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The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells.

Chen Q, Zhou H, Fang Y, Stieg AZ, Song TB, Wang HH, Xu X, Liu Y, Lu S, You J, Sun P, McKay J, Goorsky MS, Yang Y - Nat Commun (2015)

Bottom Line: Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency.Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled.The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA [2] California NanoSystems Institute, University of California, Los Angeles, California 90095, USA.

ABSTRACT
Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.

No MeSH data available.


Related in: MedlinePlus

KPFM results of perovskite films.Representative atomic force microscopy height images of (a) perovskite film of Reference and (c) Sample 1. Co-localized KPFM images of surface work function of (b) of Reference and (d) Sample 1. Cross-sectional analyses of KPFM data (insets) show variations in local CPD of up to ±160 mV in magnitude across the sample surface with minimal variation (±20–40 mV) at GBs.
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f6: KPFM results of perovskite films.Representative atomic force microscopy height images of (a) perovskite film of Reference and (c) Sample 1. Co-localized KPFM images of surface work function of (b) of Reference and (d) Sample 1. Cross-sectional analyses of KPFM data (insets) show variations in local CPD of up to ±160 mV in magnitude across the sample surface with minimal variation (±20–40 mV) at GBs.

Mentions: The properties of GBs are investigated to further clarify the nature of carrier dynamics within perovskite films on Cl incorporation. Variations in electronic properties at GBs have been reported to effectively boost device performance in conventional as well as perovskite solar cells44. Observed energy band bending at GBs is generally thought to facilitate charge separation and transport. Meanwhile, prolonged carrier lifetimes are attributed to the successful passivation of surface states45. In the current study, the contribution of surface state passivation is ruled out given the similar values of carrier lifetime in both Reference and Sample 1. The local surface potential of each sample has been characterized using KPFM to examine the effects of chlorine incorporation on energy level alignment between the various layers as well as the possibility of band bending along GBs. Spatial maps of surface topography and corresponding local surface potential for both the Reference and Sample 1 on ITO substrates are shown in Fig. 6. KPFM provides a reliable measurement of local surface potentials stemming from contact potential differences (CPD) between the tip and sample surface associated with their relative work functions. Minimal (±20–40 mV) variations in the CPD were observed between the GB and grain bulk for both samples thereby indicating that chlorine incorporation has a negligible effect on the energy band edge at GBs and thus carrier transport within the perovskite film. In addition, the magnitude of local variations in the CPD (±120–160 mV) were similar for both samples. These grain-to-grain variations appear to depend on the physical orientation of each grain and its associated crystallographic face43. Moreover, chlorine incorporation was seen to affect the mean values of the CPD in the perovskite film, which decreased from ∼0.65 to 0.45 V (Fig. 6b,d). The measured CPD of TiO2 films (0.4 V), and observed CPD shift towards that of TiO2 in perovskite films following chlorine inclusion indicates better energy band alignment, which is likely to facilitate the electron extraction along the interface. Cross-sectional KPFM was used to further assess the energy level band alignment between the absorber and transport layers (Supplementary Fig. 8). A downward shift in CPD similar to that measured at the surface of the perovskite material was confirmed for Sample 1 compared with the Reference, as seen in the corresponding line profiles (Supplementary Fig. 8c,f) and image histograms (Supplementary Fig. 9). An analogous shift in CPD of the Spiro-OMeTAD films deposited on Reference and Sample 1, respectively, was observed (Supplementary Fig. 9). The relatively larger CPD of Spiro-OMeTAD on Sample 1 indicates a deeper Fermi energy level of the hole transport materials serves to enlarge the build-in potential and improve device performance through an increase in VOC. In this regard, it is inferred that the Cl incorporation to improve optoelectronic properties of perovskite devices may associate with both interfaces.


The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells.

Chen Q, Zhou H, Fang Y, Stieg AZ, Song TB, Wang HH, Xu X, Liu Y, Lu S, You J, Sun P, McKay J, Goorsky MS, Yang Y - Nat Commun (2015)

KPFM results of perovskite films.Representative atomic force microscopy height images of (a) perovskite film of Reference and (c) Sample 1. Co-localized KPFM images of surface work function of (b) of Reference and (d) Sample 1. Cross-sectional analyses of KPFM data (insets) show variations in local CPD of up to ±160 mV in magnitude across the sample surface with minimal variation (±20–40 mV) at GBs.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4490385&req=5

f6: KPFM results of perovskite films.Representative atomic force microscopy height images of (a) perovskite film of Reference and (c) Sample 1. Co-localized KPFM images of surface work function of (b) of Reference and (d) Sample 1. Cross-sectional analyses of KPFM data (insets) show variations in local CPD of up to ±160 mV in magnitude across the sample surface with minimal variation (±20–40 mV) at GBs.
Mentions: The properties of GBs are investigated to further clarify the nature of carrier dynamics within perovskite films on Cl incorporation. Variations in electronic properties at GBs have been reported to effectively boost device performance in conventional as well as perovskite solar cells44. Observed energy band bending at GBs is generally thought to facilitate charge separation and transport. Meanwhile, prolonged carrier lifetimes are attributed to the successful passivation of surface states45. In the current study, the contribution of surface state passivation is ruled out given the similar values of carrier lifetime in both Reference and Sample 1. The local surface potential of each sample has been characterized using KPFM to examine the effects of chlorine incorporation on energy level alignment between the various layers as well as the possibility of band bending along GBs. Spatial maps of surface topography and corresponding local surface potential for both the Reference and Sample 1 on ITO substrates are shown in Fig. 6. KPFM provides a reliable measurement of local surface potentials stemming from contact potential differences (CPD) between the tip and sample surface associated with their relative work functions. Minimal (±20–40 mV) variations in the CPD were observed between the GB and grain bulk for both samples thereby indicating that chlorine incorporation has a negligible effect on the energy band edge at GBs and thus carrier transport within the perovskite film. In addition, the magnitude of local variations in the CPD (±120–160 mV) were similar for both samples. These grain-to-grain variations appear to depend on the physical orientation of each grain and its associated crystallographic face43. Moreover, chlorine incorporation was seen to affect the mean values of the CPD in the perovskite film, which decreased from ∼0.65 to 0.45 V (Fig. 6b,d). The measured CPD of TiO2 films (0.4 V), and observed CPD shift towards that of TiO2 in perovskite films following chlorine inclusion indicates better energy band alignment, which is likely to facilitate the electron extraction along the interface. Cross-sectional KPFM was used to further assess the energy level band alignment between the absorber and transport layers (Supplementary Fig. 8). A downward shift in CPD similar to that measured at the surface of the perovskite material was confirmed for Sample 1 compared with the Reference, as seen in the corresponding line profiles (Supplementary Fig. 8c,f) and image histograms (Supplementary Fig. 9). An analogous shift in CPD of the Spiro-OMeTAD films deposited on Reference and Sample 1, respectively, was observed (Supplementary Fig. 9). The relatively larger CPD of Spiro-OMeTAD on Sample 1 indicates a deeper Fermi energy level of the hole transport materials serves to enlarge the build-in potential and improve device performance through an increase in VOC. In this regard, it is inferred that the Cl incorporation to improve optoelectronic properties of perovskite devices may associate with both interfaces.

Bottom Line: Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency.Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled.The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA [2] California NanoSystems Institute, University of California, Los Angeles, California 90095, USA.

ABSTRACT
Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.

No MeSH data available.


Related in: MedlinePlus