Organometallic halide chalcogenide-based solar cells have attracted attention due to their low material cost, ease of large-area device fabrication, and high photovoltaic conversion efficiency. In the practical application environments with huge temperature difference between day and night, the temperature change will trigger the phase transition and lattice strain of the chalcogenide material, resulting in the rapid degradation of the device performance and damage, which is the key challenge and difficulty in limiting the chalcogenide solar cells to be applied. In view of this, this breakthrough work reveals that β-poly(1,1-difluoroethylene) can effectively improve the crystalline properties of chalcogenide thin films, effectively passivate the interfacial defects on the surface of thin film crystals, optimize the interfacial energy level arrangement of chalcogenide, and promote the carrier transport, thus enhancing the photovoltaic characteristics of p-i-n-structured chalcogenide devices. More importantly, in the variable temperature environment, the orderly arrangement of β-poly(1,1-difluoroethylene) at grain boundaries can effectively buffer the deformation of grain boundaries induced by grain extrusion during the temperature change process and release the lattice stress to realize a recoverable crystal structure, thus significantly improving the variable temperature stability of the device.
In order to dissect the crystallization process of chalcogenide thin films, the study was carried out based on synchrotron GIWAXS to characterize the whole process of film formation. From comparing the GIWAXS results (Fig. 1A, B), it can be seen that the diffraction signal is significantly weakened during the initial 60 s, which indicates that the initial intermediate phase of DMSO-DMF-PbX2 is suppressed. This effect was attributed to the intermediate phase isolated by the long-chain β-pV2F molecule. The study of scattering features centered at q = ~10 nm-1 along the (001) crystal plane observed during the film formation process suggests that the colloid has solidified and transformed into a black phase. It was found that the black phase of the target film appeared earlier (Δtt > Δtc) than that of the control film, implying that β-pV2F promotes the conversion of the intermediate phase to the chalcogenide black phase. The rapid phase transition was associated with a lower formation energy, possibly due to the rapid aggregation of dispersed PbX2 and organic salts by β-pV2F during the volatilization of DMSO and DMF. When crystallization was complete (stage t7), the signal of the target film was stronger than that of the control film (Fig. 1c). This indicates that the formed target chalcogenide film is more organized. Thus, β-pV2F controls the crystallization kinetics of chalcogenides by decreasing the formation energy of chalcogenides, promotes the phase transition, and results in a more ordered crystal structure.
Figure 1.Crystallization kinetics of chalcogenide thin films
In order to elucidate the source of the excellent temperature resistance of the devices, this work further investigated the effect of β-pV2F on the morphology and crystal structure of the chalcogenide thin films during the temperature change process using synchrotron radiation GIWAXS. As shown in Figure 2A and B, the temperature-induced film degradation in the target chalcogenides was suppressed, as well as the grain boundary deformation induced by grain extrusion during the temperature-change process, and the structural stability of the target chalcogenides was significantly improved. As shown in Fig. 2C, chalcogenide strain varied with temperature cycling, indicating changes in lattice parameters in chalcogenide; in contrast, the target chalcogenide exhibited stable strain cycling in a narrow range (-0.06% to 0.38%), which indicated that the target chalcogenide had a recoverable crystal structure and a releasable lattice stress.
Fig. 2. structural evolution of chalcocite during temperature cycling
Quantum Efficiency Tester
The MNPVQE-300 photovoltaic QE system is a common tool in photovoltaic research and production line quality processes for accurate determination of solar cell spectral response/EQE (IPCE) and IQE.
The MNPVQE-300 is compatible with a wide range of photovoltaic device types, materials, and architectures, including c:Si, mc:Si, a:Si, µ:Si, CdTe, CIGS, CIS, Ge, dye-sensitized, organic/polymer, tandem, multi-junction (2-, 3-, 4-junction, etc.), quantum wells, quantum dots, sulfur species, and chalcogenides.
