Rebirth of CuInS2 as hole transport material for perovskite solar cells

Copper indium sulfide, CuInS2 (CIS), has entered the field of photovoltaic cells for about three decades but somehow, they have not met their theoretically predicted efficiency as an absorber material. However, it is being employed as a hole transport layer in conjunction with organic–inorganic perovskite solar cells, which offer improved environmental stability at a reduced production cost. This commentary piece aims to draw researchers' attention to the use of CIS as hole transport layers in organic‐inorganic perovskite solar cells, with the expectation that further study and efficiency improvements will propel perovskite solar cells into the commercialization race.

In materials science, it is sometimes seen that a compound that may emerge as a promising candidate for a certain application; because of some seeming properties; may prove to be futile in that area but may become unexpectedly apposite for other purposes.Copper indium sulfide, CuInS 2 (CIS) is such a material that a few decades ago was seen as an excellent absorber layer for solar cells but because of its inability to reach high-efficiency values, could not flourish as much as was expected of it. 1For a long time, not much interesting research came to sight in this regard especially after lead halide perovskite materials reported efficiency comparable to Si-based devices.After a few years of silence, CuInS 2 suddenly surfaced as a "green" hole transport material for lead halide perovskite devices with commendable charge carrier extraction. 2 In 2014, Chen et al. 2 were the first ones to report the use of CuInS 2 as a potential hole transport layer (HTL) for planar hetrojunction (CH 3 NH 3 )PbI 3 perovskite solar cell.They saw the potential stability that CuInS 2 offers as hole transport material (HTM) to the perovskite absorber layer and synthesizing it by solution precursor method reduced the cost of the device as well.They prepared devices with different layer thicknesses for CuInS 2 were used with the highest efficiency device of more than 5% achieved.
Without a doubt, lead halide perovskite solar cells (PSCs) are an efficient absorber material for photovoltaic cells that checks all the criteria from efficient light absorption to long carrier lifetime with less recombination at interfaces.Except for one shortcoming, they have instability in weather conditions. 3The main reason for it is the use of organic HTLs like Spiro-OMeTAD, PTAA, P3HT, PANI, PEDOT:PSS, which makes the absorber layer prone to humidity and high temperature and also increases the cost of manufacture. 4Thus, with more research being done on PSCs the attention began to shift toward inorganic HTMs, especially copper-based compounds and this is when CuInS 2 was seen as a potential HTM for perovskite devices. 5tability is one of the main issues with the industrialization of PSCs.With the use of CIS HTM, atmospheric stability has been reported to be improved in comparison to organic HTMs.When compared to commonly used HTL of PEDOT:PSS which reported by Chen and coworkers, 6 the CIS film as an effective HTL has been prepared by using low temperature solution method for inverted PSCs which effectively promoted the stability of device at ambient atmosphere with air and RH of 55%.Similarly, Liu et al. 7 reported the synthesis of CIS quantum dots by modified hot injection method for HTM in PSCs as shown in Figure 1A.CIS has a good band alignment with perovskite layer (Figure 1B) and also has good film formation as shown in the scanning electron microscopy image (Figure 1C).The top-performing solar cells with CuInS 2 had a high power conversion efficiency (PCE) (18.8%), which was comparable to the PCE of the Spiro-OMeTAD-based (19.2%) optimized solar cells (Figure 1D-F).However, the biggest mark hall was the improvement in stability of noncapsulated device with CIS HTL in comparison to Spiro-OMeTAD-based devices.At 20% relative humidity, the device with CuInS 2 , the PCE of a device only degraded drastically by 9% of its initial value within 15 days of ageing in contrast with Spiro-OMeTAD device that degraded by 33% (Figure 1G-J).
CIS HTM has later been reported to have reduced interfacial charge recombination between perovskite/ HTM interface as compared to other HTMs.It suppresses the charge recombination pathways in the interface which in turn reduces the nonradiative recombination. 8As photoluminescence measurements of perovskite/CIS device in comparison to perovskite/spiro device can be seen in Figure 2A,B to have a better understanding of interfacial charge dynamics.Not only does the fluoresce quenching of the perovskite device is compatible with the spiro device but it also has a comparable fluorescence lifetime showing good separation of photogenerated carriers along the device.Similarly, by use of transient absorption spectroscopy (Figure 2C,D), it can be seen that a photobleached area at 728 nm wavelength corresponding to the photogenerated electron transitions from the ground state to the excited state can be seen in which device with CIS HTM shows faster photobleaching recovery rate proposing better charge separation and collection.
As for the film deposition, CIS HTMs are easily synthesized by chemical routes and deposited by nonvacuum techniques.As Taheri-Ledari et al. 10 have deposition CIS HTL on a perovskite absorber layer of uniform thickness (Figure 3A) and a notable efficiency of 15.75% with a significant short-circuit current density of 24.54 mA/cm 2 as seen in Figure 3B,C.Whereas, the device performance is concerned, it is a bit lower than the one with Spiro-OMeTAD, but the low cost, easy synthesis and stability to atmospheric conditions (Figure 3D) are some prominent features that can make one ignore the shortfall.The reported degradation mechanisms under extreme conditions inveterate that devices without CIS HTL displayed significant retention (94.3%) after 10 day/night cycles without device encapsulation.Thus, the perovskite thin films with a CIS HTL have shown superior resistance to humidity and improved photovoltaic stability.
Despite the potential of CIS as an HTM for PSCs, there are many obstacles, such as a lower PCE, and additional work is required before CIS can be implemented in actual PSCs devices.For this objective, understanding of interfacial recombination by sound theoretical and experimental results is crucial to develop paths for the commercialization of PSCs.Moreover, with advances made along the road, in the near future the efficiency may reach or surpass to that of devices with organic HTMs.

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I G U R E 1 (A) Schematic device structure and (B) energy band alignment of CuInS 2 hole transport material (HTM) with perovskite absorber layer.(C) Cross-sectional SEM of the device.(D) J-V characteristic (E) IPCE and (F) steady-state power output at the maximum power point of the device with CuInS 2 hole transport layer (HTL) in comparison to Spiro-OMeTAD.Time dependence of normalized performance parameters of (G) power conversion efficiency (PCE) (H) VOC (I) JSC (J) FF of the optimized solar cells with CuInS 2 and Spiro-OMeTAD as HTMs measured in ambient air.Reproduced with permission: Copyright 2019, Elsevier B.V. 7

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I G U R E 2 (A) The photoluminescence (PL) spectra, (B) PL decay curves and (C, D) the transient absorption spectra of perovskite film without hole transport material (HTM), perovskite film with CuInS 2 (CIS) HTM and perovskite film with Spiro-OMeTAD as HTM, respectively.Reproduced with permission: Copyright 2021, The Royal Society of Chemistry. 9