Graphene Interface Engineering for Perovskite Solar Modules: 12.6% Power Conversion Efficiency over 50 cm2 Active Area Downloaded via DURHAM UNIV on July 25, 2018 at 06:23:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Antonio Agresti,§ Sara Pescetelli,§ Alessandro L. Palma,§ Antonio E. Del Rio Castillo,‡ Dimitrios Konios,∥ George Kakavelakis,∥ Stefano Razza,§ Lucio Cinà,§ Emmanuel Kymakis,∥ Francesco Bonaccorso,*,‡ and Aldo Di Carlo*,§ §
C.H.O.S.E. (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy ‡ Istituto Italiano di Tecnologia, Graphene Labs, Via Morego 30, 16163 Genova, Italy ∥ Center of Materials Technology and Photonics & Electrical Engineering Department School of Applied Technology, Technological Educational Institute (T.E.I) of Crete Heraklion, 71 004 Crete, Greece S Supporting Information *
ABSTRACT: Interfaces between perovskite solar cell (PSC) layer components play a pivotal role in obtaining high-performance premium cells and large-area modules. Graphene and related twodimensional materials (GRMs) can be used to “on-demand” tune the interface properties of PSCs. We successfully used GRMs to realize large-area (active area 50.6 cm2) perovskite-based solar modules (PSMs), achieving a record high power conversion efficiency of 12.6%. We on-demand modulated the photoelectrode charge dynamic by doping the mesoporous TiO2 (mTiO2) layer with graphene flakes. Moreover, we exploited lithium-neutralized graphene oxide flakes as interlayer at the mTiO2/perovskite interface to improve charge injection. Notably, prolonged aging tests have shown the long-term stability for both small- and largearea devices using graphene-doped mTiO2. Furthermore, the possibility of producing and processing GRMs in the form of inks opens a promising route for further scale-up and stabilization of the PSM, the gateway for the commercialization of this technology. in spin-coating the perovskite layer by using a mixture of γbutyrolactone (GBL) and dimethyl sulfoxide (DMSO) as the main solvents, followed by a toluene or chlorobenzene (used as antisolvents) treatments during the spinning process.15−18 However, although uniform and pinhole-free perovskite layers are produced, this approach is hardly scalable to large and module size substrates because of the difficulty in uniformly depositing by spin coating the antisolvent on large-area substrates. To overcome such limitations, several alternative techniques for the perovskite deposition have been proposed,19 with the solution-process methods being the most promising ones, e.g., enabling cost-effective20,21 roll-to-roll production.22 In particular, the one-step deposition allows a pinhole-free perovskite deposition directly on compact TiO2 layer (cTiO2),
T
he recent development of perovskite solar cell (PSC) technology gave rise to an unprecedented power conversion efficiency (PCE) improvement from η = 3.8%1 up to 22.1%2 in less than 7 years. The demonstrated PCE values make the PSC technology competitive with secondgeneration thin-film photovoltaics such as copper indium gallium selenide (CIGS) or cadmium telluride (CdTe).3 The PSC success is strictly linked with the remarkable efforts made to improve the device’s structure, to fine control the growth and the morphology of the active perovskite layer, and to engineer the interfaces between the cell’s constituent layers.4−10 The PCE record of PSCs11 has been achieved with the archetypal mesoscopic device configuration using n-type mesoporous TiO2 (mTiO2) layer as electron transport layer (ETL).12,13 For this structure,11 a mixture of formamidinium and methylammonium as the monovalent cations with the addition of inorganic cesium has been used to grow perovskite crystals, with the layer morphology finely controlled by exploiting a solvent-engineering technique.14 The latter consists © 2016 American Chemical Society
Received: December 9, 2016 Accepted: December 27, 2016 Published: December 27, 2016 279
DOI: 10.1021/acsenergylett.6b00672 ACS Energy Lett. 2017, 2, 279−287
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ACS Energy Letters
Figure 1. Structures of small-area PSCs: (a) PSC-A with the reference mesoscopic structure FTO/cTiO2/mTiO2/perovskite/spiro-OMeTAD/ Au, (b) PSC-B with GO-Li as interlayer between perovskite and mTiO2, (c) PSC-C using graphene-doped mTiO2 layer, and (d) combined PSC-D structure FTO/cTiO2/G+mTiO2/GO-Li/perovskite/spiro-OMeTAD/Au.
treatment of the mTiO2 layer prior to the perovskite deposition32,50 and/or TiO2 doping,51,52 (ii) the use of different TiO2 nanostructures53−55 and heterostructures,56 and (iii) the modification of energy levels by the addition of interface layers.57−59 Moreover, the potential to tune the perovskite/ mTiO2 interface60 allows the reduction of the I−V curve hysteresis,61 while at the same time improving the charge collection at the PE.32 Graphene and related two-dimensional (2D) materials (GRMs) are emerging as the paradigm shift of interface engineering to boost both photovoltaic performance19,62,63 and stability of PSCs.58,64 In fact, owing to their 2D nature and the large variety of 2D crystals possessing complementary (opto)electronic properties,65 which can be on-demand tuned by chemical functionalization and edge modification,66 GRMs can be considered ideal materials for PSC interface engineering. The first experiments have demonstrated the use of graphene oxide (GO) or reduced graphene oxide (RGO) as dopants in transport layers67−69 and as interlayers between perovskite and transporting layers70,71 with the aim of improving the charge collection mechanism at the electrodes.72,73 Moreover, GRMbased inks can be produced by cheap and high-yield manufacturing processes using nontoxic solvents such as ethanol (EtOH) or IPA.74−76 This allows the integration of GRMs in an in-line production process for large-area perovskite devices and modules, with the aim of reducing performance losses experienced by PSC scale-up.77,78 In fact, the scale-up process amplifies typical problems79 undergone in perovskite films deposition such as nanoscale pinholes,80 crystal grain boundaries,81 and perovskite film roughness,82 which can severely affect the film quality and consequently the module PCE.23,83−87 The aforementioned problems have so far limited the maximum delivered power (MDP) of PSMs. The record PSM efficiency, i.e., PCE = 14.9%, has been achieved for an active area of 4 cm2 with a MDP of 58 mW;88,89 when the active area is increased to 60 cm2, the PCE reduces to 8.7%,14 with a MDP of 552 mW. In this work, we demonstrate that GRMs can indeed be the key elements for an efficient strategy of PSC scale-up.
in the so-called planar configuration, leading to the most efficient fabrication procedure (i.e., PCE approaching 18%).23 However, planar PSCs usually suffer from large current−voltage (I−V) hysteresis phenomena.24−26 The insertion of a mTiO2 scaffold25 combined with a two-step perovskite deposition,27 with the deposition of PbI2 layer prior to dipping the substrate in a methylammonium iodide (MAI)−2-propanol (IPA) solution,28−30 reduces the I−V hysteresis.25 In fact, the presence of mesoporous metal oxide scaffolds such as Al2O331 or TiO232 aids the perovskite crystal formation, hindering short circuit between photo (PE) and counter (CE) electrodes.25,33 The two-step procedure is the preferred perovskite deposition method for large-area mesoscopic PSCs and perovskite-based solar modules (PSMs), ensuring a deeper perovskite infiltration into the mTiO2 scaffolds compared to the single-step process.34 This determines a fine control of the morphology of the perovskite capping layer over interpenetrated TiO2/perovskite substrate, which is beneficial for improving the perovskite film uniformity,35 boosting the performance of the final devices.27 Despite the remarkable PCE value achieved by using mesoscopic PSCs,36−38 record efficiency (22.1%)11 is yet far from the predicted efficiency limit (∼31%).39 Notably, losses due to interfacial recombination40 negatively affect the charge injection at perovskite/transporting layer interface. Similarly, poor charge transport in electron (ETL)41 and hole (HTL)42,43 transporting layers severely limits charge collection at the electrodes. These phenomena lead to a reduction of both device short-circuit current (ISC) and fill factor (FF),44−46 thus reducing the PCE. Among the recently identified recombination mechanisms for methylammonium lead triiodide (MAPI) based PSCs,47 those involving TiO2/MAPI and MAPI/hole transport material (HTM) interfaces play a crucial role in limiting the PCE.40 In particular, (i) interfacial electron transfer from the MAPI conduction band (CB) to the HTM and/or to TiO2 surface states47 and (ii) interfacial electron transfer from TiO2 CB to the HTM and/or to MAPI need to be prevented.48,49 A fine-tuning of interface and interlayer properties is mandatory to enhance the charge transport and extraction at the mTiO2/MAPI interface by (i) chemical 280
DOI: 10.1021/acsenergylett.6b00672 ACS Energy Lett. 2017, 2, 279−287
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ACS Energy Letters
Figure 2. Photovoltaic parameters measured at 1 SUN and relative standard deviation on 12 PSCs (open-circuit voltage, VOC; short-circuit current density, JSC; fill factor, FF; and PCE reported in panels a, b, c, and d, respectively), for the four investigated PSCs.
with respect to the PSC-A, mainly due to an increase of the short-circuit current density (JSC), see Figure 2. In particular, the insertion of GO-Li, as interlayer between perovskite and mTiO2 (PSC-B), leads to a significant improvement of the average JSC values (+18%), while a 4.3% loss in the averaged open-circuit voltage (VOC) is observed. We do hypothesize that the VOC reduction is linked with the presence of the GO-Li, which induces a downward displacement of the TiO 2 conduction band (CB) with a consequent reduction of VOC72,94 (see Figure 2a). With respect to the use of graphene flakes, the GO-Li insertion within the mesoporous layer resulted in an increased JSC (+9.6%) compared to that of the PSC-A (Figure 2b). Contrary to the GO-Li case, the graphene flakes addition does not lead to a reduction in the VOC value. Finally, the type D structure shows an overall PCE improvement of about 7% with respect to the reference one, which is linked with an increase of 7% of the JSC value, retaining, at the same time, satisfying VOC values when compared to that of the PSC-A. We point out that the reduction of the PCE standard deviation achieved in the case of PSC-D is highly desirable for large-area PSCs, where local inhomogeneity of the active layer can affect the device’s PCE.34,95 To gain a deeper understanding of the effect of GIE on the PSCs performance, electro-optical characterizations and transient measurements are carried out on encapsulated PSCs (see the Supporting Information for details). The increase in JSC for PSC-B and PSC-C (see Figure 2b), with respect to PSC-A, is confirmed by the incident photon-to-current conversion efficiency (IPCE) spectra and by the extracted integrated JSC values reported in Figure S5a. In particular, the GO-Li interlayer enhances the IPCE in the spectral range between 400 and 600 nm, with respect to the PSC-A,72 up to a maximum of +7% at 440 nm (see Figure S5b). An IPCE increase of 8.6% is observed at ∼760 nm. The long-wavelength IPCE rise with respect to reference PSC is more significant for PSC having the G+mTiO2 scaffold (PSC-B), see Figure S5b. This phenomenon could be linked with two different processes, i.e., (i) efficient electron injection from perovskite to mTiO2
Graphene interface engineering (GIE) is proposed as an effective way to boost PCE of both PSCs and PSMs by limiting the charge losses occurring at the perovskite/mTiO2 interface, improving at the same time the stability. In particular, lithiumneutralized graphene oxide (GO-Li) flakes have been introduced as interlayer at the mTiO2/perovskite interface72 with the aim of improving the charge injection from the perovskite to the mTiO2, while graphene flakes have been dispersed into the mTiO2 layer58 to speed up the charge dynamic at the PE.58 This allowed us to realize a GRMs-based PSC module having a PCE of 12.6% on an over 50 cm2 active area and a MDP of 638 mW at 1 SUN illumination conditions. Small-Area Cells. Small-area solar cells (0.1 cm2) are realized to assess the influence of GIE on PSCs with respect to the reference device (Figure 1a). We exploit graphene flakes and GO-Li produced by solution processing;75,90 see the Supporting Information for both technical details and morphological characterization. In addition, Raman characterization91−93 of graphene-based mesoscopic substrates is provided in Figure S3. By using the standard two-step production procedure detailed in the Supporting Information, the small-area reference PSCs (indicated in the following as PSC-A) have shown an averaged PCE of 13.5% calculated on 12 cells. In addition, 3 PSC sets are realized by using the GIE strategy. In particular, in sample B (PSC-B), a GO-Li interlayer is spin coated onto the mTiO2 layer before the perovskite deposition (Figure 1b), while in sample C (PSC-C), see Figure 1c, graphene ink is dispersed into the mTiO2 paste to form the graphene-doped cell scaffold (G+mTiO2); see the Supporting Information for experimental details and Raman characterization (Figure S3). Finally, in device D (PSC-D), the GO-Li interlayer is deposited onto the G+mTiO2 scaffold (Figure 1d), realizing a combined structure. The different PSC configurations are reported in Figure 1 (see Figure S4 for the corresponding energy band diagram), while the statistical photovoltaic parameters measured for each set of PSCs are reported in Figure 2. The GRM-based PSCs (i.e., PSC-B, PSC-C, and PSC-D) show higher PCEs (+14.8%, +13.6%, and +6.1%, respectively) 281
DOI: 10.1021/acsenergylett.6b00672 ACS Energy Lett. 2017, 2, 279−287
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Figure 3. (a) JSC and (b) normalized VOC rise profile acquired by retaining the tested devices in open-circuit conditions, in the dark, and by suddenly (t = 0) switching on the light at 1 SUN irradiation conditions.
have been used in dye-sensitized solar cell (DSC) technology and recently adopted also for PSCs.99,100 The VOC rise test is carried out by taking the device from steady-state operating conditions under dark at open circuit, switching on the light source, and monitoring the subsequent rise in photovoltage.101 The concentration of the photoinjected electrons in the TiO2 film primarily determines the photovoltage transient profile. Moreover, the buildup of electrons is in competition with the electron recombination processes, which is detrimental for the device performance. As reported in Figure 3b, both PSC-B and PSC-D show faster VOC rise profile, which can be associated with a better charge injection at the perovskite/mTiO2 interface, with respect to the reference one. This result is correlated to the increase of JSC vs Pinc slope already reported in Figure 3a. Remarkably, PSC-C and PSC-D have shown the fastest dynamic due to the presence of graphene flakes within the mTiO2 layer. Thus, the obtained PCE values reported in Figure 2d (i.e., PCE = 16.4% for top PCE using G+mTiO2 layer) are linked with optimized charge injection processes. The influence of GIE on the stability of the PSCs is assessed by both prolonged illumination at maximum power point (MPP) and shelf life tests at open-circuit conditions (see Figure.S6 and Figure.S7). Among the tested PSCs, the PSC-C and PSC-D retain ∼88% of the initial PCE after 16 h of endurance test, showing longer lifetime with respect to the one obtained by the other types of PSCs. Stationary and transient electro-optical analyses, reported in Figures S6 and S8, show that PSC-C and PSC-D have a moderated PE degradation with respect to PSC-A, suggesting a reduced occurrence of trap sites and charge recombination paths following the light soaking test. In contrast, the GO-Li interlayer dramatically affects the device’s long-term stability because of a significant reduction of the JSC (−84% after 16.5 h of light soaking test at 1 SUN). Lithium atoms degrade the perovskite layer,69,102−104 thus compromising the perovskite/ GO-Li/mTiO2 interface and, consequently, the electron injection process. Notably, the combined use of graphene in mTiO2 and GO-Li interlayer results in a considerably improved stability with respect to the PSC-B, especially for what concerns light stress at MPP. In fact, PSC-D (Figure S6d, curve D) retains more than 70% of the initial PCE after 16 h of prolonged 1 SUN stress test. Large-Area Modules. To test the effectiveness of our proposed GIE approach, we developed large-area modules, fabricated on a 10 × 10 cm2 substrate area, consisting of eight seriesconnected PSCs (active area 6.32 cm2), with an overall active area of 50.56 cm2. The module aperture ratio, i.e., the ratio between the active area and the aperture area, is approximately
and/or (ii) increased charge transport and collection at the PE. To get an insight into the physical mechanism responsible for the IPCE increase, discriminating the charge injection process from transport and collection phenomena, we carried out transient measurements. We recorded JSC (Figure 3a) and VOC (Figure S6a) over 3 decades of incident optical power (i.e., from 0.002 to 2 SUN) by using a white LED (see the Supporting Information for details). The JSC vs Pinc plots of Figure 3a show linear trends for all the investigated PSCs, an indication of energy level matching (see Figure S4) of the device component layers. In fact, a nonlinear trend of JSC versus light intensity is linked with the existence of energy barriers within the device, negatively affecting the charge extraction process.96 Moreover, a nonlinear shape is indicative of geminate recombination mechanism involving the hole− electron exciton and/or space charge limitation at the heterojunction associated with unbalanced electron and hole mobilities.97 For our PSCs, both the insertion of graphene into the mTiO2 layer (PSC-C) and the GO-Li interlayer between perovskite and mTiO2 (PSC-B) led to an improvement of the electron injection at the perovskite/TiO2 interface, as confirmed by the increase of JSC vs Pinc slope,96 i.e., up to 18.4% (Figure 3a), with respect to that of the reference PSC-A. In fact, for PSC-B the slope of the linear fitting is 224 mA/W, for PSC-C 217 mA/W, and for PSC-D 225 mA/W, while for the reference PSC-A a value of only 190 mA/W is obtained. The optimization of charge extraction has been experimentally demonstrated recently for both graphene flakes/perovskite58 and GO-Li/perovskite72 interfaces. In particular, Volonakis and Giustino98 predicted, by means of first-principles calculations on graphene−CH3NH3PbI3 interfaces, that pristine graphene suppresses the octahedral tilt in the first perovskite monolayer, leading to a nanoscale ferroelectric distortion with a permanent polarization. This interfacial ferroelectricity drives electron extraction from the perovskite, hindering electron−hole recombination,98 a phenomenon that could explain the electron injection mechanism at the perovskite/graphene-doped mTiO2 interface experimentally demonstrated in this work. Additionally, the energy level matching between GO-Li and the mTiO2 layer together with TiO2 trap passivation72 facilitate electron injection in PSCs with a GO-Li interlayer. A detailed study of the VOC dependence on Pinc (see Figure S6a) further confirms that neither graphene flakes nor GO-Li introduce trap states in PSCs. To investigate the influence of the GRM on the electron injection and collection at the PE of the devices, we tested the dynamic performance under pulsed light conditions with transient photovoltage (TPV) measurements.99 TPV tests 282
DOI: 10.1021/acsenergylett.6b00672 ACS Energy Lett. 2017, 2, 279−287
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+mTiO2 and GO-Li interlayer in PSM-D results in the most efficient PSM, i.e., exceeding by 9% the PCE of the reference one. The as-obtained PCE values are linked with the increase of FF (+8.8%), still maintaining a VOC of 8.57 V. Differently from the results obtained with PSCs, the PSM-D has shown the best PCE performance (Table 1), confirming the crucial role of GIE in retaining high PCE uniformity passing from small-area, i.e.,
