High Open‐Circuit Voltages in Tin‐Rich Low‐Bandgap Perovskite‐Based Planar Heterojunction Photovoltaics

Low-bandgap CH3NH3(PbxSn1–x)I3 (0 ≤ x ≤ 1) hybrid perovskites (e.g., ≈1.5–1.1 eV) demonstrating high surface coverage and superior optoelectronic properties are fabricated. State-of-the-art photovoltaic (PV) performance is reported with power conversion efficiencies approaching 10% in planar heterojunction architecture with small (100 cm2 V−1 s−1) intrinsic carrier mobilities. The performance of organometallic halide (hybrid) perovskite solar cells has improved dramatically in just a few years, with photovoltaic (PV) power conversion efficiencies (PCEs) now exceeding 22% for state-of-the-art devices.1-5 This remarkable result, coupled with their low cost, tunability, and versatile low-temperature preparation methods, makes hybrid perovskites one of the most promising semiconductor classes not just for solar energy harvesting, but also for light emitting diodes (LEDs), field effect transistors (FETs), and lasers.6-12 3D organometallic halide perovskites such as those studied here adopt the classic perovskite structure ABX3, where A represents the organic cation like methylammonium (MA); B the divalent metal ion like lead (Pb2+), tin (Sn2+), germanium (Ge2+); and X the halide like chlorine (Cl−), bromine (Br−), iodine (I−), a mixture of halides, or pseudohalide/molecular ions (BF4−).12-17 The vast majority of research conducted in this area has focused on “first generation” MAPbX3-based perovskites,8, 18 mainly due to the excellent tunability of their optoelectronic properties.18, 19 Characteristics such as high mobility,20, 21 high bimolecular recombination rate for charge carriers,10 and low exciton binding energy22-24 have resulted in the demonstration of high-performance films and devices for PV, LED,9, 12, 25-27 FET,21 and lasing applications.10, 28 One potential drawback of the MAPbX3 semiconductor family, however, is its relatively wide bandgap (3.2–1.6 eV), which dramatically limits its sensitivity in the near-IR (NIR) to mid-IR region of the solar spectrum. Replacement of Pb2+ for Sn2+ can extend the absorption range to below 1.3 eV,13, 14 with intermediate bandgaps obtained via the fractional substitution of the original cation.29-31 Of relevance to this work is the binary metal perovskite CH3NH3(PbxSn1–x)I3 0 ≤ x ≤ 1.30, 31 Interestingly, the bandgap bows and becomes lower when Sn2+ is substituted by Pb2+ for samples with 80% and 60% Sn content compared to 100% Sn-based perovskite, in line with previous observations.30, 31 While such tin-based perovskites offer tunable bandgaps down to 1.1 eV, the fabrication of efficient optoelectronic devices has been impeded by factors including poor semiconductor quality and low surface coverage.30 As a consequence, solar cells made using these perovskites often exhibit very low efficiencies, with typical PCEs 3 eV),50 and the longer wavelength absorption and EQE can be increased by adding molecular iodine into the precursor solution that reduces this pure chloride perovskite phase to below 10%, hence, enhancing the longer wavelength absorption.50 This is beyond the scope of the current work and is being investigated separately. Now, taking into account the PLQE of these materials at 1 sun intensity (0.0007 ± 10−5 and 0.009 ± 10−4 for 60% and 80% Sn content thin films, respectively) the VOC loss was estimated, using kTln (PLQE),51-53 to be ≈200 and 120 meV, respectively for 60% and 80% Sn content based PVs. Now, if we account for this loss from the maximum thermodynamically achievable VOC's for the 60% and 80% Sn content based PVs (0.97 and 0.89 V, respectively, with respect to their bandgaps) yields a VOC of around ≈0.77 V and 0.77 V, respectively. We emphasize that, remarkably, the measured VOC value for the 60% Sn content matches with the estimated maximum thermodynamically achievable VOC's suggesting that we do not have any additional loss other than the losses due to the low PLQEs which can possibly be attributed to the superior semiconductor quality we achieve using elevated temperature processing of perovskite. However, for the 80% Sn content PV device there is still an additional loss of ≈110 meV that can potentially arise from the defects present in this perovskite and poor quality of morphology compared to the 60% Sn content sample (see the Supporting Information, Figure S9). Therefore, further improvements in the VOC's in these systems can possibly come from improvement of the PLQEs, paving the way for VOC's close to the values predicted from the S–Q model. The VOC loss from the bandgap (Eg – qVOC) is plotted in the Figure 3D along with the predicted VOC values using the S–Q model for each bandgap.52 The S–Q limit of VOC changes for different bandgaps and has been adapted from Nayak and Cahen.52 We can observe that for the 60% and 80% Sn content devices if we consider the losses due to the low PLQEs, the 60% Sn devices do not show any additional losses. We measured the PL lifetimes for 60% and 80% Sn content samples at an excitation density of ≈5 × 1015 cm−3 (similar to the charge carrier density ≈1 sun) and we obtain lifetimes of 0.87 and 0.95 ns, respectively, with the decay fitted using the monomolecular recombination model (see the Supporting Information, Figure S10 and Table S2). The PL decay lifetimes for the 100% Sn sample were within the instrument limit of 0.3 ns. Such low PL lifetimes especially for the 60% and 80% Sn content samples along with the low PLQE values point toward highly dominant nonradiative processes and should have resulted in very poor PV performances. However, we observe very efficient performance from the 60% and 80% Sn content based perovskite PVs. This is only possible when the intrinsic mobilities are high because that enables efficient charge separation and prevents charge carrier recombination. To validate this, we performed terahertz pump-probe measurements that can probe intrinsic charge carrier mobility values in a thin film.20 We obtained high charge carrier mobility values in the range of 100–200 cm2 V−1 s−1 for perovskite thin-film samples with Sn content >50%, which might be the reason behind the remarkable performance of the low-bandgap perovskite PVs.39 More details are available in the Supporting Information, Figure S11 and Table S3. To further elucidate the charge recombination processes in CH3NH3(PbxSn1–x)I3 0 ≤ x ≤ 1 perovskite-based PV devices, we performed light-intensity-dependent PV characterization measurement on the 60% and 80% Sn content perovskite-based PVs. Figure 4A illustrates J–V curves of the 60% and 80% Sn content PVs under different incident light intensity from 0.02 to 1 sun (for a complete set of J–V curves, please see the Supporting Information, Figure S12). The J–V curves are almost parallel to the x-axis (voltage axis) around the JSC (voltage = 0 V) indicating that the series resistance has no effect on JSC in these two devices. Furthermore, the JSC and VOC at these different light intensities are captured in Figure 4B. Figure 4B (top) demonstrates the power law dependence, , of the JSC on incident light intensity, where the data are plotted on a log–log scale. The 60% Sn content based perovskite PV demonstrates an = 0.94 that is close to unity, indicating that there are no significant energy barriers and space charge limits for the charge extraction process.54, 55 On the other hand, for the 80% Sn content based perovskite PV that demonstrates a relatively smaller value of = 0.88 compare to the 60% Sn content based perovskite PV, this points toward the presence of space charge limitations in this device possibly due to poor thin-film coverage issues compared to its 60% Sn content counterpart (see the Supporting Information, Figure S9). The light-intensity-dependent VOC provides critical insights into the mechanism of recombination processes in the PV device. At VOC there is no net flow of current (J = 0 mA cm−2), which means that all the photogenerated charge carriers should recombine in the perovskite film. The corresponding charge carrier recombination process is reflected by the value of “n” determined by the slope of the VOC versus incident light intensity; n(kT/q) where q is the elementary charge, k is the Boltzmann constant, and T is the temperature.56, 57 VOC as a function of incident light intensity (I) is plotted on a linear–log scale as shown in Figure 4B (bottom) for 60% and 80% Sn perovskite-based PVs. For bimolecular charge carrier recombination, this ideality factor n approaches unity. Whereas, when the ideality factor n approaches 2, Shockley–Read–Hall (SRH) type, trap-assisted recombination dominates.56-59 In the case of 60% Sn-based perovskite PV device, the slope of the fall in VOC results in a at low incident light intensities (up to 20 mW cm−2). Thus, resulting in n ≈ 1.77 indicates SRH type of recombination in this irradiation regime. However, at higher irradiance regime (between 20 and 200 mW cm−2) that includes standard AM 1.5 (100 mW cm−2), we observe . Thus, resulting in n ≈ 0.93 indicates a bimolecular type of carrier recombination as demonstrated previously for highly efficient CH3NH3PbI3-based perovskite PVs.32, 57-59 This further confirms that at AM 1.5 intensities, traps can be filled at thermal where the recombination is mainly due to bimolecular processes and that do not the charge us to achieve high PCEs of ≈10% and VOC's close to the prediction of the S–Q model. However, for the 80% Sn content based perovskite PV, we observe only one regime from 1 to 200 mW where . Thus, resulting in an n = indicates an SRH type trap-assisted charge carrier recombination process at AM 1.5 intensities. This could be a result of a higher electronic disorder in 80% Sn perovskites compared to 60% Sn and the poor coverage (see the Supporting Information, Figure would result in hence, with the light intensity of the poor performance of 80% Sn-based perovskite PV devices is compared to their 60% Sn In we have a study the structural and properties of CH3NH3(PbxSn1–x)I3 0 ≤ x ≤ 1 Sn with the bandgap an anomalous bowing effect and to lower bandgap eV) perovskites than the pure Sn-based a novel elevated temperature processing we demonstrate low-bandgap perovskite thin films. We also the of different to sensitive absorption and the in the results to in the case of semiconductors. We the perovskite thin films in highly efficient PVs using planar heterojunction The PV devices using 60% and 80% Sn content materials with bandgaps of ≈ and 1.19 eV, respectively, demonstrated high PCEs reaching 10% and VOC's approaching the prediction of the S–Q model were for the losses due to nonradiative recombination and lead us to the that high charge carrier mobilities are to overcome the losses due to recombination as in these perovskites to achieve high high-performance low-bandgap perovskites could potentially open up optoelectronic in and can also be potentially in all perovskite or PV which to the S–Q CH3NH3(PbxSn1–x)I3 0 ≤ x ≤ 1 precursor solution was by lead and in a in to obtain a and precursor solution was prepared from tin and The CH3NH3(PbxSn1–x)I3 0 ≤ 1 perovskite was prepared by the corresponding of pure lead and tin perovskite precursor substrates were with by in an for which the substrates were under for solution was at for and at in for of the temperature of was increased to which was prepared for elevated temperature CH3NH3(PbxSn1–x)I3 0 ≤ x ≤ 1 perovskite precursor were also on at the perovskite film was made in the filled by CH3NH3(PbxSn1–x)I3 0 ≤ x ≤ 1 perovskite precursor on substrates with from up to and at for and to at for On top of perovskite layer, was at for from a 20 solution in In the was as the top using thermal the photovoltaic measurements were using an device where devices in a density–voltage (J–V) measurements were using a in the and under a solar for using a calibrated solar from for The factor for such was obtained from the EQE measurements from the measurements were under AM 1.5 at mW measurements were under the solar using a combination of a series of that changes the light of the solar The measurements from 1 to sun were using to the irradiation the J–V measurements were using a of and that the area of More than devices were for the 60% and 80% Sn content PV devices. 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PL and PLQE The PL and PLQE of thin-film samples were measured using an described A with an excitation power of mW and a focused of was to the samples was measured using an The samples were between two were measured in a at of the A energy of was with samples in a were at into the of the incident was measured using a calibrated using data were using the in the range was on perovskites samples made on substrates with different Sn under The measurements were using the with mW focused to a the terahertz resulting in a terahertz with a of ≈1 The sample was between the terahertz source and the at and data were at an of with respect to a substrate was from the of the peak in the measurement was achieved on a substrate separately. within the substrate result in which were by only the peak in the and would like to and for the and of for the of a sample for The from the and - and the for the of would like to the and for the and for a and would like to the and for the and The data this are available at The of the and was in the and in the on As a to our and this provides by the Such materials are and be for but are not or issues from than should be to the The is not for the content or of any by the than content) should be to the corresponding for the