Long‐Term Stability of the Oxidized Hole‐Transporting Materials used in Perovskite Solar Cells

Abstract

The vast majority of the hole transporting materials require the use of chemical doping as an essential step for preparation of efficient perovskite solar cells. An oxidized organic hole-transporting material, obtained during a doping procedure, could potentially be one of the weak links in the device composition. It is not uncommon for the solar cell to heat up under summer sun; therefore, all device components must possess some degree of resistance to repetitive thermal stress. In the current publication, a series of oxidized hole-transporting materials have been synthesized and their long-term stability investigated. During thermal stability testing of the films, kept at 100 °C under an inert atmosphere, it was observed that oxidized HTMs start to degrade and partly revert to original unoxidized material. It is known that oxidized HTM, formed during doping, is responsible for the increased conductivity and ultimately for better efficiency of hole extraction process in the PSC device; therefore, observed instability of the oxidized HTMs in the thin films at elevated temperatures could be one of the causes of drop in conductivity reported for the doped spiro-OMeTAD. It could also potentially be one of the reasons why perovskite solar cells lose their efficiency under prolonged thermal stress.

Temperature Impact on Perovskite Solar Cells Under Operation

“…[20] As the glass transition temperature of the spiro-OMeTAD is 125 8C [33] and there is no degradation in pristine (non-additivated) spiro-OMeTAD, it can be conclude that the performance decrease is relatedt ot he evaporation of additives. [34] Finally,a nd as discussed before, Kasparavicius et al [25] also reported that oxidized HEL starts to degradea tt hose values of temperature and part reverts back to the original unoxidized material, and the other partr eacts with TBP. The pinholes at the spiro-OMeTAD layer should then be formed because of the fast evaporation of additives during the thermals tress tests.…”

“…Even though TBP presents ab oilingp oint of almost 200 8C, Bailie et al showed that this compound in small quantities can possessa na ppreciable equilibrium partial pressure, which allows it to evaporate at temperatures as low as 85 8Ci n dye-sensitized solar cells. [34] Finally,a nd as discussed before, Kasparavicius et al [25] also reported that oxidized HEL starts to degradea tt hose values of temperature and part reverts back to the original unoxidized material, and the other partr eacts with TBP.…”

“…According to Kim et al,a te levated temperatures iodide ions from the perovskite layer diffuse into the spiro-OMeTADa nd act as reducing agents, decreasing its oxidation and, consequently,i ts conductivity properties. More recently,M agomedov et al [24] showedt hat oxidized HEL speciesc an interact with tert-butylpyridine additive, forming pyridinated products that influence negatively the performance of the device, even with short aging times.Moreover, Kasparavicius et al [25] also showedt hat oxidized HEL starts to degradea t1 00 8Ca nd partly reverts to the original unoxidized material, and the otherp art reacts with TBP.A dditionally,L iTFSI and FK209,w hich are also responsible for the oxidation of HELs, do not affect the degradation rate. This led to the conclusion that the textural modification of the spiro-OMeTADl ayer (generation of pinholes at high temperature) and certain unidentified modifications at the perovskite/ spiro-OMeTADi nterface seem to be the main reasonsf or the performance drop of the cells at high temperature.…”

“…These authors observed that when using ad opant-free HEL [poly(3-hexylthiophene-2,5-diyl), P3HT],t he devices retained the initial efficiencya nd showed no apparent performance change after 800 hs toragei nt he temperature range 20-85 8C. More recently,M agomedov et al [24] showedt hat oxidized HEL speciesc an interact with tert-butylpyridine additive, forming pyridinated products that influence negatively the performance of the device, even with short aging times.Moreover, Kasparavicius et al [25] also showedt hat oxidized HEL starts to degradea t1 00 8Ca nd partly reverts to the original unoxidized material, and the otherp art reacts with TBP.A dditionally,L iTFSI and FK209,w hich are also responsible for the oxidation of HELs, do not affect the degradation rate. Thus, the authors concluded that even if the dopants are important to oxidize the HELs to increaset heir conductivities and consequently to obtain higherp owerc onversion efficiencies, the instability of the oxidized HELs can be responsible for the low-performing spiro-OMeTAD-based devices at elevated temperatures.…”

Application of a Tetra‐TPD‐Type Hole‐Transporting Material Fused by a Tröger's Base Core in Perovskite Solar Cells

“…In addition, the V1160 HTM was tested in a dopant‐free configuration. As dopants are known to chemically interact with the HTMs, it is advisable to search for the compounds that ensure sufficient performance without the use of additives. Interestingly, a device with dopant‐free V1160 showed better performance than that of the corresponding Spiro‐OMeTAD‐based device, resulting in a moderate PCE of 12.6%.…”