Low-temperature aqueous solution processed ZnO as an electron transporting layer for efficient perovskite solar cells

Abstract

Lead halide perovskite solar cells (PSCs) typically use high-temperature processed TiO₂ as electron transporting layers (ETLs). Here, we report a low temperature and aqueous solution processed route to prepare ZnO ETLs for perovskite solar cells. The ZnO ETL was prepared by spin coating a stable aqueous solution of an ammine–hydroxo zinc complex, [Zn(NH₃)_x](OH)₂, followed by thermal annealing at a relatively low temperature of 150 °C. The as-prepared ZnO film was highly transparent and uniform. Moreover, the relatively low work function of the ZnO film led to a high open circuit voltage (1.07 V), indicating its promise as the electron transporting layer. To further improve the photovoltaic performance, urea was introduced between the ZnO layer and the perovskite layer to improve the perovskite crystal growth. As a result, the PCE surged to 14.6%. We further exploited the low-temperature processed ZnO film for flexible PSCs, which have shown good mechanical stability with a respectable PCE of 11.9%.

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Introduction

Organic–inorganic lead halide perovskite solar cells (PSCs) have drawn broad attention in recent years.^1–8 The power conversion efficiency of PSCs has reached over 22%,^9,10 which is largely attributed to the superior photovoltaic properties of lead halide perovskites, such as a broad absorption band with a high extinction coefficient,¹¹ high electron and hole mobility,¹² and long electron–hole diffusion length.¹³ Highly efficient PSCs mainly use TiO₂ as the electron transporting layer (ETL). Typically, the TiO₂ layer is prepared by spin-coating or spray pyrolysis of a solution of a TiO₂ precursor followed by annealing at 500 °C in order to transform the amorphous oxide layer into the crystalline form, which prevents the use of flexible plastic substrates as they are unstable at high temperature. Hence, a low-temperature processed ETL material should be explored.^14,15 Among the ETL materials, ZnO is the most promising owing to its high transparency, low work function and high electron mobility.^16–18 A number of techniques, such as electro-deposition, sol–gel and solution-processing of nanoparticles, were demonstrated to prepare ZnO ETLs for perovskite solar cells. The electro-deposition method is complex,¹⁹ and the solution method is attractive in order to take advantage of the high throughput fabrication techniques. The most widely used solution processed ZnO sol–gel method²⁰ requires thermal annealing at a high temperature, typically above 200 °C, due to the high decomposition temperature of the precursor (zinc acetate), which prevents the fabrication of flexible solar cells. For flexible solar cells, a lower processing temperature is always preferred. Recently, Liu et al.²¹ reported that perovskite solar cells using low-temperature solution-processed ZnO colloidal nanoparticles as the ETL material could achieve a high power conversion efficiency (PCE) of 15.7%. But the stock solution containing ZnO colloidal nanoparticles was not stable due to the aggregation of the nanoparticles in solution.

In this study, we demonstrated a low temperature and solution processed route, based on an ammine–hydroxo zinc complex solution instead of the ZnO nanoparticle suspension, to prepare the ZnO ETLs for perovskite solar cells at a relatively low temperature of 150 °C, which exhibited a PCE of 10.6% with a high open circuit voltage of 1.07 V. To further improve the photovoltaic performance, urea was introduced between the ZnO layer and the perovskite layer to tune the perovskite crystal growth. The PCE surged to 14.6% with the modification of urea on ZnO, which is one of the highest values obtained for ZnO based planar n–i–p PSCs. Moreover, using the prepared ZnO as ETLs, efficient flexible PSCs with good mechanical stability can also be achieved. The PCE can reach a promising value of 11.9% for flexible PSCs. This renders the prepared ZnO a promising ETL for highly efficient PSCs.

Results and discussion

Aqueous solution processed ZnO film

The aqueous [Zn(NH₃)_x](OH)₂ solution was prepared according to previous report.²² The Zn(OH)₂ precipitate was firstly produced by the reaction of Zn(NO₃)₂ and NaOH. Then, the precipitate was suspended in water, followed by centrifugation and supernatant removal. Rinse and separation steps were repeated three times to remove Na⁺ and NO₃⁻. After the final centrifugation, the Zn(OH)₂ precipitate was dissolved in aqueous ammonia to form a stock solution. The Zn concentration in the stock solution was about 0.12 M. The stock solution was stable under ambient conditions for over three months. The ZnO thin films were prepared by spin coating the aqueous solution onto the substrates, followed by annealing at 150 °C for 10 min. Precipitation, dissolution, and ammonia dissociation schemes are expressed as

Zn(NO₃)₂(aq) + 2NaOH(aq) → Zn(OH)₂(s) + 2NaNO₃(aq)

Zn(OH)₂(s) + xNH₃(aq) → [Zn(NH₃)_x](OH)₂(aq)

[Zn(NH₃)_x](OH)₂(aq) → ZnO(s) + xNH₃(g) + H₂O(g)

Physical properties of the ZnO film

Fig. 1a shows the optical transmittance spectra of the ZnO film. The prepared ZnO demonstrates a slightly higher transmittance compared to the ITO substrate in the visible region. Fig. 1b shows scanning electron microscopy (SEM) images of the ZnO film. The ZnO film appears dense and continuous. These results reveal that the low-temperature, aqueous solution processed ZnO film is highly transparent and uniform, which is an ideal ETL material for perovskite solar cells.

Fig. 1 (a) Transmission spectra and (b) SEM image of the as-prepared ZnO films.

For the ETL material, a lower work function is desirable for the reduction in a charge extraction barrier. According to the UPS measurements (Fig. 2), the work function of the ZnO film prepared in this report is determined to be 3.71 eV, which is lower than the work function of the ZnO film prepared using the widely used sol–gel method (4.15 eV).²⁰ The lower work function of the prepared ZnO film improves the energy level alignment between ZnO and perovskite, which increases the carrier extraction and charge generation. Moreover, the lower cathode work function provides a larger built-in potential to facilitate the charge transport. These effects would increase V_oc of PSCs.

Fig. 2 UPS spectra of the ZnO films prepared using the aqueous solution method (this report) and the sol–gel method.

Device performance of ZnO based PSCs

We fabricated PSCs using the prepared ZnO film as ETL based on the planar n–i–p device structure of glass/ITO/ZnO/ CH₃NH₃PbI₃/spiro-OMeTAD/Ag. As shown in Fig. 3 and Table 1, the PSC with the prepared ZnO as ETL showed the best PCE of 10.6% with the high open circuit voltage (V_oc) of 1.07 V, which is higher than the V_oc of the sol–gel ZnO based PSCs (0.97 V). The higher V_oc should be ascribed to the lower work function of the prepared ZnO, indicating that the prepared ZnO film is an excellent ETL material for high performance PSCs. To further improve the photovoltaic performance, urea was introduced between the prepared ZnO film and perovskite to tune the perovskite crystal growth. The device based on the planar n–i–p device structure of glass/ITO/ZnO/urea/ CH₃NH₃PbI₃/spiro-OMeTAD/Ag was fabricated. After the modification of urea on ZnO, the PCE surged to 14.6%. For the CH₃NH₃PbI₃ perovskite film, the substrates covered with an amino group would favor the growth of smooth and high crystalline perovskite films. The introduction of urea with two amino groups resulted in about 38% enhancement of PCE and the simultaneous improvement of all the device parameters. We attributed the improved device performance to the evolved morphology of CH₃NH₃PbI₃ film on the urea surface. It should be noted that the hysteresis was observed for the PSC device (Fig. S5, ESI†), which is always shown in the PSCs with the planar n–i–p structure.

Fig. 3 J–V curves of the perovskite CH₃NH₃PbI₃-based solar cells using the ZnO ETL with the reverse scan.

Table 1 Device parameters of the PSCs with the reverse scan

	V oc [V]	J sc [mA cm^−2]	FF [%]	PCE [%]
				Best	Average
Sol–gel ZnO	0.97	16.79	57	9.3	8.5
Bare ZnO	1.07	16.81	59	10.6	9.1
ZnO/urea	1.12	19.94	66	14.6	13.4

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Urea modification

To investigate the effects of urea on the morphology of CH₃NH₃PbI₃ perovskite film, Fig. 4 shows the scanning electron microscopy (SEM) images of perovskite film surface morphology on different substrates. The SEM images of the CH₃NH₃PbI₃ film directly showed that the number of pin-holes was reduced with the deposition of urea on the ZnO. The amino groups of urea would act as the nucleation center for CH₃NH₃PbI₃ deposition and crystal growth. The well-covered perovskite on the ZnO/urea substrate can be observed in Fig. 4b. In contrast, the CH₃NH₃PbI₃ perovskite crystals show many pin-holes on the top of bare ZnO in Fig. 4a, which would lead to current leakage.

Fig. 4 SEM images of CH₃NH₃PbI₃ formed on (a) bare ZnO and (b) ZnO/urea (pin-holes are in red circles).

In order to further look into the influence of urea modification on the improvement of photovoltaic performance, we investigated charge dynamics by measuring photoluminescence (PL) and time-resolved photoluminescence (TRPL) of the perovskite film on bare ZnO and ZnO/urea. Fig. 5a shows the steady-state PL spectra of perovskite films on bare ZnO and ZnO/urea, which were excited at 450 nm. The PL intensity of the ZnO/urea/perovskite film was reduced compared with that of ZnO/perovskite film, while the absorbance of the two films is essentially the same. This result shows fast electron injection from perovskite into ZnO after the modification of urea. Furthermore, the TRPL was measured by monitoring the peak emission at 768 nm as shown in Fig. 5b. Fitting the data with tri-exponential decay yields three time constants, i.e. τ₁, τ₂ and τ₃, as summarized in Table S1 (ESI†). The decay component of τ₁ ∼ 9 ns might come from the bulk recombination in the perovskite bulk film, and the decay component of τ₂ ∼ 100 ns could be attributed to the recombination of free carriers in the radiative channel. For ZnO/perovskite, τ₁ is 9.6 ns with a 9.25% ratio and τ₂ is 142.2 ns with an 80.83% ratio. As for ZnO/urea/perovskite, both τ₁ and τ₂ were shortened to 8.8 ns and 84.7 ns, respectively. The faster charge transfer rate indicated stronger electronic coupling between the ZnO and CH₃NH₃PbI₃ perovskites with the modification of urea. This is consistent with enhanced J_sc and FF from the urea modified ZnO-based device.

Fig. 5 (a) The steady-state PL spectra of perovskite film on glass, bare ZnO and ZnO/urea. (b) TRPL decay transient spectra of ITO/ZnO/perovskite and ITO/ZnO/urea/perovskite.

Flexible perovskite solar cells

We replaced the rigid glass/ITO substrate with a flexible PET/ITO substrate and fabricated the solar cell devices using the same procedures. Flexible perovskite solar cells based on the structure of PET/ITO/ZnO/urea/perovskite/spiro-OMeTAD/Ag were fabricated. Fig. 6a shows the photocurrent density–voltage (J–V) curve of the ZnO-based flexible PSC. The J–V curve exhibits a short circuit current density (J_sc) of 18.7 mA cm⁻², an open circuit voltage (V_oc) of 1.12 V, and fill factor (FF) of 0.57, resulting in PCE of 11.9%. In addition, we performed mechanical bending tests over 500 cycles. As shown in Fig. 6b, the performance of the device retains over 80% of its initial efficiency even after 500 bending cycles.

Fig. 6 (a) J–V curves for the best flexible PSC and the photograph of the ZnO-based flexible PSC (inset). (b) Normalized PCE of ZnO-based flexible PSCs as a function of bending cycles.

Conclusions

We reported a low temperature and solution processed route to prepare the ZnO ETLs for perovskite solar cells by spin coating the aqueous solutions of an ammine–hydroxo zinc complex, [Zn(NH₃)_x](OH)₂, followed by thermal annealing at a relatively low temperature of 150 °C. The prepared ZnO shows high transparency and a low work function. Perovskite solar cells with the prepared ZnO as ETLs exhibited a PCE of 10.6% with a high open circuit voltage of 1.07 V. To further improve the photovoltaic performance, urea was introduced between the ZnO layer and the perovskite layer to tune the perovskite crystal growth. The PCE surged to 14.6% with the modification of urea on ZnO. The deposition of urea resulted in the improved morphology of the crystalline perovskite film without pin-holes and the enhanced electronic coupling between the ZnO layer and the perovskite layer. Moreover, using the prepared ZnO as ETLs, efficient flexible PSCs with good mechanical stability can also be achieved. The PCE can reach a promising value of 11.9% for flexible PSCs. These results define a promising approach to the fabrication of a high efficient electron transporting layer of the perovskite solar cell at low temperature.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21503011), the Fundamental Research Funds for the Central Universities (buctrc201516), RGC of Hong Kong (GRF No. 16300915) and HK Innovation and Technology Fund (ITS/004/14).

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Footnote

† Electronic supplementary information (ESI) available: Synthesis of ZnO precursor, fabrication and characterization of PSCs, XRD of the perovskite film, EQE spectra of the perovskite CH₃NH₃PbI₃-based solar cell using the ZnO ETL, histograms of PCEs measured for 30 cells using the ZnO ETLs with or without urea modification, the contact angle of water of the ZnO films with or without urea modification. See DOI: 10.1039/c6qm00248j

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