Small molecule organic solar cells

1.2.1 Introduction of OSC

Solar cell technology provides clean and renewable energy which converts the optical power (mainly from sun) to electric power and has attracted lots of attentions^1,2. OSC, including macromolecule (polymer) and small molecule^3,4, has advantages of low production cost, light weight and large fabrication area in consumer market comparing to the inorganic solar cells^5,6.

A milestone for OSC was proposed in 1985, by C. W. Tang⁷et al. By using two organic materials, copper phthalocyanine (CuPc) and perylenetetracarboxylic derivative (PV) as donor and acceptor materials, respectively, it was possible to achieve power conversion efficiency up to 1% under AM2.0 illumination⁸. In 1992, an evidence for photo-induced charge transfer between a conducting polymer and buckminsterfullerene was claimed by F. Wudl^9,10,11et al. Such fullerene derivatives, C60 and C70, were efficient electron acceptor materials, while the quantum yields in C₇₀ were higher than C₆₀ under the measurement of transient absorption¹². Fig. 1-1 shows the operation principle of an OSC. First, (1) incident photons excite the

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molecules to generate the singlet excitons, which are mainly the Frenkel excitons in active layer; (2) then, the excitons diffuse randomly inside the cell and reach D/A interface; (3) electron-hole pairs are generated from excitons are dissociation by built-in potential and (4) finally, electron and holes transport via acceptor and donor materials which are be collected by the electrodes¹³.

Fig. 1-1 (a) Operation principle and (b) J-V characteristic of an OSC.

There are several parameters which can be extracted from a typical J-V characteristic under light illumination, as shown in Fig. 1 (b). Open circuit voltage (V_OC) is defined as the voltage at zero current density, related to the energy offset between the highest occupied molecular orbital (HOMO) of donor and the lowest unoccupied molecular orbital (LUMO) of acceptor. The short circuit current (J_SC) represents the current under the zero applied bias, which can be improved with the use of small band-gap organic material. Along the J-V characteristics, one can find a point which delivers maximum electrical power, with voltage and current density at Vmp

and J_mp, respectively. Fill factor (FF) corresponds to the area ratio between multiplication of Vmp×Jmp and VOC×JSC. This important parameter is closely related to series resistance (R_s) and shunt resistance (R_sh), which are the mainly determined by carrier mobility and leakage recombination, respectively. The final efficiency (PCE) can be calculated as the multiplication for J_SC, V_OC and FF.

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1.2.2 Bulk heterojunction configuration

Exciton dissociation of OSC was achieved by D-A interface, which provides the potential to overcome the strongly binding energy of the exciton (e.g., ~0.3–0.5 eV)^15,18. Mixing of donor and acceptor materials can effectively improve exciton dissociation¹⁶. It was first given by M. Hiramoto et al. in 1992. They applied a p-i-n structure which i-layer was constructed by mixing p-type phthalocyanine (PC) and n-type perylene derivative (PTC). This three-layer OSC doubled the photo-generate-current compared to the two layer ones¹⁷. In 2000, T. Tsuzuki et al.

doped the titanylphthalocyanine (TiOPc) with a fullerence (C60) and achieved a PCE of 0.63%.¹⁸ In 2003, D. Gebeyehu et al. blended zinc-phthalocyanine (ZnPc) as electron donor and C60 as electron acceptor to raise the efficiency to 1.04% with a proper transporting layer¹⁹. The mixing ratio of donor and acceptor materials strongly impacted the performance. But this p-i-n architecture limited the thickness of the mix layer in the middle due to the poor carrier collection efficiency within 30 nm. As shown by S. Uchida et al. in 2004,²⁰ with a traditional organic materials copper phthalocyanine (CuPc) blending with C₆₀, the PCE can be as high as 3.6%, 3.5% and 3.3% under 0.3, 1 and 2.4 suns, respectively which showed the recombination limited behaviors.

In 2014, Y. Zou et al. used a simple bulk heterojunction for OSC to boost up the efficiency to 7.9%²¹. The active region of the OSC consisted of a mixed layer with 2-{[7-(4-N,N-ditolylaminophenylen-1-yl)-2,1,3-benzothiadiazol-4-yl]methylene}mal ononitrile (DTDCPB) as electron donor material and fullerene C₇₀ as the electron acceptor material, respectively. DTDCPB is a donor material with the molecular configuration of donor-acceptor-acceptor (D-A-A). As shown in Fig. 2 (a), the active region was sandwiched by neat donor and acceptor materials, which is called planar

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mixed heterojunction (PMHJ) structure. The neat donor and acceptor in Fig. 2 (a) can be replaced by a high mobility buffer layer MoOx in Fig. 2 (b) which reduced the resistance of the cell. This architecture can effectively reduce the non-germinate recombination for electrons and holes on the interfaces of the planar layers that usually happened in PMHJ-OSC and led a high fill factor over 0.65.

(a) (b)

Fig. 1-2 Comparison between planar mixed heterojunction and bulk heterojunction OSCs²¹

1.2.3 D-A-A system organic materials

In this section, we will introduce the donor materials with asymmetric donor-acceptor-acceptor (D-A-A) configurations²². The concept of D-A-A systems originally came from the configuration D-π-A which was used to in the dye-sensitizers solar cell^23,24. Maintaining the advantages for strong benzothiadiazole moiety and quinoid character, an inceptive compound2-{[7-(5-N,N-ditolylaminothiophen-2-yl)-

2,1,3-benzothiadiazol-4-yl]methylene}malononitrile (DTDCTB) was synthesized and

PCE up to 5.81% was obtained ²⁵ . Then,

2-{[7-(4-N,N-ditolylaminophenylen-1-yl)-2,1,3-benzothiadiazol-4-yl]methylene}mal ononitrile (DTDCPB), which replaced the thiophene groups by phenylene, showed a higher PCE for 6.8%²⁶. From the electron donor endcap architecture, the electron-rich

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and fortified quinoidal characters of thiophene functional groups of DTDCTB resulted in the redshift (50-70 nm) in absorption spectra, compared to phenylene attached molecules, DTDCPB. It increased the J_SC of DTDCTB-based OSC, compared to DTDCPB-one. This was not only due to the packing of the molecular which laid for a coplanar conformation between the thiophene and BT rings with a small dihedral angle in comparison with phenylene (with ortho−ortho steric interactions by single crystal X-ray crystallography) but also the support electron-rich of thiophene for better π-electron delocalization as shown in Fig. 1-3. However, owing to the electron-rich nature of thiophene of DTDCTB, it showed a smaller oxidation potential than phenylene-containing molecule (DTDCPB) and resulted in a lower VOC, and hence PCE. When replacing the p-tolyl substituent of DTDCPB into hydrogen bond, 2-{[7-(4-N,N-diphenylaminophenylen-1-yl)-2,1,3-benzothiadiazol-4-yl]methylene}m alononitrile (DPDCPB) exhibited a higher oxidation potential which resulted in a better VOC. And it resulted in the blue shift of absorption spectra and hence lower JSC. Compromising the J_SC and V_OC, DTDCPB exhibited the highest PCE=6.8%²⁷.

Fig. 1-3 Striking a balance between J_SC and V_OC²⁶

Further, a substitution for benzochalcogenodiazole unit was also possible to adjust the photophysical properties as shown in Fig. 1-4. Here, 2,1,3-benzoxadiazole (BO) was adopted as the central bridging acceptor of DTDCPBO, which resulted in red shift of absorption spectra ~10 nm compared to 2,1,3-benzothiadiazole (BT)

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bridged molecules, DTDCTBO, due to a larger electronegativity of the oxygen atom and a lower HOMO level, owing to the decrement of the energy for frontier orbital²⁸. However, although OSC devices based on BO materials exhibited longer absorption spectra and higher JSC, the FF was lower due to more serious bimolecular recombination, compared to those based on BT ones.

Fig. 1-4 Molecular structures of the D-A-A electron donor materials with BO and BT moieties.

1.2.4 Recombination mechanisms of OSC

There are mainly two basic recombination routes in OSCs^29,30: germinate and nongerminate (bimolecular) pair recombination. For the first-order germinate loss, recombination happens before it splits into charge. It is population independent because it is only driven by coulomb attraction. After the exciton bonding pair is totally dissociated, there is still possible for carriers to encounter (collide) and meet each other before they are collected by electrodes, which is called bimolecular recombination. This loss depends on the populations of carriers. It is noted that bimolecular recombination is likely to happen via reformation of interfacial charge transfer states. Typically, recombination mechanisms were studied by transient photoconductivity³¹, time-of-flight³². Besides, it can be investigated by light intensity dependent measurement³³.

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Eq. 1 illustrates the relation between V_OC and J_SC. Here, n and J_OL are the ideal factor and the leakage current, respectively. Typically, n equals to 1 for the ideal diode and falls on 1.0-2.0 for OSCs. For an OSC dominated by bimolecular recombination, n is 1.0. On the other hand, trap assisted Shockley-Read-Hall (SRH) recombination resulted in n=2.

(1)

(a) (b)

Fig. 1-5 (a) Germinate and non-germinate recombination. (b) Trade-off between ΔELUMO and PCE³⁴.

The lowest unoccupied molecule orbital (LUMO) energy offset between donor and acceptor, ΔELUMO, also take place in germinate pair dissociation here³⁴. This offset will not only play an important role in charge transfer (CT), but also attribute to free carriers dynamics. Literatures had been report that a higher ΔELUMO (>1 eV) resulted to a better electric properties for a promotion in FF and quantum efficiency³⁵ compared to a modest one (0.2-0.3 eV). Note that this is valid when the donor material is the main absorber. On the other hand, when the acceptor material is the main absorber, ΔEHOMO is the dominating factor. Anyway, this offset will oppositely

JOL ln Jsc q

= nkT Voc

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expend too much exciton energy during dissociation if the value lying too high, so the trade-off between for its limitation is shown in Fig. 1-5 (b).. This difference can also relative to the charge energetic driving force for separation ΔGCS=Eg - (IP_D - EA_A), which shows a charge separation dependence (~ΔELUMO)³⁶ by providing a 40%/eV enhancement in charge collection efficiency³⁷. IP_D and EA_A represented to an ionized potential and electron affinity for donor and acceptor, respectively.