Research Article | Open Access
Ziqi Xu, Nengxu Li, Xiuxiu Niu, Huifen Liu, Guilin Liu, Qi Chen, Huanping Zhou, "Balancing Energy-Level Difference for Efficient n-i-p Perovskite Solar Cells with Cu Electrode", Energy Material Advances, vol. 2022, Article ID 9781073, 8 pages, 2022. https://doi.org/10.34133/2022/9781073
Balancing Energy-Level Difference for Efficient n-i-p Perovskite Solar Cells with Cu Electrode
Developing low cost and stable metal electrode is crucial for mass production of perovskite solar cells (PSCs). As an earth-abundant element, Cu becomes an alternative candidate to replace noble metal electrodes such as Au and Ag, due to its comparable physiochemical properties with simultaneously good stability and low cost. However, the undesirable band alignment associated with the device architecture impedes the exploration of efficient Cu-based n-i-p PSCs. Here, we demonstrated the ability of tuning the Fermi level () of hole transport layer (HTL) to reduce the energy level difference (Schottky barrier) between HTLs and Cu. Further, we identified that the balance of energy level difference between HTL and adjacent layers (including perovskite and Cu) is crucial to efficient carrier transportation and photovoltaic performance improvement in the PSCs. Under the optimized condition, we achieve a device power conversion efficiency (PCE) of 20.10%, which is the highest on the planar n-i-p PSCs with Cu electrode. Meanwhile, the Cu-based PSCs can maintain 92% of their initial efficiency after 1000 h storage, which is comparable with Au-based devices. The present work not only extends the understanding on the band alignment of neighboring semiconductor functional layer in the device architecture to improve the resulting performance but also suggests great potential of Cu electrode for application in PSCs community.
Organic-inorganic hybrid PSCs have drawn enormous attention in academia and industry due to their excellent optoelectronic properties and low fabrication cost [1–3]. Up to now, the PCE skyrocketed from the initial 3.8% to 25.7% in the past few years [4–10]. However, most of the high performance PSCs are employing expensive mental (such as Au and Ag) as the back electrode, which not only increase the fabrication cost but also bring into undesirable instability issues in the resulting devices. For instance, Ag electrode and iodide ions (originated from perovskite) could easily diffuse and react with each other to form insulated AgI, which was detrimental to the long-term stability of PSCs . Meanwhile, although Au was demonstrated to be more chemically stable than Ag, the reaction with perovskites under humidity was still found at the interface level . To solve this problem, some buffer layers (i.e., MoOx, Ta-WOx, etc.) were used to prevent the direct contact of metal electrodes with the perovskite layer [13–15]. However, these buffer layers were only able to separate the perovskite and metal electrodes in a short period, leaving metal atoms still diffusing through the buffer layers after under long-term operation, leading to unfavorable degradation . Moreover, the Au electrodes were estimated to occupy over 75% of the total material cost of the PSC module, which largely hindered the commercialization of PSCs . Therefore, it is necessary to seek out an ideal electrode with substantially low cost and inert reactivity with perovskites.
Cu as an earth-abundant element is the promising candidate to be electrode for its comparable physical properties with Au and Ag and exhibits good stability with simultaneously low cost . Firstly, Cu exhibits comparable work function (~4.7 eV) with Au (~5.1 eV) and Ag (~4.6 eV) ; the conductivity of Cu ( S m−1) is also similar with Au ( S m−1) and Ag ( S m−1) at 20°C , which is suitable for electrode application. Secondly, Cu shows good environmental stability due to its excellent corrosion resistance, especially contacts with halogens and nonoxidizing acids (i.e., HI and HBr) . In addition, Cu is demonstrated to be more difficult to diffuse into the perovskite layer even under 80°C annealing, owing to the high formation energy and diffusion barrier of Cu impurity . Even if Cu can diffuse into the perovskite, the Cu impurities only behave as the benign defects which are analogous with the intrinsic vacancies in perovskites, thus cannot cause severe carriers trapping . Thirdly, Cu has the substantially cheaper price of ~0.009 $/g, compared to Ag ~0.8 $/g and Au ~55 $/g. All of these advantages indicate that Cu has the great potential to be the commercially adopted metal electrode in perovskite photovoltaics.
Actually, the adoption of Cu electrode in perovskite solar cell has been evaluated previously for both p-i-n and n-i-p architectures. For instance, Stolterfoht et al. have demonstrated that Cu could be an excellent electrode for high performance invert (p-i-n) PSCs, which showed the PCE of 21.6% (6 mm2 aperture area) and 20.3% (1 cm2 aperture area) . In addition, Huang group developed a series of efficient and stable invert PSC devices with Cu electrode, including small area PSCs (<1 cm2) [23–25], perovskite solar modules [26–28], and flexible solar cells . Surprisingly, Cu-based invert PSCs were proved to be stable under Gamma-ray radiation , which provided prospects for the application of radiation detectors and space solar cells. However, when the Cu electrode was applied into n-i-p PSCs, only few attempts were documented, and the corresponding PCE were relatively low. For example, early work achieved a PCE of 13.49% with open circuit voltage (Voc, 1.0 V) and fill factor (FF, 69.1%) based on the mesoporous n-i-p PSCs . When using the traditional 2,2,7,7-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (Spiro-OMeTAD) as the HTL, a PCE of 9.2% was obtained due to the internal resistance of HTL/Cu interface largely increased . Then, the PCE of Cu-based n-i-p PSCs was elevated to 17.3% by using tBP as the HTL additive to improve the charge selectivity at the interface . Even though, this value was much lower than the PCE obtained in p-i-n PSCs with Cu electrode . Given that the n-i-p structured PSCs currently show the most state-of-art photovoltaic performance , the demand for high-cost Au electrode makes the development of cheap and efficient Cu-based n-i-p PSCs become an important commercialization direction.
Here, we developed a feasible method, namely, “balancing energy level difference,” to optimize the band alignment in n-i-p PSCs with Cu electrode. Through material characterizations, we systematically adjusted the of HTLs to match well with both perovskite and Cu electrode. Under the optimized condition, we achieved a PCE of 20.10% with the Voc of 1.084 V and FF of 78.77%, which is the highest PCE in n-i-p PSCs with Cu electrode. Deep study revealed that a good balance of energy level difference between perovskite/HTL and HTL/Cu interface could significantly improve the charge collection and transport properties in the resultant devices, leading to improved FF. Moreover, we evaluated the stability of PSCs and found that the devices with Cu electrode could remain 92% of their initial PCE after 1000 h storage. This finding broadens our understanding on the band alignment of neighboring semiconductor functional layers in high-performing devices, which also sheds light on the application of Cu electrode in PSC commercialization.
2. Results and Discussion
In this study, the perovskite films were fabricated by the two-step solution method, where the fabrication details could be found in Supplementary Information. To obtain a suitable energy level match between perovskite/HTL/Cu, we mainly adjusted the of HTLs (combined with Spiro-OMeTAD and Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA)) through changing the relative content of different hole transport materials. The corresponding precursor solutions were prepared by adding PTAA into Spiro-OMeTAD with varied mass ratio of 0%, 13%, 17%, and 25%, which were all dissolved in chlorobenzene. For convenience, we defined these hybrid HTLs with 0 wt%, 13 wt%, 17 wt%, and 25 wt% PTAA as HTL-1, HTL-2, HTL-3, and HTL-4 for further investigation, respectively.
We firstly investigated the for different HTLs through the UPS measurements, to obtain the band alignment among perovskite/HTL/Cu, which would have a profound influence on the device performance. The measured of different HTLs are showed in Figure 1(a) (the experimental details were shown in Figure S1). It was found that the for pure Spiro-OMeTAD (HTL-1) matched well with perovskite (Figure S2) with the lowest difference of ~0.11 eV, which is expected to deliver a higher Voc with less energy loss along the HTL/perovskite interface. However, the energy difference between HTL-1 and Cu was significantly large (0.51 eV, Figure 1(b)), suggesting a higher Schottky barrier (poor charge transportation) at the HTL/Cu interface (illustrated in Figure 1(c)). To decrease the energy difference between HTL and Cu electrode, the of HTL should be increased, and in this work, we achieved it through the mixed HTLs, as discussed in the above (the for HTL-2, HTL-3, and HTL-4 is –4.31 eV, –4.40 eV, and –4.54 eV, respectively). As a result, the corresponding energy difference between HTLs and Cu was reduced to ~0.39 eV, ~0.30 eV, and ~0.16 eV (Figure 1(b)), respectively, which is beneficial for charge transportation (Figures 1(d)–1(f)). However, these changes would accordingly bring into negative effects on the band alignment between HTL and perovskite, with increased difference of ~0.23 eV, ~0.32 eV, and ~0.46 eV (Figure 1(b)), respectively, and this may lead to larger energy loss during carrier extraction process (Figures 1(d)–1(f)). Therefore, how to obtain a balanced condition on these two types of energy differences at the interfaces, including perovskite/HTL and HTL/Cu, is crucial to determine the device performance with substantially lower energy loss and considerable charge transportation. The balanced energy difference at different interfaces (i.e., for HTL-3, with similar energy difference at perovskite/HTL (0.32 eV) and HTL/Cu (0.30 eV) interfaces) would potentially promote the hole extraction and transportation across the device, thus improve the PCE of PSCs.
We then assessed the potential of these HTLs for the application in PSCs, by checking the corresponding morphology, crystallization, and photogenerated carrier behaviors. Based on the uniform polycrystalline perovskite film (Figure S3), the morphology of different HTLs (deposited on the perovskite) were revealed by scanning electron microscopy (SEM) images (Figures 2(a)–2(d)). Due to the basically amorphous state of such organic molecules, the morphology of HTLs was found to have no clear crystallization state, and all HTLs (HTL1-4) exhibited good coverage without appreciable pinholes. Moreover, we characterized the thickness of these HTLs and found that there was no obvious difference between HTL1-4 with the average thickness of ~200 nm. Then, we studied the crystallization behavior of different HTLs via the X-ray diffraction (XRD) measurements. While the pure Spiro-OMeTAD partially crystallized with small full width at half maximum (FWHM) of main diffraction peak and contained some sharp diffraction peaks, the main diffraction peak became broader (with larger FWHM), and those sharp peaks gradually disappeared after the incorporation of PTAA into the HTLs (HTL2-4, Figure S4). This result indicates that the mixed HTLs suppressed the crystallization of Spiro-OMeTAD in the composites, which was beneficial for the optoelectronic performance of PSCs. To further monitor the carrier behavior of perovskite/HTL samples, we performed the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. As shown in Figure 2(e), the perovskite/HTL-1 (only Spiro-OMeTAD) heterojunction quenched the PL signal of perovskite most efficiently than other heterojunctions, and the mixed HTLs with higher (from HTL-2 to HTL-4) showed less quenching effects, demonstrating that the hole extraction and transportation from perovskite to the HTL were more effective with lower difference at the perovskite/HTL interface [34, 35]. Furthermore, the carrier lifetime (obtained from TRPL results) of perovskite/HTL samples increased from 81.07 ns (HTL-1), 126.54 ns (HTL-2), 147.20 ns (HTL-3) to 178.96 ns (HTL-4), as shown in Figure 2(f) and Table S1, indicating the suppressed carrier extraction, which were all in consistent with the PL results. To be noted, the carrier lifetime difference between different samples was not significant; thus, the mixed HTLs still exhibited comparable carrier transfer properties with pure Spiro-OMeTAD, which provides prerequisite for the application in PSCs.
To further reveal the charge carrier dynamics within PSC devices, we then collected the PL spectra of PSCs (ITO/SnO2/perovskite/HTL/Cu) which were short-circuit connected (so that the photo carriers could easily transport into the external circuit). As shown in Figure 2(g), the PL intensity gradually decreased from HTL-1 to HTL-4, which was dramatically different from the tendency in electrode-free samples (ITO/SnO2/perovskite/HTL, Figure 2(e)). This phenomenon indicated that the carrier transport ability from HTL to Cu electrode governed the PL emission within the devices. Considering the gradually reduced Schottky barrier at HTL/Cu interface from HTL-1 to HTL-4, we inferred that most of the light-induced carriers were conducted to the external circuits, thus decreasing PL intensity. Moreover, we performed the transient photovoltage decay (TPV) and transient photocurrent decay (TPC) measurements. As shown in Figure S5 and Table S2, the charge recombination lifetime () of device based on HTL-3 (166.01 μs) was longer than other devices. This result suggested a substantially suppressed charge recombination in the whole PSC device, including the perovskite bulk and the perovskite/HTL-3/Cu interface. Meanwhile, it was observed that the device with HTL-3 showed a shorter charge transport lifetime () than devices based on other HTLs (Figure 2(h) and Table S2), indicating a much more efficient charge transfer across the device. Therefore, we can conclude that the balanced energy difference between perovskite/HTL and HTL/Cu interface enabled the more effective carrier transportation in the device, through avoiding the too much energy difference and corresponding poor carrier transport behavior at either interface.
To reveal the relationship between interface energy level difference and optoelectronic performance in PSCs, we fabricated the PSC devices with the configuration of ITO/SnO2/perovskite/HTL/Cu (as shown in the cross-sectional SEM image in Figure S6). The fabrication details could be found in Supporting Information. As shown in Figure 3(a), the Voc gradually decreased with the energy deference at perovskite/HTL interface increased (the averaged Voc for HTL-1-, HTL-2-, HTL-3-, and HTL-4-based PSCs are 1.100, 1.091, 1.088, and 1.085 V, respectively), which may be due to the enlarged interface energy loss (Figures 1(c)–1(f)). Meanwhile, the Jsc showed opposite variation trend of Voc, with the gradually increased value of 21.85, 22.88, 23.21, and 23.38 mA/cm2 for HTL1-4, respectively (Figure 3(b)). This result was basically in consistent with the TPC measurements, and we contributed it to the better carrier transportation at HTL/Cu interface with the reduced Schottky barrier. Considering the opposite tendency of Voc and Jsc, the trade-off between them still could not guarantee the device performance by different HTLs, while it further count on the FF. Interestingly, the averaged FF increased from 63.31% (HTL-1) to 74.25% (HTL-2), and up to the highest value of 77.98% based on the HTL-3, then decreased to 75.24% with HTL-4 (Figure 3(c)). This tendency was just fitted with the energy level difference at different interfaces (summarized in Figure 1(b) and Table S3), where the more balanced energy difference (meaning the less variation between perovskite/HTL and HTL/Cu interface energy difference) would lead to the higher FF in corresponding PSCs (Figure 3(e)). Notably, the Cu-based PSC with pure Spiro-OMeTAD (HTL-1) exhibited the poorest FF due to the largest energy difference variation between perovskite/HTL (~0.11 eV) and HTL/Cu (~0.51 eV) interface, which was in consistent with other work based on the same device structure . From these results, we could conclude that the “energy level difference balance” between perovskite/HTL and HTL/electrode interface indeed exhibited crucial influence on the PSC performance (especially FF), through altering the charge collection and transfer behavior at these interfaces. Eventually, the HTL-3-based PSCs showed an average PCE of 19.57%, which was higher than other HTLs based PSCs (Table S4). And the variation trend of PCE was exactly the same as that of FF (Figure 3(d)), suggesting that the FF was a key factor to influence the Cu-based PSC performance.
The best performing PSC with HTL-3 achieved a high reverse scan PCE of 20.10% with the Voc of 1.084 V, Jsc of 23.54 mA/cm2, and FF of 78.77%, as shown in Figure 3(f). In addition, the forward scan PCE of this device was 18.88%, with the Voc of 1.075 V, Jsc of 23.58 mA/cm2, and FF of 74.48% (Figure S7). The stabilized power output of this device was 19.53% (Figure S8). To our best knowledge, this was the highest PCE of n-i-p PSC with Cu electrode. The external quantum efficiency (EQE) measurement exhibited an integrated Jsc of 23.03 mA/cm2, as shown in Figure S9, which was in good agreement with the J-V measurements.
Furthermore, we monitored the stability of the PSCs based on HTL-3 and different metal electrodes (Au, Ag, and Cu), and these devices were kept under N2 atmosphere at temperature about 25-40°C. As shown in Figures 4(a)–4(d), the PSCs with Cu electrode exhibited comparable performance loss compared to the PSCs with Au electrode, which were all kept over 90% of their initial PCE after over 1000 h storage. Meanwhile, the optoelectronic performance of Ag-based PSCs dropped dramatically after 300 h storage, mainly due to the unfavorable metal diffusion and reaction with perovskite components . Therefore, the n-i-p PSCs with Cu electrode could be a promising candidate toward for efficient and stable devices after further improvement.
In summary, we innovatively provided a method, called “balancing energy level difference,” to optimize the band alignment across perovskite/HTL/Cu interface in n-i-p PSCs with Cu electrode. Combined with the material characterization and device performance, we systematically investigated the energy difference at perovskite/HTL and HTL/Cu interfaces with different HTLs and built a relationship between interface energy difference and corresponding photovoltaic parameters. Accompanied with the investigation on carrier behavior, we concluded that the balanced energy difference at these interfaces could significantly improve the charge collection and transportation properties in resultant devices. Under the optimized condition, we achieved a high PCE of 20.10% with Voc of 1.084 V and FF of 78.77%, which was the highest PCE in n-i-p PSCs with Cu electrode. Besides, we monitored the stability of PSCs with different metal electrodes and found that the devices with Cu electrode could remain 92% of their initial PCE after over 1000 h storage. The present work not only expanded our knowledge on seeking a balanced band alignment at different interfaces to improve the performance of devices but also ensured the application of Cu electrode in further PSC industrialization.
The authors declare that the main data supporting the findings in this study are available within the article and its supplementary information. Additional data are available from the corresponding authors upon reasonable request.
Conflicts of Interest
The authors declare no competing interests.
H.Z., Z.X., and N.L. conceived the idea. H.Z., Z.X., N.L., and X.N. designed the experiments. Z.X., N.L., and X.N. were involved in all the experimental parts. G.L. performed the TPC and TPV measurements. Z.X., X.N., N.L., and H.L. contributed to the fabrication of high performance perovskite solar cells. H.Z., Q.C., Z.X., N.L., and X.N. wrote and revised the manuscript. All authors were involved in the discussion for data analysis and commented on the manuscript. Z.X., N.L., and X.N. contributed equally to this work.
This work was supported by the National Natural Science Foundation of China (Grant No. 51972004), the National Key Research and Development Program of China (Grant Nos. 2020YFB1506400 and 2017YFA0206701), and the Tencent Foundation through the XPLORER PRIZE.
Figure S1: secondary electron cut-off regions of UPS spectra for different HTLs (HTL1-HTL4). Figure S2: secondary electron cut-off region of UPS spectrum for perovskite film. Figure S3: the SEM images of perovskite film under low magnification and high magnification. Figure S4: the XRD patterns of different HTLs (HTL1-HTL4). Figure S5: normalized transient photovoltage decays of PSC devices with different HTLs (HTL1-HTL4). Figure S6: the cross-sectional SEM image of perovskite solar cells. Figure S7: the J-V measurements under reverse scan (1.2 ~ –0.2 V) and forward scan (–0.2 ~ 1.2 V) of device-based different electrodes (Cu, Ag, and Au). Figure S8: steady-state power output of best Cu device under max power point tracking. Figure S9: external quantum efficiency spectrum together with the integrated current density for the HTL-3-based PSC device. Table S1: the fitted carrier lifetime of perovskites with different HTLs (HTL1-HTL4) which obtained from the TRPL measurements (Figure 2(b)). Table S2: the fitted charge recombination lifetime (τr) and charge transport lifetime (τt) of perovskite solar devices which obtained from the TPV and TPC measurements (Figures 2(c) and 2(d)). Table S3: the calculation on energy difference variation in different samples based on Figure 1(b). Table S4: the averaged photovoltaic parameters for the PSCs with different HTLs (HTL1-HTL4). (Supplementary Materials)
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