Tremendous progress has been made recently in almost every aspect of PSCs. Many empirical design rules have been extracted from these results. In this review, we examined a wide range of polymer materials to provide useful guidelines to help elucidate the complex interplay of various types of structures (chemical, morphological, and in device fabrication) and their effects on various key solar cell properties. Going from a micro- to a macrolevel, this begins with controlling the molecular structure via the choice of different donor–acceptor copolymer backbones, side-chain engineering, and heteroatom substitution in the polymer backbone. We summarized the effects that arise from these structures in terms of both active layer morphology and ultimately the physical properties which emerge when energy levels and domain sizes are optimized in harmony. On the most macroscale, we described in detail the modification of device structure via the composition of interfacial HSLs and ESLs fabricated from materials such as metal oxides and CPEs, as well as the emergence of ternary blend solar cells. Next, both the incorporation of dopants such as noble metal nanoparticles to improve absorption and maximize solar cell properties via the plasmonic effect and all-polymer solar cells made from both polymer donor and acceptor were examined. Finally, we briefly introduced recent studies and current status of stability for PSCs.
Recently, several groups predicted the thermodynamic PCE limit of PSCs to be >20% in single-junction devices.To achieve this goal, an EQE of 90% over a broad range and a FFof 80% are necessary. At this stage, encouraging EQE of ∼80% and FF of approximately 80% have already been reported in different polymer systems. Several recent studies reveal that blend nanomorphology plays an important role in determining deviceFF. For example, utilizing 1-chloronaphthalene additive to optimize the interpenetrating network for P3HT:ICBA devices results in a FF of 75%. Guo et al. showed that FF of 76–80% may be realized from highly ordered molecular packing and optimal vertical phase separations. Yan and co-workers controlled the phase separations by temperature-dependent aggregation behavior of donor polymers and achieved a FF of 77% Li et al. achieved a FF of 74% for a 300 nm thick diketopyrrolopyrrole-based polymer device with interconnecting and crossing crystalline fibrous structures for high charge carrier mobilities. Still, at this stage, there is no polymer that exhibits a PCE value more than twice that of P3HT, and all device PCEs that are worthy of mention are within 8 ± 2%. The current challenge then, is to achieve a high Jsc while maintaining a large Voc in PSC devices. From the discussion in this review, several promising routes can be proposed.
(a)
Material engineering to achieve broad absorption and high hole mobility in donor polymers. These two are the main challenges in materials design for PSCs. The absorption spectra of the active layer materials need to be extended to the near-IR region while maintaining a high absorption coefficient throughout the absorption range. Hole mobilities should be high enough to efficiently transport holes and balance charge transport. In addition, the HOMO and LUMO energy levels of polymers can be engineered to achieve better performance for PSCs.
(b)
Combining intramolecular charge separation as well as optimized morphology to facilitate charge separation. The key is to minimize hole and electron binding energy. A high dielectric constant in the polymer blend will be desirable.
(c)
Interfacial engineering to facilitate charge transport and collection in high-performance PSCs through decreased resistance, as well as improving device stability. Supramolecular chemistry might be a powerful approach to address these issues.
(d)
Mixing multiple donors or acceptors in a high-performance ternary structure to obtain complementary light absorption, increase Jsc, as well as provide better charge transport and improve Voc.
(e)
Utilizing metal nanostructures to achieve improved optical and electrical properties in PSC devices such as increased absorption and improved charge transport.
Although the design guidelines provided here are limited in detail and scope, we hope that the readers have gained useful insights into material synthesis, device engineering, and physics that may help to drive them toward the development of more efficient PSC materials and devices. An encouraging conclusion to be drawn from this review ought to be that the general trend of development in PSCs is exciting and the future seems to be bright; yet, there is much science to be done before these OPV devices can have a significant impact on our renewable energy landscape and our society.
The authors declare no competing financial interest.
http://pubs.acs.org/doi/full/10.1021/acs.chemrev.5b00098