Selective Organic Contacts for Methyl Ammonium Lead Iodide (MAPI

Letter www.acsami.org

Selective Organic Contacts for Methyl Ammonium Lead Iodide (MAPI) Perovskite Solar Cells: Influence of Layer Thickness on Carriers Extraction and Carriers Lifetime Ilario Gelmetti,† Lydia Cabau,† Núria F. Montcada,*,† and Emilio Palomares*,†,‡ †

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Avenida Països Catalans,16, Tarragona E-43007, Spain ‡ ICREA, Passeig LLuís Companys, 23, Barcelona E-08010, Spain S Supporting Information *

ABSTRACT: We have fabricated MAPI solar cells using as selective contacts PEDOT:PSS polymer for holes and PCBMC70 fullerene derivative for electrons. The thickness of MAPI, PCBM-C70, and PEDOT:PSS layers has been varied in order to evaluate the contribution of each layer to the final device performance. We have measured the devices capacitance under illumination and the charge carrier’s lifetime using photoinduced time-resolved techniques. The results show that in this kind of devices the limiting layer is the PCBM-C70 due to its relative reduced mobility compared to PEDOT:PSS that makes the control of the fullerene thickness crucial for device optimization. Moreover, capacitive measurements show differences for the devices having different PCBM-C70 layer thicknesses in contrast with the measurements on the different PEDOT:PSS thickness. These give indications about holes and electrons storage and their distribution. KEYWORDS: perovskite solar cells, carrier recombination, selective contacts, PCBM, PEDOT:PSS, transient photovoltage, differential charging, charge extraction

T

organic selective contacts (for example fullerene derivatives for electrons and PEDOT:PSS for holes, reaching 15% with iodine perovskite3,4 or 17% doping the PEDOT:PSS layer,5 18% for mixed iodine−bromine perovskite6). Comparable efficiencies have reported using an organic selective contact (for example, fullerene derivatives for electrons) in combination with doped NiOx for holes, reaching 17% efficiency.7−10 In all the cases, the major efforts, since the first perovskite report, have been the improvement of the perovskite morphologies and film quality (crystallite size, thickness, roughness, pinhole-free layers) by using different fabrication methods with the main aim to compete with record efficiencies, many methods have been described for this purpose.11−17 On the other hand, the number of semiconductor molecules and materials employed as hole transport contact layer has increased exponentially in order to replace the most commonly used spiro-OMeTAD for alternative hole transporting molecules that can be easily synthesized18−20 or that do not require the addition of dopants.21,22 In a lesser extent, a number of electron transport layers have also been investigated but

he use of hybrid inorganic−organic lead halide perovskite based thin layers has been attracting enormous interest from researchers working on solar-to-energy conversion devices devoting a huge effort in a breath-taking race for the world record efficiency. The remarkable physical properties of these new materials allowed reaching power conversion efficiencies (PCE) as high as 22.1% in less than 10 years.1 Published works reaching efficiencies greater than 20% have been achieved with architectures deriving from the Dye sensitized solar cells (DSSC) tradition, which consists of a TCO (transparent conducting oxide), often fluorine-doped tin oxide (FTO), a thin layer of dense TiO2, a mesoporous layer of nanocrystalline TiO2 nanoparticles, the perovskite layer, an organic hole transport layer, and a metal anode. In these solar cells, the TiO2 layer acts as selective contact for electrons, whereas the organic material (often a semiconductor small organic molecule or polymer) acts as the hole transport material (HTM). Nonetheless, other device architectures can be found at the scientific literature that have in common the use of the TCO and the photoactive layer but either a combination of allinorganic selective contacts (for example, TiO2 for electrons and NiOx for holes2) or, acquiring more relevance, from the Organic Photovoltaics (OPV) school, a combination of all© 2017 American Chemical Society

Received: May 11, 2017 Accepted: June 16, 2017 Published: June 16, 2017 21599

DOI: 10.1021/acsami.7b06638 ACS Appl. Mater. Interfaces 2017, 9, 21599−21605

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ACS Applied Materials & Interfaces

described in the experimental section (see the Supporting Information). The champion device performances of each configuration are listed in Table 1. Devices with Different MAPI Thickness (Variations on the Absorber Material Layer). Figure 2 illustrates the current−voltage (J−V) characterization for three representative cells with different thicknesses of MAPI. It could be observed that increasing the MAPI thickness leads to an increase of the photocurrent and, thus, the overall solar cell efficiency (see Table 1). The fill factor (FF) is not heavily influenced by MAPI layer thickness increment so that we could assume that resistances are very similar as well as the interfaces with the respective electron transporting material (ETM) and the HTM (Figure S2). To ensure that the interfaces with the MAPI are similar using different thicknesses, we have performed ESEM imaging (see Figure S5); we have demonstrated that, even though the roughness increases with the MAPI thickness, the grain size and distribution of the crystallites are almost identical.

extensively ridden by the use of TiO2 in normal structures and fullerene derivatives in inverted devices.23 Nevertheless, some groups have focused their interest in trying to disentangle some unsolved issues such as the origin of the open circuit voltage24,25 in both normal and inverted perovskite devices, the origin of the hysteresis,26−31 and more recently and scarce, the influence of the selective contacts over the carrier density and carrier recombination lifetime under device working conditions.32−35 This last would lead to an important improvement of the optimization methods and allow more efficient carrier extraction from the photoactive layer. In this work, we aim to fabricate MAPI perovskite solar cells using all-organic solution-processed selective contacts by the deposition of poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS), MAPI precursors, and [6,6]-phenylC71-butyric acid methyl ester (PCBM-C70) (Figure 1) by spin-coating and study the charge carrier accumulation and recombination lifetime under illumination.

Figure 1. Energy level distribution diagram of the MAPI devices.

Figure 2. Photocurrent vs voltage curves for MAPI solar cells with different MAPI thickness: 230 nm (1), 320 nm (2), and 440 nm (3).

Inverted structure MAPI solar cells were fabricated using organic selective contacts, the full stack can be represented as ITO/PEDOT:PSS/MAPI/PCBM-C70/Ag (see Figure S1). Different thicknesses for the PCBM-C70, perovskite and PEDOT:PSS layers were explored in order to study the effect in the final device performance, the variation of the charge storage capability and finally the changes in the recombination dynamics. All layers were deposited by solution processing and the layer thickness was tuned via spin-coating speed as

Regarding the open circuit voltage (VOC), this value also barely changed with thickness. We have to highlight that both PCBM-C70 and PEDOT:PSS layers were likewise deposited to be the same for all the devices and in order to make a fair comparison. Despite this, it could be assumed that the charge losses and the overall resistance of the devices related to the contacts were very similar and, thus, the increase of the short-

Table 1. Main Performance Solar Cell Parameters in Forward Scan Direction for the Most Representative Devices Studied in This Worka device

MAPI (nm)

PCBM-C70 (nm)

PEDOT:PSS (nm)

JSC (mA cm−2)

VOC (V)

FF

PCE (%)b

1 2 3 4 5 6 7 8 9 10

230 320 440 350

40

65

40 60 90 120 40

65

12.8 14.0 18.6 18.3 14.5 12.0 9.60 10.5 13.0 12.5

1.02 1.03 0.987 0.892 1.00 1.07 1.05 1.08 1.04 1.07

0.56 0.58 0.57 0.54 0.53 0.44 0.20 0.59 0.55 0.59

7.37 8.44 10.4 8.80 7.72 5.60 2.02 6.68 7.41 7.89

300

27 45 65

(7.34) (8.21) (9.51) (7.45) (6.18) (4.82) (1.40) (6.55) (6.78) (7.42)

For parameters from the reverse scan, see the Supporting Information. bAll devices measured under standard conditions (100 mW cm−2 sun simulated irradiation at AM1.5 G). Average PCEs reported in round brackets. For full statistics, see Figures S2−S4. a

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ACS Applied Materials & Interfaces circuit current (JSC) can only be attributed to the increase of MAPI thickness.36 Using the same solar cells and in order to confirm these assertions, we performed photo-induced differential charging (PIDC), photo-induced charge extraction (PICE), and photo-induced transient photovoltage (PI-TPV) measurements as reported before.33,37 PICE measurements were performed to measure the charge density of devices under working conditions (Figure S10a). There is a mandatory condition that gives reliability to this measurement: Decay from PICE has to be faster than voltage decay from PI-TPV transients.37 In our case, the PICE decay was slightly faster than PI-TPV; this ensures its correct interpretation (Figure S7). Alternatively, when the PICE and PI-TPV VOC decays lifetime were of the same order of magnitude, PIDC were measured giving a more representative calculated geometric capacitance (Cg) of 68, 65, and 41 nFcm−2 for devices 1, 2, and 3 respectively (Figure S10b) that are consistent with the ones obtained using PICE technique (Figure S10a). These results are, using both PICE and PIDC techniques, in perfect agreement to the parallel plate capacitor model where the Cg is inversely proportional to the distance between the two electrodes or, that is the same, the thickness of the device; this assumes that charges are stored, in open circuit conditions, at the contacts. Moreover, in order to study the charge storage and recombination in the bulk of the device, close to 1 sun conditions, the Cg was subtracted from the total capacitance; the resulting chemical capacitance (Cμ) was integrated over voltage giving the charge density presumably stored at the bulk of the device at different light intensities.33,38 PIDC measurements are depicted in Figure 3a. As can be seen, and despite the difference in thickness, the charge densities measured using PIDC in the three devices were very similar as well as the VOC, no shift in the charge distribution was observed that is in good agreement with the observed VOC from the J−V characterization. Furthermore, carrier recombination processes can be assessed using PI-TPV measurements; this technique allows the measurement of carriers’ lifetime under different light intensities at open circuit conditions. Figure 3b shows the PI-TPV measurement for devices 1, 2, and 3. As can be seen, the charge recombination follows same kinetics, in other words the three devices shown the same PITPV slope, and then no substantial differences could be considered. Thus, we have demonstrated that the increase of the JSC is directly related to the thickness of MAPI layer, with the VOC being equal for the three devices. Devices with Different PCBM-C70 Thickness (Variations on the Electron Transport Material Layer). ETM layer was also tested, and in order to ensure good coverage in all devices the thickness of the MAPI devices were slightly reduced to 350 nm, resulting in smoother MAPI and allows the study of the PCBM-C70 with a wider range of thicknesses (see Figure S13). Figure 4 illustrates the measured J−V curves for four representative cells with different thicknesses of PCBM-C70: 40 nm (device 4), 60 nm (device 5), 90 nm (device 6), and 120 nm (device 7). We could observe that device 4 presents the highest JSC of this set and a progressive decrease of the JSC with the increase in thickness of PCBM-C70, whereas the VOC show minor differences. The FF, on the contrary, drastically decrease as the ETM becomes thicker, that points out to an important resistance to transport, which is in agreement with the increase

Figure 3. (a) Charge density from PIDC (integrated chemical capacitance) at different voltages and (b) charge carrier lifetime at different charge density of devices 1 (230 nm), 2 (320 nm), and 3 (440 nm) using different thickness of MAPI.

Figure 4. Photocurrent vs voltage curves for solar cells using 40 nm (4), 60 nm (5) 90 nm (6), and 120 nm (7) of PCBM-C70.

of the series resistances reflected in the J−V curves and the rise of the S-shape using 120 nm (7) of PCBM-C70.39,40 The mobility of the fullerene has been extensively reported41 as being in the order of 1 × 10−3 cm2 V−1 s−1, which is a low mobility value compared to MAPI42 or PEDOT:PSS43 layers and becomes one of the main limitations for the inverted MAPI devices. Our results are in agreement with the previous publication of Seok et al.36 21601


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Moreover, PICE was also measured for these devices, and we found that PICE decays were, in all cases, faster than PI-TPV decays. PICE of these devices are shown in Figure S11a and a direct relationship is shown between total charges and the experimental JSC from J−V curves. It is clear that thicker PCBM-C70 layers impede the efficient collection of charges at the metal contact. Devices with Different PEDOT:PSS Thickness (Variations on the Hole Transport Material Layer). Following same procedure, PEDOT:PSS layer was also studied using different thicknesses maintaining constant the MAPI (300 nm) and PCBM-C70 (40 nm) layers. Figure 6 illustrates the J−V curves for a set of solar cells with different PEDOT:PSS layer thicknesses: 27 nm (device 8), 45 nm (device 9), and 65 nm (device 10). The JSC average (see Figure S4) reveals no evident variations due to the variation of PEDOT:PSS layer thickness as well as for the VOC and FF.

For device 7, photoinduced transient measurements have not been measured because of the extremely low performance. PIDC measurements for this set are shown in Figure 5a, in this

Figure 6. Photocurrent vs voltage curves for solar cells using 27 nm (8), 45 nm (9), and 65 nm (10) of PEDOT:PSS. Figure 5. (a) Charge density from PIDC (integrated chemical capacitance) at different voltages and (b) charge carrier lifetime at their corresponding charge density of devices 4 (40 nm), 5 (60 nm), and 6 (90 nm) using different thickness of PCBM-C70.

Furthermore, it is important to remark that the hole mobility of PEDOT:PSS is 1 order of magnitude higher (1 × 10−2 cm2 V s−1)42 than the PCBM-C70 electron mobility, that means that this layer does not present significant resistance to transport of the holes. Therefore, the thickness does not become a limitation for the device operation. PICE, PIDC, and PI-TPV were also performed to study the charge behavior changing PEDOT:PSS thickness. PICE and PIDC (Figure S12a, b) demonstrate in both cases very similar charge distribution, specially the Cg of the three devices seem to be identical around 50 nF cm−2 in PIDC and 58 nF cm−2 in PICE. Furthermore, after subtracting the Cg from the total charge (Figure 7a), for the different PEDOT:PSS layer thicknesses the obtained charge density vs voltage curve is almost identical. In PI-TPV (Figure 7b) the charge lifetime were very similar for the three cases, device 9 presents quite slower charge recombination rates at 1 sun conditions, but the differences were not as substantial as to be considered a direct consequence of the difference in PEDOT:PSS layer thickness. The conclusion that can be extracted from these results is that PEDOT:PSS thickness does not affect the charge distribution, indicating that holes are mainly stored at the ITO/PEDOT:PSS interface, in other words, PEDOT:PSS does not play any role in the charge accumulation, in contrast to the electrons that are seemed to be stored at MAPI/PCBM-C70 interface.

case the geometric capacitance was calculated to be 57, 48, and 36 nF cm−2 for 40 nm (device 4), 60 nm (device 5), and 90 nm (device 6) thick PCBM-C70 layer respectively (Figure S11b). These differences observed in the Cg (low light intensities) are a direct indication that the system can be modeled as a parallel plate capacitor, as seen for different MAPI thicknesses. Thus, both layers MAPI and PCBM-C70 are englobed in the semiconductor bulk between the two electrodes of the capacitor. We can determine that this system, far from 1 sun conditions, stores the charges at the Ag electrode. Additionally, and interestingly, we observed a shift on the measured charge distribution, from Cμ, from PIDC for the thicker PCBM-C70 (Figure 5a). Huang and co-workers25 related the difference in the band tail of the charge distribution to changes in the energy disorder of the fullerene layer because of its lack of crystallinity and random molecular orientation and interaction with the perovskite. The measured exponential curve from PIDC clearly points out to this trend having the thickest devices the most disordered layer. That drove us to think that, close to 1 sun conditions, charges are mainly stored at the MAPI/PCBM-C70 interface. 21602


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and recombination. The device with the thinnest PCBM-C70 layer presented the highest JSC and FF due to its low resistance to transport. In addition, the PIDC controversially showed higher charge densities using thicker layers of PCBM-C70 that means greater amount of charges stored at the bulk, however the recombination (at 1 sun conditions) becomes faster due to the mentioned transport losses and the extraction of these charges becomes less efficient. At this point, the geometric capacitance has been strongly influenced by both MAPI and PCBM-C70 thickness layer variation; this is an indication that the charges are mainly stored at the PCBM-C70/MAPI interface. Finally, it was demonstrated that modifying PEDOT:PSS layer do not affect the final device performance, and PIDC, PICE and PI-TPV measurements present no evaluable differences. This points out that because of the great hole mobility of the PEDOT:PSS layer, this organic layer does not limits the performance of the devices, and the holes would be mainly stored at the ITO/PEDOT:PSS interface and thus the thickness of the PEDOT:PSS do not play any role in the storage but in the extraction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06638. Experimental methods, JV statistics, roughness profiles, SEM and ESEM characterization, VOC stability, and photoinduced transient methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors Figure 7. (a) Charge density from PIDC (integrated chemical capacitance) at different voltages and b) Charge carrier lifetime at their corresponding charge density of devices 8 (27 nm), 9 (45 nm), and 10 (65 nm) using different thicknesses of PEDOT:PSS.

*E-mail: [email protected] (E.P.). *E-mail: [email protected] (N.F.M.).

In conclusion, we have fabricated hybrid perovskite solar cells using inverted structure achieving efficiencies up to 10% under standard solar simulation conditions. The maximum efficiency was obtained through the optimization of the thickness of selective contact layer for electrons (PCBM-C70) and, on the other hand, the optimization of the photoactive perovskite layer. The optimization was carried out analyzing the carrier accumulation at different light bias and the carrier lifetime between different solar cells. We observed in all cases an extended linear relationship between the measured charge and the light bias for values from 0 V (dark conditions) to 0.8 V. This implies that, under open circuit conditions and far from 1 sun conditions, the charges are mainly accumulated at the selective contact electrodes. An exponential increase, which results from the charge accumulation at the photoactive layer, appeared at potentials close to 1 sun simulated conditions (0.8−1.1 V). Moreover, for devices with different MAPI layer thickness all the charge distribution and recombination rates were very similar indicating that the device performance was mainly limited by the organic contacts, PEDOT:PSS and PCBM-C70. In these devices the increase of the measured JSC can be directly related to the increase of the MAPI layer thickness. While modifying the thickness of PCBM-C70 we found an important impact on performance as well as a different charge distribution

Author Contributions

ORCID

Emilio Palomares: 0000-0002-5092-9227 All the experimental data reported in this manuscript was provided by I.G. under the supervision of L.C., N.F.M., and E.P. The manuscript written by N.F.M. and E.P. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank MINECO for CTQ2016-80042-R project, the Severo Ochoa Excellence Accreditation 2014-2018 SEV2013-0319 (ICIQ-BIST). E.P. also acknowledges AGAUR for the SGR project 2014 SGR 763 and ICREA for economical support.

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Selective Organic Contacts for Methyl Ammonium Lead Iodide (MAPI

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