Archives for the month of: August, 2013

I recently had an opportunity to attend the 39th IEEE Photovoltaics Specialists Conference in Tampa, Florida (http://www.ieee-pvsc.org/PVSC39/). Since I am just starting to work in Photovoltaics, it was a great opportunity for me to get immersed in this very quickly developing field by listening to high quality presentations given by the leaders of the field. The presentations were divided into 11 topic areas, with a few sessions taking place at the same time. I really wanted to be in a few rooms simultaneously, but with my background in quantum theory I decided to focus mostly on attending sessions from research Area 1: Fundamentals and New Concepts of Future Technologies.

I found a talk by Megumi Yoshida from Imperial College London :“Progress towards Realizing Intermediate Band Solar Cell – Sequential Absorption of Photons in a Quantum Well Intermediate Band Solar Cell” particularly interesting. I was familiar with the concept of introducing the intermediate band (IB) into the solar cell to improve the absorption of photons with energy lower than the band gap energy (see Fig 1 (a)), however this talk took this concept one step further. The intermediate band solar cell (IBSC) can be created introducing quantum mechanically confined structures such as quantum wells and quantum dots [1] into the solar cell. In such cases, the IB arises from the confined states of the electrons in the conduction band (CB) potential.  In theory, by introducing the IB the current can be enhanced by two step absorption of long wavelength photons without reducing the voltage, hence leading to higher conversion efficiency (thermodynamical limit of 46.8% at 1 sun [2]), than a single bandgap solar cell (31.0% at 1 sun).

Figure 1 of Anna's blog

Fig. 1. Energy diagram of an (a) IBSC and (b) IBSC with photon rachet band (RB), in which extra photocurrent is produced due to sequential absorption of sub-bandgap photons increasing theoretical limit in the conversion efficiency. Reprinted with permission from [3]. Copyright 2012 AIP Publishing LLC.

 

However, experimentally obtained IBSCs suffer from a significant voltage loss resulting in lower than predicted theoretical conversion efficiency due to the short lifetime of electrons in the intermediate states. The short lifetime is caused by fast radiative and non-radiative recombination of carriers that occurs before the second photon can be absorbed. To achieve a long carrier lifetime of electrons in the IB, the authors suggest the introduction of a non-emissive (optically decoupled from the valance band (VB) [3]) “ratchet band (RB)” at an energy interval ΔE below the IB (shown in Fig. 1(b)). If there is fast thermal transition between the IB and the ratchet band, the photo-excited electrons in the IB rapidly relax into the RB where the lifetime of the carriers can be very long given that the RB is optically isolated from the VB. The increase in lifetime of the electrons enhances the probability of the second optical excitation process from the RB to the CB and increases the IB-CB generation rate. At the same time, the recombination rate of the electrons from the IB to the VB depends on the population in the IB. The presence of the RB reduces the population of carriers in the IB and the same the recombination rate leading to the increase of the photocurrent of the solar cell and its higher efficiency.

Figure 2 of Anna's blog

Fig. 2. Efficiency limit of photon ratchet IBSC of various concentrations as a function of ΔE. Efficiency at ΔE=0 corresponds to conventional IBSCs. Reprinted with permission from [3]. Copyright 2012 AIP Publishing LLC.

Authors calculate the globally optimised limiting efficiency of the photon ratchet IBSC as a function of the energy difference ΔE between the IB and RB. These dependencies are plotted for three solar concentrations in Fig. 2. Even though electrons lose the energy ΔE by transitioning from the IB to the RB, an increase in efficiency is visible as ΔE is increased. At the same time, the presence of the ratchet increases the below-bandgap and thermalization losses, but the associated efficiency gain through reduction in recombination is even larger. At 1 sun illumination, the efficiency of the IBSC is increased from 46.8% (ΔE = 0, conventional IBSC) to 48.5% with a photon ratchet at ΔE = 270 meV. At full concentration however, the introduction of the loss due to the presence of the ratchet band is not compensated by any other mechanism, leading to a decreased efficiency of the IBSC with the RB as compared to the standard IB cell (ΔE = 0).

Although it seems that the implementation of the photon ratchet IBSC is going to be challenging, I think that this concept is very interesting. Authors suggest that the RB can be built out states of indirect bandgap semiconductor separated in momentum space from the IB. Electrons would be first excited into a direct IB state, followed by a relaxation down through phonon emission to an indirect photon ratchet state, which is at a lower energy and separated by momentum k from the IB state, as well as the top of the VB.

Anna Trojnar's picture

 

-Anna Trojnar, Ph.D

Postdoctoral Fellow

Sunlab, University of Ottawa

 

[1] A. Marti, L. Cuadra, A. Luque, “Quantum dot intermediate band solar cell” Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference, p940 (2000).

[2] A. Luque and A. Martí, “Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels”, Phys. Rev. Lett. vol. 78, p5014 (1997).

[3] M. Yoshida, N. J. Ekins-Daukes, D. J. Farrell, C. C. Phillips, “Photon ratchet intermediate band solar cells,” Appl. Phys. Lett. vol. 100, p263902 (2012)

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The 39th Photovoltaic Specialists Conference in Tampa, Florida was a great conference. Photovoltaic specialists from all over the world gathered at the paradise of recreation, learning the most recent developments in this field delivered by hundreds of high quality presentations and posters. The topics span across eleven general areas from fundamental and new concepts of PV to the supporting of PV innovation with little to none overlap between each area. For me, an organic chemist focusing on developing new polymers to improve the stability of organic photovoltaics (OPV), the area that mostly interested me was, without a doubt, OPV.

Thursday of last week was the day for OPVs. In the morning, Professor Christian Körner from Heliatek GmbH gave a talk on the recent progress of organic solar cells [1].  Like a typical general review of this field, the talk started with some fundamental concepts and theories of organic semiconductors, then moved to some developments that aimed at solving the issues commonly encountered when applying organic materials to photovoltaic devices. He also covered recent improvements on the lifetime and module efficiencies of OPVs, which are not as much discussed as the efficiencies of lab scale devices. Not surprisingly, the materials presented in this talk are highly related to those being developed at Heliatek. With active materials based on small molecules and p-i-n structured devices, they could achieve 12% of photo conversion efficiency on lab scale devices and 9% on 100 cm2 scale modules that last as long as several thousand hours under intensified illumination. However, from an academic point of view, these technologies and accomplishment do not necessarily reflect the general recent progress of OPV in academia, where polymers and bulk heterojunctions are the dominant materials and device structure, respectively. Regarding the progress of OPV using conventional structures and material, there was an interesting presentation in the afternoon session given by an award winning Ph.D. candidate Biswajit Ray from Purdue University [2]. In his research work, he showed that the limiting factor of the charge transportation process are recombinations happening at the neutral, flat region of the band diagram. Conventionally, many people think the limiting factor is charge mobility. Although his evidence is not sufficient enough, it is helpful to look at a problem from a view that opposes the convention.

Apart from the presentation content, a new presentation technique was unveiled at one of the plenary talks. In it, the presenter used a facebook homepage to deliver his talk with the aid of the timeline function. At the end, an internet address that directs to that facebook page made all the slides publically accessible. It is notable that people’s attention was drawn to this presentation even though some of them were not familiar with the field.

It was a wise idea that the conference committee chose to hold it in Tampa Bay. It was the best time of the year to go there and to the beach where the water was warm and the sand was fine and soft like snow. Everyone from the network that went to the beach had a wonderful time there. Also, the Salvador Dalí Museum is definitely worth seeing, since it holds the largest number of Dalí’s masterpieces which will truly stun you.

[1] C. Körner, M. Hermenau, C. Elschner, C. Schunemann, S. Mogck, M. Riede, K. Leo, “Recent progress in organic solar cells: From a lab curiosity to a serious photovoltaic technology” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

[2] B. Ray and M. Alam “Role of charged defects on organic solar cell performance: Prospect of Heterojunction-free device design” in 2013 39th IEEE Photovoltaic Specialists Conference (PVSC), 2013.

Ben Zhang's picture

Chi Zhang (Ben)

Ph.D. Candidate, Year 3

Simon Fraser University, Burnaby, British Columbia

As Ontario’s flagship renewable energy (RE) incentive program, referred to as the Feed-in Tariff (FIT), enters its third review, it is an appropriate point to assess several recent political and policy changes which have implications for the next iteration of RE incentives. In my previous blog, I wrote about the challenges facing Photovoltaics (PV) with respect to the World Trade Organization’s (WTO) initial ruling against Ontario’s domestic content requirements, growing economic turbulence in the RE sector, political adjustments within the provincial Liberal party  as well as other industrial and policy factors that have created uncertainty surrounding the future deployment and development of PV in Ontario. These events have had lasting impacts on the prospects of PV and continue to influence future policy engagement. In conjunction with prior developments, there are new pressures and policy changes which will have serious implications for PV. Do these changes point to positive change for the future of PV?

Foremost among these pressures is the fallout around the WTO decision. In May 2013, the final ruling by the WTO stated that Ontario’s RE policy framework was in violation of international trade law[i]. The decision established that the domestic content requirement, which requires that developers source a certain percentage of RE system components from Ontario companies in order to be eligible for FIT incentives, contravenes international trade agreements. Without protection from international competition, PV companies who have invested in Ontario face an uncertain future. Will PV panel manufacturers be able to compete with Asian firms? Which parts of the PV value chain will Ontario firms occupy? Will we purchase technology from overseas and focus primarily on project installation? These are just some of the questions left unanswered as the government begins to reinvent the FIT.

The provincial government has already communicated significant changes to the FIT, which may portent further revisions presently being contemplated. In June 2013, the government announced that large RE projects (>500 kW) would no longer fall under the FIT mechanism[ii]. Instead, these projects would be contracted using a competitive procurement process. The details of this process have yet to be articulated. Moreover, the annual cap for FIT contracts has been reduced from 250 MW to 200 MW[iii]. Aside from the FIT, the political administration has also renegotiated its deal to procure a significant quantity of RE from Samsung, cutting the contract from 2,500 MW down to 1,369 MW[iv]. From this, it would appear that a deliberate effort is being made to slow further RE procurement and deployment in the province.

Political pressures are also quite notable. First there is the continued uncertainty arising from a minority government situation, where opposing parties have stated they would either abandon[v] or seriously redesign[vi] RE policies. Second, controversial gas plant cancellations continue to plague the governing party, casting shadows over the energy file and the commitment to RE deployment[vii]. Lastly, and perhaps most importantly, there are still many unanswered questions regarding the future of nuclear energy in the province[viii]. If Ontario commits to build new nuclear reactors, there is little impetus to invest in RE and efficiency.

Together, the above factors indicate that there is increasing uncertainty surrounding the RE sector in the province. Recent developments suggest that Ontario is veering away from its commitment to an economy and energy system based on RE. So, the worst may have yet to come for PV in Ontario.

Danny Rosenbloom

Daniel Rosenbloom
Ph.D Candidate

Carleton University