Addressing the Function of Easily Synthesized Hole Transporters in

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Addressing the Function of Easily Synthesized Hole Transporters in Direct and Inverted Perovskite Solar Cells Rosabianca Iacobellis, Sofia Masi, Aurora Rizzo, Roberto Grisorio, Marianna Ambrico, Silvia Colella, Paolo Francesco Ambrico, Gian Paolo Suranna, Andrea Listorti, and Luisa De Marco ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00208 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Energy Materials

Addressing the Function of Easily Synthesized Hole Transporters in Direct and Inverted Perovskite Solar Cells

Rosabianca Iacobellis,⸸† Sofia Masi,‡ Aurora Rizzo,‡ Roberto Grisorio,‡○* Marianna Ambrico,‡ Silvia Colella,‡◊ Paolo F. Ambrico,‡ Gian Paolo Suranna,‡○Andrea Listorti,‡◊* Luisa De Marco‡* ⸸

Dipartimento di Ingegneria dell'Innovazione, Università del Salento, via per Monteroni,

73100, Lecce, Italy. †

Center for Biomolecular Nanotechnologies (CBN) Fondazione Istituto Italiano di

Tecnologia, Via Barsanti 14, 73010, Arnesano, Italy. ‡

CNR-NANOTEC, Istituto di Nanotecnologia, c/o Campus Ecotekne, Università del Salento,

Via Monteroni, 73100 Lecce, Italy. e-mail: [email protected] ○

DICATECh - Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di

Chimica,

Politecnico

di

Bari,

Via

Orabona,

4

I-70125

Bari,

Italy.

e-mail:

[email protected] ◊

Dipartimento di Matematica e Fisica "E. De Giorgi", Università del Salento, Campus

Universitario via Monteroni, 73100 Lecce, Italy. e-mail: [email protected]

Keywords: hole transporting materials, organic inorganic hybrid perovskite, solar cells, device architecture, interfaces

Abstract Two simple small molecules are designed and successfully implemented here as holetransporting material (HTM) in perovskite-based solar cells (PSCs). With the aim of elucidating the interconnection between molecular structure, properties and their role in the working devices these HTMs are implemented in both thin planar direct (n-i-p) and inverse (p-i-n) geometries. It is observed how the HTM layer morphology influences the photovoltaic 1 ACS Paragon Plus Environment

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performance. Moreover, analyzing the different devices, fundamental informations are retrieved on the factors influencing small molecules hole extracting/transporting functionality in PSCs. Specifically, two determinant roles are identified: when HTMs are introduced as growing substrate (p-i-n) positively impact on the device performance via influencing perovskite formation, meanwhile their efficacy in transporting the holes governs the performances of direct configurations (n-i-p). These findings can be extended to a wide family of small molecule HTMs, providing general rules for refining the design of novel and more efficient ones.

1. Introduction Hybrid Halide Perovskite Solar Cells (PSCs) are fostering intensive interest towards photovoltaic technologies alternative to silicon, due to the unique properties of the active material like low cost manufacture, intense and extended absorption and ambipolar, efficient carriers transport. Since the seminal work of Miyasaka in 2009,

1

the power conversion

efficiency of PSCs has increased from 3.9% to 10% with the solid-state PSCs devices introduced in 2012 by the groups of Snaith, Grätzel and Park

2,3

and has rapidly reached

22.1% in the early 2016. 4 These outstanding progresses have been achieved owing to the improvement in the perovskite composition, along with the exploitation of diverse device layouts and of suitable partner materials consenting efficient charge management within the devices.

5

PSCs signify today a wide class of devices spanning from mesoscopic oxides

embodying structures 6,7 to flat ultra-thin configurations. 8 The use of organic thin substrates is particularly interesting foreseeing large scale and flexible applications. In general, planar PSC device architecture consists of a perovskite (PVK) active layer sandwiched between an electron and hole transporting layers (ETL and HTL), commonly arranged in two device categories depending on the role of the substrate: direct or inverted. In the planar direct (or n– i–p) configuration, the ETL is deposited on the bottom transparent electrode and the HTL 2 ACS Paragon Plus Environment

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underneath the top metal contact. Whereas, in the inverted (or p–i–n) architecture HTL is deposited onto the bottom transparent electrode, under the perovskite active layer, while the ETL is deposited atop. 9 Along with the optoelectronic properties of PVK, the nature of ETL and HTL is central in determining the photovoltaic efficiency in solar cells, since they drive the growth of the perovskite material and extract photogenerated charges. 9, 10 The exploitation of different architectures has implied the development of a huge number of HTMs. In particular, organic small molecule HTMs have been widely investigated owing their chemical versatility and easy processability. The most commonly used small molecule, Spiro-OMeTAD, usually leads to the highest performances, but its multi-step synthesis, highcost and complex doping processes could hamper the large scale application in PSCs. For these reasons, the development of novel alternative HTMs with low-cost, facile synthesis and easy processability is highly desirable. In this respect, different small molecules based on thiophene,

11-13

triphenylamine,

14

carbazole,

15-17

phenothiazine

18, 19

have been designed

20

and it has been demonstrated how even minor changes in the chemical structure could significantly influence the molecular geometry and the optoelectronic properties as well as the film morphology and the hole conductivity. 14-17 Common requisites for an efficient HTM are a proper energy band alignment, suitable hole mobility, chemical compatibility with the perovskite layer and pinhole-free film morphology.

20

Despite these general guidelines, an

explicit relationship between the properties of the HTMs and their photovoltaic performances is not yet fully rationalized. HTMs are typically tested in direct devices and in the presence of dopants (lithium salts or cobalt complexes) that enhance their hole mobility, enabling the use of relatively thick HTM layers.

15, 16, 18

Nevertheless, such doping process not only increases the complexity of device

fabrication but also induces deleterious effects on device stability due to the oxidation processes, the hygroscopicity of the additives and undesired ion migration introduced by dopants.

20

Here, in order to expand our comprehension of this puzzling picture and to 3 ACS Paragon Plus Environment


highlight the different roles and functionalities of HTMs, two simple small molecules, namely H1 and H2, were easily synthesized and tested in very thin layers without the use of dopants. The molecules were implemented in two planar configurations bearing fullerene derivatives as ETL: the direct (or n-i-p) C60/PVK/HTM solar cell and the inverted (or p-i-n) HTM/PVK/PCBM device. This direct comparison allowed us to focus on the role of the holetransporting layer. From the behavior of the two molecules and the referential SpiroOMeTAD in the archetypal layouts, fundamental informations on the role of molecular HTMs in different device configurations and on the factors influencing their efficiency in PSCs could be retrieved.

2. Results and discussion H1 and H2 were characterized by the presence of a rigid central core unit, carbazole or phenothiazine, respectively (Figure 1), decorated with two triphenylamine. The central building blocks were selected for their favorable photochemical properties, already successfully exploited in Dye Sensitized Solar Cells 16, 21 or in Organic Light-Emitting Diodes (OLED). 22

Figure 1: Chemical structures of H1 and H2 and side-views of the simulated optimized molecular geometry.

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These precursors present in fact several advantages such as a simple and versatile chemical structure and an easy functionalization for a straightforward tuning of their optoelectronic properties. The presence of carbazole or phenothiazine can regulate the energy levels, affect structural planarity and give different charge delocalization properties. The roughly planar geometry of the central building block is expected to enhance the π-π stacking interaction, which is beneficial for promoting the hole mobility. In addition, the introduction of an alkyl chain on HTMs molecules ensures high solubility in organic solvents and could plays an important role in the solid film assembly. 14 The synthetic routes for H1 and H2 are shown in Figure S1, with details described in the Supporting Information section. Briefly, the two target molecules were synthesized by reacting the dibromo derivatives 2 and 4 (3,6-dibromo-9-hexyl-9H-carbazole and 3,7dibromo-10-hexyl-10H-phenothiazine, respectively), with the p-methoxy triphenylamine boronic ester 123 in the Suzuki coupling conditions.

Figure 2. a) Normalized UV-Vis absorption spectra of H1, H2 and Spiro-OMeTAD in CHCl3 solution at 10-5M. b) Cyclic voltammograms of H1, H2 and Spiro-OMeTAD in nBu4BF4/CH2Cl2 at 10-4M. c) Energy diagram showing the frontier molecular orbitals computed for H1, H2 and Spiro-OMeTAD compounds at the B3LYP/6-31G** level and the corresponding DFT-calculated energies. 5 ACS Paragon Plus Environment


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The normalized UV-Vis absorption spectra of HTMs in CHCl3 (10-5 M) and in solid film are displayed in Figure 2a and S6, respectively, while the corresponding absorption properties are listed in Table 1. H1 and H2, being nearly transparent in the visible region are suitable for the implementation in p-i-n devices because do not compete with the perovskite active materials for the light harvesting. Cyclic Voltammetry (CV, Figure 2b) was employed to determine the electronic energy levels of the HTMs (these data are reported in Table 1). The down-shifted HOMO levels of H1 and H2 with respect to Spiro-OMeTAD better matched with the valence band of CH3NH3PbI3 perovskite (-5.4 eV). 24 The hole mobilities of undoped H1 and H2, extracted from the current density vs voltage curves (Figure S7),

25,26

were one order of magnitude higher than that of the undoped Spiro-

OMeTAD (see Table 1); the latter, used as reference material, was measured in the same manner and the hole mobility value resulted to be 2.4 x 10-5 cm2 V-1 s-1, similar to the data reported in literature.

16

These high mobilities are in principles compatible with the use of

undoped H1 and H2 layers in the PSC devices, further simplifying the fabrication procedure.

Table 1. Optical, electrochemical and electrical properties of H1, H2 and Spiro-OMeTAD HTM. λmax1 [nm]a

λonset [nm]a

EgOtt b [eV]

EHOMO c [eV]

Epa vs Fc c [V]

ELUMO d [eV]

EHOMO e [eV]

Hole mobility [cm2 V-1 s-1]f

H1

334 (343)

386 (404)

3.2 (3.1)

–5.22

+0.12

–2.02

–4.88

2.4 x 10-4

H2

334 (343)

417 (438)

3.0 (2.8)

–5.17

+0.07

–2.17

–4.79

1.3 x 10-4

SpiroOMeTAD

308

420

3.0

–5.02

–0.12

–2.02

–4.46

2.4 x 10-5

a

Spectra recorded from CHCl3 solution (10-5 M) and form film on quartz substrates (data recorded from thin films are showed within parentheses); b Optical band gap; c from Cyclic Voltammetry measurements measured in 0.1 M nBu4BF4 in 10-4M CH2Cl2 solution; d ELUMO = EHOMOc – EgOtt b; e from Density Functional Theory calculations; f from Space Charge Limited Current mobility measurements.

The thermal properties of H1 and H2 were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Figure S8, the decomposition temperatures corresponding to the 5% weight loss in the thermogravimetric plot were 6 ACS Paragon Plus Environment

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assessed to be 292 °C and 254 °C for H1 and H2, respectively. The relatively low thermal stability of the two HTMs could be ascribed to the presence of the alkyl chain directly bound to the core nitrogen atom. Concerning the DSC analyses, during the first heating scan, glass transitions accompanied with enthalpic relaxation were observed for H1 and H2 and no melting processes were recorded, indicating that the materials exist in an amorphous state. In fact, no crystallization was observed during the cooling and second heating steps, only the glass transition at 72.3 °C and 82.9 °C for H1 and H2, respectively. In order to gain insight into the molecular geometry as well as the frontier orbital electron density distributions of the new HTMs, density functional theory (DFT) calculations were performed using the methyl group at the replacement of the hexyl chain to reduce the computation cost. As shown in Figure 1, the direct bond between the two benzene units of the carbazole-based core in H1 forces the molecule to adopt a planar conformation with respect to the optimized geometry of H2, where the insertion of the sulfur atom induces a distortion of the molecule core. The energy of the frontier orbitals were calculated on the optimized geometry of the molecules at the B3LYP/6-311G(d,p) level of the theory including solvent effects (conductor-like polarizable continuum model method, CH2Cl2). It was found that the DFT calculations corroborate the experimental CV observation (vide supra) concerning the relative position of the HOMO energy levels of the two HTMs, notwithstanding the core distortion of the phenothiazine-based molecule in the case of H2. This observation can be rationalized by analyzing the electron density distributions of the relevant HOMOs reported in Figure 2c: in the case of H1, the HOMO seems to be uniformly distributed along the whole molecule, while the same frontier orbital in the case of H2 is mainly localized on the electronrich phenothiazine core. 18 Since the morphology of the HTM layers can affect the charge extraction and transport capability, this was firstly studied aiming at the architectures sketched in Figure 3, allowing us to discriminate its contribution to the overall device efficiency and to verify whether the 7 ACS Paragon Plus Environment


fabrication methodologies of the different devices impact on the properties of the active materials to some extent.

Figure 3. AFM images showing H1 and H2 40 nm-thick films deposited onto ITO (a and b, respectively) or perovskite (d and e, respectively) substrates along with the representative sketches of inverted and direct devices (c and f, respectively).

The morphology of the HTM layers were investigated by atomic force microscopy (AFM), as depicted in Figure 3, and by scanning electronic microscopy (SEM, Figure S9). 40 nm thick H1 and H2 film were deposited onto ITO or perovskite substrates by spin coating, as indicated in the experimental details. The AFM inspection revealed that a very compact and smooth films were obtained for both H1 and H2 deposited onto ITO substrate: the root-meansquare (RMS) roughness of H1 and H2 was ~0.7 nm and ~0.8 nm, respectively. The same molecules deposited on perovskite follow and planarize the irregular CH3NH3PbI3 polycrystals surface with a slightly coarser film and a RMS roughness of ~1.3 and ~1.8 nm for H1 and H2, respectively. At micrometric scale it is observed how H1 and H2 on both ITO and perovskite substrates are prone to produce some aggregated structures that are characterized by very low thickness, only few nanometers (see Figure S10). The AFM profiles showed flat and extended, 1-2 nm 8 ACS Paragon Plus Environment

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thick formations for H1 and H2 on ITO while the rearrangement of the small molecules on the perovskite surface seems to give slightly rougher structures, especially in the case of H2. Furthermore, the thickness of the HTMs was varied in the range of 20-40 nm adjusting the concentration of the spin-coated solutions. H1 on ITO gave a smooth and planar film over a large area, even in the case of 20 nm-thick film while H2 in the same processing conditions showed some defects and uncovered areas (see Figure S11). Planar geometries of the molecules, together with the presence of the alkyl chain, could allow extended π-π intermolecular interaction and hence a more favorable packing of the film, as already reported by Hagfeldt and co-workers for small molecules HTM having similar planar cores. 14-16 A potential problem related to the use of small molecules as underlayer, could be their dissolution upon perovskite deposition.

27

Therefore it has been verified herein that a thin

HTM layer remains after the spin coating of perovskite solution, as reported in Figure S12. From the comparison between the tilted cross section SEM images of a perovskite layer deposited directly on ITO (a) or on H1 underlayer (b) it is easy recognizable the presence of the small molecule thin film, highlighted in green, in Figure S12b. It appears as a compact, few nanometer thick layer, which uniformly cover the ITO substrate. On the other hand, the typical islands-like morphology of bare ITO is clearly visible under the perovskite polycrystalline film in Figure S12a. To the best of our knowledge, this is the first time that the presence of a small molecule underlayer is unequivocally proved in inverted devices. 28-30 The morphologies and coverage of the perovskite ﬁlms grown on H1, H2 and C60 were also verified. CH3NH3PbI3 were deposited onto the HTM film by following the solvent engineering deposition method using a Lewis base adduct 31 slightly modified 32 as reported in supporting information, resulting in a 300 nm thick perovskite layers. The top view SEM and AFM of the CH3NH3PbI3 films, shown in Figure 4, reveal that the perovskite growth on C60 and H2 is characterized by the presence of a few defects with a RMS roughness of ~7.0 nm,



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while the CH3NH3PbI3 deposited on H1 appears slightly smoother (RMS ~6.0 nm) and compact, representing a good substrate for the deposition of the following PCBM ETL.

Figure 4. SEM and AFM images showing perovskite (PVK) deposited on 40 nm-thick H1 and H2 films or on C60 substrates. SEM scale bar 500 nm. The photovoltaic performances of the novel molecules H1 and H2 were investigated in the two different device configurations: the direct (or n-i-p) PSC and inverted (or p-i-n) one, comparing the performances with those of the most popular reference small molecule, SpiroOMeTAD. As above mentioned, the direct PSC architecture had the following configuration: glassITO/C60/CH3NH3PbI3/HTM/Au

while

the

inverted

one

was:

glass-

ITO/HTM/CH3NH3PbI3/PCBM/LiF/Al, schematically shown in Figure 3f and 3c. All fabrication details can be found in the Experimental part. The JV curves of the best devices are reported in Figure 5a and 5c and their photovoltaic parameters, namely short circuit current density (JSC), open circuit voltage (VOC), fill factor (FF) and PCE are listed in Table 2. Full hysteresis scans are reported in the Supporting Information (Tables S1 and S2) while the photocurrent densities and PCE at the maximum power point as a function of time are reported in Figure 5b and 5d. 10 ACS Paragon Plus Environment

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Figure 5. IV best curves (a) and stabilized photocurrent measurements at the maximum power point and power output (b) of H1 and H2 based n-i-p devices; IV best curves (c) and stabilized photocurrent measurements at the maximum power point and power output (d) of H1 and H2 based p-i-n devices. Having ascertained the good film forming ability of these molecules and keeping in mind that undoped HTMs would more efficiently work at low thickness, we implemented very thin layers, 20 and 40 nm, with the aim to understand the behavior of the HTL and its role in the working devices. Several considerations can be gathered by the comparison of these solar cells behavior. All the inverted PSCs showed better performances than the corresponding direct devices: H1 and H2 molecules implemented in the p-i-n configuration reached 14.3% and 13.2%, respectively, whereas in the n-i-p yielded as best results 9.6% and 11.4% efficiency, respectively. The reference Spiro-OMeTAD (see Table S1 and S2 in supporting information) showed 10.1% efficiency in direct devices and 14.7% efficiency in inverted devices. It is worth noticing that these performances were achieved without the use of any dopant additive. Importantly, H1 in inverted configuration achieves comparable performances with respect to the reference, as well as to be synthesized with an easier protocol.



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Table 2: Photovoltaic parameters for H1 and H2 in different devices configurations and for different HTM thickness

Device configuration n-i-p H1 p-i-n

n-i-p H2 p-i-n

n-i-p SpiroOMeTAD p-i-n

HTM thickness [nm]

FF [%]

Voc [V]

Jsc [mA cm-2]

20

-

-

-

PCE [%] -

40

50

1.017

18.9

9.6

20

73

0.945

20.9

14.3

40

75

0.946

19.2

13.5

20

-

-

-

-

40

53

0.988

21.8

11.4

20

57

0.873

17.2

8.5

40

70

0.913

20.8

13.2

20

55

0.971

18.0

9.7

40

57

0.987

17.8

10.1

20

75

0.887

19.8

13.1

40

76

0.979

19.7

14.7

In addition, we observed that the direct devices based on 20 nm thick H1 or H2 did not work, most probably because of a not perfect coverage of the perovskite surface, leading to short circuits. It is noticeable how all the direct devices showed rather low FF values (in the range of 50-57%) while significantly higher FF (70-76%) were recorded for the inverted cells suggesting a more favorable arrangement of the molecules and/or the formation of advantageous interfaces with respect to the n-i-p configuration. This was in good agreement with the morphologies of the HTM thin layers reported in Figure 3: the compact and smooth thin films, obtained for H1 and H2 molecules on ITO affected the morphology and the interfaces of the whole stack, particularly promoting hole extraction and/or transport (see the FF and JSC values). Differently, in direct configuration the relatively rough morphology of the perovskite layer led to a slightly rougher HTM film altering in this way the interfaces with the perovskite and/or the upper gold electrode. Considering the CH3NH3PbI3 active layer, the more uniform perovskite film grown on H1 substrate (see Figure 4) likely contributed to the higher efficiency of the inverted H1 based devices. Furthermore, in inverted devices, the HTM layer quality and thickness greatly influenced the performance of the devices: H1 at 20 nm achieved 14.3% efficiency while the same molecule 12 ACS Paragon Plus Environment

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at 40 nm gave 13.5% efficiency; on the other hand, H2 at 20 nm showed an efficiency of 8.5% with a poor FF of 0.57 while at 40 nm reached an efficiency of 13.2%. This behavior could be explained as follows: the use an undoped HTM layer as thin as possible minimizes the electrical resistance along the charges path

33

and allows to yield high photovoltaic

efficiency but this is true only if a good quality HTM film is formed. This is the case of H1, whose good filming on ITO at few nm allows the highest performances for the 20 nm inverted devices; differently H2, showing some pinholes at low thickness, has a worse impact on the quality the HTM/perovskite interface, decreasing the benefits of using very thin layers. The reliability of this approach was proved testing an additional HTM, known as H101 34 that introduced as undoped 40 nm thick layer in p-i-n PSC yielded 10.4% efficiency with Jsc 20.3, Voc 0.833 and FF 61 (see the IV curves in the Figure S13); this is a good value if compared to the efficiency obtained in the cited paper

34

where a 260 nm thick undoped H101 layer in

direct configuration gave 10.6% efficiency with Jsc 18.9, Voc 0.97 and FF 57. This result strengthens our findings, confirming that a thin undoped HTM layer can yield high efficiency in inverted devices, acting as well working extracting material. The statistic distribution of the photovoltaic parameter (Figure 6) displayed a better reproducibility for H1 and H2 inverted device with respect to the reference Spiro-OMeTAD, highlighting the reliability of the novel materials. Moreover, negligible hysteresis has been recorded as shown in Figure S14 and Table S2. Devices stability has been checked upon storage in inert atmosphere for several weeks; we observed a slight reduction of the overall efficiency (within 10%) that was less intense for H1 with respect to H2 and spiro-OMeTAD.



Figure 6: Statistic distribution of the photovoltaic parameters of the H1, H2 and SpiroOMeTAD inverted devices

In order to rule out differences among the HTMs in their capability of efficiently extract the charge from the perovskite layer in the diverse configuration time resolved radiative emission from the active layer in presence or not of the charge extracting layers were performed. These analyses were carried out for both device configurations and compared H1 and H2 with the Spiro-OMeTAD reference (Figure 7 and Table S3). The pristine material showed very long PL decays indicating low electronic trap densities and balanced carriers transport.

35

In comparison, the decays associated to this emission were

strongly quenched when the perovskite active layer was coupled to a HTM in both configurations, with small differences among samples, suggesting an efficient hole-extraction process. In the direct configuration, Figure 7a, H1 and H2 performed even better than SpiroOMeTAD, see Table S3, in fresh samples, then for systems kept outside the glove box for 24 h, air exposure “activated” the Spiro-OMeTAD molecule and the charge extraction become more efficient. This effect did not occur in H1 and H2 embedding systems, which are 14 ACS Paragon Plus Environment

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unmodified by time. The effect of the Spiro-OMeTAD air exposure is known, as the formation of transient Spiro-OMeTAD-O2 complex has been proposed, which would p-dope the material improving the hole conductivity and shifting the Fermi level toward the HOMO. 36

Figure 7. a) Time resolved photoluminescence (PL) measurements of glass/CH3NH3PbI3/PMMA (black line), as made glass/CH3NH3PbI3/Spiro-OMeTAD (olive line), 1 day air exposed glass/ CH3NH3PbI3/Spiro-OMeTAD (green line), glass/ CH3NH3PbI3/H1 (blue line) and glass/ CH3NH3PbI3/H2 (red line). b) Time resolved photoluminescence measurements of glass/CH3NH3PbI3/PMMA (black line), glass/SpiroOMeTAD/ CH3NH3PbI3/PMMA (olive line), glass/H1/ CH3NH3PbI3/PMMA (blue line) and glass/H2/ CH3NH3PbI3/PMMA (red line). The decays are collected at the maximum of the perovskite emission band (780 nm).

In our systems it seems to finally configure a better alignment of the perovskites valence band to the Spiro-OMeTAD HOMO level. Noticeably air exposure is also a source of instability/irreproducibility of the device photovoltaic performances affecting both perovskite 37

and Spiro-OMeTAD layer

38

constituting one of the factors underling the non ideality of

this hole extracting small molecules for perovskite based solar cells. By contrast, in the inverted configuration, Figure 7b the three HTMs performed similarly, see Table S3. Comparing the results obtained in both configurations, it is proven how these new molecules can effectively act as hole extracting materials in both device configurations.



Conclusion

In conclusion, two simple small molecules, H1 and H2, were designed and tested in direct (ni-p) or inverted (p-i-n) PSC in order to improve the understanding of the role of HTMs in planar perovskite solar cells. A different behaviour of the two molecules related to the different device architecture were observed. In fact, H1 and H2, in the inverted configuration reached the remarkably high values of 14.3% and 13.2%, respectively, approaching the PCE achieved by the Spiro-OMeTAD in the same configuration (14.7%) while showed lower efficiencies in direct configuration. These achievements could be rationalized as follows: even if the electronic properties of novel HTMs are suitable for efficient hole extraction, the unbalanced charge transport through the layers could be detrimental since the carriers mobility in the perovskite layer would be always superior than the organic extracting layers ones. Therefore, a very thin HTM layer, with a shorter pathway, can improve carriers balance and thus devices efficiency. The inverted configuration permits the use of a 20 nm thin layer, yielding the highest performances, while for the direct layout a thicker HTM film, necessary to flatten the perovskite polycrystalline film, strongly reduces the devices efficiency. In addition, we can deduce that small molecule HTMs could take on different roles depending on the device architecture. In the direct configuration they act mainly as transporting layer and their capability of efficiently transport the charge governs the performances. On the other hand, they have a double role in inverted configurations: they are the growing substrate of the perovskite (affecting its self assembly) and the charge extraction interlayer (thinner is better). Importantly, these different functions often require different chemical structures, demanding different synthetic strategies, for these reasons our findings could represent essential guidelines for the future proper design of functional molecules for photovoltaic applications.


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Supporting Information. Experimental details on synthesis and characterization of H1 and H2, experimental details on device fabrication and characterization, UV-vis spectra on thin films, hole mobility, thermogravimetric curves, SEM and AFM images, IPCE, hysteresis data, radiative lifetimes.

Acknowledgements The authors gratefully acknowledge the project PERSEO-“PERrovskite-based Solar cells: towards high Efficiency and lOng-term stability” (Bando PRIN 2015-Italian Ministry of University and Scientific Research (MIUR) Decreto Direttoriale 4 novembre 2015 n. 2488, project number 20155LECAJ) for funding. A.R. gratefully acknowledges SIR TwoDimensional Colloidal Metal Dichalcogenides based Energy-Conversion Photovoltaics (2D ECO), Bando SIR MIUR Decreto Direttoriale 23 gennaio 2014 no. 197, project number RBSI14-FYVD. S.C. and A.L. acknowledge Regione Puglia and ARTI for funding FIR – future in research projects “PeroFlex” (project no. LSBC6N4) and “HyLight” (project no. GOWMB21). R.G. and G.P.S. acknowledge the Bridge-Early Stage COMPOSTRONICS project (cod. 5730587, Austrian Research Promotion Agency-FFG) for funding. L.D.M. acknowledge Apulia Region for project “Nanoapulia– nanofotocatalizzatori per un’atmosfera più pulita” (Bando Aiuti a Sostegno dei Cluster Tecnologici Regionali cod. MDI6SR1 – CUP B38C14001140008).

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Table of Contents (TOC)


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Addressing the Function of Easily Synthesized Hole Transporters in

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