Solar Cells Using PbS Colloidal Quantum Dot Film - ACS Publications

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6.5% Certified SbSe Solar Cells Using PbS Colloidal Quantum Dot Film as Hole Transporting Layer Chao Chen, Liang Wang, Liang Gao, Dahyun Nam, Dengbing Li, Kanghua Li, Yang Zhao, Cong Ge, Hyeonsik Cheong, Huan Liu, Haisheng Song, and Jiang Tang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00648 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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6.5% Certified Sb2Se3 Solar Cells Using PbS Colloidal Quantum Dot Film as Hole Transporting Layer Chao Chen,‡#∆// Liang Wang,‡#∆// Liang Gao,‡#∆// Dahyun Nam,┴ Dengbing Li,#∆// Kanghua Li,#∆// Yang Zhao,#∆// Cong Ge,#∆// Hyeonsik Cheong,┴ Huan Liu,∆ Haisheng Song,#∆ and Jiang Tang*#∆// #

Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and

Technology, 1037 Luoyu Road, Wuhan, 430074, Hubei, P. R. China. ∆

School of Optical and Electronic Information, Huazhong University of Science and

Technology, 1037 Luoyu Road, Wuhan, 430074, Hubei, P. R. China. //

Shenzhen R&D Center of Huazhong University of Science and Technology, Shenzhen, 518000,

P. R. China ┴

Department of Physics, Sogang University, Seoul 04107, Korea

Corresponding Author *E-mail: [email protected]

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ABSTRACT: Sb2Se3 is a promising candidate for thin film photovoltaics, with a suitable bandgap, benign grain boundaries, Earth-abundant constituents, non-toxic constituents and excellent stability. But low doping density (1013 cm-3) of Sb2Se3 absorber and back contact barrier limit its efficiency. Here we introduced PbS colloidal quantum dot (CQD) film as the hole transporting layer (HTL) to construct a n-i-p configurated device and overcame these problems. Through simulation guided optimization, we have significantly improved the efficiency of Sb2Se3 thin film solar cell to a new record of certified 6.5%. The PbS CQD HTL not only minimized carrier recombination loss at the back contact and boosted carrier collection efficiency, but also contributed photocurrent by its own near-infrared absorption. Furthermore, these n-i-p devices also demonstrated improved device uniformity, achieving 6.39% in a 1.02 cm2 device.

TOC GRAPHICS


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Simple binary and ternary semiconductors consisting of non-toxic and Earth-abundant elements such as Cu2O,1 SnS,2 Sb2Se3,3-4 CuSbS25 and CuSbSe26 have been intensively explored as absorber materials for thin film photovoltaics recently. Among them, Sb2Se3 not only has a suitable band gap (direct 1.17 eV and indirect 1.03 eV)7 enabling ~30% Shockley–Queisser efficiency limit,8 but also possesses intrinsically benign grain boundaries if properly aligned along c-axis due to its peculiar one dimensional crystal structure.4 Furthermore, it is a binary compound with high vapor pressure and easy crystallization, enabling simple evaporation to generate high quality polycrystalline films.9 Thus, Sb2Se3 is attractive as a promising light absorber for photovoltaic devices. The significant progress of Sb2Se3 solar cells has been made in superstrate, substrate and sensitized configurations during the last few years. For superstrate solar cells, our group first reported 1.9% efficient CdS/Sb2Se3 solar cells with Sb2Se3 absorber film fabricated by thermal evaporation.10 Further, based on the rapid thermal evaporation (RTE) technique, we obtained high quality Sb2Se3 films and then 5.6% certified efficiency.4 Very recently, nontoxic ZnO was applied as the buffer layer, and the ZnO/Sb2Se3 device showed certified 5.93% efficiency with exceptional stability.3 For substrate solar cells, by in-situ selenization during Sb2Se3 evaporation process, the best efficiency of 4.25% was achieved.11 In parallel, sensitized Sb2Se3 solar cells were fabricated by electrodeposition and spin-coating methods with power conversion efficiencies (PCE) as 2.1% and 3.21%, respectively.12-13 Clearly, Sb2Se3 solar cells could be produced through multiple strategies with different device configurations, and the major challenge is to further improve device efficiency.


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For the most widely studied superstrate CdS/Sb2Se3 thin film solar cells, the current device efficiency is limited by two known obstacles: low doping density and back contact barrier. The optimal doping density for the absorber layer is ~1016 cm-3 to obtain a balance between built-in potential and depletion width.14 For our RTE derived Sb2Se3 films, the doping density lies at ~1013 cm-3,15 which is even worse than CdTe (~1014 cm-3)16 and requires substantial improvement. Our group have made great efforts to improve the hole concentration but unfortunately the results are not very satisfactory possibly due to the peculiar one-dimensional crystal structure.17-18 For the second, temperature dependent current density-voltage (J-V) measurements revealed rollover at low temperature, suggesting a non-ohmic back contact (Figure S1). We attempt to introduce a HTL to construct a n-i-p configuration, because it not only resolves the back-contact issue but also circumvents the doping challenge through using the lightly doped Sb2Se3 layer as the intrinsic layer. For reference, the most efficient hybrid lead halide perovskite solar cells exclusively adopted this configuration, as either n-i-p or p-i-n using NiO19 or PTAA20 to collect holes and SnO221 or TiO222 to collect electrons. The most efficient PbS CQD solar cells also employed the n-i-p device structure, with ZnO served as the n-layer and 1,2-ethanedithiol (EDT) treated PbS CQD film as the p-layer.23 We are thus confident that the n-i-p configuration are worthy of pursuing for efficiency improvement.

In this manuscript, we presented a 6.5% certified Sb2Se3 solar cells using PbS CQD film as hole transporting layer. Firstly, we identified the requisite conditions for HTL by solar-cellcapacitance-simulator (SCAPS) simulation as ∆Ev (valence band offset) within 0.1 to -0.2 V and the hole density (p) beyond 1017 cm-3. Then, we selected PbS CQD films as HTL, and the size of CQDs, the ligand treatment and the film thickness were all well optimized. Finally, we obtained


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a n-i-p device with significantly improved efficiency (from 5.4% to a certified 6.50%) due to the improvement of all the device parameters [open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF)]. The PbS CQD HTL not only minimizes carrier recombination loss at the back contact and boosts carrier collection efficiency, but also contributes photocurrent by its own near-infrared absorption. Furthermore, these n-i-p devices also demonstrate improved device uniformity, making it possible to construct large area devices.

Figure 1. Device architecture of a prototypical Sb2Se3 solar cell a) without and b) with HTL. c) PCE, d) VOC, e) JSC, f) FF as a function of HTL carrier concentration (p) and valence band offset (∆Ev) between Sb2Se3 and HTL. Energy band diagram for g) CdS/Sb2Se3 and h) CdS/Sb2Se3/HTL devices under AM 1.5G irradiation at short-circuit conditions. The solid and


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hollow circles represent the electron and hole, respectively. The curved arrows stand for the recombination of electron-hole pair. The straight arrow represents transmission of electron and hole. The blue and red lines show the quasi-Fermi levels of electrons and holes.

To identify the best HTL for the n-i-p device, simulation using the SCPAS software was first implemented in indium-doped tin oxide (ITO)/CdS/Sb2Se3/HTL/Au device.24 After a series of validation and derivation to match the experiment results, the baseline parameters of materials for SCPAS simulations were obtained in Table S1-3, which coincide with the reported material values.15 The simulated device performance without HTL were consistent with our typical device (VOC=0.390 V, JSC=25.6 mA cm-2, FF=54.6%, PCE=5.45%), confirming our simulation is highly reasonable and reliable. Figure 1a,b are the device architectures of our Sb2Se3 solar cells without and with the HTL. For the conventional n-p device, electric field in the depletion region assists the separation and extraction of photo-generated carriers via the drift mechanism (Figure 1g). However, electrons and holes could also recombine at the back surface, leading to photocurrent loss. In contrast, when a thin HTL with appropriate band position was applied at the back surface, electrons would be blocked and hole extraction could be enhanced, further facilitating charge collection (Figure 1h). In the simulation, we mainly took two of the most critical factors into account, ∆Ev (difference value of the valence band between Sb2Se3 and HTL) which affects the hole transport and p of HTL which directly influences the built-in electric field at the back. We systematically varied ∆Ev from -0.3 to 0.3 eV and p from 1014 to 1020 cm−3. Figure 1c-f show the VOC, JSC, FF and PCE mapping as a function of ∆Ev and p. Simulation results indicate that the requirements for HTL to achieve optimal VOC, JSC, FF and PCE simultaneously are ∆Ev within 0.1 to -0.2 eV and p beyond 1017 cm-3. If the ∆Ev is out of 0.1 to -0.2 eV, the FF and JSC


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will dramatically decline due to the blocked carrier transport. If p is lower than 1017 cm-3, band bending between Sb2Se3/HTL will be too inappreciable to have obvious improvement of VOC. We thus apply these two parameters to guide the HTL selection for our Sb2Se3 solar cells.

The simulation results inspire us to explore PbS CQDs as the HTL for Sb2Se3 solar cells, because the band position and doping density of PbS CQD films are highly tunable through CQD size control and ligand adjustment.25 PbS has a Bohr radius of 18 nm and its band gap could be easily tuned from 0.38 eV to 2.0 eV through size modification.26 On top of that, the band position of PbS CQD film could vary by up to 0.9 eV between different ligand treatments due to the intrinsic dipole of ligands as well as CQD-ligand interface dipole difference.25 Furthermore, depending on the surface composition of PbS CQDs, film doping density is tunable from heavily n-type using halide passivation in inert atmosphere to highly p-type by thiols treatments carried out in air ambient.27 In addition, PbS CQDs could be spin-coated at room temperature, avoiding possible damage to the underneath Sb2Se3 layer; it could also likely contribute extra light absorption to overcome the insufficient absorption of photons beyond 900 nm by our ~400 nm thick Sb2Se3 film. All combined together, PbS CQDs are a good choice at present for the HTL and we thus carefully optimized the size, ligand treatment and film thickness. eM


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Figure 2. a) Energy level diagram between the corresponding different sized PbS CQDs and Sb2Se3 in our devices. The numbers of x-axis are the corresponding excitonic peak positions of PbS CQDs. b) Energy level diagram of PbS CQDs employing different ligand exchange. c) PCE of devices using different sized PbS CQD as HTL. d) PCE and VOC of devices with different ligand treated PbS CQDs as HTL.

We first optimized the size of PbS CQDs for the HTL. Since the band gap and band position of PbS CQDs are highly sensitive to their size, valence band offset at the Sb2Se3/PbS interface could thus be modified by regulating the size of PbS CQDs. PbS CQDs of different sizes with exciton absorption peaks at 700, 820, 935 and 1320 nm named as PbS-700, PbS-820, PbS-935, PbS-1320 were obtained (Figure S2) and applied as HTL for comparison. For the CdS/Sb2Se3 device, CdS buffer was produced using the chemical bath deposition followed by the ambient


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CdCl2 treatment, and Sb2Se3 absorber was fabricated employing our standard RTE process.4 PbS CQDs were subsequently deposited on the CdS/Sb2Se3 device surface by the layer-by-layer spincoating procedure. Band alignment between different size of PbS CQDs and Sb2Se3 is shown in Figure 2a, with the values from literature reports.28 The conduction and valence band positions of Sb2Se3 are -4.15 and -5.3 eV, respectively.9, 29 Since the valence-band maximum (VBM) of PbS CQDs all lies near -5.1 eV which is a little shallower than that of Sb2Se3 (-5.3 eV), ∆Ev (-0.2 eV) sufficiently favors hole injection from Sb2Se3 into PbS CQD HTL. On the other hand, the conduction band minimum (CBM) becomes deeper as the PbS CQD size increases. The CBM of PbS-1320 is deeper than that of Sb2Se3 (-4.15 eV), causing harmful electron injection from Sb2Se3 into PbS CQD HTL and severe recombination loss at the PbS/Sb2Se3 interface. The significance of band alignment is fully revealed by the photovoltaic device performance under standard AM 1.5G (100 mW cm2) testing conditions, as shown in Figure 2c. Clearly, despite to different extents, all devices showed improvement upon the addition of PbS CQD HTL, mainly due to the removal of back contact barrier. PbS-1320 results in the least enhancement because of its improper band alignment and hence undesirable recombination loss at the back contact. Although PbS-820 and PbS-700 have enough high CBM to prevent election injection into PbS CQD HTL, they improve the device performance less than PbS-935. This is because PbS-935 contributes more long-wavelength (900-1100 nm) photo-generated carriers than PbS-820 and PbS-700. PbS-935 HTL produced the best performance for further optimization.

As stated before, ligand exchange also plays a critical role in the band position and doping density of PbS CQD film, both of which decide the HTL effect on CdS/Sb2Se3 devices. Here we chose EDT, 3-mercaptopropionic acid (MPA), tetramethylammonium hydroxide (TMAOH) and


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tetrabutylammonium iodide (TBAI) to treat PbS CQDs of 935 nm (the optimal size for our device as discussed above). All the treatments were carried out in air ambient following the optimized procedure for PbS CQD solar cells.23 The CBM and VBM of PbS CQD film using different ligand exchange, combined with the work function measured by Kelvin probe, are shown in Figure 2b. The work functions of PbS CQD films with EDT and TMAOH treatments are closer to VBM, in accordance with the strong p-type doping (beyond 1017 cm-3).30-31 TBAI treated PbS CQD film is weakly n-type, and MPA treated film is weakly p-type, consistent with literature reports.32 As a result, Sb2Se3 solar cells using EDT or TMAOH treated PbS CQDs as HTL have higher build-in field, which is in agreement with previous theoretical analysis. Figure 2d shows PCE and VOC of devices with PbS CQD HTL treated by different ligand exchange. Data extracted from 100 devices were used for the statistics. Sb2Se3 devices employing TMAOH treated PbS CQDs as HTL shows the highest VOC, approaching 0.44 V. The devices with EDT or MPA treated PbS CQDs as HTL demonstrated higher VOC than the control devices, but devices employing TBAI treated PbS CQDs shows even lower VOC than the control devices due to the ntype nature of TBAI treated PbS CQDs. This is consistent with the trend of Fermi level position (Figure 2b), as VOC is largely dictated by the quasi-Fermi level difference between CdS and HTL upon illumination. For device performance, EDT treated PbS CQD HTL resulted in the highest efficiency while devices employing TMAOH treated PbS CQDs demonstrates unsatisfactory efficiency. As TMAOH is strongly basic, it reacted with the fragile Sb2Se3 film during the treatment and then caused severe damage to the film, leading to low efficiency. EDT is inertness toward the Sb2Se3 absorber, and EDT treated PbS CQD film particularly carried out in dry air possesses the optimal doping density and band position to serve as the best HTL for Sb2Se3 devices. Figure S3 shows the device performance with PbS CQD HTL treated in wet air, dry air


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and N2.

Figure 3. a) J-V characteristics of device with and without PbS CQD HTL. b) Histogram of device performance measured on 100 separate devices with and without HTL. The standard efficiency deviations for control devices and devices with HTL are 0.27% and 0.08%, respectively. The LBIC graph of devices c) without and d) with PbS CQD HTL, respectively. e) J-V characteristics of a large (1.02 cm2) device employing PbS CQD HTL tested under standard AM1.5G illumination. Table 1. Device characteristics of Sb2Se3 solar cells with and without PbS CQD HTL

Sample Control PbS CQD HTL (our lab) PbS CQD HTL (certified)

VOC (V) 0.398 0.427 0.427

JSC (mA cm-2) 25.8 27.7 25.5

FF (%) 52.7 58.0 59.3

PCE (%) 5.42 6.87 6.50

Further, through optimizing the thickness of PbS CQD HTL, the best device performance was


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obtained with about 30 nm PbS CQD film (Figure S4). Figure 3a presents the J-V curves of the best device with PbS CQD HTL and the control device under simulated AM 1.5G (100 mW cm2

) illumination. The detailed device parameters are also summarized in Table 1. Compared with

the control devices, VOC, JSC and FF are relatively enhanced by 7.3%, 7.4% and 10.1% after using PbS CQD film as HTL, respectively, resulting in a 27% relative improvement in PCE from 5.4% to 6.87%. We also sent one device to Newport Corporation for certification and obtained a certified device efficiency of 6.5% as VOC of 427 mV, JSC of 25.5 mA cm-2 and FF of 59.3% (Figure S5, cert. #2893.01, issued on Jan. 19, 2016). This efficiency remains the highest certified efficiency for Sb2Se3 solar cells so far. To check the reproducibility, over 100 separate devices with and without HTL were tested. The histograms of the device performance are shown in Figure 3b. The average efficiency of the control devices is 5.08% with a standard deviation of 0.27%, while the devices with optimal HTL demonstrate an average efficiency of 6.62% with a standard deviation of 0.08%. Obviously, the PbS CQD HTL not only significantly increases device efficiency, but also decreases the standard deviation of device efficiency. Light-beaminduced current (LBIC) measurements were carried to check the spatial uniformity of our devices. The photocurrent mappings, as shown in Figure 3c,d, evaluate the spatial nonuniformity and localized performance of the investigated photovoltaic devices. The scanning results made by LBIC reveal that the control device shows a larger photocurrent difference of 136 pA with a standard deviation of 1.7 pA, while the corresponding values for the device with PbS CQD HTL were 118 pA and 1.0 pA, respectively. Whereas the net value of these numbers allows no direct comparison between different samples since the system was not calibrated and the connection could be varied from sample to sample, the ratio of standard deviation to mean value is physically meaningful. The calculated ratios for the devices with and without HTL are


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8.47×10-3 and 1.25×10-2, respectively. The calculated ratio unambiguously confirmed that device homogeneity is significantly enhanced upon the employment of a PbS CQD HTL. The layer-bylayer spin-coated PbS CQD film fills the concaves of Sb2Se3 films to constitute a smoother surface, which is confirmed by our atomic force microscope (AFM) results (Figure S6). Hence, the more uniform back electrode contact leads to the more homogeneous device. The large-area (1.02 cm2) devices as shown in Figure 3e achieved the highest efficiency of 6.39%, over 18% higher than the previous record 5.4% (1.08 cm2 area).4 This result is very encouraging because there is often a substantial efficiency gap between small and relatively large sized devices particularly for some next generation photovoltaic devices. This result also means that the RTE derived Sb2Se3 absorber films are homogeneous and the back contact plays a critical role in determining device uniformity.

Figure 4. a) EQE of the champion cell with HTL and the control cell. The black line in the inset


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is the EQE difference between device with and without PbS CQD HTL, and the blue line is the product of absorption (A) of PbS CQDs HTL and the transmittance (T) of Sb2Se3 film. b) VOCdecay and c) C-2-V plot of devices with and without PbS CQD HTL. d) The VOC and PCE stability of Sb2Se3 solar cell with and without HTL stored in the dry air without encapsulation. e) J-V characteristics of a diode based on Sb2Se3 solar cell with and without HTL under dark conditions. f) Rsh parameter histogram of device measured on 100 separate devices with and without HTL. The device efficiencies for devices with and without HTL demonstrate a distribution with an average efficiency of 5.08%, 6.62%, respectively. The standard deviations of Rsh are 17.0 and 19.1 Ω cm2 for devices with and without HTL, respectively.

We now discuss the origins of device performance improvement. The external quantum efficiency (EQE) spectra of devices with and without PbS CQD HTL are shown in Figure 4a. Clearly, the introduction of the PbS CQD HTL improves the EQE value throughout the whole responsive spectrum. EQE gain between 300 and 550 nm is translated into a photocurrent increase of 0.52 mA cm-2 by integrating the EQE difference with the standard AM 1.5G solar spectrum. In this wavelength region, photons are fully absorbed near the CdS/Sb2Se3 interface, and the photon-generated electrons and holes are subsequently separated by built-in electric field, and collected by CdS layer and HTL, respectively. Electrons can be efficiently collected because these electrons are generated near the CdS layer. Holes, however, must transport a long distance to be collected by HTL. Due to the low hole mobility of Sb2Se3,15 the hole transport is limited and results in photocurrent loss.12 With the help of PbS CQD layer, the enhanced field could increase the drift length and promote hole collection efficiency. Because the back PbS CQD layer will not change the properties of CdS/Sb2Se3 interface, we believe the enhanced hole


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collection capability by the PbS CQD HTL boosts the collection of photogenerated holes near the front CdS/Sb2Se3 heterojunction and consequently results in EQE improvement at the short wavelength.12 The EQE also has a conspicuous improvement between 600 and 950 nm, corresponding to an integrated photocurrent increase of 1.38 mA cm-2. The difference between the EQE values at this range, as shown in the inset of Figure 4a with black dot line, exhibits a peak at 935 nm which is in accord with the peak of A×T (the blue dot line in the inset of Figure 4a), where A is the absorption of PbS CQDs HTL and T is the transmittance of Sb2Se3 film. This observation means that the PbS CQD layer absorbs near infrared photons passing through the Sb2Se3 absorber and partially converts them into photo-generated carriers. We must stress that since the weak absorption of PbS CQD layer contributed a small photocurrent, the PbS CQD layer does not act as a conventional HTL. In order not to confuse people, we still call PbS CQD layer as HTL for two reasons. (I) The dominant junction is provided by CdS/Sb2Se3, please see the analysis of the built-in electric field in the next content. (II) Most of light is absorbed by Sb2Se3 layer, because the photons with wavelength from 300nm to 850 nm can completely be absorbed by 400 nm thick Sb2Se3 layer.7 In addition, EQE enhancement between 600 to 850 nm is originated from the enlargement of the depletion region width (xd). Capacitance-voltage (C-V) measurements (Figure 4c) were used to deduce xd as 214 nm for devices with HTL and 195 nm for devices without HTL, respectively. Wider xd could facilitate more efficient charge collection through drifting, resulting in the EQE enhancement between 600-850 nm. Simply, the enhanced carrier collection efficiency induced by the PbS CQD HTL, combined with its own contribution to the photocurrent, renders the EQE enhancement and JSC increase. But for the devices employing PbS-820 and PbS-700 as HTLs, there was no corresponding photocurrent contribution in the near infrared range as shown in Figure S7.


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Transient VOC-decay was conducted to explore photo-generated carrier dynamics within a fully assembled device.33 The sample was irradiated by a pulsed laser to generate a photovoltage perturbation. When the laser pulse was off, VOC would gradually decay to the original steady value, which could be monitored by a high-speed oscilloscope. The carrier lifetime could be extracted by Equation (1):

F kT  dV  τ = − i  OC  q  dt 

−1

(1)

where k is the Boltzmann constant, T is temperature, q is the elementary charge, t is time and Fi is a constant between 1 at low injection and 2 at high injection, dVOC/dt is the initial slope of VOC-decay. Here we used Fi=2 for the calculation due to the strong laser intensity (photocarrier concentration ~1018 cm-3). Because the VOC-decay can be significantly influenced by the junction RC [product of resistance (R) and capacitance (C)] time constant, this interference will be reduced by measuring the small signal voltage decay with a steady-state bias light to reduce R. We thus applied a quarter of one sun illumination to generate a steady VOC of 0.2 V and applied a strong light pulse to introduce the additional ~0.2 V. As shown in Figure 4b, by fitting the VOCdecay, carrier recombination lifetime of Sb2Se3 device with PbS CQD HTL was extracted as 1.372 µs, which was twice larger than 0.508 µs of the control device. Since the heterojunction and absorber quality were identical in both devices, we ascribe the increased carrier lifetime to the positive effect of the PbS CQD HTL on band alignment (Figure 1h). The PbS CQD HTL blocks electrons from moving toward the back contact, reduces the recombination probability of electron and holes and hence prolongs the carrier lifetime. Longer carrier lifetime could enhance


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the splitting of quasi-Fermi level, leading to higher VOC.

Besides VOC decay, Mott–Schottky plots (C-2-V) were analyzed to understand the origin of VOC improvement from the built-in potential (Vbi) variation.14 As shown in Figure 4c, the Vbi of the device with PbS CQD HTL, obtained from the horizontal intercepts, increased from 0.475 V to 0.543 V compared to the control. The increased Vbi is derived from the band bending between Sb2Se3 and PbS CQD HTL rather than between CdS and Sb2Se3 due to the same slopes of the linear regions (blue lines in Figure 4c), which indicates that the carrier density in Sb2Se3 and consequently the band bending between CdS and Sb2Se3 are identical in both cases. As Vbi is positively correlated to VOC, slightly improved VOC has been therefore observed in CdS/Sb2Se3 solar cells employing PbS CQD HTL.

Last, we discussed the stability of our n-i-p Sb2Se3 solar cells with the best performance. A representative device using PbS CQD HTL without any encapsulation was stored in dry laboratory ambient with its performance periodically monitored (Figure 4d). As time passed by, the PCE actually gradually increased from 6.35% to 6.69% within 60 days, which is mainly attributed to the VOC improvement. Previous study indicated that ambient oxidization could increase the carrier density of PbS CQD film using EDT treatment which is also accord with our kelvin probe results (Figure S8).34 Similar to our previous observation, the deepened Fermi level of HTL increases the VOC and the resultant PCE. For reference, the stability of control device was shown in Figure 4d. The performance of control device demonstrated very small change after 105 days.


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Having resolved the possible origins for improved VOC and JSC, now we are discussing the J-V characteristics of our photovoltaic devices to reveal the origin of FF improvement. Obviously, the device with HTL demonstrates a better diode rectification, as shown in Figure 4e, indicating HTL reduces the leakage current. Furthermore, analysis of 100 separate devices reveals that devices with HTL show substantially higher shunt resistance (Rsh), increasing from 137.8 Ω cm2 to 211.2 Ω cm2. In addition, the back contact was improved after using PbS CQDs as HTL (Figure S1). As a result, the PbS CQD HTL improves the apparent diode quality and gives rise to better FF.

In this work, we reported the successful implementation of PbS CQD HTL for Sb2Se3 solar cells with the highest certified PCE of 6.5% in Sb2Se3 solar cells. Guided by the SCAPS simulation results, EDT treated PbS CQDs with 935 nm excitonic peak served as the best HTL. The PbS CQD HTL not only minimized carrier recombination loss at the back contact and boosted carrier collection efficiency, but also contributed photocurrent by its own absorption. The strategy presented here, introducing a HTL to build n-i-p devices, circumvents the doping challenge of Sb2Se3 film and offers a direct approach to further increase the efficiency of Sb2Se3 solar cells, which hopefully could stimulate fruitful research in Sb2Se3 photovoltaics in the near future. SUPPORTING INFORMATION

Experimental methods


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Current density-voltage (J-V) curves of control and HTL devices at 180 K under simulated AM1.5 G irradiation.

The absorption spectra of different sized PbS CQDs.

The influence PbS CQD film deposition environment on the device performance.

The dependence of PbS CQD film thickness on device performance.

The picture of certified Sb2Se3 solar cells.

AFM images of the Sb2Se3 and Sb2Se3/HTL.

The EQE of devices with PbS-820 and PbS-700.

The relationship between VOC of devices and work function of PbS CQDs.

The Basic Material parameter for SCAPS simulation.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Jiang Tang: 0000-0003-2574-2943 Author Contributions ‡

C. C., L. W., and L. G. contributed equally to this work

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Major State Basic Research Development Program of China (2016YFA0204000), the National Natural Science Foundation of China (91433105 and 51602114), and the Special Fund for Strategic New Development of Shenzhen, China (JCYJ20160414102210144), the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant (No. 20123010010130) funded by the Korea government Ministry of Trade, Industry and Energy. The authors thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices, WNLO.

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Figure 1. Device architecture of a prototypical Sb2Se3 solar cell a) without and b) with HTL. c) PCE, d) VOC, e) JSC, f) FF as a function of HTL carrier concentration (p) and valence band offset (∆Ev) between Sb2Se3 and HTL. Energy band diagram for g) CdS/Sb2Se3 and h) CdS/Sb2Se3/HTL devices under AM 1.5G irradiation at short-circuit conditions. The solid and hollow circles represent the electron and hole, respectively. The curved arrows stand for the recombination of electron-hole pair. The straight arrow represents transmission of electron and hole. The blue and red lines show the quasi-Fermi levels of electrons and holes. 462x328mm (200 x 200 DPI)


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Figure 2. a) Energy level diagram between the corresponding different sized PbS CQDs and Sb2Se3 in our devices. The numbers of x-axis are the corresponding excitonic peak positions of PbS CQDs. b) Energy level diagram of PbS CQDs employing different ligand exchange. c) PCE of devices using different sized PbS CQD as HTL. d) PCE and VOC of devices with different ligand treated PbS CQDs as HTL. 313x252mm (200 x 200 DPI)


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Figure 3. a) J-V characteristics of device with and without PbS CQD HTL. b) Histogram of device performance measured on 100 separate devices with and without HTL. The standard efficiency deviations for control devices and devices with HTL are 0.27% and 0.08%, respectively. The LBIC graph of devices c) without and d) with PbS CQD HTL, respectively. e) J-V characteristics of a large (1.02 cm2) device employing PbS CQD HTL tested under standard AM1.5G illumination. 490x289mm (300 x 300 DPI)


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Figure 4. a) EQE of the champion cell with HTL and the control cell. The black line in the inset is the EQE difference between device with and without PbS CQD HTL, and the blue line is the product of absorption (A) of PbS CQDs HTL and the transmittance (T) of Sb2Se3 film. b) VOC-decay and c) C-2-V plot of devices with and without PbS CQD HTL. d) The VOC and PCE stability of Sb2Se3 solar cell with and without HTL stored in the dry air without encapsulation. e) J-V characteristics of a diode based on Sb2Se3 solar cell with and without HTL under dark conditions. f) Rsh parameter histogram of device measured on 100 separate devices with and without HTL. The device efficiencies for devices with and without HTL demonstrate a distribution with an average efficiency of 5.08%, 6.62%, respectively. The standard deviations of Rsh are 17.0 and 19.1 Ω cm2 for devices with and without HTL, respectively. 477x268mm (300 x 300 DPI)


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Solar Cells Using PbS Colloidal Quantum Dot Film - ACS Publications

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