Toward charge extraction in all-inorganic perovskite solar cells by interfacial engineering

Jie Ding a, Jialong Duan b, Chenyang Guo c and Qunwei Tang *ad
aInstitute of New Energy Technology, College of Information Science and Technology, Jinan University, Guangzhou 510632, PR China. E-mail: tangqunwei@jnu.edu.cn
bSchool of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR China
cJiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, PR China
dJoint Laboratory for Deep Blue Fishery Engineering, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, PR China

Received 19th March 2018 , Accepted 3rd May 2018

First published on 8th May 2018


All-inorganic perovskite solar cells (PSCs) are a promising solution to address the poor stability of organic–inorganic hybrid PSC devices under humidity and thermal attacks. However, the severe interfacial charge recombination from large energy differences has markedly limited the further enhancement of power conversion efficiency. The charge extraction from perovskite layer has been improved by setting intermediate energy levels using quantum dots (QDs) through interfacial engineering. In the current study, CuInS2/ZnS QDs with tunable bandgaps and hole-transporting behavior were prepared to modify the CsPbBr3/carbon interface. Arising from the improved charge separation, a power conversion efficiency of 8.42% was achieved for QD tailored inorganic PSC in comparison with 6.01% for pristine devices. These all-inorganic PSCs present unprecedented stability under high humidity and significant improvements in the fill factor, short-circuit photocurrent, and open-circuit voltage.


Introduction

In the past few years, organic–inorganic hybrid perovskite solar cells (PSCs) have achieved the highest certified power conversion efficiency (PCE) of more than 22% since the first prototype in 2009.1–4 Despite the rapid increase in PCE associated with the creation of new perovskite materials and manufacturing techniques, the performance attenuation upon humidity and thermal attacks as well as iodide ion diffusion from lead halides to metallic electrodes remains unresolved. In addition, the poor long-term stability has been a significant burden for commercialization.5,6 A promising solution to this impasse is to replace organic–inorganic halides with a more stable light-harvester. Perovskite-structured cesium lead bromide (CsPbBr3) halide is regarded as a good candidate because of its high carrier mobility and unchanged crystal structure in high-humidity and high-temperature atmosphere; therefore, it has attracted growing interest as a light absorber for inorganic PSC platforms.7–9

Using the classical inorganic PSC architecture of electron-transporting material/CsPbBr3/carbon, the PCE of 6.7% has been achieved for the prototype.10 In comparison with organic–inorganic hybrid PSCs, the PCEs of inorganic devices are still much lower due to the wide bandgap (2.3 eV) in CsPbBr3 halide.11 Several routes including mixed-anion, addition of hole-transporting materials (HTMs) and multi-step deposition have been attempted to enhance solar cell efficiency. For example, inorganic halide perovskite with formula CsPbX3 (X = Br2I, BrI2) display narrower bandgaps (Eg = 1.73 eV and 1.92 eV, respectively), allowing a slightly broader absorption of the solar spectrum; however, the stability is drastically reduced under high humidity or high temperature.12–14 Another method is to add HTMs between a CsPbBr3 film and a carbon back-electrode. However, the addition of these HTMs not only increase the cost of the final device, but also increases the water-steam absorption, thus leading to perovskite degradation.15–17 In recent years, PbS quantum dots (QDs) have been introduced into planar hybrid PSCs as HTMs and have appropriate energy orientation.18,19 Although QDs are always used in a variety of solar cell devices for their high photovoltaic coefficients and tunable energy levels,20–24 the interfacial engineering of the energy band alignment of QDs between the perovskite and carbon layers is also highly worth understanding and has yet to be unraveled. In addition, the realization of an optimized interface for hole transportation between perovskite and carbon electrode is more challenging than facilitating electron transport because the material synthesis and processing steps should be compatible with the stability requirements of the underlying perovskite film without introducing any subsequent structural deterioration.

During the past few decades, II–VI and IV–VI QDs have attracted considerable interest because of their widespread applications.25 However, the toxicity of heavy metal species such as cadmium and lead, prompted us to develop environmental-friendly QDs without sacrificing their wide range of absorption and tunable energy levels. Ternary I–III–VI QDs are of great interest for fundamental studies and technical applications arising from their parallel optoelectronic properties and deficiency in highly toxic elements.26–29 Therefore, we would launch a strategy of making a type of CuInS2/ZnS QDs that are free of class A elements (Cd, Pb, and Hg) or class B elements (Se and As), so that they are eco-friendly, and their size can be controlled by adjusting the reaction temperature and time during the synthesis process. Moreover, CuInS2/ZnS QDs are also featured with high luminescence, tunable emission wavelength, and radiation stability. Inspired by these promises, we herein demonstrate a rational design of CuInS2/ZnS QDs for inorganic PSCs by interfacial engineering. The charge extraction and therefore the photovoltaic performances of the as-fabricated devices improved though tuning the band alignments. A notable increase of 40% in PCE from 6.01% to 8.42% suggests that this approach is promising towards efficient charge extraction.

Results and discussion

The as-synthesized CuInS2/ZnS QDs were used to modify the CsPbBr3/carbon interface, and the inorganic PSC device was fabricated by a classical solution-process method, as shown in Fig. 1a. A multi-step route was employed to fabricate the CsPbBr3 film: PbBr2 solution was spin-coated onto c-TiO2/m-TiO2 film for one time and CsBr solution was coated onto the PbBr2 layer for four cycles. Finally, conductive carbon ink was applied to cover the solar cell by a doctor-blade method.30 The cross-sectional SEM image of an all-inorganic PSC (Fig. 1b) demonstrates the average thicknesses as 100 nm, 450 nm and 650 nm for c-TiO2/m-TiO2, CsPbBr3 and carbon layer, respectively. It is noteworthy to mention that the CuInS2/ZnS QDs are closely adhered to the CsPbBr3 layer instead of forming a single layer. This can be cross-checked by the invisible interface between the CsPbBr3 layer and the carbon back-electrode (Fig. 1b). Upon sunlight illumination, the electrons in the valence band (VB) of CsPbBr3 crystals absorb photons and jump to the conduction band (CB), and subsequently flow to the CB of TiO2 layer, leaving holes to transfer to the highest occupied molecular orbital (HOMO) edge of CuInS2/ZnS QDs, as shown in Fig. 1c. The energy difference between the VB level of CsPbBr3 halide and the work function of carbon black electrode is 0.6 eV. Therefore, there are significant electron–hole recombination in the QD-free PSC device. By setting CuInS2/ZnS QDs at CsPbBr3/carbon interface, the bridging effect of these QDs rapidly extract the holes to the HOMO level of CuInS2/ZnS QDs and finally to the back electrode. To better evaluate the possibility of CuInS2/ZnS QDs for application in inorganic PSCs, the energy levels of these materials were estimated by the traditional electrochemical technique. Cyclic voltammetry was performed to determine the HOMO and the lowest unoccupied molecular orbital (LUMO) levels of CuInS2/ZnS QDs according to following equations. The oxidation potential of QDs is located at +0.83 eV. Therefore, the HOMO level is calculated according to the equation EHOMO = −(4.88 − E1/2,Fc,Fc+ + EOX), where EOX is the oxidation potential of QDs and E1/2,Fc,Fc+ is the half-wave potential of Fc/Fc+ couple. The LUMO is thus obtained by the bandgap in the absorption spectra, which is defined as the lowest energy transition (Fig. S1 and Table S1). The UV-vis absorption spectra of CuInS2/ZnS QDs in methylbenzene are shown in Fig. 1d. It is noteworthy that CuInS2/ZnS QDs exhibit extended absorption to 700 nm, and the strong absorption of CuInS2/ZnS in the range of 600–700 nm may make considerable contribution for light harvesting in the PSCs. The method developed by Tandon and Gupta was used to evaluate the bandgap energy (Eg) of CuInS2/ZnS QDs. By plotting (Ahν)2versus photon energy (),31,32 as shown in Fig. S2, the Eg values of the three CuInS2/ZnS QDs samples were determined to be 1.81, 1.71 and 1.68 eV. UV-vis absorption spectra of the as-prepared QDs in diluted aqueous solutions show reduced absorbance but same absorption peak positions with an increase in dilution (Fig. S3).
image file: c8ta02522c-f1.tif
Fig. 1 (a) Schematic illustration of the inorganic PSC device structure. (b) Cross-sectional SEM image of a total device. (c) Energy diagram of each layer in the device with energy levels given in eV. (d) Absorption spectra of CuInS2/ZnS QDs and CsPbBr3 in methylbenzene.

According to charge extraction mechanism, the optical absorption properties and energy-level structures of CuInS2/ZnS QDs are crucial in promoting electron–hole separation. Under an optimized In[thin space (1/6-em)]:[thin space (1/6-em)]Cu ratio, the particle sizes and therefore the Eg values of CuInS2 QDs can be tuned by adjusting the synthesis temperature and time via a modified hot-injection method. By controlling the energy levels of QDs, the photo-induced holes are efficiently extracted from the CsPbBr3 layer through matching with the energy levels of the perovskite film. However, the CuInS2 QDs are found to be substantially sensitive to oxygen and amine because catalytic agents will accelerate the oxidation of QDs. By introducing a ZnS shell onto the CuInS2 core, the air-stability is markedly improved, and the obtained CuInS2/ZnS core/shell QDs have greatly improved PL quantum yield of up to 30%.33 The PL spectra of CuInS2/ZnS QDs are shown in Fig. 2a, suggesting a red shift of emission peak position with the increase in reaction temperature. XRD patterns of CuInS2/ZnS QDs were also recorded, as shown in Fig. S4. According to XRD patterns, the characteristic peaks (27.86°, 46.91°, and 55.08°) of CuInS2 QDs agree well with those of the bulk CuInS2 (JCPDS no. 65-2732). This indicates the compositional homogeneity of CuInS2 rather than a simple mixture of CuS2 and In2S3. Fig. 2b shows the transmission electron microscopy (TEM) images of CuInS2/ZnS QDs with different bandgaps. The average particle size is 3.8 nm for CuInS2/ZnS QDs with Eg = 1.81 eV, which gradually increases to 5.6 nm for 1.71 eV and 6.4 nm for 1.68 eV. The increase in particle size occurs due to the crystal contraction of CuInS2/ZnS QDs at high temperatures. Unless otherwise specified, the as-prepared CuInS2/ZnS QDs are defined as 1.81 eV-QDs, 1.71 eV-QDs and 1.68 eV-QDs. The fluorescent picture of CuInS2/ZnS QDs under 365 nm irradiation is shown in Fig. 2c, which is consistent with CIE image shown in Fig. 2d. SEM mappings of Cu, Zn, In, and S elements at the CsPbBr3 film are shown in Fig. 2e and the detailed information is described in Fig. S5.


image file: c8ta02522c-f2.tif
Fig. 2 (a) Absorbance and steady-state PL spectra of CuInS2/ZnS QDs. (b) HR-TEM images and size distribution of CuInS2/ZnS QDs. (c) Fluorescent picture of CuInS2/ZnS QDs under 365 nm irradiation. (d) CIE image of CuInS2/ZnS QDs. (e) SEM mapping of Cu, In, S and Zn elements at CsPbBr3 surface.

The photocurrent–voltage (JV) curves for the all-inorganic PSC devices tailored by various CuInS2/ZnS QDs are shown in Fig. 3a, and the photovoltaic parameters are summarized in Table 1. A PCE of 6.01% (Jsc = 6.13 mA cm−2, Voc = 1.34 V, FF = 73.16%) is recorded for QDs-free inorganic PSC device, which is comparable to the state-of-the-art all-inorganic CsPbBr3 solar cells free of HTMs. Upon interfacial modification with CuInS2/ZnS QDs, the photovoltaic performances are significantly enhanced, achieving maximized Voc of 1.45 V, Jsc of 7.47 mA cm−2, FF of 77.73%, and a PCE of 8.42%. Accordingly, the improved performance mainly stems from the increased values of Jsc, FF and Voc. Notably, this is the first time that enhanced PCE was obtained for all-inorganic CsPbBr3 PSCs using lead-free I–III–VI QDs as the interfacial material. The mechanism behind this enhancement is the boosted charge extraction ability arising from the intermediate energy level between VB of perovskite and work function of the carbon electrode. Notably, the 1.81 eV-QDs have closer HOMO edge to the valence band of CuInS2/ZnS QDs. Therefore, the electron–hole recombination is efficiently reduced. It is known that a reduced recombination process will allow for an increase in FF. In this fashion, the FF of QDs-solar cells obeys the following order 1.81 eV-QDs > 1.71 eV-QDs > 1.68 eV-QDs, which is consistent with the JV characterization. The incident-photon-conversion-efficiency (IPCE) spectra for these devices are depicted in Fig. S6 to confirm the solar-to-electric conversion behaviors of CuInS2/ZnS QDs. In order to evaluate device stability, photocurrent densities at an applied bias close to the initial maximum power point is monitored as a function of time (Fig. 3b). There is an abrupt increase in photocurrent when irradiated with simulated sunlight, and a steady-state current density of 6.74 mA cm−2 and a cell efficiency of 8.40% are obtained for the optimized PSC, which are larger than 5.29 mA cm−2 and 6.00% for QDs-free PSC, respectively. The high photocurrent density along with improved stability demonstrates a positive effect of QDs in photovoltaic performances. Statistical data on a large batch of twenty solar cells (Fig. 3c) demonstrates a small deviation and high reproducibility using the method reported herein. To fully evaluate this state-of-the-art integrated PSCs, the aging tests were performed in ambient conditions (humidity = 80%, temperature = 25 °C) for the unsealed device with 1.81 eV-QDs, as shown in Fig. 3d. Apparently, CuInS2/ZnS QDs-based PSCs are stable for about one month, maintaining about 94% of original PCEs even in high-humidity conditions. The higher PCE along with improved stability demonstrates that CuInS2/ZnS QDs tailored CsPbBr3 solar cell is promising to promote further development of inorganic PSCs.


image file: c8ta02522c-f3.tif
Fig. 3 (a) JV curves of inorganic PSCs with and without CuInS2/ZnS QDs under air mass 1.5 global (AM1.5G, 100 mW cm−2) illumination. (b) The steady-state power output at the maximum power points for optimal and referenced devices. (c) Photovoltaic characteristics for 20 random PSC devices. (d) Normalized photovoltaic parameters of CuInS2/ZnS QDs tailored inorganic PSC as a function of storage time in humid air (80% RH, 25 °C) without encapsulation.
Table 1 The photovoltaic parameters of all-inorganic PSC devices. Jsc: short-circuit current density; Voc: open-circuit voltage; FF: fill factor
Devices V oc (V) J sc (mA cm−2) FF (%) PCE (%)
QDs-free 1.34 6.13 73.16 6.01
1.81 eV-QDs 1.45 7.47 77.73 8.42
1.71 eV-QDs 1.40 7.37 75.49 7.79
1.68 eV-QDs 1.37 6.48 75.02 6.66


The steady-state PL spectra (Fig. 4a) are recorded to gain insight into the defect density of perovskite films with and without CuInS2/ZnS QDs. Through modification with QDs, the PL intensities are significantly quenched in comparison with that of the pristine CsPbBr3 film. The decreased PL intensity indicates that CuInS2/ZnS QDs with Eg = 1.81 eV minimize the defect density of the perovskite film. Moreover, the time-resolved PL decay curves of CsPbBr3 and CsPbBr3/CuInS2/ZnS-QDs were measured to determine the PL decay lifetimes, as shown in Fig. 4b. The PL decay curves were fitted by the single-exponential decay function I = Ae−(tt0)/τ1, in which I is the PL intensity at time t, A is a constant, and τ1 is the faster component of the spontaneous radiative recombination time of the exciton. The PL decay lifetime of the perovskite-CuInS2/ZnS with Eg = 1.81 eV was determined to be 1.78 ns as compared to 0.3 ns for QDs-free film. The prolonged PL decay lifetime of the perovskite film is attributed to the suppressed nonradiative recombination channels. The longer PL lifetime suggests reduced charge recombination for the CuInS2/ZnS QDs tailored perovskite film. Fig. 4c represents dependence of Jsc plots on light intensity. According to JscIα (α ≤ 1), the charge recombination at CsPbBr3/CuInS2/ZnS/carbon interface should be minimum (α ≈ 1) for maximum carrier sweep at the short circuit. In comparison to α = 0.88 for QDs-free device, the α-value yielded from QDs modified PSCs is around 0.998. This result demonstrates that the pristine device exhibits significant recombination, which is minimized by modifying the CsPbBr3/carbon interface with CuInS2/ZnS QDs. The JV curves of the devices with and without QDs are shown in Fig. S7. Electrochemical impedance spectroscopy (EIS) was carried out to clarify the significant changes in FF caused by the CuInS2/ZnS QDs. Fig. 4d shows the Nyquist plots of the PSCs with and without CuInS2/ZnS QDs recorded in the dark. Only one semicircle is found in each EIS spectrum, referring to the charge transfer resistance (Rct) at the CsPbBr3/carbon interface. The reduced Rct for 1.8 eV-CuInS2/ZnS QDs modified PSC indicates that hole extraction in this cell becomes more efficient compared with that of the control cell. The VB of 1.81 eV-CuInS2/ZnS QDs lies between the VB of CsPbBr3 and the work function of the carbon electrode, thus facilitating the hole transfer from CsPbBr3 perovskite to carbon. Moreover, the decreased Rct is responsible for an increase in FF. All of these results demonstrate that CuInS2/ZnS QDs with Eg = 1.81 eV maximize charge extraction for PCE enhancement.


image file: c8ta02522c-f4.tif
Fig. 4 (a) PL spectra and (b) time-resolved PL decay curves of perovskite films with and without CuInS2/ZnS QDs. (c) Jsc as a function of illuminated light intensity for the PSCs. (d) EIS Nyquist plots and an equivalent circuit of the inorganic PSCs.

Experimental section

Crystallography synthesis of CuInS2 cores

In brief, a mixture of 0.25 mmol CuI, 1 mmol In(AC)3, 5 mL DDT, 10 mL OAm, and 10 mL 1-ODE loaded in a 50 mL three-neck flask were degassed at 80 °C for 40 min and heated to 150 °C under Ar gas flow. Then, the solution was maintained at this temperature for 15 min and heated to 230 °C to form CuInS2 cores. The growth of CuInS2 cores lasted for about 10 min at 230 °C, and the solution colour changed from yellowish to orange, red and finally to dark brown. Higher reaction temperatures and/or longer reaction times led to the formation of larger CuInS2 cores with longer emission wavelengths. Purified CuInS2 QDs were obtained by repeating the precipitation/dispersion method and then, they were dispersed in hexane for characterizations.

Synthesis of CuInS2/ZnS QDs

For the preparation of CuInS2/ZnS QDs, a mixture of 2 mmol Zn(Ac)2, 1 mL DDT, 2 mL OA, and 3 mL ODE loaded in a 25 mL flask were heated to 100 °C and maintained at this temperature for 30 min. The Zn precursor solution was poured into the CuInS2 solution when the growth of the core was complete. The mixed solution was heated to 240 °C and kept for 20 min to obtain CuInS2/ZnS QDs. The nucleation temperature was controlled at 180 °C, 200 °C and 240 °C to tune the size and bandgap of the CuInS2/ZnS QDs. A lower synthesis temperature led to smaller sized CuInS2/ZnS QDs with larger bandgaps. In this fashion, the 1.81 eV-QDs, 1.71 eV-QDs and 1.68 eV-QDs were synthesized at 180 °C, 200 °C and 240 °C, respectively. Purified CuInS2/ZnS QDs were obtained by repeating the precipitation/dispersion method and then, they were dispersed in hexane for characterization and device fabrication.

Assembly of inorganic PSC devices

All the fabrication processes for PSCs were conducted at room temperature and ambient air. Commercially available FTO glass (sheet resistance = 12 ohm per square) was etched with Zn powder and 2.0 M HCl to obtain desirable patterns. Then, it was rinsed with acetone, ethanol and deionized water. The compact TiO2 (c-TiO2) layer was deposited on the FTO substrate by spin-coating an ethanol solution of titanium isopropoxide (0.5 M) and diethanol amine (0.5 M) at 7000 rpm for 30 s and annealing in air at 500 °C for 2 h. The mesoscopic TiO2 (m-TiO2) layer was then deposited onto the c-TiO2 layer by spin-coating TiO2 paste (with average particle size of 20 nm) at 2000 rpm for 30 s and annealing in air at 450 °C for 30 min. Then, the substrate was immersed in 0.04 M TiCl4 aqueous solution at 70 °C for 30 min, cleaned with deionized water and ethanol, and finally annealed at 450 °C for another 30 min. Subsequently, N,N-dimethylformamide (DMF) solution of 1.0 M PbBr2 was spin-coated onto the m-TiO2 layer at 2000 rpm for 30 s, followed by drying at 80 °C for 30 min. Next, methanol solution of 0.07 M CsBr was spin-coated onto PbBr2 layer at 2000 rpm for 30 s. After it was dried in air, the film was heated at 250 °C for 5 min. By repeating the CsBr deposition four times, the perovskite-structured CsPbBr3 film was thus formed. Then, the CuInS2/ZnS QDs were spin-coated twice onto the CsPbBr3 film at 2000 rpm for 30 s, followed by drying at 85 °C for 5 min to evaporate cyclohexane. The concentrations of CuInS2/ZnS QDs were tuned to 10, 30, 60 and 100 mg mL−1 to optimize the cell performances. Finally, the carbon back electrode was used to cover the PSC device by coating the conductive carbon ink through a doctor-blade method and then, the electrode was heated at 100 °C for 5 min.

Measurements

The photocurrent density-voltage (JV) curves of PSCs with an active area of around 0.25 cm2 were recorded on a CHI660E electrochemical workstation under irradiation of simulated solar light in ambient atmosphere. The incident light intensity was controlled to 100 mW cm−2. The morphologies of the perovskite films were obtained using a field-emission scanning electron microscope (SEM, SU8220, Hitachi). Transmission electron microscope (TEM) images were acquired on a Tecnai G2 F20 microscope. Ultraviolet-visible (UV-vis) absorption spectra of various perovskite films were characterized employing a Meipuda UV-3200 spectrophotometer in the wavelength range of 350–800 nm. The PL spectra of perovskite films were recorded on a FluoroMax-4 spectrofluorometer and the time-resolved PL delay characterizations were conducted with a Horiba spectrometer excited by a 385 nm laser. The IPCE spectra of the as-prepared devices were recorded using an IPCE kit developed by Zolix Instruments Co., Ltd.

Conclusions

In summary, eco-friendly CuInS2/ZnS QDs with tunable particle size and energy levels were synthesized by adjusting temperature and time, and applied as buffer material to promote charge extraction in all-inorganic CsPbBr3 PSCs. Due to the enhanced hole extraction, the CuInS2/ZnS QDs-modified PSC yielded a maximized PCE of as high as 8.42% in comparison to 6.01% for the QDs-free device. Even under 80% RH for more than one month, the solar cell retains 94% of its initial efficiency. This study is far from optimized, but the new concept and excellent results provide new opportunities for minimizing charge recombination and enhancing cell performances.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support from National Natural Science Foundation of China (21503202, 61774139) and Director Foundation from Qingdao National Laboratory for Marine Science and Technology (QNLM201702).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta02522c

This journal is © The Royal Society of Chemistry 2018