Naohiko
Kato
*a,
Shinya
Moribe
a,
Masahito
Shiozawa
a,
Ryo
Suzuki
a,
Kazuo
Higuchi
a,
Akira
Suzuki
b,
Mareedu
Sreenivasu
b,
Katsuya
Tsuchimoto
b,
Koji
Tatematsu
c,
Katsuyoshi
Mizumoto
c,
Shoichi
Doi
c and
Tatsuo
Toyoda
c
aToyota Central Research and Development Laboratories, Inc., Nagakute-shi, Aichi 480-1192, Japan. E-mail: n-kato@mosk.tytlabs.co.jp
bAISIN Cosmos R&D Co.,Ltd., Kariya-shi, Aichi 448-8650, Japan
cAISIN SEIKI Co., Ltd., Kariya-shi, Aichi 448-8650, Japan
First published on 1st November 2018
To realize highly efficient solid-state dye-sensitized solar cells (SDSCs), the absorption range of the dye should be extended to the near-IR range to increase short-circuit current density (Jsc); a high Jsc in turn requires a highly conductive p-type semiconductor. A newly developed dye (DIPDAB2) with a porphyrin dimer structure provided higher absorption coefficients than the conventional dye with a similar framework (DTBC) in the long wavelength range of 700–800 nm, leading to higher incident photon-to-current conversion efficiencies. The dip in the absorption spectrum of DIPDAB2 located at 500–700 nm between the Soret band and Q band was filled by combining with two kinds of organic dyes (D131 and D358). The multi-dye consisting of the three dyes realized a high Jsc over 20 mA cm−2. The use of copper iodide that has a higher conductivity than p-type organic semiconductors and copper complexes secured a high filling factor. Introduction of Li ions into the TiO2 photoelectrodes improved the open-circuit voltage (Voc) along with a slight increase in Jsc. Light soaking also contributed to a higher Voc. The conversion efficiency of the present SDSC was as high as 10%.
Replacement of the liquid electrolytes with solid-state p-type semiconductors can realize both high efficiency and high durability,8,9 by solving the evaporation issue with securing a high hole conductivity. Therefore, studies on solid-state DSCs (SDSCs) have been very active. Table 1 summarizes electric properties of candidate p-type materials and photovoltaic properties of SDSCs. Among them, copper iodide (CuI) is highly conductive and stable.10 The highest η of SDSCs using CuI is 7.4% coupled with the N3 dye.11 Although CsSnI3 perovskite is also highly conductive and achieves a higher η of 8.5% with the N719 dye,12 it is an unstable material; easily oxidized under ambient air condition. On the other hand, conductivities of p-type organic semiconductors such as 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) and the copper complexes are much lower than those of the inorganic materials.9,13–15
p-type conductor/dye | Conductivity [mS cm−1] | VBMa [eV] | J sc [mA cm−2] | V oc [V] | Filling factor | Efficiency [%] | Stability of p-type materials |
---|---|---|---|---|---|---|---|
a Valence band maximum (VBM) was measured by photoelectron yield spectroscopy. b Ref. 22. c Ref. 11. d Ref. 12. e Ref. 13. f Ref. 9. g Ref. 14. h Ref. 15. | |||||||
CuI/D149b | 308 | −5.00 | 12.8 | 0.65 | 0.72 | 6.0 | Stable |
CuI/N3c | 308 | −5.00 | 14.5 | 0.73 | 0.70 | 7.4 | Stable |
CuI/D131 + D358 + DIPDAB2 (this work) | 308 | −5.00 | 22.0 | 0.65 | 0.71 | 10.1 | Stable |
CsSnI2.95F0.05 + 5% SnF2/N719d | 1111 | −4.90 | 15.9 | 0.72 | 0.74 | 8.5 | Unstable |
Spiro-OMeTAD + Co complex FK102/Y123e | 0.02 | −5.20 | 9.5 | 0.98 | 0.77 | 7.2 | Unstable |
Cu(dmp)2TFSI/LEG4f | 0.10 | −5.70 | 13.8 | 1.01 | 0.59 | 8.2 | Stable |
Cu(tmby)2TFSI/Y123g | 0.17 | −5.67 | 13.9 | 1.08 | 0.73 | 11.0 | Stable |
Cu(tmby)2TFSI/WS72h | 0.17 | −5.67 | 13.8 | 1.07 | 0.79 | 11.7 | Stable |
In the present study, we have focused on four items to improve η of SDSCs. The priority for this purpose is to widen the absorption range of the dye to increase the short-circuit current density (Jsc). Therefore, the first item is a new dye (DIPDAB2) with a porphyrin dimer structure to achieve higher absorption coefficients in the near-IR range. A conventional indoline dye (D149) has a narrow absorption band ranging up to 780 nm,16 resulting in a low Jsc of 13 mA cm−2. We have developed DIPDAB2 based on a donor–acceptor type porphyrin dimer dye: N-fused carbazole-zinc porphyrin-free-base porphyrin triad (DTBC) that sensitizes up to 900 nm.17 The molecular structure of DIPDAB2 is compared with that of DTBC in Fig. 1. There are refinements in DIPDAB2. A stronger electron donor unit of bis(4-hexylphenyl)amine moiety and a π conjugated system extended by a triple-bond-connected acceptor of benzoic acid red-shift the Q band. DTBC and other dyes of the porphyrin dimer structures previously developed: YDD6 (ref. 18) and LDD1 (ref. 19) are equipped with common substituents on the two porphyrin rings as shown in Fig. 1. The unique point of DIPDAB2 is that two different substituents are attached to the two porphyrin rings, which may inhibit aggregation of the dyes on the TiO2 electrodes. The second is the use of a multi-dye consisting of DIPDAB2 and two kinds of organic dyes: a yellow indoline dye (D131)20 and a red indoline rhodanine dye (D358),21 to cover a wide wavelength range from the visible to the near-IR. It has been demonstrated that co-sensitization can improve the performance of DSCs.18,19 We have selected co-sensitizers of D131 and D358 dyes to match DIPDAB2.
A LiI treatment after the dye-absorption on the TiO2 electrodes is employed to increase the open-circuit voltage (Voc), which is the third item. The last is light soaking that also increases Voc. The effects of the LiI treatment and light soaking on the present SDSCs are contrasting to those on liquid-type DSCs of improvements in Jsc with lowering in Voc.
Another important point is the selection of the p-type semiconductor. CuI and copper complexes are stable materials and hence the candidates, as stated above. Our strategy of the improvement in η is to increase Jsc, which requires high conductivity of the p-type material, because detrimental impacts of the inner resistance are more notable at a higher Jsc. Copper complexes show high efficiency,13–15 however, the conductivity is much lower than that of CuI. Therefore, we selected CuI for the p-type semiconductor. We have previously overcome the difficulties in filling the porous TiO2 electrodes with CuI by using TiO2 particles with a larger size than those for DSCs using liquid electrolytes, as shown in Fig. S2.†22
Fig. 3 (a) IPCE spectra, (b) current–voltage (I–V) curves under 1 sun irradiation of the liquid electrolyte DSCs using DIPDAB2 and DTBC. |
The second item is the use of a multi-dye. Although DIPDAB2 exhibits high absorbance in the near-IR range as described above, there is a dip between the Soret band centered at 500 nm and the Q band located at around 700 nm. To fill the gap and secure highly efficient sensitization in the whole of the visible-near-IR range, a multi-dye consisting of DIPDAB2 and two kinds of organic dyes: D131 and D358 was used. The molecular structures of D131 and D358 are displayed in Fig. S9.†Fig. 4 compares the IPCE spectrum of the SDSC using the multi-dye (DIPDAB2 + D131 + D358) with those for D149 and DIPDAB2. D131 and D358 well complemented the dip between the Soret band and Q band originating from the porphyrin dimer in DIPDAB2. As a result, the IPCE spectrum for the multi-dye covered a notably wider range than that for D149, leading to a dramatic improvement in Jsc to over 20 mA cm−2 estimated from the IPCE spectrum. We consider that the multi-dye inhibits the aggregation of the single DIPDAB2 dye.18,19 As the result, the IPCE for the multi-dye in the absorption range of the Q band was much higher than the single DIPDAB2 dye.
The third item is the post treatment using LiI. The dye-adsorbed TiO2 electrodes were immersed in LiI acetonitrile solutions. Fig. 5 shows the I–V curves of the SDSCs with and without the post treatment. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) mappings of Li displayed in Fig. S10† revealed uniform distribution and higher concentrations of Li in the TiO2 electrodes for higher concentrations of the LiI solutions. With increasing Li concentration, Voc increased from 0.59 V for no LiI treatment, and eventually reached 0.65 V using the 0.5 M LiI solution with a slight increase in Jsc.
Fig. 5 Current–voltage (I–V) curves of the SDSCs under 1 sun irradiation and in the dark with and without the 0.5 M LiI post treatment. |
It is well known that introduction of LiI into an electrolyte consisting of I−/I3− redox couples used for DSCs increases Jsc while it decreases Voc.25 Li ions adsorbed on the TiO2 electrodes lowers the conduction-band minimum (CBM) of the TiO2, leading to a higher efficiency of electron injection from the photo-excited dyes to the TiO2 and consequently a higher Jsc. On the other hand, Voc lowers as it is determined from the energy difference between the CBM of the TiO2 and the redox level. The contrasting result of increasing Voc for the present SDSCs suggests a different mechanism of the LiI treatment effect, like reduction of carrier recombination at the interfaces between the TiO2 and CuI.
The fourth item is light soaking. During the light soaking at 1 sun, Voc gradually increased with only a slight decrease in Jsc, as shown in Fig. 6a. After a half hour, Voc reached 0.65 V (an improvement by 0.07 V) with securing a high Jsc of 22 mA cm−2 for the best-performance cell. The I–V curves of the SDSCs using the multi-dye and D149 are compared in Fig. 6b. Reflecting the wider sensitivity range, Jsc for the multi-dye was much higher than that for D149. In addition, there was no degradation of the filling factor (FF) in spite of the higher Jsc, owing to the high conductivity of CuI. As a result, η for the multi-dye was significantly improved to be 10.1% compared with that for D149, and substantially exceeded the previously reported highest value of 7.4% using CuI and the N3 dye.11 The reproducibility was fairly good as summarized in Table S2.† It is worthy to note that the effect of the light soaking on the SDSCs were also contrasting to that of higher Jsc and lower Voc on the DSCs using liquid electrolytes, although the present mechanism is not clear.
Fig. 6 (a) Relation between Jsc and Voc of the SDSCs before/after the light soaking, (b) current–voltage (I–V) curves of the SDSCs under 1 sun irradiation using the multi-dye and D149. |
The photovoltaic performance of the SDSCs employing the four items is compared with previously reported ones in Table 1. The present Jsc of 22 mA cm−2 is extremely higher than those of the other SDSCs, with no remarkable degradation of FF compared with the other SDSCs using CuI. The disadvantage for CuI is lower Voc than those for the other p-type semiconductors, caused by carrier recombination at the CuI surfaces.26 It has been reported that CuI surfaces act as hole trapping sites.26 The maximal Voc determined from the difference between the CBM of TiO2 (−4.2 eV from the vacuum level) and the VBM of CuI (−5.2 eV) is 1.0 V.8 If Voc could be improved to close to the ideal value with securing a high Jsc of 22 mA cm−2, η of 15% would be attained. Thus, the SDSCs using the multi-dye consisting of the new porphyrin dimer dye DIPDAB2 and organic dyes with CuI are promising for highly efficient and cost-effective solar cells.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta06418k |
This journal is © The Royal Society of Chemistry 2018 |