Thermal annealing and precursor composition
play critical roles
in crystallinity control and morphology formation of perovskite thin
films for achieving higher photovoltaic performance. In this study
we have systematically studied the role of annealing temperature on
the crystallinity of perovskite (CHNH3PbI3)
thin films cast from single (without PbCl2) and mixed (with
PbCl2) halide precursors. Higher annealing temperature
leads to agglomeration of perovskite crystals. The effects of annealing
temperature on the performance of perovskite solar cells are different
in single and mixed halide processed films. It is observed that the
perovskite crystallinity and film formation can be altered with the
addition of lead chloride in the precursor solution. We report that
single halide perovskite solar cells show no change in morphology
and crystal size with increase in annealing temperature, which was
confirmed by UV–vis absorption spectroscopy, X-ray diffraction
(XRD), and atomic force microscopy (AFM). However, mixed halide perovskite
(CH3NH3PbI3–x
Cl
x
) solar cells show significant change
in crystal formation in the active layer when increasing annealing
temperature. In addition, heating perovskite precursor solutions at
150 °C can lead to enhancement in solar cell efficiency for both
single and mixed halide systems. Perovskite solar cells fabricated
using heated precursor solutions form dense film morphology and thus
significantly improved fill factor up to 80% with power conversion
efficiency exceeding 13% under AM 1.5 condition.
Single and double junction solar cells with high open circuit voltage were fabricated using poly{thiophene-2,5-diyl-alt-[5,6-bis(dodecyloxy)benzo[c][1,2,5]thiadiazole]-4,7-diyl} (PBT-T1) blended with fullerene derivatives in different weight ratios. The role of fullerene loading on structural and morphological changes was investigated using atomic force microscopy (AFM) and X-ray diffraction (XRD). The XRD and AFM measurements showed that a higher fullerene mixing ratio led to breaking of inter-chain packing and hence resulted in smaller disordered polymer domains. When the PBT-T1:PC60BM weight ratio was 1 : 1, the polymer retained its structural order; however, large aggregated domains formed, leading to poor device performance due to low fill factor and short circuit current density. When the ratio was increased to 1 : 2 and then 1 : 3, smaller amorphous domains were observed, which improved photovoltaic performance. The 1 : 2 blending ratio was optimal due to adequate charge transport pathways giving rise to moderate short circuit current density and fill factor. Adding 1,8-diiodooctane (DIO) additive into the 1 : 2 blend films further improved both the short circuit current density and fill factor, leading to an increased efficiency to 4.5% with PC60BM and 5.65% with PC70BM. These single junction solar cells exhibited a high open circuit voltage at ∼ 0.9 V. Photo-charge extraction by linearly increasing voltage (Photo-CELIV) measurements showed the highest charge carrier mobility in the 1 : 2 film among the three ratios, which was further enhanced by introducing the DIO. The Photo-CELIV measurements with varying delay times showed significantly higher extracted charge carrier density for cells processed with DIO. Tandem devices using P3HT:IC60BA as bottom cell and PBT-T1:PC60BM as top cell exhibited a high open circuit voltage of 1.62 V with 5.2% power conversion efficiency.
In this work, the effects of the spacer between benzo[1,2-b;3,4-b']dithiophene (BDT) and dialkoxybenzothiadiazole (ROBT) in polymers were investigated for applications in organic solar cells. Polymer PBDT-2T-ROBT has a bi-thiophene (2T) spacer between the BDT and ROBT units, while PBDT-ROBT is a direct copolymer of BDT and ROBT. The polymer/PC 70 BM solar cells using both polymers were fabricated and optimized via polymer/fullerene ratio, solvent, and solvent additives. The spacer has significantly improved solar cell performance from 1.28% (Voc =0.77 V, Jsc = 3.13 mA/cm 2 , FF = 53.11 %) to 5.11% (Voc =0.66 V, Jsc = 13.33 mA/cm 2 , FF = 58.12%). The x-ray diffraction (XRD) spectra show the PBDT-2T-ROBT/PC 70 BM blended film is semicrystalline while PBDT-ROBT/PC 70 BM film is amorphous. This indicates that the spacer facilitates polymer organization for higher carrier mobility in the film. The atomic force microscopy (AFM) topography and phase images show that PBDT-2T-ROBT/PC 70 BM films form fibrillar networks, while PBDT-ROBT/PC 70 BM films exhibit larger granular morphology. The photoinduced charge extraction by linearly increasing voltage (Photo-CELIV) measurements show thatPBDT-2T-ROBT/PC 70 BM has a mobility of 9.59×10 -5 cm 2 /Vs, which is higher than the mobility of 8.64×10 -5 cm 2 /Vs for PBDT-ROBT/PC 70 BM film.
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