Fundamentals of Solar Cell Design. Rajender Boddula
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with thiophene moiety. Acceptor and donor exhibited complementarity in their light absorption spectra. The donor-acceptor blend absorption starts at ~350 nm and ends at ~780 nm. Inverted solar cells were fabricated with configuration of: ITO/Zno/Blend of DFDT(DPP)2 + IDIC or DFDT(DPP)2 + IDC-4F/MoO3/Ag, to generate PV parameters. DFDT(DPP)2 + IDIC-4F Blend displayed PCE of 9.43% with Voc = 0.86 V; Jsc = 16.83 mA/cm2; FF = 65%. Authors inform that donor acceptor interaction in the blend leads to good crystallinity as well as improved morphology, and these are also the factors responsible for the improvement of efficiency.
Figure 1.38 Tetrathiophene linked with DPP.
Huan Li et al. synthesized [36] a new A-D-A–type small donor NDTSR molecule (Figure 1.39). NDTSR has naphthalene fused with four thiophenes is the central core, and further, it is attached to three thiophene units on both sides and with rhodanine as terminal group at both ends. IDIC and ITIC (Figure 1.39) were taken as small-molecule acceptors. Solution phase light absorption for donor and acceptors indicate the complementarity in their light absorption. NDTSR with acceptor blend exhibited light absorption covering ~350- to 780-nm region. Conventional and simple configuration was adopted for fabricating the solar cell as: ITO/ PEDOT-PSS/Blend NDTSR + IDIC or NDTSR + ITIC/Ca/Al and the PV parameters were determined. The NDSTR + IDIC blend provided poor efficiency of 1.71%, whereas NDTSR + ITIC blend gave very good result by showing 8.05% efficiency. The big difference observed in using IDIC and ITIC was attributed to the donor-acceptor molecular interactions leading to the formulation of ordered film, which could facilitate the charge mobility/migration effectively.
Figure 1.39 Fused NDTSR with rhodanine end group.
Sachin Badgujar et al. prepared [37] two small molecules, one as donor—BDT3TR, and another as acceptor—O-IDTBR (Figure 1.40), for the purpose of studying their all small-molecule solar cell efficiency. Donor and acceptor were selected because of their complementarity in their light absorption spectrum. Blend of donor-BDT3TR and acceptor–OIDTBR absorption occurred in ~350- to ~650-nm region. Solar cell device structure adopted was: ITO/PEDOT-PSS/Blend of BDT3TR + O-IDTBR/ ZnO/CPE/Al for measuring the PV parameters. An efficiency 6.96 % was recorded with other parameters as, Voc = 1.06 V; Jsc = 12.10 mA/cm2; FF = 55%. Authors mentioned that complimentary light absorption by donor and acceptor and high lying HOMO level of O-IDTBR could be the reasons behind the higher efficiency observed in these investigations. Further, they advocated that this was the first all small-molecule BHJOSCs.
Figure 1.40 Linear BDTT linked trithiophene with rhodanine end group.
Jisu Hong et al. reported [38] the synthesis of three small-molecule acceptors (Figure 1.41). These were two naphthalenediimides linked to thiophene - NDICN-T: i) NDICN-T linked to bithiophene—NDICN-BT and ii) NDICN-T linked to (E)-1,2-di(thiophene-2-yl)ethane—NDICNTVT. The small-molecule donor employed was DTS-F. UV-visible light absorption and photoluminescence spectra were recorded for three acceptors and one donor and were found to have considerable overlapping. PV properties were obtained by fabricating conventional or simple solar cell architecture like: ITO/PEDOT-PSS/Blend of DTS-F with acceptor molecule/LiF/Al. The blend DTS-F with NDICN-TVT gave satisfactory efficiency of 3.01. Furthermore, to probe the solar cell fabrications and efficiency, authors evaluated film morphology, femtosecond transient absorption on films, and charge transport dynamics. These investigations inform that NDI (Naphthalene Diimide) derivatives can be probed as lead molecules in further studies.
Figure 1.41 NDIs linked with spacer.
1.8 Conclusion
The chapter illustrates polymer donor and non-fullerene small-molecule acceptor and all small-molecule (donor and acceptor) BHJSC’s performance. It is evident from the examples cited that non-fullerene–based BHJOSCs are becoming increasingly popular and in particular all small molecule–based BHJOSCs have just started their journey and will take off further. We can say that fullerene-based BHJOSCs are becoming outdated due to their limitations and solar cell efficiency. Several examples cited in this chapter provide over 17% efficiency achieved for polymer donor and non-fullerene small-molecule acceptor and over 14% efficiency cited for all small-molecule (donor and acceptor) BHJSCs. It becomes clear that a variety of designs have been developed to make small molecules in order to test them for solar cell efficiency. It is predicted that these small-molecule BHJOSCs are expected to display higher efficiency in near future by the use of proper design of molecules and solar cell fabrications. More interestingly, all small-molecule (acceptor and donor) materials have higher advantage than polymer donor and small-molecule acceptor type solar cells. All small-molecule (acceptor and donor) design and synthesis is less expensive, and higher purity can be achieved. Their optical, thermal, and electrochemical properties are tunable by proper design of molecules. Most of the examples discussed in this chapter are not clear with stereochemistry of the terminal double bond attaching to electron withdrawing group (like rhodanine or indanone), since the E and Z isomers differ in their properties. Particularly, their dipolemoment (vector) or polarizability (tensor) may have an impact on the molecular association in the formation of films. Hence, defining the stereochemistry of double bond will improve solar cell functioning. The film morphology can be controlled by applying supramolecular chemistry interactions, while designing the small molecules. The thermal stability and photo stability coupled with high PCE will place these BHJOSCs for the possible commercial applications in the near future.
Acknowledgement
This chapter was authored purely out of academic interest to familiarize scholars about organic solar cell materials, particularly related to organic molecules. The examples covered in this review are chosen from recent literature appearing in different journals. The authors of this review are highly appreciative of the research articles published for their contributions in the area of organic solar cells. This chapter is only representative in nature and not intended to be exhaustive. Scholars are advised to go through original research publications for detailed information. The structures are also drawn briefly to give an idea about the products. The authors of this review further acknowledge the original contributors and publishers of the research articles cited here for their potential and interesting scientific work, with a larger interest in academic excellence and advancement.
VJR thanks Dr. B. Parthasaradhy Reddy, Chairman Hetero Drugs, Pvt. Ltd., and Dr. K. Ratnakar Reddy, Director HR Foundation for their encouragement. VJR also thanks CSIR, New Delhi, for Emeritus Scientist Honor.
References
1. Dongxue Liu, Ting Wang, XinKe, Nan Zheng, Zhitao Chang, ZengqiXie and Yongsheng