The stability of perovskite solar cells (PSC) is often compromised by the organic hole transport materials (HTMs). We report here the effect of WO3 as an inorganic HTM for carbon electrodes for improved stability in PSCs which are made under ambient conditions. Sequential fabrication of the PSC was performed under ambient conditions with mesoporous TiO2/Al2O3/CH3NH3PbI3 layers and, on the top of these layers, the nanoparticles embedded carbon electrode was used. Different concentrations of WO3 nanoparticles as HTM incorporated in carbon counter electrodes were tested, which varied the stability of the cell under ambient conditions. The addition of 7.5% WO3 (by volume) led to a maximum power conversion efficiency of 10.5%, whereas the stability of the cells under ambient condition was ~350 h, maintaining ~80% of the initial efficiency under light illumination. At the same time, the higher WO3 concentration exhibited a high efficiency of 9.5%, which was stable up to ~500 h with a loss of only ~15% of the initial efficiency under normal atmospheric conditions and light illumination. This work demonstrates an effective way to improve the stability of carbon based perovskite solar cells without affecting the efficiency for future applications.

The emergence of perovskite solar cells (PSCs) in a "catfish effect" of other conventional photovoltaic technologies with the massive growth of power conversion efficiency (PCE) with a simple manufacturing process has given a new direction to the entire solar energy field. Usually, PSC components such as electron transport material (ETM), perovskite sensitizer, hole transport material (HTM), and electrode materials need to be appropriately aligned according to the electron transfer and recombination process in order to achieve the best out of the device. Despite the enormous amount of research, the stability, reactivity, and cost issues of noble metal (Au, Ag) electrode-based traditional PSC devices are becoming obstacles to marketization. Due to the low fabrication cost and enhanced ambient stability, carbon counter electrode-based PSC (CPSC) evolved as a suitable alternative in such scenarios. These CPSCs are still in a stage of development where different fabrication engineering, designs and materials are being investigated to attain a comparable state with the standard commercialized photovoltaics. To date, hardly any report is available on ambient CPSC with PCE over 15% and stability of ~1000h without encapsulation, which opens up the window for more research.
The fundamental objective of this thesis work was to develop high-performance ambient CPSC with PCE > 15% under 1SUN AM 1.5 illumination, maintaining the stability of ~1000h. This was achieved using alternative ETM and HTM with strategic incorporation instead of traditional ones. Noticeably, the temperature is a crucial parameter to attain and, at the same time, retain the aimed PV performance and stability. Therefore, a physico-thermal investigation was performed to understand the effect of temperature on the fabricated CPSC devices. The understanding further helped to examine the possible futuristic application of CPSC as the semi-transparent device for energy savings build environment.
To achieve the goals of this thesis, the 1st step involved finding out suitable combination of HTM and carbon counter electrodes highlighted in chapter 3. For the first time, the fully printable mesoporous CPSCs are demonstrated with concentration-dependent WO3 (5, 7.5, and 10% by volume) nanoparticles incorporated in carbon electrodes fabricated under ambient conditions. The highest PCE ~10.5% was obtained with the 7.5% WO3/carbon device; however, the 10% WO3/carbon device exhibited better ambient stability of ~600h. Besides, graphene/ poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS) was introduced as an alternative HTM with novel light soaking and surface wettability strategies, and an enhanced PV performance with PCE >11% was achieved. In search of an alternative HTM/carbon combination with more superior performance, a novel and cost-effective synthesis process of graphitic CNP as a suitable counter electrode and its combination with NiO was visualized. The stability test of the high-temperature counter electrode strategy of CNP/NiO showed ~1000 h air stability with negligible efficiency loss having a maximum PCE of 13.2%, whereas the low-temperature strategy of CNP/NiO devices showed 14.2% PCE with ~650 h air stability. Thus CNP/NiO combination achieved performance very close to the aims of this thesis, which was enhanced to the required performance by introducing alternative ETM for the devices. Chapter 4 describes the strategic incorporation of morphology modulated BaSnO3 (BSO) and brookite TiO2 (BTO) nanostructures in place of conventional anatase TiO2 as ETM to successfully achieve PCE >13.5% and >15%, respectively, with stability >1000 h. The enhanced electron transport and reduced charge recombination by rod-based nanostructures of BSO and BTO displayed the best performance for the types to date in CPSC. Along with performance improvement, the understanding of CPSC’s temperature behaviour was considered in this thesis to understand the real-world feasibility of CPSC for the first time. The temperature coefficients (TC) of photovoltaic parameters for MAPbI3-based devices are demonstrated in chapter 5 with a detailed physico-chemical understanding. Besides CH3NH3PbI3, other perovskites such as CH3NH3PbI3-xClx and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 were applied as an alternative sensitizer for the CPSCs and studied their temperature coefficients across a wide range of real-world temperatures to obtain behavioural differences between the halide perovskites. Finally, the suitability of semi-transparent CPSC for fenestration integration was evaluated for the first time via fabrication engineering and thickness control with the highest reported average visible transmittance/PCE combination to date, as discussed in chapter 6. Finally, in this thesis work, CPSC devices are explored, which highlights fascinating ambient fabrication processes and significant device performance with new series of HTMs, ETMs and designs for futuristic applications.

In this work, carbon counter electrode perovskite was developed at the laboratory environment and building integrated photovoltaic (BIPV) window application using this material was investigated. At 1 sun (1000 W/m2) continuous incident solar radiation from an indoor simulator, this particular type of perovskite had 8.13% efficiency. Average solar and visible transmittance of this perovskite BIPV window was 30% and 20% respectively. Solar heat gain for different incident angle was evaluated for this perovskite glazing. For the University of Exeter, Penryn (50.16° N, 5.10° W) UK location, solar heat gain coefficient (SHGC) or solar factor (SF) varied from 0.14 to 0.33 at the highest and lowest incident angle respectively. Overall heat transfer coefficient (U-value) of 5.6 W/m2K was realized for this glazing while calculation was performed by window performance analysis programme, WINDOW 6.0. Daylight glare control potential of this glazing was investigated using subjective rating methods and comfortable daylight penetrated through glazing in a typical cloudy condition. Colour properties of this material showed that 20% visible transmittance is threshold limit, and below this value colour or visual comfort using this glazing is not achievable.