Highly Efficient Integrated Perovskite Solar Cells Containing a Small

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Highly Efficient Integrated Perovskite Solar Cells Containing a Small Molecule-PC BM Bulk Heterojunction Layer with an Extended Photovoltaic Response Up to 900 nm 70

Ming Cheng, Cheng Chen, Kerttu Aitola, Fuguo Zhang, Yong Hua, Gerrit Boschloo, Lars Kloo, and Licheng Sun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03564 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016

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Chemistry of Materials

Highly Efficient Integrated Perovskite Solar Cells Containing a Small Molecule-PC70BM Bulk Heterojunction Layer with an Extended Photovoltaic Response Up to 900 nm Ming Cheng,a Cheng Chen,b Kerttu Aitola,c Fuguo Zhang,d Yong Hua,b Gerrit Boschloo,c Lars Kloo, b Licheng Sun* a, d a

Department of Chemistry, Organic Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden. Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden. b

c

Uppsala University, Department of Chemistry – Ångström Laboratory, Physical Chemistry, Box 523, 751 20 Uppsala, Sweden.

d

State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT–KTH Joint Education and Research Centre on Molecular Devices, Dalian University of Technology (DUT), 116024 Dalian, China.

ABSTRACT: We demonstrate a high efficiency perovskite solar cell (PSC) integrated with a bulk heterojunction layer, based on acceptor-donor-acceptor (A-D-A) type hole transport material (HTM) and PC70BM composite, yielding improved photoresponse. Two A−D−A-structured hole-transporting materials termed M3 and M4 were designed and synthesized. Applied as HTMs in PSCs, power conversion efficiencies (PCEs) of 14.8% and 12.3% were obtained with M3 and M4, respectively. The HTMs M3 and M4 show competitive absorption, but do not contribute to photo-current, resulting in low current density. This issue was solved by mixing the HTMs with PC70BM and to form a bulk heterojunction (BHJ) layer and integrating this layer into the PSC as hole transport layer (HTL). Through careful interface optimization, the (FAPbI3)0.85(MAPbBr3)0.15/HTM:PC70BM integrated devices showed improved efficiencies of 16.2% and 15.0%, respectively. More importantly, the incident-photon-to-current conversion efficiency (IPCE) spectrum shows that the photo-response is extended to 900 nm by integrating the M4:PC70BM based BHJ and (FAPbI3)0.85(MAPbBr3)0.15 layers.

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Introduction Recently, methylammonium lead halide perovskites (MAPbI3 and MAPbI3-XClX), formamidinium lead iodide perovskites (FAPbI3) and the mixed perovskite (FAPbI3)0.85(MAPbBr3)0.15 with three-dimentional lead-halide structures, have been widely explored as efficient light absorbers/conductors for solidstate solar cells. 1-10 The high performance can be attributed to unique properties of the perovskite materials, such as direct band gap, long charge-carrier diffusion lengths and very high charge carrier mobilities. 11-14 In 2009, Miyasaka and coworkers firstly reported MAPbI3 based liquid-state perovskite solar cells (PSCs) with a power conversion efficiency (PCE) of 3.8%. 6 In late 2012, Park and Snaith et al. used MAPbI3 and MAPbI3-xClx as light harvesting materials in combination with 2, 2’, 7, 7’-tetrakis[N, N-di(4-methoxyphenyl)amino]-9, 9’-spirobifluorene (Spiro-OMeTAD) as hole transport material (HTM), achieving a PCE exceeding 9%. 5, 15 In the past four years, PCEs larger than 20% has been reported following the development of new materials [electron transport materials (ETMs) and HTMs] and engineering processes (crystal growth and device interface modification). 2, 4, 9 The quick increase in PCEs qualifies the PSC as a promising, highly efficient and cost-effective photovoltaic technology. The commonly used hybrid lead halide perovskite materials, such as MAPbI3, MAPbI3-xClx and (FAPbI3)0.85(MAPbBr3)0.15 show a light response up to 800 nm due to its optical band gap of 1.55 eV, 5, 6,

leaving a wide near-infrared region (NIR) unused. One important strategy to further enhance the PCE of PSCs is to focus on the full utilization of the solar light below 800 nm and another strategy is to broaden the light response to the unexploited longer wavelength region. By replacing the MA cation with the slightly larger FA cation, Snaith and Seok et al. reported the FAPbI3 with a bandgap of 1.48 eV, allowing a slightly broader absorption of the solar spectrum. 9, 16, 17 The FAPbI3 based PSCs have shown a maximum PCE >20%.9 Another approach is the combination of hybrid lead halide perovskite materials with complementary absorbing materials with a broad spectral coverage using a tandem cell structure. Building on long charge-carrier diffusion length, high charge carrier mobility and the ambipolar properties of MAPbI3, Ding et al. fabricated an integrated solar cell by implanting a bulkheterojunction (BHJ) layer into PSCs with the inverted device structure corresponding to ITO/PEDOT:PSS/MAPbI3/(PDPP3T:PC60BM)/Ca/Al. 18 The low bandgap polymer and PC60BM mixture remarkably broadens the photo-response of PSCs. Disappointedly, the integrated device showed lower photovoltaic performance (8.8%) than the more simple PSC only containing PC60BM as ETM (9.5%). Later, Yang et al. improved this concept and reported an interlayer free parallel tandem solar cell by simply integrating MAPbI3-xClx/BHJ together with regular planar structure. 19 In this device, the polymer based BHJ (PBDTTSeDPP:PC70BM) function as hole transport layer (HTL) and

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both the MAPbI3-xClx layer and the BHJ layer can deliver photocurrent. Correspondingly, the integrated device displayed extended response to solar spectrum and rendered a higher PCE (12.0%), compared with a PSC device based on PBDTT-SeDPP alone (9.7%). Recently, by optimizing the BHJ morphology with a novel n-type polymer (N2200) and a new solvent-processing additive (diphenyl ether), Lee and coauthors reported highly efficient integrated device (PCE = 16.36%) combining a MAPbI3 layer and an NIR absorbing BHJ. 20 The optimized perovskite/BHJ devices exhibit a dramatically increased current density (from 17.61 to 20.04 mA·cm−2) due to the additional NIR harvesting, while maintaining the high fill factor (FF) (77%) and open-circuit voltage (Voc) (1.06 V) characteristic of typical PSCs. In all of cases mentioned above, polymer materials are used as light harvesting materials in BHJ. Compared to the polymer materials, small molecular materials have more promising properties, because of their well-defined molecular structure, high purity and smaller batch-to-batch variation. To date, many acceptordonor-acceptor (A−D−A) structured small molecule materials have been explored as HTMs in PSCs. 21-29 However, none of these reported A−D−A structured HTMs show strong absorption beyond 800 nm due to the fact that deliberate adjustments of the small molecule material’s energy levels are needed. Moreover, applied as HTMs in PSCs, these neat materials show competitive but non-productive light absorption, resulting in lower short-circuit current density (Jsc). 21-29 As discussed above, this issue can be to some extent solved by forming BHJ with those reported colorful small molecule materials, but the improvement is very limited, because just very little sun-light before 800 nm escapes through perovskite layer. 19 Previously, we reported a A-D-A structured small molecular material M1 and successfully applied it as HTM in PSCs. M1 exhibit high hole mobility and conductivity, which is important for application as dopant-free HTMs in PSCs. To keep these good properties, we adopt the backbone [phenoxazine (POZ) flanked benzo[1, 2b:4, 5b’]-dithiophene (BDT)] of M1 but modify the electron-withdrawing ending groups, which has been proved to be an efficient way to adjust material energy levels. 30-32 With the introduction of very strong electronwithdrawing end group, this material M4 shows strong absorption in the UV-vis-NIR region (300 to 900 nm). M3 and M4 possesses suitable energy levels for application in PSCs as HTMs. Applied in PSCs, the device with the structure of FTO/compact TiO2 (c-TiO2) / mesoporous TiO2 (m-TiO2) /(FAPbI3)0.85(MAPbBr3)0.15/HTM/Au rendered high power conversion efficiency (PCE) of 14.8% and 12.3% at 100 mW·cm-2 AM 1.5G simulated irradiation for pristine M3 and M4, respectively. During the measurements, we found that M3 and M4 show competitive absorption but no-contribution to photo-current. One main reason might be that the photosensitive HTM cannot form bilayer heterojunction with perovskite. To solve this problem, PC70BM was introduced to hole transport layer to form BHJ heterojunction with photosensitive HTMs, facilitating the separation of excitons generated by excited M3 or M4. Through careful interface optimizations, the integrated PSCs with the structure of FTO/c-TiO2/mTiO2/(FAPbI3)0.85(MAPbBr3)0.15/BHJ/V2O5/Au showed improved efficiencies of 16.2% and 15.0% for M3 and M4 containing devices, respectively. The incident-photon-to-current conversion efficiency (IPCE) spectrum shows that the photoresponse is extended to 900 nm by integrating the narrow band

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gap small molecular material based BHJ M4:PC70BM and (FAPbI3)0.85(MAPbBr3)0.15 layers together.

Results and Discussion Molecular Structures

Figure 1 Structure of the small molecule materials M1, M3 and M4 The detailed structures of M1, M3 and M4 are shown in Figure 1. M3 and M4 are derived from M1, which is reported by our group before. 23 The same backbone, in which benzo[1, 2b:4, 5b’]-dithiophene (BDT) core building block is flanked by phenoxazine (POZ) units, is adopted to maintain semiconductor properties of M1. The POZ units are capped with different electron-withdrawing units [N-Ethylrhodanine (M1), N-(2-ethylhexyl)pyridinium (M3) and (2-(3-cyano- 5, 5dimethylfuran-2(5H)-ylidene)-malononitrile (M4)] to further adjust the energy levels of the resulting molecular entities. The capping using electron-withdrawing groups especially affect the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels, which is extremely important to get a narrow band gap molecule material. The synthetic routes of M3 and M4 and experimental details are described in the supporting information.

Absorption Properties


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Chemistry of Materials be calculated according to the relation of ELUMO = − (|EHOMO| E0-0) (eV). For TiO2, (FAPbI3)0.85(MAPbBr3)0.15 and the other components, the data are collected from literature. 33 As shown in Figure 3a, the HOMO of M3 (− 5.38 eV) and M4 (− 5.18 eV) are more positive than the valence band (VB) of (FAPbI3)0.85(MAPbBr3)0.15 (vacuum scale), and consequently holes generated in the (FAPbI3)0.85(MAPbBr3)0.15 film can be scavenged by the M3 and M4 to be collected at the Au anode. Simultaneously, the LUMO of M3 and M4 are more positive than the conduction band (CB) of (FAPbI3)0.85(MAPbBr3)0.15. Therefore, besides acting as HTMs, M3 and M4 can also be electron-blocking materials, which is important to obtain high efficiency.

Figure 2 a) Absorption spectra of M3 and M4 in CH2Cl2 solution and in the form of a thin film on FTO/compact-TiO2 substrate; b) Absorption spectra of Perovskite [(FAPbI3)0.85(MAPbBr3)0.15], Perovskite [(FAPbI3)0.85(MAPbBr3)0.15] + M3:PC70BM and Perovskite [(FAPbI3)0.85(MAPbBr3)0.15] + M4:PC70BM on FTO/compact-TiO2 substrate The UV-Vis absorption properties of M3 and M4 in solutions and film state are shown in Figure 2. The corresponding data are summarized in Table 1. For M1, the data was collected from our previous research work. 23 In dichloromethane (DCM), similar with M1, M3 shows strong optical absorption in the region of 300-600 nm. While very different from M1 and M3, M4 exhibits a strong and broad absorption in the UVvis-NIR region (300–800 nm), with distinct peaks located at 397 nm and 644 nm (see Figure 2a). Considering the same backbone was used to construct M1, M3 and M4, the differences in light absorbing properties mainly result from different π-conjunction lengths and different electron-withdrawing abilities of ending groups. Increasing the electron-withdrawing ability of capping groups seems to be an efficient way to generate narrow bandgap materials. Casting as a thin film on FTO/compact-TiO2 substrate, the absorption spectra of M1, M3 and M4 broaden in different extent and a noteworthy redshift can be observed, indicating π–π stacking interaction through aggregation in the thin film. It is noteworthy that M4 exhibits extended absorption to 900 nm in film state and the strong absorption of M4 in the NIR region may provide the additional contribution for light harvesting in the PSCs. Optical band-gaps (E0-0) were estimated from the absorption edge of the film spectra to 1.91 (M3) and 1.44 (M4) eV, respectively.

Energy Levels, Hole Mobility and Conductivity To better evaluate the possibility of M3 and M4 for application in PSCs as HTMs, the energy levels (HOMO and LUMO), hole mobility and conductivity of these two materials were roughly estimated. The HOMO levels of M3 and M4 were tested by cyclic voltammetry (CV) using the ferrocene/ferrocenium (Fc/Fc+) redox system as reference, and the CV plot is shown in Figure S1. The LUMO of M3 and M4 can

Figure 3 a) Schematic structure of PSCs containing M3 or M4 as HTMs, b) SEM image of cross-sectional structure of PSCs containing M4 as HTMs The hole mobility of M3 and M4 were estimated by using the space–charge limited current (SCLC) method with the device structure of FTO/PEDOT:PSS/HTM/Au and the test results are shown in Figure 4a. In the low voltage region, it is clear that space charge conditions are not fully evolved for M4. Therefore, for both of M3 and M4, the hole mobility was calculated by fitting the test data in high voltage region. The extracted hole mobilities of M3 and M4 are 2.67 × 10−4 cm2·V-1·s-1 and 1.29 × 10−4 cm2·V-1·s-1, respectively. To confirm M3 and M4 can extract hole efficiently from perovskite layer, steady-state photoluminescence (PL) measurements were performed with structure of FTO/glass/(FAPbI3)0.85(MAPbBr3)0.15 and FTO/glass/(FAPbI3)0.85(MAPbBr3)0.15/HTM. As shown in Figure 4c, the pristine perovskite film showed a PL peak at 774 nm. When cast HTMs on perovskite surface, the PL peak was obviously quenched, suggesting that M3 and M4 are good quenchers for (FAPbI3)0.85(MAPbBr3)0.15. The much higher quenching efficiency of M3 indicates higher hole extraction efficiency, which is consist



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Table 1. Optical and electrochemical data of M1, M3 and M4

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Material

λmax solution a / nm

ɛ / M-1·cm-1

λmax film b / nm

E0-0 c / eV

EHOMO d / eV

ELUMO e / eV

M1

521

3.7×104

557

1.84

–5.29

–3.45

M3

536

4.1×104

552

1.91

–5.38

–3.47

M4

644

4.2×104

681

1.44

–5.18

–3.74

a Absorption spectra were recorded in dichloromethane solution (2×10 -5 M). b Films of M3 and M4 were prepared by spin-coating an o-dichlorobenzene solution (10 mg/mL) of each compound onto FTO/TiO2 substrates at a spin rate of 1500 rpm. c E0-0 was estimated from the film absorption onsets. d CV measurements were carried out in dichoromethane solutions with [TBA]PF6 (0.1 M) as electrolyte, ferrocene/ferrocenium (Fc/Fc+) as an internal reference. e ELUMO = –(|EHOMO| –E0-0).

Photovoltaic Performance of PSCs Containing Pristine M3 and M4 as HTMs

Figure 5 a) J–V characteristics of PSCs employing pristine M3 and M4 as HTMs, b) IPCE spectra of PSCs employing pristine M3 and M4 as HTMs

Figure 4 a) J-V characteristics of different HTM films in holeonly devices, b) J-V characteristics of the in-plane devices containing pristine M3 and M4, c) Steady-state PL spectra of FTO glass/(FAPbI3)0.85(MAPbBr3)0.15, FTO glass/(FAPbI3)0.85(MAPbBr3)0.15/M3 and FTO glass/(FAPbI3)0.85(MAPbBr3)0.15/M4 excited from FTO side with a n excitation light of 532 nm. with the higher hole mobility of M3. The conductivity of M3 and M4 were evaluated by using electrical conductivity setup reported before and the results are shown in Figure 4b. The extracted conductivity values are 1.1 × 10−4 s·cm-1 and 8.8 × 10−5 s·cm-1 for M3 and M4, respectively.

Considering the two new molecule materials M3 and M4 possess high hole mobility, high conductivity and suitable energy levels for scavenging holes from the (FAPbI3)0.85(MAPbBr3)0.15 VB and mediating them to the Au Fermi level, M3 and M4 were firstly employed as additives free HTMs in PSCs with the detailed device structure of FTO/cTiO2/m-TiO2/(FAPbI3)0.85(MAPbBr3)0.15/HTM/Au. The crosssection SEM image of a M4 based PSC demonstrates a welldefined layer-by-layer structure (see Figure 3b). The thickness of the mesoporous TiO2, the perovskite capping layer and HTM layer were determined to about 200 nm, 450 nm and 55 nm, respectively. The photovoltaic performance, including Voc, Jsc, FF and PCE, is evaluated from current density−voltage (J−V) and the incident-photon-to-current conversion efficiency (IPCE) characteristics (see Figure 5). The electronwithdrawing capping groups exhibit big effects on


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Chemistry of Materials Table 2. J–V characteristics of the PSCs containing pristine M3 and M4 as HTM

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HTM

M3

M4

-2

scan direction

Voc / mV

Jsc / mA·cm

FF / %

η/%

from OC to SC

1085

20.7

69.7

15.6

from SC to OC

1075

20.5

63.5

14.0

from OC to SC

980

19.7

68.6

13.2

from SC to OC

985

19.6

58.8

11.4

photovoltaic performance. The previously reported material M1 showed the highest efficiency of 13.6% (see Figure S2). Under the same conditions, M3 based devices exhibited a higher PCE of 14.8% at 100 mW·cm-2 AM 1.5G simulated irradiation [scan from open circuit (OC) to short ciucuit (SC): Voc = 1085 mV, Jsc = 20.7 mA·cm-2, FF = 69.7%, PCE = 15.6%; scan from SC to OC: Voc = 1075 mV, Jsc = 20.5 mA·cm-2, FF = 63.5%, PCE = 14.0%, scan rate: 20 mV·s-1). While, the M4 based devices exhibit an inferior PCE of 12.3% (scan from OC to SC: Voc = 980 mV, Jsc = 19.7 mA·cm-2, FF = 68.6%, PCE = 13.2%; scan from SC to OC: Voc = 985 mV, Jsc = 19.6 mA·cm-2, FF = 58.8%, PCE = 11.4%, scan rate: 20 mV·s-1). For the new reported materials M3 and M4, the M3 based PSCs achieved higher Voc, Jsc and FF, leading to higher PCEs. In theory, for mesoporous PSCs, the Voc is mainly determined by the difference between the quasi-fermi level of the TiO2 substrate and the HOMO level of the HTM. As mention above, the HOMO of M3 is about 200 mV negative than that of M4, correspondingly, M3 based PSCs render higher Voc. The higher FF value is mainly resulted from the higher conductivity of M3. M3 and M4 based PSCs exhibit some differences in response to solar spectrum. Compared with M4 based PSCs, the PSCs containing M3 as HTM showed higher IPCE in the region from 600-800 nm while lower IPCE in region from 450-600 nm. The currents integrated from IPCE spectra are 19.7 mA·cm-2 and 19.2 mA·cm-2 for M3 and M4 based PSCs, respectively, matching well with J-V results. In the region where M3 and M4 have strong absorption, the IPCE values obtained is correspondingly a little lower. Besides that, it is noteworthy that although M4 exhibits extended absorption to 900 nm, M4 based PSCs don’t have response to solar spectrum beyond 800 nm. These results indicate that although material M3 and M4 have strong absorption, they don’t contribute to the photocurrent. On the contrary, the competitive absorption of HTMs results in low Jsc. This is mainly due to the generated exciton in HTM layer can’t dissociate into free charges (electron and hole) at HTM/perovskite interface. 19, 20 There are maybe two main reasons for this result. One is the insufficient driven force for the disassociation of excitons, the other is the photosensitive HTMs cannot form bilayer heterojunction with perovskite. 34, 35 As shown in our manuscript, the LUMO levels of designed HTMs are more positive than the valence band of perovskite materials, with the ∆E around 0.37-0.64 eV. For organic semiconductors, the disassociation of excitons (electron and hole pairs) needs to overcome the binding energy of the order of 0.3–0.5 eV. The driven force, at least for M3, should be enough. Based on this, we deduce maybe no heterojunction formed between HTM and perovskite. From the J-V results we can see that our devices show

average efficiency of champion devices/ %

average efficiency of 20 devices / %

14.8

13.6±0.74

12.3

12.4±0.74

large hysteresis. Previously, we found that when M1 used as HTM in planar structured PSCs in which PC70BM modified ZnO was used as electron selective layer, there is nearly no hysteresis. While here, when M1, M3 and M4 used as HTM in mesoporous structured PSCs, the device shows hysteresis behavior. Therefore, we think the hysteresis behavior may results from the TiO2 layers (compact TiO2 and mesoporous TiO2). To diminish the hysteresis behavior, modification of TiO2 layer may be needed.

Photovoltaic Performance of Integrated PSCs Containing M3 and M4 based BHJ HTLs

Figure 6 a) Schematic structure of integrated devices containing M3 or M4 based BHJ layers, b) SEM image of crosssectional structure of PSCs containing M4: PC70BM BHJ layer To avoid the competitive absorption from M3 and M4 and simultaneously make M3 and M4 contribute to photocurrent, PC70BM was introduced to HTL and integrated devices were fabricated. For the integrated devices, the optimized schematic structure is shown in Figure 6. From SEM image (Figure 6b) we can see that compared with M4 based typical PSCs, just the thickness of HTL (about 110 nm) in integrated devices increased a lot. The V2O5 layer is too thin to be detected. Due to the ambipolar transport properties of perovskite materials, the electron generated from the separated exciton at the do-



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nor−acceptor (D/A) interface in the M3:PC70BM or M4:PC70BM BHJ film can transport through electron acceptor material (here, PC70BM) network to the perovskite material to finally be collected at the TiO2/FTcathode. 19, 20 The

Table 3. J–V characteristics of the PSCs containing M3:PC70BM or M4:PC70BM BHJ HTLs

HTL

M3:PC70BM

M4:PC70BM

scan direction

Voc / mV

Jsc / mA·cm-2

FF / %

η/%

from OC to SC

1095

22.2

71.5

17.3

from SC to OC

1095

22.5

61.5

15.1

from OC to SC

1015

24.0

65.4

15.9

from SC to OC

1015

23.6

Figure 7 a) J–V characteristics of integrated devices containing M3:PC70BM or M4:PC70BM BHJ HTLs, b) IPCE spectra of PSCs containing M3:PC70BM or M4:PC70BM BHJ HTLs perovskite layer absorbs light in the range of 300−800 nm and the BHJ layer has the ability to absorb any UV−Vis light escaping through perovskite layer (Figure S4). Therefore, the photocurrent should be produced in parallel for this integrated device by both the (FAPbI3)0.85(MAPbBr3)0.15 and the BHJ (M3:PC70BM or M4:PC70BM layers). Solution processed V2O5 was employed as co-HTL (or called cathode modifier). The J−V curves of the integrated devices are shown in Figure 7a. The detailed champion solar cell performance parameters are summarized in Table 3. As shown in Figure 7a and Table 3, the integrated device containing M3:PC70BM or M4:PC70BM BHJ layers showed the highest average PCE of 16.2% (scan from OC to SC: 17.3%, Jsc = 22.2 mA·cm−2, Voc = 1095 mV and FF = 71.5 %; scan from SC to OC: 15.1%, Jsc = 22.5 mA·cm−2, Voc = 1095 mV and FF = 61.5%) and 15.0% (scan

58.4

average efficiency of champion devices / %

average efficiency of 20 devices / %

16.2

15.2±0.77

15.0

14.2±0.59

14.0

from OC to SC: 15.9%, Jsc = 24.0 mA·cm−2, Voc = 1015 mV and FF = 65.4 %; scan from SC to OC: 14.0%, Jsc = 23.6 mA·cm−2, Voc = 1015 mV and FF = 58.4%), respectively. Under the same conditions, compare with PSCs containing pristine M3 and M4 as HTMs, the higher efficiency displayed by the integrated device can mainly be ascribed to the greatly improved Jsc. From IPCE spectra shown in Figure 7b, we can see that for (FAPbI3)0.85(MAPbBr3)0.15/M3:PC70BM integrated devices, the IPCE values obtained are slightly higher than those for the PSCs containing neat M3 as HTM, especially in region of 400-600 nm. The result gives a good explanation that the better Jsc mainly originates from the more full utilization of the solar light below 800 nm. More importantly, for integrated device containing M4:PC70BM BHJ HTL, a notable extension of the IPCE response up to 900 nm was detected. Both the improvement of the IPCE and the expanded wavelength response indicate an efficient photo-response of the M4:PC70BM BHJ film boosting the photocurrent. Therefore, forming a BHJ by incorporating PC70BM with a colored HTM is indeed essential to improve the light harvesting efficiency of (FAPbI3)0.85(MAPbBr3)0.15 together with colorful materials. The currents integrated from IPCE spectra are 21.6 mA·cm-2 and 23.1 mA·cm-2 for integrated devices containing M3:PC70BM and M4:PC70BM BHJ, respectively. The differences between integrated and tested Jsc are within 3%, indicating the J-V test results are reasonable. For (FAPbI3)0.85(MAPbBr3)0.15/M3:PC70BM and (FAPbI3)0.85(MAPbBr3)0.15/M4:PC70BM integrated devices, the V2O5 layer plays an important role (vide infra) in achieving high conversion efficiencies, while for PSCs containing neat M3 and M4 as the HTMs, the effect of the V2O5 layer is negligible. The (FAPbI3)0.85(MAPbBr3)0.15/M3 and (FAPbI3)0.85(MAPbBr3)0.15/M4 based photovoltaic devices without V2O5 layers offer high efficiencies up to 14.8% and 12.3%, respectively. However, the integrated devices without a V2O5 modifying layer are inferior in performance, especially the Voc and FF are very low (see Figure S3 and Table S1), which may results from recombination losses at BHJ/Au interface. 19 Therefore, the V2O5 layer used in the integrated devices is essential to retard recombination losses. In this type of devices,


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V2O5 serves as a buffer layer between the BHJ layer and the Au electrode, facilitating hole extraction from the BHJ layer to the Au electrode. In order to check the reproducibility of the PSCs studied, 20 devices containing M3, M4, M3:PC70BM BHJ and M4:PC70BM BHJ as HTLs were investigated. As can be seen from the statistical results obtained (see Figure 8), the PCE data collected shows a reasonable standard deviation, indicating a reproducible photovoltaic performance. For all four different types of PSCs, more than 60% of devices show PCEs above averages. P-values were also calculated to confidently claim the integration of BHJ and perovskite layers together can improve devices’ PCE. 36 To get p-values, the number of standard deviations (Z-score) was firstly determined. For M3 and M3:PC70BM based PSCs, Z-score was calculated to be 9.29. By using an online calculator, the two-tailed p value obtained is less than 0.001. Similarly, the two-tailed p value for M4 and M4:PC70BM based PSCs is also less than 0.001. These results indicate that M3:PC70BM and M4:PC70BM based PSCs indeed show improved efficiencies compared with neat M3 and M4 based devices, respectively.

Charge Recombination in PSCs To better parallel the charge recombination process in the PSCs containing M3, M4, M3:PC70BM and M4:PC70BM as HTLs, light intensity dependence of J–V characteristics were conducted and the relevant results are shown in Figure 9. From the results we can see that the Jsc of all devices show the linear dependence on incident light intensity. The exponential factor (α) of M3, M4, M3:PC70BM and M4:PC70BM based devices are 0.972, 0.863, 0.929 and 0.914, respectively, indicating that non-geminate recombination in PSCs containing M4 as HTM is the strongest among these devices, correspondingly, the lowest Voc was obtained. 19, 25 The FF of all devices hardly change with light intensity, therefore, the non-geminate recombination should be the dominant loss mechanism in these solar cells. Otherwise the FF should become higher when lowering the light intensity. 19, 25

Figure 9 a) Jsc as a function of light intensity in a doublelogarithmic scale for M3, M4, M3:PC70BM and M4:PC70BM based devices and b) FF as a function of light intensity for M3, M4, M3:PC70BM and M4:PC70BM based devices

Stability of PSCs

Figure 8 Statistics of photovoltaic parameters Voc (a), Jsc (b), FF (c), and PCE (d) of 20 PSCs containing different HTL measured under AM 1.5 G illumination (100 mW·cm−2)

Figure 10 Aging test result of M3, M4, M3:PC60BM and M4:PC70BM based PSCs in ambient conditions (humidity of about 30% and temperature of 25 °C)



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To fully evaluate these two new materials and the integrated PSCs, the aging tests were performance in ambient conditions (humidity of about 30% and temperature of 25 °C) with 10 unsealed devices containing M3, M4, M3:PC70BM and M4:PC70BM based HTLs. After measurements, the PSCs were kept in dark condition. From the results we can see that for neat M3 and M4 based PSCs, the devices stay stable within one month, maintaining about 91% and 94% of original PCEs, respectively. But for M3:PC70BM and M4:PC70BM based integrated PSCs, the efficiency decayed about 21% and 28%, respectively. Compared the stability test results for M3, M4, M3:PC70BM and M4:PC70BM based PSCs, we think the serious efficiency decay of integrated devices may result from the aggregation of PC70BM in BHJ HTL, which has also been identified to be a main influence factor of organic solar cell stability. 37, 38 The micro-sized aggregated PC70BM cluster induces a negative effect such as an electron and hole trap sites, resulting in a decreased photo-current density and performance.

Conclusion In Summary, based on our previous studies, two new A−D−A structured small molecule materials M3 and M4 were further developed, in which a BDT core building block was functionalized by POZ units and subsequently capped with N-(2ethylhexyl) pyridinium and 2-(3-cyano-4,5,5-trimethylfuran2(5H)-ylidene)-malononitrile, respectively. Due to the extended π-conjunction length and strong electron-withdrawing ability of capping group, M4 shows strong absorption in the UV-vis-NIR region, providing the possibility of long wavelength light harvesting. The high hole mobility, high conductivity and good energy level alinement with (FAPbI3)0.85(MAPbBr3)0.15 qualify M3 and M4 as promising candidates for the application in PSCs as new HTMs. Applied in PSCs, devices containing neat M3 and M4 as HTMs just show efficiencies of 14.8% and 12.3%, respectively. One limitation is low Jsc obtained due to competitive light absorption with (FAPbI3)0.85(MAPbBr3)0.15 from HTL while does not contribute to the photocurrent. Incorporation of PC70BM with M3 or M4 to form a BHJ, facilitates exciton dissociation, and the (FAPbI3)0.85(MAPbBr3)0.15/BHJ integrated devices exhibited enhanced efficiencies up to 16.2% and 15.0% for M3:PC70BM and M4:PC70BM containing devices, respectively. More importantly, for integrated device containing M4:PC70BM BHJ HTL, a notable extension of the IPCE response up to 900 nm was detected. Higher photocurrent density and higher PCE can be expected by designing even more efficient narrow band gap small molecule materials to maximize the light harvesting capacity and further device optimization to minimize charge recombination losses.

SUPPORTING INFORMATION The synthesis of materials M3 and M4, Perovskite Solar Cell Fabrication supplied as Supporting Information. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Licheng Sun E-mail: [email protected]

Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by the Swedish Energy Agency, the Swedish Research Council, the Knut and Alice Wallenberg Foundation, the National Natural Science Foundation of China (21120102036, 91233201), the National Basic Research Program of China (973 program, 2014CB239402).

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Table of Contents artwork

An efficient integrated perovskite solar cells, containing narrow band gap small molecular material as light haversting donor material and PC70BM as acceptor material, was reported.


Highly Efficient Integrated Perovskite Solar Cells Containing a Small

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