No abstract

We reveal an iodine vapor-induced degradation mechanism in formamidinium–lead-iodide-based perovskite solar cells stressed under combined heat and light illumination.

the formation of defect complexes and possibly even phases of ordered defect compounds. [9][10][11][12] Increasing the Cu content toward stoi chiometry can considerably reduce the pre sence of such defects and defect complexes. It has been shown that stoichiometric Cu concentrations can be beneficial for absorber crystallinity, [13] defect density, [7,13] mobility, [14,15] and doping density. [14][15][16] Despite this, solar cells based on Cu stoichiometric CuInSe 2 (CIS) have never reached efficiencies above 13.5%, [7,[17][18][19] while Cudeficient CIS yielded efficiencies only up to 15.0%. [20] CIS cells with high CGI are mainly limited by a low open circuit voltage (V OC ), and to a lesser extent, by a reduced fill factor (FF) and current density. However, the implementation of a Cudeficient surface layer has shown to recover the V OC loss in those high CGI cells, while a decrease in current density remains attributed to tun neling recombination. [17,21] Similar improvements of the absorber surface have been achieved with alkali treatments after etching of the secondary phases in Curich samples, [22,23] although most recent results indicate that this treatment may passivates defects that were at least partly generated by the etching in the first place. [24] This suggests that the front absorber surface limits the efficiency of Cu stoichiometric devices. Therefore, it is important to reduce the front interface recombination, besides any other, in such devices.In an earlier publication, we have shown that ungraded CIS solar cells are constrained by recombination at the back interface, [25] which limits the solar cell efficiency. This back sur face recombination can be effectively suppressed by the imple mentation of a single bandgap grading achieved by adding Ga close to the Mo back contact, leading to improved efficiencies up to 16.1% with a bandgap of 1.0 eV absorber [25] and later to 18.0% with application of a RbF postdeposition treatment (PDT). [26] Those solar cells were processed with Cudeficient absorbers (CGI: 0.85-0.90). CIS solar cells processed with high Cu content show lower V OC , and therefore lower efficiency. Believing in recombination as the root cause, we hypothesize that the application of heavy alkali PDT, for example with RbF, may suppress the recombination observed for absorbers with stoichiometric Cu concentrations. To investigate this, we processed and analyzed CISbased solar cells with various Cu concentrations and amounts of RbF during PDT treatment.Here we report a large improvement in efficiency of solar cells processed with Cu composition close to stoichiometry. In State-of-the-art Cu(In,Ga)Se 2 (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe 2 sola...

This work shows that vacuum decreases operational lifetime of perovskite solar cells (ITO/SnO 2 /perovskite/Spiro-MeOTAD/Au) by accelerating perovskite decomposition starting from the grain boundaries, accompanied by outgassing and defect formation. These defects further accelerate ion migration (Li + , Au + and/ or Au 3+ , I À , and Br À ) across the device. We propose a robust perovskite solar cell structure (ITO/PTAA/perovskite/PCBM/ZnO/AZO/[Ni/Al grid]) that effectively mitigates these degradation pathways, leading to a device showing a projected T 80 lifetime of 4,750 h at its maximum power point condition, 1-sun illumination at 50 mbar.

Compositional grading has been widely exploited in highly efficient Cu(In,Ga)Se2, CdTe, GaAs, quantum dot solar cells, and this strategy has the potential to improve the performance of emerging perovskite solar cells. However, realizing and maintaining compositionally graded perovskite absorber from solution processing is challenging. Moreover, the operational stability of graded perovskite solar cells under long‐term heat/light soaking has not been demonstrated. In this study, a facile partial ion‐exchange approach is reported to achieve compositionally graded perovskite absorber layers. Incorporating compositional grading improves charge collection and suppresses interface recombination, enabling to fabricate near‐infrared‐transparent perovskite solar cells with power conversion efficiency of 16.8% in substrate configuration, and demonstrate 22.7% tandem efficiency with 3.3% absolute gain when mechanically stacked on a Cu(In,Ga)Se2 bottom cell. Non‐encapsulated graded perovskite device retains over 93% of its initial efficiency after 1000 h operation at maximum power point at 60 °C under equivalent 1 sun illumination. The results open an avenue in exploring partial ion‐exchange to design graded perovskite solar cells with improved efficiency and stability.