http://pubs.acs.org/journal/aelccp
Reducing Surface Recombination Velocities at the Electrical Contacts Will Improve Perovskite Photovoltaics Jian Wang,† Weifei Fu,‡ Sarthak Jariwala,†,‡ Irika Sinha,† Alex K.-Y. Jen,†,‡,§,∥ and David S. Ginger*,† Downloaded via OPEN UNIV OF HONG KONG on January 23, 2019 at 13:28:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Department Department § Department ∥ Department ‡
of of of of
Chemistry, University of Washington, Seattle, Washington 98195, United States Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States Chemistry, City University of Hong Kong, Kowloon, Hong Kong Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong
S Supporting Information *
ABSTRACT: We explore the effects of nonradiative recombination at the extracting contacts on the achievable performance of halide perovskite photovoltaic cells. First, we perform device simulations using standard drift-diffusion models with experimental semiconductor parameters matching those of methylammonium lead triiodide (MAPbI3). We quantify the range of surface recombination velocities (SRVs) that would allow this archetypal perovskite to reach power conversion efficiencies of 27%. In particular, for contacts with well-aligned energy levels, SRVs of ∼1−10 cm/s should enable open-circuit voltages of 1.30 V, within 96% of the Shockley−Queisser limit. Next, we use time-resolved photoluminescence to experimentally determine the SRVs on 14 different common electron- and hole-extracting contacts, including TiO2, SnO2, ZnO, PCBM, ITIC, ICBA, TPBi, PEDOT:PSS, PTAA, PVK, NiO, MoO3, WO3, and spiro-OMeTAD. These results point the way to the selection and rational engineering of better contacts as a means to achieve higher efficiencies in perovskite solar cells. the surface,7,8 resulting in a decrease in the PLQE and PL lifetime.9,10 Recently, a number of studies have discussed the roles that energy level alignment,11−13 conductivity/mobility,14 stability,15 and processing conditions16 play in selecting optimum contacts for perovskite solar cells. Surface recombination, as quantified by the surface recombination velocity (SRV),17,18 despite being a well-known important parameter that limits solar cell performance,17,19−21 has so far received less academic interest in the context of perovskite solar cells.22−24 When the atomic lattice is abruptly broken at a surface/ interface, unsatisfied dangling bonds (or foreign bonds) introduce electronic energy levels inside of the bandgap that enhance electron−hole nonradiative recombination at the surface/interface by acting as stepping stones for charge carrier transitions between the conduction and valence bands. The
O
rganic−inorganic halide perovskite photovoltaics (PVs), with a demonstrated power conversion efficiency of over 23%,1 offer a pathway for continued cost reduction and efficiency increases in thin-film solar cells. Champion cells reported in the literature already have demonstrated 97% of the theoretical limit for current density and 98% for the fill factor, leaving improvement of the opencircuit voltage (VOC) as the largest remaining opportunity for continued increases in performance.2 Approaching theoretical VOC limits requires eliminating all competing nonradiative channels. Several studies have shown that processing additives and/or small-molecule passivators can lead to remarkably long carrier lifetimes,3−5 internal photoluminescence quantum efficiencies (PLQEs) even approaching 100%, and quasiFermi level splittings of over 97% of the radiative limit in the prototypical methylammonium lead triiodide perovskite (CH3NH3PbI3, MAPbI3) thin films.6 However, achieving these values in an operational device architecture has remained elusive because contacting the perovskite with extracting contacts generally induces new, nonradiative loss pathways at © 2018 American Chemical Society
Received: October 25, 2018 Accepted: December 10, 2018 Published: December 10, 2018 222
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volume, i.e., we assume no optical or transport loss beyond the interface. All other material/device parameters in the simulations are selected based on reported literature values (experimentally determined, whenever possible), as given in Table S1. Furthermore, for simplicity, we omit an explicit discussion of the contribution of ion/ion vacancy migration, which had been shown to impact the perovskite recombination properties,4,27 though these effects are treated indirectly through our use of TOPO passivation, which is believed to passivate halide ion vacancies.3 Here, we selected the SRVs for simulation (from high to low) to span a wide range encompassing the following scenarios: (1) Metallic contacts, which typically induce SRVs on the order of >105 cm/s in many semiconductors.17,21 As a result, metallic contacts, other than establishing a built-in field, usually do not prevent surface recombination of the minority carriers. (2) In the range of 10 3 −10 4 to represent “unpassivated” perovskite interfaces without induced defects. The few reported SRVs for native perovskite (MAPbBr3 single crystal, CsSnI3 ingot) surfaces have been on the order of 103− 104 cm/s for the free surface.22,23 (3) The reported lowest SRV for Si PV after passivation is on the order of 0.1−1 cm/s.19 (4) We chose 10−2 cm/s simply as an end point to represent a near-ideal interface. Here we assume that the SRV is identical at both contacts, i.e., the hole recombination velocity at the electron collection interface and the electron recombination velocity at the hole collection interface, and vary them simultaneously in logarithmic steps. Figure 1b shows that the simulated device efficiency increases from 20 to 27% as the SRV decreases from 105 to ∼1 cm/s and saturates at ∼27% upon further decreasing the SRV to 10−2 cm/s. The saturation at 27% PCE, below the Shockley−Queisser limit of 30.2% for a 1.63 eV gap material, is the result primarily of our practical choice to use for an experimental absorption profile, with a 1000 nm thick active layer (which results in imperfect absorption and a reduced JSC) and our choice of an experimental value for the bulk recombination coefficient rather than a coefficient chosen to match detailed balance arguments (which results in a reduced VOC compared to the detailed balance limit). By increasing device thickness or reducing the bulk recombination value, we can recover the theoretical limit (see SI Figure S3); however, our focus here is to ascertain to what extent surface recombination is likely to be limiting state of the art perovskite solar cells using practical experimental parameters today. Next, we turn to examine the effects of varying the SRV on the specific PV performance parameters. Figure 1c shows that, as the SRV decreases from ∼ 105 cm/s to ∼ 103 cm/s, the JSC increases rapidly to ∼22.93 mA/cm2, which corresponds to a ∼99.9% internal quantum efficiency for this film thickness, and saturates thereafter. This result reflects the fact that JSC increases linearly with respect to the carrier extraction, i.e., the difference between the generation (G) and recombination (R) rates, where G is a constant defined by only absorption and R is proportional to SRV. It is also noted that the JSC saturation value corresponds to a 92.9% collection yield of all photons above the selected bandgap (24.67 mA/cm2, 1.63 eV), which leaves behind a 1.74 mA/cm2 uncollected photocurrent due to the insufficient absorption near its band edge. On the other hand, as the SRV is decreased, VOC increases consistently up to 1.30 V at a SRV of ∼1 cm/s (Figure 1d), with the VOC gaining approximately 45 mV per decade reduction in SRV over the range from 104 to 10 cm/s. The plateau at ∼1.30 V is
SRV, defined as the constant ratio of the surface recombination rate (Rs) to excess minority carrier concentration at the surface/interface (Δns), SRV = Rs/Δns, is the surface analogue of the minority carrier lifetime in a bulk semiconductor. This parameter provides a convenient homogeneous boundary condition for the excess minority carrier concentration, which can be used in both device modeling calculations21 and interpretation of PL lifetime measurements.17,19 Historically, many PV technologies, such as in Si,17,19 III−IV,20 and CIGS,21 had been successfully optimized for higher performance by focusing on strategies to reduce SRV. In this study, we first perform drift-diffusion simulation to ascertain the likely impact of SRV on perovskite device efficiency at the highest levels of device performance as other properties like intrinsic bulk and surface defects are eliminated. We then report experimental measurements of the SRVs at the perovskite (MAPI 3 ) surface with or without surface passivation, as well as the SRVs at various perovskite/contact layer interfaces. Finally, we compare our simulations and experimental results with the reports of device performance achieved to date and discuss possible future contact layer development strategies. Figure 1 shows device performance parameters that we obtained using drift-diffusion simulations with the SCAPS
Figure 1. (a) SCAPS simulated J−V curves for a 1000 nm thick MAPbI3 cell with varying SRV values from 105 to 10−2 cm/s in logarithmic steps. Inset: Energy level diagram for the simulation (assuming 0.1 eV offset at both contacts; see SI Figure S1 for the impact of energy barrier selection). Detailed simulation parameters are given in Table S1. (b−e) Summarized PV parameters (PCE, JSC, VOC, FF) as a function SRV values. (The simulated values plateau slightly below the Shockley−Queisser limit of 30.2% as a combined result of our selection of limited thickness and bulk recombination coefficient, as discussed in the SI; see Figure S2.)
software package25,26 for a 1000 nm thick MAPI3 solar cell with the hypothetical architecture depicted in the inset of Figure 1A, using varying assumptions about the SRV. As SRV is purely a 2D parameter, here we consider only the absorber/ electron transport layer (ETL) and the absorber/hole transport layer (HTL) interface but not the ETL and HTL 223
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usually more conductive than the semiconductor, making analysis of the signal more challenging. In any event, radiative efficiency is what ultimately matters in approaching the maximum power conversion efficiency,28 leaving PL under open-circuit conditions as a straightforward probe. In TRPL measurements of a semiconductor slab with interfaces, an effective decay lifetime (τeff) is measured. We fit the transient PL decays (τeff) using a stretched exponential, consistent with distributed kinetics in a polycrystalline sample,3 which is then related to the bulk lifetime (τb) and surface lifetime (τs)18
within 96% of the Shockley−Queisser limit (1.355 V) based on the material bandgap (1.63 eV), and this plateau occurs in our simulations because of our adoption of an experimentally determined bimolecular recombination coefficient of the MAPbI3 material (4 × 10−11 cm3/s).3 This bulk bimolecular recombination becomes the dominant loss when SRV is below 100 cm/s, which ultimately limits the simulated device performance from approaching the detailed balance limit (see Figure S3 for details). Further improvements in perovskite thin-film growth and passivation to produce films with even higher internal PL quantum yields would still benefit from reduction of the SRV to below 10 cm/s. The VOC gain as the logarithm of the SRV is also expected, the slope of which is dependent on the selected energy offset (maximum slope of ∼60 mV per decade; see SI Figure S1), a direct result of the proportionality relation between VOC and nonradiative loss (kT/e × ln(1/PLQE)). As the FF at the maximum power point represents a recombination environment between the JSC and VOC scenarios, its dependence on SRV as well shows mixed behavior between these limits, as expected (Figure 1e). Comparing our simulation results to the current record efficiency of ∼23%,1 we could estimate that a SRV of ∼5 × 103 cm/s could be tolerated before becoming the limiting factor in the performance of today’s current cells. We emphasize that, while the leading efficiency cells possess slightly different perovskite material parameters due to the composition and processing variations, these simulation results based on prototypical MAPbI3 are meant to provide order of magnitude upper limits to help frame the search for better contact materials. In summary, the simulation results clearly show that interface engineering efforts to decrease the SRV down to the order of ∼1−10 cm/s could open the door to perovskite efficiencies up to ∼27%, which would result mainly from an increase in the achievable VOC. In addition to generally benchmarking the required SRV values, we note two specific points. First, there exists an interplay between the SRV values and the contact energy alignment on device performance. Figure S1 shows that the worse the contact alignment (higher energy offset), the lower the device performance at a medium/high SRV value, while they all saturate to ∼27% at very low SRV values. This result implies that a more poorly aligned contact has even more stringent requirements for reducing the SRV, which is a practical point as often extracting contacts do not necessarily form an ideal energy alignment with the perovskite active layer (Figure S5). Second, the impacts of the two interfaces are not identical but depend on whether they are located at the front or the back contacts (Figure S2). Because of the higher generation rate at the front (illuminated) contact, a high SRV at this interface would more significantly limit the device performance, yet the back interface remains important; both interfaces must be optimized to possibly achieve the predicted ∼27% efficiency. Nevertheless, results in Figure S2 suggest that there is possibly a judicious sequence when optimizing the contactsfirst the front contact and then the back contact (see the SI for more details). Having established a quantitative criterion for the SRV in perovskite devices, we next turn to the experimental determination of this parameter. In conventional PV technology such as Si, the free surface SRV can be determined by either time-resolved photoconductance (TRPC) or TRPL measurements.17,19 However, photoconductive approaches can be difficult to apply near interfaces as the contact layers are
1 1 1 = + τeff τb τs
(1)
In eq 1, τb is a factor that depends on only the bulk material properties, while τs is related to the SRV, sample thickness (W), and diffusion constant (D) of the excess carriers. Given that any thin-film sample would have a top and bottom surface/interface, previous study has established that τs can be approximated analytically in two limiting cases.18 First, when the SRVs at the top and bottom interfaces are identical, τs is given by eq 2 τs ≅
W 1 iW y + jjj zzz 2SRV Dk π {
2
when SRV1 = SRV2
(2)
On the other hand, when the SRV at one contact (SRV2) is far greater than the SRV at the other contact (SRV1) or, equivalently, when SRV1 ≈ 0, τs is approximately given by eq 3 τs ≅
W 4 iW y + jjj zzz SRV2 Dk π {
2
when SRV1 ≅ 0
(3)
We note that the sample thickness (W) can be determined and the diffusion constant (D) can be estimated from measured carrier mobility values (μ) based on the Einstein relationship (D = μkBT), where kB and T are the Boltzmann constant and temperature, respectively. Therefore, by measuring the bulk lifetime (τb) and knowing the boundary conditions (either SRV1 = SRV2 or SRV1 = 0), the SRV of the interface in question can be assessed from the measured TRPL lifetime (τeff). One approach to determine τb is to measure and extrapolate τeff from a very thick sample series, such as a Si wafer19 or perovskite single crystal,29 where bulk recombination dominates over surface recombination. However, it is difficult to adopt this method in polycrystalline perovskite thin films, where the thickness is usually limited below 1 μm due to practical processing considerations. To address this difficulty, we adopt the strategy of Lewis base surface passivation using trioctylphosphine oxide (TOPO) passivation on MAPI3 thin films, which can yield internal PL quantum efficiencies of over ∼90% and lifetimes exceeding ∼8 μs.3,6 Because we have shown that TOPO molecules only bond to the external surface (passivating surface defects),3 we make the assumption here that the nearunity internal PLQE of this processing route implies that remaining recombination occurs nearly all in bulk. In other words, this method allows us to establish a lower limit for the corresponding bulk lifetime (τb > 8 μs) because surface passivation alone can restore well-prepared samples to such levels of performance. With this estimate in hand, we can then determine the SRVs of MAPbI3(TOPO)/air interfaces and the glass/MAPbI3 interface by plotting the experimentally 224
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by measuring PL in this system. We also apply analysis to the unpassivated perovskite/air interface (without TOPO treatment). Doing so yields the data in open squares plotted in Figure 2, which suggests a ∼1000 cm/s SRV, which is consistent with SRV values reported for unpassivated perovskite crystals (MAPbBr3 and CsSnI3) in the literature,22,23 giving us confidence in the method. Next, we apply this approach to quantify the SRV at the interface between the perovskite layer and 14 common contact layers in both bottom and top contact configurations, including spiro-OMeTAD, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine (PTAA), poly(9-vinylcarbazole) (PVK), NiO, WO3, and MoO3 as HTLs and TiO2, ZnO, SnO2, phenyl-C61-butyric acid methyl ester (PC61BM), 1,3,5tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi), 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-sindaceno[1,2-b:5,6-b′]dithiophene (ITIC), and indene-C60 bisadduct (ICBA) as ETLs. The thicknesses and band structures of these contact layers are summarized in the SI (Experimental Details section and Figure S5). For contact layers usually processed underneath of the perovskite, we process TOPO treatment on the perovskite; for contact layers processed on top of the perovskite, we omit the TOPO treatment. In either case, the interface to be evaluated is identical to that in standard devices, while the other interface (either glass/perovskite or perovskite/TOPO) possess a low SRV (≤10 cm/s) value that would not limit this evaluation process. Figure 3a,c shows the raw PL decays for several representative contacts, along with the fits used to extract the lifetimes. It can be seen that all contacts reduce the PL lifetime relative to the glass/perovskite/TOPO structure, with some, such as PEDOT:PSS, spiro-OMeTAD, or PC61BM, reducing the lifetime by factors of 1000 or more. We note that
Figure 2. Calculated (lines) and experimental (squares) effective PL lifetimes, τeff, plotted vs the perovskite layer thickness for varying SRV values. Two conditions are assumed for the calculation to provide a bracket of SRV estimates: equal recombination at both contacts, SRV1 = SRV2 (solid lines), and zero recombination at one contact, SRV1 = 0, (dashed lines). These assumptions allow us to determine SRV to within an order of magnitude.
limiting scenarios (SRV1 = SRV2 or SRV1 ≈ 0) (eqs 2 and 3), we find that our data indicate that the SRV of our TOPOpassivated samples on glass lies between SRV2 ≈ 10 cm/s, if SRV1 ≈ 0 (dashed lines), and SRV ≈ 6 cm/s, if SRV1 = SRV2 (solid lines). The τeff values are similar when measured from either excitation direction; therefore, we cannot further resolve the SRV differences between these two interfaces. However, given that SRV can vary by orders of magnitude for different electrode interfaces, this relatively small level of uncertainty in our reference samples is sufficient to allow us to benchmark the SRVs of many perovskite/electrode contacts in use today, setting an upper limit (≤10 cm/s). In other words, with a clean long-lived reference sample, we can resolve SRV numerically down to ∼10 cm/s for any perovskite/contact layer interface
Figure 3. Experimental PL decays for perovskite films in contact with various HTLs (a) and various ETLs (c). Inset: shorter time windows for low lifetime stacks. Extracted SRV values for HTLs (b) and ETLs (d). The labels (B) and (T) represent that the contact layer is placed at the bottom (B) or on top (T) of the perovskites, respectively. 225
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∼10 cm/s can be achieved at both the electron- and holeextracting contacts. We also propose that PL-based will be increasingly useful in the search for new contacts and interface passivation strategies for perovskites as a means to probe SRVs without the need to optimize full device stacks. Finally, perovskite solar cells should stand to benefit from exploration of new passivation strategies or new extraction layers, perhaps by exploring a wider range of the OLED literature and perhaps by exploration of new device geometries, similar to the designs used for Si to reduce the amount of surface area between the absorbing Si and the extracting contact. Doing so provides a clear pathway to reaching single-layer perovskite efficiencies of 27% and beyond.
occasionally authors will interpret reductions in PL intensity and lifetime at open circuit upon contacting the active layer as indicative of a “good” contact that facilitates charge extraction. However, we believe that such an interpretation is incorrect. At VOC conditions, there is no net charge extraction, and any PL quenching is indicative of new nonradiative loss pathways created by the interface and should be avoided. Therefore, combined PL analyses at both VOC and JSC conditions are underway to discern these processes toward a thorough evaluation of contact layer materials. Figure 3b,d summarizes the extracted SRVs on various ETLs and HTLs either at the bottom (labeled as B) or on top (labeled as T) of the perovskite layer, all in relevant regular n− i−p or inverted p−i−n configurations. We find that the measured SRVs vary from ∼10 to ∼5000 cm/s with different contact layer interfaces. Some contact layers that are prevalently seen in leading efficient devices,30 such as PTAA, TiO2, and NiO, which interestingly all reside underneath of the perovskite, show low SRVs of 1,000 h Operational Stability. Nature Energy 2018, 3 (1), 68−74. (16) Dunlap-Shohl, W. A.; Daunis, T. B.; Wang, X.; Wang, J.; Zhang, B.; Barrera, D.; Yan, Y.; Hsu, J. W. P.; Mitzi, D. B. Room-Temperature Fabrication of a Delafossite CuCrO2 Hole Transport Layer for Perovskite Solar Cells. J. Mater. Chem. A 2018, 6 (2), 469−477. (17) Black, L. E. New Perspectives on Surface Passivation: Understanding the Si-Al2O3 Interface; Springer: Switzerland, 2016. (18) Sproul, A. B. Dimensionless Solution of the Equation Describing the Effect of Surface Recombination on Carrier Decay in Semiconductors. J. Appl. Phys. 1994, 76 (5), 2851−2854. (19) Grant, N. E.; Niewelt, T.; Wilson, N. R.; Wheeler-Jones, E. C.; Bullock, J.; Al-Amin, M.; Schubert, M. C.; van Veen, A. C.; Javey, A.; Murphy, J. D. Superacid-Treated Silicon Surfaces: Extending the
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