Enhanced Photovoltage for Inverted Perovskite Solar Cells Using Delafossite CuCrO₂ Hole Transport Material

Ⅰ.   Introduction

Owing to the appropriate band gap and remarkable absorption covering the entire visible range of solar spectrum [1, 2], large ambipolar charge-carrier mobilities [3], and long charge carrier diffusion lengths [4, 5], halide perovskite has received extensive research attention in the field of photovoltaics. For the regular perovskite solar ells (PSCs), the power conversion efficiency (PCE) has exceeded 25% [6–8]. However, the fabrication process of TiO₂ electron transport layer (ETL), the most common ETL in regular PSCs, generally requires a high temperature of 500 ℃ or higher, which limits the development of flexible PSCs with regular structure [9–12].

Although the PCE of inverted architecture PSCs has exceeded 21% recently [13, 14], it is still inferior to that of regular architecture PSCs. For inverted PSCs, poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) usually serves as a hole transport material (HTM), nevertheless, the energy level matching between PEDOT: PSS and perovskite is usually imperfect, which leads to relatively low open-circuit voltage ($V_{{\rm{OC}}}$) [15–18]. Additionally, the hygroscopicity and acidity of PEDOT: PSS would induce the decomposition of perovskite, and further affect the long term stability of PSCs [19–22]. To address these issues, two different approaches have been usually adopted to improve the performance of inverted PSCs. One way is introduction of appropriate additives to adjust the work function of PEDOT: PSS to match the energy level of perovskite, or tune the acidity of PEDOT: PSS to increase the stability of devices. For example, PSS-Na [22] and 2, 3, 5, 6-tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (F4-TCNQ) [23] were introduced into PEDOT: PSS HTM, which adjusted the work function of HTMs and enhanced the $V_{{\rm{OC}}}$ of PSCs successively. Imidazole [24] and sodium citrate [25] were adopted as additives to adjust the acidity of PEDOT: PSS and improved the long term stability of PSCs effectively. The other method is development of appropriate p-type semiconductor materials to replace PEDOT: PSS. Inorganic p-type semiconductor materials with inherent better stability and relatively higher hole mobility are potential alternatives. Some p-type inorganic semiconductor materials such as MoO₃, V₂O$_5$, CuO, Cu₂O, NiO, CuSCN, and CuCrO₂ have been used as HTM for PSCs, and the corresponding devices exhibit excellent performance [26–30]. Among them, CuCrO₂ based PSCs exhibited relatively higher performance based on the excellent optical transmittance, high hole mobility, UV-blocking characteristic, and energy band alignment [31–35]. However, the related previous investigations are still not systematic and extensive enough, and most of them are based on the conventional perovskite CH₃NH₃PbI₃ [32–34] or regular device structure [31]. And therefore, it is necessary to further investigate CuCrO₂ as an HTM of PSCs, including the combination of CuCrO₂ with other kinds of perovskite materials.

Triple-cation mixed-halide perovskite is a type of star material in the research field of PSCs in recent years [36, 37]. Compared with CH₃NH₃PbI₃ based PSCs, triple-cation mixed-halide perovskite based PSCs generally exhibit better photovoltaic performance and better stability [38, 39]. Therefore, a triple-cation mixed-halide perovskite [(FAPbI₃)$_{0.87}$(MAPbBr₃)$_{0.13}$]$_{0.92}$[CsPbI₃]$_{0.08}$ that has been proved to have excellent performance by previous investigations [40, 41] was adopted as light harvesting material in our investigation. In addition, an inverted structure ITO/HTM/perovskite/PCBM/BCP/Ag has been adopted, and the temperature in the whole fabrcation process has not exceeded 150 ℃, which could adapt to the fabrication of flexible devices. The uniform CuCrO₂ nanoparticles with an average size of 10 nm were synthesized through hydrothermal method and used as HTM for inverted structure PSCs, as a result, the $V_{{\rm{OC}}}$ of device has been improved effectively. Subsequently, the work function of CuCrO₂ film and the charge recombination behaviour of PSCs were investigated to analyze the mechanism of $V_{{\rm{OC}}}$ improvement.

Ⅱ.   EXPERIMENTS

The CuCrO₂ nanoparticles were synthesized through hydrothermal method as described in the previous studies [42–44]. Briefly, Cr(NO₃)$_2\cdot$9H₂O (15 mmol) and Cr(NO₃)$_2\cdot$3H₂O (15 mmol) were added into the Teflon reactor containing 70 mL of deionized water and stirred for 10 min, NaOH (5.0 g) was subsequently added into the Teflon reactor and stirred for 30 min. Then, the Teflon reactor was sealed in an autoclave and placed in an oven at 240 ℃ for 60 h. The obtained dark green precipitate was washed in turn with diluted hydrochloric acid for two times, deionized water for three times, and absolute ethanol for three times. And then, the CuCrO₂ nanoparticles were stored in absolute ethanol and dispersed by ultrasonic for homogenization before spin-coating. It is worth noting that the obtained CuCrO₂ ethanol solution could keep uniform dispersion for more than one year (FIG. S1 in supplementary materials (SM)), which is different from CuCrO₂ nanoparticles dispersed in absolute isopropanol and methanol in previous literatures [31, 32]. From the perspective of cost and environmental protection, absolute ethanol is more suitable for the dispersion and storage of CuCrO₂ nanoparticles. See SM for other experimental details.

Ⅲ.   RESULTS AND DISCUSSION

A   Properties of CuCrO₂ nanoparticles and films

XRD measurements were performed to determine the crystal structure of CuCrO₂ nanoparticles and the obtained XRD pattern is shown in FIG. S2 in SM. All the diffraction peaks agree well with the rhombohedral 3R delafossite CuCrO₂ crystalline phase (JCPDS No.00-039-0247), the peaks located at 2$\theta$ of 15.5°, 31.4°, 36.4°, 62.3°, and 74.4° could be correspondingly indexed to (003), (006), (012), (110), and (202) planes, respectively, and the average crystallite sizes are calculated to be 9.6 nm using Debye-Scherrer formula. The TEM image (FIG. S3 in SM) of CuCrO₂ nanoparticles demonstrates that the CuCrO₂ nanoparticles are uniform and their average sizes are 10.89$\pm$1.95 nm, which is close to the calculation result from XRD pattern.

CuCrO₂ films were prepared via spin-coating the dispersion solution of CuCrO₂ nanoparticles with concentration of 8 mg/mL onto ITO substrates, and their thicknesses were adjusted by repeating spin-coating step. FIG. 1 displays the surface and cross-sectional FE-SEM images of CuCrO₂ films with different thicknesses deposited on ITO substrates. The repeating spin-coating step of CuCrO₂ nanoparticles solution from one time to four times are named as 1 coat, 2 coats, 3 coats, and 4 coats, respectively. It is easy to observe that CuCrO₂ nanoparticles could cover the ITO substrates uniformly and completely except for the 1 coat sample. The cross-sectional FE-SEM images show that the thickness of CuCrO₂ film increases from 43 nm to 120 nm with the spin-coating step being repeated from one to four times. Predictably, the PSCs with 1 coat CuCrO₂ hole transport layer (HTL) would exhibit poor performance caused by the incomplete coverage of CuCrO₂ films which leads to direct contact between ITO substrate and perovskite layer and results in serious charge recombination. In this investigation, the CuCrO₂ films with different thicknesses have been used as HTLs of PSCs, the effect of their thickness on the performance of PSCs and the corresponding mechanism have been investigated.

Figure  1.  FE-SEM (field emission scanning electron microscope) images of the CuCrO₂ films deposited on ITO substrates. The surface views of the samples (a) 1 coat, (c) 2 coats, (e) 3 coats, and (g) 4 coats, and their corresponding cross-sectional views (b) 1 coat (43 nm), (d) 2 coats (70 nm), (f) 3 coats (100 nm), and (h) 4 coats (120 nm). In each cross-sectional view, CuCrO₂ film region and ITO substrate region are in green and blue, respectively.

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In terms of the inverted structure PSCs, the transmittance of HTL is one of the important impact factors of cell performance. From FIG. 2, all the CuCrO₂ films with different thickness possess excellent transmittance and they allow enough solar energy to pass through and reach the perovskite light harvesting layer. The light transmittance of CuCrO₂ HTL is close to the PEDOT: PSS HTL in the visible light range except that it is lower than that of PEDOT: PSS in shortwave region. In the shortwave region, the transmittance of CuCrO₂ HTL decreases with the increase of thickness which could be ascribed to the increasing absorption and scattering. With the wavelength increasing, the transmission spectrum of each CuCrO₂ HTL is a wavy curve due to the interference effect.

Figure  2.  The optical transmission spectra of CuCrO₂ films with different thicknesses and PEDOT: PSS film deposited on ITO substrates, respectively.

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B   Photovoltaic performance of PSC devices

The photocurrent-photovoltage ($J$-$V$) curves of the representative PSCs with different thickness of CuCrO₂ HTLs and PEDOT: PSS HTL are shown in FIG. S4 in SM. The thickness of CuCrO₂ HTLs exhibits an obvious influence on the performance of PSCs, 1 coat CuCrO₂ HTLs-based PSCs possess a relatively lower $V_{{\rm{OC}}}$ of 1006 mV compared with the other CuCrO₂ HTLs based PSCs possessing higher $V_{{\rm{OC}}}$ of more than 1030 mV. The relatively lower $V_{{\rm{OC}}}$ of 1 coat CuCrO₂ HTLs based PSCs could be attributed to the direct contact between ITO substrate and perovskite layer caused by the poor coverage of CuCrO₂ films which would lead to serious charge carrier recombination inevitably. Additionally, the relatively lower short-circuit current density ($J_{{\rm{SC}}}$) of PSCs (Table S1 in SM) based on 1 coat CuCrO₂ HTLs could also be attributed to the poor coverage of 1 coat CuCrO₂ HTLs. With the spin-coating step being repeated, ITO substrates are covered completely, and the $V_{{\rm{OC}}}$ and $J_{{\rm{SC}}}$ of PSCs increase obviously. The PSCs based on 2 coats CuCrO₂ HTLs exhibit the best performance and is referred as correspondingly the champion device possessing the highest PCE. With further repeating spin-coating step, the $V_{{\rm{OC}}}$ of devices fluctuates slightly around 1030 mV, but the $J_{{\rm{SC}}}$ decreases slightly to 17.69 mA/cm$^2$ for PSCs based on 4 coats CuCrO₂ HTLs which could be ascribed to more charge recombination occurring inside thick CuCrO₂ films and series resistance increasing. On the other hand, all the PSCs based on CuCrO₂ HTLs exhibit higher $V_{{\rm{OC}}}$ than those based on PEDOT: PSS HTLs. FIG. 3(a) plot $J$-$V$ curves of the champion devices based on CuCrO₂ and PEDOT: PSS HTLs, respectively. The statistical PCE distributions of 20 samples of the two type PSCs are shown in FIG. 3(b) and the corresponding detailed photovoltaic parameters are summarized in Table S2 in SM. The champion device with 2 coats CuCrO₂ HTLs exhibits a better PCE of 15.06% with a $V_{{\rm{OC}}}$ of 1039 mV, a $J_{{\rm{SC}}}$ of 19.57 mA/cm$^2$, and a fill factor (FF) of 74.1%. The PEDOT: PSS based device yields a 13.17% PCE with a lower $V_{{\rm{OC}}}$ of 904 mV, a $J_{{\rm{SC}}}$ of 20.57 mA/cm$^2$, and an FF of 70.9%. The $J$-$V$ curves of the fresh champion device and the same device aged for 15 days are shown in FIG. S5 in SM. The performance of devices based on CuCrO₂ HTL remains almost unchanged after 15 days, while the performance of device based on PEDOT: PSS HTL decreases significantly, which indicates the devices based on CuCrO₂ HTL have better stability.

Figure  3.  (a) The $J$-$V$ curves of the champion PSCs based on CuCrO₂ and PSC based on PEDOT: PSS HTLs. (b) The statistics of PCE distribution for devices with CuCrO₂ (20 samples) HTLs and PEDOT: PSS (20 samples) HTLs.

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Generally, there are three factors affecting the $V_{{\rm{OC}}}$ of PSCs. Firstly, the composition and morphology of the under layer, such as CuCrO₂ and PEDOT: PSS HTLs, have great influence on the crystallinity and morphology of the obtained perovskite films including their grain sizes which would affect the light absorption and charge carrier recombination. Secondly, different HTMs have different energy level structures, and their energy level matching degree with the perovskite layer would be different inevitably. Thirdly, different materials possess different surface trap densities which could lead to different charge recombination. The above three factors would affect the performance of PSCs including the $V_{{\rm{OC}}}$, and the mechanism of the $V_{{\rm{OC}}}$ improvement of CuCrO₂ based PSCs has been investigated from the above aspects in this investigation.

C   Properties of perovskite films

FIG. 4 displays the SEM images of perovskite films prepared on CuCrO₂ and PEDOT: PSS layers, respectively. Both the perovskite films are uniform, but the average grain size is slightly different, the average grain size of the perovskite deposited on PEDOT: PSS HTL is 280 nm, and the CuCrO₂-based one is 200 nm. Smaller perovskite grain size would lead to the reduction of light absorption [45, 46], which is not conducive to the performance of PSCs. The thicknesses of perovskite films prepared on CuCrO₂ and PEDOT: PSS HTL are 577 and 551 nm, respectively (FIG. S6 in SM). XRD patterns of the perovskite films deposited on CuCrO₂ and PEDOT: PSS HTLs shown in FIG. S7 in SM indicate that all the diffraction peak positions of the two perovskite films are coincident, that is, the structures of the two kinds of perovskite deposited on CuCrO₂ and PEDOT: PSS HTLs are the same. Excepting the peaks of the triple-cation mixed-halide perovskite and ITO substrate, there are also two peaks belonging to $\delta$-phase FAPbI₃ and PbI₂ that could be found at 11.6° and 12.7°, respectively [47, 48]. It is difficult to convert photoinactive $\delta$-phase of FAPbI₃ into photoactive phase of FAPbI₃ completely and it is a common phenomenon in the perovskite materials containing FAPbI₃. Mercifully, the content of $\delta$-phase FAPbI₃ is very small, which would not affect the cell performance obviously. The peak belonging to PbI₂ comes from the excess PbI₂ in precursor solution and an appropriate amount of excess PbI₂ has been proved to have passivation effect, suppress charge recombination, and improve the performance of PSCs [49–52].

Figure  4.  SEM images of perovskite films deposited on (a) CuCrO₂ film and (b) PEDOT: PSS film, respectively. (c) The UV-Vis absorbance spectra of perovskite film deposited on CuCrO₂ and PEDOT: PSS HTL.

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UV-Vis-NIR absorption spectra have been performed to evaluate the light harvesting capabilities of the perovskite films prepared on PEDOT: PSS and CuCrO₂ HTLs. It could be found that the absorption of the perovskite film prepared on PEDOT: PSS HTL is slightly stronger than that on CuCrO₂ HTL (FIG. 4(c)), which could be ascribed to the larger grain size of perovskite deposited on PEDOT: PSS HTLs and the slightly higher transmittance of PEDOT: PSS HTLs in shortwave region. As a result, the PSCs with PEDOT: PSS HTLs exhibit higher $J_{{\rm{SC}}}$, compared with CuCrO₂-based PSCs. Additionally, the thickness of CuCrO₂ films does not exhibit obvious influence on the light absorption of perovskite films prepared on CuCrO₂ HTLs regardless of their thickness ranging from 43 nm to 120 nm (FIG. S8 in SM).

D   Mechanism of improving $V_{{OC}}$ of device

Ultraviolet photoelectron spectroscopy (UPS) measurements have been conducted on the CuCrO₂ films to investigate their energy level structure. As shown in FIG. 5 (a) and (b), the energy difference between Fermi level ($E_{{\rm{F}}}$) and valence band maxima (VBM) of CuCrO₂ films is 0.40 eV and the cut-off energy ($E_{ {\rm{cut-off}}}$) is 16.24 eV. The work function, $\Phi$, of CuCrO₂ film could be calculated using the following equation:

Figure  5.  (a) VBM and secondary electron cut-off of CuCrO₂ film. (b) The energy level diagrams of PSCs based on CuCrO₂ and PEDOT: PSS HTMs. (c) The representative Nyquist plots and (d) Bode-phase plots of PSCs with CuCrO₂ and PEDOT: PSS HTLs measured under the bias voltage of 0.9 V in the darkness.

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where $E_ {\rm{s}}$ is the photon energy of He I source. The calculated $\Phi$ value of CuCrO₂ film is 4.98 eV. And then, the VBM position of CuCrO₂ HTL could be obtained, $-$5.38 eV, which is similar to the previous work [31]. The VBM of CuCrO₂ film ($-$5.38 eV) is close to the highest occupied molecular orbital (HOMO) of the perovskite ($-$5.4 eV), which could effectively reduce the energy loss caused by non-radiation recombination and further enhance $V_{{\rm{OC}}}$ of PSCs.

Subsequently, to further investigate the charge dynamics in PSCs with different HTLs, electrochemical impedance spectroscopy (EIS) measurements have been carried out, and the representative EIS Nyquist plots and Bode-phase plots of the PSCs based on 2 coats CuCrO₂ HTL and PEDOT: PSS HTL obtained under the bias voltage of 0.9 V are shown in FIG. 5 (c) and (d). The semicircle in each Nyquist plot reflects the charge carrier behavior at the interface between perovskite layer and adjacent charge carrier layer. In this investigation, the change of this semicircle could be ascribed to the change of charge behavior at the HTL/perovskite interface. After using the equivalent circuit (the inset of FIG. 5(c)) to fit the Nyquist plot, the recombination resistance ($R_{{\rm{rec}}}$) could be obtained. The device based on CuCrO₂ HTL exhibits a $R_{{\rm{rec}}}$ of 1039 $\Omega$ which is much higher than that of the PSC based on PEDOT: PSS HTL (209.8 $\Omega$), and this phenomenon indicates the probability of charge recombination in CuCrO₂-based PSCs is much lower than that in PEDOT: PSS based PSCs. In addition, the charge carrier lifetime could be evaluated from the Bode-phase plots (FIG. 5(d)), the charge carrier lifetime $\tau$ could be estimated according to the following equation

where $f_{\max}$ represents the peak frequency of Bode-phase plot. The $f_{\max}$ of CuCrO₂ based device is only half that of PEDOT: PSS based device, and the calculated charge carrier lifetime of CuCrO₂-based device and PEDOT: PSS based device is 503 ns and 251 ns, respectively, the CuCrO₂ based device exhibits a longer charge carrier lifetime.

Ⅳ.   CONCLUSION

The delafossite CuCrO₂ nanoparticles with the average size of 10.89±1.95 nm have been spin-coated directly on ITO substrate and used as HTL for the inverted structure PSCs with triple-cation mixed-halide perovskite [(FAPbI₃)$_{0.87}$(MAPbBr₃)$_{0.13}$]$_{0.92}$(CsPbI₃)$_{0.08}$ light harvesting layer. The CuCrO₂ based PSCs exhibit a relatively higher $V_{{\rm{OC}}}$ of 1039 mV compared with the PSCs based on PEDOT: PSS which is the most common HTM for inverted structure PSCs. The $V_{{\rm{OC}}}$ improvement of CuCrO₂-based PSCs could be attributed to the better energy band alignment between CuCrO₂ and the perovskite. The perovskite films deposited on CuCrO₂ HTL possess much longer charge lifetime and the corresponding PSCs exhibit larger recombination resistance. The relatively stable and low-cost inorganic p-type CuCrO₂ is a good alternative HTM for inverted PSCs, and it is suitable for mass production of large area PSCs and wearable flexible PSCs.

Supplementary materials: Digital photos of CuCrO₂ nanoparticles dispersed in absolute isopropanol, methanol, and absolute ethanol, respectively (FIG. S1), XRD pattern (FIG. S2) and TEM image (FIG. S3) of the CuCrO₂ nanoparticles, $J$-$V$ curves of the PSCs with different thicknesses CuCrO₂ HTLs (FIG. S4), $J$-$V$ curves of the fresh and aged PSCs, cross-sectional SEM images of PSCs (FIG. S5), cross-sectional SEM images of CuCrO₂-based device and PEDOT: PSS-based device (FIG. S6), XRD patterns of perovskite films deposited on the CuCrO₂ and on PEDOT: PSS HTLs (FIG. S7), UV-Vis absorbance spectra of perovskite films (FIG. S8), as well as the tables of photovoltaic parameters of PSCs with different thicknesses CuCrO₂ or PEDOT: PSS HTLs are available.

Ⅴ.   ACKNOWLEDGMENTS

This work was jointly supported by the National Natural Science Foundation of China (No.62075223 and No.11674324), CAS Pioneer Hundred Talents Program of Chinese Academy of Sciences, CAS-JSPS Joint Research Projects (GJHZ1891), Director Fund of Advanced Laser Technology Laboratory of Anhui Province (AHL2020ZR02), Key Lab of Photovoltaic and Energy Conservation Materials of Chinese Academy of Sciences (PECL2019QN005 and PECL2018QN001), the Natural Science Foundation of Top Talent of Shenzhen Technology University (No.2020101), Natural Science Research Project of Higher School of Anhui Province (KJ2020A0477), Initial Scientific Research Fund of Anhui Jianzhu University (No.2018QD60), and Anhui Provincial Key Laboratory of Photonics Devices and Materials for partial experimental supports.