An Hydrophilic Anode Interlayer for Solution Processed Organohalide Perovskite Solar Cells

Abstract

between the active layer and the electrodes should lead to an improved V oc . [ 11 ] The conundrum faced in tuning the V oc is how to modify the ionization potential of the interlayer on which the organohalide perovskite layer is deposited while maintaining the optimum fi lm quality. For example, it has previously been reported that the V oc of PVSCs formed by physical vapor deposition onto ultrathin organic semiconductor interlayers could be modifi ed by changing the ionization potential of the interlayer. [ 11 ] However, the problem observed was that not all the interlayers gave the same nor indeed optimum organohalide perovskite fi lm.With the focus on solution processing for large area PVSCs, solving the ionization potential-fi lm quality issue is important. Attaining the desired organohalide perovskite fi lm essentially boils down to matching the surface energies of the deposition surface (electrode interlayer) and the evolving fi lm after casting. Titanium dioxide (TiO 2 ) in the form of either a mesoporous scaffold or a dense thin fi lm has the ideal surface energy and the required insolubility for solution processed PVSCs. [ 22,23 ] On the other hand, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) can be processed at low temperatures, and possesses orthogonal solubility to the organohalide perovskite precursors and a suitable surface energy for fi lm growth. Recently, Choi et al. reported a conjugated polyelectrolyte (CPE-K) as a hole transport layer. [ 24 ] However, their devices with PEDOT:PSS and CPE-K interlayers exhibited low V oc s (<1 V), limited by the ionization potentials of PEDOT:PSS (−5.1 eV) and CPE-K (−4.9 eV), respectively. [ 24,25 ] More recently, in a similar approach using conjugated polymers with poly(electrolyte) side chains, planar devices with V oc s of greater than 1 V have been achieved. [ 26,27 ] It is important to note that the maximum possible V oc for a nonexcitonic homojunction solar cell is ultimately defi ned by the difference in electrode work functions after the thermodynamic limit has been considered. In our experience, other organic semiconductors, including those previously used in physical vapor deposited PVSCs, are unsuitable for solution processing due to the mismatch in surface energies. That is, the casting solution of the organohalide perovskite precursors simply does not wet the work function modifying interlayers although we do note one report that claims suitable fi lms can be formed on poly-TPD if very high spin-coating speeds are used. [ 3 ] Therefore, it is critical to develop interlayers that can meet three requirements: i) have an appropriate ionization potential to match the valence band energy of the chosen organohalide perovskite; ii) provide a suitable surface energy to allow for high-quality polycrystalline fi lm formation using solution processing methods; and iii) not be soluble in the solvent used to deposit the organohalide perovskite precursors. To date, these requirements are yet to be achieved in an interlayer and are the ...

Recent Advances in Energetics of Metal Halide Perovskite Interfaces

“…Besides the delicate control over the perovskite film itself, interface engineering has shown increasing significance for highly efficient perovskite‐based optoelectronic devices . Various charge transport layers including organic and oxide materials have thus been incorporated to improve the extraction or injection efficiency of holes and electrons, device stability and processability.…”

A Polymer Hole Extraction Layer for Inverted Perovskite Solar Cells from Aqueous Solutions

“…To maximize device efficiency, high‐temperature annealing is required for V 2 O 5 (500 °C) and NiO X (300 °C), and CuI is cast from volatile organic solvents (e.g., acetonitrile) . Hence, novel organic HELs have emerged as promising alternatives to PEDOT:PSS, in particular polyelectrolytes due to their wettability by the perovskite precursor solution . For example, Choi et al reported a water/methanol‐processed polyelectrolyte as a hole extraction material in inverted perovskite solar cells, affording a maximum PCE of 12.5%, while Li et al developed a water soluble polyelectrolyte for inverted perovskite solar cells that required thermal annealing at 140 °C for 30 min to achieve a maximum PCE of 16.6% …”

Considerations for Upscaling of Organohalide Perovskite Solar Cells

“…However, the hydrophobic nature of PCDTBT and other similar conjugated polymers makes it difficult to transfer this approach of modifying the work function of the anode to solution processed devices . We have also demonstrated that using a partially converted sulfonium precursor to poly(1,4‐phenylenevinylene), it is possible to modify the work function of the anode and solution process a high‐quality CH 3 NH 3 PbI 3 organohalide perovskite junction . In this current work, we report a different approach to forming a more hydrophilic interlayer for the anode that utilizes a hydrophobic polymer.…”

“…The open‐circuit voltage ( V oc ) of planar OHP solar cells is dependent on the ionization potential of the p‐type interlayer or work function of the anode . In previous studies on evaporated OHP planar solar cells we showed that a thin p‐type interlayer of poly[ N ‐9″‐heptadecanyl‐2,7‐carbazole‐ alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) delivered a high V oc in the range of 1.0–1.1 V .…”

“…In previous studies on evaporated OHP planar solar cells we showed that a thin p‐type interlayer of poly[ N ‐9″‐heptadecanyl‐2,7‐carbazole‐ alt ‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) delivered a high V oc in the range of 1.0–1.1 V . However, the hydrophobic nature of PCDTBT and other similar conjugated polymers makes it difficult to transfer this approach of modifying the work function of the anode to solution processed devices . We have also demonstrated that using a partially converted sulfonium precursor to poly(1,4‐phenylenevinylene), it is possible to modify the work function of the anode and solution process a high‐quality CH 3 NH 3 PbI 3 organohalide perovskite junction .…”