. For investigating the electron transfer from a donor to an acceptor, the photoluminescence (PL) spectra of the mixed donor–acceptor films were tested, as shown in Fig. S1 (ESIw).

Fig. 1

Table 1 Half-wave potentials of the reduction processes, onset reduction and oxidation potentials, LUMO energy levels of the fullerene derivatives Fullerene derivatives

E1 (V)

E2 (V)

E3 (V)

LUMO (eV)

H120 PC70BM

0.63 0.60

1.02 1.01

— 1.52

3.85 3.90

After being mixed with PC70BM and H120, the PLs of P3HT films in two systems were nearly totally quenched. In PBDTTT-C and PBDTTDPP systems, both H120 and PC70BM nearly totally quenched the PL of the donor materials. The electrochemical properties of H120 and PC70BM were investigated by cyclic voltammetry, as shown in Fig. 3. Table 1 lists the half-wave potentials of the reduction processes of the two fullerene derivatives. The reduction potentials of H120 are negatively shifted in comparison with that of PC70BM. The LUMO energy levels of H120 and PC70BM were estimated from their onset reduction potentials indicated in the cyclic voltammograms. The onset reduction potentials (Ered on) of H120 and PC70BM were 0.55 V and 0.50 V vs. Ag–Ag wire, respectively. From the onset reduction potentials, the LUMO energy levels of the molecules were calculated according to the equation: LUMO = e(Ered on + 4.40). The LUMO energy levels of H120 and PC70BM calculated in this way are 3.85 eV and 3.90 eV, respectively. The higher LUMO energy level of H120 is desirable for its application as an acceptor in PSCs to get higher open-circuit voltage. PSCs were fabricated with P3HT, PBDTTT-C and PBDTTDPP as donors and H120 as an acceptor. For comparison, the counterpart PC70BM based devices also was fabricated. The donor/acceptor weight ratios in P3HT, PCBDTTT-C and PBDTTDPP systems are 1 : 1, 1 : 2 and 1 : 2, respectively. The PSC devices based on P3HT as a donor was solvent annealed and thermal annealed at 150 1C for 10 min. For the PSCs based on PBDTTT-C and PBDTTDPP as donors, 3% DIO was added into DCB as an additive. The details of device fabrication and testing are shown in the ESI.w Fig. 4a–c show the current density–voltage (J–V) curves of the devices based on P3HT, PBDTTT-C and PBDTTDPP as donors, respectively, under the illumination of AM1.5G, 100 mW cm2. The results of open-circuit voltage (Voc), short-circuit current (Jsc),

Absorption spectra of the PC70BM and H120 in THF solutions.

Fig. 2 Absorption spectra of the P3HT, PBDTTT-C and PBDTTDPP pure films, and their mixed films with H120 and PC70BM.

Chem. Commun.

Fig. 4 Current–voltage curves of the photovoltaic devices based on P3HT (a), PBDTTT-C (b) and PBDTTDPP (c) as donors, and H120 and PC70BM as acceptors.

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Table 2 Photovoltaic results of the devices based on P3HT, PBDTTT-C and PBDTTDPP as donors, and H120 and PC70BM as acceptors, under the illumination of AM1.5G, 100 mW cm2 Active layer

Voc (V) Jsc (mA cm2) FF (%) PCE (%)

P3HT:H120 P3HT:PC70BM PBDTTT-C:H120 PBDTTT-C:PC70BM PBDTTDPP:H120 PBDTTDPP:PC70BM

0.64 0.59 0.75 0.72 0.78 0.74

10.14 9.79 14.18 14.19 13.42 13.60

65 67 57 62 60 65

4.22 3.89 6.00 6.30 6.20 6.50

fill factor (FF) and power conversion efficiency (PCE) of the devices are listed in Table 2. Compared with PC70BMbased devices, all three devices based on H120 as the acceptor have higher Voc, due to the higher LUMO level of H120. In the P3HT system, H120 based devices have higher Jsc than that of PC70BM devices. In PBDTTT-C and PBDTTDPP systems, the Jsc of H120 based devices and PC70BM based devices are nearly the same. However, in these two donor systems, H120 devices have lower FF than that of PC70BM devices. The AFM phase images of these devices (Fig. S2, ESIw) show that there are good donor–acceptor phase separations in all these six films. The lower FF of H120 devices was suspected to be mainly induced by the lower electron mobility of H120. The electron mobilities of H120 and PC70BM were 6.32 104 cm2 V1 s1 and 9.55 104 cm2 V1 s1, which were tested by the SCLC method. The details are reported in the ESI.w Due to the higher Voc and Jsc, and nearly the same FF, the PCE of P3HT devices was improved from 3.89% to 4.22% by using H120 as an acceptor to replace PC70BM. However, the PCE of other donor systems with H120 as an acceptor is still slightly lower than that of the PSCs with PC70BM as an acceptor. The PCE of PBDTTT-C and PBDTTDPP with H120 as the acceptor reached 6.00% and 6.20%, and those of PC70BM devices are 6.30% and 6.50%, respectively. In this communication, we designed and synthesized one novel fullerene derivative H120 by a cyclic addition reaction, and characterized the optical, electrochemical and photovoltaic properties of H120. It has very good solubility in common organic solvents such as chlorobenzene etc. The LUMO level of H120 is 0.05 eV higher than that of PC70BM, which induced a higher Voc for the corresponding devices. The photovoltaic devices based on P3HT, PBDTTT-C and PBDTTDPP as donors, and H120 as an acceptor, have higher Jsc and FF than those of the counterpart PC70BM devices, which induced a PCE of 4.22%, 6.00 and 6.20%, respectively. To our knowledge, this is the first time a high Voc acceptor shows comparable solar cell performance with state-of-art low band gap polymers. The new acceptor system surely deserves further investigation to improve the current polymer solar cell to a new level. This work was financially supported by Solarmer Energy Inc., UC Discovery Grant.

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