Azhar Fakharuddin

A perspective on the production of dye-sensitized solar modules

Dye-sensitized solar cells (DSCs) are well researched globally due to their potential as low-cost photovoltaic (PV) devices especially suited for building and automobile integrated PV (BIPV, AIPV) and portable or indoor light harvesting applications. Since 1991, large monetary and intellectual investments have been made to develop DSCs into deployable technologies, creating a wealth of knowledge about nano-interfaces and devices through an increasing number of research reports. In response to these investments, the dawn of the new millennium witnessed the emergence of a corporate sector of DSC development. Advances in their design, their incorporation on flexible substrates, the development of solid state modules, their enhanced stability in outdoor environments, and their scalable fabrication tools and techniques have allowed DSCs to move from the laboratory to real-life applications. Although photoconversion efficiencies are not on a par with commercially available CIGS or single crystalline silicon solar cells, they possess many features that compel the further development of DSC modules, including transparency, light weight, flexibility, conformability, workability under low-light conditions, and easy integration in buildings as solar windows. In fact, DSC panels have been shown to deliver even more electricity than their silicon and thin film counterparts of similar power ratings when exposed to low light operating conditions due to their workability in such conditions; thus, they are potential market leaders in BIPV and indoor light harvesting photovoltaic technology. However, large area dye-solar modules lack in performance compared to their laboratory scale devices and also suffer from long term stability issues. Herein, we discuss the main factors behind their inferior photovoltaic performance and identify possible opportunities for the design of more efficient DSC modules.

Progress, challenges and perspectives in flexible perovskite solar cells

Perovskite solar cells have attracted enormous interest since their discovery only a few years ago because they are able to combine the benefits of high efficiency and remarkable ease of processing over large areas. Whereas most of research has been carried out on glass, perovskite deposition and synthesis is carried out at low temperatures (<150 °C) to convert precursors into its final semiconducting form. Thus, developing the technology on flexible substrates can be considered a suitable and exciting arena both from the manufacturing view point (e.g. web processing, low embodied energy manufacturing) and that of the applications (e.g. flexible, lightweight, portable, easy to integrate over both small, large and curved surfaces). Research has been accelerating on flexible PSCs and has achieved notable milestones including PCEs of 15.6% on laboratory cells, the first modules being manufactured, ultralight cells with record power per gram ratios, and even cells made on fibres. Reviewing the literature, it becomes apparent that more work can be carried out in closing the efficiency gap with glass based counterparts especially at the large-area module level and, in particular, investigating and improving the lifetime of these devices which are built on inherently permeable plastic films. Here we review and provide a perspective on the issues pertaining progress in materials, processes, devices, industrialization and costs of flexible perovskite solar cells.

Vertical TiO2 Nanorods as a Medium for Stable and High-Efficiency Perovskite Solar Modules

Perovskite solar cells employing CH3NH3PbI3-xClx active layers show power conversion efficiency (PCE) as high as 20% in single cells and 13% in large area modules. However, their operational stability has often been limited due to degradation of the CH3NH3PbI3-xClx active layer. Here, we report a perovskite solar module (PSM, best and av. PCE 10.5 and 8.1%), employing solution-grown TiO2 nanorods (NRs) as the electron transport layer, which showed an increase in performance (∼5%) even after shelf-life investigation for 2500 h. A crucial issue on the module fabrication was the patterning of the TiO2 NRs, which was solved by interfacial engineering during the growth process and using an optimized laser pulse for patterning. A shelf-life comparison with PSMs built on TiO2 nanoparticles (NPs, best and av. PCE 7.9 and 5.5%) of similar thickness and on a compact TiO2 layer (CL, best and av. PCE 5.8 and 4.9%) shows, in contrast to that observed for NR PSMs, that PCE in NPs and CL PSMs dropped by ∼50 and ∼90%, respectively. This is due to the fact that the CH3NH3PbI3-xClx active layer shows superior phase stability when incorporated in devices with TiO2 NR scaffolds.

Multiporous nanofibers of SnO2 by electrospinning for high efficiency dye-sensitized solar cells

Various one-dimensional nano-morphologies, such as multiporous nanofibers (MPNFs), porous nanofibers (PNFs), and nanowires (NWs) of SnO2, are synthesized using electrospinning technique by controlling the tin precursor concentration. The MPNFs have ∼8-fold higher surface area compared to the other morphologies. Dye-sensitized solar cells (DSCs) were fabricated using these nanostructures as photoanodes and their performance was compared. The MPNFs surpass the performance of PNFs and NWs as well as conventional TiO2 paste. Record photoconversion efficiency (PCE) of ∼7.4% was realized in MPNFs DSCs, which was twice to that achieved using PNFs (∼3.5%). Furthermore, the MPNFs showed over >80% incident photon to current conversion efficiency (22% higher than that achieved by spherical P25 TiO2 particles) and also demonstrated ∼3 times longer electron lifetime and electron diffusion length. Owing to the possibility to produce large quantities using electrospinning technique, huge commercial potential of SnO2 nanostructures, and promising results achieved herein, the MPNFs are expected soon to be utilized in commercial devices.

Standardization of photoelectrode area of dye-sensitized solar cells

This study is aimed to provide new insights on the scalability of dye-sensitized solar cells (DSCs). The DSCs of electrode area up to ∼2 cm2 were fabricated using commercially available P25 TiO2 particles, N3 dye, and iodide/triiodide electrolyte. The photovoltaic conversion efficiency follows a biexponential decay, the main contributor to which is the short circuit current density (JSC). Interesting features were observed in the electrochemical impedance spectra and charge transport parameters in the devices as the photoelectrode areas were increased. Results show that electrons from an area above a threshold are not collected due to varied choice of diffusion pathways. Furthermore, this study identifies that the area of the photoelectrode for reporting the efficiency needs to be fixed at ∼0.5 cm2 for 25 nm TiO2 particles because below this value it strongly varies. On the other hand, the study provides opportunities to build high efficiency dye-sensitized solar cells using the current choice of materials.

Mesoporous titania-vertical nanorod films with interfacial engineering for high performance dye-sensitized solar cells

Working electrode (WE) fabrication offers significant challenges in terms of achieving high-efficiency dye-sensitized solar cells (DSCs). We have combined the beneficial effects of vertical nanorods grown on conducting glass substrate for charge transport and mesoporous particles for dye loading and have achieved a high photoconversion efficiency of (η) > 11% with an internal quantum efficiency of ∼93% in electrode films of thickness ∼7 ± 0.5 μm. Controlling the interface between the vertical nanorods and the mesoporous film is a crucial step in attaining high η. We identify three parameters, viz., large surface area of nanoparticles, increased light scattering of the nanorod-nanoparticle layer, and superior charge transport of nanorods, that simultaneously contribute to the improved photovoltaic performance of the WE developed.

Research Update: Behind the high efficiency of hybrid perovskite solar cells

Perovskite solar cells (PSCs) marked tremendous progress in a short period of time and offer bright hopes for cheap solar electricity. Despite high power conversion efficiency >20%, its poor operational stability as well as involvement of toxic, volatile, and less-abundant materials hinders its practical deployment. The fact that degradation and toxicity are typically observed in the most successful perovskite involving organic cation and toxic lead, i.e., CH3NH3PbX3, requires a deep understanding of their role in photovoltaic performance in order to envisage if a non-toxic, stable yet highly efficient device is feasible. Towards this, we first provide an overview of the basic chemistry and physics of halide perovskites and its correlation with its extraordinary properties such as crystal structure, bandgap, ferroelectricity, and electronic transport. We then discuss device related aspects such as the various device designs in PSCs and role of interfaces in origin of PV parameters particularly open circuit voltage, various film processing methods and their effect on morphology and characteristics of perovskite films, and the origin and elimination of hysteresis and operational stability in these devices. We then identify future perspectives for stable and efficient PSCs for practical deployment.

Channeling of electron transport to improve collection efficiency in mesoporous titanium dioxide dye sensitized solar cell stacks

Dye-sensitized solar cell (DSC) modules are generally made by interconnecting large photoelectrode strips with optimized thickness (∼14 μm) and show lower current density (JSC ) compared with their single cells. We found out that the key to achieving higher JSC in large area devices is optimized photoelectrode volume (VD ), viz., thickness and area which facilitate the electron channeling towards working electrode. By imposing constraints on electronic path in a DSC stack, we achieved >50% increased JSC and ∼60% increment in photoelectric conversion efficiency in photoelectrodes of similar VD (∼3.36 × 10−4 cm3) without using any metallic grid or a special interconnections.

Charge transport through split photoelectrodes in dye-sensitized solar cells

Charge transport and recombination are relatively ignored parameters while upscaling dye-sensitized solar cells (DSCs). Enhanced photovoltaic parameters are anticipated by merely widening the devices physical dimensions, viz., thickness and area as evident from the device design adopted in reported large area DSCs. These strip designs lead to ≤50% loss in photocurrent compared to the high efficiency lab scale devices. Herein, we report that the key to achieving higher current density (JSC ) is optimized diffusion volume rather than the increased photoelectrode area because kinetics of the devices is strongly influenced by the varied choices of diffusion pathways upon increasing the electrode area. For a given electrode area and thickness, we altered the photoelectrode design by splitting the electrode into multiple fractions to restrict the electron diffusion pathways. We observed a correlation between the device physical dimensions and its charge collection efficiency via current-voltage and impedance spectroscopy measurements. The modified electrode designs showed >50% increased JSC due to shorter transport time, higher recombination resistance and enhanced charge collection efficiency compared to the conventional ones despite their similar active volume (∼3.36 × 10−4 cm3). A detailed charge transport characteristic of the split devices and their comparison with single electrode configuration is described in this article.

A silanol-functionalized polyoxometalate with excellent electron transfer mediating behavior to ZnO and TiO 2 cathode interlayers for highly efficient and extremely stable polymer solar cells

Combining high efficiency and long lifetime under ambient conditions still poses a major challenge towards commercialization of polymer solar cells. Here we report a facile strategy that can simultaneously enhance the efficiency and temporal stability of inverted photovoltaic architectures. Inclusion of a silanol-functionalized organic–inorganic hybrid polyoxometalate derived from a PW9O34 lacunary phosphotungstate anion, namely (nBu4N)3[PW9O34(tBuSiOH)3], significantly increases the effectiveness of the electron collecting interface, which consists of a metal oxide such as titanium dioxide or zinc oxide, and leads to a high efficiency of 6.51% for single-junction structures based on poly(3-hexylthiophene):indene-C60 bisadduct (P3HT:IC60BA) blends. The above favourable outcome stems from a large decrease in the work function, an effective surface passivation and a decrease in the surface energy of metal oxides which synergistically result in the outstanding electron transfer mediating capability of the functionalized polyoxometalate. In addition, the insertion of a silanol-functionalized polyoxometalate layer significantly enhances the ambient stability of unencapsulated devices which retain nearly 90% of their original efficiencies (T90) after 1000 hours.