Configurations

There are many different materials and configurations that encompass current state-of-the-art perovskite solar cells, ranging from all organic to all inorganic contacts, with or without the inclusion of a mesoporous scaffold. Some of the most efficient cells reported to date comprise a fluorine-doped tin oxide-FTO-coated glass, a 20–50-nm-thick TiO₂ compact layer, a 100–200-nm-thick mesoporous TiO₂ layer, which acts as an electron transport material and scaffold infilled with perovskite, a solid perovskite overlayer of 200–300 nm thickness, an additional light-absorbing layer, a hole-transporting layer, and a metal back electrode. ^{Reference Jeon, Noh, Yang, Kim, Ryu, Seo and Seok10} Often, mesoporous Al₂O₃ is used in place of TiO₂, or the scaffold is completely removed in planar heterojunction cells. A planar heterojunction device architecture typically consists of glass/FTO/compact metal oxide/perovskite/hole transporter/gold contact.

The most commonly used hole-transporting materials in perovskite solar cells are spiro-OMeTAD (2,2′7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) ^{Reference Im, Jang, Pellet, Grätzel and Park11} and PTAA (polytertiary aryl amine) polymer. ^{Reference Jeon, Noh, Kim, Yang, Ryu and Seok12} The electronic properties of the perovskite and hole-transporting material have been extensively investigated, including the optimization of the performance by chemical p-doping of spiro-OMeTAD.

Optimization of the thickness of the various layers and the morphology of these layers affects overall device performance. For example, by varying the thickness of the mesoporous TiO₂ layer and the perovskite overlayer, the absorption of the device can be tuned, yielding significant enhancement in the short-circuit current. A too thin perovskite overlayer (less than 200 nm) results in low photocurrent due to insufficient absorption. However, an excessively thick overlayer results in a lower fill factor. The fill factor is the ratio of the maximum obtainable power to the product of the open-circuit voltage and short-circuit current density.

The grain size of the perovskite crystals also plays an important role. Typically, the thickness of the hole-transporting material in perovskite solar cells is in the range of 150 nm to 200 nm. If this is too thin, then it provokes short circuits, whereas if it is too thick, this decreases the fill factor. ^{Reference Nie, Tsai, Asadpour, Blancon, Neukirch, Gupta, Crochet, Chhowalla, Tretiak, Alam, Wang and Mohite13} In fact, state-of-the-art perovskite devices that incorporate mesoporous TiO₂ can be considered as a combination of a mesoporous TiO₂/perovskite bulk heterojunction layer and a flat solid perovskite layer.

There are now many research teams advancing different cell configurations in parallel with insulating or semiconducting mesoporous scaffolds and with organic or inorganic semiconductor contacts for planar heterojunctions. A single dominant architecture (or contact material and configuration) may emerge, but it also may be possible that a number of suitable perovskite cell “technologies” can emerge, similar to the handful of commercial silicon cell technologies that currently exist.

Perovskite material

The typical crystalline structure of perovskite material is ABX₃, where A = MA cation, B = lead di-cation, and X = iodide; this is called MA lead triiodide perovskite. The MA lead triiodide perovskite exhibits direct bandgap absorption from the top of the valence band to the bottom of the conduction band. The valence band is dominated by iodide p orbitals, and the conduction band consists of Pb p orbitals, therefore the transitions are p-to-p orbitals. ^{Reference Park14} The bandgap of ABX₃ perovskite can be tuned by varying A, B, and X. ^{Reference Gao, Grätzel and Nazeeruddin15} Further, the bandgap can be modulated by appropriate selection of mixed cations and halogens. ^{Reference Pellet, Gao, Gregori, Yang, Nazeeruddin, Maier and Grätzel16,Reference Noh, Im, Heo, Mandal and Seok17}

Among these combinations, perovskites containing MA and formamidinium (FA) as the A cation and metal Pb²⁺ as the B cation have raised interest due to their excellent optoelectronic properties, potential for low-temperature solution processable films, and stability. ^{Reference Burschka, Pellet, Moon, Humphry-Baker, Gao, Nazeeruddin and Grätzel18,Reference Pang, Hu, Zhang, Lv, Yu, Wei, Qin, Xu, Liu and Cui19} In addition, ABX₃ properties are amenable to the formation and nature of the growth of perovskite materials. ^{Reference Lee, Teuscher, Miyasaka, Murakami and Snaith4} The MA lead iodide (CH₃NH₃PbI₃) perovskite material has a panchromatic absorption down to ca. 800 nm, direct bandgap of 1.57 eV, large absorption coefficient of 1.5 × 10⁴ cm^–1 at 550 nm, ^{Reference Kojima, Teshima, Shirai and Miyasaka2} low exciton binding energy (<10 meV in MAPbI₃), ^{Reference Savenije, Ponseca, Kunneman, Abdellah, Zheng, Tian, Zhu, Canton, Scheblykin, Pullerits, Yartsev and Sundström20} very high charge-carrier mobility (66 cm²/Vs for MAPbI₃), ^{Reference Savenije, Ponseca, Kunneman, Abdellah, Zheng, Tian, Zhu, Canton, Scheblykin, Pullerits, Yartsev and Sundström20} ambipolar charge transport, ^{Reference Stranks, Eperon, Grancini, Menelaou, Alcocer, Leijtens, Herz, Petrozza and Snaith21} large electron and hole diffusion lengths (over 1 μm and potentially over 100 μm), ^{Reference Dong, Fang, Shao, Mulligan, Qiu, Cao and Huang22} and low nonradiative recombination rates. ^{Reference Wetzelaer, Scheepers, Sempere, Momblona, Ávila and Bolink23}

Perovskite solar cells show unreliable efficiencies due to scan speed–dependent hysteresis observed in the photocurrent–voltage curves of the cells, which could be a bottleneck in the development of photovoltaic technology based on them ( Figure 2 ). Various studies regarding the origin of the hysteresis have been documented, such as ferroelectricity caused by dipoles of MA molecules, ^{Reference Gottesman, Haltzi, Gouda, Tirosh, Bouhadana, Zaban, Mosconi and De Angelis24} ion migration, ^{Reference Almora, Zarazua, Mas-Marza, Mora-Sero, Bisquert and Garcia-Belmonte25} and unbalanced charge collection rate at the interfaces. ^{Reference Ponseca, Savenije, Abdellah, Zheng, Yartsev, Pascher, Harlang, Chabera, Pullerits, Stepanov, Wolf and Sundström26} Among these, unbalanced charge collection and ion migration seem to be realistic causes for hysteresis behavior.

Figure 2. Representative current–voltage characteristics of the champion cell—the best performing cell to date using a methylammonium lead triiodide perovskite—(a) and current density–voltage (J–V) curve hysteresis by scanning forward and backward (b). The current–voltage data in (a) show the best efficiency obtained with an optimized perovskite morphology, and the (b) J–V curves at different scanning directions exhibit small hysteresis. Note: J _sc, short-circuit current density; V _oc, open-circuit voltage; FF, fill factor; PCE, power-conversion efficiency. Reprinted with permission from Reference 35. ©2015 Wiley.

The most striking evidence to support the imbalanced charge extraction comes from the p-i-n structures (inverted perovskite solar cell), where the hysteresis is significantly lower compared to the n-i-p (normal perovskite solar cells) structures. ^{Reference Shao, Xiao, Bi, Yuan and Huang27} The imbalanced charge collection is probably related to the electrical character of the perovskite materials and charge accumulation at the interface between perovskite and n-type/p-type layers. ^{Reference Dymshits, Henning, Segev, Rosenwaks and Etgar28} Therefore, the control of perovskite crystal morphology and growth, passivation of the perovskite layer by interfacial engineering, and development of new n- and p-type materials for efficient charge extraction at the heterojunctions are paramount to further improve the power-conversion efficiency, specifically long-term stabilized power output of perovskite solar cells. Figure 2 shows an optimized perovskite solar cell (Figure 2a) and significantly reduced hysteresis (Figure 2b).

Fabrication

Besides the excellent PV performance of the organic–inorganic halide perovskite, the perovskite material is attractive because of its versatility in terms of fabrication techniques. There are several deposition methods available for perovskite films, including one-step precursor solution deposition, ^{Reference Kim, Lee, Im, Lee, Moehl, Marchioro, Moon, Humphry-Baker, Yum, Moser, Grätzel and Park5} two-step sequential deposition, ^{Reference Burschka, Pellet, Moon, Humphry-Baker, Gao, Nazeeruddin and Grätzel18} dual-source vapor deposition, ^{Reference Liu, Johnston and Snaith29} and vapor-assisted solution procession. ^{Reference Chen, Zhou, Hong, Luo, Duan, Wang, Liu, Li and Yang30}

In the one-step precursor solution deposition, the MA lead iodide perovskite is deposited onto the mesoporous metal oxide film by spin coating of a precursor solution of 1:1 CH₃NH₃I and PbI₂ in γ-butyrolactone or dimethylformamide (DMF) or a mixture of dimethyl sulfoxide and DMF solvents on top of the 200–400-nm-thick film of TiO₂ (anatase) nanoparticles ( Figure 3 a). Burschka et al. reported a 15% power-conversion efficiency, where the perovskite was deposited by a novel sequential deposition technique on a mesoporous TiO₂ film (Figure 3b). ^{Reference Burschka, Pellet, Moon, Humphry-Baker, Gao, Nazeeruddin and Grätzel18}

Figure 3. Four general methods to prepare perovskite active layers. (a) One-step precursor deposition; (b) sequential deposition method; (c) dual-source vapor deposition; and (d) vapor-assisted solution process. Reprinted with permission from Reference 15. © 2014 Royal Society of Chemistry.

In an inverted layout, similar to that employed in all organic polymeric solar cells, the perovskite layer was deposited using a dual source sublimation method and used as the electron donor in combination with an electron acceptor [6,6]-phenyl C₆₁-butyric acid methylester PCBM (Figure 3c). ^{Reference Malinkiewicz, Yella, Lee, Espallargas, Graetzel, Nazeeruddin and Bolink31} Holes are extracted via the transparent conductor poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS). In this configuration, the sublimated MA lead iodide perovskite layers are sandwiched between two very thin electron and hole blocking layers consisting of polytertiary arylamine and PCBM organic molecules. The organic materials were deposited using solution-based processes, whereas the CH₃NH₃PbI₃ perovskite and the metal contact were deposited using thermal evaporation under vacuum, which reached efficiencies as high as 12% at 100 mW cm⁻². ^{Reference Malinkiewicz, Yella, Lee, Espallargas, Graetzel, Nazeeruddin and Bolink31}

Yang et al. reported a low-temperature approach for the deposition of the perovskite absorbing layer called vapor-assisted solution processing (VASP), which is similar to a combination of the sequential deposition method and dual-source vapor deposition. ^{Reference Xiao, Liu, Qi, Zhang, Wang, Gao, Qiu, Ma, Cui, Chen, Zhan, Yu, Qin, Hu and Zhu32} In this process, PbI₂ films are deposited onto a FTO glass coated with a compact layer of TiO₂, followed by annealing in methylammonium iodide vapor at 150°C in a N₂ atmosphere for 2 h to form perovskite films (Figure 3d). The perovskite film derived from this approach exhibits full surface coverage, uniform grain structure with grain size of up to a few nanometers, and 100% precursor transformation. VASP presents a simple, controllable, and versatile approach to the pursuit of high-quality perovskite films and the resulting high-performance PV devices.

Several groups have investigated charge transfer dynamics by using different wavelengths and fluorescence of femtosecond excitation of perovskite films and by probing the visible-near-infrared transient absorptions of the generated charge carriers. If the scaffold material is a mesoporous layer of TiO₂, the data show sub-picosecond electron injection into titania in 170 fs, and electron and hole relaxations in the range of 0.5–0.7 ps within perovskite and titania conduction bands. ^{Reference Douhal, Piatkowski, Cohen, Ramos, Di Nunzio, Nazeeruddin, Grätzel and Ahmad33} The contribution of trap states in the recombination processes is small, thus increasing the survival probability of the charge in the excited perovskite collecting at the respective electrodes.