Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Yuchuan Shao; Zhengguo Xiao; Cheng Bi; Yongbo Yuan; Jinsong Huang

doi:10.1038/ncomms6784

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Shao, Yuchuan; Xiao, Zhengguo; Bi, Cheng; Yuan, Yongbo; Huang, Jinsong 2014-12-15 00:00:00 ARTICLE Received 22 Apr 2014 | Accepted 7 Nov 2014 | Published 15 Dec 2014 DOI: 10.1038/ncomms6784 Origin and elimination of photocurrent hysteresis by fullerene passivation in CH NH PbI planar 3 3 3 heterojunction solar cells 1 1 1 1 1 Yuchuan Shao , Zhengguo Xiao , Cheng Bi , Yongbo Yuan & Jinsong Huang The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efﬁciency of CH NH PbI solar cells. The elucidation of the origin 3 3 3 of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efﬁciency. Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0656, USA. Correspondence and requests for materials should be addressed to J.H. (email: [email protected]). NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 rganometal trihalide perovskites (OTPs) have recently Results drawn tremendous research interest as they have Eliminating photocurrent hysteresis by PCBM passivation. The Odemonstrated increased power conversion efﬁciency structure of the PHJ devices in this study is indium tin oxide 1–9 (PCE; above 15%) in o5 years of research . One of the (ITO)/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) most attractive features of OTP materials is their ability to form (PEDOT:PSS)/MAPbI (280B320 nm)/Phenyl-C61-butyric acid very good polycrystals through low-temperature solution methyl ester (PCBM) (20 nm)/C (20 nm)/2,9-dimethyl-4,7- 10–12 processes . Thus, the density of bulk crystal defects in diphenyl-1,10-phenanthroline (BCP, 8 nm)/aluminum (Al, perovskite materials can be much smaller than organic 100 nm), as shown in Fig. 1a. The MAPbI layers were formed by semiconductors in bulk-heterojunction solar cells, contributing a low-temperature solution process, interdiffusion of lead iodide to many of their intriguing properties, such as large charge (PbI ) and a methyl ammonium iodide (CH NH I, CH NH ¼ 2 3 3 3 3 13,14 14 carrier mobility , long carrier lifetime and recently observed MA) stacking layer followed by a solvent annealing process, 15 17,18 lasing . One big mystery in perovskite solar cell characterization which was recently developed in our research group . It forms is the presence of large photocurrent hysteresis in many smooth, compact perovskite ﬁlms with 100% surface coverage perovskite solar cells, which was recorded by many groups by and gives extremely small low leakage current on the order of 16 4 3 2 scanning photocurrents with increasing or decreasing voltages . 10 B10 mA cm . The solvent annealing markedly Generally, a much larger short circuit current density (J ) and increases the grain size to be comparable to or larger than ﬁlm SC open circuit voltage (V ) is recorded if the photocurrent sweep thickness, paving the way for the study of the fullerene OC starts from the forward bias (4V ) rather than from the reverse passivation effect. A cross-section scanning electron microscope OC bias. Sometimes a ﬁll factor (FF) 4100% was observed in our (SEM) image is shown in Fig. 2b, revealing the polycrystalline study. Photocurrent hysteresis imposes a serious issue on the nature of MAPbI . accurate determination of perovskite solar cell efﬁciencies and A PCBM/C double fullerene layer was inserted between the stability concerns. There are concerns on whether photocurrent perovskite and cathode as an electron acceptor and collection 19,20 hysteresis is an intrinsic property of perovskite materials, which layer . The PCBM layer was spun onto the perovskite layer might originate from the possible ferroelectricity of OTPs or the followed by low-temperature thermal annealing at 100 C for electromigration of ions in OTPs . varied durations of 0 to 60 min and the C layer was thermal In this manuscript, we identify the presence of a large density evaporated. We have previously shown that the spun PCBM layer of charge traps as the origin of the notorious photocurrent can conformably cover perovskite with intimate contact and hysteresis in the planar heterojunction (PHJ) perovskite solar permeate into the perovskite layer along the grain boundaries as cells, and demonstrate that these traps can be passivated by the the thermal annealing proceeds . Controlled devices without fullerene layers deposited on the perovskite layers, which PCBM layers were also fabricated with exactly the same eliminate photocurrent hysteresis and improve device parameters except for the PCBM layer. The photocurrents of performance. the control devices show a large hysteresis when scanning the Al BCP Al PCBM Fullerene Perovskite PEDOT:PSS Grain ITO Perovskite boundary Glass PEDOT:PSS ITO HV mag HFW pressure tilt WD 500 nm 1.49 µm 10.00 kV 200,000 × 1.10e-3 Pa 1 ° 8.9 mm W/O PCBM , scanning from - to + bias 1.0 –2 W/O PCBM , scanning from - to + bias PCBM 15min TA, scanning from - to + bias –4 PCBm 15min TA, scanning from + to - bias 0.8 PCBM 45min TA, scanning from - to + bias –6 PCBM 45min TA, scanning from + to - bias –8 Light off Light on Light off 0.6 –10 –12 W/O PCBM 0.4 PCBM 45min TA –14 –16 0.2 –18 –20 0.0 –22 0.0 0.2 0.4 0.6 0.8 1.0 –50 0 50 100 150 200 Voltage (V) Time (s) Figure 1 | Perovskite photovoltaic device structure and performance. (a) Device structure with PCBM layer. (b) Cross-section SEM image of perovskite devices with 45 min thermal annealing (TA) PCBM layer. (c) Photocurrents for devices without a PCBM layer (orange), with PCBM layers thermally annealed for 15 min (green) and 45 min (blue), respectively. Hollow triangles and solid triangles represent the scanning direction from negative to positive bias and from positive to negative bias, respectively. (d) Photocurrent rising process on turning on and turning off the incident light for the devices without PCBM layer (yellow) and with PCBM layer after 45 min thermal annealing (blue). 2 NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –2 Current density (mA cm ) Normalized current density NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 ARTICLE Conduction Band Air side Trap states PL transition before PCBM passivation PL transition after PCBM passivation Valence band ITO side N(E) 1.0 1.6 From ITO side 778 nm From air side 0.8 1.4 Fitted PL 1.2 778 nm PL from bulk 0.6 PL from trap states 1.0 0.4 0.8 0.6 0.2 Band transition 0.4 0.0 0.2 775 nm 782 nm Trap states (× 3) –0.2 0.0 700 750 800 850 750 775 800 825 850 875 Wavelength (nm) Wavelength (nm) Figure 2 | Photoluminescence for PCBM passivation effect from different incident directions. (a) Schematic of PL peaks blue-shift from passivation effect. (b) Experiment set-up. (c) The PL spectra for samples with 45 min thermal annealing PCBM layer with 532 nm green laser as excitation source from the air side (dark blue), from the ITO side (pink), and samples W/O PCBM layer from the air side (orange), from the ITO side (sky blue). (d) The PL spectra of the samples with 45 min thermal annealing PCBM layer excited by a 650-nm red laser, with incident light from the air side (red circle line) and from the ITO side (blue circle line, shifted for clarity). The PL spectrum with incident light from the air side were deconvoluted to two peaks, which can be assigned to emission from band transition (green dash line), and from trap states (orange dash line), which was magniﬁed by three times for clarity. photocurrents with increased or decreased bias at a scan rate of Origin and the location of the charge traps in MAPbI .An 0.05 V s , as shown in Fig. 1c. Such photocurrent hysteresis is important issue that must be addressed is to ﬁnd out where the also typically observed in many kinds of electronic devices, traps are in the bulk or at the surface/grain boundaries. Here we particularly defect-rich, organic-based electronic devices, used a photoluminescence (PL) study to identify the location of which contain a non-negligible amount of charge traps . The the charge traps by conﬁning the PL light excitation region close current hysteresis can be explained by the dynamic electric to the surface of perovskite layer or through the perovskite ﬁlms. ﬁeld/charge injection modulated charge trapping and detrapping Generally, the spontaneous radiative recombination between processes and is intentionally designed for some functional trap states leads to a red-shifted emission peak compared with 23,24 electronic devices, such as bistable memory . The that from the band edge transition and passivation of these trap photocurrent hysteresis decreases with a spun PCBM layer on states can blue-shift the PL peak, which is illustrated in Fig. 2a. top of a perovskite layer and completely disappears after We ﬁrst used an excitation light of 532 nm, which has a pene- annealing the PCBM layer for 45 min. A high PCE of 14.9% tration length of 80 nm, much less than the thickness of the under air mass 1.5 global (AM 1.5G) illumination was perovskite ﬁlms (280B320 nm). The geometry of the PL mea- obtained. Clearly, the application of a PCBM layer with surement is shown in Fig. 2b, in which the incident excitation appropriate thermal annealing duration markedly increased the light either from the ITO side or from the air side, and the PL PCE by 204% from 7.3 to 14.9%, which comes mainly from the from perovskite ﬁlms with and without PCBM layers were enhanced J and FF. measured. As shown in Fig. 2c, the perovskite ﬁlm without a SC These results demonstrate the presence of a large density of PCBM layer had a PL peak at 782 nm, which is independent of charge traps in annealed MAPbI ﬁlms, and PCBM can effectively the incident light directions, indicating the top and bottom passivate them. This conclusion is also supported by the increased surfaces have the same optical property. As expected, the per- device response speed after passivation. As shown in Fig. 1d, the ovskite passivated by PCBM had a blue-shifted PL peak from control devices without PCBM show a slow rising of photo- 782 to 775 nm when the top surface was excited, while the PL current to maximum value during a long duration (75 s) on peak did not show any shift when the incident light came from turning on the illumination, corresponding to the trap ﬁlling the ITO side. The results indicate that the PCBM can passivate process, while the photocurrent turns on almost instantly in the the trap states close to the top surface and/or along the grain optimized devices. The slow rising of photocurrent to maximum boundaries, and the permeation depth of PCBM in perovskite also explains J calculated from external quantum efﬁciency ﬁlms after 45 min of thermal annealing should be no more than SC curves in many publications. This does not agree with the 200 nm, otherwise the passivation effect should be detected at measured J under steady illumination because external the ITO side. To verify this hypothesis, a 650 nm continuous red SC quantum efﬁciency measurement is generally conducted at a laser, which has a much longer penetration depth comparable to much higher lock-in frequency. the MAPbI ﬁlm’s thickness, was used to excite the perovskite NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. PCBM Perovskite PEDOT:PSS ITO Normalized intensity (a.u.) Normalized PL intensity (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 ﬁlms passivated by PCBM. As shown in Fig. 2d, the PL peak 1E20 W/O PCBM shifted to 778 nm, which lies between the 775 and 782 nm, which No thermal annealing are assigned to the recombination from the band transition and Thermal annealing 15 min 1E19 Thermal annealing 45 min the trap states, respectively. The PL spectra did not show an obvious shift after changing the incident light directions from 1E18 the air side to the ITO side. These characterizations can be explained by the fact that the PL emission under red laser excitation comes from the entire ﬁlm. The PL spectra can be well 1E17 deconvoluted into two 775 and 782 nm peaks, as shown in Fig. 2d. In addition, a quench of perovskite PL by the PCBM of 1E16 70% was also observed, smaller than previously reported results, Band 1 Band 2 Band 3 which may stem from the different thicknesses of the perovskite 1E15 and PCBM layers and different thermal annealing processes . 0.30 0.35 0.40 0.45 0.50 0.55 The PL measurements, with an excitation light of different E() (eV) wavelengths clearly veriﬁed that the majority of trap states are close to the surface of the MAPbI ﬁlms. It can also be inferred Figure 3 | Trap density of states (tDOS) obtained by thermal admittance that theunpassivatedchargetraps closetothe ITOsidehave spectroscopy. tDOS for devices without PCBM (orange), with PCBM but no negligible hindrance to the hole collection, and thus the charge thermal annealing (red), with 15 min thermal annealing PCBM (green), traps are most likely electron traps. 45 min thermal annealing PCBM (blue). The presence of a large trap concentration close to the top surface of perovskite thin ﬁlms can be explained by the low thermal stability of these materials. Compared with the Schottky analysis. The applied angular frequency o deﬁnes an traditional inorganic semiconductors, OTPs are hybrid materials energetic demarcation, with much lower thermal stability. MAPbI has a reported decomposition temperature of 300 C(ref. 10), but the E ¼ k Tln ð2Þ o B decomposition at surfaces or grain boundaries may occur at much lower temperatures. A recent study by M. Gra¨tzel group where o is the attempt-to-escape frequency. The trap states showed that MAPbI decomposed after thermal annealing for a below the energy demarcation can capture or emit charges with short time at 150 C, and we found that perovoskites the given o and contribute to the capacitance. As shown in decomposed to PbI at an even lower thermal annealing Fig. 3, there was a relatively large density of defect states on the 17 19 3 temperature of 105 C if the thermal annealing duration was order of 1 10 to 1 10 m in the devices without any as long as 3 h (ref. 26). We infer that the MAPbI ﬁlm surfaces fullerene passivation, which explained the large hysteresis of and grain boundaries should be decomposed at a much shorter photocurrents observed. The tDOS with an energy level above thermal annealing time, which yields non-stoichiometry 0.40 eV (Band 2 and Band 3) decreased by nearly two orders of composition and dangling bonds and can cause midgap states magnitude just after the spin coating of PCBM on the perovskite and charge traps. As all of the MAPbI ﬁlms formed by the ﬁlms even without thermal annealing. The marked decrease of interdiffusion method used in this study went through thermal the tDOS was consistent with the decreased photocurrent annealing of 2 h, it is very likely that these traps originated from hysteresis, indicating the effective passivation of charge traps in the surface decomposition. perovskite by PCBM. It was noted that the relatively shallow trap states (Band 1, 0.35–0.40 eV) were only slightly passivated by PCBM without thermal annealing. However, after thermal Discussion annealingsothat the PCBM diffusedintoperovskite layers along After establishing that PCBM passivation can eliminate the grain boundaries, the density of states in Band 1 reduced photocurrent hysteresis in perovskite solar cells, we present signiﬁcantly, while there was barely any further reduction of evidences of the PCBM passivation effect by direct measurement deep-trap density. These results indicate that the deep traps of trap density before and after passivation, then the electronic located at the surface of the perovskite ﬁlms can be passivated transport property change of perovskite ﬁlms by Hall and without thermal annealing, while the shallow traps stay deeper transient photocurrent measurements and ﬁnally demonstrate the in the perovskite ﬁlms, such as grain boundaries, which can only reduced surface charge recombination in devices by impedance be passivated by the diffusion of PCBM into the perovskite spectroscopy (IS) modelling. layers. Thermal admittance spectroscopy (TAS) analysis was To ﬁndout theinﬂuenceofPCBMpassivation on thetransport used to quantize the reduction of trap states in perovskite properties of the perovskite ﬁlms, the charge carrier mobility and ﬁlms by the passivation of PCBM. TAS is a well-established, concentration was investigated using Hall Effect measurements. All effective technique for characterizing both shallow and of the perovskite ﬁlms fabricated by the interdiffusion method deep defects, which has been broadly applied in understanding 27,28 29 showed p-type behaviour. It was shown by our previous study that defects in thin ﬁlm and organic solar cells .The 2 1 1 hole mobility was increased from 2.5 to around 30 cm V s energetic proﬁle of trap density of states (tDOS) can be after thermal annealing of perovskite ﬁlms for 1.5 h, saturated derived from the angular frequency dependent capacitance with further extended annealing up to 3 h (ref. 26). The perovskite using the equation: ﬁlms in this study were thermal annealed for 2 h before PCBM V dC o bi deposition, so the mobility change of the perovkite ﬁlm by N ðE Þ¼ ð1Þ T o thermal annealing can be ignored. Figure 4a shows the variation qW do k T of the carrier concentration and mobility as the duration of where C is the capacitance, o is the angular frequency, q is the PCBM thermal annealing increased, from 15 min to 1 h. Hole 2 1 1 elementary charge, k is the Boltzmann’s constant and T is the mobility continued to increase to 114 cm V s after 45 min temperature. V and W are the built-in potential and depletion of annealing, which is the highest Hall mobility reported . The bi width, respectively, which were extracted from the Mott– hole concentration continuously decreased from 2.5 10 to 4 NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –3 –1 DOS (m eV ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 ARTICLE 3.0 1.0 PCBM TA for 45 min 2.5 W/O PCBM 100 0.8 PCBM TA for 45min 2.0 0.6 Mobility 80 Carrier 1.5 concentration 0.4 0.48 µs 1.0 0.2 0.5 0.0 0.38 µs 0.0 20 –0.2 020 40 60 –2 –1 0 123 4 PCBM thermal annealing time (min) Time (µs) Figure 4 | Electronic properties of perovskite ﬁlms without PCBM and with varied PCBM thermal annealing time. (a) Charge carrier concentration (orange) and mobility (blue) by Hall Effect measurements. (b) Charge transient time for samples without PCBM layer (orange) and with 45 min thermal annealing PCBM layer (blue) by transient photocurrent measurement. 13 3 4.3 10 cm as the thermal annealing duration increased charge recombination is much slower because the OTP thin ﬁlms from 0 to 45 min. We speculate that the perovskite top surfaces or were shown to have excellent crystallinity and low density of bulk the grain boundaries tended to decompose on thermal annealing, defects . t and t extracted under different bias are bulk surface which introduced more n-doping I vacancies and compensated shown in Fig. 5a,b. As expected, t is around two orders of surface the p-doping in the original perovskite ﬁlms. This was conﬁrmed magnitude smaller than t at zero bias and one order of bulk by the reduction in work function with Fermi energy moving magnitude smaller at V , and the overall charge recombination OC closer to midgap on thermal annealing, as observed by a previous in the devices is dominated by the surface recombination. t bulk ultraviolet photoelectron spectroscopy (UPS) study . After showed little variation under different PCBM passivation further increasing the thermal annealing time to 60 min, the conditions, while t increased about three times in whole- surface hole mobility and the carrier concentration became almost bias range after optimizing PCBM annealing conditions, proving invariant. The variation trend of the carrier mobility correlates our scenario that the PCBM passivation can effectively reduce the well with the variation of tDOS: when the tDOS decreases, the surface recombination but have little inﬂuence on the bulk hole mobility increases. As Hall effect measurement characterizes recombination. t is much more sensitive to the applied bias bulk the electronic property of the entire ﬁlm rather than the top than t , as the bulk charge recombination is sensitive to total surface surface, it is inferred that PCBM passivates the grain boundaries injected charge density , while the interface charge and reduces the energy barrier between grain domains, facilitating recombination is mainly determined by the surface trap density. the hole transportation in the plane direction. Therefore, the t increases with the decrease of the surface surface The Hall mobility represents carrier transportation behaviour trap density, matching the tDOS results from the TAS in the plane direction, but, under device operating conditions, the measurements. The IS modelling result demonstrated that charges transport in the out of plane direction, which means the surface recombination accounts for the major portion of Hall mobility does not characterize the charge extraction process. recombination in the devices, and PCBM passivation can To ﬁnd out how passivation affects the charge collection process, effectively reduce the surface recombination and increase device transient photocurrent (TPC) measurement was conducted to efﬁciency. Neverthereless, the t after pasivation is still much surface measure the average charge transit time across the ﬁlms after smaller than the bulk recombination t , indicating an bulk charge generation. As seen in Fig. 4b, the devices with 45 min of opportunity to further improve the device performance with thermal annealing a PCBM layer had a shorter charge transient more sophisticated surface passivaiton techniques. time of 0.38 ms than the devices without a PCBM layer (0.48 ms), The passivation effect of PCBM on the device operation is corresponding to an enhanced vertical charge carrier transit summarized by the device model shown in Fig. 5c. As the carrier 3 3 2 1 1 mobility from 1.9 10 to 2.4 10 cm V s . diffusion length in OTP bulk material (721 nm), calculated from We continued to measure the devices under real operating t and transit mobility, is much larger in terms of ﬁlm bulk conditions using IS and used modelling to identify the main thickness, the charges generated in OTPs should be able to reach recombination pathways in OPTs. IS has been widely utilized in the surface with negligible loss. The long diffusion length in the many photovoltaic systems, such as dye-sensitized solar cells , bulk of OTP ﬁlms is reasonable, because the optimized perovskite 33 34,35 organic solar cells and perovskite solar cells . The technique ﬁlm thickness is B600–700 nm while its highest efﬁciency is is a valuable tool to decouple electronic parameters, such as 15.6% (ref. 18). The electron recombination at the perovskite capacitances and resistances in photovoltaic devices, by analysing surface severely reduces the electron extraction efﬁciency at the the frequency-dependent alternating current response with cathode side, while PCBM on the perovskite ﬁlms can effectively appropriate equivalent circuits. The inset of Fig. 5a shows the passivate these electron traps and markedly reduce the interface equivalent circuit used for our curve ﬁtting, which is similar to charge recombination, which boosts the device J and FF. This is SC that used in organic solar cells by ref. 33 with modiﬁcation. Here further supported by the ﬁvefold smaller series resistance and the resistor-capacitor (RC) circuits of the bulk and interface almost comparable shunt resistance after PCBM passivation, as recombination were separated to ﬁnd out their individual summarized Supplementary Table 1. contribution to total charge recombination. The experiment We have revealed and highlighted the importance of the PCBM data can be well ﬁtted using this equavelent circuit, as shown in surface passivation effect to improve perovskite solar cell device Supplementary Fig. 1. Two time constants were extracted from performance. The mitigation of defect states is effective, which the IS modelling, which were assigned to the surface charge can be deduced from the signiﬁcant increase of photocurrent recombination with a short charge recombination lifetime response speed and decrease of the tDOS. Improving electronic (t ) and bulk charge recombination with a much longer properties of perovskite ﬁlms with optimum fullerene thermal surface charge recombination lifetime (t ). We assumed that the bulk annealing, including a reduced interface charge recombination, bulk NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. Carrier concentration *10 –3 (cm ) –2 –1 –1 Mobility (cm V s ) Normalized transient current Al BCP Al BCP PCBM Perovskite Perovskite PEDOT:PSS PEDOT:PSS ITO ITO ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 recombination within the crystal grains is negligible when the 10,000 geometry carrier diffusion length is longer than the crystal grain size. The passivation strategy was proved its merits in the OTP system. series R R bulk surface Most importantly, our work determined the origin of the 1,000 internal photocurrent hysteresis in perovskite solar cells and provides a simple and effective method to eliminate it paving the way for C C bulk surface accurate PCE measurements and further efﬁciency improvements. W/O PCBM PCBM no TA PCBM 15 min Methods PCBM 45 min Film formation and device fabrication. PEDOT:PSS (Baytron-P 4083, used as 0.0 0.2 0.4 0.6 0.8 1.0 received) was spun on a clean ITO substrate at a speed of 3,000 rounds per minute Voltage (V) (r.p.m.). The ﬁlm was then annealed at 110 C for 20 min. PbI and MAI were dissolved into dimethylformamide (DMF) and 2-propanol with concentrations of 1 1 450 mg ml for PbI and 45 mg ml for MAI, respectively. Both solutions were heated at 100 C for around 10 min before being used to make sure both MAI and PbI were fully dissolved. The PbI solution was spun on a PEDOT:PSS layer at 2 2 6,000 (r.p.m.) for 35 s. Then the PbI ﬁlm was transferred onto a 70 C hot plate for quick drying. The MAI solution was then spun on top of dried PbI ﬁlm at 6,000 r.p.m. for 35 s at room temperature to achieve ﬁlms with thicknesses ranging from 280–320 nm. The spin-coated PbI /MAI stacking ﬁlms were annealed at 100 C for 2 h. After they were cooled down to room temperature, PCBM (dis- solved into Dichlorobenzene (DCB), 2 wt %) was spun on top of the formed perovskite layers (for samples with a PCBM layer). After that, the ﬁlm was W/O PCBM annealed at 100 C for different thermal annealing times to let the PCBM crystalize PCBM no TA and diffuse into the perovskite layer. The device was ﬁnished by thermal evapor- PCBM TA 15 min PCBM TA 45 min ating C (20 nm), BCP (8 nm) and aluminum (100 nm) in sequential order. The 1 2 device area is the overlap of the ITO substrate and aluminum electrodes (6 mm ). 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Film and device characterization. Simulated AM 1.5G irradiation (100 mW cm ) was produced by a Xenon-lamp-based solar simulator (Oriel Weak bulk charge 67005, 150 W Solar Simulator) for current-voltage measurement. The light recombination intensity was calibrated by a silicon (Si) diode (Hamamatsu S1133) equipped with a Schott visible-colour glass-ﬁltered (KG5 colour-ﬁltered). The PL spectrum was measured by iHR320 Photoluminescence Spectroscopy at room temperature. A 532-nm green laser with an intensity of 10 mW cm from Laserglow Tech- nologies was used as the excitation source in PL measurement. The thermal admittance spectroscopy was performed using an E4980A Precision LCR Meter from Agilent at frequencies between 0.1 to 1,000 kHz. Hall effect and conductivity Electron measurements were performed with the six contacts van der Pauw method. The traps contacts were deposited by thermally evaporating 100 nm gold (Au) layers. Indium was used to attach the copper (Cu) wires on the Au contacts. The magnetic ﬁeld Weak bulk charge was kept invariant as 0.3 T through the measurements. Keithely 2400 source meter recombination was used to apply DC bias current, and a Keithely 4200 Model was used to record the Hall voltage. All samples were measured in air, under a dark environment, and at room temperature. The validity of the measurement was veriﬁed by measuring a 14 3 standard n-Si sample with 1.8 10 cm carrier concentration. Impedance spectroscopy was recorded by the E4980A Precision LCR Meter from Agilent with homemade software. For transient photocurrent measurement, 337-nm laser pulses Reduced electron with 4 ns in width and low intensity were applied to the short circuited devices in traps by passivation the dark. The photocurrent produced a transient voltage signal on a 50 O resistor, which was recorded by an oscilloscope. Hole Electron Photon Vertical transit mobility calculation. The transit mobility m is determined by Figure 5 | Impedance spectroscopy characterization of charge carrier m ¼ L /t V , where L is the thickness of the devices; t is the transit time extracted t t bi t recombination by PCBM passivation. (a) Bulk recombination time and from the transit photocurrent decay curve and V is the built-in potential, which is bi extracted from the Mott–Schottky analysis. (b) surface recombination time with different PCBM passivation conditions under different applied bias. Inset of (a): the equivalent circuits for impedance spectroscopy ﬁtting. (c) Schematic of the surface recombination References 1. Burschka, J. et al. Sequential deposition as a route to high-performance reduction by passivating the trap states. perovskite-sensitized solar cells. Nature 499, 316–319 (2013). 2. Liu, M., Johnston, M. B. & Snaith, H. J. Efﬁcient planar heterojunction perovskite solar cells by vapour deposition. 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Competing ﬁnancial interests: The authors declare no competing ﬁnancial interests. Appl. Phys. Lett. 85, 5655–5657 (2004). 25. Dualeh, A. et al. Effect of annealing temperature on ﬁlm morphology of Reprints and permission information is available online at http://npg.nature.com/ organic–inorganic hybrid pervoskite solid-state solar cells. Adv. Funct. Mater. reprintsandpermissions/ 24, 3250–3258 (2014). 26. Cheng, B. et al. Understanding the Formation and Evolution of Interdiffusion How to cite this article: Shao, Y. et al. Origin and elimination of photocurrent Grown Organolead Halide Perovskite Thin Films by Thermal Annealing. hysteresis by fullerene passivation in CH NH PbI planar heterojunction solar cells. 3 3 3 J. Mater. Chem. A 2, 18508–18514 (2014). Nat. Commun. 5:5784 doi: 10.1038/ncomms6784 (2014). NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. 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Publisher: Springer Journals
Copyright: Copyright © 2014 by Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.
Subject: Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
eISSN: 2041-1723
DOI: 10.1038/ncomms6784
Publisher site: See Article on Publisher Site

Abstract

ARTICLE Received 22 Apr 2014 | Accepted 7 Nov 2014 | Published 15 Dec 2014 DOI: 10.1038/ncomms6784 Origin and elimination of photocurrent hysteresis by fullerene passivation in CH NH PbI planar 3 3 3 heterojunction solar cells 1 1 1 1 1 Yuchuan Shao , Zhengguo Xiao , Cheng Bi , Yongbo Yuan & Jinsong Huang The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efﬁciency of CH NH PbI solar cells. The elucidation of the origin 3 3 3 of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efﬁciency. Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0656, USA. Correspondence and requests for materials should be addressed to J.H. (email: [email protected]). NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 rganometal trihalide perovskites (OTPs) have recently Results drawn tremendous research interest as they have Eliminating photocurrent hysteresis by PCBM passivation. The Odemonstrated increased power conversion efﬁciency structure of the PHJ devices in this study is indium tin oxide 1–9 (PCE; above 15%) in o5 years of research . One of the (ITO)/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) most attractive features of OTP materials is their ability to form (PEDOT:PSS)/MAPbI (280B320 nm)/Phenyl-C61-butyric acid very good polycrystals through low-temperature solution methyl ester (PCBM) (20 nm)/C (20 nm)/2,9-dimethyl-4,7- 10–12 processes . Thus, the density of bulk crystal defects in diphenyl-1,10-phenanthroline (BCP, 8 nm)/aluminum (Al, perovskite materials can be much smaller than organic 100 nm), as shown in Fig. 1a. The MAPbI layers were formed by semiconductors in bulk-heterojunction solar cells, contributing a low-temperature solution process, interdiffusion of lead iodide to many of their intriguing properties, such as large charge (PbI ) and a methyl ammonium iodide (CH NH I, CH NH ¼ 2 3 3 3 3 13,14 14 carrier mobility , long carrier lifetime and recently observed MA) stacking layer followed by a solvent annealing process, 15 17,18 lasing . One big mystery in perovskite solar cell characterization which was recently developed in our research group . It forms is the presence of large photocurrent hysteresis in many smooth, compact perovskite ﬁlms with 100% surface coverage perovskite solar cells, which was recorded by many groups by and gives extremely small low leakage current on the order of 16 4 3 2 scanning photocurrents with increasing or decreasing voltages . 10 B10 mA cm . The solvent annealing markedly Generally, a much larger short circuit current density (J ) and increases the grain size to be comparable to or larger than ﬁlm SC open circuit voltage (V ) is recorded if the photocurrent sweep thickness, paving the way for the study of the fullerene OC starts from the forward bias (4V ) rather than from the reverse passivation effect. A cross-section scanning electron microscope OC bias. Sometimes a ﬁll factor (FF) 4100% was observed in our (SEM) image is shown in Fig. 2b, revealing the polycrystalline study. Photocurrent hysteresis imposes a serious issue on the nature of MAPbI . accurate determination of perovskite solar cell efﬁciencies and A PCBM/C double fullerene layer was inserted between the stability concerns. There are concerns on whether photocurrent perovskite and cathode as an electron acceptor and collection 19,20 hysteresis is an intrinsic property of perovskite materials, which layer . The PCBM layer was spun onto the perovskite layer might originate from the possible ferroelectricity of OTPs or the followed by low-temperature thermal annealing at 100 C for electromigration of ions in OTPs . varied durations of 0 to 60 min and the C layer was thermal In this manuscript, we identify the presence of a large density evaporated. We have previously shown that the spun PCBM layer of charge traps as the origin of the notorious photocurrent can conformably cover perovskite with intimate contact and hysteresis in the planar heterojunction (PHJ) perovskite solar permeate into the perovskite layer along the grain boundaries as cells, and demonstrate that these traps can be passivated by the the thermal annealing proceeds . Controlled devices without fullerene layers deposited on the perovskite layers, which PCBM layers were also fabricated with exactly the same eliminate photocurrent hysteresis and improve device parameters except for the PCBM layer. The photocurrents of performance. the control devices show a large hysteresis when scanning the Al BCP Al PCBM Fullerene Perovskite PEDOT:PSS Grain ITO Perovskite boundary Glass PEDOT:PSS ITO HV mag HFW pressure tilt WD 500 nm 1.49 µm 10.00 kV 200,000 × 1.10e-3 Pa 1 ° 8.9 mm W/O PCBM , scanning from - to + bias 1.0 –2 W/O PCBM , scanning from - to + bias PCBM 15min TA, scanning from - to + bias –4 PCBm 15min TA, scanning from + to - bias 0.8 PCBM 45min TA, scanning from - to + bias –6 PCBM 45min TA, scanning from + to - bias –8 Light off Light on Light off 0.6 –10 –12 W/O PCBM 0.4 PCBM 45min TA –14 –16 0.2 –18 –20 0.0 –22 0.0 0.2 0.4 0.6 0.8 1.0 –50 0 50 100 150 200 Voltage (V) Time (s) Figure 1 | Perovskite photovoltaic device structure and performance. (a) Device structure with PCBM layer. (b) Cross-section SEM image of perovskite devices with 45 min thermal annealing (TA) PCBM layer. (c) Photocurrents for devices without a PCBM layer (orange), with PCBM layers thermally annealed for 15 min (green) and 45 min (blue), respectively. Hollow triangles and solid triangles represent the scanning direction from negative to positive bias and from positive to negative bias, respectively. (d) Photocurrent rising process on turning on and turning off the incident light for the devices without PCBM layer (yellow) and with PCBM layer after 45 min thermal annealing (blue). 2 NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –2 Current density (mA cm ) Normalized current density NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 ARTICLE Conduction Band Air side Trap states PL transition before PCBM passivation PL transition after PCBM passivation Valence band ITO side N(E) 1.0 1.6 From ITO side 778 nm From air side 0.8 1.4 Fitted PL 1.2 778 nm PL from bulk 0.6 PL from trap states 1.0 0.4 0.8 0.6 0.2 Band transition 0.4 0.0 0.2 775 nm 782 nm Trap states (× 3) –0.2 0.0 700 750 800 850 750 775 800 825 850 875 Wavelength (nm) Wavelength (nm) Figure 2 | Photoluminescence for PCBM passivation effect from different incident directions. (a) Schematic of PL peaks blue-shift from passivation effect. (b) Experiment set-up. (c) The PL spectra for samples with 45 min thermal annealing PCBM layer with 532 nm green laser as excitation source from the air side (dark blue), from the ITO side (pink), and samples W/O PCBM layer from the air side (orange), from the ITO side (sky blue). (d) The PL spectra of the samples with 45 min thermal annealing PCBM layer excited by a 650-nm red laser, with incident light from the air side (red circle line) and from the ITO side (blue circle line, shifted for clarity). The PL spectrum with incident light from the air side were deconvoluted to two peaks, which can be assigned to emission from band transition (green dash line), and from trap states (orange dash line), which was magniﬁed by three times for clarity. photocurrents with increased or decreased bias at a scan rate of Origin and the location of the charge traps in MAPbI .An 0.05 V s , as shown in Fig. 1c. Such photocurrent hysteresis is important issue that must be addressed is to ﬁnd out where the also typically observed in many kinds of electronic devices, traps are in the bulk or at the surface/grain boundaries. Here we particularly defect-rich, organic-based electronic devices, used a photoluminescence (PL) study to identify the location of which contain a non-negligible amount of charge traps . The the charge traps by conﬁning the PL light excitation region close current hysteresis can be explained by the dynamic electric to the surface of perovskite layer or through the perovskite ﬁlms. ﬁeld/charge injection modulated charge trapping and detrapping Generally, the spontaneous radiative recombination between processes and is intentionally designed for some functional trap states leads to a red-shifted emission peak compared with 23,24 electronic devices, such as bistable memory . The that from the band edge transition and passivation of these trap photocurrent hysteresis decreases with a spun PCBM layer on states can blue-shift the PL peak, which is illustrated in Fig. 2a. top of a perovskite layer and completely disappears after We ﬁrst used an excitation light of 532 nm, which has a pene- annealing the PCBM layer for 45 min. A high PCE of 14.9% tration length of 80 nm, much less than the thickness of the under air mass 1.5 global (AM 1.5G) illumination was perovskite ﬁlms (280B320 nm). The geometry of the PL mea- obtained. Clearly, the application of a PCBM layer with surement is shown in Fig. 2b, in which the incident excitation appropriate thermal annealing duration markedly increased the light either from the ITO side or from the air side, and the PL PCE by 204% from 7.3 to 14.9%, which comes mainly from the from perovskite ﬁlms with and without PCBM layers were enhanced J and FF. measured. As shown in Fig. 2c, the perovskite ﬁlm without a SC These results demonstrate the presence of a large density of PCBM layer had a PL peak at 782 nm, which is independent of charge traps in annealed MAPbI ﬁlms, and PCBM can effectively the incident light directions, indicating the top and bottom passivate them. This conclusion is also supported by the increased surfaces have the same optical property. As expected, the per- device response speed after passivation. As shown in Fig. 1d, the ovskite passivated by PCBM had a blue-shifted PL peak from control devices without PCBM show a slow rising of photo- 782 to 775 nm when the top surface was excited, while the PL current to maximum value during a long duration (75 s) on peak did not show any shift when the incident light came from turning on the illumination, corresponding to the trap ﬁlling the ITO side. The results indicate that the PCBM can passivate process, while the photocurrent turns on almost instantly in the the trap states close to the top surface and/or along the grain optimized devices. The slow rising of photocurrent to maximum boundaries, and the permeation depth of PCBM in perovskite also explains J calculated from external quantum efﬁciency ﬁlms after 45 min of thermal annealing should be no more than SC curves in many publications. This does not agree with the 200 nm, otherwise the passivation effect should be detected at measured J under steady illumination because external the ITO side. To verify this hypothesis, a 650 nm continuous red SC quantum efﬁciency measurement is generally conducted at a laser, which has a much longer penetration depth comparable to much higher lock-in frequency. the MAPbI ﬁlm’s thickness, was used to excite the perovskite NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 3 & 2014 Macmillan Publishers Limited. All rights reserved. PCBM Perovskite PEDOT:PSS ITO Normalized intensity (a.u.) Normalized PL intensity (a.u.) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 ﬁlms passivated by PCBM. As shown in Fig. 2d, the PL peak 1E20 W/O PCBM shifted to 778 nm, which lies between the 775 and 782 nm, which No thermal annealing are assigned to the recombination from the band transition and Thermal annealing 15 min 1E19 Thermal annealing 45 min the trap states, respectively. The PL spectra did not show an obvious shift after changing the incident light directions from 1E18 the air side to the ITO side. These characterizations can be explained by the fact that the PL emission under red laser excitation comes from the entire ﬁlm. The PL spectra can be well 1E17 deconvoluted into two 775 and 782 nm peaks, as shown in Fig. 2d. In addition, a quench of perovskite PL by the PCBM of 1E16 70% was also observed, smaller than previously reported results, Band 1 Band 2 Band 3 which may stem from the different thicknesses of the perovskite 1E15 and PCBM layers and different thermal annealing processes . 0.30 0.35 0.40 0.45 0.50 0.55 The PL measurements, with an excitation light of different E() (eV) wavelengths clearly veriﬁed that the majority of trap states are close to the surface of the MAPbI ﬁlms. It can also be inferred Figure 3 | Trap density of states (tDOS) obtained by thermal admittance that theunpassivatedchargetraps closetothe ITOsidehave spectroscopy. tDOS for devices without PCBM (orange), with PCBM but no negligible hindrance to the hole collection, and thus the charge thermal annealing (red), with 15 min thermal annealing PCBM (green), traps are most likely electron traps. 45 min thermal annealing PCBM (blue). The presence of a large trap concentration close to the top surface of perovskite thin ﬁlms can be explained by the low thermal stability of these materials. Compared with the Schottky analysis. The applied angular frequency o deﬁnes an traditional inorganic semiconductors, OTPs are hybrid materials energetic demarcation, with much lower thermal stability. MAPbI has a reported decomposition temperature of 300 C(ref. 10), but the E ¼ k Tln ð2Þ o B decomposition at surfaces or grain boundaries may occur at much lower temperatures. A recent study by M. Gra¨tzel group where o is the attempt-to-escape frequency. The trap states showed that MAPbI decomposed after thermal annealing for a below the energy demarcation can capture or emit charges with short time at 150 C, and we found that perovoskites the given o and contribute to the capacitance. As shown in decomposed to PbI at an even lower thermal annealing Fig. 3, there was a relatively large density of defect states on the 17 19 3 temperature of 105 C if the thermal annealing duration was order of 1 10 to 1 10 m in the devices without any as long as 3 h (ref. 26). We infer that the MAPbI ﬁlm surfaces fullerene passivation, which explained the large hysteresis of and grain boundaries should be decomposed at a much shorter photocurrents observed. The tDOS with an energy level above thermal annealing time, which yields non-stoichiometry 0.40 eV (Band 2 and Band 3) decreased by nearly two orders of composition and dangling bonds and can cause midgap states magnitude just after the spin coating of PCBM on the perovskite and charge traps. As all of the MAPbI ﬁlms formed by the ﬁlms even without thermal annealing. The marked decrease of interdiffusion method used in this study went through thermal the tDOS was consistent with the decreased photocurrent annealing of 2 h, it is very likely that these traps originated from hysteresis, indicating the effective passivation of charge traps in the surface decomposition. perovskite by PCBM. It was noted that the relatively shallow trap states (Band 1, 0.35–0.40 eV) were only slightly passivated by PCBM without thermal annealing. However, after thermal Discussion annealingsothat the PCBM diffusedintoperovskite layers along After establishing that PCBM passivation can eliminate the grain boundaries, the density of states in Band 1 reduced photocurrent hysteresis in perovskite solar cells, we present signiﬁcantly, while there was barely any further reduction of evidences of the PCBM passivation effect by direct measurement deep-trap density. These results indicate that the deep traps of trap density before and after passivation, then the electronic located at the surface of the perovskite ﬁlms can be passivated transport property change of perovskite ﬁlms by Hall and without thermal annealing, while the shallow traps stay deeper transient photocurrent measurements and ﬁnally demonstrate the in the perovskite ﬁlms, such as grain boundaries, which can only reduced surface charge recombination in devices by impedance be passivated by the diffusion of PCBM into the perovskite spectroscopy (IS) modelling. layers. Thermal admittance spectroscopy (TAS) analysis was To ﬁndout theinﬂuenceofPCBMpassivation on thetransport used to quantize the reduction of trap states in perovskite properties of the perovskite ﬁlms, the charge carrier mobility and ﬁlms by the passivation of PCBM. TAS is a well-established, concentration was investigated using Hall Effect measurements. All effective technique for characterizing both shallow and of the perovskite ﬁlms fabricated by the interdiffusion method deep defects, which has been broadly applied in understanding 27,28 29 showed p-type behaviour. It was shown by our previous study that defects in thin ﬁlm and organic solar cells .The 2 1 1 hole mobility was increased from 2.5 to around 30 cm V s energetic proﬁle of trap density of states (tDOS) can be after thermal annealing of perovskite ﬁlms for 1.5 h, saturated derived from the angular frequency dependent capacitance with further extended annealing up to 3 h (ref. 26). The perovskite using the equation: ﬁlms in this study were thermal annealed for 2 h before PCBM V dC o bi deposition, so the mobility change of the perovkite ﬁlm by N ðE Þ¼ ð1Þ T o thermal annealing can be ignored. Figure 4a shows the variation qW do k T of the carrier concentration and mobility as the duration of where C is the capacitance, o is the angular frequency, q is the PCBM thermal annealing increased, from 15 min to 1 h. Hole 2 1 1 elementary charge, k is the Boltzmann’s constant and T is the mobility continued to increase to 114 cm V s after 45 min temperature. V and W are the built-in potential and depletion of annealing, which is the highest Hall mobility reported . The bi width, respectively, which were extracted from the Mott– hole concentration continuously decreased from 2.5 10 to 4 NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. –3 –1 DOS (m eV ) NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 ARTICLE 3.0 1.0 PCBM TA for 45 min 2.5 W/O PCBM 100 0.8 PCBM TA for 45min 2.0 0.6 Mobility 80 Carrier 1.5 concentration 0.4 0.48 µs 1.0 0.2 0.5 0.0 0.38 µs 0.0 20 –0.2 020 40 60 –2 –1 0 123 4 PCBM thermal annealing time (min) Time (µs) Figure 4 | Electronic properties of perovskite ﬁlms without PCBM and with varied PCBM thermal annealing time. (a) Charge carrier concentration (orange) and mobility (blue) by Hall Effect measurements. (b) Charge transient time for samples without PCBM layer (orange) and with 45 min thermal annealing PCBM layer (blue) by transient photocurrent measurement. 13 3 4.3 10 cm as the thermal annealing duration increased charge recombination is much slower because the OTP thin ﬁlms from 0 to 45 min. We speculate that the perovskite top surfaces or were shown to have excellent crystallinity and low density of bulk the grain boundaries tended to decompose on thermal annealing, defects . t and t extracted under different bias are bulk surface which introduced more n-doping I vacancies and compensated shown in Fig. 5a,b. As expected, t is around two orders of surface the p-doping in the original perovskite ﬁlms. This was conﬁrmed magnitude smaller than t at zero bias and one order of bulk by the reduction in work function with Fermi energy moving magnitude smaller at V , and the overall charge recombination OC closer to midgap on thermal annealing, as observed by a previous in the devices is dominated by the surface recombination. t bulk ultraviolet photoelectron spectroscopy (UPS) study . After showed little variation under different PCBM passivation further increasing the thermal annealing time to 60 min, the conditions, while t increased about three times in whole- surface hole mobility and the carrier concentration became almost bias range after optimizing PCBM annealing conditions, proving invariant. The variation trend of the carrier mobility correlates our scenario that the PCBM passivation can effectively reduce the well with the variation of tDOS: when the tDOS decreases, the surface recombination but have little inﬂuence on the bulk hole mobility increases. As Hall effect measurement characterizes recombination. t is much more sensitive to the applied bias bulk the electronic property of the entire ﬁlm rather than the top than t , as the bulk charge recombination is sensitive to total surface surface, it is inferred that PCBM passivates the grain boundaries injected charge density , while the interface charge and reduces the energy barrier between grain domains, facilitating recombination is mainly determined by the surface trap density. the hole transportation in the plane direction. Therefore, the t increases with the decrease of the surface surface The Hall mobility represents carrier transportation behaviour trap density, matching the tDOS results from the TAS in the plane direction, but, under device operating conditions, the measurements. The IS modelling result demonstrated that charges transport in the out of plane direction, which means the surface recombination accounts for the major portion of Hall mobility does not characterize the charge extraction process. recombination in the devices, and PCBM passivation can To ﬁnd out how passivation affects the charge collection process, effectively reduce the surface recombination and increase device transient photocurrent (TPC) measurement was conducted to efﬁciency. Neverthereless, the t after pasivation is still much surface measure the average charge transit time across the ﬁlms after smaller than the bulk recombination t , indicating an bulk charge generation. As seen in Fig. 4b, the devices with 45 min of opportunity to further improve the device performance with thermal annealing a PCBM layer had a shorter charge transient more sophisticated surface passivaiton techniques. time of 0.38 ms than the devices without a PCBM layer (0.48 ms), The passivation effect of PCBM on the device operation is corresponding to an enhanced vertical charge carrier transit summarized by the device model shown in Fig. 5c. As the carrier 3 3 2 1 1 mobility from 1.9 10 to 2.4 10 cm V s . diffusion length in OTP bulk material (721 nm), calculated from We continued to measure the devices under real operating t and transit mobility, is much larger in terms of ﬁlm bulk conditions using IS and used modelling to identify the main thickness, the charges generated in OTPs should be able to reach recombination pathways in OPTs. IS has been widely utilized in the surface with negligible loss. The long diffusion length in the many photovoltaic systems, such as dye-sensitized solar cells , bulk of OTP ﬁlms is reasonable, because the optimized perovskite 33 34,35 organic solar cells and perovskite solar cells . The technique ﬁlm thickness is B600–700 nm while its highest efﬁciency is is a valuable tool to decouple electronic parameters, such as 15.6% (ref. 18). The electron recombination at the perovskite capacitances and resistances in photovoltaic devices, by analysing surface severely reduces the electron extraction efﬁciency at the the frequency-dependent alternating current response with cathode side, while PCBM on the perovskite ﬁlms can effectively appropriate equivalent circuits. The inset of Fig. 5a shows the passivate these electron traps and markedly reduce the interface equivalent circuit used for our curve ﬁtting, which is similar to charge recombination, which boosts the device J and FF. This is SC that used in organic solar cells by ref. 33 with modiﬁcation. Here further supported by the ﬁvefold smaller series resistance and the resistor-capacitor (RC) circuits of the bulk and interface almost comparable shunt resistance after PCBM passivation, as recombination were separated to ﬁnd out their individual summarized Supplementary Table 1. contribution to total charge recombination. The experiment We have revealed and highlighted the importance of the PCBM data can be well ﬁtted using this equavelent circuit, as shown in surface passivation effect to improve perovskite solar cell device Supplementary Fig. 1. Two time constants were extracted from performance. The mitigation of defect states is effective, which the IS modelling, which were assigned to the surface charge can be deduced from the signiﬁcant increase of photocurrent recombination with a short charge recombination lifetime response speed and decrease of the tDOS. Improving electronic (t ) and bulk charge recombination with a much longer properties of perovskite ﬁlms with optimum fullerene thermal surface charge recombination lifetime (t ). We assumed that the bulk annealing, including a reduced interface charge recombination, bulk NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 5 & 2014 Macmillan Publishers Limited. All rights reserved. Carrier concentration *10 –3 (cm ) –2 –1 –1 Mobility (cm V s ) Normalized transient current Al BCP Al BCP PCBM Perovskite Perovskite PEDOT:PSS PEDOT:PSS ITO ITO ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms6784 recombination within the crystal grains is negligible when the 10,000 geometry carrier diffusion length is longer than the crystal grain size. The passivation strategy was proved its merits in the OTP system. series R R bulk surface Most importantly, our work determined the origin of the 1,000 internal photocurrent hysteresis in perovskite solar cells and provides a simple and effective method to eliminate it paving the way for C C bulk surface accurate PCE measurements and further efﬁciency improvements. W/O PCBM PCBM no TA PCBM 15 min Methods PCBM 45 min Film formation and device fabrication. PEDOT:PSS (Baytron-P 4083, used as 0.0 0.2 0.4 0.6 0.8 1.0 received) was spun on a clean ITO substrate at a speed of 3,000 rounds per minute Voltage (V) (r.p.m.). The ﬁlm was then annealed at 110 C for 20 min. PbI and MAI were dissolved into dimethylformamide (DMF) and 2-propanol with concentrations of 1 1 450 mg ml for PbI and 45 mg ml for MAI, respectively. Both solutions were heated at 100 C for around 10 min before being used to make sure both MAI and PbI were fully dissolved. The PbI solution was spun on a PEDOT:PSS layer at 2 2 6,000 (r.p.m.) for 35 s. Then the PbI ﬁlm was transferred onto a 70 C hot plate for quick drying. The MAI solution was then spun on top of dried PbI ﬁlm at 6,000 r.p.m. for 35 s at room temperature to achieve ﬁlms with thicknesses ranging from 280–320 nm. The spin-coated PbI /MAI stacking ﬁlms were annealed at 100 C for 2 h. After they were cooled down to room temperature, PCBM (dis- solved into Dichlorobenzene (DCB), 2 wt %) was spun on top of the formed perovskite layers (for samples with a PCBM layer). After that, the ﬁlm was W/O PCBM annealed at 100 C for different thermal annealing times to let the PCBM crystalize PCBM no TA and diffuse into the perovskite layer. The device was ﬁnished by thermal evapor- PCBM TA 15 min PCBM TA 45 min ating C (20 nm), BCP (8 nm) and aluminum (100 nm) in sequential order. The 1 2 device area is the overlap of the ITO substrate and aluminum electrodes (6 mm ). 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V) Film and device characterization. Simulated AM 1.5G irradiation (100 mW cm ) was produced by a Xenon-lamp-based solar simulator (Oriel Weak bulk charge 67005, 150 W Solar Simulator) for current-voltage measurement. The light recombination intensity was calibrated by a silicon (Si) diode (Hamamatsu S1133) equipped with a Schott visible-colour glass-ﬁltered (KG5 colour-ﬁltered). The PL spectrum was measured by iHR320 Photoluminescence Spectroscopy at room temperature. A 532-nm green laser with an intensity of 10 mW cm from Laserglow Tech- nologies was used as the excitation source in PL measurement. The thermal admittance spectroscopy was performed using an E4980A Precision LCR Meter from Agilent at frequencies between 0.1 to 1,000 kHz. Hall effect and conductivity Electron measurements were performed with the six contacts van der Pauw method. The traps contacts were deposited by thermally evaporating 100 nm gold (Au) layers. Indium was used to attach the copper (Cu) wires on the Au contacts. The magnetic ﬁeld Weak bulk charge was kept invariant as 0.3 T through the measurements. Keithely 2400 source meter recombination was used to apply DC bias current, and a Keithely 4200 Model was used to record the Hall voltage. All samples were measured in air, under a dark environment, and at room temperature. The validity of the measurement was veriﬁed by measuring a 14 3 standard n-Si sample with 1.8 10 cm carrier concentration. Impedance spectroscopy was recorded by the E4980A Precision LCR Meter from Agilent with homemade software. For transient photocurrent measurement, 337-nm laser pulses Reduced electron with 4 ns in width and low intensity were applied to the short circuited devices in traps by passivation the dark. The photocurrent produced a transient voltage signal on a 50 O resistor, which was recorded by an oscilloscope. 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Competing ﬁnancial interests: The authors declare no competing ﬁnancial interests. Appl. Phys. Lett. 85, 5655–5657 (2004). 25. Dualeh, A. et al. Effect of annealing temperature on ﬁlm morphology of Reprints and permission information is available online at http://npg.nature.com/ organic–inorganic hybrid pervoskite solid-state solar cells. Adv. Funct. Mater. reprintsandpermissions/ 24, 3250–3258 (2014). 26. Cheng, B. et al. Understanding the Formation and Evolution of Interdiffusion How to cite this article: Shao, Y. et al. Origin and elimination of photocurrent Grown Organolead Halide Perovskite Thin Films by Thermal Annealing. hysteresis by fullerene passivation in CH NH PbI planar heterojunction solar cells. 3 3 3 J. Mater. Chem. A 2, 18508–18514 (2014). Nat. Commun. 5:5784 doi: 10.1038/ncomms6784 (2014). NATURE COMMUNICATIONS | 5:5784 | DOI: 10.1038/ncomms6784 | www.nature.com/naturecommunications 7 & 2014 Macmillan Publishers Limited. All rights reserved.

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Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

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Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

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