| Author: | Wang, Chenhao |
| Title: | Observation of hot-carrier photocurrent in Ruddlesden-Popper (RP) perovskite/MoS2 heterostructure |
| Advisors: | Li, Mingjie (AP) Yu, Siu Fung (AP) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Hot carriers Perovskite materials Perovskite solar cells Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xvii, 104 pages : color illustrations |
| Language: | English |
| Abstract: | Hot carrier relaxation has emerged as a prominent energy loss pathway in solar cell design, significantly impeding the advancement of photovoltaic performance. Consequently, harnessing the full potential of hot carriers has become crucial for surpassing the Shockley-Queisser (SQ) limit. In this study, we synthesized a single crystal RP perovskite with n=4 and constructing a MoS2/2D RP perovskite/h-BN device, investigating the hot-carrier-extraction process and exploring the associated hot carrier-induced photocurrent. The goal was to enhance the power conversion efficiency of the perovskite photovoltaic device. To accomplish this, we employed transient reflection measurements as a key tool for studying these phenomena. Firstly, we synthesized the single crystal RP perovskite BA2MAn-1PbnI3n+1 with varying n values from 1 to 4 and conducted morphological and optical characterizations. Based on the obtained results, we selected the RP perovskite with n=4 for further analysis. Recognizing the moisture sensitivity of perovskite, we employed a protective h-BN layer to encapsulate the perovskite, enabling the fabrication of a MoS2/perovskite heterostructure covered by h-BN layer and corresponding device for subsequent investigations. In our analysis, we observed a significant quenching of photoluminescence (PL) intensity in the heterostructure compared to the pure perovskite counterpart. This observation suggests the occurrence of carrier transfer from the perovskite layer to MoS2 layer. The short hot carrier cooling time observed in the MoS2, in comparison to the longer hot carrier cooling time of perovskite, lends support to the hypothesis of hot carrier extraction from the perovskite to the MoS2 layer. We conducted electrical measurements on the MoS2/perovskite/h-BN device to investigate its performance. For comparison, we also fabricated and tested separate devices using MoS2 and perovskite multi-layers. We utilized a continuous wave (CW) laser with wavelengths of 375nm, 450nm, 520nm and 660nm to stimulate the hot carriers mainly in perovskite. As the power density increased, the photocurrent exhibited an upward trend. Significantly, the open circuit voltage (Voc) and short circuit current (Isc) exhibited an increase with the rise in power intensity for wavelengths of 375nm, 450nm, and 520nm. However, no increase was observed under 660nm excitation. It has been observed that hot carrier photocurrent is generated when the excitation photon energy exceeds the bandgap energy (Eg). To further examine hot carrier extraction, we maintained a constant incident photon density per square centimeter and per second across the different wavelengths. Notably, we observed an enhanced Voc and Isc as the wavelength decreased under the same photon density, providing evidence of hot carrier extraction. Based on the results, the EQE values for the MoS2/perovskite device were calculated to be 36.1% at 375nm, 30% at 450nm, and 7.7% at 520nm. Furthermore, we applied external voltage to the MoS2 layer, allowing for relative tuning of the Fermi level. This enabled control over the barrier for hot carrier extraction. The application of a negative external voltage led to an increased Voc, which can be attributed to the decreased barrier height at the valence band edge between MoS2 and perovskite, in contrast to the positive voltage condition. In addition to electrical measurements, the hot carrier extraction was experimentally demonstrated using transient reflection measurements. The measurements were conducted both without and with an applied bias voltage, which was pumped by photons with an energy of 3.1eV. Perovskite exhibits a longer hot carrier cooling time, while the hot carrier temperature is lower in the MoS2/perovskite heterostructure compared to individual perovskite. These findings suggest the extraction of hot carriers from perovskite by MoS2. The devices were subjected to both 5V and -5V, and the transient reflection spectra were obtained simultaneously. Notably, when a negative voltage was applied to the MoS2/perovskite device, the MoS2 peak exhibited enhancement while the perovskite peak decreased. The hot carrier temperature was determined by extracting it from the fitted high-energy tail observed at the perovskite peak. It was found that the hot carrier temperature is lower under negative external voltage compared to positive external voltage, which can be attributed to the reduced barrier between MoS2 and perovskite. Theoretical density functional theory (DFT) calculations reveal that applying an electric field to the MoS2/perovskite heterojunction results in a decrease in the barrier between the valence band edge of MoS2 and perovskite. Additionally, There a distribution of hot carriers within the MoS2 layer. This reduction is advantageous for facilitating the hot carrier extraction process. The DFT calculations align with the findings of electrical measurements and transient reflection data. |
| Rights: | All rights reserved |
| Access: | open access |
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