To further improve the performance of PSCs and realize their portable applications, a better understanding of the structure-property-performance relationship and the difference between fullerenes and SMAs in PSCs is needed. A comparative study of PC71BM and m-ITIC was also performed in pairs with a high performance benzodithiophene (BDT) based polymer as donor.
Introduction
Current States of Organic Photovoltaics (OPVs)
To overcome these drawbacks associated with fullerenes, low-band-gap non-fullerene acceptors are emerging, e.g. n-type conjugated polymers and small molecules, with synthetic flexibility that are reinventing PSC technologies.9 Through judicious molecular optimization and appropriate donor/acceptor combination, PCEs ranging from 8% to a record 13.1% in individual PSCs have been achieved in in a very short time. have been shown to be much more mechanically robust compared to the fullerene counterparts, making them better candidates for flexible and portable electronics.16-17.
OPVs Working Mechanism, Structural Classification, and Characterization
- Working Mechanism
- Device Structure
- Device Characterization
Short circuit current density (JSC) is the current density flowing in a displaced cell only through the internal field instead of the applied external field. The open circuit voltage (VOC) is the maximum voltage at which the current is zero.
Organic Photoactive Materials
- p-type materials
- n-type materials
- Fullerene derivatives
- Non-fullerene acceptors
- Empirical Matching Criterion
- General Polymerization Methods
In the early stage, only the widely used cyano-substituted poly(phenylvinylene) (CN-PPV) acceptor. In the two-phase mixing system, mixing between donor and acceptor is of crucial importance for achieving an optical morphology.
D-A alternating copolymers made by direct arylation for OPVs
Research background and motivation for the work
We then compared their structural defects and properties, as well as OPV properties, with the polymer obtained from standard Stille polymerization. Compared to its Stille analogue, the OPVs based on P4 prepared by DAP under conventional heating exhibit an improved PCE of up to 5.1%, thanks to the relatively small amount of structural defects resulting from unselective C–H activation and/ or homolinkages within the polymer backbone.
Results and discussion
- Material Synthesis and Characterization
- Optical and Electrochemical Properties
- Theoretical Calculation
- Photovoltaic Properties
- Morphological Properties
- Charge Recombination and Transport Properties
Ultraviolet photoelectron spectroscopy (UPS) was also used to measure the ionization potential (IP) levels of the three polymers (Figure 2.6d). The morphology of the polymer:PC71BM blend films with the optimized weight ratio (1:3) was investigated by tapping mode atomic force microscopy (AFM) (Figure 2.9).
Conclusion
To further investigate the charge dissociation and collection process, the photocurrent density (Jph) as a function of the effective applied voltage (Veff) was examined for the three polymer devices (Figure 2.11d). The Jph is defined as Jlight – Jdark, where Jlight and Jdark are the current densities under illumination and in the dark, respectively, and the Veff is defined as V0 – V, where V0 is the compensation voltage at Jph = 0 and V is the applied bias.52 , 53 As shown in Figure 7d, Jph reaches saturation (Jsat) at a sufficiently large reverse voltage (i.e. Veff > 2.0 V). Under short-circuit conditions, mixtures based on P4, starting from Jsat at 2.5 Veff, showed a higher Jph/Jsat value of 81% than those of the other samples (78% and 77% for P1 and P7 respectively), indicating higher charge dissociation efficiency.
More interestingly, under the maximum power output, P4 devices again show a slightly higher Jph/Jsat value of 79%, compared to both P1 and P7 devices, implying that alternate structures with relatively higher purity also enhance charge extraction and collection can improve. Not only do all the results observed above demonstrate that the charge recombination and transport characteristics are highly sensitive to the relative amount of structural defects within the polymer distribution, but they are also consistent with the trends in the performance of the polymer-based devices tested (vide above).
Mapping suitable donor-acceptor couples in NF-PSCs
Modulating the molecular packing and nanophase blending via random terpolymerization
- Research background and motivation for the work
- Results and discussion
- Polymer Design and Properties
- Photovoltaic Properties
- Morphology Characterization
- Charge Generation, Separation and Transport Properties
- Conclusion
The change in VOC values matched the trend of the HOMO levels of the corresponding PTPTI-Tx polymers. Additionally, the d-spacings and coherence lengths (CCLs)124 of the face-on (010) diffraction peaks in the blend films were calculated (Figure 3.1.4d). Furthermore, it is clear that the P (E,T) values in the m-ITIC systems are higher than those of the corresponding PC71BM ones.
In addition, a change in the molecular packing of the acceptor polymers as a function of Mn is also observed. However, with further increase of the PDMS-S content in the matrix, a severe decrease in the FF was observed (Figure 6.2d).
Enhancing acceptor order ranges in an interpenetrated 3-D texture toward 12% efficiency
- Research background and motivation for the work
- Results and discussion
- Material Design and Properties
- Photovoltaic Properties
- Morphology Characterization
- Charge Transport and Recombination Dynamics
- Photophysics
- Conclusion
Comparing non-fullerene acceptor with fullerene in OPVs
Research background and motivation for the work
Currently, polymer solar cells (PSCs) based on conjugated polymers such as electron donors mixed with fullerenes (e.g. [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)) as electron acceptors are the leading candidates in organic photovoltaic technology and have power conversion efficiencies (PCE) of more than 10% achieved using a single bulk heterojunction architecture.129-136 Such major advances in PSCs based on fullerene acceptors are essentially fueled by their high electron mobility and isotropic electron transfer nature. 137-139 However, fullerenes are not ideal electron acceptors due to many intrinsic issues such as poor light absorption and unoptimized energy levels, which limit the design adaptability of the electron donor pair. Therefore, there is considerable interest in the development of alternative non-fullerene acceptors to solve these drawbacks and to ultimately overcome the PCE bottleneck associated with fullerene-based PSCs.140-150. A breakthrough in non-fullerene acceptors has recently occurred with the development of fused-aromatic-ring-based molecules such as ITIC and m-ITIC by Zhan et al.
Then followed a series of publications that shed light on their many advantages over fullerenes, such as synthetic flexibility, easy tuning of optical and electronic properties, and good morphology stability.153-162 However, in most cases, due to the different conjugated skeletons of fullerene-based acceptors and ITICs, (nearly isotropic π‒orbitals vs. highly anisotropic π‒orbitals), these state-of-the-art donor polymers in fullerene-based PSCs may not be the best-matched donors in fullerene-free PSCs and vice versa.163-167 Therefore, it is very challenging to develop high-performance polymer donors that are simultaneously compatible with PC71BM and ITIC based acceptors in PSC. Recently, random copolymerization has emerged as a promising synthetic strategy for fine-tuning the morphology (e.g., crystallinity and orientation), charge transfer ability, electronic energy levels, and band gaps.168-172 This can be easily related to photovoltaic parameters such as JSC, VOC and FF to optimize the molecular structure to achieve high photovoltaic efficiency.
Results and discussion
- Synthesis and Characterization of Copolymers
- Photovoltaic Properties
- Film Morphology
- Charge Generation, Dissociation, and Transport Properties
- Photophysics
In both cases, the same trend is observed for each solar cell parameter as a function of the TT content in the main backbone. The results can be understood from the point of view of the overall higher fraction of the face-on oriented crystallites in the PC71BM-based blends, as shown by GIWAXS. For example, in the case of PC71BM-based devices, high crystallinity and enhanced face-on crystallite fraction are responsible for higher JSC and PCE.
When using the pump wavelength of 730 nm, only m-ITIC was excited, as confirmed by the absence of the TA signal in the pure PBDB-TT5 film. In the PC71BM-based mixing system, the spectral overlap of the absorption in PBDB-TT5 and PC71BM makes it difficult to selectively excite the donor or acceptor components.
Conclusion
The spectral coverages are consistent with the GSB properties observed in pure PBDB-TT5 under ump at 500 nm. It is safe to assign this excitation transfer to a hole transfer process from m-ITIC to PBDB-TT5, which is also favored by the energy alignment of the LUMO bands in both materials. The early phase lifetime is shortened from ~0.5 ps in the pure film to ~0.15 ps in the mixed film, implying very efficient hole transfer on the sub-ps time scale in the fullerene-free PBDB-TT5:m-ITIC system.
The charge transfer process manifests as a delay in the growth of PBDB-TT5 GSB signals (Figure 4.9e) and the photoinduced absorption in PC71BM (Figure 4.9f). Nevertheless, compared to the GSB dynamics of PBDB-TT5 in the m-ITIC and PC71BM based mixing systems, the build-up of GSB dynamics is found to be much faster in the PBDB-TT5:m-ITIC mixing, providing a more efficient gap implies. transfer in the PBDB-TT5:m-ITIC combination.
A synergetic effect of molecular weight and fluorine in all-PSCs
Research background and motivation for the work
Results and discussion
- Synthesis, Optical and Electrochemical Properties of Polymers
- Photovoltaic Properties
- Morphological Properties
- Charge Recombination and Transport Properties
Clearly, the VOCs of the all-PSCs increase monotonically with higher F content in the donor backbone due to the deepening of the HOMO levels, almost regardless of the Mn variation in the acceptor polymer. Interestingly, for any given donor polymer, both JSC and FF gradually increase with increasing Mns of the acceptor polymer, while maintaining similar VOCs. The enhancement of JSCs may be somehow related to the higher absorption coefficients observed with increasing Mns of the acceptor polymer.
Furthermore, the EQE data show that the JSCs are strongly dependent on the Mns of the acceptor polymer. For TQ-based blend films, increasing the acceptor Mns leads not only to the increase in the intensity of the (010) peaks, but also to the decrease of the d(010) spacing, indicating that a strong crystallization process in TQ: H-P( occurs) NDI2OD-T2).
Conclusion
Taken together, all the above data show that not only can the charge transport properties be strongly dependent on the variations in both Mn and F, but the exciton dissociation can also be optimized by using the correct F content in the donor in conjunction with a high Mn. acceptor. As a result, compared to all other samples, TQ-F:H-P(NDI2OD-T2) preferentially suppressed recombination and showed enhanced exciton dissociation behavior, which simultaneously contributed to the improved FF and JSC values in the device. (c) slope values of Voc as a function of light intensity (I); (d) ratio of net photocurrent (Jph) to saturated photocurrent (Jsat) at the short circuit condition on the different polymer donor and acceptor compositions in all-PSCs.
This study not only provides new insights into the structure-property relationship with respect to the simultaneous effects of Mn and F, but also leads to a mechanistic understanding of the parameters to aid in the future PCE optimization of all PSCs.
Stretchable photoactive layer in all-PSCs
Research background and motivation for the work
Results and discussion
A pseudo-freestanding tensile test204 was performed to directly assess the intrinsic mechanical properties of the four blend films (Figure 6.1b). Further increasing the PDMS-S fraction results in monotonically decreased elastic modulus and tensile strength; however, the elongation at break increases to its peak value of 53.15% in the TQ-F:P(NDI2OD-T2):PDMS-S20 film and then decreases to 42.96% in the TQ-F:P(NDI2OD-T2): PDMS-S50 one, leading to overall decreasing toughness values (Figure 6.1d). The variations in mechanical properties are closely correlated with the evolutionary microstructures caused by the different amounts of PDMS-S embedded (Figure 6.2a).
Especially in the optimal TQ-F:P(NDI2OD-T2):PDMS-S10 film, a well-mixed bulk morphology with percolated PDMS-S networks was observed, which is primarily responsible for the excellent mechanical compliance. Moreover, the preferred face-on oriented molecular packing in FTQ:N2200:PDMS-S10 film (Figure 6.2b), as evidenced by the invisible (h00) lamellar diffraction in the out-of-plane direction in contrast to the intensified (100 ) diffraction in the in-plane direction benefits charge transfer through the active layer to the electrodes, which is beneficial for photovoltaic application.
Conclusion
Experimental section
The resulting mixture was kept at 110°C for 24 hours and then quenched with water and extracted with diethyl ether. After the addition, the mixture was stirred overnight at room temperature and then quenched with water and extracted with ethyl acetate. After pouring into water, the mixture was extracted with ethyl acetate and dried over MgSO4.
The resulting mixture was stirred overnight at room temperature overnight and then quenched with water and extracted with diethyl ether. Then 5 ml of anhydrous toluene was added and the mixture was heated to 110°C for 72 hours.
Acknowledgements
