Characterization of amorphous Si, CdTe, and Cu(InGa)Se 2 -based thin-film solar cells is described with focus on the deviations in device behavior from standard device models. Quantum efficiency (QE), current-voltage (J-V), and admittance measurements are reviewed with regard to aspects of interpretation unique to the thin-film solar cells. In each case, methods are presented for characterizing parasitic effects common in these solar cells in order to identify loss mechanisms and reveal fundamental device properties. Differences between these thin-film solar cells and idealized devices are largely due to a high density of defect states in the absorbing layers and to parasitic losses due to the device structure and contacts. There is also commonly a voltage-dependent photocurrent collection which affects J-V and QE measurements. The voltage and light bias dependence of these measurements can be used to diagnose specific losses. Examples of how these losses impact the QE, J-V, and admittance characterization are shown for each type of solar cell.Solar cell operation, either crystalline or thin film, can be described by identifying loss mechanisms. These can be divided into three categories. First are recombination losses which limit the open-circuit voltage V OC . Second are parasitic losses, such as series resistance, shunt conductance, and voltage-dependent current collection, which primarily impact the fill factor (FF), but can also reduce short circuit current J SC and V OC . Finally, there are optical losses which limit generation of carriers and, therefore, J SC . We focus on losses largely unique to TFSCs.Physical and electrical properties of TFSCs which cause them to have different losses from the standard 'textbook' crystalline Si (c-Si) cells include: * TFSC absorber layers have much higher absorption coefficients than c-Si so a large fraction of the photogeneration occurs near the interface and in the high field space charge region (SCR). This enables high currents, even with relatively small collection lengths; * the semiconductor films often have a range of shallow and deep defect levels or defect bands within the bandgap. These result from imperfect crystallinity or amorphous structure and from the use of low-cost materials and processes optimized for high throughput and low cost as much as for high device efficiency. This can create different recombination mechanisms than radiative band-to-band recombination commonly found in ideal crystalline semiconductor devices; * poor minority carrier lifetime, due to the above factors, leads to increased reliance on the electric field for sufficient minority carrier collection rather than diffusion alone. This often results in voltage-dependent collection of light-generated current; * TFSCs are heterojunction device structures with high densities of defect states at interfaces which can provide a path for interface recombination; * the grain boundaries in polycrystalline Cu(InGa)Se 2 and CdTe devices may act as high recombination surfaces or shunt paths. This l...

Thin film solar cells (TFSC) have passed adolescence and are ready to make a substantial contribution to the world's electricity generation. They can have advantages over c-Si solar modules in ease of large area, lower cost manufacturing and in several types of applications. Factors which limit TFSC module performance relative to champion cell performance are discussed along with the importance of increased throughput and yield. The consensus of several studies is that all TFSC can achieve costs below 1 $/W if manufactured at sufficiently large scale >100 MW using parallel lines of cloned equipment with high material utilization and spray-on encapsulants. There is significant new commercial interest in TFSC from small investors and large corporations, validating the thin film approach. Unique characteristics are discussed which give TFSC an advantage over c-Si in two specific markets: small rural solar home systems and building integrated photovoltaic installations. TFSC have outperformed c-Si in annual energy production (kWhrs/kW), have demonstrated outdoor durability comparable to c-Si and are being used in MW scale installations worldwide. The merits of the thin film approach cannot be judged on the basis of efficiency alone but must also account for module performance and potential for low cost. TFSC advocates should promote their unique virtues compared to c-Si: lower cost, higher kWhr/kW output, higher battery charging current, attractive visual appearance, flexible substrates, long-term stability comparable to c-Si, and multiple pathways for deposition with room for innovation and evolutionary improvement. There is a huge market for TFSC even at today's efficiency if costs can be reduced. A brief window of opportunity exists for TFSC over the next few years due the Si shortage. The demonstrated capabilities and advantages of TFSC must be proclaimed more persistently to funding decision-makers and customers without minimizing the remaining challenges.

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The voltage dependence of the photocurrent JL(V) of CdTe/CdS solar cells has been characterized by separating the forward current from the photocurrent at several illumination intensities. JL(V) reduces the fill factor (FF) of typical cells by 10–15 points, the open circuit voltage (VOC) by 20–50 mV, and the efficiency by 2–4 points. Eliminating the effect of JL(V) establishes superposition between light and dark J(V) curves for some cells. Two models for voltage dependent collection give reasonable fits to the data: (1) a single carrier Hecht model developed for drift collection in p‐i‐n solar cells in which fitting yields a parameter consistent with lifetimes of 10−9 s as measured by others; or (2) the standard depletion region and bulk diffusion length model fits almost as well. The simple Hecht‐like drift collection model for photocurrent gives very good agreement to J(V) curves measured under AM1·5 light on CdTe/CdS solar cells with FF from 53% to 70%, CdTe thickness from 1·8 to 7·0 µm, in initial and stressed states. Accelerated thermal and bias stressing increases JL(V) losses as does insufficient Cu. This method provides a new metric for tracking device performance, characterizes transport in the high field depletion region, and quantifies a significant FF loss in CdTe solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (VOC) due to the surface passivation and heterojunction contacts, and high short circuit current density (JSC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence VOC and JSC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since JSC and VOC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.