I am from Dhaka, Bangladesh. I completed my undergrads in Electrical and Electronic Engineering from BUET in 2009. After completing my PhD in May 2016 from Purdue University, I have been working as a Post-Doctoral research associate with Professor M. A. Alam in Alam CEED Group.
Research: Solar cells
Thermodynamic analysis of PV operation provides insights on how an ideal PV should perform. Detailed balance calculations including non-idealities redefine the limits of solar cells, and demonstrate under what conditions the non-ideal PV limits can reach close to ideal performance. Along with these understandings, a systematic and detailed device analysis will help us address the major bottle-necks of PV operation.
Coupled opto-electronic studies help us design and model structures that can have enhanced photovoltaic (PV) performances—at the same time, the structure has to be easily fabricated using existing materials.
Performance limit of converting one form of energy to another preferred form for any engine is primarily addressed by thermodynamic arguments. We can think of solar cells as ‘photon engines’ which convert light to electrical energy—thus its performance limit would also be understood from thermodynamic analysis. We start from a 2-energy-level model for understanding of solar cell operation. The simple model gives a lot of insight into how an ideal solar cell works. By adding practical limitations and non-idealities the loss components cumulate and help predict the efficiencies of currently available solar cells.
- Effect of Heterojunctions (HJ). The thermodynamic calculations for heterojunction solar cells (e.g., BHJ-OPV or planar HJ-OPV) offer some interesting conclusions. In case of poor mobility materials (e.g., OPVs), most of the recombination will occur at the cross-gap of the HJ. The cross-gap (lower than the bandgap), limits the open circuit voltage and in turn the overall efficiency of the cell.
- Exciton dissociation in OPVs. We find that exciton bottleneck may arise under normal PV operation if and only if the exciton binding energy, EB is above a critical value, EB(critical). We use a generalized formulation to derive a simple expression for EB(critical). Based on EB found in literature for common OPV materials, it is highly unlikely that excitons would greatly limit OPV performance. The results suggest that we should reconsider the implications of EB obtained from pump-probe experiments in interpreting the efficiency limit of single semiconductor solar cells.
- Imperfect absorption and non-radiative absorption. The S-Q limit calculated from the principle of detailed balance assumes perfect absorption above the bandgap energies, and emission only by radiative recombination, i.e., non-radiative recombination due to defects and carrier-carrier scattering are absent. None of these assumptions are satisfied by any of the practical solar cell materials. We have studied the effect of these non-ideal optical responses on the PV limits (JSC, VOC, FF, and efficiency limits) following the detailed balance formalism. Interestingly, light trapping is not required for PV operation close to S-Q limit. For example, a GaAs solar cell with L~3μm can operate close to the maximum possible efficiency limit. Solar cells with low external radiative efficiency (ERE) have room for improvement in by optical model focused on tailoring the emission.
Cell Performance: Carrier collection
The extremely poor electronic properties (i.e., poor mobility (μ) and low lifetime (τ)) of organic and polymer materials degrade JSC from the values expected from the absorption. The low μτ product results in high recombination and thus low FF. We have proposed structures that focus on enhancing carrier collection efficacy:
- Nano-structured electrode. Optimized nano-structured electrode (e.g., fin-like structure) provides shorter collection paths for carriers thus minimizing recombination.
- 'Layer-split' tandem OPV. We demonstrate that OPV sub-cells made of the single polymer (or organic semiconductor) and arranged in a series tandem configuration can lead to impressive (factor of 2-5) efficiency gains, provided that the sub-cells can be thinned for optimum carrier collection and stacked for improved light absorption.
Cell Performance: Light management
For certain PV technologies (e.g., a-Si), light absorption is the primary bottleneck rather than carrier collection. Besides, photon management schemes can provide an extra handle for enhancing PV efficiency by providing improved absorption with structures that also provide shorter carrier transport paths.
Photon management is one of the major steps towards efficient PV performance. Good coupling to the incident light (i.e., low reflection) followed by trapping and high absorption in the active layer is the aim for the solar cell photonic design.
- Branched nanowire (BNW) array. Nanowire (NW) array with sub-wavelength diameter show very low reflectance. This provides a strong coupling of the light into the structure. However, the absorption length increases in the NW array for larger wavelengths. Thus the absorption in the longer wavelength range is low in a finite length NW array. Hence, for broadband absorption, there is always a trade-off between reflection and total absorption while designing the diameter of the NW array. Our proposed BNW structure eliminates this compromise by allowing independent tuning of reflectance and absorptance. The BNW configuration is essentially a cascaded collection of NW array layers comprising of different diameters. The small diameter NWs at the top allow low reflection and the consecutive layers (higher diameter NW arrays) ensures high absorption.
- Meta-mirrored planar cell. The idea of absorption enhancement in the geometric limit involves isotropically scattering the incident light using a randomly textured surface. However, extensive texturing of the top surface is difficult to integrate with a thin film solar cell. Hence a large variety of nano-patterning has been explored for absorption improvement in thin dielectrics. Recent developments in the concepts of meta-surface have shown the possibility of rearranging light propagation in planar structures. Meta-surface designed for appropriate absorption and emission management can be merged with the existing high efficiency solar cells. Our proposed meta-mirror scheme provides means for trapping the incident light indefinitely—therefore, even a very poor absorber will have perfect absorption.
M. Ryyan Khan and M. A. Alam, “Critical Binding Energy for Exciton Dissociation and its Implications for the Thermodynamic Limit of Organic Photovoltaics,” in 2014, 72nd Device Research Conference (DRC). (Accepted) [pdf]
M. Ryyan Khan, P. Bermel, and M. A. Alam, “Thermodynamic Limits of Solar Cells with Non-ideal Optical Response,” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013. [pdf]
X. Wang, M. Ryyan Khan, M. A. Alam, and M. Lundstrom, “Approaching the Shockley-Queisser limit in GaAs solar cells,” in 2012 38th IEEE Photovoltaic Specialists Conference (PVSC), 2012, pp. 002117 –002121. [Link]
Asaduzzaman Mohammad, Suprem Das, M. Ryyan Khan, Muhammad Alam, David Janes, “Reflection and Transmission Properties of Thin Branched InSb NW Arrays Formed by Electrodeposition”, Electronic Materials Conference, 2011.
B. Ray, A. G. Baradwaj, M. R. Khan, B. W. Boudouris, and M. A. Alam, “Collection-limited theory interprets the extraordinary response of single semiconductor organic solar cells,” PNAS, p. 201506699, Aug. 2015. [Link]
Reza Asadpour*, Raghu V. K. Chavali*, M. Ryyan Khan*, and Muhammad A. Alam, “Bifacial Si heterojunction-perovskite organic-inorganic tandem to produce highly efficient (ηT* ∼ 33%) solar cell,” Applied Physics Letters, vol. 106, no. 24, p. 243902, Jun. 2015. (*equal contribution) [Link]
M. Ryyan Khan, X. Wang, P. Bermel, and M. A. Alam, “Enhanced light trapping in solar cells with a meta-mirror following generalized Snell’s law,” Opt. Express, vol. 22, no. S3, pp. A973–A985, May 2014. [Link]
X. Wang, M. Ryyan Khan, M. Lundstrom, and P. Bermel, “Performance-limiting factors for GaAs-based single nanowire photovoltaics,” Opt. Express, vol. 22, no. S2, pp. A344–A358, Mar. 2014. [Link]
M. Ryyan Khan, B. Ray, and M. A. Alam, “Prospects of layer-split tandem cells for high-efficiency OPV,” Sol. Energy Mater. Sol. Cells, vol. 120, Part B, pp. 716–723, Jan. 2014. [Link]
Ruiyi Chen, Suprem R. Das, Changwook Jeong, M. Ryyan Khan, David B. Janes, and Muhammad A. Alam, “Co-percolating Graphene-Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes,” Advanced Functional Materials, 2013. [Link]
M. A. Alam and M. Ryyan Khan, “Fundamentals of PV efficiency interpreted by a two-level model,” American Journal of Physics, vol. 81, no. 9, pp. 655–662, Sep. 2013. [Link]
M. A. Alam, B. Ray, M. Ryyan Khan, and S. Dongaonkar, “The essence and efficiency limits of bulk-heterostructure organic solar cells: A polymer-to-panel perspective,” Journal of Materials Research, vol. 28, no. 04, pp. 541–557, Feb. 2013. [Link]
X. Wang, M. Ryyan Khan, J. L. Gray, M. A. Alam, and M. S. Lundstrom, “Design of GaAs Solar Cells Operating Close to the Shockley–Queisser Limit,” IEEE Journal of Photovoltaics, vol. PP, no. 99, pp. 1 –8, 2013. [Link]
M. Ryyan Khan, X. Wang, and M. A. Alam, “Fundamentals of PV Efficiency: Limits for Light Absorption,” arXiv:1212.2897, Dec. 2012. [Link]
A. Mohammad*, S. R. Das*, M. Ryyan Khan*, M. A. Alam, and D. B. Janes, “Wavelength-Dependent Absorption in Structurally Tailored Randomly Branched Vertical Arrays of InSb Nanowires,” Nano Lett., Nov. 2012. (*equal contribution) [Link]
J. E. Allen, B. Ray, M. Ryyan Khan, K. G. Yager, M. A. Alam, and C. T. Black, “Self-assembly of single dielectric nanoparticle layers and integration in polymer-based solar cells,” Applied Physics Letters, vol. 101, no. 6, pp. 063105–063105–4, Aug. 2012. [Link]
B. Ray, M. Ryyan Khan, C. Black, and M. A. Alam, “Nanostructured Electrodes for Organic Solar Cells: Analysis and Design Fundamentals,” IEEE Journal of Photovoltaics, vol. PP, no. 99, pp. 1 –12, 2012. [Link]
M. Ryyan Khan and M. K. Hasan, “A novel model for show-through in scan of duplex printed documents,” SIViP, vol. 6, no. 4, pp. 625–645, Nov. 2012. [Link]
M. Ryyan Khan, H. Imtiaz, and M. K. Hasan, “Show-through correction in scanned images using joint histogram,” SIViP, vol. 4, no. 3, pp. 337–351, Sep. 2010. [Link]
M. Ryyan Khan, T. Hasan, and M. Rezwan Khan, “Iterative noise power subtraction technique for improved speech quality,” in International Conference on Electrical and Computer Engineering, 2008. ICECE 2008, 2008, pp. 391 –394. [Link]