Enhanced charge carrier extraction and transport with interface modification for efficient tin-based perovskite solar cells

Abstract

Tin-based perovskites have become the most promising non-lead perovskites due to their ideal band gap and low toxicity. Although the open circuit voltage of tin-based perovskite solar cells (TPSCs) continues to approach the theoretical value, the short-circuit current is still far from the theoretical value. Here, we describe an interface modification method by regulating the property of hole transport layer, PEDOT:PSS, which improves the surface molecular morphology and the energy level alignment of PEDOT:PSS/perovskite interface. Advanced GIWAXS and IR s-SNOM characterization are conducted to achieve multi-dimensional characterization of nanoscale surface morphology and chemical distribution of PEDOT:PSS. With the multi-attribute optimization, charge carrier extraction and non-radiative recombination are also improved. The resultant TPSCs exhibit a higher power conversion efficiency of 13.32% in compared with the control device of 10.50%, accompanied with an increase in the short-circuit current from 18.10 to 20.50 mA cm⁻² and FF from 68.23% to 76.43%. This work demonstrates a reliable strategy for improving charge carrier extraction and device performance for lead-free TPSCs.

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Introduction

Perovskites have gained significant attention in the field of solar cells,^1–5 detectors,^6,7 transistors^8,9 and light-emitting diodes (LEDs)^10,11 due to their remarkable photoelectric properties. In particular, lead-based perovskite solar cells (PSCs) have shown power conversion efficiencies of 26.7% (ref. 12) comparable to traditional silicon cells. However, concerns regarding the heavy metal toxicity of lead-based perovskites have hindered their development. As a result, tin-based perovskites have emerged as highly promising alternatives, thanks to an ideal band gap and lower toxicity levels.¹³

However, the highest certified efficiency achieved by TPSCs currently stands at 15.67% (V_OC = 0.974 V, J_SC = 21.7 mA cm⁻², FF = 74.14%),¹⁴ which is still considerably below the theoretical limit of 32.3%.¹⁵ The disparity between actual and theoretical efficiency primarily stems from losses in short-circuit current (J_SC) and fill factor. Specifically, the open circuit voltage of practical devices reaches 91.46% of its theoretical value (1.065 V), while the J_SC and fill factor are only 63.27% and 84.09% of their respective theoretical values (34.3 mA cm⁻², 88%). Consequently, enhancing the short-circuit current represents a crucial aspect in improving the efficiency of TPSCs.

Some studies have reported the enhancement of J_SC and FF in TPSCs. Ning et al. devised a synthetic route to generates a highly coordinated SnI₂·(DMSO)_X adduct, which directs out-of-plane crystal orientation and achieves a more homogeneous structure in poly crystalline perovskite thin films.¹⁶ The J_SC of the TPSCs increased from 16.3 mA cm⁻² to 20.1 mA cm⁻². Mi et al. introduced trimethylthiourea to the spin coating of FASnI₃ films. This bifunctional ligand greatly improved the morphology and texture of FASnI₃ films by spreading and joining individual crystal grains.¹⁷ The optimization has brought an improvement in J_SC from 17.2 mA cm⁻² to 20.3 mA cm⁻² and FF from 73% to 77%. Wang et al. presented a post-growth treatment of 3-aminomethylbenzo[b]thiophene on the perovskite film, which react with exposed FA⁺ on the perovskite surface and suppresses the formation of iodine vacancy defects, leading to a reduction in trap density.¹⁸ The J_SC and FF achieved increasement from 19.38 mA cm⁻² to 21.42 mA cm⁻² and 62% to 66%, respectively. Most of these studies focus on the perovskite crystallization and electron transport layers optimization. However, little effort has concentrated on the improvement of hole transport layer (HTL) and HTL/perovskite heterojunction interface quality. Up to now, most high-efficiency TPSCs are based on inverted device structures with PEDOT:PSS as HTL.^17,19–26 The performance of TPSCs is still hindered by the PEDOT:PSS interface with highly hygroscopic and acidic PSS molecules.^27–29 Deprotonation of the –SO₃H group in PSS is one of the reasons of interfacial degradation due to its low acid dissociation constant, leading to acid erosion and iodine volatilization in perovskites.³⁰ Therefore, improving HTL/perovskite heterojunction interface contact is crucial to improving the efficiency of TPSCs.

In this study, we developed an interface modification strategy to enhance the charge carrier extraction and transport in TPSCs. By utilizing a simple post-treatment procedure on the PEDOT:PSS film, the interfacial energy level alignment was improved, increasing the short-circuit current and minimizing non-radiative recombination in TPSCs. Synchrotron radiation-based X-ray scattering and near field optical microscopy both revealed that the PSS content of the post-treated PEDOT:PSS film decreased, with the increase of the proportion of PEDOT. In addition, the crystallization and morphology of perovskite film was also optimized. The PCE of the devices significantly improved from 10.50% to 13.32%, mainly due to the increase in the short-circuit current from 18.10 to 20.50 mA cm⁻² and FF from 68.23% to 74.37%. Our work provides a reliable strategy for improving short-circuit current losses and the device performance for planar heterojunction in TPSCs.

Results

As shown in Fig. 1a, TPSCs were prepared based on the commonly used structure of ITO/PEDOT:PSS/perovskite/ICBA/Ag device configuration. After spin-coating the PEDOT:PSS film, we treated the PEDOT:PSS film with three different solvents: isopropyl alcohol (IPA), ethanol (EtOH) and trifluoroethanol (TFE). A schematic diagram of the changes in PEDOT:PSS film after post-treatment process is shown in Fig. 1b. After solvent treatment, the PEDOT ratio and crystallinity was increased. To evaluate the crystallization of the PEDOT:PSS films with and without post-treatment, we performed grazing incidence wide-angle X-ray scattering (GIWAXS) on the PEDOT:PSS films (Fig. 1c), in which the inner ring represents insulating PSS and the outer ring corresponds to conductive PEDOT.^31,32 The out-of-plane integration of the q-space-converted 2D GIWAXS images are shown in Fig. 1d, where the solvent-treated PEDOT:PSS film shows a higher intensity. In general, PEDOT:PSS conductive polymer films do not exhibit sharp crystalline reflections. Therefore, it is difficult to extract accurate information about grain size. Using the reflection information of GIWAXS, we can determine stacking distances from the centroid of the reflections. The information about the size of crystalline grains is contained in the width of the diffraction peaks, and at the simplest level the Scherrer equation can be employed to extract a rough grain size. By further analyzing the sector integrals of the GIWAXS measurements, we estimated the ratio of PEDOT to PEDOT:PSS, molecular interchain coupling (d-spacing) and a rough estimate of the crystal size with Gaussian functions and Scherrer analysis, as shown in Fig. 1e and Table S1.† The ratio of PEDOT in PEDOT:PSS varies from 0.38 (control) to 0.39 (IPA-treated), 0.40 (TFE-treated), and 0.42 (EtOH-treated), indicating the conductivity improvement of PEDOT:PSS after treatment. According to these data, the TFE-treated PEDOT:PSS film possesses the largest PEDOT and the smallest PSS crystalline size, indicating that TFE-treated films have a better molecular arrangement.

Fig. 1 (a) Schematic diagram of the inverted structure TPSCs. (b) The schematic diagram of changes in PEDOT:PSS film after post-treatment process. (c) 2D GIWAXS patterns of PEDOT:PSS films treated with different solvents. (d) Out-of-plane integration of 2D GIWAXS data. (e) An overview of the crystallite sizes, d-spacing, and the proportion of PEDOT in PEDOT:PSS films treated with different solvents. (f) Energy levels measured with UPS.

To further investigate the effect of post-treatment on the energy levels, ultraviolet photoelectron spectroscopy (UPS) measurements were performed, as depicted in Fig. 1f. The valence bands of PEDOT:PSS films are 4.76 eV (control), 4.80 eV (EtOH-treated), 4.82 eV (TFE-treated) and 4.85 eV (IPA-treated), respectively. The energy gap between the valence bands of HTL and perovskite becomes smaller after treatment, resulting in better energy level alignment across the heterojunction interface, which is beneficial for charge carrier transport. The energy levels calculated from the UPS images (Fig. S1†) are shown in Table S2.†

To unravel the effect of the different solvent treatments on the surface molecular morphology of the resulting PEDOT:PSS films, we applied infrared scattering scanning nearfield optical microscopy (IR s-SNOM). Basically, this technique combines atomic force microscopy measurements revealing film surface topography with the local spectral analysis by collecting the signal from the sample area comparable with the cantilever tip size (15–25 nm).³³ Thus, by scanning the samples at the characteristic IR vibration bands we could map independently the distribution of PEDOT and PSS counterparts of PEDOT:PSS as well as reveal the presence of residual treatment reagent (EtOH, IPA or TFE) and its primary localization. The obtained results are presented in Fig. 2. The top row images (Fig. 2a) show the nanoscale morphology of control (non-modified) PEDOT:PSS. One could notice that domains of PEDOT and PSS are both appearing on the surface with random distribution, which suggests a poor order of the as-deposited PEDOT:PSS film. The treatment with EtOH results in the substantial enrichment and dispersed distribution of PEDOT on the film surface. The residual amount of EtOH treatment solvent co-localizes with PSS domains, which confirms high mutual affinity of EtOH and PSS. The IPA treatment results in an increased amount of PEDOT and PSS components on the film surface and a more dispersed distribution compared to the control. The residual IPA content is higher than that of EtOH and TFE, homogeneously distributed within the film and does not show any obvious co-localization with either PEDOT or PSS components. The TFE treatment results in a reduced amount of PEDOT component and a more dispersed PSS component on the film surface relative to the control. The residual TFE shows a clear co-localization with PEDOT, indicating that TFE has a high affinity for PEDOT. The TFE treatment also leads to the remarkable ordering of the PEDOT phase with the formation of multiple lamellas with apparently unidirectional orientation. GIWAXS and IR s-SNOM together illustrate the crystallization and morphological changes of the PEDOT:PSS films in the vertical and horizontal directions.

Fig. 2 The IR s-SNOM data obtained for differently processed PEDOT:PSS films: (a) control, (b) EtOH, (c) IPA and (d) TFE-treated. From left to right: PEDOT distribution, PSS distribution, and the distribution of the treatment reagent (if used).

The atomic force microscopy (AFM) images show the surface morphology of PEDOT:PSS films with and without post-treatment, as shown in Fig. 3a–d. The root mean square (RMS) roughness of TFE-treated PEDOT:PSS films exhibit lowest surface roughness (1.17 nm) compared with IPA (1.2 nm), EtOH (1.45 nm) and control (1.36 nm), suggesting improved interface contact to extract charge carriers. To further study the carrier extraction at the HTL/perovskite heterojunction interface, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) were employed to measure the mechanism of charge carrier kinetics. The devices with post-treatment show an obvious decrease in the intensity compared to the control one (Fig. 3e), indicating that electrons and holes can move freely without losing energy or being trapped. The TFE-treated device exhibited the best carrier extraction capability. Moreover, the carrier lifetime of solvent-treated films determined from the TRPL curves shows an obvious reduction from 7.07 ns (control) to 3.89 ns (EtOH-treated), 3.54 ns (IPA-treated) and 2.62 ns (TFE-treated), as shown in Fig. 3f. The improved charge carrier extraction and separation of PEDOT:PSS/perovskite heterojunction interface indicates that the post-treatment results in suitable energy levels and a stronger binding between perovskite and HTL. The fitting parameters of the TRPL spectra are given in Table S3.†

Fig. 3 (a–d) AFM images of control and post-treated PEDOT:PSS films. The scale of AFM images is 2 × 2 μm. (e) The steady-state PL and (f) TRPL spectra of glass/PEDOT:PSS/perovskite films.

The characterization results have revealed a remarkable effect of the surface treatment on the nanoscale morphology of the PEDOT:PSS films, which has great influence on the optoelectronic and electrical properties of the material. Consequently, these effects can influence the photovoltaic performance of TPSCs based on different PEDOT:PSS films. To verify this hypothesis, we fabricated TPSCs using the device structure shown in Fig. 1a. The TPSCs based on post-treated PEDOT:PSS show higher PCE and J_SC than the control as shown in Fig. S2 and Table S4.† The V_OC of the device treated with EtOH did not increase significantly, which is consistent with its weak energy level modification effect in the UPS test. The J_SC of the device treated with IPA is lower than that of the devices treated with EtOH and TFE, which may be related to the excessive amount of IPA residue shown in the IR s-SNOM test. Among them, devices based on TFE-treated PEDOT:PSS exhibit the best PCE, therefore, we define TPSCs based on the TFE-treated PEDOT:PSS film as the target while the devices without solvent treatment are used as the control in the following text.

The hydrophobicity of the substrates determines the morphology of the deposited perovskite films. Therefore, we investigated contact angles of PEDOT:PSS films (Fig. S3†). The target PEDOT:PSS film has a much higher value of 25.98° than the control (6.94°). The increase in contact angle may be due to the lower hydrogen bonding ability of TFE, which comes from the two lone pairs of the oxygen with low electron donicity.^34,35 It was found that non-wetting substrate suppresses heterogeneous nucleation and thus results in less dense nuclei and consequently larger grain size and high-aspect-ratio crystalline grain growth of the perovskite layer.^36–38 We then observed the morphology of perovskites deposited on PEDOT:PSS films of the control and target devices. The scanning electron microscopy (SEM) characterization of the control and target perovskite films are shown in Fig. 4a and b. The target perovskite film shows a reduced number of pinholes compared to the control film, implying an improvement in the film quality. We used XRD tests to characterize the effect of the PEDOT:PSS treatment on crystallization. As shown in Fig. S4,† the target film exhibited a higher crystallization peak compared to the control. After calculation and analysis, the half of the maximum (FWHM) of the main peaks for the control and target films were 0.1441 and 0.1185, respectively, indicating that the target film has superior crystallinity. The crystallization of the control and target perovskite films was also investigated by GIWAXS, as shown in Fig. 4c and d. We conducted in-plane and out-of-plane integrals on the GIWAXS 2D patterns and obtained the results shown in Fig. 4e and f. The target has stronger out-of-plane oriented crystallization than the control, while the control has stronger in-plane oriented crystallization than the target, which indicates that the TFE-treated substrate guides the perovskite crystals to grow in the out-of-plane direction, which makes the target group more conducive to the longitudinal transport of carriers. We obtained SEM cross-sectional images of both the control and target devices at different magnifications, as shown in Fig. S5.† The target device exhibits a more uniform morphology of perovskite/PEDOT:PSS interface compared to the control. As shown in the red circle marked regions, the bottom surface of the PEDOT:PSS in the target device shows better contact with the perovskite grain boundaries, whereas the control device display the opposite interaction. For the control sample, there are still some voids imbedded at the perovskite/PEDOT:PSS interface while there are almost no voids for the target sample. Perovskite devices with such voids are generally less efficient and stable under illumination.³⁹ This is also consistent with our device performance, as our target device delivered higher efficiency and better device stability.

Fig. 4 (a and b) SEM images of control and target perovskite films. (c and d) 2D GIWAXS patterns of the control and target perovskite films. (e) In-plane and (f) out-of-plane integrals of the 2D GIWAXS data.

We characterized the photoelectric properties of the control and target TPSCs. The incident monochromatic photon–electron conversion efficiency (IPCE) is shown in Fig. 5a. The J_SC of the target device increased from 17.63 mA cm⁻² to 19.56 mA cm⁻² in comparison to that of the control device, demonstrating that post-treatment can enhance charge carrier extraction and transport. The target device also shows a reduced dark current density of 1.13 × 10⁻⁶ mA cm⁻² in comparison to the 1.0 × 10⁻⁵ mA cm⁻² for the control device, as shown in Fig. S6a.† To further understand the charge recombination mechanism, we compared the J–V characteristics of control and target devices under light intensities ranging from 10 mW cm⁻² to 100 mW cm⁻². Fig. 5b shows the logarithmic relationship between V_OC and light intensity. As reported, the slope deviating from kT/q is closely related to trap-assisted recombination.⁴⁰ The target device exhibits a slope of 1.17 kT/q, while that of the control device is 1.59 kT/q. The smaller slope close to 1 for the target device indicates a reduction in trap-assisted recombination compared with that of the control device, demonstrating that post-treatment can improve interface quality by reducing non-radiative recombination. Space-charge-limited current measurements were also performed on hole-only devices with a structure of ITO/PEDOT:PSS/perovskite/PTAA/Au to evaluate the elimination effect of the trap states in the perovskite films by the solvent treatment. Fig. S6b† shows the I–V curves of the control and the target perovskite devices in the dark on a double-logarithmic scale. The trap-state density (n_trap) can be calculated by: n_trap = 2εε₀V_TFL/eL², where ε is the relative dielectric constant (typically 35 for perovskites), ε₀ the vacuum permittivity, V_TFL the onset voltage of TFL region, e the elementary charge, and L the thickness of the perovskite films. The V_TFL values of the target and control devices can be deduced at 0.27 V and 0.43 V, respectively. The corresponding n_trap values are estimated to be 6.98 × 10¹⁵ and 1.11 × 10¹⁶ cm⁻³, respectively, confirming a significant decrease in the trap density caused by solvent treatment. The reduction of non-radiative recombination and trap density is consistent with the improvement in the quality of the PEDOT:PSS/perovskite heterojunction interface after post-treatment, which is consistent with the improvement of FF.^41–44 We also conducted transient photocurrent (TPC) and transient photovoltage (TPV) measurements on the control and target TPSCs to examine the carrier dynamics within the devices. As shown in the TPC results, the target device has a shorter decay time of 0.18 μs, confirming an enhanced charge transport process in comparison to the control device with a decay time of 0.47 μs in Fig. 5c. In terms of the TPV measurement in Fig. 5d, the target device shows a longer electron lifetime of 2.56 μs in comparison to the control (0.84 μs). The increase in the photovoltage decay time in the target device can be attributed to improved interfacial contacts that enable efficient carrier flow between the perovskite and HTL, consistent with the improvement of PL, TRPL and J_SC.^45–50

Fig. 5 (a) IPCE spectra and (b) the linear relationship between V_OC and the logarithm of the light intensity. (c) TPC and (d) TPV decay kinetics of perovskites based on different PEDOT:PSS. (e) J–V curves and (f) operational stability of the control and target device under simulated AM 1.5G solar illumination at 100 mW cm⁻² by maximum power point tracking (MPPT).

The PCE of the target device is 13.32% (V_OC = 0.85 V, J_SC = 20.50 mA cm⁻², FF = 76.43%), and that of the control device is 10.50% (V_OC = 0.85 V, J_SC = 18.10 mA cm⁻², FF = 68.23%), as shown in Fig. 4e. To demonstrate repeatability, we prepared and tested 80 tin-based perovskite solar cell devices. As shown in Fig. S7,† the experimental results strongly supported our conclusions. Compared with the control device, the target device achieved a much-enhanced J_SC, resulting from the high-quality heterojunction interface. The improvement of the crystallization quality of the perovskite film and the improvement of the bottom interface contact caused by the bottom interface treatment also improves the stability of the device. Steady-state PCE measurements were carried out to evaluate the operational stability of the device. As shown in Fig. 4f, the efficiency of the target device remains over 95% of its initial value after maximum power point tracking (MPPT) for 100 min in nitrogen environment. After being stored in nitrogen atmosphere for 2880 hours, the target device maintains above 85% of the initial efficiency, as shown in Fig. S8,† demonstrating excellent stability compared with the control device.

Conclusion

In conclusion, our study focused on the interface modification of the PEDOT:PSS/perovskite. Through this post-treatment strategy, we successfully modified the distribution of PEDOT and PSS molecules, and simultaneously achieved a more consistent energy level alignment with the Sn-based perovskite. This improvement in the PEDOT:PSS/perovskite interfacial contact greatly enhanced the carrier extraction efficiency. The perovskite devices fabricated on the modified HTL exhibited remarkable enhancement in short-circuit current, fill factor, as well as reduced defect density and recombination rate. Overall, our research demonstrates an efficient interface modification method for regulating the property of hole transport layer (PEDOT:PSS) and provides valuable insights into interface engineering for Sn-based PSCs.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Conceptualization, Zhenzhu Zhao and Qin Hu; formal analysis, Zhenzhu Zhao, Mulin Sun, Chengjian Yuan and Fang Xiang; funding acquisition, Qin Hu, Yuqian Yang and Pavel A. Troshin; investigation, Zhenzhu Zhao, Fang Xiang, Xuefei Wu, Zachary Fink, Zongming Huang, Junyao Gao, Honghe Ding, Pengju Tan, Nikita A. Emelianov and Lyubov A. Frolova; project administration, Qin Hu and Yu Li; resources, Zhengguo Xiao, Pavel A. Troshin, Thomas P. Russell and Junfa Zhu; supervision, Qin Hu; validation, Zhenzhu Zhao; writing-original draft, Zhenzhu Zhao; writing-review & editing, Qin Hu, Yu Li and Pavel A. Troshin.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (62104221), the Fundamental Research Funds for the Central Universities (WK2100000025), the USTC Research Funds of the Double First-Class Initiative (YD2100002007), and the USTC National Synchrotron Radiation Laboratory (KY2190000002). Yuqian Yang acknowledges the financial support from the China and Germany Exchange Postdoctoral Program. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. We acknowledge the GIWAX measurements at beamline 7.3.3 of Advanced Light Source (LBNL). Thanks are due to Prof. Xinglin Lu and Mr Zhiyang Zhu for assistance with the measurements of contact angles. This work was partially carried out at the USTC Center for Micro- and Nanoscale Research and Fabrication. This work was partially carried out at the Instruments Center for Physical Science, University of Science and Technology of China. PAT, NAE and LAF acknowledge the support from Russian Ministry of Science and Higher Education (grant no. 075-15-2022-1216).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06046f

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