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Register a new userRole of Carrier Mobility and Band Alignment Engineering on the Efficiency of Colloidal Quantum Dot Solar Cells
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We investigate physics based design of colloidal quantum dot (CQD) solar cells using self-consistent computational modeling. The significance of band alignment engineering and optimized carrier mobility are quantitatively explored as a function of sub bandgap defect densities (N_t) in the bulk CQD. For $N_t leq 10^{15} cm^{-3}$, band alignment engineering near the interface of CQD and the metal contact could significantly improve open circuit voltage by suppressing the forward bias dark current. This effect could enhance cell efficiency up to ~37% for thinner $(< 1 mu m)$ CQD layers. For thicker $(> 1 mu m)$ CQD layer, the effect of band engineering is diminished as the forward bias dark current becomes diffusion-limited and less dependent on the interfacial band offsets. An optimal carrier mobility in CQD lies in the range ~ 10^{-2} cm^2/Vs - 10^0 cm^2/Vs and shows variation as a function of CQD layer thickness and the interfacial band offset. For $N_t approx 10^{14} cm^{-3}$, an optimally designed cell could provide ~20% efficiency under AM1.5G solar spectrum without employing advanced structural optimizations such as the nanostructured electrodes. These physical insights contribute to a better understanding of quantum dot solar cell design, allowing a step further towards a highly efficient and a low cost solar cell technology.
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In this research, the effect of Magnesium Fluoride (MgF2) Anti-Reflection (AR) layer was investigated in quantum dot sensitized solar cells (QDSCs). MgF2 nanoparticles with the dominant size of 20 nm were grown by a thermal evaporation method and a thin layer was formed on the front side of the fluorine-doped tin oxide (FTO) substrate. In order to study the effect of the AR layer on the efficiency of solar cells, this substrate was utilized in CdS QDSCs. In this conventional structure of QDSC, TiO2 nanocrystals (NCs) were applied on the FTO substrate, and then it was sensitized with CdS quantum dots (QDs). According to the results, the QDSCs with MgF2 AR layer represented the maximum Power Conversion Efficiency (PCE) of 3%. This efficiency was increased by about 47% compared to the reference cell without the AR layer. The reason is attributed to the presence of the AR layer and the reduction of incident light reflected from the surface of the solar cell.
The quantum well solar cell (QWSC) has been proposed as a flexible means to ensuring current matching for tandem cells. This paper explores the further advantage afforded by the indication that QWSCs operate in the radiative limit because radiative contribution to the dark current is seen to dominate in experimental data at biases corresponding to operation under concentration. The dark currents of QWSCs are analysed in terms of a light and dark current model. The model calculates the spectral response (QE) from field bearing regions and charge neutral layers and from the quantum wells by calculating the confined densities of states and absorption coefficient, and solving transport equations analytically. The total dark current is expressed as the sum of depletion layer and charge neutral radiative and non radiative currents consistent with parameter values extracted from QE fits to data. The depletion layer dark current is a sum of Shockley-Read-Hall non radiative, and radiative contributions. The charge neutral region contribution is expressed in terms of the ideal Shockley radiative and non-radiative currents modified to include surface recombination. This analysis shows that the QWSC is inherently subject to the fundamental radiative efficiency limit at high currents where the radiative dark current dominates, whereas good homojunction cells are well described by the ideal Shockley picture where the limit is determined by radiative and non radiative recombination in the charge neutral layers of the cell.
The spin-split indirect bandgap in hybrid-halide perovskites provides a momentum-space realisation of a photon-ratchet intermediate band. Excited electrons thermalise to recombination-protected Rashba pockets offset in momentum space, building up the charge density to have sufficient flux to the higher lying conduction band. This effect could be used to form an intrinsic intermediate band solar cell with efficiencies beyond the Shockley-Queisser limit if a selective low-electron affinity contact can be made to the higher conduction state. This concept is supported by analysis of the many-body electronic structure. Production of above-bandgap voltages under illumination would affirm the physical mechanism proposed here.
As the race towards higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport towards higher power efficiency has been urgently demanded. Here, we unravel a hidden role of A-site cation of PSCs in carrier transport which has been largely neglected, i.e., tuning the Frohlich electron-phonon (e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Frohlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ion. The coordination to I alleviates electron-phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses lower electron-phonon coupling by several promising organic cations including hydroxyl-ammonium cation (NH$_3$OH$^+$) and possibly Li$^+$ solvating methylamine (Li$^+$NH$_2$CH$_3$) than methyl-ammonium cation. A new perspective on the role of A-site cation could help in improving power efficiency and accelerating the application of PSCs.
The performance of kesterite thin-film solar cells is limited by a low open-circuit voltage due to defect-mediated electron-hole recombination. We calculate the non-radiative carrier-capture cross sections and Shockley-Read-Hall recombination coefficients of deep-level point defects in Cu$_2$ZnSnS$_4$ (CZTS) from first-principles. While the oxidation state of Sn is +4 in stoichiometric CZTS, inert lone pair (5$s^2$) formation lowers the oxidation state to +2. The stability of the lone pair suppresses the ionization of certain point defects, inducing charge transition levels deep in the band gap. We find large lattice distortions associated with the lone-pair defect centers due to the difference in ionic radii between Sn(II) and Sn(IV). The combination of a deep trap level and large lattice distortion facilitates efficient non-radiative carrier capture, with capture cross-sections exceeding $10^{-12}$ cm$^2$. The results highlight a connection between redox active cations and `killer defect centres that form giant carrier traps. This lone pair effect will be relevant to other emerging photovoltaic materials containing n$s^2$ cations.
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