Elucidating the Origins of Subgap Tail States and Open‐Circuit Voltage in Methylammonium Lead Triiodide Perovskite Solar Cells

Abstract

Recombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, the impact of active layer crystallinity on the accumulated charge and opencircuit voltage (V oc ) in solar cells based on methylammonium lead triiodide (CH 3 NH 3 PbI 3 , MAPI) is demonstrated. It is shown that MAPI crystallinitycan be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device V oc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased V oc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photoluminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombination pathways. Fundamental insights into the origin of V oc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high-performance devices.

Origin of Open‐Circuit Voltage Enhancements in Planar Perovskite Solar Cells Induced by Addition of Bulky Organic Cations

“…We note that the effect of excess MAI on device performance was found to be dependent on HTL employed. Using PEDOT:PSS as HTL, excess MAI results in enhanced device V oc, as we have previously reported, whilst with PTAA/PFN excess MAI did not enhance V oc . The origin of this dependence is unclear but may be associated with the differing impact of the HTL on the perovskite film morphology and crystallinity.…”

“…Integrating the photoemission signal for energies below the VBE (shaded areas in Figure f) indicate that this trap state density is some 70% larger for MAPbI 3 compared to NMA‐MAPbI 3 (0.25% NMA), providing further evidence that NMA is efficient at trap state passivation. Increased trap state densities have been correlated with lower device V oc , as trapped charge functions as recombination centers reducing quasi‐Fermi level splitting in the perovskite layer . Supporting this correlation, Figure h plots τ 1 (assigned primarily to charge trapping) determined from the perovskite film TCSPC data (Figure e) and device V oc versus NMA concentration.…”

“…Increased trap state densities have been correlated with lower device V oc , as trapped charge functions as recombination centers reducing quasi-Fermi level splitting in the perovskite layer. [53] Supporting this correlation, Figure 4h plots τ 1 (assigned primarily to charge trapping) determined from the perovskite film TCSPC data (Figure 4e) and device V oc versus NMA concentration. It is apparent that suppressed charge trapping (observed as a longer τ 1 ) correlates with a higher device V oc (the small loss of V oc at high NMA concentrations most probably is associated with a loss of charge transfer efficiency).…”

3 D NiO Nanowall Hole‐Transporting Layer for the Passivation of Interfacial Contact in Inverted Perovskite Solar Cells

“…The V oc and FF are mainly related to the recombination behavior within the perovskite layer and the interface. The presence of bulk/interface defects would induce severe charge recombination and energy loss . As discussed above, the unavoidable film issues for the pristine NiO NCs film (agglomeration or discontinuity), would increase the defect density in the NiO/perovskite interface, resulting in the unnecessary V oc /FF loss.…”

Tuning the Optical Properties of Already Crystalized Hybrid Perovskite

“…An increase in the PL intensity can be seen for both the pure FAI and 75/25 post‐treated films, whereas in the case of an excess of FABr in the dipping solution (including the 50/50 sample), the PL intensity was low compared with the standard film. The decreased PL intensity in the case of an excess of FABr is mainly due to an increase in the nonradiative SRH recombination and the density of traps, which results from the structural distortion in the perovskite lattice …”