high photoexcitation intensity can lead to room-temperature magneto-photoluminescence and magneto-photocurrent with negative and positive signs, respectively, below the fi eld of 200 mT. Our results provide evidence that the charge recombination and dissociation are spin dependent at room temperature in OMHPs. Essentially, our results indicate that applying a magnetic fi eld can suppress the spin mixing between antiparallel and parallel spin states in electron-hole pairs and consequently decreases the antiparallel spin states but increases the parallel spin states. The change between different spin states in electron-hole pairs can eventually modify the exciton formation when the electron-hole pairs relax into excitons. Because of Pauli Exclusion Principle applied onto excitonic states, the singlet and triplet excitons can have high and low annihilation rates. As a result, decreasing the antiparallel spin states in electron-hole pairs by suppressing the spin mixing can weaken the singlet exciton formation, consequently leading to a decrease on photoluminescence toward the development of negative magneto-photoluminescence. On the contrary, decreasing the exciton formation can slow down the consumption on the electron-hole pairs. This can lead to more electron-hole pairs ready for charge dissociation to generate a photocurrent, generating a positive magneto-photocurrent. Clearly, the spin polarizations can be used as a new approach to control the charge recombination and dissociation in OMHPs. Furthermore, by using the observed magneto-photoluminescence and magneto-photocurrent, we investigate the dissociation effects in electron-hole pair states at different excitation intensities to further understand charge recombination and dissociation at different densities at device-operating condition. We fi nd that the critical bias required to completely quench the magneto-photocurrent and magneto-photoluminescence signals is increased with increasing photoexcitation intensity in OMHPs. This implies that, as the charge density increases with increasing the photoexcitation intensity, the formation of electron-hole pairs is enhanced, requiring a stronger fi eld to complete the charge dissociation toward generating photocurrent. Simultaneously, the line-shape on magneto-photocurrent and magnetophotoluminescence shows a narrowing phenomenon with increasing photoexcitation intensity. This indicates that the electron-hole pairs experience mutual inter-pair interactions and consequently changes the internal interactions within each electron-hole pairs. Clearly, our magneto-optical studies can provide critical understanding on controlling spin-dependent charge recombination and dissociation toward improving the photovoltaic actions in perovskite solar cells. Figure 1 a shows the magneto-photocurrents at different excitation intensities from the continuous wave (CW) laser beam of 532 nm in OMHP solar cells with the device architecture of ITO/PEDOT:PSS/CH 3 NH 3 PbI 3x Cl x /PC 71 BM/TiO x /Al. We Organometal halide perovskites (OM...
Polarization and spin-dependent excited states and charge transport.
to the electronic and morphological parameters in the active perovskite layer, the electron transport layer (ETL) and hole transport layer (HTL) are significantly important in determining the photovoltaic performance in perovskite solar cells. [3][4][5][6] Specifically, the role of a transport layer in solar cells is to facilitate charge transport and assist charge collection toward the respective electrodes as well as inhibiting from recombination of charge carriers on the path to the electrodes. [3] Meanwhile, the open circuit voltage (V oc ) is normally determined by the energy differences between the Fermi levels of the ETL and the HTL. There are certain prerequisites for a hole transport material in order to perform more efficiently in perovskite solar cells, such as energy band alignment, suitable hole mobility, chemical compatibility with the perovskite layer as well as good thermal and photochemical stability. [5,6] In addition, morphology and crystallinity of this layer plays an important role. The importance and influence of these parameters in the transport layer on perovskite solar cells performance have been extensively investigated. [3,[5][6][7][8][9][10] In this article, we demonstrate the importance of photogenerated dipoles in the HTL as a new strategy to improve the photovoltaic performance of planar perovskite solar cells. In order to explore this effect, we select two HTLs with similar conjugated thiophene backbones, namely PTB7 and P3HT with significant and negligible optically generated dipoles. Moreover, our measurements based on space charge limited current model indicate that PTB7 and P3HT have hole mobilities in the same order in agreement with previous publications (Figure S1, Supporting Information). [11][12][13][14] It has been shown that PTB7 has a relatively large internal dipoles through intrachain charge transfer from benzodithiophene to thienothiophene moieties under photoexcitation. [15] The dipole moments (D) in ground and excite states have been measured to be 3.76 and 7.13 for PTB7 and 0.19 and 0.43 for P3HT, respectively. [15] In general, perovskites have the advantage of both bulk polarization and semiconducting properties. [16][17][18][19][20] It was shown that the presence of a polar molecule, methylammonium at the center of perovskite introduces the possibility of orientational P3HT-poly(3-hexylthiophene) are separately used as the HTL with significant and negligible photoinduced dipoles, respectively. Electric field-induced photoluminescence quenching provides the first-hand evidence to indicate that the photoinduced dipoles are partially aligned in the amorphous PTB7 layer under the influence of device built-in field. By monitoring the recombination process through magneto-photocurrent measurements under device operation condition, it is shown that the photoinduced dipoles in PTB7 layer can decrease the recombination of photogenerated carriers in the active layer in perovskite solar cells. Furthermore, the capacitance measurements suggest that the photoinduced dipoles ...
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