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Solar cells boosted by an improved charge-carrying material

The commercialization of a promising class of solar cell has been hindered by issues associated with the components needed to construct it. A possible solution has now been reported.
Liyuan Han is at the State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China.
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The most promising technology for the next generation of solar cells is based on a class of material known as perovskites. Perovskite solar cells can convert light into electricity with high efficiency (about 22%)1, but only when polymers known as polytriarylamine (PTAA)or 2,2ʹ,7,7ʹ-tetrakis(N,N-di-p-methoxyphenylamine)-9,9ʹ-spirobifluorene (spiro-OMeTAD) are used to transport holes — quasiparticles that bear a positive charge and are produced as part of the power-generating mechanism — within the cells. The high cost of these polymers limits their use in commercial solar cells. Another issue is that trace quantities of compounds called dopants need to be added to the polymers to enhance hole transport, but such dopants cause degradation of the perovskite layer in the devices2,3. In a paper in Nature, Jung et al.4 report an architecture for a perovskite solar cell that uses a cheaper, dopant-free polymer as the hole-transport material, and that has a truly impressive efficiency of 22.7%.

The problems associated with PTAA and spiro-OMeTAD have stimulated the search for alternatives. Cheaper, dopant-free materials for transporting holes have been reported57, as well as new stable dopants8, but the power-conversion efficiencies of perovskite solar cells made using these materials cannot compete with those of devices that use PTAA or spiro-OMeTAD. Finding low-cost hole-transport materials that provide both high efficiency and stability, and that are compatible with the industrial processes used to make solar cells, remains challenging.

One alternative candidate is poly(3-hexylthiophene) (P3HT; ref. 2)2. This polymer is cheap, has optoelectronic properties that are perfect for solar cells, and could be used in industrial-scale manufacturing processes. However, no efficiencies higher than 20% have been reported for perovskite solar cells made using P3HT. To understand the problems associated with P3HT, let’s consider how hole-transport materials are used in perovskite solar cells.

The general principle of solar cells is that light absorbed by an ‘active’ material, such as a perovskite, generates a pair of charge carriers — a negatively charged electron and a positively charged hole (Fig. 1). These charge carriers are then separated and carried to different electrodes in a circuit, thereby generating a current. One way of achieving all this is to sandwich the perovskite between a material that carries the holes to an electrode and another material that carries the electrons to a separate electrode.

Figure 1 | An extra layer for perovskite solar cells. In solar cells, light absorbed by an active material, such as a perovskite, generates electron–hole pairs; holes are quasiparticles formed by the absence of an electron. The electrons and holes separate and pass through electron- or hole-transport materials, respectively, until they reach an electrode. In this example, the holes pass through to a gold electrode, whereas the electrons travel to a transparent conducting film that acts as an electrode. A current is generated when the electrodes are connected to a circuit. The polymer poly(3-hexylthiophene) (P3HT) is a cheap hole-transport material, but solar cells made using P3HT have had low power-conversion efficiency. Jung et al.4 inserted a material called n-hexyl trimethyl ammonium bromide (HTAB) between P3HT and the perovskite layer. Interdigitation of molecular chains in HTAB and P3HT causes the polymer to self-assemble into fibrils that have excellent hole-transport properties, thereby increasing the efficiency of the solar cell. The extra layer also improves the stability of the device.

Part of the problem with using P3HT as the hole-transport material is that it makes only poor physical contact with perovskites, which limits the transfer of holes between the materials9. Another issue is that electrons and holes can recombine — a process called non-radiative recombination — at the perovskite–P3HT interface10, which results in energy losses.

Jung and colleagues’ innovation is to overlay the perovskite layer with a material that conducts electrons poorly; the authors refer to this material as a wide-bandgap halide (WBH). The WBH blocks the transfer of electrons between the perovskite layer and P3HT, and therefore reduces charge recombination at the interface.

The WBH layer was formed in situ by the reaction of a compound called n-hexyl trimethyl ammonium bromide (HTAB) with the surface of the perovskite layer. The molecules that comprise HTAB consist of a hydrophilic head connected to a hydrophobic tail. The tails of HTAB interact strongly with the hydrophobic side chains of P3HT, as a result of van der Waals forces (Fig. 1). These interactions cause the molecules in P3HT to self-assemble into fibrils on the surface of the WBH (Fig. 2).

Figure 2 | The bulk structure of a hole-transport material.a, The polymer P3HT can be used to transport charge carriers called holes in solar cells, and is typically amorphous. b, Jung and colleagues’ process4 for making solar cells causes P3HT to self-assemble into fibrils. Hole mobility in fibrillar P3HT is about 10,000 times higher than in the amorphous form. Scale bars, 600 nanometres.

The bulk structure of P3HT affects its charge-transporting properties: hole mobility in fibrillar P3HT (ref. 11)11 is about 10,000 times higher than that in the amorphous form of the material12. This meant that Jung et al. did not need to use dopants in their solar cells to improve hole transport. Moreover, the HTAB molecules effectively neutralize charged defects on the surface of the perovskite crystal. This neutralization helps to reduce the amount of charge recombination that occurs at the perovskite–P3HT interface.

The combined effects of using a fibrillar WBH layer in the perovskite solar cells enabled Jung and colleagues to obtain the outstanding efficiency of 22.7%. Their cells showed greatly improved stability compared with a control device that lacked a WBH layer — a WBH-containing cell encapsulated in plastic maintained more than 95% of its initial power-conversion efficiency after 1,370 hours of continuous illumination using light that simulates the intensity and spectrum of sunlight at Earth’s surface. This stability can be attributed to the use of dopant-free P3HT. Non-encapsulated cells also showed greater moisture resistance than did unencapsulated control devices lacking WBH, as a result of the hydrophobic tails of HTAB.

Finally, Jung and co-workers showed that two widely used industrial methods for preparing thin films of material — spin coating and bar coating — could be used to prepare modules of perovskite coated with WBH and P3HT, with an area of about 25 square centimetres. By comparison, the solar cells characterized in the rest of the study had an area of about 0.09 cm2. Solar cells made from the larger modules using both industrial methods all had almost identical power-conversion efficiencies, indicating that the authors’ solar-cell architecture could be reliably mass-produced for commercial applications.

The low cost and remarkable efficiency of perovskite solar cells make it reasonable to assume that such devices will become a commercially viable alternative to silicon solar cells, which are widely used at present. The greatest challenge to their commercialization is stability — more work is needed to improve the stability not only of perovskites, but also of the charge-transporting materials and the electrodes. By demonstrating how P3HT can be used as a stable and effective charge-transporting material, Jung and colleagues might have helped to accelerate the progress of perovskite solar cells to the market.

Nature 567, 465-467 (2019)

doi: 10.1038/d41586-019-00936-x

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